Flash drought alters the nonstructural carbohydrate pool composition of mature red maple (Acer rubrum) and red oak (Quercus rubra) trees

Link To Share This Poster: https://cdmcd.co/B79Gad

Emilia Pelegano-Titmuss and Sara Poon, The Bronx High School of Science, New York, NY, Clare Kohler and Andrew Reinmann, Environmental Sciences Initiative, Advanced Science Research Center at the Graduate Center, City University of New York, Andrew Reinmann, Department of Geography and Environmental Science, Hunter College

Background/Question/Methods
In temperate forests of the northeastern U.S., annual precipitation is projected to increase, but late-growing season water stress and the severity of droughts are also likely to increase. Water limitation can negatively impact tree physiological processes and reduce overall forest carbon sequestration. During periods of water stress, trees can use their nonstructural carbohydrate (NSC) pools (starches and sugars) as a source of carbon to maintain growth, metabolism, transport, and osmoregulation. As such, NSC pools can play an important role in tree and ecosystem resistance to drought. However, how the size and composition of tree NSC pools respond to water stress remain uncertain. We have been monitoring NSCs in mature trees in a temperate deciduous forest at Black Rock Forest in the Hudson Highlands Region of New York since 2018. A late-growing season flash drought occurred in 2019 and we used this anomalous event to advance understanding of the response of tree NSC pool size and composition to drought. The mountainous terrain of this region creates natural gradients in soil moisture. We established three study sites along a ~1 km long transect spanning dry south and north-facing hillslopes and a mesic valley. At each site, we have been monitoring soil moisture, soil temperature, and air temperature since 2018. Each site includes 2-3 mature red maples (Acer rubrum) and 3 mature red oaks (Quercus rubra). These species vary in their response to water stress—red maples are isohydric while red oaks are anisohydric—and are two of the most common tree species in the region. During November (i.e., after fall leaf senescence) of 2018-2020, we collected tree cores at breast height from each tree for analyses of sapwood starch and sugar concentrations.
Results/Conclusions
This drought altered soil moisture across the gradient, but declines were substantially larger at the north-facing and south-facing hillslopes (80% and 50% decline) compared to the valley (15% decline). We find that while overall NSC concentrations remained unchanged between 2018 (“average” year) and 2019 (drought year), the composition changed. Starch concentrations decreased by 68.9% in red maples and 53.7% in red oaks, but sugar concentrations increased by 166.9% in red maples and 131.6% in red oaks, which induced large changes in starch:sugar ratios. These findings suggest that during drought stress, trees convert starches to sugars, likely to help maintain osmotic gradients and hydraulics systems. This work provides additional evidence for the importance of NSCs in buffering trees against environmental stress.