Publisher Summary This chapter demonstrates two structural properties of plant communities—canopy leaf area and growth efficiency. It develops four basic premises that (1) canopy leaf area can be related to competition for light, (2) growth efficiency is responsive to canopy leaf area and other identified environmental factors, (3) potential productivity or site capability can be estimated from knowing maximum canopy leaf area, and (4) canopy leaf areas for trees or stands can be nondestructively estimated by determining sapwood cross-sectional area at a convenient reference height. Canopy leaf area and its vertical distribution can be accurately estimated through correlations with conducting sapwood area throughout the crown. Species within the same genera have widely differing ratios of leaf area to sapwood area. To estimate the canopy leaf area on large trees, the linear taper in sapwood area from breast height (1.37 m) to the base of the crown must be determined.

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Estimating Forest Growth and Efficiency in Relation to Canopy Leaf Area

Spatial patterns in natural {sup 15}N abundance ({sup o}{sup 15}N) in soil, soil solutions, and non-N{sub 2}-fixing plants were studied in the deciduous forest on Walker Branch Watershed near Oak Ridge, Tennessee. This study was undertaken to test the hypothesis that foliar {sup o}{sup 15}N values are related to the availability of inorganic nitrogen in mineral soil. Soils collected in or near valley bottoms on the watershed had higher levels of net nitrogen mineralization and net nitrification potential than those sampled from ridges and slopes. More positive foliar {sup o}{sup 15}N values occurred in valley bottoms, which, relative to other positions on the watershed, were characterized by greater availability of soil nitrogen and lower C-to-N ratios in the O{sub i}-horizon, in the surface mineral soil, and in autumn leaf fall. Although leaf nitrogen concentrations changed significantly over the course of the growing season, there was little seasonal variation in foliar {sup o}{sup 15}N values. A hypothesis about the relative importance of different sources of nitrogen to the forest and how nitrogen cycling varies with topography in this nitrogen-deficient ecosystem was derived, in part, from spatial patterns in natural {sup 15}N abundance. There appear to be two processes affecting the topographicmore » patterns in foliar {sup 15}N abundance on this watershed: (1) greater uptake from isotopically heavy pools of inorganic soil nitrogen by plants in valley bottoms, and (2) uptake of isotopically light ammonium-N in atmospheric deposition by plants on ridges and slopes (where the availability of inorganic soil nitrogen to plant roots is more limited). Results from this study indicate that foliar {sup o}{sup 15}N values are positively correlated with net nitrification potential in surface soil.« less

Variation in Foliar 15N Abundance and the Availability of Soil Nitrogen on Walker Branch Watershed

1 Introduction.- 2 Site Description.- 2.1 Location.- 2.2 Climate.- 2.3 Air Quality.- 2.4 Geology.- 2.5 Soils.- 2.6 Vegetation.- 2.7 Land-Use History.- References.- 3 Forest Meteorology.- 3.1 Introduction.- 3.2 Site Description.- 3.3 Characterization of Boundary Conditions.- 3.4 Canopy Radiation Transfers and Regimes.- 3.5 Canopy-Atmosphere Turbulent Exchange.- References.- 4 Atmospheric Chemistry, Deposition, and Canopy Interactions.- 4.1 Introduction.- 4.2 Methodology.- 4.3 Major Ions.- 4.4 Trace Metals.- References.- 5 Water.- 5.1 Introduction.- 5.2 Hydrologie Features.- 5.3 Hydrologie Monitoring.- 5.4 Annual Water Budget.- 5.5 Precipitation.- 5.6 Soil-Plant-Atmosphere Processes.- 5.7 Streamflow.- 5.8 Watershed Comparisons.- 5.9 Summary.- References.- 6 Carbon Dynamic and Productivity.- 6.1 Introduction.- 6.2 Forest Biomass and Productivity.- 6.3 Carbon Fluxes.- 6.4 Comparisons with Other Forests.- 6.5 Summary and Conclusions.- References.- 7 Terrestrial Nutrient Cycling.- 7.1 Introduction.- 7.2 Nutrient Cycles: Basic Characteristics.- 7.3 Perturbations.- 7.4 Comparison of Walker Branch Watershed with Other Forest Ecosystems.- 7.5 Summary and Conclusions.- References.- 8 Streams: Water Chemistry and Ecology.- 8.1 Introduction.- 8.2 Site Description.- 8.3 Stream Water Chemistry.- 8.4 Nutrient Limitation and Phosphorus Spiraling.- 8.5 Summary.- References.- 9 Modeling Chemical Transport, Uptake, and Effects in the Soil-Plant-Litter System.- 9.1 Introduction.- 9.2 Landscape Context.- 9.3 Modeling the Soil-Plant-Litter System.- 9.4 Model Applications.- 9.5 Pollutants and the Diurnal Cycle.- 9.6 Walker Branch Watershed Perspective.- 9.7 Summary.- References.- 10 Implications of Walker Branch Watershed Research.- References.

