Biological Conservation 143 (2010) 144–155 Contents lists available at ScienceDirect Biological Conservation journal homepage: www.elsevier.com/locate/biocon Extent and conservation of tropical dry forests in the Americas C.A. Portillo-Quintero, G.A. Sánchez-Azofeifa * Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada T6G 2E3 article info abstract Article history: This paper shows the results of an assessment on the current extent of Neotropical dry forests based on a Received 26 May 2009 supervised classification of MODIS surface reflectance imagery at 500-m resolution. Our findings show Received in revised form 16 September that tropical dry forests extend for 519,597 km2 across North and South America. Mexico, Brazil and Boli- 2009 via harbor the largest and best-preserved dry forest fragments. Mexico contains the largest extent at Accepted 24 September 2009 181,461 km2 (38% of the total), although it remains poorly represented under protected areas. On the Available online 29 October 2009 other hand, Brazil and Bolivia contain the largest proportion of protected tropical dry forests and the larg- est extent in continuous forest fragments. We found that five single ecoregions account for more than half Keywords: of the tropical dry forests in the Americas (continental and insular) and these ecoregions are: the Chiquit- Conservation Deforestation ano dry forests, located in Bolivia and Brazil (27.5%), the Atlantic dry forests (10.2%), the Sinaloan dry Extent forests in Mexico (9.7%), the Cuban dry forests (7.1%) and the Bajio dry forests in Mexico (7%). Chiquitano 2 2 Fragmentation dry forests alone contain 142,941 km of dry forests. Of the approximately 23,000 km of dry forest under MODIS legal protection, 15,000 km2 are located in just two countries, Bolivia and Brazil. In fact, Bolivia protects Tropical dry forests 10,609 km2 of dry forests, where 7600 km2 are located within the Chiquitano dry forest ecoregion and protected by a single park. Low extent and high fragmentation of dry forests in countries like Guatemala, Nicaragua, Ecuador, Costa Rica and Peru means that these forests are at a higher risk from human disturbance and deforestation. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction characteristics are attractive for human settlement and develop- ment in the tropics (Tosi and Voertman, 1964; Sánchez-Azofeifa Tropical dry forests are among the most threatened ecosystems et al., 2005). Furthermore, most scientific efforts for the study in the world as a consequence of intensive anthropogenic distur- and conservation of tropical vegetation have focused on tropical bance (Janzen, 1988; Hoekstra et al., 2005). Ewel (1999) explains rain forests, while little attention has been paid to tropical dry for- that in this particular ecosystem, the environmental constraints ests despite its high species richness and endemism of woody on human development are low in comparison to others. Here, an- plants, especially in continental and oceanic islands (Trejo and nual rainfall does not deviate greatly from potential evapotranspi- Dirzo, 2000; Sánchez-Azofeifa et al., 2005; Gillespie et al., 2005; ration, irrigation water is needed in modest amounts, yet rainfall is Kier et al., 2005). not so high that pest and nutrient leaching are overbearing prob- Several authors have defined tropical dry forests in different lems (Ewel, 1999). This ecosystem has historically supported high ways, based on similar or different criteria. Mooney et al. (1995) human population densities given that its climatic and edaphic defines the tropical dry forest simply as forests occurring in tropi- cal regions characterized by pronounced seasonality in rainfall dis- tribution with several months of drought. Sánchez-Azofeifa et al. Abbreviations: AVHRR, Advanced Very High Resolution Radiometer; ESRI, (2005) describes tropical dry forests as a vegetation type typically Environmental Systems Research Institute; ETM, enhanced thematic mapper; dominated by deciduous trees where at least 50% of trees present GHRC, Global Historical Climatology Network; GLCF, Global Land Cover Facility; are drought deciduous, the mean annual temperature is >25 °C, ha, hectares; IR, infrared; IUCN, International Union for the Conservation of Nature; total annual precipitation ranges between 700 and 2000 mm, and km, kilometers; km2, square kilometers; m, meters; mm, millimeters; MODIS, Moderate Resolution Imaging Spectroradiometer; NGOs, non-governmental orga- there are three or more dry months every year (precipitation nizations; NLCD, National Land Cover Database; PES, payment for environmental <100 mm). Pennington et al. (2006) uses a wider interpretation services; SPOT, Systeme Pour l’Observation de la