Current understanding of which traits confer drought resilience versus susceptibility in trees is still limited. The California "hot drought" of 2012-2016 was more severe than any observed in the previous 1200 years. This drought stress left an estimated 130 million standing dead trees in the Sierra Nevada, and was followed by only one year of recovery prior to a return to currently dry conditions. I sampled Pinus ponderosa and P. jeffreyi in multiple stands that experienced extensive drought-induced mortality in the Sierra Nevada to ask: Which cellular traits determine pine tree survival or mortality, and how do these cellular traits scale to whole-tree and stand-level climate responses? I compared annual rings with measurements of xylem cell (tracheid) dimensions and tracheid cell wall thickness. I then asked how climatic variables such as drought stress and temperature influenced observed ring traits, and how these traits varied between living and dead trees.
Results/Conclusions
Annual growth in living trees increased slightly or stayed constant during and following the drought, while dead trees rapidly decreased growth prior to mortality. Fall precipitation explained growth patterns in drought survivors, but not dead trees. Growth was also positively associated with warm fall and cold winter temperatures. At the cellular level, average tracheid diameter was positively correlated with previous November temperatures and negatively correlated with January temperatures. The hydraulic safety factor (HSF) is the ratio of tracheid wall thickness to tracheid diameter, and is a representation of resistance to collapse or cavitation under drought stress and freeze-induced embolism. HSF increased over the length of the drought, driven by decreased tracheid diameter, and was higher in living trees than drought-killed trees. Mixed linear models showed that living trees grew larger rings in years with colder winters, but that this growth was also associated with increasing HSF. This demonstrates that trees that grow a high density of small tracheids with high HSF are more drought-resilient than trees that grow fewer tracheids with lower HSF. Temperature relationships show that pines need cold, wet winters to maintain increased growth of safe xylem.