W. S. F. Schuster 1
Schuster, W.S.F
1,2 , K.L. Griffin 2 , K.J. Brown 3 , M.H. Turnbull 4 , D. Whitehead 5 , and D.T.
Tissue 6
1 Black Rock Forest Consortium, 129 Continental Road, Cornwall, New York 12518, USA,
845-534-4517, fax 845-534-6975, schuster@ldeo.columbia.edu
2 Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, USA.
3 Ohio University, Athens, Ohio, USA
3 University of Canterbury, Christchurch, New Zealand
4 Landcare Research, New Zealand
5 Texas Tech University, Lubbock, TX, USA.

W. S. F. Schuster 2
Abstract
We sought to quantify forest biomass and net carbon (C) uptake rates over 70 years in an aggrading deciduous forest in southeastern New York, USA. Annual increment in live, aboveground tree biomass (AGB), estimated using species-specific allometric equations, was as high as 6.9 Mg ha -1 y -1 on undisturbed long-term plots in the 15-km 2 Black Rock Forest. Between the early 1930s and 2000, average annual AGB increment on these plots after all losses was 2.17
Mg ha -1 y -1 , equivalent to an annual sequestration of 1.08 Mg C ha -1 y -1 . Forest-wide AGB accumulation, estimated from inventory measurements, averaged 1.25 Mg ha -1 y -1 (0.62 Mg C ha -
1 y -1 ) between 1930 and 1985 despite periodic logging. A subset of 51 plots averaged further
AGB accumulation of 1.85 Mg ha -1 y -1 between 1985 and 2000. We found some indication of decreasing carbon sequestration with stand maturation, but not on all plots. AGB increment was most rapid (2.91 Mg ha -1 y -1 ) between the early 1930s and the early-mid 1960s, and least rapid
(1.28 Mg ha -1 y -1 ) from the early 1960s through the mid 1980s. AGB increment was positively correlated with soil depth and pH, and negatively correlated with slope steepness, initial stand age, and biomass. Red oak ( Quercus rubra L.) canopy trees stored C at twice the rate of similarsized canopy trees of other species.

W. S. F. Schuster 3
Introduction
Forests cycle and store most of the earth’s terrestrial biomass and thus play a dominant role in the global carbon (C) cycle (Dixon et al. 1994). Temperate deciduous forests in the northern hemisphere comprise some of the world’s most substantial C sinks (Ciais et al. 1995,
Myneni et al. 2001), thereby acting to counter anthropogenic increases of atmospheric CO
2
and the associated consequences. However, estimates of average annual C sequestration for North
American temperate forests range widely ( i.e.
1.7 Pg y -1 , Fan et al.1998; 0.04 Pg y -1 , Schimel et al. 2000), and some suggest they may be approaching saturation (Field and Fung 1999). While some forests in eastern North America have been sequestering C at substantial rates in recent years (Goulden et al. 1996), other forests in the region have experienced reductions in biomass accumulation and C sequestration (Likens et al. 1994, 1996). Clearly, empirical studies are needed to provide better spatial and temporal resolution of forest C fluxes to enable further elucidation of controlling factors (Scurlock et al. 1999).
Eddy-covariance studies provide an important means of tracking seasonal and annual C flux ( e.g.
Valentini et al. 2000; Janssens et al. 2001). Estimates of net annual ecosystem production (NEP, equal to net primary production (NPP) minus heterotrophic respiration) generally range upward from 2 Mg C ha -1 y -1 for temperate forests. However, short-term NEP studies in forests are not sufficient to determine net C flux over longer periods and larger regions, termed net biome production (NBP, equal to NEP minus losses to human activities and all other causes over long time periods; IGBP 1998). Furthermore, factors controlling long-term C flux may differ from those controlling annual C flux (Barford et al. 2001). For example, successional age and changes in community composition can interact with other biotic and abiotic factors to regulate long-term forest C storage (Gower et al. 1996, Ryan et al. 1997). Ecosystem theory

W. S. F. Schuster 4 states that net C uptake will decrease to zero as ecosystems approach full maturity (Odum 1969,
1971). However, Carey et al. (2001) have argued that, while forests older than 100 years have been considered to be insignificant C sinks, net primary production (NPP) continues to increase in some forests over 200 years old, and other studies have also emphasized the C-sink potential of older forests ( e.g.
Schulze et al. 2000).
Here we examine long-term biomass changes in a North American temperate deciduous forest by analyzing long-term plot and inventory records compiled over a 70-year period of forest maturation. We analyze records from the Black Rock Forest, a 1530-ha oak-dominated forest in a region where forests have been aggrading following recovery from repeated clearcutting and conversion to agricultural use during the 19 th and early 20 th centuries. We use forest inventory records and repeated measurements from undisturbed plots between 1930 and 2000 in conjunction with species-specific allometric equations to study how aboveground biomass has changed over this period of regrowth. We address two questions: (i) how have forest biomass and net C uptake rates varied over this time period, and (ii) have these been decreasing with increasing stand age? We also analyze temporal patterns in growth rates to determine how factors such as environmental conditions and community composition have impacted long-term forest C flux.
Methods
Site Description
The Black Rock Forest (BRF) is a 1530-ha oak-dominated forest in southeastern New
York State, located within the Highlands Physiographic Province, an approximately 8000 km 2 forested uplands underlain by the Reading Prong geologic formation (Fig. 1; Braun 1967).
Average annual precipitation is 1.2 m and air temperature is strongly seasonal, with monthly

