Understanding forests' response to drought allows a better prediction of how resilient they will be in an accelerated climate change scenario. There seems to be a consensus that decreasing tree growth and survival are major and pervasive drought negative impacts in forests. In Amazonia, almost 50% of forest grows on shallow water tables (WT), which could act as a direct water source for plants during droughts, modulating the whole ecosystem dynamics and responses. However, Amazonian drought reports come mainly from deep WT forests, implying that our knowledge of how the Amazon responds to drought is incomplete.
We hypothesize that shallow WT acts as a buffer of drought negative effects commonly reported in deep WT areas, modulating Amazonian forest dynamics and responses to drought.
We calculated tree mortality and recruitment rates and variation in tree diameter increment and aboveground biomass (productivity) based on long-term inventory plots distributed across Amazonia, covering major drought events in last decades and comprising a high range of soil-hydrological conditions (from shallow to deep WT forests). WT depths for each plot were obtained from field measurements or estimated from large-scale hydrological models or remote-sensing products.
While in non-drought periods shallow WT forests show higher mortality rates and ~18% lower aboveground biomass productivity than deep WT forests, this pattern was reversed during droughts. Increased drought severity over the last two decades did not lead to decreasing tree growth nor increasing mortality over shallow WT forests. On the other hand, in deep WT forests trees decreased growth up to half and increased mortality up to six times when compared to non-drought periods. Evaluating forest responses to the severe 2015–2016 drought, shallow WT forests did not show changes in mortality rates neither lose biomass, but tree recruitment rates increased by half compared to pre-drought rates. Moreover, using monthly growth rate observations, trees in shallow WT forests maintained more constant growth during drought than trees in deeper WT forests, which had 53.9% reduction of monthly increment and 66.5% reduction of the annual stem increment during that drought period.
Groundwater in shallow WT counteracts drought negative effects on forest dynamics and productivity. These findings highlight the need of better understanding shallow WT forest responses to drought, with the large portion of the Amazon covered by shallow WT indicating a large potential hydrological refugia and implying that previous projections of drought impacts may be overestimated.
Amazonia, Biomass, Drought, Forest dynamics, Hydrological refugia, Productivity, Water table