Patterns in water flux and carbon cycle controls along land use and climate gradients
Soil Sciences
Ecology and Biodiversity of Plants and Ecosystems
Final Report Abstract
Seasonality in micrometeorology of the Kilimanjaro environment has a strong influence on the way different ecosystems distributed along its elevation gradient use water. Our sapflow measurements were based on single trees and the results provided here are limited to single tree data. In the next steps, these data sets will be up-scaled to provide a picture of entire ecosystem water use. More time is required in order to accomplish this assignment. The results, however, show a strong seasonality in tree water use, controlled by soil moisture availability, radiation and VPD. We also observed strong differences among species in some ecosystems. In the lower montane forest for example, tree species vary in their growth characteristics and this determines their access to the top crown. Slow growing secondary succession species therefore remain below the canopy and exhibit relatively low flux rates compared to pioneers that dominate the top canopy layer. In the home garden, different functional types grow together, but they exhibit different growth forms resulting in a vertical canopy layering. Vertical layering has a significant influence on radiation distribution within the canopy such that very low light intensities are experienced at the forest floor. In addition, there was also significant variation in VPD between the upper canopy layer and within the canopy. These changes in radiation and VPD strongly influence water use by the different components of this ecosystem. This in turn translated in different flux rates in the respective layers. In the savanna, trees are widely spaced and radiation is abundant. Soil moisture availability is however restricted to only a short period of the year. Trees in the savanna, therefore, employ a conservative approach to water use. Despite strong differences in precipitation along the elevation of Kilimanjaro, rates of xylem sap flux remain relatively conservative, ranging 0.1-0.2 Kg cm^-1 day^-1. In this respect, the total amount of water that a tree will consume is, therefore, strongly dependant on tree size, which determines the conducting sapwood area. Trees in dry areas, such savanna tend to remain small in order to limit water use. We are already determining sapwood areas (SA) for the respective tree species considered in our sapflow measurements. With the help of a predictive model, single trees SA will then be scaled to plot level. Based on the predictive equation suggested by Ewers et al. 2002: Ec = SFD * As/Gs, where Ec is canopy transpiration, SFD is sap flux density, As is sapwood area and Gs is ground area, we shall estimate Ec for the respective ecosystem types. These results will be compared with those determined from a second parametric model that includes the climate variables. These approaches will provide more information for better understanding of the Kilimanjaro ecosystems.
