Elevation modulates the phenotypic responses to light of four co-occurring Pyrenean forest tree species

Abstract

Elevation markedly modified the species-specific phenotypic responses of trees to light. The effects from elevation mostly occurred at organ level (SLA) for the montane species, but at whole-plant level (allocation traits) for the subalpine ones. The subalpine species allocate more carbon to roots under closed canopies at low elevation. Contrary to expectations, most species displayed smaller specific leaf area at high elevation. The most drought-intolerant species was more severely affected by drought under shade than in open microsites. Understanding the phenotypic responses of mountain tree species to different levels of local irradiance can be of critical importance for elucidating their capacity to relocate above their current distributional limit in response to environmental changes. To evaluate whether the response of different co-occurring forest tree species to local irradiance varies when they are growing at various elevations or beyond their current distributional limit. Seedlings of four tree species (Betula pendula Roth., Pinus sylvestris L., Abies alba Mill., and Pinus uncinata Ram. ex DC.) were planted under different irradiance levels (forest understory vs natural gaps) and at various elevation (montane–subalpine ecotone and subalpine belt). After four growing seasons, 48 plants per species were excavated to assess allocational (biomass distribution) and morphological (specific leaf area) traits. Midday leaf water potential was recorded during a period of intense drought. The subalpine species (A. alba and P. uncinata) increased their allocation to the root system at low elevation and under dense canopy. We observed constant or higher SLA in all species when they develop in the subalpine belt. B. pendula was affected more severely by drought at low elevations and under shade than in open microsites. We found marked species-specific phenotypic variability of tree seedlings to increasing irradiance, with these responses modulated by the elevation at which the trees were growing.