Analysis of biogeochemical cycling processes in Walker Branch Watershed.

To determine whether autumnal nutrient resorption contributes significantly to plant fitness, resorption was prevented by defoliating scrub oak (Quercus ilicifolia) plants ("defoliated plants") just prior to the onset of foliar senescence in the years 1985 through 1987. At the same time, to account for the potential influence of the reduced photosynthetic season on defoliated plants, another group ("shaded plants") was subjected to darkness, resulting in senescence in the absence of concomitant photosynthesis. In 1988, senescence and abscission were allowed to proceed normally in all study plants. In the three years that followed treatment years, 1986-1988, radial stem growth and yearly increases in foliar biomass of defoliated plants averaged 41 and 54% less than in controls. Acorn production in defoliated plants in 1989 was only 10% of that in either controls or shaded plants. In contrast to such marked responses in growth and reproduction, content of nitrogen, phos- phorus, copper, manganese, and zinc in presenescent (green) leaves exhibited little response to defoliation in the previous year. In 1988, when resorption efficiencies could be measured in plants previously subjected to defoliation, there were no significant differences between control and defoliated plants for nitrogen, copper, manganese, or zinc. The efficiency of phosphorus resorption was 10% lower in defoliated plants. In shaded plants, resorption efficiencies of nitrogen, phosphorus, and copper, but not of manganese and zinc, were substantially reduced relative to controls in treatment years, indicating both quantitative and qualitative differences between dark-induced and normal senescence. The significant negative effects on growth and reproduction that resulted from preventing resorption in Q. ilicifolia offer strong support for the hypothesis that resorption contributes significantly to plant fitness.

Effects of Preventing Nutrient Resorption on Plant Fitness and Foliar Nutrient Dynamics

Eight major species of southern oaks: a bibliography, 1876-1987

The translocation of nutrients into and out of outer canopy leaves of ten eastern deciduous forest species was calculated from the temporal patterns of foliar nutrient pools sampled through a growing season. The calculations accounted for average chemical leaching effects due to rainfall. There were no significant differences in translocation rate between species within the evergreen, understory, or overstory-deciduous tree groups. Evergreen species had lower translocation rates than deciduous trees. Translocation rates into leaves of deciduous species showed a very rapid increase during spring; however, by late May, foliar phosphorus was being translocated at a slow rate back to stems. A similar trend was established for nitrogen by mid-June. An internal storage pool is suggested as the major source of foliar nitrogen during the spring flush since a simulation of nitrogen uptake from soil could only account for one-fourth of the quantity of nitrogen transported to leaves by the end of May. Simulation further showed that trace levels of soluble nitrogen (0.01 ppm) in soil were sufficient to supply a deciduous forest with an estimated nitrogen uptake of 100 kg N ha/sup -1/ year/sup -1/.

R. H. Strand

Papers

Nutrient Translocation in the Outer Canopy and Understory of an Eastern Deciduous Forest