Abstracts Biological Sciences

Changes in eco-hydrological functioning after tropical rainforest transformation to rubber and oil palm plantations

by Alexander Röll

This work presents findings from eco-hydrological research carried out in Jambi, Sumatra, Indonesia in the framework of the CRC990 (Sub-Project A02). Our results point to substantial changes in central ecosystem water fluxes after tropical rainforest transformation to monoculture plantations, which affects the availability of water at the landscape scale and thus impacts on the well-being of rural communities. Rainforest transformation to agricultural systems is generally expected to alter ecosystem water cycles at local and regional scales. Transpiration, i.e. water use by plants, is central to the hydrological cycle and biosphere-atmosphere feedback mechanisms. In Amazonia, e.g., substantial reductions in land-atmosphere water fluxes after large-scale rainforest transformation to pasture altered precipitation patterns. The hot spot of current rainforest transformation is the ‘Maritime Continent’ of Indonesia. In contrast to Amazonian rainforest transformation to pasture or soy-bean, rainforests in Indonesia are largely being transformed to rubber (Hevea brasiliensis Müll.) and oil palm (Elaeis guineensis Jacq.) monocultures. Scattered in between, locally some traditional agroforestry systems (e.g. ‘jungle rubber’) remain in the landscape. They are considered a glimmer of hope regarding the balancing of economics and ecosystem services. For the ‘maritime’ tropics, eco-hydrological consequences of rainforest transformation to the prevalent productive systems (i.e. oil palm and rubber) have not yet been convincingly addressed. To assess changes in the magnitude of the central water flux of stand transpiration as well as in its spatial and temporal variability after rainforest transformation, we simultaneously studied four tropical lowland land use types in Jambi, Indonesia with a sap flux technique. Our 39 study sites were located in reference forests, ‘jungle rubber’ agroforests and rubber and oil palm monocultures. The main objectives were (1) to verify and if necessary adjust a broadly used sap flux technique for measurements on different species in a lowland landscape in Jambi, Indonesia, (2) to shed first light on the thus far relatively unknown water use characteristics of oil palm as a globally rapidly expanding crop species, and (3) to assess the consequences of the continuing ‘maritime’ rainforest transformation to monoculture plantations for landscape-scale eco-hydrological functioning. First, a commonly applied sap flux technique, the thermal dissipation probe method (TDP, Granier 1985), was tested and adjusted for measurements on several tropical monocot and dicot species. Experiments confirmed that the method should be calibrated when working on previously unstudied (monocot) species. Using the original Granier calibration equation, good agreement was found between TDP derived water use rates and reference gravimetric measurements for four tropical tree species including rubber, but substantial deviations became apparent for oil palm leaves. We thus derived new, oil-palm specific parameters for the TDP… Advisors/Committee Members: Hölscher, Dirk (advisor), Hölscher, Dirk (referee), Knohl, Alexander (referee), Leuschner, Christoph (referee).