Water transport in needles of alpine conifers

Winter at the alpine timberline causes excessive embolism in the (secondary) xylem of conifers. In previous studies, embolism induction, adaptation mechanisms and effects on tree life at the timberline were analysed. These studies focused on stem and twig xylem, while information on water transport in one of the trees most important organs, the needle, is lacking. The project presented deals with the properties of the water transport system in conifer needles. We hypothesise that the xylem of needles is more vulnerable to drought and freeze-thaw induced embolism than the xylem of twigs but we expect a similar hydraulic efficiency. We suppose excessive winter-embolism at the alpine timberline causing needle damage and effects on the photosynthetic capacity of the needle biomass. On the other hand, we expect needles to play a role in refilling processes. Field observations at the alpine timberline as well as laboratory studies are planned to prove these hypotheses. Based on a new technique to measure needle xylem conductivity, the annual course of embolism will be studied in Norway spruce and Stone pine trees. These measurements will also focus on refilling processes in spring. Furthermore, embolism patterns in winter will be correlated with patterns of needle damage visible in spring. In the laboratory, hydraulic efficiency and vulnerability to drought and freeze-thaw events of several conifer species will be assessed and correlated with anatomical measurements. In an additional experiment, needles will be artificially embolised with pressure collars to estimate effects on photosynthesis. The project presented will enable the development and application of new methods to analyse the water transport system of conifers and give insights into the hydraulic properties of conifer needles. It will increase our knowledge on conifer water relations at the alpine timberline and their relevance for timberline formation.

Needles are essential organs for the photosynthesis of conifers, but they are also relevant resistances within the water transport system of trees. Dysfunction of this transport system, which is situated in the xylem, causes water deficits, stomatal closure and tissue damage. Previous projects revealed that the transport system in stem and branches of conifers growing at the alpine timberline is affected by embolism (breakage of water columns) during winter, but information on needle water relations was lacking. Therefore, the hydraulic of conifer needles, with a focus on their hydraulic conductivity and safety, the occurrence of embolism at the timberline, and the role of needles for repair processes (refilling) was studied. Measurements of the hydraulic resistance of conifer needles were enabled by a new hydraulic method, based on embedding of needles in a paraffin block. New methodical developments were also made for ultrasonic emission measurements and the centrifuge technique, both used for the analysis of the resistance to embolism formation. Measurements revealed that main hydraulic resistances within conifer needles are situated outside the xylem, while conductivity in needle xylem was similar to branch hydraulic conductivities. In contrast, needles of several species showed higher vulnerability to embolism than branch xylem. Accordingly, measurements indicated high embolism in needles at the timberline during winter. A correlation of embolism intensity and the extent of needle damage in the consecutive spring was observed. Seasonal courses and experiments at the timberline also showed that needles play an important role for embolism repair during late winter and spring. Refilling started in distal branch parts, and deuterium labelling of water revealed that up to 50% of xylem sap resulted from water uptake over the branch surface. This project demonstrated that needle hydraulics are critical for conifer water relations. Needles are composed of a complex and interrelated system of resistances which are balanced with other resistances in the tree. This study also improved our knowledge of conifer water relations at the alpine timberline and especially of refilling processes observed every year after excessive winter embolism. 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.