From agriculture back to tropical rainforest: When does ecological succession stabilize hot and dry extremes?

Forests are an important component of nature-based solutions for climate mitigation as trees both regulate local microclimate and store carbon. Forest restoration provides a critical link between the biodiversity and climate crises. This is especially important in the tropics where shifting agriculture is one of the biggest drivers of deforestation. However, an important knowledge gap remains how quickly ecological succession stabilizes understory microclimate and restores the buffering capacity of forests to reduce climatic extremes. In the Ecuadorian Chocó, we examined natural ecological succession after previous agricultural land use for pasture and cacao along a 40-year chronosequence, comparing the effects of increasing vegetation cover on microclimate to understory conditions in old-growth stands. In this wet lowland rainforest ecosystem, trees regulated hot and dry extremes during the day, whereas nighttime conditions did not vary with land use. In open agricultural areas, maximum temperatures (Tmax) decreased -4.6°C during the first 1-2 decades of succession, at a rate of -0.7 °C per 10% increase in canopy cover. Tmax was also -1.2 °C cooler in cacao plots relative to pasture, as small Theobroma trees helped to stabilize the understory microclimate. In later successional stages, Tmax decreased another -2.1 °C until full recovery at 30–40 years for cacao compared to estimated recovery of 50–80 years for pasture. Plant transpiration stabilized daytime relative humidity (RHmin) and vapor pressure deficit (VPDmax) at least a decade before Tmax, which was sensitive to high solar radiation and reduced cloud cover. In this system, cloud cover increased at higher elevation maintaining cool ridgetop microclimates important for threatened endemics. Although natural regeneration stabilized microclimate in the forest understory, our results indicate that if the cloud bank changes, agricultural conversion may increase the long-term vulnerability of tropical rainforests to climatic extremes.