Can functional traits explain the dynamic of undesirable species in tropical grasslands

In wet tropical conditions, the improved pastures are usually implanted after deforestation and most times with a single forage grass (mainly C4 and exotic grasses) and more rarely with legumes. These artificial grasslands ecosystems are more unstable than native pastures. Grasslands can be degraded by the colonization of "undesirable" plants. These species are unpalatable and so reduce the production of the grasslands ("agronomical impact"). Some of these species are also invasive and colonize also the natural forests ("ecological impacts"). Understand the dynamics of undesirable species regarding the climate, soil, management and the vegetation is the key to propose sustainable management of tropical grasslands. The studies of undesirable plants are generally using species approaches within one region. Considering the diversity of tropical area, the undesirable species may differ from one region to another. These species approaches limit the generality of the results of these studies. The objective of this work is to test the use of functional trait approach for the understanding of the dynamics of undesirables' species. We used datasets from three different tropical regions: the Reunion Island in Indian Ocean New Caledonia Island in the Pacific Ocean and the French Guyana in the Amazonian region. The islands are biodiversity hotspots threaten by biological invasion. The grassland ecosystems studied are a major element of plants invasion process. In the French Guyana the grassland are implanted after deforestation of Amazonian forest. In each region, the grassland botanical composition and the dynamic of undesirable species were monitored during several years: 1994,1995,1996,2001 and 2004 for the Reunion, from 1999 to 2005 for New Caledonia and from1997 to 2000 and from 2010 to 2013 for the Guyana. The trait values of 84 functional traits were extracted from the TRY database (www.try-db.org) from the 367 species presented in the surveys. We imputed some of the missing value using different methods. Our first step was to compare the functional traits of the different undesirables to see if we can identify common functional strategies along the three regions. Secondly, we try to understand the dynamics of these different groups and the effect of management and climate on the different undesirable strategies. In conclusion, we discuss the interest and the limits of using functional traits for the understanding and the mangement of undesirable species. (Texte integral)