Climate change predictions anticipate global increases in temperature. Additionally, rising aridity in already dry environments may create novel hydraulic and metabolic stress for terrestrial vegetation. We investigated how such changes may impact the carbon status of trees. Using data from the Los Alamos Survival-Mortality (SUMO) field experiment, we asked four questions related to the non-structural carbohydrate (NSC) dynamics of two dominant tree species with differing drought tolerances: (1) is growth always the first to stop during drought (sink limitation)?, (2) does NSC increase during sink limitation?, (3) how often do trees experience sink and source limitation, and (4) how do NSC dynamics and the frequency of source/sink limitation vary under different environmental conditions and between species? Individuals of Juniperus monosperma and Pinus edulis were placed into ambient, drought, heat and heat+drought treatments for four years. Seasonal measurements of predawn water potential (Ψpd), photosynthesis (A), growth, leaf + twig, bole and root NSC were collected. We used linear regression to calculate the Ψpd when A stops for each tree in each treatment, and ANOVA to test for changes in NSC and the percentage of the study period that each tree spent in a given limitation status.
Results/Conclusions
We found significant differences between the Ψpd at stomatal closure between both J. monosperma and P. edulis. The Ψpd at source limitation was significantly different between all treatments. Ambient and droughted P. edulis showed the greatest and lowest sensitivities to Ψpd, respectively. This suggests a possible acclimation of A to long-term drought. P. edulis NSC varied across all treatments, with no change occurring for the ambient treatment. NSC declined only for drought trees under sink limitation, and for heat and heat + drought under source limitation. No NSC increases were detected during sink limitation for either species. For J. monosperma, starch was converted into sugars, with no total NSC change, potentially as a mechanism of anisohydry. Time spent in sink and source limitation varied between species and treatments, with droughted P. edulis spending the largest percentage of the study in sink limitation. No differences in time spent in source limitation was detected between treatments. These results highlight a lack of support for an increase in NSC when growth has stopped due to water stress. Furthermore, our results suggest that global change type drought may not be enough to significantly alter the carbon status of trees in semi-arid habitats.