Print PageExtensive tree die-off in forested ecosystems has been recently documented around the world in association with drought and warmer temperatures. More frequent drought is projected for the next century, occurring concurrent with increased temperatures, and could cause more die-off, potentially altering biogeochemical and hydrologic cycles, with implications for the land-surface feedback on climate and ecosystem services for society. Long-term ecological responses to die-off that include recovery of pre-mortality forest conditions, shifts in forest composition, or conversion to non-forest, will determine the persistence these effects. Surprisingly, the sensitivity of drought-induced mortality in seedlings has not been evaluated with respect to a broad range of higher temperatures projected to accompany future extreme drought, yet seedling establishment is a key demographic process controlling forest structure and function. Here we assess the sensitivity of drought-induced mortality for tree seedlings of two species, the more drought-tolerant Pinus edulis and the less drought-tolerant Pinus ponderosa, both of which have experienced recent, large-scale tree die-off. We experimentally simulated drought for these seedlings at ambient and elevated (+3.6, +5.9, +7.7°C) temperatures, while measuring CO2 and H2O gas exchange weekly, until all seedlings died.
Results/Conclusions
Drought-induced mortality in both pine species was highly sensitive to temperature, exhibiting earlier mortality of 4.4 %/°C for P. edulis and 3.6 %/°C for P. ponderosa. Notably, while P. edulis showed greater drought resistance, outlasting P. ponderosa at all temperatures, P. edulis also demonstrated a higher degree of within-species temperature- sensitivity to drought than P. ponderosa. For both species, earlier mortality during drought was associated with a greater effect of temperature on stomatal conductance, net photosynthesis, and respiration. Both species closed their stomata prior to death, consistent with a similar drought tolerance strategy, but P. edulis outlasted P. ponderosa, and lower temperature trees outlasted those at higher temperatures during this period. Greater seedling respiration rates during the period of stomatal closure were correlated with a reduction in time-to mortality under lethal drought. At higher temperatures, more frequent, shorter droughts become sufficient to cause mortality so episodes of seedling mortality are expected to increase from future climate warming alone, even without an increase in drought frequency. This vulnerability of seedlings to drought at higher temperatures suggests that seedling recruitment may not offset overstory adult tree mortality and that woodland and forest communities may undergo substantial future changes.
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