12^th Annual Harvard Forest Ecology Symposium

Monday, April 23, 2001

Development and Lateral Expansion of Peatlands in Central New England
R. Anderson This project is investigating the developmental history of forested peatlands in central New England by examining two questions. 1. What is the influence of climate and topography on the development of forested peatlands? 2. Is lateral expansion across adjacent uplands, i.e. paludification, an important part of this history? With adequate moisture, peatlands form through the mechanisms of terrestrialization, when a waterbody fills with peat, and paludification, when the peat mass laterally expands onto dry land. The relative importance of these two mechanisms can be distinguished by characterizing the peat deposits and developing detailed chronologies for individual sites. Terrestrialization is identified by the presence of open-water sediments while paludification is indicated by progressively younger basal peat sediments towards the peatland edge. Both climate and topography function as fundamental controls on the development of peatlands and the dynamics of paludification. When climate is a driving force across a region, multiple sites may be expected to have similar dates of peat initiation or lateral spread. Topography influences paludification by directing lateral expansion towards the shallowest slopes. If the surrounding slopes are very steep, paludification may be greatly limited. To address these questions, three forested peatlands were studied. First, basin morphometery and sediment stratigraphy were mapped in detail. Basin depth was defined as depth to mineral sediments, either till or glacial clay. Table 1 presents the field classification scheme utilized and interpretations of the sediments types. To examine sediment patterns, an average stratigraphy was determined for the four stratigraphy types (Figures 1, 2, and 3). Basin shape typically varied gradually, although in some instances there were very abrupt changes. At all three sites, areas less than 1 m in depth were characterized by woody peat deposited directly on glacial till. This is potential evidence of lateral expansion. Slightly deeper areas typically had woody peat deposited over gyttja (aquatic sediments). In the deep basins, the characteristic stratigraphy was woody and shrub peat over gyttja and clay. Overall, there were no apparent large-scale reversals from peat sediments to aquatic sediments. Sediment patterns were consistent across all three sites, suggesting similar developmental pathways. Based on this information, basal sediment samples were taken from transects running from the deepest part of the basin to the upland edge. In addition, samples were taken at sediment transitions from a core from the deepest basin of each site. These samples are currently being AMS radiocarbondated, and the results will help interpret the role of climate in development.

Eddy Covariance and Biometric Measurements of CO₂ Exchange at the Harvard Forest
C. Barford, S. Urbanski, L. Hutyra, E. Pyle, F. Frizzell, S. Heath, J. Munger and S. Wofsy

Forest inventories and atmospheric studies both indicate that forests in northern mid-latitudes have sequestered significant atmospheric CO₂ since 1980, although the magnitude and distribution of the sink are the subjects of lively debate. Factors controlling net carbon uptake must be understood in order to predict future growth rates of atmospheric CO₂, and to enable management of regional carbon budgets. Recent analyses have attributed net C uptake to land use, fire history, longer growing seasons, and fertilization by anthropogenic CO₂ and N. Our carbon exchange studies at Harvard Forest use eddy covariance and biometry, two independent methods, in order to: (1) monitor net ecosystem exchange (NEE) of CO₂ over time scales from hours to several years, (2) place CO₂ exchange measurements in the context of past land use, disturbance and current tree species demography, (3) facilitate comparisons with other forested ecosystems.

Over a seven-year period (1993-2000), annual NEE averaged -1.9 Mg C ha^-1 yr^-1, with +/- 50% inter-annual variations. Biometric measurements in the eddy covariance footprint compared well with long-term NEE, with 60-70% of mean NEE attributed to above-ground wood increment (AGWI), and the balance to soils and coarse woody debris. However, the ratio AGWI/(-NEE) for individual years with detailed dendrometry was 1 (1998), 0.6 (1999) and 0.7 (2000). This variation arises from asynchrony between carbon cycle processes, such as lagging respiration of leaf litter in dry years, and use of stored carbohydrate for tree growth. Tree mortality also contributed to inter-annual variation in the carbon budget (mortality = 0.4, 1.0, and 0.3 Mg C ha^-1 yr^-1 in 1998-2000, respectively).

Current C sequestration at Harvard Forest may be attributed to ecosystem characteristics, which have been strongly influenced by land-use and disturbance history. The 1938 hurricane and subsequent salvage allowed establishment of fast-growing, shade-intolerant northern red oak (Quercus rubra L.). These oaks are now approximately half mature size and constitute half of above-ground woody biomass and AGWI. Their growth rate is relatively slow for the species, which argues against significant fertilization by CO₂ or N deposition. Harvard Forest soils are strongly N-limited, which decreases the potential for CO₂ fertilization at ambient levels. Anthropogenic N deposition rates at Harvard Forest are modest, contributing on the order of 10% of the annual N mineralization rate, but chronic deposition over decades has likely contributed to C storage at the site.

We find that decadal mean C uptake rates were controlled by stand age and composition, the legacies of prior disturbance. Inter-annual fluctuations reflected ecosystem response to climate variations, through changes in litter decay rates and tree mortality. Given the broad spatial cohesion of global climate anomalies and high variability in the atmospheric CO₂ increase rate, it seems likely that year-to-year variations in C sequestration by the terrestrial biosphere are also influenced by climatic factors quite different from the ecological factors regulating long-term sequestration.

Long-Term Forest Monitoring at Harvard Forest
A. Barker Plotkin and D. Foster

Permanent plots are a key component of a long-term ecological research program. They provide direct insight into forest development, complement reconstructive and space-for-time techniques, and serve as controls to experimental areas. Sites in which the trees are mapped provide detail into the mechanisms of forest change and spatial patterns of forest dynamics. Permanent plot studies have been part of the Harvard Forest’s mission since it was established.

Since 1909, quantitative forest inventories of the Harvard Forest have been undertaken every 10-30 years. One of these inventories was completed in 1937, fortuitously providing a detailed set of baseline data to compare to the post-1938 hurricane forest. In 1992, establishing permanent plots in the approximate locations of the 1937 inventory plots, and sampling all vascular species and soil chemical and physical properties augmented this inventory. This study helped to elucidate persistent effects of past land-use on current forest structure and function (Motzkin et al. 1999, Compton and Boone 2000). Another set of permanent plots (121 20x20m plots and a 4.5 ha stem-mapped area) was established at the Montague Sand Plain for a similar study of the effects of land-use on modern vegetation and ecosystem processes in an environment with little environmental variation (Motzkin et al. 1996, Compton and Boone 1998).

Research at the Harvard Tract of the Pisgah Forest, an 8 hectare area of old-growth hemlock-white pine forest in southwestern New Hampshire, has been ongoing since the 1920s. The current set of permanent forest plots (fourteen 20x20m plots) was established in 1984 (Foster 1988) and was most recently remeasured in Fall 2000. This stand was severely damaged by the 1938 hurricane; data from these permanent plots shows continuing forest development with major trends of increasing basal area, decreasing stem numbers and a decline in tree species richness as short-lived or light-demanding species become less important over time. At Harvard Forest, fourteen permanent plots (0.025-0.1 ha) were established to document vegetation recovery directly after the 1938 hurricane; these plots illustrate successional trends in both the trees and understory vegetation (Spurr 1956, Hibbs 1983, Mabry and Korsgren 1998).

