Elevational variation of the seasonal dynamic of carbohydrate reserves in an alpine plant of Mediterranean mountains

In alpine plants, the temporal variation in the concentrations of non-structural carbohydrates (NSC) is closely related to the growth phenology, which is largely controlled by annual variations in temperature. However, in alpine areas of Mediterranean-type climate regions, plants growing at low elevations are also exposed to seasonal drought. Given the influence of drought on growth phenology and gas exchange, we hypothesize that the seasonal dynamics of growth and NSC concentrations in alpine plants of Mediterranean biomes is co-controlled by elevational gradients of temperature and soil moisture. If so, the end of the growing season and the maximum NSC concentrations at lower elevations should coincide with the occurrence of drought. We characterized the seasonal dynamics of photosynthesis capacity, growth and NSC concentrations, in an alpine plant species of the Andes of central Chile (Phacelia secunda Gmel.) at 1600 and 3600 m. We found that the length of the growing season was similar between elevations, but the timings differed. Whilst at 3600 m, the number of leaves and the mean leaf length progressively increased from December to February, at 1600 m, in contrast, they increased from the October to December. Likewise, maximum NSC concentrations at 3600 were observed at autumn along with growth cessation. Conversely, at 1600 m, the highest NSC concentration and the growth cessation were found towards mid-summer, and coincided with a drastic drop in both stomatal conductance and photosynthesis which were not observed at 3600 m. These results demonstrate that temperature alone does not control the growth phenology and the seasonal dynamics of NSC concentrations in alpine plants of Mediterranean biomes. Rather, summer drought also exerts a significant influence in the timing of the growing season and the NSC dynamics.