Samuel B. McLaughlin

Evaluating environmental consequences of producing herbaceous crops for bioenergy.

Abstract The environmental costs and benefits of producing bioenergy crops can be measured both in terms of the relative effects on soil, water and wildlife habitat quality of replacing alternate cropping systems with the designated bioenergy system, and in terms of the quality and amount of energy that is produced per unit of energy expended. While many forms of herbaceous and woody energy crops will likely contribute to future biofuels systems, The Department of Energys Bioenegy Feedstock Development Program (BFDP), has chosen to focus its primary herbaceous crops research emphasis on a perennial grass species, switchgrass ( Panicum virgatum) . The choice of switchgrass as a model bioenergy species was based on its high yields, high nutrient use efficiency and wide geographic distribution. Another important consideration was its positive environmental attributes. The latter include its positive effects on soil quality and stability, its cover value for wildlife, and relatively low inputs of energy, water and agrochemicals required per unit of energy produced. A comparison of the energy budgets for corn, which is the primary current source of bioethanol, and switchgrass reveals that the efficiency of energy production for a perennial grass system can exceed that for an energy intensive annual row crop by as much as 15 times. In addition potential reductions in CO 2 emissions, tied to the energetic efficiency of producing transportation fuels and replacing non-renewable petrochemical fuels with ethanol derived from grasses are very promising. Calculated carbon sequestration rates may exceed those of annual crops by as much as 20–30 times, due in part to carbon storage in the soil. These differences have major implications for both the rate and efficiency with which fossil energy sources can be replaced with cleaner burning biofuels. Current research is emphasizing quantification of changes in soil nutrients and soil organic matter to provide improved understanding of the long term changes in soil quality associated with annual removal of high yields of herbaceous energy crops.

Switchgrass as a sustainable bioenergy crop

Switchgrass (Panicum virgatum L.) shows potential as a sustainable herbaceous energy crop from which a renewable source of transportation fuel and/or biomass-generated electricity could be derived. In 1992, a new research program focused on developing switchgrass as a biomass energy feedstock was initiated by the U.S. Department of Energy in five of the southern United States. The multifaceted, multi-institution research addresses breeding for improved biomass yields, regional field tests, cultural practices, physiology and tissue culture. Recent progress is highlighted in this paper. Preliminary results from the breeding program indicate that recurrent restricted phenotypic selection could lead to development of new cultivars. A technique for regenerating switchgrass plants via tissue culture has been proven and new populations of regenerated plants have been established in the field. Performance trials at three regional cultivar testing centers in Virginia, Alabama and Texas have shown that ‘Alamo’ switchgrass has higher biomass yield and broader adaptability than other cultivars tested. Research on management practices designed to maximize biomass yield has shown that multiple harvests of switchgrass may reduce total seasonal yields in some instances and that responses to fertilizer inputs vary with the environment. Seed dormancy often retards rapid establishment of competitive stands of switchgrass. Our research has indicated that seed dormancy can be modified, resulting in increased seed germination and a greater number of switchgrass plants. Research on the physiology of switchgrass has shown that lowland and upland ecotypes differ in photosynthetic rate but not in respiration rate. Findings in each of these areas can contribute to development of switchgrass as a sustainable bioenergy crop. Future research will address molecular biology techniques for exploiting genetic variation, explore canopy architecture and carbon allocation patterns affecting biomass yield, elucidate key factors in successful establishment of switchgrass and provide technology transfer that facilitates scale-up of switchgrass production for commercial energy production.

Projecting Yield and Utilization Potential of Switchgrass as an Energy Crop

The potential utilization of switchgrass (Panicum virgatum L.) as a cellulosic energy crop was evaluated as a component of a projected future national network of biorefineries designed to increase national reliance on renewable energy from American farms. Empirical data on yields of switchgrass from a network of experimental plots were coupled with data on switchgrass physiology and switchgrass breeding progress to provide reasonable expectations for rates of improvement over current yields. Historical breeding success with maize (Zea mays L.) was found to provide a reasonable model for projected linear rates of yield improvement of switchgrass based on documented progress to date. A physiologically based crop production model, ALMANAC, and an econometric model, POLYSYS, were utilized to estimate variability in switchgrass yield and resource utilization across the eastern two‐thirds of the United States. ALMANAC provided yield estimates across 27 regional soil types and 13 years of weather data to estimate variability in relative rates of production and water use between switchgrass and maize. Current and future yield projections were used with POLYSYS to forecast rates of adaptation and economic impacts on regional agricultural markets. Significant positive impacts on US markets, including significant increases in farm income and significant reduction in the need for government subsidies, were projected. This was based on expected technological progress in developing biorefineries that will significantly increase national energy self‐sufficiency by producing feed protein, transportation fuel, and electrical power from cellulosic feedstocks.

