Functional groups behaviour and non-stable natural edges of gallery forest

Abstract

Gallery forests possess natural edges between forest and grasslands, offering a unique opportunity to examine how species performance varies across natural environmental gradients and to study forest expansion into grasslands and the effect of positive interaction on this expansion. Here we asked whether demographic rates of tree functional groups varied with distance to the edge, whether we could explain differences in plant strategies through functional traits and which traits increase growth and allow survival. We also asked whether gallery forests are expanding into grasslands and whether a facilitation process fosters forest expansion. Also, we examined whether there is a change from positive to negative interactions in these short gradients as a consequence of the change in microclimate. For this purpose, we examine mortality and recruitment for 30 species existing in the first few meters (10 m) of natural edges of 8 different gallery forests, and use demographic data from 5 annual inventories (2009 - 2013). The shade-tolerant group had the lowest mortality rates and basal area loss while both pioneer and light-demanding species had similar behavior for these rates. There was no difference between groups for recruitment the rates and basal area gain. Distance from the edge differentially affected the functional groups in terms of survival and growth. Survival of the pioneer group was enhanced near the edge, but for the light-demanding and shade-tolerant, survival increased toward the forest interior. All groups had higher growth in the grassland and reduced growth in the edge. Those differences in behavior could be explained by differences in functional traits. A great part of the species in natural edges have an acquisition strategy. Growth was enhanced by acquisitive traits. However, mortality selected both strategies, but in distinct zones of the edge. We found forest expanding over grasslands with the pioneer species having the spearhead role, followed by the light-demanding species and, lastly, by the shade-tolerant in a clear succession process, that could suggest a facilitation process. However, the presence of facilitators did not foster forest expansion since the more facilitators, the less forest expansion. Also, there were no changes from positive to negative interactions, but even in the grassland presence of neighbors reduced survival. Our results showed how strong and sharp the edge gradient is and how the functional groups had growth and survival differentially affected by it. Although we did not find evidence of facilitation, negative interactions do not completely suppress expansion, since there was expansion in most of the sites.