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

Abstract

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.

The other side of droughts: wet extremes and topography as buffers of negative drought effects in an Amazonian forest

“…Moreover, there has been a significant rainfall increase over the past four decades in most of the northern Peruvian and Brazilian regions, and there are projected increases in both the frequency and extent of extreme wet periods for the coming decades (Gloor et al ., 2013; Duffy et al ., 2015; Espinoza et al ., 2019). This study, the first to our knowledge showing buffers of drought effects on Amazonian tree species along a high range of hydrological and topographic conditions (also see Sousa et al ., 2020 for a region dominated by SWT forests), highlights the importance of considering both extreme wetness episodes and hydro‐topographic influences when evaluating drought impacts. These have relevant consequences for understanding and predicting the impact of current and future climate: first, the groundwater memory effect can potentially rescue Amazon forests from extreme dry conditions when droughts are preceded by extreme wet periods; second, the large portion covered by SWT forests indicates large potential hydrological refugia.…”

“…Notwithstanding that, tree species in these environments actually suffered less than those in DWT forests during extreme drought conditions (as in 2015–16). These differences in tree responses along the hydrological gradient must be linked to the buffering capacity of the SWT levels on lowlands, which may function in drought conditions as hydrological refugia, that is, locations on the landscape that support populations of a species while the surrounding climatic conditions become unsuitable for that species (McLaughlin et al ., 2017; Sousa et al ., 2020). In valleys, the water table level is shallow and can supply moisture to roots year‐round, even in drought years (Hodnett et al ., 1997; Cuartas, 2008).…”

Linking plant hydraulics and the fast–slow continuum to understand resilience to drought in tropical ecosystems

“…Furthermore, landscape features and community level properties (e.g. root depth diversity) likely interact, increasing the spatial and temporal heterogeneity of water availability and allowing the coexistence of divergent hydraulic strategies (Lima et al ., 2018; Sousa et al ., 2020).…”

“…Furthermore, landscape features and community level properties (e.g. root depth diversity) likely interact, increasing the spatial and temporal heterogeneity of water availability and allowing the coexistence of divergent hydraulic strategies (Lima et al, 2018;Sousa et al, 2020). Deciduousness imposes a major energetic drawback to plants, because it restricts photosynthesis to the period when plants retain their active crown.…”

Increasing aridity will not offset CO2 fertilization in fast‐growing eucalypts with access to deep soil water

“…Recent findings suggest that when trees are able to access shallow water tables, even extreme sporadic drought has little impact on tree growth (Sousa et al, 2020). Nevertheless, in the long term, deep soil water availability is tightly linked to precipitation patterns over land (Berg et al, 2017).…”