EFFECTS OF SOIL-TEMPERATURE ON GROWTH, BIOMASS ALLOCATION AND RESOURCE ACQUISITION OF ANDROPOGON-GERARDII VITMAN

The effects of root temperature on growth and resource acquisition (carbon, nitrogen and phosphorus) were examined in Andropogon gerardii, a dominant C4 grass of tallgrass prairies of the midcontinental USA. Soil temperature (5.9 to 35.5-degrees-C) was manipulated independently from air temperature (c. 25/20-degrees-C day/night) in a greenhouse. Total biomass at the end of the experiment and relative growth rate (RGR) were maximum at 25-degrees-C soil temperature (T(soil)) and decreased at higher and lower temperatures. T(soil) had no effect on leaf area ratio (the ratio of leaf area to total biomass); thus the effect of T(soil) on RGR was primarily through its influence on net assimilation rate (NAR). Different mechanisms may have contributed to decreases in NAR at sub- and supraoptimal T(soil). Soil temperatures below 20-degrees-C caused significant reductions in foliar nitrogen (N) and phosphorus (P) concentration. High concentrations of these nutrients in roots suggest that nutrient transport was more strongly inhibited than uptake, and low foliar N and P may have contributed to reduced photosynthetic rates observed at 5 and 10-degrees-C T(soil). Net photosynthesis was < 12-mu-mol m-2 s-1 at 5 and 10-degrees-C T(soil) and > 20-mu-mol m-2 s-1 at 15-40-degrees-C. Changes in net photosynthesis did not therefore contribute to the reduction in NAR at supra-optimal T(soil). The strong growth dependence of A. gerardii on T(soil) indicates that this abiotic factor may significantly limit productivity in tallgrass prairie, particularly early in the growing season.