Walter Lyford, a soil scientist at the Harvard Forest for many years, established a three hectare mapped forest site on the Prospect Hill tract of Harvard Forest. The forest is a typical mixed hardwood site with some variation in soils, particularly drainage. This area has been remeasured decadally since 1969; we plan to remeasure the site in Summer 2001, and translate the hand-drawn maps to a Geographic Information System. The Mapped Overstory Plots and Hemlock Woodlot Plot were established in 1990 as part of the Harvard Forest Long-Term Ecological Research program. Both sites include large mapped areas. The Mapped Overstory Plots include four 50x50m plots in which all stems >5cm are mapped and measured; this area is due to be revisited soon. The Hemlock Woodlot is a 0.72 ha site dominated by hemlock. This area has been subject to intensive paleoecological reconstructions (Foster et al. 1992, Foster and Zebryk 1993). The site was remeasured in 1999 with particular attention paid to evaluating the vigor of the overstory hemlock trees. This should provide valuable baseline data in anticipation of the arrival of the hemlock woolly adelgid to the site. So far, no evidence of adelgid infestation has been found

These sites help us develop an understanding of the long-term processes that affect forest development and the role of disturbances on these systems (e.g., hurricanes and pathogens). Such studies become ever more valuable over time.

Compton, J. E., R. D. Boone, G. Motzkin, and D. R. Foster 1998. Soil carbon and nitrogen in a pine-oak sand plain in central Massachusetts: role of vegetation and land-use history. Oecologia 116: 536-542.
Compton, J.E. and R. D. Boone 2000. Long-term impacts of agriculture on soil carbon and nitrogen in a New England forest. Ecology 81: 2314-2330.
Foster, D. R. and T. M. Zebryk 1993. Long-term vegetation dynamics and disturbance history of a Tsuga-dominated forest in New England. Ecology 74: 982-998.
Foster, D.R. 1988. Disturbance history, community organization and vegetation dynamics of the old-growth Pisgah forest, south-western New Hampshire, U.S.A. Journal of Ecology 76:105-134.
Foster, D.R., T. Zebryk, P. Schoonmaker and A. Lezberg. 1992. Post-settlement history of human land-use and vegetation dynamics of a Tsuga canadensis (hemlock) woodlot in central New England. Journal of Ecology 80: 773-786.
Hibbs, D.E. 1983. Forty years of forest succession in central New England. Ecology 64: 1394-1401.
Mabry, C. and T. Korsgren. 1998. A permanent plot study of vegetation and vegetation-site factors fifty-three years following disturbance in central New England, U.S.A. Ecoscience 5: 232-240.
Motzkin, G., P. Wilson, D.R. Foster and A. Allen. 1999. Vegetation patterns in heterogeneous landscapes: the importance of history and environment. Journal of Vegetation Science 10: 903-920.
Motzkin, G., D.R. Foster, A. Allen, J. Harrod and R.D. Boone. 1996. Controlling site to evaluate history: vegetation patterns of a New England sand plain. Ecological Monographs 66: 345-365.
Spurr, S.H. 1956. Natural restocking of forests following the 1938 hurricane in central New England. Ecology 37: 443-451.

Understory Dynamics in the Experimental Hurricane
A. Barker Plotkin and D. Foster

The hurricane experiment at Harvard Forest was designed to simulate the impacts of a catastrophic storm like the 1938 New England Hurricane to mature hardwood forest. In October 1990, canopy trees were pulled over using a winch, resulting in direct and indirect damage to nearly 70% of the stand. In addition to the dramatic effects on the arboreal layer of the stand, this disturbance altered conditions in the forest understory. More light reached the forest floor for several years, and uprooting of trees resulted in patches of disturbed soil. After the manipulation, pits and mounds covered 8% of the site (Cooper-Ellis et al. 1999). We have periodically assessed the shrub and herb layer response to the manipulation for ten years, from 1990 – 2000.

Composition and abundance of understory vegetation was assessed in 1990 (before the manipulation), 1991, 1992, 1995 and 2000 in the experimental and control sites within nested 10m² (shrubs) and 1m² (herbs) plots placed along each of three transects in the 0.8 ha pulldown site and one transect in the 0.6 ha control area. In addition to cover estimates for individual species and total cover of life-form categories, the height of the tallest individual shrub of each species on each plot was recorded and cover of substrates (leaf litter, woody debris, rock, exposed soil, pit and mound) was estimated.

Most species were found in the experimental area both before and after the manipulation, but some species turnover has been observed throughout the ten years (Table 1). Two club moss species dropped out of the experimental site, and a few other species disappeared from the site but have returned. Several new species, many of which tend to colonize disturbed areas, have come in to the site. Some were transients, but others have persisted. Many of these species were found in plots containing a pit or mound (Cooper-Ellis et al. 1999). Two exotic and potentially invasive species (Lonicera morowii and Celastrus orbiculatus) have been found in one part of the experimental site in recent years; this area will be monitored closely but since the forest canopy is re-establishing well, these species are not expected to greatly increase.

A few species showed changes in abundance after the manipulation. Rubus species appeared in the experimental site, probably from buried seed; by 2000, they had begun to decline (Fig. 1). Hay-scented fern (Dennstaedtia punctilobula) and starflower (Trientalis borealis) increased markedly after the manipulation, but are also now declining, indicating that after 10 years, understory conditions in the experimental site may be more similar to pre-disturbance conditions. However, two abundant shrub species, Amelanchier spp. and Corylus cornuta, continue to increase. When species are grouped more broadly in life-form classes, tall shrubs and saplings show higher abundance in the experimental site versus the control (Fig. 2). Overall, changes in understory flora in the hurricane experiment are relatively modest and concentrated in areas with disturbed soil. The landscape context of the site, which is surrounded by intact forest, and rapid recovery of the forest canopy via advance regeneration and sprouting, contribute to this stability.

Cooper-Ellis, S. M., D.R. Foster, G. Carlton and A. Lezberg. 1999. Forest response to catastrophic wind: results from an experimental hurricane. Ecology 80: 2683-2696.

Forest Development Ten Years after an Experimental Hurricane
A. Barker Plotkin, S. Martell and D. Foster

The hurricane experiment at Harvard Forest was designed to simulate the impacts of a catastrophic storm like the 1938 New England Hurricane to mature hardwood forest. In October 1990, canopy trees were pulled over using a winch, resulting in direct and indirect damage to nearly 70% of the stand. One goal of the experiment is to study regeneration mechanisms and changes in species composition. In summer 2000, we surveyed all stems that had grown above 5 cm dbh across the experimental and control sites.

The number of new stems is far greater in the experimental site (630 stems/ha) than in the control (27 stems/ha). The importance of sprouts (72 stems/ha; 12%) is less than that of saplings (558 stems/ha; 88%). Most of these saplings are advance regeneration, stems that were present in the understory of the stand before the manipulation. The greatest number of new stems was found in the center of the site with fewer to the north and south, possibly an edge effect.

Stem density has recovered to original levels (Table 1), but species composition has shifted in the experimental site from red oak – red maple to a black birch - red maple forest. Regeneration in the manipulation is dominated by black birch (Betula lenta), followed by red maple (Acer rubrum) and yellow birch (Betula alleghaniensis) (Fig. 1). Red oak (Quercus rubra) was a dominant species in the pre-manipulation forest, but only one red oak has grown into the 5 cm size class since 1990. This compositional shift is dampened somewhat by the component of the original canopy that has survived the manipulation. Surviving canopy trees form the new stand along with stems recruited since the manipulation. Currently, more than one-third of trees >= 5 cm dbh are survivors from the original overstory. These survivors include a few low-vigor prostrate stems, bent and leaning trees that are rebuilding their crowns, and vigorous undamaged trees. The largest trees in the stand are undamaged red oaks (46 stems/ha), and many smaller red maples have persisted as well. The current forest stand has a multi-layered structure, with dense new stems forming a lower stratum below a middle stratum of small surviving and recovering trees and scattered large emergent stems from the original stand.

