Fixing nitrogen in a changing climate:
Minucci, Jeffrey Michael
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Climate change is increasing drought frequency, aridity, and temperature, which may affect symbiotic nitrogen fixation (SNF), a process which facilitates recovery from disturbance in terrestrial ecosystems. The impacts of these climate stressors may occur through their effects on the physiology, competitive ability, and geographic distribution of symbiotic N2-fixing plants. In this dissertation, I examine how these stressors affect Robinia pseudoacacia L., the dominant N2-fixing tree in Eastern US forests. I first tested how drought frequency affects the growth, physiology, and SNF rate of R. pseudoacacia, by growing seedlings in the greenhouse under soil moisture regimes which varied in drought frequency but not mean moisture. I found that, overall, R. pseudoacacia growth and SNF rate were resistant to both single prolonged droughts and frequent but brief droughts events, however drought frequency determined the physiological drought response strategy. I then examined how increased growing season aridity altered the competitive ability of R. pseudoacacia in an early successional forest, by experimentally reducing soil moisture over three years. I found that while R. pseudoacacia presence promoted greater soil nitrogen (N) availability and productivity of non-fixing trees, its growth and abundance were reduced in drier soils. Finally, I asked how aridity interacts with temperature and other abiotic factors to constrain the geographic distribution of R. pseudoacacia across the Eastern US, using forest inventory data. I found that presence of R. pseudoacacia was strongly associated with high mean annual temperature but only moderate summer temperature, conditions which generally promote a high value of N relative to carbon and minimize water stress. Using my model to project habitat suitability of R. pseudoacacia in 2050, I found that this species may gain substantial suitable habitat at the north edge of its range, while losing habitat at the southern edge as a result of higher temperatures. Collectively, the findings of this dissertation suggest that increasing aridity and temperature due to climate change will affect growth, competitive ability, and geographic distribution of the biogeochemical keystone species R. pseudoacacia, with implications for future N cycle dynamics and forest resilience to disturbance.