Response of carbon fluxes to water relations in a savanna ecosystem in South Africa

W L Kutsch,I Mchugh,H Eckhardt,N Hanan,C Williams,B Scholes,W Kubheka

doi:10.5194/bg-5-1797-2008

Abstract

Abstract. The principal mechanisms that connect carbon fluxes with water relations in savanna ecosystems were studied by using eddy covariance method in a savanna ecosystem at Kruger National Park, South Africa. Since the annual drought and rewetting cycle is a major factor influencing the function of savanna ecosystems, this work focused on the close inter-connection between water relations and carbon fluxes. Data from a nine-month measuring campaign lasting from the early wet season to the late dry season were used. Total ecosystem respiration showed highest values at the onset of the growing season, a slightly lower plateau during the main part of the growing season and a continuous decrease during the transition towards the dry season. The regulation of canopy conductance was changed in two ways: changes due to phenology during the course of the growing season and short-term acclimation to soil water conditions. The most constant parameter was water use efficiency that was influenced by VPD during the day but the VPD response curve of water usage did change only slightly during the course of the growing season and decreased by about 30% during the transition from wet to dry season. The regulation of canopy conductance and photosynthetic capacity were closely related. This observation meets recent leaf-level findings that stomatal closure triggers down-regulation of Rubisco during drought. Our results may show the effects of these processes on the ecosystem scale.

Highlights

Savannas are defined as tropical and sub-tropical vegetation types where woody plants and grasses co-dominate
The principal mechanisms that connect carbon fluxes with water relations in savanna ecosystems were studied by using eddy covariance method in a savanna ecosystem at Kruger National Park, South Africa
Since the annual drought and rewetting cycle is a major factor influencing the function of savanna ecosystems, this work focused on the close inter-connection between water relations and carbon fluxes

Summary

Introduction

Savannas are defined as tropical and sub-tropical vegetation types where woody plants and grasses co-dominate They are characterized by a seasonal change of water availability with distinct rainy and dry seasons. Increasing our understanding of the physical and physiological controls on savanna functioning using eddy covariance measurements is of great importance both for fundamental ecological understanding as well as for improved management and conservation of these important ecosystems. Such measurements will, provide basic data for refining models of ecosystem carbon dynamics and facilitate improved understanding of the role of tropical

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Conclusion