Evaluation of Greenhouse and Ozone-Depleting Gases in Rural New England
D. Barnes and S. Wofsy

The Montreal Protocol on Substances that Deplete the Ozone Layer in 1987 and its subsequent London (1990) and Copenhagen (1992) Amendments mandated control measures on the production and consumption of ozone-depleting substances (UNEP, 1985-1997). The majority of the substances, including CFC11, CFC12, CFC113, Halon-1211, CCl₄, and CH₃CCl₃, were scheduled for 100 percent reductions in production and sales by 1 January 1996 in developed countries. The success of the Protocol in the U.S. has thus far been determined by inventory estimates only. Given that today emissions violate international agreements, they may not be reported willingly. The quality of these inventories and the recent urban pollution history has yet to be independently established for the post-1996 ban years.

To address this deficiency, this study provides an independent measure of emissions from a major emitting region of ozone-depleting species for the years 1996 through 1998, the first three years after the full implementation of the Montreal Protocol. The measurements were taken every 24 minutes at Harvard Forest, MA, downwind of the Northeast urban-industrial corridor, including the greater metropolitan region of New York City. Using the well-documented EPA carbon monoxide emissions, which are reported on a per county basis, and a composite PCE inventory, derived from the EPA/TRI records and the McCulloch and Midgley sales-based country-level tallies (McCulloch et. al. 1996 and P. Midgley, pers. comm.), we estimate the annual and seasonal urban/industrial emissions of CFC11 (CCl3F), CFC12 (CCl2F₂), CFC113 (C₂Cl₃F₃), methyl chloroform (CH₃CCl3), and halon-1211 (CBrClF2) all on a per capita basis. The results of this study have confirmed the accuracy of the above listed inventories for the New York City – Washington, D.C. corridor (Barnes 2000).

The annual urban pollution emissions for each species indicate that a number of the species exhibit distinct inter-annual trends. Our data indicate that, following the full Montreal Protocol ban in 1996, emissions of CFC12, CFC113, and CH₃CCl₃ have continued to decline, as expected. Halon-1211, a fire-extinguishant for which no adequate replacement has yet been found, does not exhibit any distinguishable pattern. For CFC11, the trend is positive, which is a surprise especially given the ban on its production by the Montreal Protocol, the decline in its atmospheric growth rate witnessed as early as 1989 (Elkins et. al., 1993), and its close seasonal correspondence to CFC12 whose emissions are decreasing, not increasing.

Also measured over the three year experiment was molecular hydrogen, a major trace gas with an ambient concentration of around 530 ppb. Although a major by-product of combustion that is involved in a range of processes that directly impact on the levels of OH and O₃ in the troposphere and stratosphere, it is not itself deleterious to the atmosphere and has received little attention in the literature.

With a number of field stations reporting a correlation between observed H2 enhancements and local traffic flow (Schmidt et. al. 1970; Scranton et. al. 1980; Novelli et. al. 1999), the focus of anthropogenic H2 source studies has centered almost entirely on automobiles, with little reference to other technological processes. Remarkably, the only direct evidence of the presence of H2 in automobile exhaust dates back to a motor emission study by the Society of Automotive Engineers in 1965 (Starkman et. al. 1965), before the era of emission standards, fuel additives, catalytic converters, and other automotive changes. At that time, Starkman et al. attributed the hydrogen they observed to the well-known water-gas equilibrium reaction found in engines, with a H_²/CO yield of about 40% (Penner et. al. 1977).

We use here our three years of continuous H₂ and CO measurements, taken downwind of the major urban/industrial region of the New York City-Washington, D. C. corridor, to provide robust estimates for H₂/CO ratios by season for polluted air in the late 1990s. Despite changes in the automotive industry and advances in our understanding of the intricacies of tropospheric chemistry, the average H₂/CO value of 0.403 ppb/ppb (or 0.0290 kg/kg) observed at Harvard Forest for winds issuing from the southwest is entirely in harmony with that predicted by Starkman et. al. (1965) from the water-gas reaction.

On the basis of the EPA CO emissions inventory for 1996 (www.epa.gov/ceis), we can create county-level maps of H₂ fossil fuel releases for the Northeastern U.S. in 1996 (Fig. A). As with CO, for which the anthropogenic emissions are likewise dominated by road transportation and combustion, those counties which contain or neighbor large cities are responsible for the bulk of the H₂ anthropogenic output. The largely rural counties in Virginia and West Virginia have the lowest absolute H₂ fossil fuel emissions. On a per capita basis (Fig. B), New York City and environs have the lowest emission rates and are surrounded by counties whose values increase slowly with distance from New York City. The greatest per capita emitters are at the furthest distances from New York City, predominantly in Virginia and West Virginia. That such a pattern should emerge between H₂ and population is not unexpected given that cars are a major source of anthropogenic emissions for both gases. The low per capita H2 emissions for New York City in particular may be explained by the high population density and the greater reliance on public transportation in that area.

Barnes, D. H. 2000. Quantifying Urban/Industrial Emissions of Greenhouse and Ozone-Depleting Gases Based on Atmospheric Observations. Ph.D. thesis, Harvard University.
Elkins, J. W., T. M Thompson, T. H. Swanson, J. H. Butler, B. D. Hall, S. O. Cummings, D. A. Fisher, and A. G. Raffo. 1977. Decrease in the growth rates of atmospheric chlorofluorocarbons 11 and 12. Nature 364: 780-783.
McCulloch, A. and P. M. Midgley. 1996. The production and global distribution of emissions of trichloroethene, tetrachloro-ethene and dichloromethane over the period 1988-1992. Atmospheric Environment 30: 601-608.
Novelli, P. C., P. M. Lang, K. A. Masarie, D. F. Hurst, R. Myers, and J. W. Elkins. 1999. Molecular hydrogen in the troposphere: Global distribution and budget. J. Geophys. Res. 104: 30427-30444.
Penner, J. E., M. B. McElroy, and S. C. Wofsy. 1977. Sources and sinks for atmospheric H2: A current analysis with projections for the influence of anthropogenic activity. Planet. Space Sci. 25: 521-540.
Schmidt, U. and W. Seiler. 1970. A new method for recording molecular hydrogen in atmospheric air. J. Geophys. Res. 75, 1713-1716.
Scranton, M. I., W. R. Barger, and F. L. Herr. 1980. Molecular hydrogen in the urban troposphere: Measurement of seasonal variability. J. Geophys. Res. 85: 5575-5580.
Starkman, E. S. and H. K. Newhall. 1965. Characteristics of the expansion of reactive gas mixtures, Society of Automotive Engineers, Paper 650509.
United Nations Environmental Programme, 1985. Vienna Convention for the Protection of the Ozone Layer, 1987 Montreal Protocol to Reduce Substances that Deplete the Ozone layer Report, Final Report (New York), 1990 London Amendment, 1992 Copenhagen Amendment, 1997 Production and Consumption of Ozone Depleting Substances, 1986-1995, Nairobi, Kenya.

