ARTICLES https://doi.org/10.1038/s41561-018-0279-1 Mangrove canopy height globally related to precipitation, temperature and cyclone frequency Marc Simard 1*, Lola Fatoyinbo 2*, Charlotte Smetanka1,3, Victor H. Rivera-Monroy4, Edward Castañeda-Moya4,5, Nathan Thomas2,6 and Tom Van der Stocken 1 Mangrove wetlands are among the most productive and carbon-dense ecosystems in the world. Their structural attributes vary considerably across spatial scales, yielding large uncertainties in regional and global estimates of carbon stocks. Here, we present a global analysis of mangrove canopy height gradients and aboveground carbon stocks based on remotely sensed mea- surements and field data. Our study highlights that precipitation, temperature and cyclone frequency explain 74% of the global trends in maximum canopy height, with other geophysical factors influencing the observed variability at local and regional scales. We find the tallest mangrove forests in Gabon, equatorial Africa, where stands attain 62.8 m. The total global man- grove carbon stock (above- and belowground biomass, and soil) is estimated at 5.03 Pg, with a quarter of this value stored in Indonesia. Our analysis implies sensitivity of mangrove structure to climate change, and offers a baseline to monitor national and regional trends in mangrove carbon stocks. angroves are forested wetlands that represent a functional derived from space-borne remote sensing data and in situ measure- link between the terrestrial and oceanic carbon cycles1, stor- ments, to perform a global analysis of the spatial patterns and vari- ing up to four times as much carbon per unit area in com- ability in mangrove forest structure. M 2 parison to terrestrial forest ecosystems . Mangroves contribute an estimated 10–15% of the global carbon storage in the coastal ocean, Global distribution of mangrove canopy height with ~50% of mangrove litterfall production being transported to We used the global mangrove extent map26, the Shuttle Radar adjacent coastal zones and accounting for 10–11% of the global Topography Mission (SRTM) 30 m resolution global digital eleva- export of particulate terrestrial carbon to the ocean3,4. Furthermore, tion model (DEM), and Geoscience Laser Altimeter System (GLAS) mangrove forests provide a wealth of ecosystem services to coastal global Lidar altimetry products to produce two baseline canopy communities, including habitat for fisheries, firewood and timber, height maps for the year 2000: a map of maximum canopy height all valuable resources in local markets5. Despite this, mangroves are (that is, height of the tallest tree; Fig. 1) and a map of basal area impacted by anthropogenically driven disturbances such as defor- weighted height (that is, individual tree heights weighted in propor- estation, conversion to aquaculture and urban development6–8, and tion to their basal area). The latter map was used to generate the coastline transgression due to relative sea level rise9–11. Recent esti- aboveground mangrove biomass map (see Methods). Our analysis mates of global mangrove loss rates range between 0.16% and 0.39% of mangrove canopy height distribution is based on the maximum annually, and may be up to 8.08% in Southeast Asia12. As a conse- canopy height map. Both maps were validated using in situ field mea- quence, large amounts of previously stored carbon may be released surements of tree height from 331 plots (Supplementary Table 1), into the atmosphere, contributing substantially to net global resulting in overall root-mean-square errors of 3.6 m and 6.3 m, carbon emissions13–15. respectively (Supplementary Fig. 1). The maximum canopy height Global mangrove carbon stocks2 and aboveground biomass map shows that half of the world’s maximum mangrove canopy (AGB)16,17 have been estimated previously, providing AGB values height is shorter than 13.2 m (Fig. 2). The maximum canopy height derived from climate-based16 or latitudinal relationships17. The exceeds 62 ±​ 6.8 m (Fig. 2), rivaling maximum tree heights found spatially explicit distribution in forest structural attributes such as in upland tropical forests27. Equatorial regions of the West African mangrove canopy height is rarely considered in these estimates. and South American coasts stand out as hotspots with the tallest Mangrove canopy height is highly correlated with carbon turnover mangroves (Table 1a and Supplementary Tables 2–6). The top five via leaf or litterfall production18 and is therefore an important vari- countries (Table 1a) with the tallest mangroves are Gabon (62.8 m, able in quantifying contemporary global aboveground productivity Fig. 3), Equatorial Guinea (57.7 m), Colombia (54.3 m), Venezuela and carbon sequestration rates. Productivity