Floodplains as an Achilles’ heel of Amazonian forest resilience

Abstract

The massive forests of central Amazonia are often considered relatively resilient against climatic variation, but this view is challenged by the wildfires invoked by recent droughts. The impact of such fires that spread from pervasive sources of ignition may reveal where forests are less likely to persist in a drier future. Here we combine field observations with remotely sensed information for the whole Amazon to show that the annually inundated lowland forests that run through the heart of the system may be trapped relatively easily into a fire-dominated savanna state. This lower forest resilience on floodplains is suggested by patterns of tree cover distribution across the basin, and supported by our field and remote sensing studies showing that floodplain fires have a stronger and longer-lasting impact on forest structure as well as soil fertility. Although floodplains cover only 14% of the Amazon basin, their fires can have substantial cascading effects because forests and peatlands may release large amounts of carbon, and wildfires can spread to adjacent uplands. Floodplains are thus an Achilles' heel of the Amazon system when it comes to the risk of large-scale climatedriven transitions.T he sensitivity of the Amazon rainforest to climate change is a central issue in global change research (1, 2). In particular, there are concerns that a drier climate may promote a shift from forest to savanna (3, 4). All studies so far suggest that this is most likely at the southern and eastern peripheral regions where precipitation is relatively low and seasonal (2-5) and the risk of wildfires is higher (4, 6-11). Although rainfall is a dominant factor explaining forest resilience, other environmental factors obviously play a role (12)(13)(14). Arguably, the most striking variation in the nature of forests in the wet Amazonian system is related to seasonal inundations. Approximately one-seventh of the Amazon is inundated a substantial part of the year (15), causing these ecosystems to differ in many ways from the dominant upland terra firme forests (SI Brief Ecology of Floodplain and Upland Ecosystems). Here we ask whether these differences related to seasonal inundation affect forest resilience and the risk of shifting into a fire-dominated savanna state. We used two approaches to contrast the resilience of floodplain and upland forests across the Amazon (Fig. 1 A and B). First, we estimated the long-term relative resilience of forest and savanna in both ecosystems from the density distributions of tree cover (3, 16) using MODIS vegetation continuous field (VCF) data at 250 m resolution (SI Methods). Second, we studied postfire recovery of both forest types using field and remote sensing data (Fig. S1). Using annual MODIS VCF data, we measured the recovery of over 250 sites that burned during the severe droughts of 1997 and 2005 ( Fig. S1; Table S1; and Dataset S1). Using field data on tree basal area and soil variables from multiple burned forests in the central Amazon region, we validated the basin-wide analyses of ...

Resilience of tropical forest and savanna: bridging theory and observation

“…In the western Amazon, the regional-scale level of photosynthetic activity requires multiple years to recover from extreme drought (Maeda et al, 2015). This is also a region where extensive floodplain forests are particularly vulnerable to fire, because they regenerate slowly or even remain under arrested succession once burnt (Flores et al, 2017). Because fire occurrence in the Amazon increases exponentially with dry-season rainfall deficit (Aragão et al, 2008), the drought-buffering capacity of the forest could greatly reduce the risk of such fires.…”

“…We calculated the tree cover growth rate r (yr -1 ) using time series of annual tree cover data to determine for each burnt cell the growth rate after a fire has occurred (n = 60724 burns with subsequent unburned tree cover recovery) (Flores et al, 2017) where we assumed logistic growth towards carrying capacity K(MAP) (Waring et al, 1981). We thus obtained a location-specific growth rate, but although the tree cover product that we use has already been successfully used to compute post-fire recovery (Flores et al, 2017) it contains considerable uncertainties, resulting in high variation in estimates of growth rates.…”

“…We thus obtained a location-specific growth rate, but although the tree cover product that we use has already been successfully used to compute post-fire recovery (Flores et al, 2017) it contains considerable uncertainties, resulting in high variation in estimates of growth rates. Therefore, we took the median growth rate (across all values of MAP) as growth rate r (yr -1 ) in the logistic growth function.…”

“…Also not accounted for is interception evaporation from tree canopies; however, this is mainly relevant in already wet periods rather than dry ones ( Van der Ent et al, 2014). Lastly, spatial heterogeneity in conditions may enhance the overall resilience of the Amazon rainforest by counteracting possible cascading forestsavanna transitions (Zemp et al, 2017a), but may also introduce weak spots that could act as nuclei from which such transitions could occur (Flores et al, 2017).…”

“…The factors driving the resilience of these ecosystems are manifold and complex. However, several recent theoretical (Dirmeyer & Brubaker, 1999;Van Nes et al, 2014;Schertzer et al, 2015;Lasslop et al, 2016) and empirical advances (Staver et al, 2011b;Hoffmann et al, 2012a;Murphy & Bowman, 2012;Dantas et al, 2016;Flores et al, 2017) support the hypothesis that firevegetation feedbacks provide one of the dominant mechanisms explaining the main patterns of tree-cover distributions across the tropics.…”