Rooting for forest resilience : Implications of climate and land-use change on the tropical rainforests

Abstract: Tropical rainforests in the Amazon and Congo River basins and their climate are mutually dependent. Evaporation from these forests help regulate the regional and global water cycle. Furthermore, these rainforests themselves depend on precipitation to sustain their structure and functions. However, the rapid increase in human activities (such as burning fossil fuels and deforestation) has significantly changed the rainforests’ climate. Due to the effect of human-induced perturbations on moisture feedbacks (i.e., precipitation and evaporation patterns), these rainforests risk tipping to a savanna or treeless state.Understanding how these forests respond to climate change will aid in assessing their resilience to water-induced perturbations as well as in anticipating and preparing for potential tipping risks in the future. However, our understanding of how vegetation responds to climate change is fragmented, which limits our capacity to predict these risks. Previous studies have primarily relied on precipitation data to understand these forest-to-savanna transitions. However, ecosystem transition risks are also associated with water-stress, which depends on the vegetation’s capacity to adapt to drier conditions by storing water in its root zone. This thesis investigates the effect of hydroclimatic changes on root zone adaptation and its implications for forest resilience.Paper I uses remote sensing data to analyse water-stress and drought coping strategies across the rainforest-savanna transects. Paper II uses the root zone storage capacity to quantify the resilience of forest ecosystems. Using the empirical understanding of root zone forest dynamics and hydroclimatic estimates from Earth System Models, Paper III projects future forest transitions and estimates tipping risks by the end of the 21st century under four different shared socio-economic pathways. Paper IV uses atmospheric moisture tracking data to investigate the leverage landholders in South America have over precipitation and the resilience of forest ecosystems. Papers I and II reveal the non-linear relationship between the ecosystem’s above-ground structure and root zone storage capacity. These studies indicate that, under hydroclimatic changes, the ecosystem’s root zone storage capacity is much more dynamic than its above-ground forest structure and is more representative of the ecosystem’s transient state than precipitation. Ignoring this root zone adaptive capacity can underestimate forest resilience, primarily observed in the Congo rainforest. Paper III projects that the risk of forest-savanna transition will increase with climate change severity, most prominently observed in the Amazon rainforest. Paper IV finds that all landholders have equal leverage over the moisture precipitating locally and over farther-downwind land systems. According to this study, smallholders have a disproportionately larger influence over forest rainfall. However, large landholders have a larger influence on forest resilience as well as over the moisture precipitating on croplands and pastures. These results warrant the need for policies to factor in the impact of deforestation on downwind actors and promote effective ecosystem stewardship. The insights from this thesis highlight the importance of understanding and assessing ecosystem dynamics under a rapidly changing climate for strengthening management and conservation efforts across the globe.