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Global Change Effects on Humid Tropical Forests

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Global Change Effects on Humid Tropical Forests PUBLICATIONS Reviews of Geophysics REVIEW ARTICLE Global change effects on humid tropical forests: Evidence 10.1002/2015RG000510 for biogeochemical and biodiversity shifts Key Points: at an ecosystem scale • Negative effects of all global change factors were found for humid tropical Daniela F. Cusack1, Jason Karpman2, Daniel Ashdown1, Qian Cao1, Mark Ciochina1, Sarah Halterman1, forest biogeochemical processes 1 1 • All global change factors except Scott Lydon , and Avishesh Neupane carbon dioxide fertilization are likely 1 2 to promote warming and/or Department of Geography, University of California, Los Angeles, California, USA, Department of Urban Planning, greenhouse gas emissions University of California, Los Angeles, California, USA • Effects of drying and deforestation are relatively clear; effects of CO2 fertilization and N deposition are Abstract Government and international agencies have highlighted the need to focus global change less certain research efforts on tropical ecosystems. However, no recent comprehensive review exists synthesizing humid tropical forest responses across global change factors, including warming, decreased precipitation, Supporting Information: carbon dioxide fertilization, nitrogen deposition, and land use/land cover changes. This paper assesses • Supporting Information S1 fi • Table S1 research across spatial and temporal scales for the tropics, including modeling, eld, and controlled laboratory studies. The review aims to (1) provide a broad understanding of how a suite of global change Correspondence to: factors are altering humid tropical forest ecosystem properties and biogeochemical processes; (2) assess D. F. Cusack, spatial variability in responses to global change factors among humid tropical regions; (3) synthesize results [email protected] from across humid tropical regions to identify emergent trends in ecosystem responses; (4) identify research and management priorities for the humid tropics in the context of global change. Ecosystem responses Citation: covered here include plant growth, carbon storage, nutrient cycling, biodiversity, and disturbance regime Cusack, D. F., J. Karpman, D. Ashdown, Q. Cao, M. Ciochina, S. Halterman, shifts. The review demonstrates overall negative effects of global change on all ecosystem properties, with S. Lydon, and A. Neupane (2016), Global the greatest uncertainty and variability in nutrient cycling responses. Generally, all global change factors change effects on humid tropical reviewed, except for carbon dioxide fertilization, demonstrate great potential to trigger positive feedbacks to forests: Evidence for biogeochemical and biodiversity shifts at an ecosystem global warming via greenhouse gas emissions and biogeophysical changes that cause regional warming. scale, Rev. Geophys., 54, doi:10.1002/ This assessment demonstrates that effects of decreased rainfall and deforestation on tropical forests are 2015RG000510. relatively well understood, whereas the potential effects of warming, carbon dioxide fertilization, nitrogen deposition, and plant species invasions require more cross-site, mechanistic research to predict tropical forest Received 31 OCT 2015 Accepted 5 JUL 2016 responses at regional and global scales. Accepted article online 12 JUL 2016 1. Introduction Humid tropical forests are one of the planet’s greatest natural resources, serving as a terrestrial warehouse for organic carbon (C), protecting nearby communities from runoff and soil erosion, and providing habitat for a spectacular diversity of living organisms. The potential for anthropogenic climate change to disrupt ecosys- tem processes has long been recognized [Vitousek, 1994], but the effects of individual and interacting global change factors have not been comprehensively reviewed. The need for such a review is particularly urgent. Human populations and the extraction of resources continue to increase [Lutz et al., 2001; Watson et al., 2001], and a suite of global-scale consequences are rapidly unfolding on ecosystems across the globe. While climate change is broadly recognized as the most important global change that humans and natural ecosystems face, other factors are also likely to have important effects on all ecosystems, with unique and potentially more accelerated effects on tropical ecosystems. This paper is organized around different aspects of global change that are impacting humid tropical ecosystems. The global change factors are presented in two broad groups that each includes several specific aspects of change. Within (2.1) Changing Cycles we include (2.1.1) Climate Change, (2.1.2) CO2 Fertilization, and (2.1.3) Nitrogen (N) Deposition. Within (2.2) Land Use/Land Cover