CACAO AGROFORESTRY UNDER AMBIENT AND REDUCED THROUGHFALL: TREE WATER USE CHARACTERISTICS AND STAND WATER BUDGETING

Abstract

The world s tropical regions have faced dramatic losses in forest cover during recent decades. Although high rates of rainforest conversion continue today, reforestation is becoming more common. A variety of reforestation types exists, including reforestations with native trees, mixed reforestations with exotic trees, commercial monocultures or agroforests, which are combinations of trees and shrubs with crops and/or livestock. The reasons for reforestation are manifold: tree plantations and agroforestry provide means of income for farmers and thus ensure food security for the local population. Reforestation can also restore degraded land. Reforested areas often function as connectors between protected areas and areas under human occupation, and they have the capacity to capture and store carbon dioxide, helping mitigate climate change. Additionally, reforestation can aid in controlling erosion, help protect soil and water resources and may also facilitate the conservation of biodiversity.However, reforestations are under critical debate because they may decrease stream water flow and groundwater recharge if their level of evapotranspiration is higher than that of previous vegetation. This could be especially problematic in areas where water resources are already under pressure due to water use for irrigation or other human needs. It is also important to note that evidence indicates global warming may lead to increasing frequency and severity of droughts in the (sub-) tropics, further aggravating problems of water scarcity. Therefore, species selection in terms of water use, competitiveness and drought resistance might be of great importance in the future management of agroforests or tree plantations. Unfortunately, few studies have been conducted on single tree water use rates of tropical species, even if such trees are commonly used in reforestations. In addition, limited information has been gathered on the capacity of tropical spe-cies or ecosystems to cope with abnormal water scarcity. Even more problematic, recent studies in old growth forests showed that species-independent relationships exist between water use and tree size, which would leave little space for species selection as a management option. Little is known about the environmental and arboreal structural components that drive water use patterns in tropical trees used for reforestations or agroforests.In this dissertation these fields of study were addressed by conducting a large scale field ex-periment in a cacao/Gliricidia agroforest in Central Sulawesi. To begin this experiment, sap flux characteristics and tree water use were studied under ambient conditions and compared to other stands across the tropics in a second step. Further, a replicated throughfall reduction experiment was conducted within this agroforest thereafter, where net precipitation was reduced by 71% of the control for 13 months. After beginning the throughfall reduction experiment, sap flux re-sponse to declining soil moisture was described, and the soil water budget of the throughfall reduction scenario was compared to a control. The main questions I addressed in this dissertation were:This research was conducted in the framework of the Stability of Rainforest Margins in Indonesia (STORMA) project but is also based in part on data provided by other members of the Tropical Silviculture and Forest Ecology working group (respective contributions are indicated in the Chapter Overview Publications ). The dissertation compiles three publications (Chapters 3-5): The first addresses the environmental and arboreal structural components driving sap flux characteristics and tree water use of the cacao/Gliricidia agroforest in Central Sulawesi, Indonesia under ambient soil moisture conditions. Granier heat dissipation sap flux gauges were employed to measure sap flux densities in up to 18 individuals per species. Tree sap flux response to global radiation and vapor pressure deficit were measured and described using a Jarvis-Type sap flux model. The resulting model parameters and cumulative sap flux densities were then analyzed in relation to species identity and tree structure. A second publication evolved from the compilation of the modeling results from the Indo-nesian agroforest with results from a separate study done on mixed reforestation stands in the Philippines and another done on mono-specific reforestation stands in Panama, resulting in a cross continental comparison of sap flux characteristics of more than 100 individual trees belonging to 17 species of similar size, age and management regimes. The aim here was to test for universal relationships between tree structure and water use within similar tropical land use systems and to see if different species would exhibit detectable differences in water use and sap flux response to environmental parameters.After the sap flux of cacao and Gliricidia trees in the Indonesian agroforest was assessed un-der ambient soil water conditions, the stand was subdivided into six plots, three of which ( roof plots ) were covered with a sub-canopy roof made of bamboo frames and clear PVC foil to arti-ficially reduce net precipitation. The third study discusses the effects of reduced throughfall on tree sap flux of both tree species. Data on stand transpiration, incoming throughfall, and meas-ured soil water content are further brought together to parameterize a one dimensional soil water model, which describes the effects of reduced throughfall on the soil water budget in both the ambient and the reduced throughfall scenarios.The main results of the study were the following:In summary, it can be said that plant sap flux and water use characteristics clearly differed be-tween tree species in forest stands in Indonesia, Panama and the Philippines, but structural stand components such as shading regimes and tree exposure also influenced tree water use. The data further suggests that shade trees may increase and enhance stand transpiration both through their own water use and by increasing water use rates of associated cacao trees. This suggests that in addition to species selection, management practices are also valuable tools to manage stand water use in reforestations or agroforestry stands. The throughfall reduction study showed that competition between cacao and Gliricidia for water resources during long periods with limited precipitation might be either minimal or slightly in favor of cacao. Water uptake partitioning and low stand transpiration rates may have reduced competition for available water resources and may have contributed to the relatively high tolerance of this agroforest to reduced throughfall.