Jeffrey J. Lee

A Carbon Budget for Forests of the Conterminous United States

The potential need for national-level comparisons of greenhouse gas emis- sions, 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 bud- gets 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 mag- nitude 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.

Integrated Land-Use Systems: Assessment of Promising Agroforest and Alternative Land-Use Practices to Enhance Carbon Conservation and Sequestration

Degraded or sub-standard soils and marginal lands occupy a significant proportion of boreal, temperate and tropical biomes. Management of these lands with a wide range of existing, site-specific, integrated, agroforest systems represents a significant global opportunity to reduce the accumulation of greenhouse gases in the atmosphere. Establishment of extensive agricultural, agroforest, and alternative land-use systems on marginal or degraded lands could sequester 0.82–2.2 Pg carbon (C) per year, globally, over a 50-year time-frame. Moreover, slowing soil degradation by alternative grassland management and by impeding desertification could conserve up to 0.5–1.5 Pg C annually. A global analysis of biologic and economic data from 94 nations representing diverse climatic and edaphic conditions reveals a range of integrated land-use systems which could be used to establish and manage vegetation on marginal or degraded lands. Promising land-use systems and practices identified to conserve and temporarily store C include agroforestry systems, fuelwood and fiber plantations, bioreserves, intercropping systems, and shelterbelts/windbreaks. For example, successful establishment of low-intensity agroforestry systems can store up to 70 Mg C/ha in boreal, temperate and tropical ecoregions. The mean initial cost of soil rehabilitation and revegetation ranges from

Effect of simulated sulfuric acid rain on yield, growth and foliar injury of several crops

500–3,000/ha for the 94 nations surveyed. Natural regeneration of woody vegetation or agro-afforestation establishment costs were less than

Agricultural Sources and Sinks of Carbon

1000/ha in temperate and tropical regions. The costs of C sequestration in soil and vegetation systems range from

The effect of trends in tillage practices on erosion and carbon content of soils in the US corn belt

1-69/Mg C, which compares favorably with other options to reduce greenhouse gas emissions to the atmosphere. Although agroforestry system projects were recently established to conserve and sequester C in Guatemala and Malaysia, constraints to wide-spread implementation include social conditions (demographic factors, land tenure issues, market conditions, lack of infrastructure), economic obstacles (difficulty of demonstrating benefits of alternative systems, capital requirements, lack of financial incentives) and, ecologic considerations (limited knowledge of impacts and sustainability of some systems).

Sensitivity of the US corn belt to climate change and elevated CO2: I. Corn and soybean yields

Abstract This study was designed to reveal patterns of response of major United States crops to sulfuric acid rain. Potted plants were grown in field chambers and exposed to simulated sulfuric acid rain (pH 3.0, 3.5 or 4.0) or to a control rain (pH 5.6). At harvest, the weights of the marketable portion, total aboveground portion and roots were determined for 28 crops. Of these, marketable yield production was inhibited for 5 crops (radish, beet, carrot, mustard greens, broccoli), stimulated for 6 crops (tomato, green pepper, strawberry, alfalfa, orchardgrass, timothy), and ambiguously affected for 1 crop (potato). In addition, stem and leaf production of sweet corn was stimulated. Visible injury of tomatoes might have decreased their marketability. No statistically significant effects on yield were observed for the other 15 crops. The results suggest that the likehood of yoeld being affected by acid rain depends on the part of the plant utilized, as well as on species. Effects on the aboveground portion of crops and on roots are also presented. Plants were regularly examined for foliar injury associated with acid rain. Of the 35 cultivars examined, the foliage of 31 was injured at pH 3.0, 28 at pH 3.5, and 5 at pH 4.0. Foliar injury was not generally related to effects on yield. However, foliar injury of swiss chard, mustard greens and spinach was severe enough to adversely affect marketability.

Sensitivity of the US corn belt to climate change and elevated CO2: II. Soil erosion and organic carbon

Most existing agricultural lands have been in production for sufficiently long periods that C inputs and outputs are nearly balanced and they are neither a major source nor sink of atmospheric C. As population increases, food requirements and the need for more crop land increase accordingly. An annual conversion of previously uncultivated lands up to 1.5 × 107 hectares may be expected. It is this new agricultural land which suffers the greatest losses of C during and subsequent to its conversion. The primary focus for analysis of future C fluxes in agroecosystems needs to be on current changes in land use and management as well as on direct effects of CO2 and climate change. A valid assessment of C pools and fluxes in agroecosystems requires a global soils data base and comprehensive information on land use and management practices. A comprehensive effort to assemble and analyze this information is urgently needed.

