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.

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Acid precipitation falls on vast areas of forested land, including most of the eastern deciduous forest of the United States. Forest productivity, ground‐water quality, and surface waters might all be affected. To document and quantify ecosystem response to the onset of acid precipitation, simulated sulfuric acid (H2SO4) rain was applied to model forest plots of sugar maple (Acer saccharum Marsh.) and red alder (Alnus rubra Bong). One set of four plots (two alder and two maple) received a control rain consisting of a stock solution equilibrated with atmospheric CO2 to approximately pH 5.7. For three sets of four plots, sufficient H2SO4 was added to the stock solution to lower the pH to 4.0, 3.5, or 3.0. Nozzles were used to apply simulated acid rain at the rate of 2.8 mm/hour (maple) or 3.7 mm/hour (alder) for 3 hours/day, 3 days/week, throughout the year. Water samples were collected above and below the canopy, below the litter layer, and from 20‐cm and 1‐m depths below the surface of the soil. While constituents of the throughfall were similar to those of the simulated rain, the litter leachate (the actual input to the soil) had consistently higher concentrations of SO42−, Ca2+, and Mg2+, and higher pH, than the acid rain. For the first 6 months, SO42− adsorption by the soil prevented any apparent differences in the SO42−, Ca2+, or Mg2+ concentrations or the pH of samples taken from a depth of 20 cm from plots receiving either acid or control treatments. Then, SO42− concentrations at this depth on plots receiving the most acidic rain (pH 3.0) became increasingly higher than on the other plots until, after 3 years, they were approximately equal to SO42− concentrations in the rain. Soil solutions from the pH 3.5 and 4.0 treatments responded similarly starting 10 months and 1 year, respectively, after initiation of exposure to acid rain. Increased Ca2+, Mg2+, and, to a lesser extent, K+ concentrations and lowered pH in solutions from the 20‐cm depths occurred simultaneously with increased SO42− concentrations. At a depth of 1 m, no acid‐rain‐related effects were evident after 2.5 years exposure to pH 3.0 H2SO4 rain. Results showed that a hardwood canopy and litter layer can alter the input of chemicals to the soil from acid rain, that the relative sensitivity of ions to mobilization by acid rain is not the same for all components of an ecosystem, and that acid rain can alter the chemical composition of water within the root zone within a few years, even if the soil is a strong sulfate adsorber.