KROOF 3 consists of a throughfall exclusion experiment (throughfall exclusion experiment, TEE) and an experiment with a natural precipitation gradient (PGR).
Three research groups are expected to work together in KROOF 3, focusing on "Growth & Scaling", "Ecophysiology & Hydraulics" and "Rhizosphere & Carbon, in addition some further associated partners will be included. The experimental research approaches will be accompanied by modelling in all parts in order to expand the spatial and temporal scales.
TEE: throughfall exclusion experiment
In the next phase of the throughfall exclusion experiment (TEE) in the Kranzberg Forest, the trees are repeatedly as well as for the first time exposed to extreme experimental drought. The previous experimental design of KROOF is now being expanded to include additional drought-stressed plots (TE1). The previously drought-stressed plots (TE2) will now be dried out again (Fig. 1).
With this concept, a comparison of legacy effects of drought on beech and spruce trees is possible between trees that experience extreme drought for the first time (TE1) and trees that have experienced it for the second time (TE2). The experiments are accompanied trees on control plots (CO) that are exposed to natural precipitation.
The key question is whether and to what extent the trees that have survived the extreme experimental drought in the throughfall exclusion experiment (TEE) will be able to restore their functions and better adapt to future drought events, or whether they will be damaged and soon die in future extreme droughts. Feed-back mechanisms within the soil-plant-atmosphere continuum (SPAC) can either mitigate or accelerate drought-induced effects up to tree mortality. The processes and interactions at the branch, stem and root level and the effects on carbon transport and allocation are incompletely understood, also in the context of tree mortality.
PGR: precipitation gradient
The experiment with a natural precipitation gradient (PGR) consists of four plots with triplets of pure and mixed stands of beech and spruce along a precipitation gradient in Bavaria and the TEE plot in Kranzberg Forest. Ten beech and ten spruce trees are excluded from natural precipitation in each of the four PGR plots. (Fig. 2).
Previous KROOF studies have shown that the growth of spruce in mixed stands was significantly less affected by drought compared to monospecific stands. Similarly, beech grew better in monospecific stands, but the ability to recover from drought was higher in mixed stands. With the PGR concept, the effects of tree species mixture on tree hydraulics and water use are to be investigated in more detail. Advanced statistical methods will be used to analyse and understand the contrasting response of pure and mixed stands to drought over spatial and temporal scales. In addition, process-based modelling of individual tree growth (using BALANCE) will accompany all sections of the experiment.
Research questions and objectives:
Objective I: Impact of legacy effects on drought tolerance and mortality under repeated extreme drought. The aim is to clarify the role of physiological and morphological legacy effects for recovery and survival under repeated extreme drought.
Objective II: Investigation of ecophysiological responses of trees to drought and assessment of critical phases of water transport along the soil-plant-atmosphere continuum (SPAC) under extreme drought.
The aim is to analyse the sequence of events leading to drought mortality, which is still poorly understood. Hydraulic conductivity will be measured at different points along the SPAC, including the root-soil contact. It is likely that root water uptake and transport through the roots will fail before water transport through the stem fails, with failure of fine roots playing an important role.
Objective III: Quantification of water loss through bark transpiration during drought.
The aim is to quantify the daily and seasonal dynamics of bark transpiration from trunks and branches and to determine how water loss affects the water relations of trees during drought.
Objective III: Quantifizierung des Wasserverlust durch Rinden-Transpiration bei Trockenheit.
Ziel ist es, die tägliche und saisonale Dynamik der Rinden-Transpiration von Stämmen und Ästen zu quantifizieren und festzustellen, wie sich der Wasserverlust auf die Wasserverhältnisse der Bäume bei Trockenheit auswirkt.
Objective IV: Effects of tree species mixture on tree hydraulics and water use.
The aim here is to improve our limited understanding of the interaction of species in resource allocation and utilisation and the underlying hydraulic mechanisms.