Research Group Dr. Christoph Reisdorff

• Tree species and their varieties are quite differently suitable for being planted in urban environments. The roots particularly of roadside trees are faced with physical barriers and traffic derived soil contaminants. Climate change is being worsening the already harsh conditions for these trees. Reduced summer precipitation and increased incidence of heavy rains will exacerbate the water shortage for the root systems of roadside trees in their limited soil volumes. Thus, it is to be questioned whether the past and present array of tree species planted in urban environments will be capable to cope with this additional stress; or should other species and varieties be preferred for present and future roadside plantings? We tackle this question from an ecophysiological point of view e.g. by conducting field experiments and by in-situ monitoring of roadside trees. These activities are part of the project  "Stadtbäume im Klimawandel" (Roadside Trees in Changing Climate), a cooperation with the Institute of Soil Sciences of Universität Hamburg, the HafenCity-University Hamburg und the Hamburg State Department for Environment and Energy (Behörde für Umwelt und Energie).

Quite recently the phenomenon of night-time transpiration attracted the interest of ecophysiologists, since it is apparently in contradiction to the paradigm, that transpiration is nothing else than the unavoidable consequence of CO2-uptake - this is why it was claimed that stomata should be closed at night. The recent findings raise the question, what advantage it could have for a plant when it loses water during night time (in comparison to a plant, which does not). We are comparing the effect of nutrients with contrasting properties concerning their soil mobility on night time transpiration of tree species.

Eller, F., Jensen, K., Reisdorff, C. 2016: Nighttime stomatal conductance differs with nutrient availability in two temperate floodplain tree species. Tree Physiology, DOI 10.1093/treephys/tpw113

Plants reduce the stomatal conductance and/or the water potential of their leaves (osmotic adjustment) when soil water availability decreases beneath a certain threshold. The former strategy retards soil drying and the latter sustains extraction of water in dryer soils. The base of suchlike adaptive reactions is that the leaves get informed on the soil water status. The hormonal nature of root to shoot drought signaling is known to consist mainly of changing ABA and proton concentrations in the xylem sap. But little is known about the components of lowered water availability in the soil, which are sensed by the plant roots. In this context we follow the question, whether constituent parts of the rhizosphere are involved in drought sensing.

Hydraulic redistribution (HR) is called the phenomenon when water flows passively from wetter to dryer soil compartments through plant conduits (Richards & Caldwell 1987, Burgess et al. 1998). An obvious benefit of wetting rooted soil volumes during night is that the water availability can be improved, and this not only for the HR performing but also for neighboring plants (Caldwell & Richards 1989, Dawson 1993). In addition to this direct impact some studies have shown that HR also can positively affect nutrient mobility and the viability of fine roots and of mycorrhiza networks (Bauerle et al. 2008, Prieto et al. 2012a, Qerejeta et al. 2003, Egerton-Warburton et al. 2007). HR was studied mainly in dryer ecosystems. However, the few studies dealing with HR in ecosystems of humid or even perhumid climates reveal that HR is relevant not only under dryer conditions but might be a more general ecosystem function (Oliveira et al. 2005, Nadezhdina et al. 2006; Zapater et al. 2011). We undertake efforts to study HR and its biological relevance in floodplains and urban environments.