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The potential need for national-level comparisons of greenhouse gas emissions, and the desirability of understanding terrestrial sources and sinks of carbon, has prompted interest in quantifying national forest carbon budgets. In this study, we link a forest inventory database, a set of stand-level carbon budgets, and information on harvest levels in order to estimate the current pools and flux of carbon in forests of the conterminous United States. The forest inventory specifies the region, forest type, age class, productivity class, management intensity, and ownership of all timberland. The stand-level carbon budgets are based on growth and yield tables, in combination with additional information on carbon in soils, the forest floor, woody debris, and the understory. Total carbon in forests of the conterminous U.S. is estimated at 36.7 Pg, with half of that in the soil compartment. Tree carbon represents 33% of the total, followed by woody debris (10%), the forest floor (6%), and the understory (1%). The carbon uptake associated with net annual growth is 331 Tg, however, much of that is balanced by harvest-related mortality (266 Tg) and decomposition of woody debris. The forest land base at the national level is accumulating 79 Tg/yr, with the largest carbon gain in the Northeast region. The similarity in the magnitude of the biologically driven flux and the harvest-related flux indicates the importance of employing an age-class-based inventory, and of including effects associated with forest harvest and harvest residue, when modeling national carbon budgets in the temperate zone.

Forests are major components of the global carbon cycle, providing substantial feedback to atmospheric greenhouse gas concentrations (1). Our ability to understand and predict changes in the forest carbon cycle--particularly net primary productivity and carbon storage--increasingly relies on models that represent biological processes across several scales of biological organization, from tree leaves to forest stands (2,3). Yet, despite advances in our understanding of productivity at the scales of leaves and stands, no consensus exists about the nature of productivity at the scale of the individual tree (4-7), in part because we lack a broad empirical assessment of whether rates of absolute tree mass growth (and thus carbon accumulation) decrease, remain constant, or increase as trees increase in size and age. Here we present a global analysis of 403 tropical and temperate tree species, showing that for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree. The apparent paradoxes of individual tree growth increasing with tree size despite declining leaf-level (8-10) and stand-level (10) productivity can be explained, respectively, by increases in a tree's total leaf area that outpace declines in productivity per unit of leaf area and, among other factors, age-related reductions in population density. Our results resolve conflicting assumptions about the nature of tree growth, inform efforts to undertand and model forest carbon dynamics, and have additional implications for theories of resource allocation (11) and plant senescence (12).

Simulations of carbon storage suggest that conversion of old-growth forests to young fast-growing forests will not decrease atmospheric carbon dioxide (CO2) in general as has been suggested recently. During simulated timber harvest, on-site carbon storage is reduced considerably and does not approach old-growth storage capacity for at least 200 years. Even when sequestration of carbon in wooden buildings is included in the models, timber harvest results in a net flux of CO2 to the atmosphere. To offset this effect, the production of lumber and other long-term wood products, as well as the lifespan of buildings, would have to increase markedly. Mass balance calculations indicate that the conversion of 5 x 10(6) hectares of old-growth forests to younger plantations in western Oregon and Washington in the last 100 years has added 1.5 x 10(9) to 1.8 x 10(9) megagrams of carbon to the atmosphere

We document post‐fire succession on xeric sites in the southern Appalachian Mountains, USA and assess effects of 20th century reduction in fire frequency on vegetation structure and composition. Successional studies over 18 yr on permanent plots that had burned in 1976–1977 indicate that tree mortality and vegetation response varied with fuel load and fire season. In the first three years after fire, hardwood sprouts dominated tree regeneration. On sites where summer and autumn fires reduced litter depth to less than 1 cm, densities of shade‐intolerant Pinus seedlings increased steadily over this period. 4 to 8 yr after fire, large numbers of newly established seedlings and sprouts had grown to 1 – 10 cm DBH. By year 18 growth of these saplings led to canopy closure on most sites. Herbaceous cover and richness peaked in the first decade after fire, then declined. On similar sites that had not burned in more than 50 yr, regeneration of shade‐intolerant Pinus spp. and mean cover and richness of herbs were considerably lower than those observed on recently burned plots. Reconstructions of landscape conditions based on observed post‐fire succession and 20th century changes in fire regime suggest that reductions in fire frequency circa 1940 led to substantial changes in forest structure and decreases in cover and richness of herbaceous species.

We used a new model, STANDCARB, to examine effects of various treatments on carbon (C) pools in the Pacific Northwest forest sector. Simulation experiments, with five replicates of each treatment, were used to investigate the effects of initial conditions, tree establishment rates, rotation length, tree utilization level, and slash burning on ecosystem and forest products C stores. The forest examined was typical of the Cascades of Oregon and dominated by Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla (Raf.) Sarg). Simulations were run until a C steady state was reached at the landscape level, and results were rescaled relative to the potential maximum C stored in a landscape. Simulation experiments indicated agricultural fields stored the least (15% of the maximum) and forests protected from fire stored the greatest amount (93% of the maximum) of landscape-level C. Conversion of old-growth forests to any other management or disturbance regime resulted in a net loss of C, whereas conversion of agricultural systems to forest systems had the opposite effect. The three factors, in order of increasing importance, most crucial in developing an optimum C storage system were (i) rotation length, (ii) amount of live mass harvested, and (iii) amount of detritus removed by slash burning. Carbon stores increased as rotation length increased but decreased as fraction of trees harvested and detritus removed increased. Simulations indicate partial harvest and minimal fire use may provide as many forest products as the traditional clearcut-broadcast-burn system while increasing C stores. Therefore, an adequate supply of wood products may not be incompatible with a system that increases C stores.

