TIPTROPTRANS · Looking for Tipping Points on Tropical Forest-Savanna Transitions

Looking for Tipping Points on Tropical Forest-Savanna Transitions

Summary description of the project objectives Tropical forest-savanna transitions are regions where tropical forests and savannas converge and both exist as discrete “patches” under similar climatic conditions (Veenendaal et al. 2014). They are often referred as “Zones of Tension” (ZOT), following Clements’ concept of the ecotones as zones of tension (Clements 1905). Tropical forests and tropical savannas are the two most biodiverse biomes of the world (Bass et al. 2010) and account for 60% of all terrestrial gross primary productivity (Beer et al. 2010). Therefore, any shifts in their distributions will have important consequences on local, regional and global carbon balances (Field et al 1998; Sitch et al 2003), climate feedbacks (Malhi et al 2008; Better et al 2008), and on potential tipping points (Mace et al. 2014). Most studies use global models or remote sensing approaches that represent the transition from forest to savanna as a competition between a few functional groups (often just two groups: tropical grasses and tropical trees). There is plenty evidence that the transition between tropical forest and non-forest biomes is not gradual, but a threshold phenomenon between two stable states that can occur at the same level of rainfall (de L. Dantas et al. 2013). The switch between these two stable states has been suggested to be primarily mediated by fire (Hirota et al. 2011, Lehmann et al. 2011, Staver et al. 2011), although soil, climate, light, and herbivory also have a key role in this transition (Lehmann et al. 2014, Veenendaal et al. 2014). This project aimed at gathering a detailed ecological and ecophysiological data for understanding the dynamic processes operating at tropical forest-savanna transition, at a number of sites of Brazil and Africa. The study was framed in the context of the forest-savanna fire thresholds from a functional trait perspective, with the aim of testing and further developing the role of fire and its interaction with drought and resources availability. Description of the work The project conducted detailed plot surveys to quantify functional diversity, and manipulative experiments to understand and quantify seedlings responses to fire, drought, light and nutrient availability. The study areas were located in Nova Xavantina (Brazil) and Kogyae Strict Nature Reserve (Ghana) (Figure 1). In each area, a number of plots (four in Brazil and three in Ghana) were sampled along a forest-savanna transitions for the functional traits described in Table 1. Fieldwork was conducted in 2014. Two manipulative experiments (one field based at Kogyae, and the other at a greenhouse in Wageningen, Netherlands) we also performed in 2014 and 2015. The field manipulative experiment conducted at Kogyae look at differences in survival and establishment of five forest tree species in different vegetation types (forest, transition, savanna). The greenhouse experiment tested the effects of water availability and fire on a set of 27 African species ranging from wet forest to dry savanna. Main results achieved Through the campaigns and experiments we have gathered a large database on functional diversity along forest-savanna transitions of both adult and juvenile tree species. At the adult level, we have gathered an exhaustive forest-savanna functional trait dataset (see summary list of traits collected on adult trees representing up to 80% of the plot’s basal area at each permanent plot) that will enable us to scale from individual traits to ecosystem functioning properties, such as primary productivity and nutrient cycling. At the time of this report results are still preliminary and a set of scientific publications is under preparation. A Special Issue in a high-impact scientific Journal will be published during 2016. The field manipulative experiment in Ghana led to two MSc thesis (Anabel Cardoso, U. Oxford and J. Medina-Vega, Wageningen University) and a publication is currently under review at a scientific journal. The greenhouse experiment allowed us to test a new methodology for applying fire treatments on ecological research under experimental conditions. The experiment was harvested at the end of November 2015. Expected final results and their potential impact and use • A Special Issue in a high-impact scientific Journal will be published during 2016. • A new methodology for performing fire treatments on manipulative greenhouse experiments. • An extensive characterization of the functional diversity in forest-savanna transitions for use in Dynamic Vegetation models,. • Dataset to be incorporated in global datasets such a TRY and GEM. • Accurate information of the vulnerability of these transitions to ongoing changes in climate and fire regimes.

Data: CORDIS, © European Union