Is Increased Nitrogen Availability Predictive for Long-term Forest Carbon Sequestration?
G.A. Bauer, R. Minocha, G. Berntson, J. Aber and F. Bazzaz

Temperate forests are predicted to play a key role as important sinks for atmospheric carbon dioxide. This sink could be enhanced by atmospheric nitrogen (N) deposition. However, the predicted response may vary for deciduous and coniferous trees due to differences in photosynthetic nitrogen use efficiency. Experimental evidence to suggest that the impact of N deposition on temperate forest productivity may not be as great as originally assumed. This is in part due to the limited information on processes, which take place in the canopy. We investigated how changes in N deposition rate effects the partitioning of organic N into different physiological pools, and how this in turn will affect photosynthetic capacity and foliage productivity. Our study is based at the Harvard Forest Chronic N Experiment, where a 12-year addition of N on both coniferous and deciduous forests is underway. The measurements within a Pinus resinosa stand demonstrate that foliar N content has significantly increased in this species, and that this increase is accompanied by a decoupling of the photosynthesis-N relationship. Conifers of the high N treatment do not use the surplus of N to synthesize more Rubisco, which would allow them to have a higher photosynthetic capacity. Instead this N is being accumulated as putrescine (a common polyamine and also a stress indicator) and its precursor amino acid, arginine. Gas exchange measurements confirmed this observation, showing that trees from the high N treatment had significantly lower photosynthetic capacity than the control trees. These results indicate that the increase in leaf N is not accompanied by a greater capacity for carbon assimilation in the high N treatment.

Investigating the Role of History in the Modern Distribution and Species Composition of the Rich Mesic Forest Community in Western Massachusetts
J. Bellemare, G. Motzkin and D. Foster

The effects of historical factors, such as past land-use, on modern plant species distributions in eastern North America has proved difficult to document, due in part to a lack of records detailing historical land-use. Despite this difficulty, the importance of understanding the long-term impacts of human disturbance on cannot be overstated. Our research has focused on a single upland community type, rich mesic forests (RMF), which are known for their diverse assemblage of woodland herbs, including numerous species with limited dispersal ability. The RMF community is considered to be a northern variant of the Mixed Mesophytic Forest Type of the southern Appalachian Mountains as described by Braun (1950). In Massachusetts, RMF sites are most common in the western portion of the state, where their distribution largely coincides with the occurrence of calcareous bedrock.

The RMF community has been the subject of previous speculation as to the effects of past human land-use on woodland herb species, however the absence of detailed land-use records throughout much of the eastern United States has precluded an accurate assessment of these effects. Unlike other regions, land-use maps of our study area in western Massachusetts do exist and allow for the spatial reconstruction of forest cover during the past two centuries. As in much of southern New England, forest cover in the study area decreased rapidly following European settlement, reaching a low of 15-30% in the early 19^th century. During the past century forest cover has come to dominate the landscape as numerous farms have been abandoned.

Vegetation sampling on sites with differing histories has enabled us to document varied species responses to past land-use. These responses appear to result from species specific life history traits, including dispersal mode, growth form and tolerance to exposure. Herb species with seeds lacking adaptations for dispersal and those with ant-dispersed seeds tend to be strongly associated with primary forest and bedrock outcrop refugia in secondary forest, whereas endozoochorous and wind dispersed species tend to be present in both primary and secondary stands. Notable exceptions to this pattern do exist: Sanguinaria canadensis, an ant-dispersed species often considered an indicator of RMF, has persisted and thrived in hedgerows allowing for its successful re-colonization of many secondary stands. The species is now significantly more frequent in secondary forest and hedgerows than in primary forest. Spatial data from the study area stresses the long-term significance of small-scale refugia, such as bedrock outcrops and hedgerows, which have allowed for the persistence in situ of certain woodland plant species in otherwise heavily modified agricultural landscapes.

Meteorological Station
E. Boose

The new Harvard Forest Meteorological Station became operational on 11 Feb 2001. The new station records air temperature, relative humidity, dew point, precipitation (water equivalent of snow), global solar radiation, barometric pressure (corrected for elevation = 340m), horizontal scalar wind speed, vector wind speed, peak gust speed (1-second), vector wind direction, standard deviation of wind direction (wind measurements at 10m height), and soil temperature (10cm depth). Instruments are scanned once per second, and hourly and daily values are calculated and stored by a datalogger.

The station is connected to Shaler Hall via underground conduit, which contains 110VAC (for power supply and heated rain gage) and category 5 data cable (for communication with computer via short-haul modem). Hourly and daily data are posted on the Harvard Forest web page. Recent data are updated hourly but are not checked. Older data are checked and missing, questionable, or estimated values are flagged (following methods of LTER ClimbDB project). A log of events affecting station measurements (e.g., instrument repair and calibration, ice storms, lightning) is also posted.

The new station is located in the pasture north of the Community House, about 200m north of the old station next to Shaler Hall. The new site was chosen to minimize the angle of surrounding trees above the horizon for solar radiation and wind measurements (currently 15-25 degrees from the station at breast height). The old station, which measures daily minimum and maximum temperature and daily precipitation (8am-8am observation period), will be run in parallel with the new station for a full year and then discontinued.

Response of CO₂ Release from O-horizon During Drying and Wetting Cycles
W. Borken, E. Davidson, K. Savage and K. Angeloni

The water content of O-horizons in temperate forests varies widely during summer droughts and rainfall events, and this temporal variation in water content may affect microbial decomposition of organic matter and release of CO₂. Low water content can limit the growth rate and activity of microorganisms, as well as the diffusion of nutrients and carbon substrates in water films. It is well known that wetting of dry mineral soil can cause a pulse of CO₂ release due to altered availability of carbon and microbial activity. However, little attention has been paid to the short-term response of microorganisms in O-horizons to drying and wetting cycles.

In a laboratory study, we measured CO₂ release of dry O-horizon from the mixed hardwood stand at Harvard Forest following wetting. The amounts of added water were 0.5, 1.0, 2.0, 4.0 and 8.0 mm and were sprayed onto the surface in less than 5 min. Only the 8.0 mm wetting treatment required two wetting events that were applied within 1 hour. Air temperature was constant (17 ±1°C) during the entire experimental period. Soil moisture (g g^-1) was semi-quantitatively measured in the litter layer at 2 cm and 5 cm depths using DC-halfbridges.

The CO₂ release of the dry O-horizon was less than 10 mg C m^-2 hr^-1. Surprisingly, a pulse of CO₂ release peaked in less than 5 min after the water was added to the dry O-horizon (Fig. 1). Similar peak CO₂ fluxes were observed at water additions of 0.5 mm (32 mg C m^-2 hr^-1), 1.0 mm (44 mg C m^-2 hr^-1), and 2.0 mm (35 mg C m^-2 hr^-1). Larger peak CO₂ fluxes were observed at 4mm (50 mg C m^-2 hr^-1) and 8mm (68 mg C m^-2 hr^-1) water additions. The rate of CO₂ release dropped with decreasing water content to pre-wetting levels in 1-10 days, depending on the amount of water that had been added. The rather short-term CO₂-peak observed after small water additions indicates a certain amount of easily decomposable carbon that becomes available for microorganisms after the O-horizon is wetted. Because of the relatively strong response at only 0.5 mm water addition, which mostly wetted only the surface layer of intact leaf litter, it is likely that much of the easily decomposable carbon mineralized after wetting resides in this surface litter layer. Pulses of CO₂ emissions in the range of 30-70 mg C m^-2 hr^-1 represent 10-30% of average summertime emissions. Our results indicate that even small rainfall amounts significantly increase the CO₂ emissions of dry forest soils.

