The storage of nonstructural carbohydrates (NSC) is hypothesized to have evolved as part of a conservative life history strategy (sensu the “resource economics spectrum”). According to this idea, plants growing in extreme environments are locally adapted to actively allocate carbon to storage at the expense of other sinks, such as growth. These carbon stores may serve as a metabolic or osmolytic buffer against environmentally-induced stress that inhibits photosynthetic supply, such as severe drought or freezing temperatures. Studies attempting to address this hypothesis via experimental and observational droughting have demonstrated that plants will draw-down NSC stores under water limitation and having higher storage concentrations may prolong life under stress. However, to date the hypothesis remains unresolved as no study has directly addressed its underlying evolutionary mechanisms. Thus, we used a common garden design to measure genetic variation in traits related to carbon supply, growth, defense, and storage and map evolutionary tradeoffs amongst them. Our study contains 917 black cottonwood trees (Populus trichocarpa) from 19 different populations, all replicated 3x each and with full genome sequences available.
Results/Conclusions
Contrary to our predictions, we found high NSC storage to be aligned with other productive life history traits such as longer growing seasons, high growth, and defense and is locally-adapted to macroclimate. These productive life traits trade-off with conservative traits such as high wood-density and early budset. However, once variation in growing-season length is accounted for, we did find a tradeoff between high storage and high growth. This tradeoff is not locally adapted, but maintained across all populations, likely by micro-climatic variation. Our results demonstrate both locally adapted and wide-spread evolutionary tradeoffs between NSCs and other key carbon allocation traits, such as growth.