Drought tolerance in the context of ongoing Eastern US oak decline: Are Quercus species less likely to die from drought than co-occurring non-oak species?

Oak forests are a critical part of the Eastern US deciduous forest biome and provide numerous ecosystem services related to wildlife and habitat quality. Unfortunately, oak forests are endangered by an ongoing decline in the relative abundance and importance of oak species with a concomitant increase in mesophytic species over the last century. In an effort to preserve oak forests, many studies have sought to identify the main causes of oak disappearance from forest landscapes. The decline of oaks is likely driven by a complex suite of drivers including fire suppression, recruitment failure, over-browsing from foragers, and long-term trends of increasingly wet conditions that have characterized the region during the era of Quercus decline. Hydrologic stress from drought has also been recognized as an inciting factor for regional episodic mortality of mature oak trees. However, drought mortality is often not considered an important factor to explain macro-scale oak declines because oaks possess traits that are indicative of drought-tolerance.

The perspective that oaks are more drought tolerant than other common Eastern US forest species is partly attributed to broad-scale forest inventory analyses, which have often found greater annual mortality for non-oak species in response to increased standardized drought metrics. However, such analyses may not adequately assess relative mortality patterns due to site-specific factors which render the magnitude of physiological drought regionally variable (e.g., soil texture, depth to bedrock, etc.). To overcome this, we reassessed oak drought tolerance using a novel approach by comparing annual mortality on a relative species basis in stands where specific oak and non-oak species co-occur. This analysis was conducted across ~7,500 Forest Service Inventory Analysis (FIA) plots in the four state region of Illinois, Indiana, Kentucky, and Missouri, where relative mortality in response to the severe 2012 drought was calculated for 20 commonly occurring tree species, including Q. alba, Q. rubra, Q. velutina, and Q. stellata. Using this approach, we found that oak species had similar and/or greater mortality than the majority of co-occurring non-oak species, suggesting oaks are similarly susceptible to drought-induced mortality than other Eastern US forest species.