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

Abstract

There is a consensus about negative impacts of droughts in Amazonia. Yet, extreme wet episodes, which are becoming as severe and frequent as droughts, are overlooked and their impacts remain poorly understood. Moreover, drought reports are mostly based on forests over a deep water table (DWT), which may be particularly sensitive to dry conditions. Based on demographic responses of 30 abundant tree species over the past two decades, in this study we analyzed the impacts of severe droughts but also of concurrent extreme wet periods, and how topographic affiliation (to shallow-SWTs-or deep-DWTs-water tables), together with species functional traits, mediated climate effects on trees. Dry and wet extremes decreased growth and increased tree mortality, but interactions of these climatic anomalies had the highest and most positive impact, mitigating the simple negative effects. Despite being more drought-tolerant, species in DWT forests were more negatively affected than hydraulically vulnerable species in SWT forests. Interaction of wet-dry extremes and SWT depth modulated tree responses to climate, providing buffers to droughts in Amazonia. As extreme wet periods are projected to increase and at least 36% of the Amazon comprises SWT forests, our results highlight the importance of considering these factors in order to improve our knowledge about forest resilience to climate change.

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

“…Plants topographically closer to the water table converge to lower embolism resistance (higher P50), while plants on plateaus have higher and more variable embolism resistance (Oliveira et al ., 2019). In fact, species in valleys have higher rates of drought‐induced mortality in response to El‐Nino drought in Borneo (Itoh et al ., 2012) and in the Colombian Amazon (Zuleta et al ., 2017) but lower mortality in central Amazonian forests where the water table level is more stable (Esteban et al ., 2020). Hydraulic redistribution and shifts in plant water source from surface to deep soils in the dry season have been reported for tropical savannahs, dry forests and rainforests and can buffer tropical ecosystems from extended droughts (Oliveira et al ., 2005; Markewitz et al ., 2010; Broedel et al ., 2017).…”

Precipitation–productivity relationships and the duration of precipitation anomalies: An underappreciated dimension of climate change

“…Prominent contemporary examples include Australia's ~1997 to 2010 Millennium (Herberger, 2012) and 2017–2019 ‘Big Dry’ Droughts (De Kauwe et al, 2020), the California drought from ~2011 to 2015 (Goulden & Bales, 2019), and the ongoing megadrought in southwestern North America since the turn of the 21st century which, intriguingly, was preceded by the wettest 19‐year period in the last millennia for that region (Williams et al, 2020). Such prolonged wet periods seem to receive less attention than droughts but can also alter vegetation demography and NPP dynamics, from tropical forests (Esteben et al, 2020) to dryland ecosystems (Chen et al, 2017; Loik et al, 2004; Peters et al, 2012).…”

Delayed effects of climate on vital rates lead to demographic divergence in Amazonian forest fragments

“…Climate anomalies are known to have immediate effects on the growth, survival, or reproduction of plants (Esteban et al, 2021; Wright & Calderon, 2006), including Heliconia (Stiles, 1975; Westerband et al, 2017) and other tropical herbs (Wright, 1992). These effects can be complex or even contradictory—mild droughts can increase the growth rates of tropical trees and seedling survival, perhaps due to reductions in cloud cover and concomitant increases in solar radiation (Alfaro-Sánchez et al, 2017; Condit et al, 2004; Huete et al, 2006; Jones et al, 2014; Uriarte et al, 2018), but in severe drought years growth can be extremely low and mortality can be sharply elevated (Connell & Green, 2000; Edwards & Krockenberger, 2006; Engelbrecht et al, 2002).…”

“…While the demographic consequences of climate change for tropical species are expected to be similarly severe (Brodie et al, 2012; Scheffers et al, 2017), surprisingly little is known about the responses of these species to climatic variability (Paniw et al, 2021). Tropical plants may be particularly sensitive to climate change—they typically have narrow ranges of climatic tolerance (Feeley et al, 2012), and recent results suggest increases in the frequency and severity of extreme precipitation events reduce survival and reproduction (Esteban et al, 2021; Gaoue et al, 2019). This sensitivity to climatic fluctuations, coupled with evidence that plant growth and survivorship are lower in fragments (Bruna et al, 2002; Laurance et al, 1998; Zartman et al, 2015), has led to speculation that plants in forest fragments will be especially susceptible to climate change (Laurance et al, 2001; Opdam & Wascher, 2004; Selwood et al, 2015).…”