Not all trees can make a forest: Tree species composition and competition control forest encroachment in a tropical savanna

1. Forest encroachment into savannas is a widespread phenomenon, the rate of which may depend on soil conditions, species composition or changes in stand structure. As savanna specialist trees are replaced by generalist species, rates of stand development may increase. Because generalists can persist in forests, they are likely to grow more quickly and survive longer in dense stands, compared to savanna specialists. Furthermore, the faster growth rates of generalists may allow them to overtop and outcompete savanna specialists, causing rapid species turnover. 2. We measured growth and survival of 6,147 individuals of 112 species of savanna and generalist tree species over a period of 10 years in an ecological reserve in Assis, Sao Paulo State, Brazil. We modelled growth and mortality as a function of soil texture and nutrients, tree size, competitive neighbourhood, and membership in savanna or generalist (species which can persist in forests and savannas) functional groups. 3. Tree growth and survival was strongly influenced by competition, as estimated by the basal area of trees taller than a focal tree. At the stand level, savanna species are unable to contribute basal area growth in closed stands, while generalist species continue to increase in basal area even at high stand basal area. This phenomenon is driven by differences in growth and mortality. Generalists grew faster than savanna species, both in height and diameter. This difference in growth rates led to savanna species becoming suppressed more rapidly than generalists. When suppressed, savanna species were more than twice as likely to die than were generalists. Soils had inconsistent and mostly weak effects which were difficult to separate from gradients of stand structure. 4. Synthesis. We demonstrate that the presence of generalist trees accelerates the rates of basal area accumulation due to their greater growth rates and tolerance of shading. Generalists outcompete savanna trees by growing faster in the open and overtopping savanna specialists. Due to the slow growth and high mortality of savanna species in the shade, they are unable to form closed-canopy stands. Accounting for differences among functional types and development of vegetation structure is critical for modelling forest encroachment.