Relevance of green stem tissues to hydraulics of urban trees

Trees in cities provide manifold services, such as structuring urban environments, storage of carbon or regulating local climate. Though they are also exposed to intense stress periods, caused e.g. by heat salt or frost, during which tree hydraulics (i.e. water transport from roots to crown, which occurs under negative pressure) are known to be crucial for tree vitality. This project deals with the relevance of green stem tissues to tree hydraulics of urban trees. Green, photosynthetic tissues in tree stems were suggested to support water transport functions and help avoid and repair critical bubble formation, so-called embolism, blocking tree water transport. Trees in cities are exposed to high light intensities and thus photosynthetic activity of green tissues may be essential to overcome stress. We hypothesise that green stem tissues play a role to withstand summer drought as well as frost-induced embolism and respective hydraulic limitation. The functional importance of these tissues during drought and frost will be tested on potted trees of several species differing in green tissue content: pine (conifer, thick bark), beech (diffuse-porous, thin bark), birch (stem pressure in spring, thin bark), oak (ring-porous, thick bark). Analyses will be based on numerous methods, such as sap flow and stem diameter measurements or acoustic emission analyses. Manipulation experiments, including exposure of potted trees to drought, salt stress, frost cycles and artificial stem shading (covering of green stem tissues) will enable additional insights. On adult sycamore trees in Ghent, Belgium and Innsbruck, Austria novel real-time tree monitors will be installed to compare their hydraulic performance under natural drought and winter stress in a maritime versus Alpine climate. Furthermore, citizens will participate in monitoring of tree growth in the two cities based on a Citizen Science approach. The project will enable new methodical developments and insights into the functional role of green stem tissues in tree hydraulics. It will also enable to assess changes in tree hydraulics over seasons, variation across years, and compare city-specific situations with respect to drought and frost stress. Results will also be relevant for future management of urban trees, and the Citizen Science approach offers numerous participation and science communication possibilities. The project is a cooperation of Ghent University and the University of Innsbruck. Based on the unique expertise of involved research groups, pioneering results with high impact potential for both basic and applied research are expected.