THE ECOLOGICAL ROLE OF CONSUMERS-AN AGGREGATED SYSTEMS VIEW'

The EPIC model was used to simulate soil erosion and soil C content at 100 randomly selected sites in the US corn belt. Four management scenarios were run for 100 years: (1) current mix of tillage practices maintained; (2) current trend of conversion to mulch-till and no-till maintained; (3) trend to increased no-till; (4) trend to increased no-till with addition of winter wheat cover crop. As expected, the three alternative scenarios resulted in substantial decreases in soil erosion compared to the current mix of tillage practices. C content of the top 15 cm of soil increased for the alternative scenarios, while remaining approximately constant for the current tillage mix. However, total soil C to a depth of 1 m from the original surface decreased for all scenarios except for the no-till plus winter wheat cover crop scenario. Extrapolated to the entire US corn belt, the model results suggest that, under the current mix of tillage practices, soils used for corn and/or soybean production will lose 3.2 × 106 tons of C per year for the next 100 years. About 21% of this loss will be C transported off-site by soil erosion; an unknown fraction of this C will be released to the atmosphere. For the base trend and increased no-till trend, these soils are projected to lose 2.2 × 106 t-C yr−1 and 1.0 × 106 t-C yr−1, respectively. Under the increased no-till plus cover crop scenario, these soils become a small sink of 0.1 × 106 t-C yr−1. Thus, a shift from current tillage practices to widespread use of no-till plus winter cover could conserve and sequester a total of 3.3 × 106 t-C yr−1 in the soil for the next 100 years.

Potential Carbon Benefits of the Conservation Reserve Program in the United States

Abstract Climate models indicate that increasing atmospheric concentrations of CO2 and other greenhouse gases could alter climate globally. The EPIC (Erosion Productivity Impact Calculator) model was used to examine the sensitivity of corn and soybean yields over the US corn belt to changes in temperature, precipitation, wind, and atmospheric CO2 concentration. A statistically representative sample of 100 corn and soybean production sites was selected from the 1987 National Resources Inventory (NRI). One-hundred-year simulations were run for each site under 36 different climate/CO2 scenarios. The results were area weighted according to the NRI area expansion factors to produce a regionally aggregated estimate of yields. EPIC did an excellent job of reproducing current regional mean expected yields under the baseline scenario. There were 3% decreases in both corn and soybean yields in response to a 2°C temperature increase at baseline precipitation levels, with larger and smaller temperature effects under drier and wetter conditions, respectively. Crop yields increased and decreased in response to increases and decreases of 10% or 20% precipitation. A 10% precipitation increase roughly balanced the negative effect of the 2°C temperature increase. Whether the precipitation changes resulted from altered precipitation event frequency or amount per event had little effect on mean crop yields; however interannual yield variability was higher when precipitation decreases were due to frequency rather than intensity. The opposite was true, though to a lesser extent, for precipitation increases. Potential evapotranspiration responded linearly to changes in mean wind speed, leading to modest changes of 1–3 days of water stress per growing season, yield increases of up to 2% for decreased wind, and yield decreases of up to 6% for increased wind. Elevated CO2 concentrations of 625 ppmv gave the greatest yield increases, + 17% for corn and + 27% for soybean at baseline temperature and precipitation levels. The relative CO2 effect was larger under drier conditions.

Watershed surveys to support an assessment of the regional effects of acidic deposition on surface water chemistry

Abstract Climate models indicate that increasing atmospheric concentrations of carbon dioxide and other greenhouse gases could alter climate globally. The EPIC (Erosion/Productivity Impact Calculator) model was used to examine the sensitivity of soil erosion (wind, water) and soil organic carbon (SOC) (15 cm and 1 m depth) across the US corn belt to changes in temperature (+ 2°C), precipitation (±10%, ±20%), wind speed (±10%, ±20%), and atmospheric CO2 concentration (350, 625 ppmv). One-hundred-year simulations were run for each of 100 sites under 36 climate/CO2 regimes. The 100-year regionally aggregated mean water erosion rates increased linearly with precipitation, whereas the wind erosion rates decreased and total erosion rates increased non-linearly. Increasing temperature by 2°C (with CO2 and mean wind speed held constant) decreased water erosion by 3–5%, whereas wind erosion increased by 15–18%. Total erosion increased with increased temperature. Increasing CO2 from 350 to 625 ppmv (with temperature increased by 2°C and mean wind speed held constant) had no effect on water erosion, despite increases in annual total and peak runoff; this was attributed to increased vegetation cover. Wind erosion decreased by 4–11% under increased CO2. Wind erosion was very sensitive to mean wind speed, increasing four-fold and decreasing 10-fold for a 20% increase or decrease in mean wind speed, respectively. This was attributed to a threshold effect. SOC to 1 m decreased 4·8 Mg-C ha−1 from an initial value of 18·1 Mg-C ha−1 during the 100-year baseline simulation. About 50% of this loss (2·3 Mg-C ha−1) was due to transport off-site by soil erosion. SOC in the top 15 cm decreased 0·8 Mg-C ha−1 from an initial value of 4·9 Mg-C ha−1. Increased temperature and precipitation accelerated these losses of SOC, whereas increased CO2 slowed the losses.