Many climate change scenarios predict areas in the U.S. will experience more frequently occurring extreme droughts over the next century. These droughts will have major effects on the physiological functioning of important tree species in the U.S., and in some cases may push trees past their physiological limits with the potential to negatively impact their growth, reproduction, and survival. Additionally, the physiological functioning of tree species with large distributions across the U.S. will likely vary across populations located throughout the species home range in response to extreme droughts; yet, little is understood about how extreme droughts will impact the physiological functioning of trees at the intraspecific level. Such understanding is critical to accurately predict physiological responses of tree species under future climate change scenarios. Investigations of intraspecific responses using common gardens to study important and potentially adaptive physiological traits can elucidate information on the potential of populations to respond to rapid and extreme environmental changes across space and time. The overall objective of this study was to investigate population-level variation in the physiological functioning of Fraxinus americana (white ash) during one of the most extremely dry and warm years (2012) at a common garden located in Lawrence, KS.
We conducted an intraspecific analysis of leaf-level δ13C in a common garden of white ash (43 populations total) located in Lawrence, KS across 7 years, including the warmest year in U.S. history (2012), which was also one of the driest years on record at this site. Interestingly, we found that average leaf-level δ13C across all populations during the extreme year of 2012 (-28.7‰) was not significantly different from non-extreme years (-29.1‰) suggesting little environmentally induced change in physiological responses. We also found a significant relationship between leaf-level δ13C and growing season VPD for 6 of the 7 years (p<0.001; R2 = 0.64), whereas this relationship was significantly weakened when data from the non-extreme year of 2013 were included (R2 = 0.26). This suggests that effects of extreme years on physiology may persist even during future years with more favorable environmental conditions. Furthermore, we found rank order in leaf-level δ13C among populations was maintained across extreme and non-extreme years suggesting we can expect a similar relative “line-up” in leaf-level δ13C among populations during future extreme years. These results show strong population-level effects in δ13C for white ash, and this variability could have implications for the future success of this species.