Species-specific differences in climate memory may determine whether trees survive drought

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Rohan David Boone and Kiona Ogle, School of Informatics, Computing, and Cyber Systems, Northern Arizona University, Flagstaff, AZ, Larissa Yocom, Department of Wildland Resources and Ecology Center, Utah State University, Logan, UT, Drew Peltier and Kiona Ogle, Center for Ecosystem Science & Society, Northern Arizona University, Flagstaff, AZ

Background/Question/Methods
Severe drought can lead to tree decline and death, but why some trees die during drought while others survive is poorly understood. Recent work has shown, however, that antecedent (past) climate conditions can continue to affect tree growth for several years, a phenomenon referred to as the climatic memory of tree growth. This raises the question: do differences in climate memory determine which trees survive versus die during a drought? To answer this question, we leveraged a dataset of annual tree-ring widths from 31 sites, representing three species from diverse and prominent genera: Pinus sylvestris, Populus tremuloides, Quercus rubra. At each site, tree cores were extracted from living trees and from trees that died as a result of a drought. We fit a Bayesian mixed effects model to these data to determine the memory effects of monthly precipitation, mean temperature, and their interaction on ring widths (growth) prior to the mortality event. As part of this model, we estimated the relative importance of monthly climate during the year of ring formation and during the four years prior in determining the overall effect of climate on annual growth. By comparing climate effects estimated by the model for trees that survived versus those that died, we identified key differences in climatic memory that may be related to mortality risk.
Results/Conclusions
Averaged across all sites, trees that died during a drought grew about half as much annually as those that lived (0.57 mm/year versus 1.14 mm/year, respectively). Model estimates show Pinus sylvestris individuals that died during drought were the only group to show a significant effect of antecedent temperature on ring width, temperature having had a large, negative effect of ring width. Warm antecedent temperatures reduced growth in Pinus sylvestris that subsequently died during drought, but not in those that lived. All species showed a significant, positive effect of precipitation on annual growth, with no significant difference between living and dead trees. However, the analysis revealed differences in the importance of the timing of precipitation in determining ring growth. In particular, Populus tremuloides showed longer precipitation memory in trees that survived versus those that died during drought. Overall, our results suggest that the relationship between climatic memory and drought-related mortality is species-dependent. For example, in some species, intraspecific variation in precipitation memory suggests that past precipitation inputs can potentially buffer some, but not all, trees against mortality during subsequent drought periods.