The Historical Landscape of Southern New England in Early Records
J. Burk, G. Motzkin and D. Foster

In order to determine forest composition across southern New England at the time of European settlement, we have been gathering data from a variety of early references. The majority of towns in the region cited "witness" trees as reference points in early land division (proprietor), deed, town boundary, and road surveys. The most useful and detailed sources are proprietor and deed records, which usually have more citations and cover a greater geographic area than road and boundary surveys. Records were located at a variety of sources including town halls, libraries, historical societies, registries of deeds, state archives and libraries, and individual residences.

In general, tree citations were more common in towns settled during the eighteenth century and less so in the coastal and valley areas which were settled earlier. Settlement and town division histories were compiled for all three states, and several historical town maps were added to the archives.

Near-complete proprietor coverage was gathered in Massachusetts for the North and South Shore communities. Road and boundary surveys were tallied in the greater Boston area, where proprietor records did not cite trees or were missing. For coastal towns such as Boston and Gloucester, records dated as far back as the 1620s. In all, nearly 58,000 trees were tallied from 292 of the 351 current towns (84 %). Eight books had over 2,000 trees, with the largest total 3,155 in Andover.

In Connecticut, 138 of 186 (74 %) towns had useful records totaling 44000 citations. Three surveys had over 3,000 trees, with 4,500 recorded in Killingworth alone. Near complete coverage was found east of the Connecticut River and the northwest hills, with gaps in Hartford and Fairfield Counties, where trees were not used as reference markers in early surveys. Dates ranged from mid-1630s for coast and Connecticut Valley towns to 1830s for some hill towns. Coverage was also found for the majority of Rhode Island, with the only gap in the southeast coast region including Newport, Jamestown, and Block Island. Large data sets were compiled from the Providence, Little Compton, and Tiverton records. Data from southeast Massachusetts records were used for several towns annexed to Rhode Island in the eighteenth century. In Rhode Island surveys, 33 of 39 (85 %) had large data sets, totaling 2,700 trees.

In addition to the witness tree project, in order to facilitate analysis of Massachusetts historical information, a number of databases were created from 19th and early 20th C historical records. These data represent an unusual data source and provide the first opportunity to evaluate controls on vegetation composition in the early period. Census data for every town from 1801 and 1845-1905, which included agricultural land-use and woodland acreage, were summarized in a large table. A separate database was created for forest cutting data by species from information in the 1885 census. Summary tables were created for the early 1900s county forest surveys, the 1907 state forester's report, and the Umass/MaConnell data books.

Archives Development, 2000-2001
John Burk

While the majority of time spent on archival work was devoted to active research, several projects and additions continued the development of the facility, which is entering its fifth year of operation.

Harvard Forest’s information about the Sanderson family, which farmed portions of Prospect Hill in the 18^th and 19^th C, was strengthened by the purchase of an original account book from a local rare book dealer, providing a firsthand account of early agricultural activity in the area. In addition, Kathleen Hunter, a descendant of the Sanderson family, made several visits and donated a copy of her family genealogy research to the archives.

All Harvard Forest property records including deed, correspondence, and buildings and grounds records were researched for land-use citations in order to help develop a long-term plan for the tracts. An enlarged and simplified property file was created, combining related documents from the various sources.

As part of an ongoing effort to create a unified database, all archive databases, including maps, samples, photos and slides, aerial photos, research files, and rare books, were converted to ProCite bibliographic format. The enhanced features of Procite have facilitated data entry and research. During this process all related collections were inventoried and updated, with new paper listings printed.

Additions to the map collection included comprehensive coverage of the coast from New York to Cape Cod in 19^th C Coast and Geodetic surveys, and 20^th century USGS topographic quadrangles. Copies of the final maps from the 1830s Massachusetts series were acquired, giving Harvard Forest a complete set of this valuable resource. Over fifty boxes of soils and tree cores were added to the soil and sample archive, mainly from the Cape and Islands project. Other material included soils from MBL and the adelgid studies, and tree cookies and soils from the ancient Gribben Forest in Michigan. Historical and current slides and photographs were used to create a photo gallery on the Harvard Forest web page. The lantern slides were sorted and cataloged, and several successful enlargements were produced from the glass originals, enhancing the collection’s value as an archival resource. A series of 1938 Cape Cod images was added to the aerial photo collection.

The archive facilities were used daily by staff researchers on projects such as the 1830s mapping effort, and regularly by students, planners, and historians from outside institutions including Mount Wachusett Community College, the University of Massachusetts, Harvard University, the Trustees of Reservations, and the towns of Petersham, Hardwick and New Braintree.

The Effects of Hemlock Woolly Adelgid Infestation on Foliar Decomposition in Eastern Hemlock Forests of Southern New England
R. Cobb, D. Orwig and S. Currie

Forests dominated by eastern hemlock (Tsuga canadensis) are cool and deeply shaded ecosystems found throughout the New England landscape. Due to favorable climate and lack of effective native predators, the Hemlock Woolly Adelgid (Adelges tsugae, HWA) is spreading throughout southern New England’s hemlock forests with little impediment. Eastern hemlock has no natural resistance to this insect and tree mortality typically occurs within 5 to 10 years after infestation. In a recent study, Jenkins et al. (1999) demonstrated a strong linkage between HWA related canopy damage and altered community structure and ecosystem function. These authors hypothesize that changes in decomposition dynamics may be contributing to functional changes in HWA damaged stands. Insect attack can effect decomposition indirectly by altering microclimate and directly by altering foliar quality. The objectives of this study are (i) to compare relative rates of foliar decomposition in eastern hemlock forests with varying stages of adelgid caused damage and (ii) examine how HWA herbivory influences this functional process.

We studied surface decomposition for 18 months in hemlock forests ranging from uninfested controls to stands with significant HWA related canopy damage. Hemlock foliage was collected from 8 study sites where we have developed a detailed 3 year database of soil temperature, soil moisture, and N dynamics (see Orwig et al., this volume). These stands include 2 sites with canopy damage, four sites with little damage but with large HWA populations, and 2 sites without adelgid (control). At each site, we distributed foliage from each respective stand and uninfested foliage from Harvard Forest.

Sites with severe damage tended to have lower rates of mass loss and lower rates of %C and %N accumulation relative to low damage and control sites (Fig. 1). This trend was associated with dryer forest floors suggesting that reduced soil moisture in the surface soils of infested stands may be limiting fungal hyphae establishment and thereby inhibiting decomposition. Foliage type also influenced the dynamics of decomposition however, these effects were most pronounced at control sites where both sets of foliage were uninfested by HWA. This suggests that site factors have a greater influence on foliar quality than HWA herbivory alone.

Reduced decomposition at damaged sites is counter to the dynamics hypothesized by Jenkins et al. (1999) and our own expectations. Soil N mineralization has been greatly increased at these damaged sites relative to the controls, a pattern opposite of surface decomposition. However, reduced soil moisture is most pronounced in the forest floor, suggesting that subterranean processes may not be limited by moisture status and may be responding to increased soil temperature also associated with damaged stands. To better understand functional changes associated with this insect, we are preparing to make relative comparisons of surface and subsurface decomposition at a larger subset of hemlock dominated sites.

