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 ...