Dispersal limitation and fire feedback maintain mesic savannas in Madagascar: Reply

Savannas, characterized by a continuous flammable C4 grassy layer interspersed with woody vegetation, are widespread across the tropics, ranging from open grasslands to grassy woodlands (Oliveras Veldman et al., 2015). Despite their ubiquity, the origin and maintenance of savannas are fiercely debated. This is particularly true in the Central Highlands of Madagascar. Some argue that these grassy ecosystems can climatically support closed-canopy forests and, consequently, are likely a product of anthropogenic deforestation over the last two millennia (Humbert, 1949; Joseph et al., 2024). Others challenge the deforestation hypothesis (Bond et al., 2008, 2023; Phelps et al., 2022; Solofondranohatra et al., 2020; Vorontsova et al., 2016), arguing that paleoecological and phylogeographic data suggest that these high-rainfall grassy ecosystems might be ancient and predate human arrival. To reconcile the scientific evidence supporting these opposing viewpoints, we proposed an ecological mechanism—range pinning—to explain how the savannas in Madagascar could have been naturally maintained in mesic regions that can climatically support forests (Goel, Van Vleck, et al., 2020). In particular, we showed that the combination of dispersal limitation and fire-vegetation feedback could have restricted Eastern rainforests from expanding onto the Central Highlands and other mesic regions of Madagascar. Joseph and Seymour (2023) (hereafter referred to as J Douglass et al., 2019). Our simulations consistently show that, for all initial conditions, forests fail to expand onto the Central Highlands (Figure 1E–G). These results are best interpreted within the context of global paleoclimate change since the LGM. At the LGM, the Earth's climate was relatively cold and dry, which considerably reduced the extent of forests in Madagascar, restricting them to the northeastern part of the island (Figure 1A). Following the LGM, as the climate became warmer and wetter, forests expanded globally (Adams Lewis see Figure 1B in Goel, Van Vleck, et al. 2020) provide valuable context for understanding the interplay of climate and human activity in shaping the historical distribution of high-rainfall grassy biomes in Madagascar. Nevertheless, we agree that more paleodata are always better for understanding how climate and human activity have shaped vegetation in the Central Highlands. Recently published paleoecological records from Central Madagascar (Broothaerts et al., 2023; Razafimanantsoa et al., 2024) and previously published snapshots of sedimentary records (not included in our paper) (Samonds et al., 2019; Straka, 1996) suggest that vegetation patterns around these Central Highlands sites are broadly consistent with those inferred from paleoecological data considered in our paper (Gasse 8400 14C years BP; and 3800 14C years BP; Burney (1987)) indicate that fire was present in the landscape, with charcoal morphology reflecting grassy fuel type. A new record from Razafimanantsoa et al. (2024) documents a similar trend at Lake Dangovavy, where grass pollen (>30%) persisted from 6000 calibrated years BP onward (Figure 2B). Thus, paleo evidence supports the idea that fires were present in these grassy ecosystems long before the onset of rapid anthropogenic habitat transformation around 2000 14C years BP (Figure 2). While human activity likely has intensified fire regimes and landscape openness in Madagascar's Central Highlands over the last two millennia, assuming that Malagasy grassy ecosystems were entirely forested before human settlement contradicts both empirical and theoretical studies of savanna–forest coexistence. Instead, these grassy ecosystems exhibit resilience to forest expansion across millennia, perhaps in part due to range pinning. These ancient ecosystems are worth our attention and care. Nikunj Goel conducted biome simulations. Julie C. Aleman collated paleo evidence. All authors wrote the manuscript. This work was supported by NSF DMS-1615531. The authors declare no conflicts of interest. Data and code (Goel, 2025) are available in Zenodo at https://doi.org/10.5281/zenodo.16385547.