B4 - Drought effects on soil water regime and water partitioning in cacao agroforest

Abstract
The main objectives of this sub-project are (1) to analyse the rates and depths of soil water depletion in natural forest and agroforest, (2) to study separately the water use and partitioning of shade trees and cacao plants in the agroforestry system, and (3) to test for both land use systems whether hydraulic redistribution is an important mechanism to cope with drought. The results shall help to evaluate the response of forest and agroforestry systems to drought and improve our understanding of the ecological role of trees in agroforestry systems.

Summary
A major goal of the proposed study is to evaluate the drought response of the cacao agroforestry system and natural forest with respect to soil water resource use. The effect of drought on many ecosystem processes may depend on the amount of water stored in the soil profile, the plants’ ability to extract this water, and species specific physiological mechanisms to withstand the stress induced due to dry conditions. The uptake of water from deep soil may play a major role herewith. Further, hydraulic redistribution of soil moisture by plant root systems from relatively wet soil layers to dryer soil layers may be an important mechanism to cope with drought. In terms of its contribution to total evapotranspiration hydrological redistribution may not be very relevant, but it may play a critical role in delaying soil drying to the point that finer roots in the upper layers lose their activity. However, there is limited information about the drought response and involved mechanisms for natural forests in Southeast Asia and cacao agroforestry systems. A second goal of our study is to analyse the role of shade trees in the soil water use of the cacao agroforestry system. Complementarity among agricultural plants and trees could reduce competition, e.g. because trees take up water from greater soil depths.

The proposed project is part of focus 3 that studies the economic and ecosystem response to drought in agroforest and natural forest. Our specific objectives within the drought experiment are (1) to analyze the rates and depths of soil water depletion in natural forest and agroforest, (2) to study separately the water use and partitioning of shade trees and cacao plants in the agroforestry system, and (3) to test for both land use systems whether hydraulic redistribution is an important mechanism to cope with drought. The methods will include volumetric soil moisture measurements by TDR technique and soil water potential measurements with psychrometers in both the natural forest and the agroforest. These measurements will also provide data essential to all groups participating in the drought experiment. Stem sap flux studies will be done in cacao plants and shade trees in the agroforestry system in order to assess their specific changes in water use due to the induced drought. Further, a deuterium labelling experiment in the agroforestry system will be used to determine from which depth shade trees and cacao plants take up the water and thus, to distinguish complementary or competitive water use.

The results shall help to evaluate the response of natural forest and agroforest to drought and improve our understanding of the ecological role of trees in agroforestry systems. Overall, this project contributes to the integrative analysis of the systems’ drought response.