Summary The global climate change is affecting forests worldwide with effects regarding biodiversity and ecological services. Knowledge of tropical humid and dry forests are in this context of utmost importance. The main aim of the present thesis is the analysis of tree water relations and drought responses with a focus on adaptation strategies and plant functional types. The studies were conducted in two different locations, a seasonally dry tropical forest and an evergreen montane forest. Several hundred increment cores of trees were taken and high-resolution dendrometer measurements were recorded to analyze short and long term dynamics of circumference changes in connection with extreme climatic events. To support our analyses sap-flux data, volumetric soil water content, canopy coverage change and leaf phenology were also recorded. In the dry forest the tree species Ceiba trichistandra (leaf deciduous, stem succulent), Eriotheca ruizii (leaf deciduous, root succulent) and Erythrina velutina (leaf deciduous) were studied. C. trichistandra responded very sensitive to fluctuating moisture regimes with leaf phenology, sap flux and stem diameter variations, and can be regarded as a sensitive indicator for assessing climatic variations. In the montane rain forest, the focus was on dry intervals with a minimum of four consecutive days to examine how different tree species respond to drought stress. The averaged species-specific stem shrinkage rates and recovery times during and after dry periods were analyzed. The two deciduous broadleaved species Cedrela montana and Handroanthus chrysanthus showed the biggest stem shrinkage of up to 2 mm after 10 consecutive dry days. Moreover, great differences of recovery times after longer-lasting (i.e., eight to ten days) drought events between the two evergreen broadleaved species Vismia cavanillesiana and Tapirira guianensis were found. While V. cavanillesiana replenished to pre-drought stem circumference after only 5 days, T. guianensis needed 52 days on average to restore its circumference. Hence, a higher frequency of droughts might increase inter-species competition, species-specific mortality and therefore finally alter the species composition of the ecosystem.

In light of the findings presented here regarding possible effects of climate change on different tree species, this dissertation serves as important contribution to enhance the understanding of tropical forest ecosystems.