ALBERT

Notes: Summary Studies on the effects of stem girdling of a tulip poplar (Liriodendron tulipifera L.) dominated mixed deciduous forest revealed that trees continued to grow above the point of girdling for more than two years after girdling. Soil core samples showed that root biomass was not significantly reduced until two years after trees were girdled. Litterfall during the fall of 1977 from trees girdled in Semptember 1975 and allowed to develop stump sprouts (plot no. 1) was 72% of the control plot. Litterfall during the same time from trees girdled in May 1976 but with stump sprouts removed (plot no. 2) was 82% of the control. Diameter growth during the growing season of 1977 was 53% of control in plot no. 1 and 68% of control in plot no. 2. No significant differences (P〉0.05) in forest floor CO2 efflux rates were observed between plots in the field. However, respiration rates were found to be higher in the girdled plot soil than in the control soil when roots and litter were removed by sieving and CO2 efflux from just the mineral soil and associated detritus was measured. Increased leaching rates of nutrients revealed the effects of girdling on biogeochemical cycles of the forest. The most pronounced effect was increased concentrations of NO3 ions in the soil water of the girdled plots, resulting in losses of NO3 ions below the root zone (〉60 cm) during the second year. These losses amounted to 25.4 kg ha-1 in plot no. 2 and 9.0 kg ha-1 in plot no. 1 as compared to 0.15 kg/ha-1 from the control plot. Calcium losses below the root zone during the second year were ≃10 kg ha-1 greater in plot no. 2 than the control plot, while losses from plot no. 1 were about equal to the control. Calcium and nitrogen uptake by the sprouts in plot no. 1 offset the slightly reduced uptake by the girdled trees and in fact uptake in plot no. 1 may have exceeded the control during the first year. Differences in uptake could account for only a portion of the differences in NO3 ion concentrations based on litterfall and diameter growth in girdled plots.

Notes: Summary Nitrogen mineralization and net nitrification rates were 3–7 times greater in soil incubations from a girdled Liriodendron tulipifera (L.) stand than in a control stand. Neither litter nor root extracts had an inhibitory effect on nitrogen mineralization or nitrification rate. A lack of nitrification inhibitors also was demonstrated by the fact that ammonium added to the control stand was completely converted to nitrate upon incubation. Additions of sucrose increased CO2 evolution and decreased nitrogen mineralization and nitrification rates in the girdled plot soil, suggesting that nitrification could be effectively controlled by competition for NH 4 + supplies by heterotrophic soil organisms. CO2 evolution rates during incubation showed that heterotrophic as well as nitrifier activities were greater in the girdled plot soil than in the ungirdled plot soil, but the ratio of C to N mineralized was lower in the girdled plot soil. These results collectively indicate that nitrification is regulated by the availability of NH 4 + in these stands, and that the latter is strongly regulated by heterotrophic demand for N.

Notes: Abstract Previously published results from a multidisciplinary research program, Response of Plants to Interacting Stress (ROPIS), initiated by the Electric Power Research Insitute are summarized here. The overall objective of the ROPIS program was to develop a general mechanistic theory of plant response to air pollutants and other stresses. Direct and indirect phytotoxic impacts of O3 combined with induced deficiencies of key nutrients as a consequence of acidic deposition are important components in many of the hypotheses used to explain reported declines in forest growth. In order to address these concerns as they relate to loblolly pine (Pinus taeda L.) growth and develop a greater level of mechanistic understanding of stress response, a study was formulated with two major objectives: (i) over a multi-yr period evaluate the role of loblolly pine genotype in governing loblolly growth response to O3; and (ii) determine the underlying physiological and edaphic basis for loblolly growth response to O3, acidic precipitation, and soil Mg status. An open-top chamber facility located at Oak Ridge, TN provided controlled O3 exposure for the genotype screening study (1986–88) and controlled O3 exposure and rainfall exclusion and addition for the O3-rainfall acidity-soil Mg interaction study (1987–89). A variety of experimental techniques, measurements, and statistical procedures were used over a 4-yr period to quantify various aspects of plant growth, physiology, and soil-plant relationships. Results from the genotype screening study indicate that although family-specific O3 effects were observed at the end of the first year, no statistically significant O3 effects on diameter, height, or total biomass were evident at the end of three growing seasons; nor were any significant O3-family interactions found. In the interaction study, rainfall acidity and soil Mg level had only minimal affects on seedling growth and physiology. Ozone exposure produced significant changes in many variables, the most important being a net retention of carbon in above-ground biomass and a subsequent reduction in carbon allocation to the root system. This change could have important longterm implications for the tree's ability to obtain water and nutrients, maintain important rhizosphere organisms, and achieve a level of vigor that protects against disease and insect attack.