W. S. F. Schuster 5 averages ranging from –2.7

C in January to 23.4

C in July (Ross 1958, Turnbull et al. 2001).
The topography is rocky with steep slopes and elevations ranging from 110 to 450 m above sea level. The soils are predominantly brown forest soils, with bedrock or glacial till parent material at depths ranging from 0.25 to 1 m (Olsson 1981). Soil reaction is acidic, availability of nutrients is low, and site index ranges from poor to average (Table1; Lorimer 1981).
European settlement in the area began around 1700 and the land was repeatedly logged, with a small proportion of the forest completely cleared for conversion to agriculture and livestock pasture and then abandoned before 1900 (Raup 1938). Frequent clearcuts and fires resulted in a preponderance of hardwood sprout regeneration (Tryon 1939). BRF became established as a research forest in 1928 and was a unit of the Harvard University Forest system from 1949-1989. Since that time, BRF has been operated as a field station and nature preserve by the Black Rock Forest Consortium, a group of academic institutions from the surrounding region
(Tryon 1930, Mahar 2000).
Inventory Methods and Experimental Plots
In 1930, an inventory of trees of “cordwood size”, roughly greater than 0.1 m diameter at breast height (dbh), was made of the whole forest, which at the time comprised an area of 1260 ha (Tryon 1930). The forest was subdivided into 150 stands based on species composition and average tree density. Stand area, stand age (based on historical records and tree ring counts), density by species, and average wood volume (1 cord = roughly 2.26 m 3 ) were determined for each stand.
Between 1931 and 1936 a series of long-term plots ranging from 0.04-ha to 0.1-ha were established in which forest growth was monitored (Tryon 1939). Eight of these plots have remained undisturbed and all trees greater than 25.4 mm dbh on these plots have been measured

W. S. F. Schuster 6 approximately every five years for dbh and crown class (dominant: trees receiving full light from above and partial sidelight; codominant: trees receiving full light from above only; intermediate: trees receiving only partial toplight; suppressed: trees receiving no direct light). Since 1994, annual measurements of dbh and crown class have been performed. The height of all trees was measured in 1998. Most measurements were made in July or later months, when the majority of diameter growth had already occurred (Karnig and Stout 1969), and therefore represent conditions at the end of the growing season for that year. In four of the years, measurements were made in April through early May, and therefore represent conditions at the end of the previous year’s growing season.
The eight remaining long-term plots are located at intermediate forest elevations and exhibit typical forest characteristics (plots 1 – 8 in Fig. 1). Table 1 lists stand age (determined from increment cores; Lorimer 1981, D’Arrigo et al. 2002), average height of the canopy trees, slope and soil characteristics, and approximate site index (calculated from the heights of dominant and codominant oaks in 1998; Schnur 1937) for these plots. Based on these data, half of the plots are on sites of “good” quality, with the other half ranging from “fair” to “poor”, similar to the range in site quality across the forest (Lorimer 1981). The plots were originally established in pairs, one within an area that was experimentally thinned and the other in a nearby area left undisturbed as a control. Thinning operations removed dead, diseased, and dying trees, as well as those species considered to be less economically desirable ( e.g.
, gray birch ( Betula papyrifera Marsh.), bigtooth aspen ( Populus grandidentata Michx.). However, among the thinned plots ( i.e.
, plots 2, 4, 6, and 8) only plots 2 and 6 were significantly reduced in density and biomass compared to their paired control plots.
A second forest-wide inventory was completed in the summer of 1985 (Friday and Friday
1985), by which time the forest had increased in size to 1416 ha. A total of 56 experimental cuts

