III. Carbon and Nitrogen Interactions with Land Use and Environmental Change Background

The long-term trajectory and magnitude of forest growth and carbon storage in New England and the Eastern US are uncertain. To assess the current and future role of these forests in the global carbon cycle information is required defining (1) the successional status of the forests across the landscape, (2) current rates of carbon sequestration and evolution for the principal vegetation assemblages (3) the potential effects of climate variation and human-imposed disturbance and stress (e.g. N deposition, logging, forest conversion), and (4) the mechanisms controlling C and N dynamics. Past studies in HF LTER I and II place us in an ideal position to address these issues for a representative forest and surrounding lands through intensive site measurements and experiments and to make regional projections through extensive cross-site studies supported by regional ecosystem modeling.

The challenges facing an assessment of current and projected carbon storage in New England are great. Traditional measurements of forest growth and carbon uptake such as forest inventories, cannot define rates for such basic processes as accumulation of organic matter below-ground and relationship of carbon storage to past land-use activity and ongoing environmental change. The New England states currently support greater forest area than at any time in the past 200 years (Irland 1999), but increasingly these forests are being logged for wood products or converted to suburban housing and sprawl -processes that greatly alter local and regional carbon balances. Thus one objective of our work in LTER III will be to assess the impact of forest management and land use change on carbon uptake. We plan to determine the regional extent of logging and forest conversion, and to study in detail the effects on carbon stocks of a typical harvest. The goal is to be able to compare quantitatively the carbon budgets on disturbed lands with rates and extent of forest growth and succession on undisturbed lands..

Fossil fuel combustion and fertilizer production introduce vast quantities of fixed nitrogen from to the environment, most of which enters and eutrophies terrestrial ecosystems (Galloway et al.1994,Melillo 1996). Ecosystem models and extrapolations from field studies suggest that these N inputs currently stimulate significant C storage in mid-latitude forests (Melillo et al.1990, Townsend et al.1996, Holland et al.1997, Nadelhoffer et al.1999). Increasing temperatures may both incease the length of the growing season and fertilize forests as accelerating decay of soil organic matter increases N availability for uptake and storage in plants. Since the C:N ratio in plants is substantially greater than in soil organic matter, warming may increase net C storage, especially in forests by transferring N from soils to vegetation (Melillo et al.1995, Melillo 1996). Magnitudes of such increases depend on how plant C balance is affected by other factors, including the potentially deleterious effects that increased N deposition may exert on tree growth and ecosystem processes (i.e. Nitrogen Saturation; cf. Aber et al.199x,1998) and other aspects of climate change (e.g. water availability, temperature effects on photosynthesis and respiration).

Meanwhile, results from our long-term experiments underscore the fact that many fundamental mechanisms controlling nitrogen and carbon dynamics in temperate forest ecosystems remain poorly understood (Bernstein 199x, Nadelhoffer et al.199x, Aber et al.199x). Consequently in HF LTER III we propose to expand on our long-term measurements, experiments, regional comparative studies, and modeling activities to address central aspects of nitrogen and carbon cycles in New England forests especially as they pertain to current and prospective environmental concerns and to the global carbon cycle. We will focus on the responsiveness of C and N dynamics to vegetation change and succession, climate variability, forest management, and stress, especially nitrogen deposition. At the heart of this work will be intensive site-based studies extended to pertinent sub-regional, regional, and even global scales through extensive measurement and modeling activity.

