Challenges of living in the treeline. An ecophysiological approach to the study of plants at high elevation in tenerife under the climate change context

Resumen

Trees growing in the upper margins and plants at high elevations have attracted the scientist’s attention for several decades, partly because they are remarkable natural laboratories to test the plant responses to environmental constraints. In Tenerife (Canary Islands, Spain), the high elevations are characterized by a semiarid Mediterranean climate where drought occupies a leading role as a particular feature due to the existence of an inversion layer which produces that the highest altitudes receive less precipitation than the lowlands. Moreover, precipitation inter-annual variability is also a climatic characteristic. So at high elevation in Tenerife, drought is combined with the rest of environmental constraints (high radiation and low temperatures). In this specific harsh environment, the endemic pine Pinus canariensis Sweet ex Spreng. is currently forming the treeline as unique tree species. Above that, alpine ecosystem is present with a high degree of endemic species where Spartocytisus supranubius (L. f.) Christ ex G. Kunkel is leading the landscape. In this doctoral thesis, several ecophysiological techniques have been used in an attempt to advance in the understanding of processes-related to carbon and water relations, and for instance, in photosynthetic performance and growth of this particular endemic pine growing at treeline. Special emphasis in the main environmental controls on them has been also accomplished. A better understanding of the current functioning of this treeline ecosystem is essential in order to comprehend the effect of global climatic change and the associated feedback mechanisms. For that, CO2 effluxes (stem, soil and foliar), photosynthesis, growth (in short-term and long-term), transpiration, water use efficiency, as well as main photoprotective strategies have been characterized for this pine. Furthermore, aspects related to stress physiology (mainly photoprotection) were also considered in six alpine endemic species growing in the treeline and in the open alpine ecosystem in Tenerife. Carbon relations at P. canariensis treeline were characterized by low ecosystem respiratory rates, mainly controlled by temperature where the main component, soil CO2 efflux, was also controlled by soil water availability. The carbon gain showed a strong seasonality exacerbated in dry years and was constrained by drought and high evaporative demand due to stomatal limitations. Consequently, growth also showed seasonality taking place under mild temperatures and soil water availability and being strongly dependent on previous wet-season precipitation which controls the length of growing season. Transpiration rates at treeline were also low, and robust seasonality in dry years was detected. A strong stomatal control under high evaporative demands was detected and this was more pronounced in limited water supply causing strong inter-annual variability dependent on the water stored in deeper soil layers. Improvement of water use efficiency under increasing aridity, in short-term and log-term, allow maintenance or even the increase of growth at treeline. So, carbon and water relations were strongly dependent on deep water resources, and for that under increasing aridity, the serious stomatal limitations together with the upturn in ecosystem CO2 effluxes will be strongly limiting the carbon gain at treeline forest. Great plasticity in morphological and physiological performance was detected in P. canariensis where most of the variations were found in winter to cope with low temperatures and high light. Moreover, alpine species showed special morphological adaptations accompanied by photoprotection system activation. The strategies of photoprotection showed seasonal response in alpine plants being more marked in open area where higher light conditions are present. S. supranubius and E. scoparium were the best adapted species to cope with the environmental constraints imposed.