OOS 56-4 - Non-structural carbohydrate depletion impairs forest tree water relations in the field

Abstract: Climate change is increasing drought frequency and intensity in many forested regions across the globe. These conditions often push trees over critical physiological thresholds (leaf turgor loss, hydraulic failure, cell death) leading to excess rates of mortality. Loss of hydraulic function is considered the apparent driver of drought-induced mortality because it is a pervasive precursor to tree death. Still, generalizable predictions of tree mortality are not reliable due to complex co-variation of anatomical and physiological traits that contribute to hydraulic function both within and among species. The availability of non-structural carbohydrates (NSC) – carbon storage compounds formed from photosynthesis – may explain this variation in drought vulnerability and rates of physiological decline in light of emerging evidence implicating the role of NSC in maintaining osmoregulation and hydraulic function amid accumulating water deficits.

Here, we characterized the relationships between NSC content and water status indicators in naturally occurring ponderosa pine saplings. Trees receiving natural precipitation or experimental drought via precipitation exclusion were subjected to either full sun or 8 weeks of shade-induced NSC depletion. We measured NSC content in needles, along with physiological water status parameters (pre-dawn needle water potential, osmotic potential, and turgor pressure) prior to shading and at weekly intervals during the NSC-depletion treatment. Though NSC content was higher in droughted trees relative to non-droughted trees (likely due to cessation of NSC-consuming growth at the onset of drought), shading reduced NSC content in needles under both ambient precipitation and drought conditions. Trees that experienced greater declines in NSC relative to initial values exhibited poorer physiological water status illustrated by diminished leaf turgor pressure which was in turn explained by lower concentrations of osmotically active solutes. The degree to which NSC depletion impaired water relations was also related to the extent that trees were already affected by drought, suggesting a strong inter-relation between NSC content and maintenance of water status – particularly when water is limiting.

Our work provides the first evidence demonstrating that NSC availability mediates water relations in naturally occurring trees in the field – both under natural precipitation and imposed drought. More generally, these results suggest that low NSC availability may pre-dispose individuals to increased vulnerability to drought – particularly under recurring or prolonged droughts as is expected with ongoing climate change. Therefore, accounting for variation in NSC pools and their influence on water relations among individuals and species may improve predictions of drought-induced mortality risk.