Water storage in alpine conifers

Conifers growing at the alpine timberline are exposed to harsh conditions during winter leading to excessive impairments of the water transport system (embolism) as well as needle damage. Previous studies indicated tree water reservoirs to play an important role for avoidance and repair strategies. These water reservoirs will be analysed in the proposed study. We hypothesise that there are large reservoirs in the xylem, bark and needles. The water content in these tissues will show a seasonal course and explain observed within-tree patterns of water potentials. Stored water is expected to minimize or avoid drought stress during the winter season and to be a prerequisite for refilling in late winter and spring. We will study two conifers dominating the timberline in the Central Alps, Picea abies and Pinus cembra. Picea abies is known to exhibit low water potentials and high embolism rates during winter while Pinus cembra avoids critical water losses by low transpiration rates and possibly by large water reservoirs. Besides standard hydraulic methods (e.g. Scholander method, Sperry method, gravimetrical measurements), the centrifuge technique to induce embolism and a portable ultrasonic system to detect embolism formation will be used. Needle damage will be quantified optically and with the electrolyte leakage method. Water reservoirs in both species will be quantified and the relationship of water potentials and water content will be determined. Vulnerability of needles to drought stress and a seasonal course of needle damage will be analysed. Based on these data, the role and dynamics of internal water reservoirs will be studied. In a test field with young and adult trees, the soil and xylem sections will be artificially frozen and water potential as well as embolism patterns, internal water shifts and needle damage will be analysed. Measurements should result in a model approach in which microclimatic data will be used to simulate tree water relations. Predictions made by the model will be tested on adult trees growing at the alpine timberline. The proposed project will substantially improve our knowledge on water relations in alpine conifers. This is important as the physiological reason for the altitudinal limit of the life form "tree" is still unknown. Furthermore, timberline trees are excellent model plants to study hydraulic aspects which are relevant for trees in general.

Conifers growing at the alpine timberline are exposed to harsh conditions during winter leading to excessive impairments of the water transport system (blockage of the conducting system by gas, embolism) and to damage of living tissues. Previous studies indicated that internal water reservoirs are important for avoidance and/or repair of damage.In this project, water storage in the aboveground biomass of two conifers dominating the timberline in the Central Alps, Norway spruce (Picea abies) and Stone Pine (Pinus cembra), were studied. Water reservoirs in the xylem, bark and needles of the crown were quantified and linked to water potentials, which are a central measure in plant hydraulics. Based on these data, a model, which enables an estimation of water contents in studied conifers, was developed. These data were related to analyses on drought resistance of living tissues and the resistance to xylem (wood) embolism to estimate the efficiency and importance of internal water reservoirs. A focus was set on the dynamics of water storage in late winter, as embolism is removed from the xylem during this period. Results show that internal water reservoirs buffer drought stress, enable a rapid improvement of plant water status in late winter and are a prerequisite for xylem repair. The ecophysiological relevance was also demonstrated in an experiment with artificially prolonged soil frost. Only few additional weeks with soil subzero soil temperatures in spring caused excessive damage of trees. Within the project, additional studies on the hydraulics of conifer needles, on the combinatorial effects of drought and mechanical stress, on the improvements of ultrasonic emission analysis, on hydraulics of other species as well as dendrometer and stem tomography analyses were performed.The project enabled new and essential insights into tree hydraulics and the fascinating life and survival of conifers at the alpine timberline. Water stored in axes and needles enables these trees to reduce drought stress during winter and to recover from damage. This is of relevance not only for basic research on the hydraulic architecture and refilling processes of trees but also for applied aspects of alpine forests and their manifold socio-ecological functions. Based on this project, follow-up studies with a focus on the effects of climate change and intra-specific adaptations are planned.