Mortality of large trees and lianas following experimental drought in an Amazon Forest

Woods Hole Research Center, 149 Woods Hole Road, Falmouth, Massachusetts 02540-1644, USA.
Ecology (Impact Factor: 4.66). 09/2007; 88(9):2259-69. DOI: 10.1890/06-1046.1
Source: PubMed


Severe drought episodes such as those associated with El Niño Southern Oscillation (ENSO) events influence large areas of tropical forest and may become more frequent in the future. One of the most important forest responses to severe drought is tree mortality, which alters forest structure, composition, carbon content, and flammability, and which varies widely. This study tests the hypothesis that tree mortality increases abruptly during drought episodes when plant-available soil water (PAW) declines below a critical minimum threshold. It also examines the effect of tree size, plant life form (palm, liana, tree) and potential canopy position (understory, midcanopy, overstory) on drought-induced plant mortality. A severe, four-year drought episode was simulated by excluding 60% of incoming throughfall during each wet season using plastic panels installed in the understory of a 1-ha forest treatment plot, while a 1-ha control plot received normal rainfall. After 3.2 years, the treatment resulted in a 38% increase in mortality rates across all stems >2 cm dbh. Mortality rates increased 4.5-fold among large trees (>30 cm dbh) and twofold among medium trees (10-30 cm dbh) in response to the treatment, whereas the smallest stems were less responsive. Recruitment rates did not compensate for the elevated mortality of larger-diameter stems in the treatment plot. Overall, lianas proved more susceptible to drought-induced mortality than trees or palms, and potential overstory tree species were more vulnerable than midcanopy and understory species. Large stems contributed to 90% of the pretreatment live aboveground biomass in both plots. Large-tree mortality resulting from the treatment generated 3.4 times more dead biomass than the control plot. The dramatic mortality response suggests significant, adverse impacts on the global carbon cycle if climatic changes follow current trends.

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    • "The results of the remaining nine soil texture classes, with no significant relationships between precipitation levels and NPP, suggest that ∼41% of the terrestrial areas are less vulnerable to drought. Many publications have linked factors other than soil texture and soil drought tolerance in controlling forest species dominance, mortality rates and productivity (e.g., nutrient delivery capacity, soil chemistry, aluminum concentrations, rooting depth; e.g., Sanchez et al., 1982, Jha and Singh, 1990; Vogt et al., 1995; Palmiotto et al., 2004; Nepstad et al., 2007; Paoli et al., 2008; Quesada et al., 2012; Jiménez et al., 2014; Baldos et al., 2015). It would be worth expanding the database used in this study and to specifically search for other multiple variable(s) combinations to explain NPP levels. "
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    Environmental Research 11/2015; DOI:10.1016/j.envres.2015.10.022 · 4.37 Impact Factor
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    • "The resilience of Amazonian rainforests to changes in precipitation, temperature, and humidity over the basin still remains poorly understood: for example, satellitederived observations indicate that Amazon forests green up during droughts due to increased availability of sunlight (Saleska et al., 2007) that stimulates leaf flushing (Brando et al., 2010). However, this has been disputed REF, and forest inventory studies indicate increased tree mortality both during severe natural droughts (Phillips et al., 2009) and under long-term experimental droughts (Nepstad et al., 2007; Da Costa et al., 2010). Land-use change is also impacting on Amazonian ecosystems. "
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    • "Indeed, forests and woodlands of the Southwestern US have lost more than 20 % cover to drought-driven tree die-off events and wildfire (Williams et al. 2010). In the tropics, the Amazon forest carbon sink appears to be declining, attributed in part to increasing plant mortality (Brienen et al. 2015), with large trees experiencing higher mortality rates (Fisher et al. 2007; Nepstad et al. 2007; da Costa et al. 2010); this may be associated with drought related to tropical North Atlantic ocean anomalies (Aragão et al. 2007; Marengo et al. 2008; Phillips et al. 2009). However, increased mortality rates to date have not been at the scale of tree die-off there (i.e., loss of [30 % cover), except when associated with fire (Aragão et al. 2007). "
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