Intraspecific trait variation, growth, and altered soil conditions at tree species distribution limits: From the alpine treeline to the rear edge

Alpine treelines are expected to shift upward due to climate warming, whereas warmer conditions can have negative impacts on forests located near the xeric, equatorward limit of the distribution of tree species (rear edge). We compare tree populations forming the distribution limits of mountain pine (Pinus uncinata) in northeastern Spain: two cold-limited alpine treeline populations, and a rear-edge drought-prone stand. In the treelines, trees were sampled in three altitudinal belts to evaluate within-site variations considering the forest, the transitional ecotone and the treeline. Tree growth was markedly higher in one of the treeline sites (Tesso), particularly in the forest belt, as compared to the other treeline (Las Cutas), and to the warmest rear-edge stand. Tree growth in the rear-edge population depended on June precipitation. Trees presented also comparatively smaller leaves with higher Specific Leaf Area (SLA) there than in the two treelines. Regarding the differences between altitudinal belts within the treelines, treeline soils showed lower N concentrations and a lower content of sand than in forests. Soil microbiota was dominated by bacteria in the treeline and by fungi in the forest, although with differences between sites. Soil characteristics and microbial composition were strongly related, whereas its relationship with tree growth and functional traits was less clear. The marked differences in leaf traits and growth response to climate found when comparing treeline and rearedge sites highlights the biogeographical uniqueness of sites forming the equatorward distribution limit. The greater tree growth in the Tesso treeline as compared to Las Cutas indicate that interactions between climate and physical and chemical properties of the soil influence tree growth and its interactions with soil microbial communities in the treeline. A better understanding of plant-soil feedbacks may help to understand the future dynamics of tree populations forming the species' climatic and geographic limits of distribution.

Automated summary

This study compares tree populations forming the distribution limits of mountain pine in northeastern Spain. The results show that tree growth is influenced by climate and soil properties. There are marked differences in leaf traits and growth response to climate between treeline and rearedge sites. The soil microbiota in treelines is dominated by bacteria, while in forests it is dominated by fungi.