Drought and freezing vulnerability of the isolated hybrid aspen Populus x smithii relative to its parental species, P.tremuloides and P. grandidentata.

Nicholas J Deacon,Jeannine Cavender‐Bares,Jake J Grossman

doi:10.1002/ece3.5364

Abstract

AimWe assessed the vulnerability of an isolated, relictual Pleistocene hybrid aspen population of conservation interest (Populus x. smithii) and the nearest populations of its parent species (Populus grandidentata and Populus tremuloides) to springtime post‐bud break freezing and growing season drought stress. Response to these stressors in the three taxa was compared in terms of avoidance and tolerance.LocationNorth American Midwest; USA.MethodsUnique genets from the hybrid Niobrara River population and from the two parental populations were propagated in a common garden from rhizome cuttings. We tracked their phenology before and after bud break and measured their vulnerability to freezing (stem electrolyte leakage and leaf chlorophyll fluorescence) and to drought (stem hydraulic conductance, leaf osmotic potential, stomatal pore index, and gas exchange).Results Populus grandidentata was slower to leaf out, showed lower vulnerability to stem freezing and drought‐induced cavitation, but exhibited a lower capacity to tolerate drought stress through leaf resistance traits compared to P. tremuloides. Hybrids were similar to P. grandidentata in their overwintering strategy, exhibiting later bud break, and in their higher resistance to stem freezing damage, but they were more similar to P. tremuloides in their higher vulnerability to drought‐induced cavitation. The hybrids shared various leaf‐level gas exchange traits with both parents. All aspens showed limited loss of leaf photosynthetic function following moderate freezing.Main ConclusionsThe Niobrara River hybrid population is vulnerable to drought due to its combination of inherited drought avoidance and tolerance traits. As climate changes, P. x smithii will likely suffer from increased drought stress, while being unaffected by frost during warmer springs. The two parental species contrast in their survival mechanisms in response to climatic stress, with P. tremuloides tending toward freezing tolerance but drought avoidance and P. grandidentata tending toward freezing avoidance and drought tolerance.

Highlights

Given the low tempera‐ tures P. tremuloides and P. grandidentata are both exposed to within their ranges, we would expect stems of both species to survive severe freezing, either through freezing tolerance mechanisms or by freezing avoidance mediated by the timing of spring bud break and leaf development phenology, which determines the exposure of leaves to freezing temperatures
P. grandidentata and the hybrid P. x smithii aspens were more resistant than P. tremulloides, experi‐ encing only about 29% damage when frozen in May, compared to 37% damage for P. tremuloides averaged across all freezing tem‐ peratures according to the Index of Injury calculation (Figure 4; df = 5, 183; F = 34.8, p < 0.001)
Delayed bud break and leafout phenology play an important role in the avoidance of potential late‐spring freeze events in P. grandidentata, and its stems were not as injured in our freezing experiments compared to P. tremuloides

Summary

| INTRODUCTION

Deciduous tree species face environmental challenges in continen‐ tal environments, with freezing and drought representing two main stressors (Medeiros & Pockman, 2011; Menon, Barnes, & Olson, 2015). One hybrid population in particular, formed in northern Nebraska, USA during the last glacial maximum, is the focus of this study It has persisted in isolated north‐facing canyons along the Niobrara River (Kantak, 1995; Kaul, Kantak, & Churchill, 1988) and lies well outside of P. grandidentata's current range. We assessed stem and leaf responses to drought stress, which is predicted to occur in some climate change scenarios for the region (Mathys, Coops, & Waring, 2017) Based on their distributions (Figure 1a), P. tremuloides experiences more variable environments (Figure 1b). Given the low tempera‐ tures P. tremuloides and P. grandidentata are both exposed to within their ranges, we would expect stems of both species to survive severe freezing, either through freezing tolerance mechanisms or by freezing avoidance mediated by the timing of spring bud break and leaf development phenology, which determines the exposure of leaves to freezing temperatures. We expected that the hybrids would share physiological freezing and drought vulnerability with both parents, and potentially be more similar to P. grandidentata

| METHODS

Findings

| DISCUSSION