Tyler K. Refsland and Jennifer M. Fraterrigo
Hotter temperatures and altered precipitation patterns with climate change are increasing drought stress and mortality of forests in many regions. The frequency of forest fires is also increasing as a result of climate change and human ignitions. These changes highlight the need to understand how fire and drought interact to affect forest ecosystems, especially the recruitment of young trees which are vulnerable to fire damage and drought stress. Fire may have opposing effects on drought vulnerability of young trees. By killing aboveground tissue and promoting resprouting, fire can increase the ratio of a tree’s root biomass to shoot biomass, leading to an increase in the availability of water to the tree and reducing drought vulnerability. However, repeat fires can also increase sunlight in the understory, prompting an increase in air temperatures and evaporation which may intensify drought stress in young trees. We have a poor understanding of how these fire-driven effects influence the vulnerability of young trees to drought.
In this experiment, we show that small white oak (Quercus alba) trees experienced greater drought stress in burned than unburned forests, consistent with higher air temperatures and evaporative demand in the understory of burned forests. The positive effects of resprouting after fire on drought vulnerability was confined to only the most recent resprouts. These results suggest that fire-driven changes to the forest microclimate overwhelm the shorter-term benefits of resprouting on the drought vulnerability of young trees. Our findings indicate that fire can interact with drought in multiple, contrasting ways to influence the growth of young trees. As a result, the interaction of fire with drought will have consequences for future forest regeneration under climate change.