The proposed work represents expands HF LTER I and II activities in which we established a series of unique field measurements, large-scale experiments, cross-site studies, and modeling activities designed to increase our understanding of carbon and nitrogen dynamics in temperate hardwood forest ecosystems. Our permanent plot measurements are coupled with a unique series of long-term eddy flux measurements made at the Environmental Measurement Station (EMS) where exchanges of carbon, nitrogen compounds, trace gases, water vapor, and energy between the biosphere and atmosphere (Wofsy et al.1993, Goulden et al.1996, Hollinger et al.2000). Large-scale and long-term experiments include: DIRT (Detritus Inputs and Removal Treatments), in which above- and below-ground litter inputs are manipulated to evaluate processes and mechanisms of organic matter formation; Soil Warming, in which soil temperature is raised 5 degrees C. above ambient to assess the effect of warming on soil carbon and nitrogen dynamics (Peterjohn et al.1994,Melillo et al.1995,in press); and N Saturation, in which chronic N additions to forest plots assesses the effect of anthropogenic enhancement of N inputs on forest structure and function (Aber et al.1993, Magill et al.1997,in press). These intensive site-based activities are broadened through comparative studies at Hubbard Brook, Bear Brook and Howland Forest, Maine, and national and international networks of ecosystem studies (e.g. AmeriFlux, Hollinger et al.2000, NITREX).

In LTER III we will expand these efforts to address four primary questions:

1. What are the current rates of carbon storage in mixed temperate forests and how are these influenced by annual to decadal climate variability, succession, and disturbance by forest harvesting and conversion?
2. How would direct (chronic N inputs) and indirect (within ecosystem N redistribution due to climate warming) influences on the Northeast's N cycle affect the capacity of the region's forests to store carbon?
3. What above- and below-ground mechanisms mediate the ability of temperate forests to store carbon and to process fixed nitrogen?
4. What is the regional pattern of forest productivity in New England and how is this affected by legacies of prior land-use and current activity, forest stress, and nitrogen deposition?

Question 1 will be addressed using the unique eddy-covariance measurements that have been taken continuously at Harvard Forest since 1990, combined with intensive biometry, measurements of soil flux, etc. on permanent plots in the footprint of the tower. This work will be complemented by assessments of logging and forest conversion at a site and at sub-regional and regional scale. Question 2 will be addressed through a set of field and laboratory activities related to the chronic nitrogen and soil warming experiments. Question 3 will be explored through a set of ongoing and new studies and undertaken on the DIRT, chronic N, and Soil warming experiments. Question 4 will be examined through comparative studies across New England and through regional ecosystem modeling based on intensive and extensive measurements at the Harvard Forest and regionally.

A. Long-term measurement of C storage in relationship to climate variability, vegetation development, and forest harvesting and conversion

1. Environmental Measurement Station (EMS)

We plan to continue and enhance long-term measurements of Net Ecosystem Exchange (NEE) using the eddy-covariance system at the EMS, extending the longest continuous time series for NEE in the world. The flux data, in combination with comprehensive ecological measurements, provide the foundation for interdisciplinary investigations of carbon balance, legacies of prior land use, vegetation development, effects of management, and biosphere-atmosphere exchanges of pollutants and greenhouse gases by addressing the following questions:

1) What are the important biological processes controlling NEE in a temperate forest?
2) How do these processes vary with climate over annual to decadal time scales? 3) What are the quantitative relationships between NEE, forest structure (e.g. canopy
nitrogen content, LAI, coarse woody debris, stand age), species composition, and climatic variations over longer time scales (inter-annual, inter-decadal)?
4) Do seasonal anomalies in climate affect photosynthetic capacity or pools of short-lived organic matter, thus affecting annual carbon sequestration in subsequent years?

Measurements in permanent plots of carbon cycle processes, including aboveground wood increment, soil respiration, and production of litter and coarse woody debris, will complement ongoing eddy-covariance measurements of fluxes of CO₂, H₂O, sensible heat, O₃, and NO_y and observations of environmental parameters (e.g. net radiation, PAR, soil and air temperatures, wind profiles) (Wofsy et al.1993,Goulden et al.1996a,Munger et al. 1996). Individual biological C fluxes will be summed and reconciled with the forest NEE. Our 10-year data record provides a very accurate measure of average seasonal fluxes and defines mean response of the ecosystem to environmental forcing by incident light, temperature, length of the growing season, etc. Seasonal and interannual anomalies in NEE and its principal component fluxes can then be compared in detail to climatic anomalies (e.g. drought) and biological perturbations (e.g. defoliators, ice or wind damage) to test concepts of causal relationships as described in Results of Prior Research.

