Ecosystem heterogeneity determines the ecological resilience of the Amazon to climate change

Abstract

Amazon forests, which store ∼50% of tropical forest carbon and play a vital role in global water, energy, and carbon cycling, are predicted to experience both longer and more intense dry seasons by the end of the 21st century. However, the climate sensitivity of this ecosystem remains uncertain: several studies have predicted large-scale dieback of the Amazon, whereas several more recent studies predict that the biome will remain largely intact. Combining remote-sensing and ground-based observations with a size-and age-structured terrestrial ecosystem model, we explore the sensitivity and ecological resilience of these forests to changes in climate. We demonstrate that water stress operating at the scale of individual plants, combined with spatial variation in soil texture, explains observed patterns of variation in ecosystem biomass, composition, and dynamics across the region, and strongly influences the ecosystem's resilience to changes in dry season length. Specifically, our analysis suggests that in contrast to existing predictions of either stability or catastrophic biomass loss, the Amazon forest's response to a drying regional climate is likely to be an immediate, graded, heterogeneous transition from high-biomass moist forests to transitional dry forests and woody savannah-like states. Fire, logging, and other anthropogenic disturbances may, however, exacerbate these climate change-induced ecosystem transitions.Amazon forests | biomass | ecological resilience | climate change | ecosystem heterogeneity A mazonia consists of 815 million ha of rainforest, transitional forest, and tropical savannahs; stores approximately half of tropical forest carbon (1); and plays a vital role in global water, energy, and carbon cycling (2). Although uncertainties in climate predictions for the region remain large (3), recent analyses imply that significant portions of the basin will experience both longer and more intense dry seasons by the end of the 21st century (3-6). There is particular concern about southern Amazonian forests that experience longer dry seasons than forests in central and western Amazonia (3) and where a trend of increasing dry season length (DSL) and intensity has already been observed (7). Despite the importance of this region for regional and global climate, the climate sensitivity of the Amazon forests remains uncertain: model predictions range from a large-scale die-back of the Amazon (8, 9) to predictions that the biome will remain largely intact, and may even increase in biomass (10-12). Although some of these differences can be attributed to differences in the predicted future climate forcing of the region (13, 14), accurate predictions of how changes in climate will affect Amazonian forests also rely on an accurate characterization of how the ecosystem is affected by a given change in climate forcing. In this study, we examine the climate sensitivity of the Amazon ecosystem, focusing on the mechanisms underpinning changes in forest dynamics and their implications for the timing and nature of...

Forests, atmospheric water and an uncertain future: the new biology of the global water cycle

“…Some also identify positive feedbacks and the possibility of switching states (e.g., Boers et al 2017;Wright et al 2017). Others suggest that more gradual transitions are possible (Levine et al 2016;Zemp et al 2017a). It is crucial to distinguish which theory or theories best conform to reality and the risk of climate switching.…”

Resilience changes in watershed systems: A new perspective to quantify long-term hydrological shifts under perturbations

“…At the global scale, watersheds are now suffering from severe perturbations due to both climate change and anthropogenic disturbances, and notable examples include the Amazon basin and the Mississippi River basin (Davidson et al, 2012;Kidder, 2006;Levine et al, 2016;Liu and Zheng, 2002). In the Amazon basin, which is the location of the world's largest rainforest, unprecedented deforestation has caused a series of changes at the local and regional scales that have involved alterations in energy and water cycles (Davidson et al, 2012).…”

“…As a representative of the capacity of a watershed to absorb and recover from perturbations or disturbances (Folke et al, 2010;Hoque et al, 2012;Randhir, 2014;Wilson and Browning, 2012), watershed resilience and associated transitions in watershed systems has becomes a critical topic in river basin conservation and management (Davidson et al, 2012). In order to understand the vulnerability and resilience of river basins in the face of change, numerous studies have strived to improve knowledge of the linkages between natural variability, drivers of change, ecosystem responses, and feedbacks within a watershed system or its subsystems; these studies have employed both tendency analysis of perturbations and watershed structure (Costa et al, 2003;Zhang et al, 2014a;Zhao et al, 2015a) and process-based model simulations (Coe et al, 2002(Coe et al, , 2011Hu et al, 2015;Levine et al, 2016). For example, Nemec et al (2014) calculated watershed resilience with nine watershed structure properties, i.e.…”

“…Hirota et al (2011) recognized forest, savanna, and treeless state as three distinct attractors and empirically reconstructed the basin of attraction through analyzing the response of the global tree abundance pattern to precipitation. Levine et al (2016) predicted the stability of the Amazon rainforest to climate change with coupled vegetation-climate models. Furthermore, Cosens and Williams (2012) analyzed resilience mechanisms in the Amudarya River basin through an agent-based model.…”

Resilience of tropical forest and savanna: bridging theory and observation

“…Examples of such heterogeneity include local differences in water availability, differential responses of species to water stress and differences in soil properties (Levine et al, 2016). Examples include gallery forests, which occur in savanna landscapes where local water availability is high.…”