PS 15-201 - Population based differences in cold tolerance affected by drought and temperature acclimation in Pinus strobiformis

Abstract: Drought and cold stress invoke similar responses in conifers, indicating drought and cold tolerance may act via the same biological pathways. Pinus strobiformis experiences drought and cold stress in its natural habitat that varies by population. We expect previous exposure to temperature and drought to affect cold tolerance of seedlings. We hypothesize: 1) Trees from northern populations will be more cold tolerant than trees from southern populations 2) Trees subjected to cold temperatures before cold tolerance testing will be more cold tolerant than those that are not 3) Trees from southern populations will be more drought tolerant and have higher leaf mass area than trees from northern populations. We set up two experiments using seedlings germinated from northern and southern populations. Our temperature acclimation experiment exposed seedlings to a temperature manipulation treatment in a greenhouse and our drought experiment exposed seedlings to drought in two common gardens. Cold tolerance was measured by electrolyte leakage, a metric of cellular damage caused by low temperatures. We also measured leaf mass area, a common indicator of healthy plant function though increased starch/sugar content for growth or environmental stress mitigation. We determined trees from warmer populations were significantly more likely to have lower cellular damage upon exposure to freezing temperatures when put through a cold acclimation treatment, simulating fall hardening, than when left in a warm environment. Trees from colder populations performed better in the freezing treatments overall, showing they remain fairly cold tolerant even through the summer. In the drought experiment, we used rainout shelters to simulate drought and control conditions at warm and cold common garden sites to examine the interaction between cold and drought stress. We found that coming out of winter and into spring, trees in a wetter, colder environment maintained higher leaf mass area and greater cold tolerance than trees in a warmer, drier environment. Our experiments concluded that P. strobiformis cold tolerance can be influenced by an acclimation treatment before winter stress to increase survival through winter and by avoiding drought to maintain hardiness through winter into spring. Our research indicates there are differences between how northern and southern populations tolerate cold stress when experiencing resource depletion as a result of drought stress. Our findings benefit conservation efforts for P. strobiformis under climate change.