The proposed work will deliver basic biometric data, detailed carbon budgets, observations of the sensitivity of CO₂ exchange to environmental forcing and to the legacies of land use, and integrated synthesis of ecosystem control over temperate forest NEE, including:

i. Process measurements and compartment inventories (see Table 2).
ii. Integration of biological/ecological measurements with climate and NEE data from the EMS tower.
iii. Determination of control over long-term patterns of CO₂ exchange from i and ii.

TABLE 2. PROCESS MEASUREMENTS AND COMPARTMENT C INVENTORIES.

Process	Observation
NEP	Annual sum of NEE
GEE + Tree Respiration	Species-specific wood increment data
GEE + Tree Respiration	Leaf litter production data
Soil Respiration	Direct soil collar measurements
Tree mortality and decay	Annual tree and coarse woody debris surveys

Our focus on defining quantitatively the implications of past and ongoing human and natural disturbance (prior agricultural use and hurricane damage) for carbon dynamics is a new direction for the EMS that provides strong linkage to major thrusts in HF LTER. The measurements of leaf and wood production, respiration, and net carbon storage in wood track seasonal and inter-annual feedback between climate and C allocation in principal forest compartments and define the contributions of major species in the forest assemblage. These observations will provide a unique opportunity for direct, quantitative assessment of the influence on the carbon cycle of prior land use, and interactions between land cover change and climate in a representative biome.

2. The Role of Forest Management in Carbon Dynamics in Central New England

Forest management plays a key role in sequestration or release of atmospheric CO₂ by forests (Brown et al. 1996). Although there are many models of timber harvest effects on biomass and carbon balances (e.g., Harmon et al.1990;Dewar 1991 Row & Phelps 1996, Houghton 1999,Schlamadinger & Marland 1999) accurate observations of carbon fluxes associated with harvests are lacking, for both short and long time scales. In central New England, timber harvesting has increased as a consequence of forest growth and maturation and market conditions, becoming the pre-dominant forest disturbance. Determination of harvest impact on carbon storage and the pattern of forest cutting in the landscape is therefore a critical problem. Thus we will expand on past efforts to include a detailed assessment of the fate of carbon following logging in a typical forest. We will also analyze the intensity, distribution, and impacts of logging and forest conversion in central Massachusetts, in the entire state, and across New England. In addition to providing direct information on forest disturbance and C storage these analyses will provide essential input for regional modeling activities.

i. Dynamics and fate of carbon in a commercial forest harvest

Logging impacts will be assessed in a commercial harvest in a mature mixed hardwood forest, adjacent to and structurally and compositionally similar to, those at the EMS and other LTER experiments. The proposed logged and control areas have already been sampled since 1993 in 46 plots (9 harvest and 37 control), each 300 m2 in area, including litter by species, LAI, year-round soil respiration, soil moisture, and high-frequency band dendrometry on ~800 trees. In 1999 we surveyed coarse woody debris in the 9 harvest plots. In order to maintain an accurate carbon budget harvest-related measurements will carefully account for removals by weighing all product and inventorying woody residue and slash. Long-term post-harvest measurements will pay particular attention to soil moisture and temperature, soil respiration, and soil organic matter profiles (total carbon and C/N ratio). We will also measure species-specific growth increments, including shifts in growth rate between species due to competitive release and the composition of species newly growing into the >5 cm size-class, and species-specific tree mortality and decay.

The proposed work for the timber harvest will deliver the following products:

1. Detailed accounting of the short-term carbon balance associated with timber harvest and comparison with adjacent undisturbed sites (for which NEE is measured by eddy correlation).
2. Accounting of the initial fate of below-ground soil carbon following harvest.
3. Integrated understanding of how both ecological and climate responses to forest harvest control subsequent evolution of forest carbon balance.
4. Quantification of the relative magnitudes of the effects of human and long-term (10-year) natural disturbance within the ecosystem.

ii. Regional Assessment of Forest Management and Conversion

To place our site-based measurements in a broad context and to ascertain the impact of forest harvesting and conversion on C storage regionally, we will: (1) develop a spatially explicit, long-term (1984 onwards) assessment of logging activity in central Massachusetts and (2) analyze Massachusetts and New England-wide patterns of forest cutting, forest conversion to other land uses, and forest growth versus removals.

