Provenance Variation in Functional Traits of European Forest Trees: Meta-Analysis Reveals Effects of Taxa and Age Despite Critical Research Gaps.

Climate change is driving profound transformations in European forests. Understanding the adaptive potential of tree species is a key challenge for conservation and adaptation measures. A critical component of this adaptive potential lies in the intraspecific variation of functional traits. The long tradition in ecological genetics has resulted in a plethora of studies across species, regions, age classes, and traits. Prior syntheses have rarely quantified trait-specific patterns and their variation across taxa and tree age. We conducted a systematic literature search to examine intraspecific variation in natural European tree populations. We identified four approaches to study intraspecific variation (i.e., provenance effects, provenance environment interaction effects, clinal effects and transfer effects). For each approach, we compared their prevalence to show an effect while also accounting for species, species group and age. Our results found that intraspecific variation is common in European tree species, with tested traits showing significant provenance effects (73%), provenance-environment interaction effects (45%), linear clinal effects (30%) and linear transfer effects (38%). While growth traits were predominantly studied, several other traits showed higher frequencies of significant results. Specifically, reproduction, survival, phenology, plant morphology, plasticity, drought, and frost tolerance are highly relevant but still understudied in comparison to growth. Conifer species demonstrated a higher prevalence of intraspecific variation compared to broadleaves. Despite the research clearly focusing on young trials, older trials tended to show higher frequencies of effects in phenology, growth, plant morphology, and survival, suggesting accumulating environmental selection with growing tree age. Europe lacks essential information on intraspecific variation of tree species for the diversification, conservation, and adaptation of its forests, especially in southern and southeastern parts, where many species harbour high genetic diversity and are most vulnerable. The significant influence of age urges for a reanalysis, reestablishment, and maintenance of long-term trials. These trials should consider species and environmental conditions relevant for future scenarios.