and forest structure are (52.6 m) and Panama (50.9 m). These productive forests are signifi- controlled by local environmental gradients (for example, nutrient cantly taller than previously reported values16,18,28 and are located in availability and salinity) and hydrology19,20, along with regional cli- estuarine environments of the world’s most remote, cloudiest, wet- mate and geomorphology17,19–22, resulting in a range of mangrove test (precipitation > 500 cm yr−1) and hottest (mean air temperature ecotypes, from scrub (< 3 m) to tall (> 15 m) forest stands23–25. Here, 25.6 °C, ref. 29) regions. In addition, these wetlands grow in river- we produce global maps of mangrove canopy height and AGB dominated coastal settings with low human population densities, 1Radar Science and Engineering Section, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. 2Biospheric Sciences Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA. 3Department of Applied Geomatics, Université de Sherbrooke, Sherbrooke, Québec, Canada. 4Department of Oceanography and Coastal Sciences, College of the Coast and Environment, Lousiana State University, Baton Rouge, LA, USA. 5Present address: Southeast Environmental Research Center, Florida International University, Miami, FL, USA. 6Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD, USA. *e-mail: [email protected]; [email protected] 40 NATURE GEOscIENCE | VOL 12 | JANUARY 2019 | 40–45 | www.nature.com/naturegeoscience NATURE GEOSCIENCE ARTICLES a 3 11 5 10 2 Maximum 8 1 canopy 4 b 14 height 9 (m) c e 60 7 d 12 13 Field campaigns 6 1 Insets locations b d 15.5° S 2.5° N 16.0° S 0 20 km 0 20 km 78.5° W 78.0° W 77.5° W 46.0° E 46.5° E 47.0° E c e 0.5° S 2.5° S 1.0° S 0 20 km 0 20 km 48.0° W 47.5° W 47.0° W 133.0° E 133.5° E 134.0° E Fig. 1 | Global map of mangrove maximum canopy height and location of sampling sites (numbers) where in situ data were collected. a, Green colours show tallest maximum mangrove canopy height found within 1° cells. The map also shows the locations of the field sites and the locations of the high-resolution insets in b–e. b, Coastal Nariño and Cauca (Colombia). c, Coastal Pará (Brazil). d, Bombetoka Bay (Madagascar). e, Bintuni Bay (West Papua, Indonesia). 100 We analysed global trends in mangrove canopy height with 10 latitude, cyclone landfall frequency, precipitation, temperature, sea ) 2 80 Cumulative sum (% surface salinity (SSS) and tidal range. Globally, the distribution of 8 maximum mangrove canopy height follows a Gaussian latitudinal 2 60 trend (R =​ 0.91), peaking at 1.13° N (Fig. 4a), similar to trends of 6 Histogram Cumulative sum precipitation and temperature. The global distribution of canopy 40 height suggests that cyclone landfall frequency may limit the growth 4 of mangrove forests (Fig. 4a). Cyclone disturbance has been shown ) 31 2 20 to be important at more regional scales . However, the impact may Global extent (×1,000 km be confounded by other environmental factors (Fig. 4b). Our results 0 0 indicate that coastline-specific trends in maximum canopy height 0102030 40 50 60 reflect the important role of precipitation (Fig. 4b) in controlling Mangrove maximum height (m) mangrove structure and distribution, as shown recently by Osland and colleagues32. For example, the trends reflect similar differ- Fig. 2 | Global distribution of maximum mangrove canopy height extent. ences between the east and west coasts of the Americas and Africa. Areal extent of forests within each height class at 1.7 m intervals (black) While large-scale SSS appears to align with mangrove canopy height and cumulative sum of the percentages (red). (Fig. 4b), the explanatory role of this factor remains unclear as it varies strongly over short distances in estuarine environments, and potentially high nutrient availability, reduced soil salinity values and is regulated by precipitation, evapotranspiration, riverine input and significant protection from cyclone-induced high-energy winds ocean circulation33. We did not find a significant relationship of and waves17,30. canopy height variability with local tidal range (see Methods and NATURE GEOscIENCE | VOL 12 | JANUARY 2019 | 40–45 | www.nature.com/naturegeoscience 41 ARTICLES NATURE GEOSCIENCE Table 1 | Distribution of mangrove canopy height and total AGB (a) Ten countries with the tallest maximum mangrove canopy height Country Max Mean Max AGB Mean AGB Total Total Mangrove height (m) height (m) (Mg ha−1) (Mg ha−1) AGB (Mg) carbon (Mg) area (ha) 1 Gabon 62.8 23.5 910.5 244.0 33,578,276 61,504,323 137,597 2 Equatorial Guinea 57.7 21.6 800.0 208.6 2,630,892 5,337,399 12,613 3 Colombia 54.3 24.0 413.3 129.5 26,648,548 75,973,344 205,179 4 Venezuela 52.6 30.7 392.8 184.0 45,505,364