Five decades of vegetation dynamics and fire-mediated alternative stable states across forest-savanna boundaries in northern Cambodia

Seasonally dry tropical forests and mesic savannas form complex mosaic landscapes throughout mainland Southeast Asia. In these mosaics, forests and savannas are separated by abrupt boundaries, leading to questions regarding the long-term stability of ecosystems and the processes that maintain their distributions. Because these ecosystems are threatened by deforestation, agriculture, and changing fire regimes, understanding the drivers that maintain vegetation distributions in these landscapes is necessary for their conservation. To investigate vegetation dynamics, we compared historical (1976) and contemporary (2024) satellite imagery from two forest-savanna mosaics in northern Cambodia that have undergone contrasting fire management. We quantified changes in forest and savanna extent and shifts in boundary position over time. To assess the potential role of edaphic factors in structuring forest and savanna distributions, we collected soil samples along six transects perpendicular to forest-savanna boundaries across three landscapes. Transects extended 100 m into each vegetation type and were analyzed for soil texture, nutrient availability, carbon content, stable carbon isotopes, and macro-charcoal. Both landscapes faced drastically different anthropogenic pressures, with widespread deforestation and small-scale agriculture at one site and fire suppression at the other. Still, both landscapes showed remarkable stability in terms of the spatial distribution of forest and savanna and the position of the vegetation boundaries. Forest encroachment was more common at the site with fire suppression while savanna encroachment was more common at the site with widespread deforestation. Soil characteristics showed no consistent differences across the vegetation boundaries within or between sites, suggesting that soil texture and nutrients do not constrain distributions of forest and savanna. Charcoal was present in most soils in the forest and savanna, indicating that fires may cross the forest-savanna boundary. Our findings support the interpretation that, in Southeast Asia, forests and savannas behave as alternative stable states maintained by fire vegetation feedbacks. We found no consistent evidence that soils constrain vegetation distributions. Meanwhile, our findings highlight the important role of fire in structuring vegetation distributions within mosaics.