W. S. F. Schuster 7 and thinnings were accomplished between 1930 and 1985, impacting approximately one-third of the forest ( e.g.
Tryon and Finn 1949, Harrington and Karnig 1975). These areas were included in this inventory along with 10 ha of conifer trees that were planted in the forest in the 1930s and
1940s. The inventory subdivided the forest into 71 stands based on species composition, canopy height, and canopy cover. Three sample points (or occasionally more) were located at regular intervals along the long axis of each stand, producing a total of 218 plots (star symbols in Fig. 1).
All trees greater than 50.8 mm dbh on these plots were tallied using a 10-factor basal area prism, totaling 2078 trees of 37 species. Data recorded for each tree included species, dbh, and crown class. In 2000 (or in a few cases 1999 or 2001) a total of 51 of the plots were remeasured for various purposes (circled stars in Fig. 1). Measurement methods were the same as in the 1985 inventory, enabling quantification of tree growth, mortality and regeneration between 1985 and
2000 for these plots.
Biomass estimation
We used previously derived regression equations to estimate live aboveground tree biomass (AGB) from tree dbh measurements. For most trees, we used species-specific equations developed at sites that were nearby and/or edaphically similar, and from studies that had a similar dbh range and a relatively large number of samples. For red oak ( Quercus rubra L.), sugar maple
( Acer saccharum Marsh.), red maple ( A. rubrum L.), yellow birch ( Betula alleghaniensis Britt.
( B. lutea Michx.f.)), bigtooth aspen, quaking aspen ( Populus tremuloides Michx.), and beech
( Fagus grandifolia Ehrh.), we used equations from Monteith (1979). For white oak ( Quercus alba L.) and scarlet oak ( Q. coccinea Muenchh.) we used the equations of Whittaker and
Woodwell (1968). For chestnut oak ( Quercus prinus L.), black birch ( Betula lenta L), black cherry ( Prunus serotina Ehrh.), hickories ( Carya spp.), white ash ( Fraxinus americana L.),

W. S. F. Schuster 8 yellow poplar ( Liriodendron tulipifera L.), eastern hemlock ( Tsuga canadensis (L.) Carr.), and basswood ( Tilia americana L.), we used the equations of Brenneman et al. (1978). For the remaining, uncommon species we used the general equations of Monteith (1979) for either hardwoods or softwoods. Biomass in tree stumps and roots, standing dead trees and woody debris, understory vegetation, forest floor and soil are not included in these formulae.
Forest-wide biomass analyses
We estimated AGB for the 150 stands inventoried in 1930 by using diameter measurements and volume estimates made in the 1930s on 12 control plots (numbers 1, 3, 5, 7, and others indicated by squares in Fig. 1). For each tree on each plot, we estimated aboveground biomass using the allometric equations and then estimated total aboveground biomass density
(AGB) by summing for all trees on each plot and dividing by plot area. For plots established between 1931 and 1936, we estimated AGB in 1930 by subtracting the average annual AGB increment for that plot between the first two measurements multiplied by the number of years after 1930. We then regressed the estimated 1930 AGB for each of these plots on the average wood volume in “cords” (equivalent to 2.27 m 3 ) per hectare for the stand in which each plot was located. The resulting regression relationship, B a
= 2.173 * V, where B a
is aboveground biomass in Mg ha -1 and V is wood volume in cords per hectare had an r 2 of 0.61 (p< 0.002). AGB in 1930 was calculated for each of the 150 stands using this regression equation, and forest-wide AGB was estimated as the area-weighted average of these values. 1985 forest-wide AGB was estimated as the area-weighted average of the AGB calculated for each of the 71 stands in the
1985 inventory, and the difference was taken to represent the overall change in forest AGB over
55 years.

W. S. F. Schuster 9
AGB change between 1985 and 2000 was calculated in similar fashion for the 51 plots inventoried in both of these years. However, since these plots were not selected to represent the entire forest, simple means were calculated to quantify AGB change over 15 years on this subset of plots.
Long-term plot analyses
We estimated AGB for each plot by summing the individual tree AGB estimates and dividing by plot area. Transformations of these biomass data were not used since the data were normally distributed and only mildly heteroscedastic (Sokal and Rohlf 1981). We used analysis of covariance with time as a covariate to analyze AGB trends and to compare biomass estimates and regression slopes among sites. Plot size differences resulted in a bias toward finding significant site effects. LSD tests were used for post-hoc comparison of differences.
Differences in average annual biomass accumulation rates among sites and time periods
(plot initiation to the early 1960s, early 1960s to the mid-1980s, mid-1980s to 2000) were analyzed with two-way analysis of variance, using Scheffe’s tests for post-hoc comparisons.
AGB was estimated to be zero at the time of last clearcutting for each control plot to calculate initial growth rates for comparison to rates during the measurement period.
Biomass increment rates for individual tree species were analyzed in two ways. First, the proportion of total estimated aboveground biomass attributable to each species was determined for each plot at each measurement period, using linear interpolation to estimate AGB for plots not measured in a given year. These proportions were then averaged across plots for each species in each measurement year. Second, to compare the biomass increment rates of typical canopy trees of different species, data were selected for all dominant and codominant trees with initial dbh between 0.1 and 0.2 m that survived until 2000. AGB was estimated for each of these trees