Massachusetts is unique in requiring, since 1984, Forest Cutting Plans for all commercial timber harvests >25 Mbf. These documents provide a database for developing GIS overlays characterizing the spatial characteristics, intensity, motivation, landowner, and volume of all logging activities in our central Massachusetts sub-region. In LTER III we will assemble this database and undertake field sampling of areas harvested in the past 15 years in order to confirm the accuracy of the data and to evaluate the compositional and successional response of forests to logging. These data will be analyzed to:

i. Determine the extent and geographic pattern of harvesting in relationship to cultural, natural, ownership, and land-use factors and evaluate broad successional trajectories.
ii. Assess temporal trends of wood volumes and quality removed in relationship to regional economic factors.
iii. Evaluate land-use patterns and behavior in a landscape predominantly owned and controlled by non-industrial, private forest owners.
iv. Make current our regional analysis of forest disturbance developed in LTER I and II and including natural and human factors.

USDA Forest Service Forest Inventory Analysis (FIA)will be used to compare these detailed and spatially explicit data with state- and New England-wide assessments of forest cutting, conversion, and forest growth versus removal. FIA provides state-level average estimates of growth removals, and forest area. Included in removals are: "volume harvested or killed when harvested, thinnings or land clearing, and the net growing stock volume on land reclassified from timberland to noncommercial forest or non-forest between surveys". Our spatial time-series data enable us to estimate harvest rates of a large amount of non-industrial private forest, to compare this to FIA removal rates, and to develop factors to apply to FIA removal rates elsewhere. This can be combined with land change data to better characterize the amount of removals attributable to harvesting versus change in land use. Other New England states have varying degrees of timber harvesting regulation and data that can provide indriect data harvest and conversion. In addition, FIA crews recently measured the EMS footprint, providing a baseline for comparison of data from the Eastern US with eddy flux data and forest composition and stand characteristics at the LTER.

B. Mechanisms of Carbon and Nitrogen Dynamics in Relationship to Climate Change and Nitrogen Deposition - Long-term Experiments

Long-term experiments initiated in LTER I to follow the course of carbon and nitrogen dynamics in relationship to disturbance, climate change, and nitrogen deposition become increasingly important as we extend our research into the effects of land use, into mechanisms of C and N cycling, and into broader geographical scope. Importantly, as these studies move well into their second decade they enable us both to evaluate the sensitivity of C and N dynamics to climate variability and to investigate above- and below-ground processes that control N and C cycling in temperate forests. Equally importantly, these experiments have critical cross-linkages with our land-use studies, hurricane experiment, permanent plots, and measurements made at the EMS and in the proposed logging assessment. They also provide key input to the regional and global comparative studies that we undertake and to the synthetic modeling approaches described later in this proposal. Thus we will continue and expand these experiments in LTER III as described below.

1. DIRT- Plant litter influences on soil organic matter genesis and function

We will continue chronic manipulations of above- and below-ground litter inputs to forest soils by maintaining and expanding the DIRT (Detritus Input and Removal Treatments) experiment, which is developing an understanding of the long-term (years to centuries) influences of plant inputs on soil organic matter development and dynamics. Treatments are conducted on replicate (n = 3) 3m ´ 3m plots and include doubling of leaf litter inputs, preventing leaf litter inputs, preventing root inputs (with root-ingrowth barriers), and preventing all (leaf and root) plant inputs. A fifth treatment, involves experimental "impoverishment" of soil by replacing the top 20 cm of O and A horizons with B horizon soil.

