Ground measurements of biomass and environmental parameters are an important complement to our tower-based carbon flux measurements. Flux measurements tell us whether carbon dioxide is being absorbed or emitted by the forest. The plot measurements are intended to quantify how much carbon is accumulating or being lost from above ground biomass and how tree growth and production of foliage varies among species.
In 1993, we installed 40 circular, 10 m radius biometric plots in the footprint of the EMS tower on Prospect Hill. We randomly placed the plots within 100 m increments along ten 500 m transects that extend from the tower in the northwest and southwest directions. In 2001, we removed three plots (G3, H3, H4) from the datasets and ceased measurements there due to their inundation by a beaver pond. In 1999, we installed 6 additional circular, 10 m radius biometric plots on the the Simmes Lot, adjacent to Prospect Hill to study the effects of a selective harvest that occurred there in the winter of 2000-01. In the summer of 2001, we expanded the harvested plots in size to 15 m radius and ceased measurements at one plot (X4) because it was unaffected by the harvest. The harvest also affected three of the original tower plots (A4, A5, B5), which were expanded in size as a part of the harvest plot group. Consequently, there are 34 tower plots and 8 harvest plots. We have taken the following ecological measurements at each site: tree growth, woody debris, litter, leaf area increment (LAI), leaf nitrogen, and soil respiration and moisture.
Methods:
LAI
Leaf area index (LAI) data was collected at the biometric plots of the EMS site in 1998, 1999, 2005, 2006, 2007, 2008.
An LAI2000 sensor was used to make 5 measurements at each plot: one at the center, and four more at points 2 meters away from the center, in the 4 cardinal directions (N,E,S,W). All measurements were made at sunrise or sunset when the sun was close to the horizon. Plot-based LAI values were calculated using the standard methods of the LiCor program C2000. LAI values were calculated by averaging the 5 subplot values for each plot, and by masking the outer ring of the sensor (ring 5) so as to block out possible sunspots which may have hit the outer ring as the sun rose up from the horizon.
Leaf Chemistry
Foliar chemistry data was collected from leaf litter and green canopy leaf samples in 1998-2000, 2006, 2007 and 2008 at the Harvard Forest EMS biometric plots. Samples were dried at 65 degrees C for 3 days, ground, and analyzed at the Harvard Forest on a Fison Nitrogen Analyzer NA 1500 in 2000 and on a Elementar Vario Micro CHNS analyzer in 2007 and 2008.
Green leaf samples were collected by shooting them down from the canopy with buckshot.
Litter has been collected, dried, and weighed annually at all plots since 1998. A portion of the collected leaves, fruits and flowers, and wood pieces from 1998, 1999, and 2000 were analyzed for N and C content.
Litterfall
Three baskets with a radial area of 0.13 m^2 were randomly placed in each plot at the EMS tower footprint and Harvested sites. Litter collection began at the EMS site (plots A-H, 1-5) in 1998 and the Harvested site (plots X 1-6) in 1999.
Each year litter was collected throughout the fall and once the following spring, dried and weighed. Each spring's measurement is included in the previous year's dataset.
Trees
All live trees greater than or equal to 10 cm DBH were measured in the tower plots and greater than or equal to 5 cm DBH in the harvested plots. Each tree's DBH (cm) was calculated from measurements of their dendrometer's expansion (or contraction) (mm), which were taken with digital calipers. Species specific allometric equations were used to calculate each tree's weight (kg) from their DBH. Each tree's carbon content was assumed to be one half of its weight. The trees' weight in C was summed to yield aboveground woody biomass (AGWB) and increment (AGWI), and biomass lost to mortality and gained through recruitment, all in MgC/ha/yr, for each year and site. Net primary production (ANPP) was calculated as the sum of the increment of surviving trees (AGWI) and the sum of the increment of ingrowth + the sum of fine litterfall using the methods described in Clark et al. 2001. Note that measurements for years 1994-1997 at the tower site represent multi-year averages.
The understory data includes saplings and shrubs greater than or equal to 1 cm DBH and less than 10 cm DBH at the tower plots and less than 5 cm DBH at the harvested plots. Saplings and shrubs were measured in the tower plots in November 2004 and 2006 and in the harvested plots in early June and November 2006. Diameters were measured with digital calipers and recorded in centimeters. For the 2004 survey, trees were measured in 1 cm increments (ie. greater than or equal to 1 to less than 2, greater than or equal to 2 to less than 3, etc.) and the number of trees in each plot, species, and size group were tallied in the column labelled "nindivs". For the 2006 surveys all stem measurements were listed individually. Note that yellow and black birch trees are difficult to tell apart when they are small and should be considered interchangeable in the understory data.
Woody Debris
Coarse woody debris (CWD) is all pieces greater than or equal to 7.5 cm in diameter. CWD surveys involved measuring all CWD pieces in our biometeric plots in the footprint of the EMS tower and at our cut site on the Simmes Trust land.
Fine woody debris is all pieces greater than or equal to 2 cm in diameter and less than 7.5 cm in diameter. FWD was measured using the line transect method from Harmon and Sexton 1996, which gives the following formula for calculating volume per unit area (m^3*m^-2): V=9.869 * sum(d^2/8*L), where d is the piece diameter in m and L is the transect length in m. Line transects were randomly placed in different locations across the tract of land for each survey and every other 10 m segment on the transect was surveyed for FWD.
Annual means and 95% confidence intervals (where plots and segments are the unit of measurement) of volume and biomass are summarized in the files labeled "summary". Biomass was calculated from volumes using decay-class specific densities from Liu et al. 2006. For CWD pieces categorized as whole tree snags, biomass was calculated using species-specific allometric equations for live trees, which are listed for our live tree data. Volumes of whole tree snags were calculated from biomass estimates using the inverse of Liu et al. 2006's densities.
Soil moisture is measured at 4 depths in two backfilled pits near the EMS tower.