Savannas are not old fields: functional trajectories of forest expansion in a fire-suppressed Brazilian savanna are driven by habitat generalists

Flake, Samuel; Abreu, Rodolfo; Durigan, Giselda; Hoffman, William

An aerial view of one of the three studied savanna-forest gradients. Image source: Google Earth / Maxar Technologies
An aerial view of one of the three studied savanna-forest gradients. Image source: Google Earth / Maxar Technologies

Savannas cover much of the tropics, but with widespread fire suppression, forests have begun to encroach on formerly species-rich old-growth savannas. The replacement of savannas by forests has large impacts on ecosystem function, associated with changes in the traits of the trees and shrubs present – their size, resistance to fire, ability to capture light, and their light requirements as juveniles. In our study, we asked “what are the traits of the species which drive forest encroachment”?
We first addressed these questions by classifying tree and shrub species into savanna specialists, forest specialists, and generalists, and then comparing functional trait values among those groups. Generalist species, which occur both in savannas and in forests, made up the majority of the tree basal area across the gradients from savanna to forest. These species warrant a greater focus due to their dominant role in forest encroachment.
What traits differentiate generalists from savanna and forest specialists? Among all the species, we identified a primary axis of variation from species with conservative, defensive strategies (thick bark, thick leaves, and short stature) at one end and acquisitive strategies (thinner or less dense leaves, leading to higher specific leaf area, greater maximum height, greater height:diameter ratio) at the other. Generalists tended to have trait values between those of savanna and forest specialists, but they were also highly variable. These traits may allow generalists to survive fires and droughts in open areas but still reach the canopy in young forests.
Overall, due to both shifting abundance of species and the differences in traits among those species, forest encroachment leads to a shift in the average value of several traits as tree abundance increases, from conservative to acquisitive. Many of the patterns of trait change are similar to those found in other ecosystems, such as old-field succession, but some trait patterns such as decreasing bark thickness and increasing specific leaf area suggest that the dynamics of savanna-forest transitions differ fundamentally from other ecosystems.

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