Directional Selection on Tree Seedling Traits Driven by Experimental Drought Differs Between Mesic and Dry Populations

João Costa E Silva,Suzanne M Prober,Brad M Potts,Elizabeth Pinkard,Rebecca Jordan

doi:10.3389/fevo.2021.722964

João Costa E Silva, Suzanne M Prober + Show 3 more

Open Access

https://doi.org/10.3389/fevo.2021.722964

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Abstract

We evaluated population differences and drought-induced phenotypic selection on four seedling traits of the Australian forest treeEucalyptus pauciflorausing a glasshouse dry-down experiment. We compared dry and mesic populations and tested for directional selection on lamina length (reflecting leaf size), leaf shape, the node of ontogenetic transition to the petiolate leaf (reflecting the loss of vegetative juvenility), and lignotuber size (reflecting a recovery trait). On average, the dry population had smaller and broader leaves, greater retention of the juvenile leaf state and larger lignotubers than the mesic population, but the populations did not differ in seedling survival. While there was statistical support for directional selection acting on the focal traits in one or other population, and for differences between populations in selection gradient estimates for two traits, only one trait—lamina length—exhibited a pattern of directional selection consistent with the observed population differences being a result of past adaptation to reduce seedling susceptibility to acute drought. The observed directional selection for lamina length in the mesic population suggests that future increases in drought risk in the wild will shift the mean of the mesic population toward that of the dry population. Further, we provide evidence suggesting an early age trade-off between drought damage and recovery traits, with phenotypes which develop larger lignotubers early being more susceptible to drought death. Such trade-offs could have contributed to the absence of population mean differences in survival, despite marked differentiation in seedling traits.

Highlights

Global warming is leading to major shifts in the distribution of plant and animal species globally (Chen et al, 2011)
Results from F-tests indicated that the fixed-effect term for the population-by-block interaction was significant (p < 0.05) for seedling survival, but not for vegetative juvenility; the interaction term was excluded in a final generalized linear mixed model (GLMM) for the latter trait only
For comparison with residual pseudo-likelihood (RSPL) estimates, family variances were estimated for seedling survival and vegetative juvenility using maximum likelihood (ML), which enabled their statistical significance to be assessed by one-sided likelihood ratio tests

Summary

Introduction

Global warming is leading to major shifts in the distribution of plant and animal species globally (Chen et al, 2011). Sessile species, such as forest trees, are among the most vulnerable owing to their often poor dispersal abilities and long life histories (Petit and Hampe, 2006). Detecting adaptive changes in local populations of longlived organisms, such as forest trees, is difficult due to numerous factors (Alberto et al, 2013), including long-life cycles (Petit and Hampe, 2006), ontogenetic (Brunner et al, 2016) and plastic (Nicotra et al, 2010; McLean et al, 2014) changes in phenotype, superimposed on the normal selective filtering of mal-adapted inbred progeny during stand development (Koelewijn et al, 1999; Costa e Silva et al, 2011; Griffin et al, 2019).

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