Palms and trees resist extreme drought in Amazon forests with shallow water tables

The intensity and frequency of severe droughts in the Amazon region have increased in the recent decades. These extreme events are associated with changes in forest dynamics, biomass and floristic composition. However, most studies of drought response have focused on upland forests with deep water tables, which may be especially sensitive to drought. Palms, which tend to dominate the less well-drained soils, have also been neglected. The relative neglect of shallow water tables and palms is a significant concern for our understanding of tropical drought impacts, especially as one-third of Amazon forests grow on shallow water tables (<5 m deep). We evaluated the drought response of palms and trees in forests distributed over a 600 km transect in central-southern Amazonia, where the landscape is dominated by shallow water table forests (SWTF). We compared vegetation dynamics before and following the 2015-2016 El Nino drought, the hottest and driest on record for the region (-214 mm of cumulative water deficit). We observed no change in stand mortality rates and no biomass loss in response to drought in these forests. Instead, we observed an increase in recruitment rates, which doubled to 6.78% year(-1) +/- 4.40 (M +/- SD) during 2015-2016 for palms and increased by half for trees (to 2.92% year(-1) +/- 1.21), compared to rates in the pre-El-Nino interval. Within these SWTF, mortality and recruitment rates varied as a function of climatic drought intensity and water table depth for both palms and trees, with mortality being greatest in climatically and hydrologically wetter environments and recruitment greatest in drier environments. Across our transect, there was a significant increase over time in tree biomass. Synthesis. Our results indicate that forests growing over shallow water tables-relatively under-studied vegetation that nonetheless occupies one-third of Amazon forests-are remarkably resistant to drought. These findings are consistent with the hypothesis that local hydrology and its interactions with climate strongly constrain forest drought effects, and has implications for climate change feedbacks. This work enhances our understanding of integrated drought effects on tropical forest dynamics and highlights the importance of incorporating neglected forest types into both the modelling of forest climate responses and into public decisions about priorities for conservation.