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

Abstract

We evaluated population differences and drought-induced phenotypic selection on four seedling traits of the Australian forest tree Eucalyptus pauciflora using 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.

Leaf Economic and Hydraulic Traits Signal Disparate Climate Adaptation Patterns in Two Co-Occurring Woodland Eucalypts

“…It is also possible that, within a species, differences among populations in natural selection may have shaped the genetic (co)variance patterns within populations over evolutionary time, thus leading to an alignment between genetic architecture and population trajectories [ 71 , 72 , 73 ]. For example, for a set of seedling traits, different directional selection gradient estimates arising from imposed acute drought stress have been reported for glasshouse-grown mesic and arid populations of E. pauciflora [ 74 ]. Similarly, at the species level, two intermixed oak species in the same stand have been shown to exhibit significant differences in direction of the selection response and potential for adaptive evolution [ 75 ].…”

Linking leaf economic and hydraulic traits with early-age growth performance and survival of Eucalyptus pauciflora

“…In the latter sense, it is unlikely that maternal effects related to seed provisioning are important, as eucalypt seed is small and seed mass effects in another eucalypt species were not found to be statistically significant after two years of field growth ( López et al., 2003 ). Nevertheless, the effects of differences among plants in resource allocation strategies ( Wright et al., 2010 ; Costa e Silva et al., 2021 ) and unmeasured functional traits ( Choat et al., 2018 ) cannot be dismissed.…”

“…The effects of functional traits on performance attributes and/or fitness components may also change with many factors, including tree age and canopy position ( Prado-Junior et al., 2017 ), growth strategies ( Wright et al., 2010 ), environment ( Ramírez-Valiente et al., 2014 ; Siepielski et al., 2017 ; Ramírez-Valiente et al., 2021 ) and population ( Costa e Silva et al., 2021 ; Ramírez-Valiente et al., 2021 ). Our results pertain to the average within-population effects of traits on a given response variable, and the applied fixed-effects model specification attempted to control for any differences in characteristics of the populations (see Supplementary Methods 1 ).…”

“…Derivative-based estimates of selection gradients (e.g. Janzen and Stern, 1998 ; Morrissey and Sakrejda, 2013 ; Franklin and Morrissey, 2017 ; Costa e Silva et al., 2021 ) were obtained for either performance measure from a given fitted logistic model. An average directional selection gradient was derived under both definitions of η ijk (with a linear term, or with both linear and quadratic terms, for performance), whereas an average quadratic selection gradient was derived only when η ijk had a quadratic term for performance.…”

“…Accordingly, quantitative inferences about natural selection acting on traits of adaptive value have been based on fitness components (e.g. tree survival and/or fecundity), usually assessed during early stages of the life cycle ( Gómez, 2004 ; Ramírez-Valiente et al., 2014 ; Costa e Silva et al., 2018 ; Warwell and Shaw, 2018 ; Warwell and Shaw, 2019 ; Costa e Silva et al., 2021 ; Ramírez-Valiente et al., 2021 ).…”