We will conduct our 3rd post-treatment sampling in the early summer of 2001 after 10 years of manipulations. Sampling in years 0 (pre-treatment), 1, and 5 provided information on C and N contents, basic soil properties (CEC, base saturation, pH, texture), and potentially mineralizable C and N pools (methods of Stanford & Smith 1972, as modified by Nadelhoffer 1990; see data via ftp at TO BE INSERTED). Year 5 sampling and analyses (Nadelhoffer et al. submitted) were expanded to include soil microfauna, bacteria and fungal counts (methods of Ingham et al.1985) and measurements of dissolved organic N and C dynamics (Aitkenhead et al. in prep), which will be repeated in 2001. In addition, we are collaborating with Dr. Maura Meade, a molecular ecologist at Allegheny College to explore the use of molecular probes for assessing how variations in plant inputs to soil influencing microbial functional groups. Field measurements of soil respiration (Bowden et al.1993, Boone et al.1998) and soil solution chemistry (Aitkenhead et al. in prep) will be continued during alternate growing seasons, or more often if resources allow. Our accumulated field C flux and soil solution data will complement our existing data on temporal changes in soil properties and mineralizable organic matter pools. Our combined field and laboratory studies will provide information needed to test simulation models of plant-soil interactions (e.g. GEM; Rastetter et al.1991) and soil organic matter dynamics (e.g. DOCMOD; Currie & Aber 1997). In addition, we plan two new, related activities. First, we will install 'impoverished' soil plots (see above) in three additional forest types dominated by red pine, white pine, and sugar maple. Once installed, these O-A Less plots require no maintenance as natural rates of root ingrowth and litter deposition are allowed. Periodic sampling (Years 0, 1, 5, 10, 20, 30…) and assaying of soil properties and mineralizable C and N pools will allow us to characterize patterns of soil process and property recovery under different forest types.

Second, we will expand linkages to related experiments in other regions. Currently we collaborate actively with the H.J. Andrews LTER (temperate conifer forest; Drs. Kate Lajtha, Phil Sollins, etc.) and Allegheny College, PA (base-rich sugar maple forest; Dr. Richard Bowden), which have modified DIRT treatments. We have begun discussions with collaborators working at the Coweeta (GA) and Luquillo (PR) LTERs (January 2000), Hungarian investigators (lead by Kate Lajtha at the June 1999 East European ILTER symposium in Budapest), and Korean and other East Asian ILTER investigators (Nadelhoffer, October 1999 East Asia ILTER meeting in Seoul), which are considering similar experiments. Ultimately, we anticipate an array of DIRT-type experiments in a variety of climatic and vegetation zones to provide information needed to build a process level understanding of soil organic matter genesis and function at local and global scales.

2. Chronic Nitrogen Addition Experiment

We will continue treatments and core measurements on Chronic N with the primary goal of assessing the complete, long-term sequence of responses. Of particular interest is documenting the timing and extent of increased nitrate loss to determine whether this ecosystem response is driven by the intensity or the cumulative dose of additions. If cumulative dose is the key driver, then the low N plots should take three times longer to show elevated nitrate leaching than the high N plots. We will also continue the N+S plots without the S addition, making them a replicate for low N treatment. Such replication is rare in long-term, large-plot experiments.

LTER support allows continuation of the experimental manipulation and core measurements. However, we are pursuing support for two additional process-level initiatives (G. Berntson, PI).

Effects of N Deposition on Kinetics and Magnitude of Soil N Immobilization
We have developed new short-term ¹⁵N tracer methods to quantify the kinetics and magnitude of N immobilization and to quantify more clearly the contributions of "fast" and "slow" processes to gross N immobilization. In hardwood and conifer stands rapid immobilization accounts for a significant fraction of total N immobilized (1/3-2/3 of the total) and long-term N deposition led to a reduction in N immobilization. For the conifer forest, this reduction was due to a loss in the amount of N immobilized during the fast phase, whereas in the hardwood stand it was due to slow phase immobilization. A strong relationship exists between NO3- leaching losses and measured N immobilization rates, but only by explicitly including both rapid and slow phases of N immobilization. We plan to quantify fast vs. slow N immobilization in a larger number of sites, over longer periods of time, with greater replication, and with coupled field and laboratory experiments. We plan to characterize the biotic and abiotic components of these different immobilization phases through manipulation (soil sterilization) and analysis of the linkage between carbon and N dynamics during immobilization (soil CO₂ flux, microbial uptake).