W. S. F. Schuster 10 in each measurement year, again using linear interpolation between measurements, and these values were averaged by species.
We assessed the potential importance of five factors (initial stand age, initial stand biomass, average plot slope, soil pH, percent red oak in the canopy) on biomass gain on the longterm plots, by calculating Spearman’s Rank Order Correlations between these factors and two measures of biomass increment: average annual rate of AGB increase and percentage increase over initial AGB.
Results
Forest-wide results
Stand AGB in the Black Rock Forest in 1930 varied from 0 to as much as 161 Mg ha -1 .
Mean area-weighted AGB was 76.1 Mg ha -1 in 1930 with a standard deviation of 35.7 Mg ha -1
(Fig. 2). Stand age at the time ranged from 20 to 80 years and wood volume ranged from 0 to 168 m 3 ha -1 (Tryon 1930). The 1985 forest inventory documented mean area-weighted AGB of 144.9
Mg ha -1 forest-wide, with a standard deviation of 43.1 Mg ha -1 (Fig. 2). These data indicate an average forest-wide AGB increase of 68.8 Mg ha -1 in the 55 years between 1930 and 1985, comprising an average long-term AGB accumulation rate of 1.25 Mg ha -1 y -1 . Since aboveground deciduous tree biomass is composed of about 50% C (0.498 g C/g wood; Birdsey 1992), this equals an aboveground NBP rate of 0.62 Mg C ha -1 y -1 . Mortality was substantial during this period due to natural thinning, the effects of drought, and experimental cutting ( e.g.
Tryon and
Finn 1949, Karnig and Lyford 1968). Increases in biomass were primarily due to the growth of surviving trees, as ingrowth was minimal during most of this period.
A subset of 51 inventory plots further increased from a mean AGB of 167.0 Mg ha -1 in
1985 to 194.7 Mg ha -1 (with a standard deviation of 53.7 Mg ha -1 ) in 2000. The increase in

W. S. F. Schuster 11 estimated AGB on these plots between 1985 and 2000 averaged 27.7 Mg ha -1 , representing an average annual increase of 1.85 Mg ha -1 y -1 . These plots included more than one dozen mature stands of eastern hemlock and had higher AGB in 1985, on average, than the other inventory plots. Their average AGB in 2000 likely remains higher than the forest-wide average, although reductions in eastern hemlock growth and some mortality have occurred since the exotic hemlock wooly adelgid ( Adelges tsugae Annand) was first noted on trees in the forest in 1992 (Kimple and Schuster, 2002).
Long-term plot results
The eight undisturbed long-term plots had initial AGB of 20 – 130 Mg ha -1 when first established in the 1930s (Fig. 3). By 2000 all had increased substantially in AGB, ranging from
170 to 280 Mg ha -1 . This represented a two- to more than seven-fold increase in aboveground biomass over the period. Regression lines differed significantly among sites (p < 0.001). Longterm net AGB increment rate averaged 2.17 Mg ha -1 y -1 and ranged from 1.46 Mg ha -1 y -1 for plot
3 to 2.77 Mg ha -1 y -1 for plots 2 and 5 (Table 2). Initial AGB increment rates on thinned plots 2 and 6 were higher than those in their paired control plots, reaching comparable AGB at 50 and 30 years, respectively, after thinning. Estimated biomass increment rates in the initial period after the control plots were clearcut and before the first measurements were made ( i.e.
the first 46 years for plots 1 and 3 and the first 26 years for plots 5 and 7) were similar to long-term rates over the subsequent measurement period, averaging 2.13 Mg ha -1 y -1 . The period of highest longterm average AGB increment was from plot establishment in the 1930s to the early-mid 1960s, with an average rate of 2.91 Mg ha -1 y -1 and a range from 2.09 to 4.20 Mg ha -1 y -1 among plots.
Biomass increment was significantly lower (Scheffe’s test, p < 0.005; Table 2) during the subsequent period from the early 1960s to the early 1980s, averaging only 1.28 Mg ha -1 y -1 , with