Temperature-independent diet variations of respiration rates in Quercus alba and Liriodendron tulipifera

Diel cycles of bole and root respiration were observed in white oak and tulip poplar trees with highest rates occurring between 7 P.M. and midnight, and lowest rates between noon and 3 P.M. Calculated Q10 values, using daily mean temperatures and daily mean respiration rates, ranged from 1.9 to 4.8 and averaged 3.2 during several days in April and May. Respiration rates predicted from temperatures, in which Q10 was assumed equal to 2, followed a diel cycle 180 degrees out of phase with measured rates. It is suggested that any use of the classical Q1o relationship to predict respiration rates in a natural forest environment be considered suspect unless other controlling variables are considered. Reducing sugar concentrations in tulip poplar boles followed a similar diel cycle as respiration rates, suggesting a correlation between reducing sugar concentrations and respiration rates. In girdling experiments, respiration rates immediately above the incision of a girdled tulip poplar were up to five times higher than the control, while rates below the girdle dropped to about one-third of the control by the twelfth week after the tree was girdled. Diel respiration cycles were observed both above and below the girdle. We suggest that trees possess a diel scheme of food utilization whereby food catabolism is synchronized with growth processes at night when moisture availability is high.

Dendroecological applications in air pollution and environmental chemistry: research needs

Summary During the past two decades, dendrochronology has evolved in new dimensions that have helped address both the extent and causes of impacts of regional scale environmental pollution on the productivity and function of forest ecosystems. Initial focus on the magnitude and timing of alterations of baseline growth levels of individual forest trees has now broadened to include characterization of the geographic extent of effects, their distribution among species, and their relationship to soils and biogeochemical cycles. Increasingly dendrochronology has benefitted from and contributed to improved understanding of the physiological and biogeochemical basis of air pollution effects on forest ecological processes. In addition, the need to consider levels and types of remedial action has raised concerns about the relative roles of anthropogenic and natural causative factors. The subdisciplines of dendroecology and dendrochemistry have evolved in response to those needs. Such applications have extended the field from its initial primary focus on historical growth and climatic reconstruction to an emerging role as an exploratory research tool with the potential to address basic questions about the roles of air pollution in modifying relationships between the amount, timing, distribution, and quality of tree growth and biogeochemical and atmospheric processes. In this paper we focus on two regional scale air quality issues, acidic deposition and tropospheric ozone, as stressors. We evaluate past success, current limitations, and future potential of dendrochronology as an investigative tool for both quantifying and understanding the effects of these stressors on forests. Important issues related to the use of dendrochemistry to evaluate effects of acidic deposition include the role of natural vs anthropogenic processes in cation mobilization in soils; biological and geochemical significance of increases in potentially phytotoxic metals and depletion of essential base cations in stem wood; and quantitative vs qualitative interpretation of patterns of element changes in wood related to metal mobility and species differences in accumulation. Shifts in root growth, function, and distribution and increased sensitivity of tree growth to temperature stress are important indicators that cation depletion can alter forest function and the dendroclimatic signal. Critical challenges in evaluating forest responses to ozone, include defining the relative roles of episodic and chronic exposures in seasonal and annual growth cycles, and the quantifying impacts of ozone on the water relations of trees and stands. Here high-resolution measures of diurnal growth and water use patterns have the potential to identify critical features of both pollutant exposure and plant response. These insights should enhance our analytical capabilities in examining annual-scale measures of growth and provide needed understanding of changes in relationships of growth to climate. We conclude that dendrochronology, when coupled with mechanistic understanding of underlying ecological processes influencing growth, has been, and will continue to be, a valuable monitoring and investigative tool for exploring relationships between trees and their growing environment. We expect this role to become even more important in the future as better ways are sought to evaluate and predict forest growth and function in a changing global environment.

The Impacts of Acidic Deposition and Global Change on High Elevation Southern Appalachian Spruce-Fir Forests

The present distribution of high elevation spruce-fir forests in the southern Appalachian Mountains is the result of a retreat of red spruce (Picea rubens. Sarg.), Fraser fir (Abies fraseri Poir.), and associated species during the last post-glacial era to the coolest and most moist locations in this mountain range (White and Cogbill, 1992). The original range of red spruce in this region is estimated to have been from 450 km2 to 2000 km2. That range became more restricted, to approximately 300 km2, and discontinuous within the region as the climate warmed to its present level. Thus, this forest type represents what is perhaps the most sensitive forest system in the eastern United States to climate change, particularly climate warming.

Chapter 7 Evaluating Ozone Effects on Growth of Mature Forest Trees with High-Resolution Dendrometer Systems

Abstract Both manual and electromechanical dendrometer techniques have been used to define growth patterns of mature forest trees at scales ranging from hourly to seasonal, and to evaluate the roles of ozone and physical climate as contributors to observed variability in growth. This chapter addresses the issues of quantifying and modeling the specific effects of ozone in the presence of co-varying influences of other important environmental variables. A variety of statistical models have been developed that provide strongly converging evidence that short-term variations in ozone exposure, although they contribute only about 2% to hourly scale variations, have strongly accumulative effects over the growing season. Model predictions of growth loss in the range of 50% in high ozone years agreed well with observed growth changes with similar levels of ozone increase for selected sample trees. Observed and predicted growth losses in a high ozone year greatly exceed levels typically assumed for mature forest trees based on controlled studies with seedling trees.