Global change effects on humid tropical forests: Evidence for biogeochemical and biodiversity shifts at an ecosystem scale

PUBLICATIONS Reviews of Geophysics REVIEW ARTICLE 10.1002/2015RG000510 Key Points: • Negative effects of all global change factors were found for humid tropical forest biogeochemical processes • All global change factors except carbon dioxide fertilization are likely to promote warming and/or greenhouse gas emissions • Effects of drying and deforestation are relatively clear; effects of CO 2 fertilization and N deposition are less certain Supporting Information: • Supporting Information S1 • Table S1 Correspondence to: D. F. Cusack, dcusack@geog.ucla.edu Citation: Cusack, D. F., J. Karpman, D. Ashdown, Q. Cao, M. Ciochina, S. Halterman, S. Lydon, and A. Neupane (2016), Global change effects on humid tropical forests: Evidence for biogeochemical and biodiversity shifts at an ecosystem scale, Rev. Geophys., 54, doi:10.1002/ 2015RG000510. Received 31 OCT 2015 Accepted 5 JUL 2016 Accepted article online 12 JUL 2016 Global change effects on humid tropical forests: Evidence for biogeochemical and biodiversity shifts at an ecosystem scale Daniela F. Cusack 1 , Jason Karpman 2 , Daniel Ashdown 1 , Qian Cao 1 , Mark Ciochina 1 , Sarah Halterman 1 , Scott Lydon 1 , and Avishesh Neupane 1 Department of Geography, University of California, Los Angeles, California, USA, 2 Department of Urban Planning, University of California, Los Angeles, California, USA Abstract Government and international agencies have highlighted the need to focus global change research efforts on tropical ecosystems. However, no recent comprehensive review exists synthesizing humid tropical forest responses across global change factors, including warming, decreased precipitation, carbon dioxide fertilization, nitrogen deposition, and land use/land cover changes. This paper assesses research across spatial and temporal scales for the tropics, including modeling, ﬁeld, and controlled laboratory studies. The review aims to (1) provide a broad understanding of how a suite of global change factors are altering humid tropical forest ecosystem properties and biogeochemical processes; (2) assess spatial variability in responses to global change factors among humid tropical regions; (3) synthesize results 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 covered here include plant growth, carbon storage, nutrient cycling, biodiversity, and disturbance regime shifts. The review demonstrates overall negative effects of global change on all ecosystem properties, with the greatest uncertainty and variability in nutrient cycling responses. Generally, all global change factors reviewed, except for carbon dioxide fertilization, demonstrate great potential to trigger positive feedbacks to global warming via greenhouse gas emissions and biogeophysical changes that cause regional warming. This assessment demonstrates that effects of decreased rainfall and deforestation on tropical forests are 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 responses at regional and global scales. 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) CO 2 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. ©2016. American Geophysical Union. All Rights Reserved. CUSACK ET AL. Within each of these ﬁve sections, we synthesize how a particular global change factor is currently altering key ecosystem processes and properties. We focus on ﬁve ecosystem properties that are crucial for ecosystem function and which will determine whether tropical humid forests of the future will resemble those of the past TROPICAL FORESTS AND GLOBAL CHANGE