Shade Trees in Cacao Agroforestry Systems: Influence on Water Resource Use

Cacao (Theobroma cacao L.) is one of the most important tropical cash crops, and naturally grows in the understory of neotropical rainforests. Traditional cultivation systems are established by planting cacao under old-growth forest or advanced secondary forest with minor modifications to the upper forest canopy. In cacao agroforestry competition between cacao and shade trees for limited resources may reduce yields. In particular water availability is regarded a main determinant of cacao yield. Thus we asked how water use of cacao trees and stand water availability is influenced by shade tree admixture.

Specific research objectives:
In the present study, our questions were

• Does cacao water use vary with quality and quantity of shading?
• How big is the contribution of shade trees to stand transpiration rates?
• Is soil water uptake depth of cacao trees dependent on shade tree admixture?

Methodology:
The study site is located in the Kulawi valley, in the margin zone of the Lore Lindu National Park in Central Sulawesi, Indonesia.

Tree water use:

We studied sap flux of cacao trees (if present, of shade trees) grown in a) monoculture, b) in combination with Gliricidia trees of the same age, and, c) in mixture with a multi-species tree assemblage with bigger trees and a dense canopy cover with Granier type sap flux gauges.

Air humidity and air temperature (CS215, Campbell), global radiation (SKS1110, Skye Instruments) and precipitation (ARG100, Environmental Measurements) were measured and stored every 30 min on a CR800 data logger (Campbell Scientific Inc., Logan, UT, USA).

Volumetric soil water content was continuously measured on each plot with time domain reflectrometry probes (TDR, CS616, Campbell) at different soil depths according to the sampling depths for the stable isotope study.