Timing Effects of Heat-Stress on Plant Ecophysiological Characteristics and Growth

Heat waves with higher intensity and frequency and longer durations are expected in the future due to global warming, which could have dramatic impacts in agriculture, economy and ecology. This field study examined how plant responded to heat-stress (HS) treatment at different timing in naturally occurring vegetation. HS treatment (5 days at 40.5 degrees C) were applied to 12 1 m(2) plots in restored prairie vegetation dominated by a warm-season C-4 grass, Andropogon gerardii, and a warm-season C-3 forb, Solidago canadensis, at different growing stages. During and after each heat stress (HS) treatment, temperature were monitored for air, canopy, and soil; net CO2 assimilation (A(net)), quantum yield of photosystem II (Phi(PSII)), stomatal conductance (g(s)), and internal CO2 level (C-i), specific leaf area (SLA), and chlorophyll content of the dominant species were measured. One week after the last HS treatment, all plots were harvested and the biomass of above-ground tissue and flower weight of the two dominant species were determined. HS decreased physiological performance and growth for both species, with S. canadensis being affected more than A. gerardii, indicated by negative HS effect on both physiological and growth responses for S. canadensis. There were significant timing effect of HS on the two species, with greater reductions in the net photosynthetic rate and productivity occurred when HS was applied at later-growing season. The reduction in aboveground productivity in S. canadensis but not A. gerarodii could have important implications for plant community structure by increasing the competitive advantage of A. gerardii in this grassland. The present experiment showed that HS, though ephemeral, may promote long-term effects on plant community structure, vegetation dynamics, biodiversity, and ecosystem functioning of terrestrial biomes when more frequent and severe HS occur in the future.