Jenkins, J.C., Aber, J.D., Canham, C.D. 1999. Hemlock woolly adelgid impacts on community structure and N cycling rates in eastern hemlock forests. Can. J. For. Res. 29:630-645.

Decadal Scale Recovery of ¹⁵N Tracers in Bolewood at the Harvard Forest Chronic N Study
B. Colman, K. Nadelhoffer and W. Currie

Wood is posited to be an important carbon sink in the North Temperate Zone, and elevated nitrogen deposition on temperate forests could contribute to carbon storage by stimulating wood production. This hypothesized fertilization effect is dependent on the uptake of nitrogen deposition by trees and allocation to bolewood growth.

In order to determine the fate of elevated nitrogen inputs in temperate forest ecosystems, ¹⁵N-tracers were added to Control (ambient deposition) and Low-N addition (50kg N ha^-1yr^-1 since 1988) plots in 1991 and 1992 at the Harvard Forest LTER Chronic N Amendment Study. In both forest types studied (oak and red pine) ambient and fertilized plots received NH₄¹⁵NO₃ tracer on one half and ¹⁵NH₄NO₃ on the other. Recovery and redistribution of ¹⁵N tracers in the first few years following labeling was reported previously (Nadelhoffer et al. 1999). Bole wood, green foliage, soil, roots, woody debris, and understory-canopy foliage were sampled again, 8 years following the mid-point of ¹⁵N labeling (in 1999) in order to assess decadal scale patterns of ¹⁵N movements and C / N interactions.

The measured recoveries from all pools will be compared to modeled recoveries as predicted by the TRACE model. TRACE combines the vegetation production and allocation of PnET (Aber and Federer 1992) with a detailed model of decomposition, humification, and production of dissolved organics, with an emphasis on C and N interactions and an ability to predict ¹⁵N tracer redistributions in vegetation and soils (Currie et al. 1999). Direct model-data comparisons of ¹⁵N redistributions help to test and refine the model (Currie and Nadelhoffer 1999). Previous work showed that ¹⁵N tracer recovery in bolewood at the end of the two-year tracer addition ranged from 0.1% to 4.4% when averaged across tracer forms (Fig. 1). Tracer recovery varied with forest type, N-input rate, and ionic form of ¹⁵N added (Fig. 2). Overall, more ¹⁵N accumulated in wood when ¹⁵NO₃^- was applied, oaks accumulated more tracer than pines, and the percent recoveries increased with N input rate.

In 1999, seven years after the end of tracer application, recoveries were larger in bolewood than at the end of the two-year tracer addition, ranging from 0.8% to 5.1% averaged across tracer forms. The overall trend from 1992 still holds, with tracer recovery depending on forest type, rate of N-input, and form of tracer addition. Tracers have continued to accumulate in wood for almost a decade after being added to the plots, as was predicted by TRACE before these data were obtained (Currie and Nadelhoffer 1999). At the same time, the low ¹⁵N recoveries in bolewood show that N deposition may be having a small influence on wood production in these forests. Next steps in this project include working up a complete budget for C and ¹⁵N in these forests over decadal time scales, as well as incorporating lessons learned in model-data comparisons to refine and revise TRACE to apply the model across other sites. TRACE will be used to predict ¹⁵N redistributions at other intensive-study sites including Klosterhede, Denmark, and Bear Brooks Watershed, Maine. The model will also be used to scale up C and N interactions to landscape scales in the northeastern U.S. Aber, J. D. and C. A. Federer. 1992. A generalized, lumped-parameter model of photosynthesis, evapotranspiration and net primary production in temperate and boreal forest ecosystems. Oecologia 92: 463-474.

Currie, W. S. and K. N. Nadelhoffer. 1999. Dynamic redistribution of isotopically labelled cohorts of nitrogen inputs in two temperate forests. Ecosystems 2: 4-18.
Currie, W. S., K. J. Nadelhoffer and J. D. Aber. 1999. Soil detrital processes controlling the movement of 15N tracers to forest vegetation. Ecol. Appl. 9: 87-102.
Nadelhoffer, K. J., M. Downs and B. Fry. 1999. Sinks for 15N-enriched additions to an oak forest and a red pine plantation. Ecol. Appl. 9L 72-86.

Woody Detritus, Land-use History, and Long-term C and N Interactions
W. Currie, K. Nadelhoffer, and B. Colman

Depending on the particulars of forest management and disturbance, wood in varying size classes and amounts enters detrital pools in soil where it contributes to energy flow, C (carbon) storage, N (nitrogen) storage, and N cycling. Two key questions in the study of large-scale C and N cycling in temperate forests are how N cycling in soil detritus controls ecosystem-level retention of elevated N deposition, and whether elevated N deposition is likely to cause increases in C pools. The large C:N ratios in woody detritus make it a potentially important contributor to N retention, if N immobilization increases, and a potentially important contributor to C sequestration, if pool sizes increase. Both fine woody debris (FWD) and coarse woody debris (CWD) are often excluded from ecosystem-level studies of C and N dynamics, because these are difficult to sample methodically and they comprise relatively small fractions of C and N stocks. High variabilities across size classes, across decay classes, and in spatial distributions create challenges for the measurement of pool sizes. However, producing a complete C or N budget at a particular site requires the measurement of woody pools at that site. Pools of woody detritus are difficult to generalize to any given site from elsewhere because inputs are heterogeneous in space and time, and pool sizes reflect the particulars of disturbance and land use history at any site.

Long-term movement of ¹⁵N tracers into fine woody debris The contribution woody debris makes to N retention in the forest floor under elevated N inputs is unknown. Two very different mechanisms are plausible for increased N retention in this material under increased N inputs. First, N immobilization would provide a direct mechanism for increased N retention in the forest floor. Woody debris has higher C:N ratios than other litter, but the extent to which woody debris can be expected to immobilize N as it decays is a matter of current debate. Second, any increase in N uptake and the flux of N in woody litter could provide an ecosystem-level mechanism of N retention.

We studied N concentrations, C:N ratios, and pool sizes of N and biomass in fine woody debris (FWD < 5 cm diam.) 12 years into the Chronic N Study (Aber et al. 1993, Magill et al. 2000), a long-term N-amendment study in two contrasting forests: a naturally-regenerated forest dominated by Quercus spp., and a 63-yr old plantation of Pinus resinosa. We also quantitatively recovered ¹⁵N tracers (originally applied as ¹⁵NH₄ and ¹⁵NO₃) in FWD, eight years following their application in the same study, in both ambient and N-amended plots. We used these data to test predictions of tracer redistributions made by TRACE, a biogeochemical process model incorporating the vegetation processes of PnET (Aber and Federer 1992), and predicting redistributions of ¹⁵N (Currie et al. 1999).

