B04 - Diversity of tree hydraulic strategies in intensively used and natural tropical landscapes

Even though species-rich tropical old-growth forests are rapidly replaced by agroforestry systems with low species number, it is not known so far as to whether the loss of functional diversity and in speciesspecific hydraulic strategies has negative effects on the resilience of the systems when exposed to seasonal or episodic droughts. In order to assess the resilience of different tropical land use systems to climate variability, a direct comparison of the hydraulic safety margins (HSM) of plant communities with presence of different hydraulic strategies is needed. These strategies include the mutual interrelationship between growth performance, water consumption, wood anatomy, cavitation resistance of the branch wood, leaf-related water status traits, and the sensitivity of stomatal water loss control (isovs. anisohydric strategy), the latter being partly reflected in the foliar isotope signatures of C and O. By measuring these processes and state factors of growth and water consumption control in mature trees across different land-use systems (natural forest - rubber plantation - oil palm plantation), and within each system at topographically different sites, this study examines whether the diversity loss in hydraulic strategies from the species-rich natural forest to a monoculture results in lower resistance to, and resilience upon, inter-annual climate variation in intensively used tropical landscapes.
During the first phase of the CRC, we focused on the carbon cycle and sequestration across four different land-use systems in order to link altered patterns in tree species diversity, identity, and forest structure to ecosystem functioning. These measurements will be continued in the 2nd phase in order to monitor not only the spatial, but additionally the temporal (inter-annual), variability of aboveground C sequestration. Based on a record of 7 years of productivity measurement (in 2019), we will quantify the inter-annual variation in productivity in the different systems and relate the variation to the diversity of hydraulic strategies being present. In search for complementarity in the hydraulic strategies of cooccurring rainforest trees, we will study 24 tree species (plus rubber and oil palm) in detail, which represent gradients in wood density, height at maturity and growth performance. Key parameters measured in the trees are the xylem pressure at which 50% of hydraulic conductance is lost (P₅₀ value), minimum xylem water potential, hydraulic conductivity, xylem anatomical properties, the mode of stomatal regulation and HSM. Our subproject tests the `diversity - community resistance hypothesis` with respect to the role of tree hydraulic diversity for safeguarding productivity under a variable climate and contributes to the overall goals of the CRC by studying changes in the important productivity function across all forest land-use systems.