African forests advancing faster than scientists thought, study shows
Trees are conquering African savannahs at speeds five to ten times faster than previously documented, according to new satellite analysis from Cameroon. The finding challenges assumptions about tropical land use and has major implications for carbon accounting, agriculture planning, and conservation strategies across the continent.
Originaltitel: Savannah–forest dynamics: encroachment speed, model inference and spatial simulations
Skogsövertagandet av savannen accelererar snabbare än tidigare antagits. Forskare från Sveriges Lantbruksuniversitet kalibrerade en reaktions-diffusionsmodell med satellitdata från Kamerun och måtte skogsfrontens framryckningshastighet till 5–7 meter per år — två till tio gånger högre än tidigare uppskattat. Studien använder 50 års Landsat-bilder från Mpem och Djim nationalpark för att kvantifiera växtwöde sprids genom landskapet. Modellen simulerar växelspelet mellan gräs- och vedartad biomassa och ger första gången uppskattningar av trädens dispersionskoefficienter. Resultaten är centrala för klimat- och markanvändningsplanering i tropikalt Afrika: snabbare skogsexpansion förändrar koldioxidlagring, vattenbalans och marknyttningskonflikt. För investerare och infrastrukturplanerare blir det kritiskt att uppdatera prognoser för vegetationsdynamik i savannsystemet under kommande två decennier.
Abstract Forest encroachment over savannahs has been recurrently reported in the tropics over the last few decades, especially in northern tropical Africa. However, process-based, spatially explicit modelling of the phenomenon is still trailing broad-scale empirical observations. In this paper, we use remotely sensed diachronic data from Central Cameroon to calibrate a simple reaction–diffusion model, embodying dynamical interactions between grass and woody biomasses in the savannah biome. Landsat satellite image series over the Mpem and Djim National Park (MDNP) (Cameroon) witnessed a dramatic extension of forest over the last five decades, and our estimates of forest front speeds based on randomly sampled transects indeed yield higher values (5–7 m yr-1) than in the existing literature (0.5–2 m yr-1). We use model simulations to provide the first estimates of woody biomass dispersal coefficients. Since the study region did not provide examples of savannah progression, estimates of grass dispersal proved inconsistent and we reverted to literature-based historical data to reach rough estimates. We demonstrate that broad-scale remote sensing data allows for calibrating simple reaction–diffusion models of vegetation dynamics in the savannah biome. Such calibrated models become a general baseline of expected changes and a valuable tool to understand how spatial environmental factors may locally modulate the overall dynamics.