Results from the N pool-size analysis and the ¹⁵N tracer-recovery analysis indicated that under elevated N inputs of 5 g N m^-2 yr^-1 (as NH₄NO₃) over the decadal time period, only 0.15% to 0.76% of the elevated N inputs were recovered in FWD of N-amended plots relative to ambient. Any increase in N immobilization in wood appeared to be minimal, in agreement with predictions made by the TRACE model (Currie and Nadelhoffer 1999). Under N amendments, pool sizes of C in FWD were not significantly different from ambient, whereas pool sizes of N were marginally higher. Patterns of ¹⁵NH₄ vs. ¹⁵NO₃ recovery, treatment differences, and forest-type differences suggested that plant uptake, rather than detrital immobilization, was the dominant mechanism of ¹⁵N tracer movement into FWD. This result indicates that plant-soil cycling operating over a decadal time scale or longer controls C:N ratios and N pool sizes in woody debris.

Land Use History: Contrasting Patterns of C and N Stocks in Woody Detritus Data on C and N cycling from the Harvard Forest have played a key role in the development of numerous models with a wide range of purposes. In addition, some models of generalized processes across types of terrestrial ecosystems have used data from Harvard Forest to parameterize temperate forest regions. Ecosystem budgets and synthetic models applied at the Harvard Forest have relied on woody detrital measurements from other temperate forests. In 1999, by combining quadrat-sampling methods with line-intercept methods on long transects, we completed quantitative measurements of pool sizes of mass, C, and N in downed fine and coarse woody debris in two forest types in the Prospect Hill tract: the two forest stands containing the Chronic N Study. Given the knowledge of long-term management and disturbance history of land at the Harvard Forest (Foster and Boose 1992, Foster et al. 1992), we also sought to quantify relationships between site history and current ecosystem structure in the woody detrital pools.

One of the most striking patterns in our data was the difference, between these two forest stands, in the patterns of size and decay classes in coarse woody debris (CWD) (Fig. 1). In the oak forest, detritus was distributed across all size categories, and nearly all of the CWD mass was in the most advanced decay classes. Patterns of CWD mass in the pine stand stood in direct contrast to this in two ways. First, all of the CWD mass in the pine stand was in the smallest size class (< 25 cm). Second, the material was distributed across all five stages of decay, including the two most sound (least-decayed) categories.

Most of the differences we observed between stands can be attributed to differences in land use and disturbance histories. The even-aged nature of the pine stand is evident in the large masses of rotten woody debris in the largest size class of FWD and smallest size class of CWD; these are highly decayed stems that fell during the stem exclusion phase of forest development. The lack of any debris in larger categories corresponds in a clear way to the fact that this stand was pastured into the 20^th century, became a pine plantation in 1926, experienced little damage from the 1938 hurricane, and is still in its first episode of forest growth after reversion from agriculture. There are numerous living trees and some snags present in larger sizes, but since the trees have attained that size there has been no disturbance severe enough to cause extensive mortality or windthrow.

The oak forest, in contrast, was one of the first patches of the Prospect Hill tract to revert to forest following agriculture. Over a 150-yr period this hardwood stand has experienced numerous disturbances; it was lightly logged at the turn of the century, suffered extensive damage from the 1938 hurricane, and experienced mortality from the Chestnut blight (Foster and Boose 1992, Foster et al. 1992). The forest has naturally regenerated following these disturbances. The patterns of CWD that we observed (Fig. 1) show that with such a history, large well-decayed pieces of woody debris are present; it is this debris that contains the bulk of the N in woody detritus (data not shown) because of the narrow C:N ratio of this highly decayed material.

These results have important implications for the use of Harvard Forest C and N data in models that are used to extrapolate across landscapes. It is well known that climate and litter quality control decay rates, and thus pool sizes, of detritus; for woody debris, more so than for other types of litter inputs, it is also true that input rates and pool sizes depend on particulars of disturbance and forest history over long time periods. To scale results from intensive-study sites up to a grid cell or region, models could include links between land use history and woody pools, and apply those links across landscapes with data layers representing patches of history and disturbance. In any case, history should not be neglected in modeling pool sizes of C and N in woody detritus.

Aber, J. D. and C. A. Federer. 1992. A generalized, lumped-parameter model of photosynthesis, evapotranspiration and net primary production in temperate and boreal forest ecosystems. Oecologia 92: 463-474.
Currie, W. S. and K. J. Nadelhoffer. The imprint of land use history: Contrasting patterns of Carbon and Nitrogen stocks in woody detritus at the Harvard Forest. Ecosystems (In Review).
Currie, W. S. and K. N. Nadelhoffer. 1999. Dynamic redistribution of isotopically labelled cohorts of nitrogen inputs in two temperate forests. Ecosystems 2: 4-18.
Currie, W. S., K. J. Nadelhoffer and J. D. Aber. 1999. Soil detrital processes controlling the movement of 15N tracers to forest vegetation. Ecol. Appl. 9: 87-102.
Currie, W. S., K. J. Nadelhoffer and B. Colman. 2001.Long-term movement of ¹⁵N tracers into fine woody debris: increased N sinks under chronically elevated inputs. Plant Soil (In Review).
Foster, D. R. and E. R. Boose. 1992. Patterns of forest damage resulting from catastrophic wind in central New England, USA. J. Ecol. 80: 79-98.
Foster, D. R., T. Zebryk, P. Schoonmaker and A. Lezberg. 1992. Post-settlement history of human land-use and vegetation dynamics of a Tsuga candensis (hemlock) woodlot in central New England. J. Ecol. 80: 773-786.

Rapid abiotic transformation of nitrate in an acid forest soil
D. Dail, E. Davidson, J. Chorover, K. Dria and P. Hatcher

Nitrate immobilization into organic matter is thought to require catalysis by the enzymes of soil microorganisms. However, recent studies at the Harvard Forest suggest that nitrate added to soil is immobilized rapidly and this process may include abiotic pathways (Berntson and Aber, 2000). We amended living and sterilized soil with ¹⁵N-labeled nitrate and nitrite to investigate biotic and abiotic immobilization. We report rapid transformation of nitrate in incubations of the O layer of forest soils that have been sterilized to prevent microbial activity and to denature microbial enzymes. Approximately 30, 40, and 60% of the ¹⁵N-labeled nitrate added to live, irradiated, or autoclaved organic horizon soil disappeared from the extractable inorganic-N pool in less than 15 minutes. About 5% or less of the nitrate was recovered as insoluble organic N in live and sterilized soil, and the remainder was determined to be soluble organic N (DON) (Fig. 1). Added ¹⁵N-nitrite, however, was either lost to gaseous N or incorporated into an insoluble organic N form in both live and sterile organic soils (Fig 2). Hence, the fate and pathway of apparent abiotic nitrate immobilization differs from the better-known mechanisms of nitrite reactions with soil organic matter. Nitrate and nitrite added to live A-horizon soil was largely recovered in the form added, suggesting that rapid conversion of nitrate to soluble organic-N may be limited to C-rich organic horizons. The processes by which this temperate forest soil transforms added nitrate to soluble organic-N cannot be explained by established mechanisms, but appears to be due to abiotic processes in the organic horizon.

Berntson G. M. & J. J. D. Aber. 2000. Fast nitrate immobilization in N saturated temperate forest soils. Soil Biol. Biochem. 32: 151-156.

