PS 15-215 - Phenological physiology: Woody plant water deficit stress tolerance increases over the temperate growing season

Abstract: Is a tree’s capacity to withstand water deficit stress stationary in time? This question has become pressing as anthropogenic climate change is likely to increase the frequency and intensity of droughts in many temperate environments. Decades of ad hoc observations have suggested that woody plants can physiologically acclimate in response to dry soil conditions, but the degree, phylogenetic extent, and timing of such acclimation remain poorly understood. Our goals in this research were to a) compare tolerance to water stress deficit across diverse temperate woody taxa and b) document the degree to which these taxa show acclimation (increased tolerance) over the temperate growing season.

Our approach was two-fold. First, over three growing seasons, we regularly assessed water deficit stress tolerance (turgor loss point) of trees planted in the living collections of the Arnold Arboretum of Harvard University (Boston, MA, USA) and the Scott Arboretum of Swarthmore College (Swarthmore, PA, USA). We sampled across the angiosperm phylogeny, assessing turgor loss point in 30 species of maples (Acer; rosids), hollies (Ilex; asterids), and magnolias (Magnolia; basal angiosperms). Second, we used a growth chamber experiment to test whether soil water limitation could independently produce a signal of water deficit stress acclimation in Japanese maple (Acer palmatum). We characterized water deficit stress tolerance across all study species from May/June to August/September for field-collected samples and over a three-month “growing season” in the growth chamber experiment.

Despite some variation within and among taxa, we found a significant, angiosperm-wide signal of acclimation (declining turgor loss point with time) across study sites and in the growth chamber experiment. The patterns we documented help consolidate our understanding of the “phenological physiology” of water stress tolerance, in which trees become more resistant to stress when exposed to seasonal stressors. This work indicates that early growing season droughts may be especially hazardous for temperate woody plants and also guides species selection for climate-adaptive horticultural plantings, forestry, and restoration.