W. S. F. Schuster 12 losses on most plots in the early-mid 1960s and in the early 1980s. AGB increment rate then increased to an average of 2.06 Mg ha -1 y -1 between the early 1980s and 2000. Since 1994, annual values for aboveground biomass increment have ranged from strongly negative ( e.g.
loss due to mortality on plot 6) to increases as large as 6.9 Mg ha -1 y -1 .
Proportional increase in aboveground biomass on long-term plots over the measurement period was significantly negatively correlated with initial stand age and biomass (Table 3). Slope steepness was negatively correlated with percent biomass increment, but the correlation was only marginally significant (p = .07). Soil pH was positively correlated with proportional increase in
AGB (p = .03). Percentage of red oak among canopy trees exhibited a similar positive correlation with measures of biomass increment.
Among canopy trees with initial diameters between 0.1 and 0.2 m in the 1930s that survived to 2000, red oaks gained much more biomass and sequestered C at more than twice the average rate for trees of all other species (Fig. 4A). Chestnut oak growth rates were less than those for red oak, but still greater than for other species. These patterns were reflected in the increase in contribution of red oak to total aboveground biomass, averaged across all plots, from
27% to 44% over this period (Fig. 4B). Chestnut oak aboveground biomass remained at about
34% of total AGB through the entire period, while sugar maple decreased from 14% to about 7% of total. Red maple and black birch remained at around 4% of total biomass and most other species experienced modest declines.
Discussion
Aboveground biomass and carbon content
In 2000, a selection of inventory plots in the Black Rock Forest averaged 195 Mg ha -1 in live aboveground biomass and a series of long-term plots averaged 217 Mg ha -1 , 90 – 115 years

W. S. F. Schuster 13 after being clearcut. Based on calculated biomass increment rates and including areas in all successional states, we estimate average forest-wide AGB in 2000 to be approximately 170 Mg ha -1 . Basal area on the long-term plots averaged 28.31 m 2 ha -1 in 2000 (Schuster 2002). These figures are typical for mature North American deciduous forests. Jenkins et al. (2001), analyzed the USDA Forest Service Forest Inventory and Analysis (FIA) database for the mid-Atlantic region, and reported an average AGB of 199 Mg ha -1 for mature, closed-canopy oak-hickory timberlands that had not experienced recent losses due to logging, fire, disease or insects. Their estimates included stump, standing dead, and coarse woody debris biomass, all of which were excluded in our measurements. AGB varies substantially across the Black Rock Forest due to differences in age, growth rates, disturbance, and management practices, and some stands have
AGB densities over 300 Mg ha -1 . These higher biomass densities are within the range reported for old-growth eastern hardwood forests (220 to 330 Mg ha -1 ; Jenkins et al. 2001). The range in
Black Rock Forest AGB is similar to that at the Harvard Forest, located 200 km to the northeast.
The hardwood forest at the Harvard Forest eddy-covariance tower had a total AGB of 200 Mg ha -
1 in 2000 (Barford et al. 2001) and older hemlock-white pine stands had average total AGB of
320 Mg ha -1 (J. Hadley, pers. comm.). The experimental forest at Hubbard Brook, New
Hampshire, located 340 km northeast of BRF, had average AGB of 162 Mg ha -1 in low elevation northern hardwood stands in the early 1970s and 197.8 Mg ha -1 in one control watershed in 1992
(Likens et al. 1994).
Since deciduous tree biomass is composed of about 50% carbon (0.498 g C/g wood;
Birdsey 1992), estimated aboveground C in live trees in the BRF increased from an average of 38
Mg C ha -1 in 1930, to 72 Mg C ha -1 in 1985, to approximately 85 Mg C ha -1 in 2000, with some stands containing more than 150 Mg C ha -1 . However, belowground C stores, however, generally dominate ecosystem C (Dixon et al. 1994). If we consider that our average value of 85 Mg C ha -1

W. S. F. Schuster 14 in 2000 represents approximately 81.5% of total tree C (Whittaker et al. 1974; Jenkins et al.
2001), and that total tree C represents on average only one-third of total system C in United
States forests (Turner et al. 1995), then we estimate an average of 312 Mg C ha -1 for BRF total ecosystem carbon content in 2000, with century-old undisturbed areas as high as 550 Mg C ha -1 .
In comparison, Birdsey and Heath (1995) estimated that total ecosystem C across all northeastern
U.S. “timberlands” averaged 217.5 Mg C ha -1 .
Annual and long-term biomass and carbon increment rates
Between 1994 and 2000, annual AGB increment was as high as 6.9 Mg ha -1 y -1 on Black
Rock Forest long-term experimental plots and averaged 2.3 Mg ha -1 y -1 , somewhat higher than the 1.6 Mg ha -1 y -1 average annual AGB increment reported from the Harvard Forest between
1993 and 2000 (Barford et al. 2001). Eddy-covariance studies in that part of the Harvard Forest indicate a net ecosystem uptake of 2.0 Mg C ha -1 y -1 from the atmosphere by the forest since the early 1990s, with aboveground woody growth accounting for over half of the C uptake (Barford et al. 2001). This estimated rate of net ecosystem production (NEP) is near the median reported from eddy covariance studies worldwide (Canadell et al. 2000). Reported carbon uptake rates in
Europe have been slightly higher than in North America: over a period of six years the Euroflux network reported an average NEP of 2.8 Mg C ha -1 y -1 (Valentini et al. 2000, Janssens et al.
2001).
While annual aboveground increment may be significantly correlated with wholeecosystem C flux ( e.g.
Gower et al. 2001), it is tenuous to compare BRF AGB increment rates with NEP estimates from eddy covariance studies without information on how other key C stores
(e.g. roots, woody debris, soil organic matter) vary over time. Soil C, which generally represents the largest C reservoir in ecosystems, is often assumed to remain constant over time (Turner et al.