Terre; SRTM, Shuttle Radar of tropical dry forests which includes vegetation that experience Topography Mission; TM, Thematic Mapper. a minimum dry season period of 5–6 months with concomitant * Corresponding author. Address: Earth Observation Systems Laboratory, Depart- strongly seasonal ecological processes and functions. This defini- ment of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3. Tel.: +1 780 492 1822; fax: +1 780 492 2030. tion includes diverse formations such as forests within grasslands, E-mail address: [email protected] (G.A. Sánchez-Azofeifa). shrublands and savanna ecosystems; from tall forests on the 0006-3207/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2009.09.020 C.A. Portillo-Quintero, G.A. Sánchez-Azofeifa / Biological Conservation 143 (2010) 144–155 145 more humid dry forest sites to cactus scrub on the driest. Sánchez- 2. Methods Azofeifa et al. (2005) also considers savannas, gallery forests, coast- lines and mangroves among the associated vegetation types that 2.1. Definition of tropical dry forest can occur within the matrix of tropical dry forests. Despite the many different definitions, it is essential to map the extent and Rather than limiting our study to a single definition of tropical geographical distribution of these important tropical forest dry forest, our analysis intends to examine the extent of tropical formations. dry forest in a broad sense. We took into account main bioclimatic More than half of tropical dry forests occur in the Americas and phenological characteristics as indicated by several authors (approximately 54%, according to Miles et al., 2006). These are re- (Olson et al., 2001; Sánchez-Azofeifa et al., 2005; Pennington ferred to as Neotropical dry forests. At present, Neotropical dry for- et al., 2006, Mooney et al., 1995), which are more relevant from ests exist as fragments of a once large, contiguous forest that the remote sensing point of view. Bioclimatic constraints for trop- reached from Mexico to Northern Argentina, reduced primarily ical dry forests were used by the adopting Olson et al. (2001) global by the timber industry, indigenous fuel–wood extraction, and the biomes classification and Sánchez-Azofeifa et al. (2005) dry forest expansion of cattle ranching (Fajardo et al., 2005). In spite of our definition. Patterns of leaf phenology were also crucial during the awareness of the rapid destruction of dry forests, we still know image classification process in order to differentiate the spectral very little about the true current extent and degree of fragmenta- response of deciduous vegetation from the spectral response of tion of Neotropical dry forests. evergreen vegetation and also from other types of land use. The Coarse-scale resolution maps (>1 km spatial resolution) based use of tropical dry forest biome boundaries by Olson et al. (2001) on the analysis of multispectral imagery from sensors like Ad- and information on the patterns of leaf phenology during the im- vanced Very High Resolution Radiometer (AVHRR), Systeme Pour age-processing phase is further explained in the next section. l’Observation de la Terre (SPOT) and Moderate Resolution Imaging The definition of tropical dry forest proposed by Olson et al. Spectroradiometer (MODIS) have provided comprehensive global (2001) excludes deciduous vegetation types that authors consider views of the distribution of Neotropical dry forests. Nonetheless, part of the dry forest ecosystem. For example, Caatinga forest is when examined closely, these maps are inaccurate and tend to characterized as low height forest consisting mostly of small decid- overestimate the extent of tropical humid forests and underesti- uous trees and shrubs, frequently displaying twisted trunks and mate the extent of tropical dry forests. Such errors affect the accu- thorns (Queiroz, 2006). Due to ecological and floristic similarities, racy of carbon stocks estimates based on forest maps, which are Pennington et al. (2000, 2006) considers Caatinga vegetation as essential for the implementation of conservation strategies such part of the Neotropical dry forests. However, Olson et al. (2001) as the Payments for Environmental Services (PES) initiatives (Kala- classifies this region as a shrubland ecosystem. The Olson et al. cska, 2005). Limitations to these analyses include spatial resolu- (2001) definition of dry forest neither includes deciduous vegeta- tion, image classification methodology, the tropical dry forest tion in savannas nor Dry Chaco vegetation as part of the distribu- definition used, and also imagery selection procedures that do tion of tropical dry forests. Yet the Llanos savannas, the Beni not account for latitudinal differences in leaf phenology patterns