Effects of Long-Term N Deposition on Canopy Structure and Photosynthesis
Preliminary measurements from access towers in the high N pine plots suggest that treatments have changed canopy production, turnover rate, and a critical N:A_max (??) relationship. Collaborating with the USDA Forest Service, we found that while foliar N has increased by more than 100% and soluble protein has increased by 85% in the pine high canopy, maximum photosynthesis did not change. We also found that needle retention times were reduced by 40%, and LAI reduced by 16%, while litterfall rates increased by approx. 20%. These data suggest that increased N deposition has severely reduced the pine forest's gross photosynthetic capacity through reductions in standing foliage via increased turnover rates with no physiological compensation, providing an explanation for observed reductions in tree diameter increments and soil respiration rates. We will extend these measurements to low N pine plots and all hardwood plots.

3. Soil Warming

We will continue to study the effects of soil warming on soil and plant processes by maintaining our extant soil-warming experiment and by adding a new one during the next six-year LTER period. Our current study, established in18 six by six meter plots in 1991, was designed to explore the consequences of temperature increases on soil processes including soil respiration, organic matter decomposition, methane production and consumption, net nitrogen mineralization and net nitrification, and nitrous oxide production (e.g., Peterjohn et al.1994, Melillo et al.1995). The plots are grouped into six blocks, each with three plots assigned to one of three treatments. The treatments are: (1) heated plots in which the average soil temperature at 5cm is elevated 5oC above ambient using buried heating cables; (2) disturbance-control plots that are identical to heated plots except that they receive no electric power; and (3) control plots that have been left in their natural state. For at least the first three years, 2001 through 2003, we will continue to measure soil respiration, trace gas fluxes, nitrogen mineralization and nitrification, and nitrogen leaching to extend our documentation of the effects of warming on soil carbon and nitrogen stocks and cycling rates

In addition we are planning a new warming experiment to address a central question: Can soil warming change ecosystem N distribution and thereby change the capacity of forests to store C? We will directly test the idea that warming can result in the redistribution of N from the soil to plants and thereby enhance net ecosystem carbon storage (Melillo et al.1995). Using our warming protocol in Sweden, Professor Sune Linder (pers. comm.) has observed a 30% increase in wood increment in response to a 5oC soil warming over five yrs. In our new, larger, long-term soil-warming experiment we will make a series of plant and soil measurements that will quantify changes in C storage to provide insight as to how these changes may be related to N redistribution from soil to plants. The larger plots are required to include enough trees in a treatment to capture the plant soil interactions. In 2001, we will establish three, 30x30 m plots adjacent to the current soil warming study and will make 3 yrs of baseline plant and soil measurements on all plots. In 2004, we will initiate a single treatment on each plot - one heated, one disturbance-control, and one control, following the protocols in our initial warming experiment. We will use baseline time-series data in place of plot replication to determine treatment effects in the same way that whole-watershed studies do.

Plant measurements will include: (1) annual woody increment, (2) the C:N ratio in the increment, the roots and the leaves, and (3) natural abundance ¹⁵N content in the various plant parts. Measurements of woody increment will provide an estimate of the degree to which soil warming and the acceleration of the N cycle stimulate C storage in the trees. The measurements of C:N ratio in woody increment, roots and leaves will give insight into how "plastic" the stochiometry of various plant tissues is in response to acceleration of the N cycle through warming. We will also measure ¹⁵N in plant tissues (wood, roots, leaves) on all plots to determine whether plants on the warmed plots are acquiring N that has been mineralized from more refractory (¹⁵N-enriched) soil organic matter pools.

