Reframing tropical savannization: linking changes in canopy structure to energy balance alterations that impact climate

Abstract

Tropical ecosystems are undergoing unprecedented rates of degradation from deforestation, fire, and drought disturbances. The collective effects of these disturbances threaten to shift large portions of tropical ecosystems such as Amazon forests into savanna‐like structure via tree loss, functional changes, and the emergence of fire (savannization). Changes from forest states to a more open savanna‐like structure can affect local microclimates, surface energy fluxes, and biosphere–atmosphere interactions. A predominant type of ecosystem state change is the loss of tree cover and structural complexity in disturbed forest. Although important advances have been made contrasting energy fluxes between historically distinct old‐growth forest and savanna systems, the emergence of secondary forests and savanna‐like ecosystems necessitates a reframing to consider gradients of tree structure that span forest to savanna‐like states at multiple scales. In this Innovative Viewpoint, we draw from the literature on forest–grassland continua to develop a framework to assess the consequences of tropical forest degradation on surface energy fluxes and canopy structure. We illustrate this framework for forest sites with contrasting canopy structure that ranges from simple, open, and savanna‐like to complex and closed, representative of tropical wet forest, within two climatically distinct regions in the Amazon. Using a recently developed rapid field assessment approach, we quantify differences in cover, leaf area vertical profiles, surface roughness, albedo, and energy balance partitioning between adjacent sites and compare canopy structure with adjacent old‐growth forest; more structurally simple forests displayed lower net radiation. To address forest–atmosphere feedback, we also consider the effects of canopy structure change on susceptibility to additional future disturbance. We illustrate a converse transition—recovery in structure following disturbance—measuring forest canopy structure 10 yr after the imposition of a 5‐yr drought in the ground‐breaking Seca Floresta experiment. Our approach strategically enables rapid characterization of surface properties relevant to vegetation models following degradation, and advances links between surface properties and canopy structure variables, increasingly available from remote sensing. Concluding, we hypothesize that understanding surface energy balance and microclimate change across degraded tropical forest states not only reveals critical atmospheric forcing, but also critical local‐scale feedbacks from forest sensitivity to additional climate‐linked disturbance.

Drought-driven wildfire impacts on structure and dynamics in a wet Central Amazonian forest

“…Continued monitoring of these plots in regular intervals (annual or biannual) is important to improve our understanding of the related carbon fluxes (emissions and uptake) [ 58 ], the recovery time to pre-fire states [ 20 ] and/or eventual disruption of carbon dynamics by tree mortality (e.g. caused by additional drought and fire events) [ 59 ].…”

“…Prior studies have already shown the effects of these variables on post-fire tree mortality in tropical forests [ 21 , 22 ]; however, our results quantitatively characterize how trees with certain resilient plant traits (greater sizes and WD) still may not be able to survive under high-intensity fires. Since fire intensity is linked to drought severity [ 9 ], forest fire impact and extent in the less seasonal Amazon may increase in the future with widespread anthropogenic disturbances degrading forest canopies and creating hotter drier understorey conditions and with more widespread and intense droughts forecasted as a result of climate change [ 2 , 59 ].…”

Vegetation‐type conversion of evergreen chaparral shrublands to savannahs dominated by exotic annual herbs: causes and consequences for ecosystem function

“…In terrestrial ecosystems, such shifts occur when the dominant vegetation type is replaced by a different functional type. Prominent examples include the conversion of tropical forests to savannahs and grasslands after being burned and cleared (Stark et al, 2020) and the invasion of grassland ecosystems by woody plants, creating a savannah ecosystem (Breshears, 2006). These changes imply altered ecosystem function and services (Folke et al, 2004), and if the new vegetation type is persistent, then it represents an alternative stable state (Suding et al, 2004).…”

Effects of consecutive dry and wet days on the forest–savanna boundary in north‐west South America

“…The transition between humid tropical forest and savanna, one of the most striking ecotones on Earth, is currently subject to multiple drivers of change, including increased climate variability, rising atmospheric CO 2 and land‐use change (Aleman & Fayolle, 2020; Oliveras & Malhi, 2016 and references therein). Changes in this transition have important implications for land–atmosphere interactions through its effects on biogeochemical cycles, energy budget and hydrology (Bond, 2008; Stark et al., 2020). The existence of this transition has been attributed to multiple abiotic and biotic factors, such as the amount and seasonality of precipitation, fire regimes, herbivory pressure, soil properties, species adaptations and competitive strategies, and human pressure (Aleman et al., 2020; Bond, 2008; D’Onofrio et al., 2019; Hirota et al., 2011; Hoffmann et al., 2012; Langan et al., 2017; Murphy & Bowman, 2012; Oliveras & Malhi, 2016; Staal et al., 2016; Staver et al., 2011; Wuyts et al., 2017; Xu et al., 2018).…”

“…The existence of this transition has been attributed to multiple abiotic and biotic factors, such as the amount and seasonality of precipitation, fire regimes, herbivory pressure, soil properties, species adaptations and competitive strategies, and human pressure (Aleman et al., 2020; Bond, 2008; D’Onofrio et al., 2019; Hirota et al., 2011; Hoffmann et al., 2012; Langan et al., 2017; Murphy & Bowman, 2012; Oliveras & Malhi, 2016; Staal et al., 2016; Staver et al., 2011; Wuyts et al., 2017; Xu et al., 2018). There is typically an array of abiotic factors that determine where humid forest or savanna can exist, constrained mostly by water limitations and fire (Oliveras & Malhi, 2016; Scheffer et al., 2001; Stark et al., 2020). Forests predominate in conditions of high annual rainfall and a shorter dry season, whereas savannas prevail with lower annual rainfall, a longer dry season and frequent fire occurrence (Hirota et al., 2011; Staver et al., 2011).…”