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The first result would imply one single ancient event that fragmented populations into two groups, which evolved independently and without contact northwest and southeast of the Amazon basin. The second result may instead support the expansion of the SDTF into the Amazon basin sometime in the recent past.

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This is also congruent with Linares-Palomino et al. Distribution of trnH-psbA and trnS-trnG combined haplotypes in the 38 populations of Geoffroea spinosa. It is obvious from the previous sections that each species presents a unique colonization history within this region. The differentiation levels reported between the Chaco and Caatinga domains for the two species 0.

For A. Table Therefore, our study seems to agree with the Pleistocenic Arc theory in eastern South America Prado and Gibbs , which is also well supported by the levels of floristic similarity between the SDTF in this area chap. The way the three SDTF nuclei within the Chaco domain, namely the Misiones, the Chiquitano, and the Piedmont, have been historically related is more complex to assess, given the difficulty of allocating populations to one or the other of the nuclei.

But more important than the possible isolation between these SDTF nuclei is actually the role that the whole domain seems to have had in the maintenance of the genetic variability for both A. The two species agree in identifying the Chaco domain as the region with the highest variability. The Chaco plain itself is characterized by saline soils and a highly seasonal climate, which effectively represent a hostile environment for the presence of the SDTF species.

The cerros, hills, cordilleras, and net of gallery forests in between the chaco matrix Spichiger et al. Hence, and in agreement with Spichiger et al. It is difficult at this point to place the ancestral populations in space and time, and impossible to date precisely the different colonization events proposed above. If the colonization had occurred by long-distance dispersal, it is indeed odd to find such homogeneity in the geographical patterns, especially between the Chaco and the Caatinga domains. Hence, our results support the Pleistocenic Arc as proposed by Prado and Gibbs Nevertheless, dating with pollen cores or with a suitable molecular clock is necessary to confirm that the expansion of the SDTF between the Chaco and the Caatinga domains, as suggested by both species, overlaps with the Pleistocene period.

Acknowledgments We thank Professor R. Spichiger for his support; T. Pennington, D. Prado, A. Teixeira-Filho, S. Beck, and F. Silveira, K. Elizeche, L. Oakley, R. Santos, A. Daza, and E. Rosero for their help with sampling; D.

Wyler for the maps. High degrees of degradation are reported, not only for the Neotropics, but also in the old tropics Miles et al. Causes and consequences of such degradation are known on a limited basis, and much needs to be learned in terms of gaining a full understanding of what controls environmental deterioration trends in these ecosystems and their impact on ecosystem services. Furthermore, current knowledge on the extent and degree of fragmentation of tropical dry forests is constrained because of the low priority for conservation within governmental and nongovernmental funding agencies.

The current imbalance in scientific knowledge is reflected in terms of the estimation of the current extent of tropical dry forests and their degree of degradation and fragmentation. The NASA Pathfinder study is one of those cases in which little or no effort has been placed to better understand the extent of tropical dry R.

Furthermore, in general, large mapping efforts tend to confuse tropical dry forests with woody savannas and agricultural fields, contributing to a misinformed estimation of their extent and degree of fragmentation Pfaff et al. In general, tropical dry forests have received much less attention from the scientific community and management planners Masera et al.

Tropical dry forests are, in general, located in areas with good to excellent conditions for agricultural and cattle development Fajardo et al. It is estimated that more than 50 percent of all tropical dry forests in Costa Rica were cut to promote the cattle industry in Costa Rica between the late s and the early s Calvo-Alvarado et al.

Similar observations, but with different time frames, have been documented for Mexico, Venezuela, Brazil, Bolivia, and Paraguay. In this chapter, we aim to document two fundamental issues that can be considered critical to the conservation of tropical dry forests. The first one deals with the challenges associated with mapping the extent of tropical dry forests.

We explore the different methodologies used to extract their extent and discuss their major limitations and problems. We also compare current estimates of tropical dry forests among the main sources of information currently available. The second section of this chapter deals with an analysis of the forces of change currently present in tropical dry forest environments. In this section, we divide the analysis into three subsections: all tropical dry forests continental and insular , continental level, and insular level.

Recent efforts conducted by Muchoney et al. Nonetheless, their work has been a valuable approach to assessing the actual extent and geographical distribution of Neotropical dry forest.

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One of the most important landcover mapping efforts, the Global Land Cover GLC landcover mapping initiative, produced a land cover map of South America and Mesoamerica using remotely sensed satellite data Bartholome et al. Figure shows the distribution of tropical dry forests in the Neotropics by including forested area detected by the GLC within the tropical dry forest biome limits in the Americas, as defined by Olson et al.

The GLC calculates that tropical dry forests in Mesoamerica comprise , square kilometers. In South America, tropical dry forests are distributed among Venezuela, Colombia, Peru, Bolivia, and Brazil, with larger and more continuous fragments occurring in an area between eastern Bolivia and southwestern Brazil. Colombia and Venezuela also have important extensions of dry forests. The GLC estimates that 1,, square kilometers of tropical dry forests exist in South America.

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  7. That gives a total of 1,, square kilometers of land covered by tropical dry forests in the Neotropical region. On the other hand, Miles et al.

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    Tropical dry forest was defined as those tropical grid cells with at least 40 percent forest cover, based on the MODIS data set. The map includes all forest land covers within the tropical dry forest biome, as defined by Olson et al. Their results showed considerably different area estimates. The work by the GLC is based on processing satellite imagery with coarser spatial resolution 1 kilometer by 1 kilometer grid cells. Large proportion errors can arise as landscapes are represented at increasingly coarse scales Moody and Woodcock The mixture of agricultural lands, urban landscapes, and other vegetation covers with forested areas in a single class increases as the spatial resolution of the imagery decreases.

    Nonetheless, their work made several assumptions and generalizations grid cells with greater than 40 percent tree cover within four different semiarid biomes that imply the aggregation of shrublands and woodlands with tropical dry forest in a single class, suggesting the possibility of important overestimation of SDTF extent. Further improvement of SDTF extent estimates is needed. The problems associated with incorrect estimations of tropical dry forests go beyond the simple estimation of extent and spatial configuration. Kalacska et al.

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    Nevertheless, despite the preliminary nature of these previous assessments, we can derive some salient characteristics of the SDTF extent and spatial distribution in the Americas. SDTF along the Pacific coastline is highly fragmented and exposed to higher degrees of human degradation Galicia et al.

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    The largest and most continuous fragments occur in Bolivia the Bolivian montane dry forest and the Chiquitano dry forest and eastern Brazil Atlantic dry forest. The study of Miles et al. Even though this might be a much better approximation to the actual extent, other recent studies suggest that this is still an overestimation Kalacska et al. Further research is necessary in order to reduce error sources and to offer a more confident estimate of SDTF extent for regional and national decisionmaking as a baseline for studies on biodiversity, as well as national or regional policy regarding ecosystem protection and restoration, payments for environmental services, and carbon capture and flux research.

    Deforestation Rates Deforestation analysis in the Neotropical region has focused on the humid tropical forest Achard et al. Scientific studies for the determination of global deforestation rates for tropical dry forest are very scarce. It has been suggested, however, that at least 48 percent of its extent has already been converted to other land uses, compared with the 32 percent that has been converted from humid tropical forests Hoekstra et al. Fajardo et al. At the global scale, studies on deforestation are limited or not very recent.

    For example, Achard et al. The global assessment of Miles et al. A few scientific assessments of deforestation rates within Latin America are available at the national or site level. However, these studies have been performed using different methodologies, and also for periods that are considerably different among regions, all of which makes comparisons difficult.

    For example, for Mexico, Masera et al.