Forest drought vulnerability after fire: How fire-induced resprouting and an altered microclimate affect Quercus water relations under an experimental drought

Background/Question/Methods

Shifts in rainfall patterns due to climate change are expected to increase drought stress and mortality in forests, with widespread, negative consequences for forest productivity. Additionally, natural and anthropogenic fire regimes are rapidly changing, highlighting the need to understand the interactive effects of fire and drought on forest productivity and recruitment. Using rainout shelters, we imposed summer drought on in situ populations of Quercus alba seedlings and saplings located in periodically burned and unburned sites in Shawnee National Forest, IL, USA to investigate the effects of prescribed fire on drought tolerance of tree seedlings, the life history stage most vulnerable to drought and a bottleneck for overstory recruitment. We monitored leaf water potential (Ψ_pd), leaf gas exchange rates, and relative growth rate (RGR) of seedlings for two years. We tested two alternate hypotheses: (1) fire positively influences tree-water relations by reducing tree density and increasing root to shoot ratios of resprouting trees; (2) fire reduced tree drought tolerance by promoting a warmer, drier microclimate.

Results/Conclusions

Average ambient conditions in burned stands were warmer and drier than unburned stands. Burned stands maintained 3-5 °C higher maximum air temperatures and 12% (± 10%) lower soil water content than unburned stands. Averaging across fire treatment, the rainout shelters reduced mean soil water content (0 – 0.5m depth) by 28% (± 8%) relative to ambient conditions and led to a significant reduction in Ψ_pd of in situ seedlings (P < 0.01). Drought effects on seedling were size-dependent. Small, establishing seedlings experienced 5-30% greater drought-induced declines in leaf gas exchange rates and RGR than larger seedlings and saplings, with the largest declines occurring for individuals in burned stands (drought x fire: P < 0.05). These patterns suggest that changes to the forest microclimate (i.e. warmer, drier conditions) promoted by repeat burning may outweigh any positive effects (i.e. resprouting, reduced competitor density) on tree water relations, especially for shallow-rooted seedlings. We conclude that fire-driven changes to local environmental conditions may further constrain forest recruitment under future scenarios of rainfall variability.