Drought enhances symbiotic dinitrogen fixation and competitive ability of a temperate forest tree
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General circulation models project more intense and frequent droughts over the next century, but 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 wherein symbiotic dinitrogen (N2)-fixing plants play a critical role. In experimental mesocosms we manipulated soil moisture to study the effect of drought on the physiology, growth and competitive interactions of four co-occurring North American tree species, one of which (Robinia pseudoacacia) is a symbiotic N2-fixer. We hypothesized that drought would reduce growth by decreasing stomatal conductance, hydraulic conductance and increasing the 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). We further hypothesized that N2 fixation by R. pseudoacacia would decline with drought, reducing its competitive ability. Under drought, growth declined across all species; but, growth and physiological responses did not correspond to species’ hydraulic architecture. Drought triggered an 80 % increase in nodule biomass and N accrual for 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 competition with non-fixing species when both water and N were limiting. Under scenarios of moderate drought, 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.
KeywordsClimate change Biogeochemistry Hydraulic conductance Plant physiology Stomatal conductance
This study was supported by the United States Department of Agriculture Forest Service, Southern Research Station, and by cooperative agreement number 11-CA-11330140-095 to N. Wurzburger at the University of Georgia. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the authors and do not necessarily reflect the views of the USDA Forest Service or the University of Georgia. We thank Neal Muldoon, Shialoh Wilson, Sheena Zhang, Courtney Collins, Steven T. Brantley, and Jeff Minucci for their assistance with this research. We are grateful to Lindsay Boring, Ford Ballantyne and two anonymous reviewers for their constructive comments on the manuscript.
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