OOS 36-4
Effects of precipitation and temperature manipulations on carbohydrate and mortality dynamics in piñon-juniper woodlands
Drought- and temperature-associated forest mortality are expected to increase in frequency with future climate change, yet understanding of mortality mechanisms remains incomplete. Depletion of non-structural carbohydrate (NSC) stores has been implicated, but rarely field-tested, as an important mortality mechanism. Here, we present NSC results from two ecosystem-scale experiments that manipulated precipitation to two co-occurring conifers with different water regulation strategies—the relatively drought-sensitive piñon pine (Pinus edulis) and the relatively drought tolerant one-seed juniper (Juniperus monosperma). The first experiment was a strict precipitation manipulation with three treatments including ambient control, irrigation, and lethal drought. The second experiment manipulated both precipitation and temperature with four treatments including ambient control, drought, heating, and a combined drought + heat treatment. No mortality occurred in the second experiment after one year of treatment.
Results/Conclusions
In the first experiment, droughted junipers survived twice as long as droughted piñons. Droughted junipers in both experiments converted foliar NSC to glucose and fructose, and maintained three times more NSC in the foliage than twigs, consistent with osmoregulation requirements to maintain higher photosynthesis during drought. Conversely, piñon foliar glucose and fructose declined with increased temperature in both experiments (ambient temperature increases in the first), likely due to increased respiration or stomatal closure.
Despite species differences in soluble sugars, drought caused a decrease in foliar starch in dying trees of both species in the first experiment, and average leaf starch over the treatment period was a good predictor of drought-survival duration. In the second experiment, juniper leaf starch was significantly lower in treated than ambient trees after one year of treatment. The dry 2012 monsoon season and 2012-’13 winter also lead to decreased leaf and root starch in all piñons in the second experiment, with significantly lower starch and total NSC across all tissues in heated piñons. Following trees with reduced starch pools through mortality in the second experiment will provide a strong test of the predictive power of the relationship between starch and survival time from the first experiment.