Trees are susceptible to the interactive effects of climate and individual ecology due to their long life span and lack of mobility. Tree response to climatic stress may be tempered or exacerbated by competition and individual physiology. The 2012-2016 California drought is the most severe in half a century in terms of snowpack and water availability, and has resulted in at least 27 million dead trees. Surviving trees are ostensibly more drought-resilient than dead trees, and may or may not exhibit more drought-resilient traits, but little work has been conducted to specifically test this premise. I used dendrochronological approaches to examine anatomy of tracheids (the primary water-conducting structure in conifers) in Pinus ponderosa and Pinus jeffreyi in the Sierra Nevada mountains. The hydraulic safety factor (HSF, the ratio of tracheid wall thickness to tracheid diameter) is a representation of resistance to xylem collapse or cavitation under drought stress. I tested for differences in HSF between drought-killed and surviving individuals along an elevation and drought stress gradient. I then quantified the degree of drought resilience (a measure of both drought tolerance and recovery from drought) afforded by various trait combinations.
Results/Conclusions
I find that drought stress induces thicker tracheid walls both during and after drought. P. ponderosa has a lower HSF than P. jeffreyi across all samples. Living and dead trees exhibited differences in HSF. Trees of both species killed in the last year have a lower HSF and larger tracheid diameter than living trees, but also had lower HSF than trees killed in prior years, demonstrating that HSF itself does not predict drought tolerance. Living P. ponderosa trees have a higher HSF at moist higher elevation areas than at more stressed sites. The opposite was observed in P. jeffreyi. Overall drought resilience is higher at more stressed sites, with pre-stressed trees more able to recover following returns to mesic conditions. These results demonstrate the range of drought tolerance strategies used by pines. No one strategy confers better drought resilience, but living trees employ intermediate HSFs, demonstrating potential tradeoffs between drought defenses, growth, and other resource uses. Current models of hydraulic architecture predict direct relationships between hydraulic safety and resistance to drought, but this was not consistently observed. Further exploration of these traits will allow development of trait-based models of forest response to climate change that scale from the cell through the individual to the stand.