Interannual Variation of CO₂ Production within the O Horizon and Its Implications for Annual Net Ecosystem Productivity
E. Davidson, K. Savage and S. Trumbore

Carbon in the litter layer of the Harvard Forest is probably no longer accumulating significantly from one decade to the next, but there may be important variation from year to year that could affect annual estimates of net ecosystem productivity (NEP). Goulden et al. (1996) reported surprising high values of NEP during years with dry summers, apparently because summer drought suppressed respiration more than photosynthesis, causing above average annual net storage of ecosystem C. Savage and Davidson (2001) have reported significantly lower rates of soil respiration during the dry summers of 1995, 1997, and 1999 compared to the wet years of 1996, 1998, and 2000 (Fig. 1a). If low rates of respiration during dry years contribute to interannual variation of NEP, then it is important to know where within the soil/forest floor system respiration is most variable annually. We hypothesize that the forest floor (the O horizon) is most susceptible to drought-induced suppression of decomposition, resulting in transient C sinks in this horizon. The objective of this study is to estimate interannual variation of CO₂ production within the O horizon.

In addition to chamber measurements of soil respiration during the last six years, we have also measured profiles of CO₂ concentrations, water content, and temperature within the soil on a weekly basis during the growing season and less frequently during the other seasons (Fig. 1b,c,d). We estimate effective diffusivity of CO₂ within the soil as a function of water content, temperature, and porosity. Applying Fick’s first law, we estimate CO₂ flux at various soil depths from estimates of diffusivity and CO₂ concentration gradients. Production of CO₂ within each mineral soil horizon is then estimated from the difference between the upward flux moving out of that horizon and the flux from the horizon below. For the O horizon, CO₂ production is estimated from the difference between mean CO₂ emissions for the site measured with chambers and the CO₂ flux out of the top of the mineral soil estimated from diffusivity and the concentration gradient.

Our estimates of CO₂ production within the O horizon are 2.1, 1.6, 3.1, 0.4, and 3.2 Mg C ha^-1 yr^-1 for the successive years from 1996 through 2000. In contrast, litterfall-C has a much smaller interannual range: 1.7, 2.0, 2.2, and 2.0 Mg C ha^-1 yr^-1 for the years 1996 through 1999. Although variable root respiration may contribute to interannual variation in soil respiration, the large interannual variation of CO₂ production within the O horizon implies that decomposition of leaf litter is inhibited by summer drought, resulting in a sink of C in the forest floor that could be on the order of 1 Mg C ha^-1 for the year. This sink is probably transient because the litter layer produces above average CO₂ release during wet years when some of the decomposable material from the previous drought year is decomposed under favorable conditions.

Goulden, M.L., J.W. Munger, S.-M. Fan, B.C. Daube, and S.C. Wofsy. 1996. Exchange of carbon dioxide by a deciduous forest: response to interannual climate variability. Science 271: 1576-1578.
Savage, K.E., and E.A. Davidson. 2001. Interannual variation of soil respiration in two New England forests. Global Biogeochemical Cycles, in press.

Changes Observed by NMR and Pyrolysis GC/MS of Harvard Forest Soils and Their Associated Plant Components Caused by Ten Years of Heavy Nitrogen Fertilization
K. Dria, D. Dail, J. Chorover, E. Davidson, and P. Hatcher

Samples from canopy-to-soil stratigraphic profiles, containing leaf, fine root and organic and mineral soil samples, were collected from the Harvard Forest Chronic N plots during 1989, 1996 and 1999 by Nadelhoffer and coworkers and Aber and coworkers. DOM from 1997 leachate samples, obtained from these plots using zero tension lysimeters (ZTL), is currently being characterized to evaluate the changes in DOM content and composition that results from varying levels of N fertilization. All samples were analyzed for chemical structural information by solid-state ¹³C NMR and molecular level detail by pyrolysis GC/MS. Select samples were analyzed by solid-state ¹⁵N NMR and liquid-state ¹³C NMR. Samples from the 1989 profile were used as a control for comparison with samples from 1996 and 1999 to determine compositional changes caused by N fertilization over time. Additionally, samples were compared across the treatment plots of control, low N and high N to differentiate changes associated with forest changes and N fertilization. ¹³C NMR spectra from all solid samples contain predominantly signals associated with paraffinic (0-45ppm) and carbohydrate (60-90 ppm) structures and low amounts of aromatic (90-160 ppm) structures. ¹⁵N NMR spectra reveal signals associated with amide structures. The mineral soil samples exhibit minor amounts of aromatic structures, suggesting little contribution of lignin structures to the organic matter. Lignin added to the soil by plant inputs may have been oxidized, degraded and/or leached from the soil profile. Primary changes observed in each set of the canopy-to-soil profiles were a loss of carbohydrates and a persistence of the paraffinic carbon regions of the NMR spectra (Fig. 1).

Of particular interest are the effects of N fertilization on the types of leaf carbon and litter decomposition in the various soil horizons. Increased levels of methylene structures (30 and 32 ppm) are observed in the NMR spectra of black oak green leaf samples with increased fertilization in 1999 and to less of an extent in 1996. Spectra of 1999 soil samples collected from both hardwood and pine high N plots from the Oe and Oa horizons reveal greater amounts of carbohydrate, lignin and aliphatic amide carbons relative to paraffinic carbons (Pine shown, Fig. 2). This carbohydrate and lignin increase suggests that the rate of decomposition has decreased with increased N fertilization, and this conclusion is supported by results of a litter decomposition study performed by Magill and Aber (1998). Pyrolysis GC/MS data support the NMR results. Preliminary ZTL analyses indicate a DOM composition that is predominantly aromatic- and carbohydrate-type carbons.

Magill, A.H. and Aber, J.D. 1998. Plant and Soil 203: 301-311.

Land-Use Legacies in the Sand Plain Vegetation of Cape Cod National Seashore
R. Eberhardt, D. Foster, G. Motkzin, and B. Hall

Despite increased reliance in conservation planning on an understanding of landscape dynamics in response to natural disturbance, the pervasive impact of historical land-use is often under appreciated in the management and restoration of conservation areas and natural resources. We used historical and ecological approaches to determine the relative roles of past land-use, fire, and site conditions on woodland vegetation patterns in Cape Cod National Seashore (CCNS), the largest protected area of sand plain vegetation in the New England-New York region. Coastal sand plains are the focus of intense conservation activity since they support uncommon plant and animal assemblages that are dynamic as a result of past disturbance and ongoing human impacts. The study evaluated established theories underlying the interpretation and management of sand plain landscapes that emphasize fire as a controlling process.

Although predominantly wooded at European settlement, the towns of Eastham, Wellfleet, and Truro were extensively settled and 80% open at the peak of New England agriculture in the mid-19^th C. Historical maps and modern soil profiles indicate that the 5000 ha of sandplain woodlands in CCNS experienced varied land-use before agricultural abandonment and natural reforestation. Approximately 44% was plowed for crops or pasture, 42% logged repeatedly but never cleared, and 14% open and subjected to diverse uses. Relationships between modern vegetation and 19^th C land use are striking and largely independent of site conditions. Continuously wooded areas support pine-oak woodlands with abundant ericaceous shrubs, whereas previously plowed sites have less canopy oak, more pine, few ericaceous shrubs, and a distinct understory including the grass Deschampsia flexuosa and the shade-intolerant shrub Arctostaphylos uva-ursi (Fig. 1). Current composition and historical sources suggest that past agriculture generated extensive heathland and grassland habitats, much of which has subsequently reforested. In contrast to many interpretations and management guidelines, the persistent influence of fire is principally in the canopy composition and structure of former woodlots. The results highlight a need (1) to integrate an understanding of past land-use into ecological models underlying the management of biological reserves; and (2) to consider the use of management approaches that mimic past agricultural practices in order to maintain and restore important sand plain habitats.