Trait selection and the adaptive potential of 𝘗𝘪𝘤𝘦𝘢 𝘮𝘢𝘳𝘪𝘢𝘯𝘢 in the face of climate change

Abstract

The local persistence of long-lived organisms is at risk as climate change drives a rapid shift in selection regimes world-wide. Although adaptive evolution is one of the main mechanisms by which populations persist in changing environments, we have little information regarding how selection regimes will shift in response to continued climate change, nor on the potential for trees to evolve adaptively under novel selection pressures. To address these gaps, here we assessed the changes in selection in three sites along a spatial climate gradient which mimics expected temporal changes in climate and determined whether trait covariance might accelerate or impede the rate of adaptive evolution of seven P. mariana populations in the warmer and drier environment. In three common garden sites established 50 years ago, we measured an array of traits which represent water use, response to temperature, structural investment, and metabolic efficiency. Our findings reveal that all 10 traits measured in this study were under selection in at least one site. We also find different traits are under selection in each site, with four instances where the shift in selection gradient is consistent with shifts in climate: water use efficiency (WUE); needle carbon to nitrogen ratio (CN); the interaction between WUE and CN; and the interaction between CN and huber value. In the warm and dry site, traits under selection were largely uncorrelated, with only four of the 49 trait combinations under selection exhibiting intra population trait covariances. The shifts in selection gradient suggest that climate change may select for needles with higher WUE, higher structural carbon and higher hydraulic supply to the needles. The few trait-trait correlations indicate that phenotypic integration should neither impede nor facilitate adaptive evolution, leaving P. mariana provenances with the evolutionary flexibility to respond to climate change regardless of the direction to selection.