W. S. F. Schuster 15
1995). However, forest regrowth may be accompanied by increased C storage in soil organic matter, especially in areas reverting from agriculture to forest (Houghton et al. 1983, Birdsey and
Heath, 1995). Reported soil uptake rates have ranged from 0.2 to 0.6 Mg C ha -1 y -1 (Richter et al.
1999, Post and Kwon 2000). Woody debris biomass is also expected to increase in early aggradation in many forest ecosystems (Bormann and Likens, 1979). Even without uptake estimates for these other components, the 1994 – 2000 estimated annual BRF aboveground C sequestration rate of 1.2 Mg C ha -1 y -1 is nearly twice as large as a recent uptake estimate across all of North America (0.66 Mg C ha -1 y -1 ; Myneni et al. 2001).
The critical measure of long-term ecosystem C flux is net biome production (NBP), equivalent to NEP minus all non-respiratory losses of C over periods of decades or more (IGBP
1998). Since shorter-term studies generally do not account for biomass losses to factors such as herbivory, fire, windthrow, drought, disease and human activities, the biotic and abiotic factors determining long-term NBP may differ from those that regulate annual NEP (Field and Fung
1999, Barford et al. 2001). Data from the Hubbard Brook experimental forest demonstrate dramatic variation in NEP in different time periods, ranging from 4.85 Mg ha -1 y -1 in 1965-1977 to nearly zero in 1982-1992 (Likens et al. 1994, 1996). The authors of that study suggest that this reduction was a consequence of soil degradation caused by cation leaching due to anthropogenically-acidified precipitation. Black Rock Forest data also exhibit temporal variation in long-term AGB increment rates, with some differences in pattern. While AGB accumulation rates have sometimes averaged more than 4 Mg ha -1 y -1 over several years, the average over the past seven decades has been only 2.17 Mg ha -1 y -1 . Even at this lower long-term rate, forest-wide aboveground biomass increased by a factor of nearly 2.5 over the past 70 years. Despite fluctuations due to factors discussed below, Black Rock and similar forests clearly functioned as

W. S. F. Schuster 16 substantial C sinks for most of the 20 th century, while globally there was a net release of C from the biosphere to the atmosphere (Houghton et al. 1983).
Impact of stand age and other factors on long-term biomass gain
Our findings provide some evidence of decreasing annual forest carbon storage associated with stand maturation, but the patterns differ strongly from a monotonic decline with age. Some stands at ages of more than 100 years or more have exhibited undiminished aboveground biomass increment rates. Extrapolations indicate that biomass increment rates during the initial
25 - 45 years after clearcutting were similar to long-term average rates. The most rapid aggradation rates on most plots occurred between the early 1930s and the early 1960s, when stand ages increased from 25-45 years to 55-75 years. The period of lowest overall net biomass gain then ensued from the early 1960s to the mid-1980s, but this was apparently caused by external biotic and abiotic factors rather than increasing stand age. Biomass increments rates have rebounded over the past 15-16 years, averaging 1.85 Mg ha -1 y -1 in mature stands of eastern hemlock, and 2.25 Mg ha -1 y -1 in oak-dominated stands between the ages of 100 and 115 years.
These results demonstrate that NBP has been little impacted by increasing stand age during a century of aggradation, contrasting with the findings of Gower et al. (2001) in boreal forests over a similar age range.
Environmental factors such as precipitation, temperature, and length of growing season can significantly impact annual ecosystem C flux ( e.g. Goulden et al. 1996, Braswell et al. 1997,
White et al. 1999). Our results indicate that the regional drought that lasted from 1962 to 1966 had a major impact on long-term NBP as well as NEP. Aboveground tree growth was consistently reduced during this period, a pattern also documented in annual rings in eastern hemlock and chestnut oak trees (D’Arrigo et al. 2002). The concomitant high mortality in oak

