Intraspecific trait variation in a dryland tree species (Pinus monophylla) suggests adaptive capacity in response to climate change

Abstract

Forest health is jeopardized worldwide, as human-caused climate change continues to generate high rates of global tree mortality driven by more severe and frequent droughts. As temperatures rise and unpredictable precipitation patterns prevail, tree species located within arid and semi-arid regions are disproportionately threatened by drought, and will rely on their adaptive capacity, or ability to cope with, adjust to, and persist in current and future climate conditions. However, because of intraspecific trait variation, the adaptive capacity is not expected to be uniform for all populations within a species’ range. Conservation actions to protect tree species will require creating accurate predictions for where across a species’ range local populations are most vulnerable, as well as identifying potential areas of refugia. Furthermore, forest resilience and recovery will depend on successful seedling establishment. The degree of intraspecific trait variation, both for adults and juveniles, can serve as a proxy for how much tolerance for environmental change is present within particular tree populations. We investigated patterns of trait variation for Pinus monophylla, a coniferous tree species which is foundational to semi-arid Great Basin woodlands in the western US, to understand (1) how intraspecific trait variation is structured geographically and elevationally, and if it coincides with broad environmental gradients, and (2) seedling intraspecific trait variation in relation to a gradient of water availability. We selected 23 sampling locations in nine mountain ranges, stratified across broad-scale gradients of mean annual precipitation and monsoonality and local gradients of elevation. At each site, we sampled six maternal trees and harvested cones and needles from each tree to measure nine morphological traits, both reproductive and vegetative. Using variance component analysis on the maternal trees, we identified how much trait variation was explained at each scale (mountain range, site, tree, tree growth year). We then used two-block partial least squares (2B-PLS) analysis to compare morphological traits to environmental characteristics of each tree’s habitat.To explore seedling intraspecific trait variation to a gradient of available moisture, we propagated seeds from each maternal tree to plant in a common garden greenhouse experiment subjected to four watering treatments. At the end of the experiment, we measured ten seedling traits and calculated three biomass allocation ratios. We ran linear mixed effects models to understand interacting effects of watering treatment and seed source environment on measured traits and allocation ratios. We also calculated the relative distance plasticity index (RDPI) for each trait to quantify trait plasticity, and computed new linear mixed effects models with trait plasticity as its own dependent variable.We found that both adult and juvenile traits had high intraspecific variation, and that some of this variation was structured along broad environmental gradients suggesting local adaptations. Most of the variance for adult trees was explained at the tree ecological scale, however our 2B-PLS analysis identified two major plant strategies related to broad-scale environmental gradients: seed size/seed number related to a drought stress gradient, and needle investment (acquisitive/conservative) related to seasonality of precipitation. Seedlings from drier provenances that were also established in regions characterized by monsoonal precipitation patterns (i.e. greater summer rainfall rather than winter rainfall), grew both greater aboveground and belowground biomass, as well as had higher trait plasticity in response to the different watering treatments. Our results show that high trait variation in P. monophylla, both at the adult and seedling stage, could facilitate persistence of this species in future drought conditions by having a range of phenotypes that can buffer populations from widespread die-off. Populations from the drier sites, despite their trailing edge locations, may exhibit greater persistence potential by having greater trait variation and plasticity. Our study shows that understanding intraspecific trait variation, both at different life history stages and across biological scales, can be important for predicting a species’ response to climate change.