Fire increases drought vulnerability of Quercus alba juveniles by altering forest microclimate and nitrogen availability

1. Shifts in rainfall patterns due to climate change are expected to increase drought stress and mortality in forests. Natural and anthropogenic fire regimes are also changing, highlighting the need to understand the interactive effects of fire and drought on tree ecophysiological response and growth. 2. Using rainout shelters, we imposed summer drought on natural and planted populations of Quercus alba juveniles located in periodically burned and unburned sites in Shawnee National Forest, IL, USA. A subset of planted juveniles was clipped to simulate topkill. We measured predawn leaf water potential (Ψpd), leaf gas exchange (Amax, gmax), and relative growth rate (RGR) across treatments to test two hypotheses: (H1) fire reduces juvenile drought stress by improving water relations through increased root to shoot ratios after topkill, or (H2) fire exacerbates juvenile drought stress by promoting a warmer, drier microclimate or amplifying drought-induced nitrogen (N) limitation. 3. Bur...