W. S. F. Schuster 17 trees (Karnig and Lyford 1968) had a large impact on biomass stores, as three of the eight longterm plots lost AGB during this period. Site-specific environmental factors also clearly impacted biomass accumulation, as annual AGB increment consistently differed among sites over seven decades, ranging from 1.68 to 2.72 Mg ha -1 y -1 . Correlation analyses identified a trend toward higher biomass increment on sites with higher soil pH and on flatter sites, suggesting a controlling influence of soil water availability on long-term C sequestration. Large-scale environmental changes, such as increasing atmospheric CO
2
(McKenzie et al. 2001), increasing temperatures (Medlyn et al. 2000), and anthropogenic nitrogen (N) deposition (Aber et al. 2001) can also change the productivity and C storage patterns of forest ecosystems. The Black Rock
Forest currently receives high annual input of N from the atmosphere, with an average wet deposition rate of 6 – 7 kg N ha -1 y -1 (National Acid Deposition Program 2002).
In addition to stand age and environmental factors, species composition can also significantly influence long-term C storage. Biomass increment rates varied among different species of canopy trees on the long-term plots by a factor of seven. The percentage of red oak among canopy trees was significantly correlated with proportional AGB increase over 70 years, and red oak canopy trees stored C at twice the average rate for similar-sized trees of other species. Chestnut oak also exhibited greater long-term biomass increment compared to cooccurring species of birch and maple. These patterns are consistent with recent studies of the photosynthetic and respiratory responses of dominant BRF tree species to environmental variation (Turnbull et al. 2001, 2002). Abrams (1998) has documented widespread expansion of red maple, which now dominates the understory and mid-canopy of many eastern deciduous forests. However, while common in the Black Rock Forest, red maple has contributed little to long-term C storage. If the current oak-dominated canopy is eventually replaced by red maple

W. S. F. Schuster 18 and black birch, the current understory dominants, the change will likely be accompanied by a net release of C as canopy trees decompose, and could potentially lead to reduced carbon uptake.
Among biotic factors, herbivory has also played an important role in long-term biomass accretion. Regional outbreaks of the introduced gypsy moth caterpillar ( Lymantria dispar L.) in the 1970s and early 1980s were accompanied by widespread defoliation, and a majority of the long-term plots experienced increased mortality and reductions in aboveground biomass during this period. The decimation of American chestnut ( Castanea dentata (Marsh.) Borkh.) by the chestnut blight ( Cryphonectria parasitica (Murrill) Barr) between 1915 and 1918 caused a substantial loss of biomass, although it was followed by some “growth increase” in stem and roots of the remaining trees (Stout 1956). Recently, outbreaks of the introduced hemlock wooly adelgid have caused increased eastern hemlock mortality in the Black Rock Forest and elsewhere
(Orwig and Foster 1998, Kimple and Schuster 2002).
Human impact on forest C flux through tree removal for forest management and experimental purposes is a likely factor in the relatively low forest-wide AGB increment of 1.25
Mg ha -1 y -1 between 1930 and 1985. Thinning of stands initially lowers C stores, but has been shown to increase increment growth in remaining trees for 6-8 years (Karnig and Stout 1969).
Our results suggest that this effect may persist for 30-50 years.
Implications and additional research
This study concurs with other evidence indicating that North American temperate forests have functioned as significant C sinks for many decades. Black Rock and other forests in the
Highlands Province are clearly still aggrading and sequestering carbon, especially in areas with canopies dominated by red and chestnut oaks. Since site conditions such as steep slopes and highly acidic soils were correlated with lower carbon storage rates, the sink capacity of other

W. S. F. Schuster 19 portions of the eastern deciduous forest may exceed the rates reported here.
Additional biometric studies using allometric equations could add to our spatial and temporal knowledge of the magnitude of C sequestration within the region. When carefully selected, allometric equations have been shown to estimate aboveground tree biomass with little bias or error (Wharton and Griffith 1993, Arthur et al. 2001). Our approach combined repeated measures on a small number of fixed plots with inventory data from hundreds of small plot measurements over a larger area (15 km 2 ), thus minimizing disadvantages such as the potential unrepresentative nature of fixed plots and the low accuracy typical of large-scale inventory data
(Brown et al. 1997). However, while aboveground biomass likely represents the major component of NBP in aggrading forests, it remains critical to determine how components such as soil and detrital C co-vary with aboveground biomass to more precisely quantify ecosystem C flux over time.
For these and similar forests, long-term C storage rates are determined by a suite of biotic and abiotic factors that reduce long-term NBP well below annual NEP rates. Manipulative studies could substantially improve our understanding of the relative importance of these factors for both annual and long-term C storage. Our results support the conclusions of Brown et al.
(1997) and Myneni et al. (2001) that forest regrowth has significantly increased the C pool of the
United States in recent decades, with the potential for significant additional accumulations. In addition to the apparent capacity of the current canopy trees to continue gaining biomass, these forests have not yet accumulated the quantities of detritus and long-residence soil C typical of true old growth forests. Carey et al. (2001) and Schulze et al. (2000) have emphasized that even forests in an old-growth state may continue to absorb C. We project that forests in this region will continue to add biomass and sequester C, and may not become saturated for decades or even centuries, although disturbance or environmental changes may alter the trajectory.