A chronosequence of three species of logs (Pinus sylvestris L., Picea abies (L.) Karst, and Betula pendula Roth.) from northwestern Russia was resampled to develop a new method to estimate rates of biomass, volume, and density loss. We call this resampling of a chronosequence the decomposition-vector method, and it represents a hybrid between the chronosequence and time-series approaches The decomposition-vector method with a 3-year resampling interval gave decomposition rates statistically similar to those of the one-time chronosequence method. This indicated that, for most cases, a negative exponential pattern of biomass, volume, and density loss occurred. In the case of biomass loss of P. sylvestris, however, polynomial regression indicated decomposition rates were initially low, then increased, and then decreased as biomass was lost. This strongly suggests three distinct phases: the first when decomposers colonized the woody detritus, a second period of rapid exponential mass loss, and a third period of slow decomposition. The consequences for this complex pattern of decomposition were explored at the ecosystem level using a simple model. We found that a single rate constant can be used if inputs vary within a factor of 10, but that this approach is problematical if inputs are more variable.

This study investigated the effect of coarse woody debris (CWD) on mineral soils at the H.J. Andrews Experimental Forest in the central Cascade Range of Oregon, U.S.A. Nutrients in CWD leachates were compared with (i) forest floor (control) leachates, (ii) over a decay chronosequence, and (iii) among CWD of four species. There were few differences among CWD leachates and forest floor leachates. Soils under CWD were warmer but not wetter than control soils. Water-soluble organic carbon was higher in soils under CWD than in controls at 5-15 cm depth (p < 0.02), but soil C concentrations did not differ. Gross N mineralization was faster in control soils. We found no differences in N, P, microbial biomass, Biolog plate assays, or enzyme activity in soils. Nutrient leachate differences among CWD species were small. Differences in solutions and in soils among CWD and controls were largest during the middle decay classes. This study suggests that either (i) CWD has no long-term effect and does not contribute large amounts of organic matter to the soil profile or (ii) the effect of CWD is so prolonged that no spatial affect is noticeable because all soils have been affected by CWD at some time.

Forests may have an important role to play in removing carbon dioxide from the atmosphere. However, the extent of their role depends not only on the area available but also the management system that is applied and whether it is based on sound scientific principles, including those of basic ecosystem science. One aspect of ecosystem science that generally has been overlooked in forestry-related carbon projects is that of scale. By paying closer attention to scale, seemingly contradictory statements concerning forest management and carbon sequestration can be resolved, which can lead to the development of a viable carbon sequestration policy.

Decomposition constants (k) for above-ground logs and stumps and sub-surface coarse roots originating from harvested old-growth forest (estimated age 400–600 y) were assessed by volume–density change methods along a 70-y chronosequence of clearcuts on the Wind River Ranger District, Washington, USA. Principal species sampled were Tsuga heterophylla and Pseudotsuga menziesii. Wood and bark tissue densities were weighted by sample fraction, adjusted for fragmentation, then regressed to determine k by tissue type for each species. After accounting for stand age, no significant differences were found between log and stump density within species, but P. menziesii decomposed more slowly (k = 0.015·y−1) than T. heterophylla (k = 0.036·y−1), a species pattern repeated both above- and below-ground. Small-diameter (1–3 cm) P. menziesii roots decomposed faster (k = 0.014·y−1) than large-diameter (3–8 cm) roots (k = 0.008·y−1), a pattern echoed by T. heterophylla roots (1–3 cm, k = 0.023·y−1; 3–8 cm, k = 0.017·y−1), suggesting a relationship between diameter and k. Given our mean k and mean mass of coarse woody debris stores in each stand (determined earlier), we estimate decomposing logs, stumps, and snags are releasing back to the atmosphere between 0.3 and 0.9 Mg C·ha−1·y−1 (assuming all coarse woody debris is P. menziesii) or 0.8–2.3 Mg C·ha−1·y−1 (assuming all coarse woody debris is T. heterophylla). Including coarse roots increases these loss calculations (averages of all decomposition classes for the study year) to 0.5–1.9 Mg C·ha−1·y−1 or 1.0–3.5 Mg C·ha−1·y−1, respectively. Our results support substitution of log k in C flux models when stump k is unknown. Substitution of log k for coarse root k could, however, substantially overestimate C flux back to the atmosphere from these forests.

Decomposition of woody roots in Sitka spruce (Picea sitchensis (Bong.) Carriere), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and ponderosa pine (Pinus ponderosa P. Laws. Ex C. Laws.) dominated forests in Oregon, U.S.A. was studied using a chronosequence. Roots of five coniferous species were excavated from stumps with ages up to 46 years old. In order of increasing decomposition rate constant (k) the species were Douglas-fir < Sitka spruce < lodgepole pine (Pinus contorta Dougl. ex Loud.) < western hemlock (Tsuga heterophylla (Raf.) Sarg) < ponderosa pine. Variation in the proportion of bark, wood, and resin cores was correlated to these differences. Root wood showed the highest k, root bark the second, and resin cores the lowest. The occurrence of resin cores in woody roots of Douglas-fir, Sitka spruce, and lodgepole pine greatly slowed the decomposition of these species. White rots occurred frequently in ponderosa pine and lodgepole pine, whereas brown rots mostly appeared in Douglas-fir and Sitka spruce. Species with white rot had a higher k than those with brown rot. Decomposing woody roots started to release N after 20-30% mass loss, a point when the dead root C/N ratio averaged 140.