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 relative abundance and importance and a concomitant increase in mesophytic species over the last century. In an effort to preserve oak forests, many studies have sought to identify the cause of oak disappearance from the forest landscapes. The decline of oak species are likely caused by a suite of drivers including fire suppression, recruitment failure, over-browsing from foragers, and relatively wet conditions during this period. Hydrologic stress from drought has also been recognized as an inciting factor for regional episodic mortality of mature oak trees. However, because oaks possess traits that are indicative of drought-tolerance, drought mortality is often not considered an important factor to explain broad scale patterns of oak forest decline. This perspective is partly attributed to broad-scale forest inventory analyses, which have often found greater mortality for non-oak species in response to increased standardized drought metrics. However, such analyses which aggregate broad mortality patterns in response to meteorological drought may be spatially-biased by not accounting for site-specific factors which render physiological drought regionally variable (e.g., soil texture, depth to bedrock, etc.). To minimize this bias, we reassessed oak drought tolerance on a relative species basis by using Forest Service Inventory Analysis data to compare annual mortality in stands where only a specific oak species and specific non-oak species co-occur. This analysis was conducted in the four state region of Illinois, Indiana, Kentucky, and Missouri, where annual mortality in response to the severe 2012 drought was calculated for 18 common tree species relative to Q. alba, Q. rubra, Q. velutina, Q. stellate, and Q. coccinea.
Results/Conclusions
Using this approach, we found that the majority of co-occurring species experienced less mortality than oaks in response to drought. Moreover, mesophytic species which have increased over the last century (e.g., A. saccharum) were found to be similarly drought tolerant to oaks such that drier conditions prior to the 20th century could have been comparably favorable for their establishment. Overall, these results suggest that oak species are relatively susceptible to drought, and drought-induced mortality may be an important factor contributing to their declines.