Drought Enhances Symbiotic Di-Nitrogen Fixation and Competitive Ability of a Temperate Forest Tree.

General circulation models project more intense and frequent droughts over the next century and many questions remain about how terrestrial ecosystems will respond. Of particular importance, is to understand how drought will alter the species composition of regenerating temperate forests where the soil-based process of symbiotic di-nitrogen (N₂)-fixation plays a critical role by introducing new N to the ecosystem. In experimental mesocosms, we manipulated soil moisture to study the effect of drought on the physiology, growth and competitive relationships of four co-occurring North American tree species, one of which (Robinia pseudoacacia) is a symbiotic N₂-fixer. We hypothesized that drought would negatively influence growth and physiology (stomatal conductance, hydraulic conductance and water use efficiency) of species with larger diameter xylem vessel elements (Quercus rubra, R. pseudoacacia) relative to those with smaller elements (Acer rubrum and Liriodendron tulipifera) because of their vulnerability to cavitate. We further hypothesized that N₂ fixation by R. pseudoacacia would decline with drought, reducing its ability to compete with other species. 

Under drought, growth declined across all species by 40%; but, growth and physiological responses did not correspond consistently to species’ hydraulic architecture. R. pseudoacacia accrued over 3 times more N relative to other species and drought triggered an 80% increase in nodule biomass of R. pseudoacacia, improving its growth relative to other species. These results suggest that drought intensified soil N deficiency and that R. pseudoacacia’s ability to fix N₂ facilitated its competitive success when both water and N were limiting. Previous field observations of drought-induced mortality of R. pseudoacacia suggest that a threshold of physiological tolerance exists for this species. Under scenarios of moderate drought however, N₂ fixation may alleviate the N constraints resulting from low soil moisture and improve competitive ability of N₂-fixing species, and as a result, supply more new N to the ecosystem.