High-elevation five-needle pine seedling traits vary according to climatic gradients

Lacey E. Hankin and Sarah M Bisbing, Department of Natural Resources and Environmental Sciences, University of Nevada - Reno, Reno, NV, Elizabeth A. Leger, Department of Biology, University of Nevada - Reno, Reno, NV

Background/Question/Methods
Species’ persistence potential under rapid, ongoing climate change will depend on their capacity to adapt, migrate, or acclimate via phenotypic plasticity. Rates of migration and adaptation in long-lived tree species are expected to lag behind the pace of climate change, and in situ conservation may therefore be a pre-requisite of long-term persistence. The variation in and distribution of seedling traits conferring early success, particularly those related to moisture/nutrient acquisition, is a key knowledge gap for restoration and conservation of forest ecosystems. We therefore quantified variation in seedling traits and evaluated evidence for local adaptation and plasticity in five-needle pine populations across the Great Basin using paired greenhouse and field common gardens. We specifically asked: a) how do seedling traits vary within- and among-populations across environmental gradients? and b) to what extent do populations exhibit local adaptation and/or plasticity to growing conditions? We planted seeds from three whitebark (Pinus albicaulis), four limber (Pinus flexilis), and four Great Basin bristlecone pine (Pinus longaeva) populations in all source locations and in field-collected soils in the greenhouse and assessed germination, growth, and survival. We used linear mixed effects models to evaluate trait variation and quantified plasticity to soils using a relative difference plasticity index.
Results/Conclusions
Our findings highlight interspecific seedling strategies for allocating biomass, yet high intraspecific trait variation across broad climatic gradients. These divergent strategies are consistent with tradeoffs in physiological responses to temperature and water limitations in semi-arid, high-elevation environments. Bristlecone showed significantly higher specific leaf area and root length but lower growth rates and total biomass. Whitebark invested relatively more in root mass when sourced from drier conditions, while limber reduced fine root production but produced faster and more growth when sourced from warmer and drier conditions. Growth rate and total biomass also increased with source water deficit in bristlecone, consistent with interspecific niche differentiation. Notably, limber populations germinated at significantly higher rates in both the greenhouse and field gardens regardless of conditions, suggesting potential advantages of this generalist species over co-occurring bristlecone pine under increasingly stressful environmental conditions. Finally, we did not find evidence of local adaptation to soils in the greenhouse nor environmental conditions in the field. However, significant population- and species-level differences in trait responses across climatic gradients and plasticity to soils suggest that successful restoration of climate change-impacted forests may hinge upon species-soils interactions and selection of climate-informed seed sources.