Abstract

Wetlands are important providers of ecosystem services and key regulators of climate change. They positively contribute to global warming through their greenhouse gas emissions, and negatively through the accumulation of organic material in histosols, particularly in peatlands. Our understanding of wetlands' services is currently constrained by limited knowledge on their distribution, extent, volume, interannual flood variability and disturbance levels. We present an expert system approach to estimate wetland and peatland areas, depths and volumes, which relies on three biophysical indices related to wetland and peat formation: (1) long-term water supply exceeding atmospheric water demand; (2) annually or seasonally water-logged soils; and (3) a geomorphological position where water is supplied and retained. Tropical and subtropical wetlands estimates reach 4.7million km2 (Mkm2 ). In line with current understanding, the American continent is the major contributor (45%), and Brazil, with its Amazonian interfluvial region, contains the largest tropical wetland area (800,720km2 ). Our model suggests, however, unprecedented extents and volumes of peatland in the tropics (1.7Mkm2 and 7,268 (6,076-7,368) km3 ), which more than threefold current estimates. Unlike current understanding, our estimates suggest that South America and not Asia contributes the most to tropical peatland area and volume (ca. 44% for both) partly related to some yet unaccounted extended deep deposits but mainly to extended but shallow peat in the Amazon Basin. Brazil leads the peatland area and volume contribution. Asia hosts 38% of both tropical peat area and volume with Indonesia as the main regional contributor and still the holder of the deepest and most extended peat areas in the tropics. Africa hosts more peat than previously reported but climatic and topographic contexts leave it as the least peat-forming continent. Our results suggest large biases in our current understanding of the distribution, area and volumes of tropical peat and their continental contributions.

Highlights

  • Wetlands are global hotspots of biological diversity (Gibbs, 2000; Junk et al, 2006), ecosystem productivity (Rocha & Goulden, 2009) and economic activity

  • All wetland data sets agreed on four top contributors: Brazil, Indonesia, Argentina and the tropical/subtropical United States (Figure 3)

  • Among the top contributors to pantropical peat area and volume are, in this order: Brazil (18%, 20%), Indonesia (13% and 18%), DRC (7%, 10%), China (5%, 3%), Colombia (4%, 5%), Peru (4%, 6%), United States (4%, 2%), Bangladesh (3%, 3%), India (3%, 2%) or Venezuela (3%, 4%), among others (Figs S5–S8)

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Summary

Introduction

Wetlands are global hotspots of biological diversity (Gibbs, 2000; Junk et al, 2006), ecosystem productivity (Rocha & Goulden, 2009) and economic activity (aquaculture, tourism, timber; Junk et al, 2014). Under favourable hydrological conditions undisturbed wetlands are reported to act as moderate CH4 and N2O sources (Frolking et al, 2011) or to counterbalance their CH4 emissions (Petrescu et al, 2015), while acting as long-term soil carbon reservoirs dating back to the Holocene (L€ahteenoja & Roucoux, 2010; Yu, Loisel, Brosseau, & Beilman, 2010) For their multiple ecosystem services, the need for wetland conservation is widely recognized (i.e. the Ramsar convention, Ramsar 2013) but has long been challenged by national development policies and short-term economic priorities (An et al, 2007; Junk et al, 2013; Keddy et al, 2009; Paulson Report, 2015). Drainage, fire and conversion to agriculture and agroforestry are presently turning wetlands and peatlands into net emission sources of GHG (Frolking et al, 2011; Page et al, 2002; Petrescu et al, 2015; Turetsky et al, 2015; Van der Werf et al, 2010), and doing so at an accelerating pace (Davidson, 2014; Junk et al, 2013)

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