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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Available from: Paulo Moutinho, Sep 30, 2015
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    • "This shift is associated with the contrasting vulnerability of tropical species to drought: for certain taxa, or size classes, or growth characteristics (fast vs. slow growing ), or successional status (early vs. late stage), some species are subject to disproportionate mortality. Through-fall exclusion experiments have also revealed a strong variability among tree species, or tree genii, in their mortality rates (da Costa et al. 2010; Nepstad et al. 2007 "
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    ABSTRACT: Key message We review the recent findings on the influence of drought on tree mortality, growth or ecosystem functioning in tropical rainforests. Drought plays a major role in shaping tropical rainforests and the response mechanisms are highly diverse and complex. The numerous gaps identified here require the international scientific community to combine efforts in order to conduct comprehensive studies in tropical rainforests on the three continents. These results are essential to simulate the future of these ecosystems under diverse climate scenarios and to predict the future of the global earth carbon balance. & Context Tropical rainforest ecosystems are characterized by high annual rainfall. Nevertheless, rainfall regularly fluctuates during the year and seasonal soil droughts do occur. Over the past decades, a number of extreme droughts have hit tropical rainforests, not only in Amazonia but also in Asia and Africa. The influence of drought events on tree mortality and growth or on ecosystem functioning (carbon and water fluxes) in tropical rainforest ecosystems has been studied intensively, but the response mechanisms are complex. & Aims Herein, we review the recent findings related to the response of tropical forest ecosystems to seasonal and extreme droughts and the current knowledge about the future of these ecosystems. & Results This review emphasizes the progress made over recent years and the importance of the studies conducted under extreme drought conditions or in through-fall exclusion experiments in understanding the response of these ecosystems. It also points to the great diversity and complexity of the response of tropical rainforest ecosystems to drought. & Conclusion The numerous gaps identified here require the international scientific community to combine efforts in order to conduct comprehensive studies in tropical forest regions. These results are essential to simulate the future of these ecosystems under diverse climate scenarios and to predict the future of the global earth carbon balance.
    Annals of Forest Science 10/2015; DOI:10.1007/s13595-015-0522-5 · 1.98 Impact Factor
    • "Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | cidence of drought are increasing (Allen et al., 2010; Dai et al., 2013, but see Sheffield et al., 2012). Drought and any ensuing vegetation mortality events have the potential to change land ecosystems from a sink to source (Lewis et al., 2011), and the dominant mechanisms governing the ecosystem responses to drought can vary from reducing stomatal conductance (Xu and Baldocchi, 2003) to increasing tree mortality (Lewis 5 et al., 2010) and changing community species composition (Nepstad et al., 2007). Our ability to model drought effect on vegetation function is currently limited (Galbraith et al., 2010; Egea et al., 2011; Powell et al., 2013). "
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    ABSTRACT: Future climate change has the potential to increase drought in many regions of the globe, making it essential that land surface models (LSMs) used in coupled climate models, realistically capture the drought responses of vegetation. Recent data syntheses show that drought sensitivity varies considerably among plants from different climate zones, but state-of-the-art LSMs currently assume the same drought sensitivity for all vegetation. We tested whether variable drought sensitivities are needed to explain the observed large-scale patterns of drought impact. We implemented data-driven drought sensitivities in the Community Atmosphere Biosphere Land Exchange (CABLE) LSM and evaluated alternative sensitivities across a latitudinal gradient in Europe during the 2003 heatwave. The model predicted an overly abrupt onset of drought unless average soil water potential was calculated with dynamic weighting across soil layers. We found that high drought sensitivity at the northernmost sites, and low drought sensitivity at the southernmost sites, was necessary to accurately model responses during drought. Our results indicate that LSMs will over-estimate drought impacts in drier climates unless different sensitivity of vegetation to drought is taken into account.
    Biogeosciences Discussions 08/2015; 12(15):12349-12393. DOI:10.5194/bgd-12-12349-2015
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    • "In tropical rainforest, where species diversity is high and drought-sensitivity has been shown to vary with taxonomic identity (da Costa et al., 2010, Nepstad et al., 2007), there is likely to be a range of photosynthetic and respiratory acclimation responses to drought in any forest stand (Stahl et al., 2013). Understanding these processes and their possible acclimation in the context of long-term drought in tropical rainforests is key to constraining the variability which currently exists in model predictions of such processes in future climate change scenarios (Galbraith et al., 2010, Huntingford et al., 2013, Powell et al., 2013, Rowland et al., 2015), or in more conceptual models (Meir et al., 2015a). "
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    ABSTRACT: Determining climate change feedbacks from tropical rainforests requires an understanding of how carbon gain through photosynthesis and loss through respiration will be altered. One of the key changes that tropical rainforests may experience under future climate change scenarios is reduced soil moisture availability. In this study we examine if and how both leaf photosynthesis and leaf dark respiration acclimate following more than 12 years of experimental soil moisture deficit, via a through-fall exclusion experiment (TFE) in an eastern Amazonian rainforest. We find that experimentally drought-stressed trees and taxa maintain the same maximum leaf photosynthetic capacity as trees in corresponding control forest, independent of their susceptibility to drought-induced mortality. We hypothesise that photosynthetic capacity is maintained across all treatments and taxa to take advantage of short-lived periods of high moisture availability, when stomatal conductance (gs ) and photosynthesis can increase rapidly, potentially compensating for reduced assimilate supply at other times. Average leaf dark respiration (Rd ) was elevated in the TFE-treated forest trees relative to the control by 28.2±2.8% (mean ± one standard error). This mean Rd value was dominated by a 48.5±3.6% increase in the Rd of drought-sensitive taxa, and likely reflects the need for additional metabolic support required for stress-related repair, and hydraulic or osmotic maintenance processes. Following soil moisture deficit that is maintained for several years, our data suggest that changes in respiration drive greater shifts in the canopy carbon balance, than changes in photosynthetic capacity. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.
    Global Change Biology 07/2015; DOI:10.1111/gcb.13035 · 8.04 Impact Factor
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