W. S. F. Schuster 20
Acknowledgements
We thank Hal Tryon for initial work that enabled this study, Ben Stout and Jack Karnig for continuing the measurements from 1949 through 1992, and Kathleen and James Friday for conducting the 1985 survey. John Brady, Aaron Kimple, John Canella, Michael White, Kristina
Kipping, Sarah Helm, Kathy DeWitt, Matt Munson, and Joy Felio capably assisted fieldwork and data management. We also thank Michael Grant for statistical advice, Frances Schuster for map making, NIGEC, Barnard College, and the Pew Fellowship program for supporting student fellows, J.T. Mates-Muchin and J.D. Lewis for sharing data, and Ernest G. Stillman and William
T. Golden for their committed support of science in the Black Rock Forest. This work was funded in part by the Andrew W. Mellon Foundation and the Foundation for Research, Science and Technology, New Zealand.

W. S. F. Schuster 21
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W. S. F. Schuster 31
Table 1. Site conditions and stand data for eight long-term plots examined in this study.
Stand Age
Plot # in 2000 (yrs) Slope (%) Aspect Soil pH a
1
4
5
2
3
6
7
8
115
115
115
115
95
95
90
90
11
9
11
8
2
2
11
9
NW
NW
NW
NW
W
NE
NE
NE
3.65
3.70
3.85
4.00
4.55
4.25
3.90
3.85
Avg. Ht. (m)
18.2
15.8
23.8
24.6
20.6
22.2
b b ac ac d cd
(0.8) b in 1998 (+/- s.e.)
24.2
a (0.7)
24.7
a
(0.8)
(1.0)
(0.9)
(1.4)
(0.8)
(0.4)
Site Index c
(1998 est.)
61 (average)
61 (average)
43 (poor)
39 (poor)
61 (average)
62 (average)
53 (fair)
57 (fair-average) a
Mean of top and subsoil composites from eight subsamples per plot.
b
Mean heights followed by same letter do not differ significantly at alpha = 0.05, LSD test.
c
For mixed oaks (Schnur 1937).

W. S. F. Schuster 32
Table 2. Rates of biomass increment (t/ha/yr) for various time periods on long-term plots.
Overall
Plot (193X
*
- 2000)
1 1.80
2
3
4
2.77
1.46
1.89
5
6
7
8
2.77
2.41
2.21
2.07
Mean 2.17
Before 193X
2.93
1.56
2.21
1.82
#
Time period
193X
*
- 196X
&
196X
&
- 198X
$
2.91
3.32
2.17
2.09
0.05
1.83
0.93
0.88
3.01
4.20
2.52
3.06
2.91
a
2.79
1.69
1.62
0.47
1.28
b
198X
$
- 2000
2.22
3.07
0.88
2.84
2.29
0.09
2.51
2.59
2.06
ab
2.13
*
1931, except 1936 for plots 7 and 8
#
from last clearcut to 1931, except to 1936 for plot 7
&
1961, except 1964 for plots 7 and 8
$
1984, except 1983 for plots 3 and 4 and 1986 for plots 7 and 8 a,b
means followed by the same letter do not differ significantly at alpha = 0.05, Scheffe's test

Table 3. Spearman's rank order correlation of aboveground biomass increment with various plot characteristics (p values in parentheses).
Characteristic
Initial stand age
Initial biomass
Slope %
Soil pH
% Red oak
Aboveground Biomass
Increment Rate Percent Increase
-0.55 (.16)
-0.26 (.53)
-0.44 (.27)
0.36 (.38)
0.19 (.65)
-0.87 (.01)
-0.83 (.01)
-0.67 (.07)
0.75 (.03)
0.74 (.04)
W. S. F. Schuster 33

W. S. F. Schuster 34
Figure captions
Figure 1. Map of the Black Rock Forest showing inventory plots (1985: stars; 2000: circled stars) and long-term plots (numbered and unnumbered squares) and inset of surrounding region showing Highlands Physiographic Province (shaded).
Figure 2. Mean aboveground biomass (+/- 1 s.e.) for the Black Rock Forest in 1930 and 1985 and for a selection of 51 inventory plots measured in 1985 and remeasured in 2000.
Figure 3. Aboveground biomass estimated for 8 long-term experimental plots in the Black Rock
Forest from time of last cutting (clearcuts in 1885 – 1910) to 2000. Plots 2 and 6 received significant experimental thinning in 1931. Data points represent estimates made from diameter measurements in that year and solid lines are linear extrapolations of biomass between measurements.
Figure 4A. Average estimated aboveground biomass per tree for canopy tree species persisting on long-term plots throughout the measurement period. 4B. Proportion of total aboveground live tree biomass on long-term plots in trees of selected species.