Nonstructural Carbohydrate Depletion Impairs Ponderosa Pine Water Relations in the Field

Authors

Alex Goke¹, Jacob Kleimann¹, Ylva Lekberg^1,2, Gerard Sapés³, Roger Koide⁴, Anna Sala¹* ([email protected])

Institutions

¹University of Montana–Missoula, MT; ²MPG Ranch, Missoula, MT; ³University of Florida, Gainesville, FL; ⁴Brigham Young University, Provo, UT

Abstract

Ongoing climate change is increasing the frequency and intensity of hot droughts in many forested regions across the globe. These conditions lead to progressive dehydration, which often pushes trees over critical physiological thresholds (e.g., permanent turgor loss, hydraulic failure, and cell death), leading to increased mortality rates. However, predicting when, where, and which trees will die of drought remains a challenge in part because interdependent processes leading to drought-induced mortality are not fully understood. Although loss of water transport capacity (hydraulic failure) is pervasive across dead trees and considered the driver of tree death, it remains unclear whether other processes during dehydration may contribute to hydraulic failure. In greenhouse experiments, stored nonstructural carbohydrates (NSC) have been shown to affect osmoregulation and turgor maintenance, which could in turn affect soil–plant–atmosphere hydraulic function. However, whether NSC depletion impairs osmoregulation in trees in the field has not been tested.

Here, researchers tested whether NSC availability influences water relations in naturally occurring ponderosa pine saplings. Trees receiving ambient levels of precipitation or experimental drought were subjected to either full sun or to 8 weeks of shade-induced NSC depletion. The team measured NSC content in needles, along with predawn needle water potential, osmotic potential, and turgor pressure prior to shading and at weekly intervals during the NSC-depletion treatment. Preliminary results indicate higher NSC content in drought trees relative to nondrought trees, likely due to cessation of growth at the onset of drought. Shading reduced NSC content in needles under both ambient precipitation and experimental drought. Drought trees exhibited lower total and osmotic potential (more negative) than trees under ambient precipitation. As summer drought intensified, osmotic potential in trees under ambient light decreased regardless of drought treatment. In contrast, in shaded trees, osmotic potential remained constant, indicating that the depletion of NSC decreased osmoregulation capacity and turgor maintenance.

This work provides first tentative evidence demonstrating that NSC availability mediates water relations in naturally occurring trees in the field—both under natural precipitation and imposed drought stress. More generally, results suggest that under recurring or prolonged droughts, as is expected with ongoing climate change, consumption of NSC may predispose individuals to increased vulnerability to drought. 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.