Hemlock Experimental Manipulation - Simes Tract

Hemlock decline in New England is caused by direct and indirect effects of invasion of the hemlock woolly adelgid. Direct damage from the insect is causing gradual mortality of hemlock, and widespread harvesting of hemlock in advance of mortality creates a contrasting disturbance. Although both processes affect thousands of acres of forest annually we have only a limited understanding of their effects on forest ecosystem function and productivity and the nature of the subsequent forest community. We anticipate that harvesting will yield different consequences than gradual mortality from the insect. Therefore we have designed an experiment to simulate the impact of both in order to contrast them. To simulate some of the effects of the adelgid (e.g., progressive mortality, retention of the wood on the site) we are girdling all hemlocks in a hemlock-dominated stand. In the adjacent area we are conducting a commercial harvesting of hemlock. Results from both experimental treatments will be compared to the changes observed in forests that are being infested by the adelgid, and can also be included in integrated analyses of a suite of large experiments that form a core component of the Harvard Forest LTER program.

Plot Design

General Layout

Intensive study plots will be 30 x 30 m lying in the center of 90 x 90 m treatment units. There will be two replicates of four treatments: control, hardwood control, commercial harvesting and girdling. Intensive measurements will focus on the central plots, but the buffers and outlying areas will provide additional area for other studies and manipulations including transplants. Eventually the control will become a HWA plot and that the effects of HWA will overlay all treatments.

Control

No manipulation other than the standard suite of low intrusiveness sampling.

Hardwood Control

Intended to simulate a likely future forest condition after HWA kills all hemlock in a stand and deciduous trees grow into the site. No manipulation other than the standard suite of low intrusiveness sampling.

Commerical Logging

Intended to simulate an intensive commercial logging operation with the intent of removing hemlock and other commercially valuable material. This would include the removal of larger hardwoods and pine for saw logs as well as the removal of smaller stems that a logger might take in order to: improve future stand quality, facilitate skidding and general operation, or initiate a new cohort of sprouts. The result would be a substantial removal of basal area, leaving only the next generation of well-formed hardwoods and pine such as a landowner would leave expecting to return in 30 years. Commercial logging equipment would be used with the expectation that there would be soil disturbance, damage to some residual material, etc. Slash would be scattered/clumped across the area. Cordwood (i.e. smaller diameter boles and branches) would be removed. The intent is to generate an extreme response that is within the range of those seen on real commercial cuts. This treatment took place in Winter 2004-05.

Girdling

Intended to simulate some of the characteristics of HWA, but differing in key ways. Treatment would be to girdle all (and only) hemlock, with the intent of killing these over a period of weeks to months. The result would be standing dead hemlock that gradually lose their leaves, twigs, fine branches, larger branches, and then fall apart in place. Thus the change in overstory and microenvironmental conditions would be gradual (i.e., over 1-2 years) and much less severe than that of the harvesting. Important characteristics of HWA would be missing. Most notably the very lengthy period of decline during which the plant is undergoing physiological stress and metabolic imbalance that may induce biogeochemical and microbial changes on the site. No mechanical site disturbance, removal of material, etc. This treatment took place in late May 2005.

Data Collection

Vegetation Structure and Function

Overstory: Trees >=5cm (species, diameter at breast height, crown class, XY coordinates within plot); entire plot; includes dead trees if solid and taller than about 5m. Response to treatments measured as vigor of hemlock, mortality and sprouting (began 2003, ongoing; Barker Plotkin et al.). Tree heights and crown dimensions recorded for a subset of trees in each core area (Summer 2004; Albani).

Saplings: Trees >1.3m tall but <5cm dbh tallied by species in the core area of each plot (began Winter 2004-2005, ongoing; Barker Plotkin et al.)

Understory vegetation: percent cover of herbaceous and shrub vegetation (vascular); percent cover and density of tree seedlings; organic layer depth; 10, 1m2 plots within the core area of each plot (late August 2003 and late June 2004)

Seed dispersal and recruitment: (began Fall 2004, ongoing; Ellison and Record)

Seed bank: seed bank sampled from each plot core area (June 2004; Ellison and Sullivan)

Coarse woody debris: downed and standing dead wood sampled in all plots (began Summer 2005, ongoing; Barker Plotkin)

Litterfall: Five litter baskets in each plot (began Fall 2005, ongoing; Barker Plotkin et al.)

Tree sapflow: in pilot girdle plot only (Summer 2004; Phillips et al.)

Fauna

Ants: ant fauna sampled in each core area using pit traps and litter sampling (began Summer 2003, ongoing; Ellison et al.)

Aerial insects: focus on beetles - a canopy insect trap installed in plots 5, 6 and 7 (began Summer 2004, ongoing; Ellison et al.)

Salamanders: salamanders sampled along 2, 90-m transects in the buffer area of each plot. The focus is on red efts and red-backed salamanders (2003-2005; Mathewson)

Carbon and Nitrogen Dynamics

Soil N dynamics: Nitrogen dynamics sampled in 4 plots within the core area of each plot, using soil core incubations and resin bags. Soil temperature, pH, CN sampled at these sites (began Summer 2003, ongoing; Orwig and Lux)

Soil respiration: soil respiration measured at 4 sites in the core area of each plot. Also at these sites, C & N pools determined. Soil temperature at top 10cm of soil measured during respiration sampling. (began Summer 2003, ongoing; Davidson and Savage)

Soil pits, in which soil T and moisture would be sampled continuously, are planned (Davidson and Savage)

Canopy throughfall chemistry: Pilot girdled plot only (Summer 2004; Stadler)

Environmental Variables

Light: hemispherical photos on a 15m grid in each plot (began Winter 2004, ongoing; Ellison et al.). Also, hemispherical photographs were taken at the center of each plot in Summer 2003 (Albani and Moorcroft)

Air temperature: Continuously monitored at 1m from the ground in the 6 hemlock plots only (began November 2004, ongoing; Ellison et al.)

Soil temperature also continuously measured at 10cm depth in the 6 hemlock plots (began November 2004, ongoing; Ellison et al.)