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

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. 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 (Ψ_pd), leaf gas exchange (A_max, g_max) 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. Burned stands had higher vapour pressure deficits and 13% lower soil inorganic N availability than unburned stands. Rainout shelters reduced soil moisture (0–45 cm) by 10%–24% relative to ambient conditions. Consistent with H2, small, natural resprouts in burned stands experienced greater drought stress than unburned juveniles, with a 7% decrease in leaf nitrogen content (LNC), a 21%–29% reduction in A_max and g_max and a 41% reduction in RGR under drought. Drought effects on unburned juveniles, in contrast, were limited to a 5% reduction in LNC and a neutral to positive effect on leaf gas exchange and RGR. Large natural juveniles were largely unaffected by drought. Recent resprouts (i.e., clipped, planted juveniles) experienced less drought stress than unclipped juveniles, providing partial support for H1. Collectively, these results suggest that resprouting after fire can temporarily improve leaf water relations until root-to-shoot ratios re-equilibrate. In contrast, fire can exacerbate drought-driven declines in the growth of small juveniles by both promoting a warmer, drier microclimate and intensifying N limitation. Our results suggest that despite the high drought tolerance of Quercus spp., fire-driven changes to local microclimate and resource conditions could limit tree recruitment under future scenarios of rainfall variability. A plain language summary is available for this article.