Soil measurements in the new study will include: (1) C and N stocks in the bulk soil and in the light density (LD) and heavy density (HD) soil fractions; (2) the ¹⁴C and ¹⁵N content of the bulk soil, the soil fractions and the fine roots; (3) CO₂ and ¹⁴CO₂ efflux from the soil surface; and (4) the lignase enzyme activity in the various soil horizons; and (5) net nitrogen mineralization and nitrification rates. Measurements of C and N stocks in the soil and respired CO₂ are essential background information for developing budgets using ¹⁴C and ¹⁵N. We propose to use ¹⁴C (Trumbore et al.1996, Gaudinski et al.1997, Paul et al.1997, Trumbore et al.1997) and lignase enzyme activities (Sinsabaugh et al.1991,1992) to examine the turnover of meta-stable soil organic matter. We want to determine whether soil warming has increased or decreased the decomposability of meta-stable soil organic matter. Measurements of N mineralization and nitrification will provide insights into the effects of warming on the N cycle and essential background information for interpreting our ¹⁵N natural abundance study.

4. Regional Assessment of Carbon and Nitrogen Dynamics in Relationship to Land Use and Forest Stress

We have applied PnET models to Harvard Forest sites and spatially across the New York/New England region and have validated these against measurements for gross ecosystem C flux, net primary production, and water yield at HF and Hubbard Brook LTER, and regional data for NPP and water yield. The individual effects of several components of global change (CO₂, temperature, precipitation, tropospheric ozone, N deposition and land use) have been analyzed separately, and predicted responses have been published in conjunction with US Forest Service Northern and Southern Global Change Programs.

In LTER III we will extend this analysis spatially and integrate the separate effects of the different stressors listed above into a single model to provide an integrated analysis of the response of Eastern U.S. forest ecosystems to the full suite of major environmental change factors. This work will continue in cooperation with the US Forest Service through the following steps.

First, we will improve PnET's photosynthesis routine to integrate the combined effects of CO₂ and ozone, based on studies of the relationship between internal leaf CO₂ concentration and maximum photosynthetic rate. This relationship can be described by a michaelis-menten-type equation that can be used to alter the maximum rate of photosynthesis achievable at a given foliar nitrogen concentration. This approach is compatible with the realization of ozone effects on photosynthesis currently in the model and offers the potential for alternative descriptions of stomatal response to both ozone and CO₂ concentrations.

Second, we will extend the spatial extent of our study to include all forests in the Eastern US using the valuable dataset developed by the VEMAP program (Kittel xxx), which provides interpolated climate time series data from 1895 at 0.5x0.5^o resolution. Due to discrepancies we've discovered between the VEMAP 1 radiation data set and a spatial interpolation based on direct measurements within the northeastern US (Jenkins et al. 2000) we have developed and will use a summary relationship based on the limited number of actual observations made over time (Fig xx).

We will also develop data planes for N deposition and tropospheric ozone concentration. N deposition will be estimated using the NADP data base with records for over 100 stations in the eastern US (http://www.nadp.sws.uiuc.edu/nadpdata/) along with the detailed measurements of speciated dry deposition at the EMS tower to confirm the assumed relationship between dry deposition and total (wet + dry) deposition. We will use three USEPA sources to derive a spatially explicit, monthly time-step representation of ozone dose over the Eastern US for the period of measurement: documents on mapping and forecasting ozone concentrations (EPA/454/R-99/009 and EPA/625/R-99/007), daily ozone forecasts from the Hysplit_4 model (http://www.arl.noaa.gov/ready/ozone.html), and the AIRS data set (update December 1999) of surface ozone measurements from 1982 to the present. The Northeast States for Coordinated Air Use Management (NESCAUM, http://www.nescaum.org/) also compiles and maps ozone concentrations that we will use. These regional estimates will be linked to the more detailed understanding of chemical, physical, and biological processes that are inferred from the detailed measurements at the EMS tower.

Observations and modeling of reactive nitrogen deposition, and monitoring and analysis of concentrations of selected trace gases that influence the energy balance or oxidant capacity of the atmosphere will complement the nitrogen addition, ozone impact, and carbon cycling studies.