Change we include (2.2.1) Deforestation and (2.2.2) Unmanaged Land Cover Change, which explore successional trajectories of lands abandoned postdeforestation. A brief introduction to each of these global change factors is provided at the beginning of each section. Within each of these five sections, we synthesize how a particular global change factor is currently altering key fi ©2016. American Geophysical Union. ecosystem processes and properties. We focus on ve ecosystem properties that are crucial for ecosystem All Rights Reserved. function and which will determine whether tropical humid forests of the future will resemble those of the past CUSACK ET AL. TROPICAL FORESTS AND GLOBAL CHANGE 1 Reviews of Geophysics 10.1002/2015RG000510 and present. The ecosystem properties we review for each global change factor are (i) plant growth, including rates of photosynthesis, plant respiration, growth of individual plants, and ecosystem rates of net primary pro- ductivity (NPP); (ii) carbon storage at an ecosystem scale in standing plant biomass and in soils, with attention also to changes in ecosystem-scale C loss (e.g., plant mortality, net ecosystem respiration, and/or soil respira- tion); (iii) nutrient availability, including discussions of plant nutrition, soil nutrient levels, and nutrient recy- cling; (iv) plant and animal biodiversity, particularly in relation to ecosystem function; and (v) disturbance regimes, with attention to how each global change factor may alter or create new disturbance regimes in tro- pical forests. Below, the importance of these five ecosystem processes and properties is described, followed by a comprehensive review of how different global change factors are altering humid tropical ecosystems. The first and second ecosystem properties, tropical plant growth and ecosystem C storage, are closely intertwined. Plant growth removes CO2 from the atmosphere via photosynthesis and provides the basis of ecosystem food webs. Plant growth is discussed with reference to changes in photosynthetic uptake of C and changes in plant respiration of CO2 back to the atmosphere. Much of the available data focuses on responses by individual plants. Where possible, we also review how the different global change factors alter ecosystem-scale rates of NPP. For C storage, we focus on global change effects on standing stocks of plant biomass aboveground and in roots, as well as detrital plant material stored as soil organic matter (SOM). In our discussion of C storage, we review how global change may alter ecosystem-scale C losses (e.g., via plant mortality, net ecosystem respiration, and/or soil respiration). In contrast to plant growth, which focuses on plant physiology and the flux of C into ecosystems, C storage focuses on the net effect to carbon pools. Carbon storage is particularly important in tropical forests at a global scale. They are among the most C dense ecosystems, storing up to 55% of the planet’s total aboveground C, and act as a major C sink world- wide [Pan et al., 2011]. Canopy trees comprise the greatest portion of living biomass [Slik et al., 2013], hold- ing an estimated 247 Pg of C in aboveground and belowground biomass [Saatchi et al., 2011]. Tropical forests also appear to act as an ongoing C sink, taking up more C on an annual basis than is lost via respiration. Using forest inventory data, tropical forests have been estimated to take up 1.2 Pg C/yr during the 1990–2007 period [Pan et al., 2011], although this is an upper estimate, since it assumes that tropical forest cover is consistently comprised of tall, closed canopy forests, which is not the case worldwide [Wright, 2013]. In addition, approximately 30% of global soil C stocks (totaling >2000 Pg-C in the top three meters) is stored in tropical forests as dead organic matter (i.e., plant and microbial tissues that have been incorporated into soils) [Jobbagy and Jackson, 2000], with soils storing more C globally than plants and the atmosphere combined [Post et al., 1982; Tamocai et al., 2009]. Soil C storage is generally a longer-term glo- bal C sink than C storage in living aboveground plant biomass because of its slower overall turnover time [Mayer, 1994; Torn et al., 1997; von Lutzow et al., 2006]. After aboveground biomass and soils, roots are the third greatest C pool and account for an estimated 21% of total forest biomass in tropical ecosystems [Cairns et al., 1997]. Thus, biome-scale changes in plant growth and C storage in tropical forests are of broad concern. The ability of tropical ecosystems to continue functioning as C repositories depends greatly on how they will respond to global change. The third ecosystem function we assess is nutrient availability. Available nutrients for plant and microbial uptake are generally found as simple inorganic compounds in soils. Inorganic nutrients are also
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