Editor’s Choice: Volume 110 Issue 2

The editor’s choice for our February issue is “Not all trees can make a forest: Tree species composition and competition control forest encroachment in a tropical savanna” by Flake et al. Here, Associate Editor María Natalia Umaña explains the importance of this research. 

As a tropical forest ecologist, I’ve been concerned about the increasing deforestation rates and the disappearance of trees from their natural habitats. However, non-forest ecosystems like savannas might be suffering from the opposite problem: forest encroachment into savannas is becoming widespread. Yet, ecologists still lack a comprehensive understating of dynamics in the forest-savanna ecotone.

Figure 1: Study area map. (a) The study site (orange dot) is located in western Sao Paulo state, Brazil (blue line), at the edge of the Cerrado biome (hashed area). (b) Within the study area (solid black line), plots (orange dots) were established across a gradient from low tree cover to high tree cover. (c) A closed-canopy cerradão forest with ~25 m2/ha of tree basal area. (d) An open-canopy cerrado sensu stricto savanna with ~5 m2/ha of tree basal area. Landsat-8 base layer (b), courtesy U.S. Geological Survey. Images (c) and (d) by Giselda Durigan

In this paper, Flake et al. offer new perspectives into the forest-savanna dynamics by examining the variation in mortality and growth rates over a savanna-forest gradient and compares the demography between specialist (species that only occur in savannas) and generalist tree species (species that grow in both savanna and forests). They use an impressive dataset from more than 6000 individuals from over a hundred species that were monitored for growth and survival over 10 years. They found that, over the savanna-forest gradient, mortality rates increased substantially. This trend was more pronounced specialists than for generalist tree species. Furthermore, specialists grew slower and became suppressed by generalists suggesting a demographic advantage of generalists over specialist species.

These results offer new insights about the underlying demographic processes driving forest encroachment into savannas and open further exciting questions. What are the mechanisms underlying the demographic differences between specialist and generalist trees? How does the rising atmospheric CO2 affect growth in these two groups of species? I would be excited to see more studies of these fascinating systems.

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