Deposition of fixed nitrogen and effects of forest emissions and canopy deposition on atmospheric reactive nitrogen and related species

The proposed measurements will help define the input fluxes of nitrogen oxides (wet and dry) and the factors controlling these fluxes. Inputs of reactive N represent potentially major factors in the growth and health of northeastern forests, which are proximate to major source regions. We will determine individual components of fixed nitrogen, using a new instrument to measure NO₂ and HNO₃ at high frequency (funded by DoE), combined with the ongoing flux measurements, to: (1) quantify separately rates of production and loss of major components of reactive N in the atmosphere; (2) determine deposition rates of individual components of reactive N and compare the sum to total deposition; (3) assess the role of biogenic emissions of hydrocarbons in regulating deposition of fixed N; (4) identify seasonal and interannual trends in the rates of NO_x oxidation and deposition, and corrsponding ozone formation, relate those patterns to environmental and biological controlling factors, and examine interactions between inputs of atmospheric fixed N and nutrient cycles in the forest.

SYNTHESIS - Approaches and Products

Table XX. Innovative Research Approaches

Table X. Long Term Measurements on Permanent Plots at Harvard Forest

(1) Quantitative vegetation inventories (3000 acres) every 10-30 years since 1909; permanent plots locations from 1937 plots; augmented with soil chemical and physical properties (Motzkin et al. 1999, http://lternet.lternet.edu/hfr/data/hf015/hf015.html

(2) Post-1938 hurricane permanent plots (14 total; 0.025-0.1 ha) document forest recovery; trees and understory vegetation; measured in 1940, 1948, 1978, 1991 (Spurr 1956, Hibbs 1983, Mabry and Korsgren 1998).

(3) Mapped tracts in which tree species dbh, canopy class and condition are sampled every decade: (a) 4-ha upland red oak-red maple forest (est. 1962; LINK); (b) 1-ha mixed hardwood forest (est. 1990; LINK); (c) a 0.7-ha primary hemlock forest (est. 1990; Foster et al. 1992, LINK), and (d) four 30 x 30 m hemlock-dominated plots (est.1995; McLachlan et al. 2000, LINK).

(4) 8-ha Pisgah Tract of old-growth forest in SW New Hampshire with a 30 x 30 m grid and fourteen 20 x 20 m plots (est. 1984; remeasured in 1990 and 1995 and every 5 years; Foster 1988, LINK).

(5)Montague Sand Plain with 121 400 m² plots (est. 1993; Motzkin et al. 1996, http://lternet.lternet.edu/hfr/data/hf017/hf017.html ). Complete tree and understory maps of a 4.5 ha area (1997-1998) (http://lternet.lternet.edu/hfr/data/hf018/hf018.html ). Sampled every 10 years.

Table XX. Cross-Site Studies in HF LTER III

Yucatan x Puerto Rico x HF
DIRT
Hubbard Brook - Various
Ameriflux
Soil Warmings
Ireland
BigFoot
LIDET
Other?

Table 3. Long-term Meteorological Measurements at the Harvard Forest

LTER I and II
Continuous daily precipitation since 1913 and daily minimum and maximum temperature since 1963 (HF/NOAA weather station; http://lternet.edu/hfr/data/hf001/hf001.html).

LTER III
Upgrade with automated weather station and direct data downloading. Hourly and daily air temperature, precipitation (heated rain gauge), wind speed, wind direction, relative humidity, solar radiation, soil temperature, and atmospheric pressure.

Regional to Global Modeling use soil warming study data to modify our whole-ecosystem model (TEM - Terrestrial Ecosystem Model) which evaluates potential impacts of climate change (e.g., McGuire et al.1992; Melillo et al.1993,1995). Comparisons of TEM and PnET (Aber & Federer 1992) yield similar estimates of regional NPP for contemporary climate (PnET = 145 Tg C/yr; TEM = 140 Tg C/yr; Jenkins et al. 2000). TEM has been used to explore transient climate scenarios based on the United Kingdom's Meteorological Office model (UKMO).