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Novel tropical forests: response to global change

2017; Wiley; Volume: 213; Issue: 3 Linguagem: Inglês

10.1111/nph.14407

1469-8137

Jennifer A. Holm, Lara M. Kueppers, Jeffrey Q. Chambers,

Ecology and Vegetation Dynamics Studies

Warming climates in the twenty-first century are expected to push tropical ecosystems, which currently reside at the warm and wet edge of bioclimatic life zones, into novel states that have no analog on Earth today (Fig. 1). Shifting precipitation patterns will expose some tropical forest regions to more frequent drought conditions that may reduce carbon (C) assimilation and lead to vegetation dieback, and all regions will experience higher temperatures never encountered by extant taxa. At the same time, direct anthropogenic disturbances, such as the hunting of large seed dispersers, use of fire for land management, deforestation, and agricultural land abandonment, are affecting the capacity of forests to mitigate climate change. '… long-term mortality rates and disturbance events are increasing in tropical forests; therefore, being able to capture disturbances in vegetation modeling is a top priority.' Currently, intact tropical forests are estimated to be a major sink for atmospheric CO2, globally accounting for up to 50% of terrestrial C uptake (Baker et al., 2004a; Lewis et al., 2009; Pan et al., 2011), yet the sink strength appears to be weakening (Brienen et al., 2015). Uncertainties in tropical forest responses to climate and land-use change, nutrient limitations to CO2 fertilization, and secondary forest development contribute to the difficulty in predicting the stability of this C sink and C-climate feedbacks over the coming centuries. In addition, because most ecological models are parameterized and structurally based on modern observations, they may fail to predict the ecological responses to novel climates that could contribute further uncertainty to tropical forest projections. A number of research groups are pursuing key research priorities needed to improve the representation of tropical ecosystems in models and decrease the uncertainties in representing future no-analog communities. To highlight recent advances in addressing these uncertainties and build stronger collaborations, an organized oral session was held at the 2016 Ecological Society of America meeting 'Novel ecosystems in the Anthropocene', which was held in Fort Lauderdale, FL, USA. This individual session, entitled 'Novel tropical ecosystems: response to global change', brought together experimental/process scientists and modeling scientists with a shared interest in advancing the understanding of novel states in tropical ecosystems. Combining knowledge from observations and modeling can be a strong tool for improving our predictive understanding of tropical ecosystems and climate feedbacks, while informing the development of next-generation modeling science. This session was motivated, in part, by the newly developed Next-Generation Ecosystem Experiment – Tropics (NGEE-Tropics), a 10-year project aimed at improving Earth System Model (ESM) projections by developing a better process representation of tropical forest ecosystem responses to a changing atmosphere and warming climate. Key research themes included tropical forest responses to drought, responses to rising temperatures (see Cavaleri et al., 2015), species migration and compositional shifts, and advances in modeling tree mortality and transient dynamics during post-disturbance vegetation recovery. Tropical forests have diverse responses to increasing temperatures. Leaf-level processes include the acclimation of photosynthesis and shifts in the allocation of resources to biogenic volatile organic compounds (BVOCs) as a response to heat stress. For example, Jardine et al. (2014) reported that maximum net photosynthesis (Pnmax) of a tropical mango tree was achieved between 30.0 and 32.5°C, but the production of other compounds such as isoprene, which may act as an antioxidant, continued to increase with still higher temperatures, becoming markedly uncoupled from photosynthesis. This is an issue of concern because a widely used land surface model (Community Land Model, CLM) failed to reproduce leaf temperatures >33°C for a central Amazon forest, when temperatures in these forests can currently reach up to 45°C (Holm et al., 2014b). Jeff Chambers (Lawrence Berkeley National Laboratory) highlighted the increasing evidence that isoprene and other BVOCs protect tropical trees by scavenging reactive oxygen species (ROS), thereby reducing tissue damage and ultimately tree mortality under high temperature stress. As temperatures rise and the stress becomes more severe, BVOC emissions change and may provide a chemical fingerprint of the underlying biochemical and physiological processes that signal elevated stress (Jardine et al., 2015). The primary approaches to studying tropical forest responses to increased temperature to date have been growth chamber and glasshouse studies; however, Molly Cavaleri (Michigan Technological University) reported on a new in situ warming experiment in Puerto Rico, which is in a higher precipitation and temperature regime than any previous warming experiment. It focuses on warming individual biologically active components, such as roots, soil, and canopy leaves, and addresses uncertainties in specific plant processes, such as the acclimation of photosynthesis. Pre-treatment data have been collected to determine the optimum temperature of net photosynthesis, foliar dark respiration, and root respiration. If successful, this process-focused approach may provide a valuable alternative to whole-ecosystem temperature manipulations for tropical forests. Scientific advances in our understanding of the impact of drought on tropical forests are ongoing, with major knowledge gaps ranging from plant physiological or soil biogeochemical responses to patterns of tree mortality. Basic questions still persist, such as (1) which systems are most vulnerable to drought, (2) which plant functional traits enhance or limit drought resistance, and (3) can models effectively simulate drought-induced tree mortality? There is increasing evidence that large trees will suffer more from drought (Nepstad et al., 2007; Bennett et al., 2015), but is this true of all tropical locations, and can models accurately simulate the behavior of large trees? The Ecosystem Demography model version 2 (ED2), which represents mechanisms of drought-induced mortality (Xu et al., 2016), was used to simulate mortality rates related to a 2015 El Niño/Southern Oscillation (ENSO) drought, with promising results. The capacity of models to predict vegetation response to drought still needs improvement though (Powell et al., 2013; Medlyn et al., 2016), as there are spatial and magnitude discrepancies between observations and model predictions of the drought response (Fig. 2). Similarly, recent in situ monitoring in the Puerto Rico moist forest by Christine O'Connell and Whendee Silver (University of California Berkeley) revealed changes in soil moisture, soil oxygen, inorganic phosphorus, and methane before, during, and following a 2015 drought, with lags in the return to pre-drought conditions. Topographic position exerted strong controls on the drought response, and biogeochemical thresholds contributed to the complex dynamics of the system. In addition to previous reports (Saleska et al., 2007), Scott Saleska (University of Arizona) described how the Amazon 'green-up' phenomenon also occurred during the 2010 Amazonian drought (as recorded by eddy flux measurements) due to an increased photosynthetic capacity, but was followed by an increase in vegetation mortality. This highlights the interesting phenomenon that both green-up and mortality can be responses to drought, mechanisms that are not clearly represented in the current models. The reorganization of forest species composition and geographic distribution takes a longer time than the immediate physiological response of the ecosystem to climate variation. Yet, studies have found that climate change is already driving species migrations and shifts to higher elevations (Chen et al., 2011); however, only two studies in this synthesis were from tropical locations, and they did not include tropical trees. Kenneth Feeley (Florida International University) hypothesized that increasing temperatures will lead to increasing relative abundances of lowland tropical species at higher elevations, which are becoming warmer, resulting in community thermophilization (Feeley et al., 2011; Duque et al., 2015). This shift would be driven by a greater mortality of high-elevation, cold-loving species suffering from greater heat stress, which raises the concern that species may not have sufficient time to adapt or migrate with climate change, resulting in their extinction. For many lowland tropical tree species, shifts in geographic range (as opposed to a change in relative abundance) are dependent on seed transport by animals, but terrestrial mammals can also act as biotic barriers via seed predation, counteracting the uphill migration of some tropical plants as described by Erin Kuprewicz (University of Connecticut; Kuprewicz, 2015). There is currently little data on the rate of this process for tropical trees, which is not represented in current large-scale demographic models. Most ecosystem theoretical models have focused on long-term forest dynamics, such as coexistence (Kohyama, 1992) or equilibrium abundances (Strigul et al., 2008). Trevor Caughlin (University of Florida) demonstrated how a better understanding of near-term dynamics could improve our predictions of how tropical forests and reforestation will respond to anthropogenic changes during the next several decades, with direct implications for C storage and biodiversity conservation (Chazdon et al., 2016). A recently developed forest-dynamics model for early succession demonstrates the value of a theoretical model for near-term dynamics (Caughlin et al., 2016). The analytically-tractable model can be solved to understand how tree canopy closure, a critical turning point during secondary succession, depends on demographic parameters, such as growth, survival and seed rain. Beyond a threshold value of seed rain necessary to guarantee tree canopy closure, a wide range of time-to-canopy-closure is possible, ranging from > 50 years to < 10 years. This 40-year difference in forest recovery rate has major implications for C sequestration and restoration design. There is mounting evidence that long-term mortality rates and disturbance events are increasing in tropical forests (Phillips et al., 2004; Malhi et al., 2008; Brienen et al., 2015); therefore, being able to capture disturbances in vegetation modeling is a top priority. A study by Holm et al. (2014a) used a tropical individual-based demographic model to double the tree mortality rate in the central Amazon to match that of the north-western Amazon and found that the resulting 42% decrease in aboveground biomass was due to a decrease in basal area. A synthesis of inventory plots did not find a significant difference in basal area across the Amazon Basin, but rather that differences in wood density and floristic composition contribute to the variation in biomass (Baker et al., 2004b; ter Steege et al., 2006). More recent results (Brienen et al., 2015) using a larger dataset show a significant decrease in the net basal area over c.25 years across the Amazon Basin, and indicate that the net biomass change over time is related to the basal area and change in stem numbers, but not wood density. Understanding shifts in demographic rates, including from increasing disturbance, and their ecological consequences should be regarded as a priority for projecting near- and long-term C accumulation. A 2012 Department of Energy (DOE) workshop report, 'Research priorities for tropical ecosystems under climate change', laid out high-level research priorities for understanding and projecting tropical forest responses to global change (USDOE, 2012). Some priorities have been further refined and are beginning to be addressed, but others have not yet seen much progress. A key theme that emerged was the vulnerability and response of tropical ecosystems to drought. The majority of drought-related inventory datasets for the tropics have come from throughfall exclusion experiments (Meir et al., 2008); however, measurements from the 2015–2016 ENSO event will provide valuable new data from throughout the tropics. Another opportunity is the remote-sensing observations of forest reflectance, and therefore vigor, during seasonal or episodic droughts (Asner et al., 2004; Doughty & Goulden, 2008; Fig. 2a), although repeated studies are lacking. Finally, there have been important advances in the large-scale model representation of physiological water stress and drought-induced mortality (Powell et al., 2013; Christoffersen et al., 2016; Xu et al., 2016) relative to just a few years ago (McDowell et al., 2011), yet additional testing and refinement is required. While not all critical topics were addressed in this organized oral session (e.g. nutrient limitation, belowground plant processes, and land-use change were not well-represented), the presenters further elevated key priorities. In order to understand how tropical forests will respond to the novel conditions of the Anthropocene and to accurately represent the complexity of tropical systems in large-scale models, collaboration is required among tropical biologists, ecologists, Earth system scientists, and modelers to tackle the complex interactions. Bridging the gaps between multiple disciplines and research approaches is an urgent challenge that must be met to reliably predict interactions between tropical ecosystems and novel climates.