Tropical Forests, Savannas and Grasslands: Bridging the Knowledge Gap Between Ecology and Dynamic Global Vegetation Models

Friday, 19 December 2014: 10:50 AM
Mara Baudena1, Stefan C Dekker2, Peter M. van Bodegom3, Barbara Cuesta4, Steven I. Higgins5, Veiko Lehsten6, Christian H. Reick7, Max Rietkerk1, Simon Scheiter8, Zun Yin9, Miguel A. Zavala4 and Victor Brovkin10, (1)Utrecht University, Copernicus Institute, Utrecht, Netherlands, (2)Utrecht University, Copernicus Institute, Utrecht, 3584, Netherlands, (3)Free University of Amsterdam, Amsterdam, Netherlands, (4)University of Alcalá, Madrid, Spain, (5)University of Otago, Dunedin, New Zealand, (6)Lund University, Lund, Sweden, (7)Max Planck Institute for Meteorology, Hamburg, Germany, (8)Senckenberg, Frankfurt, Germany, (9)Institute for Marine and Atmospheric Research Utrecht, Utrecht, Netherlands, (10)MPI for Meteorology, Hamburg, Germany
Due to global climate change, tropical forest, savanna, and grassland biomes, and the transitions between them, are expected to undergo major changes in the future. Dynamic Global Vegetation Models (DGVMs) are largely used to understand vegetation dynamics under present climate, and to predict its changes under future conditions. However, several DGVMs display high uncertainty in predicting vegetation in tropical areas. Here we present the results of a comparative analysis of three different DGVMs (JSBACH, LPJ-GUESS-SPITFIRE and aDGVM) with regard to their different representations of the ecological mechanisms and feedbacks that determine the forest, savanna and grassland biomes, in an attempt to bridge the knowledge gap between ecology and global modelling. We compared model outcomes to observed tree cover along a mean annual precipitation gradient in Africa. Through these comparisons, and by drawing on the large number of recent studies that have delivered new insights into the ecology of tropical ecosystems in general, and of savannas in particular, we identify two main mechanisms that need an improved representation in the DGVMs. The first mechanism encompasses water limitation to tree growth, and tree-grass competition for water, which are key factors in determining savanna occurrence in arid and semi-arid areas. The second is a grass-fire feedback, which maintains both forest and savannas in mesic areas. Grasses constitute the majority of the fuel load, and at the same time benefit from the openness of the landscape after fires, since they recover faster than trees. Additionally, these two mechanisms are better represented when the models also include tree life stages (adults and seedlings), and distinguish between fire-prone and shade-tolerant savanna trees, and fire-resistant and shade-intolerant forest trees. Including these basic elements could improve the predictive ability of the DGVMs, not only under current climate conditions but also and especially under future scenarios.