The southern Yucatán peninsular region contains the largest and most rapidly disappearing continuous tract of tropical forest in Mexico (Flores and Espejel Carvajal 1994; Delfín Gonzales, Parra, and Echazarreta 1995; Acopa and Boege 1998). Vegetation in the region is a mosaic of forest types with different structural appearances (Flores and Espejel Carvajal 1994; Hernández-Xolocotzi 1959; Miranda 1958) that primarily reflect variation in environmental and edaphic conditions (Ibarra-Manríquez 1996). However, the structure and tree composition of forests in the region, as elsewhere in the central Maya lowlands, has been and remains strongly influenced by human activity (Ch. 2). In spite of the abundance of botanical work throughout the Yucatán peninsula, little attention has been devoted to characterizing the forests in this frontier region quantitatively, and the variation and distribution of forests remain poorly documented. Yet, it is precisely this kind of documentation that is required for integrated land studies of the kind that the SYPR project is undertaking (Turner et al. 2001). Since the third decade of the twentieth century, botanical interest has focused on the flora of the Yucatán Peninsula, especially that located in the historically more accessible portion of the peninsula (Ibarra-Manríquez 1996). Early twentieth-century studies (Lundell 1938; Standley 1930) led to a broad classification of the primary vegetation as deciduous tropical forests (Miranda 1958), or evergreen tropical forests (Rzedowski 1981), controlled in distribution by the northwest to southeast precipitation gradient, distinctive dry season, and karstic terrain (Ch. 2). Today, the entire region is appropriately labeled a seasonally dry tropical forest (Bullock, Mooney, and Medina 1995). During the rainy season (May–October) most species have their canopies fully displayed and light is a limiting factor in the forest understory (Martínez-Ramos 1985, 1994). For the remainder of the year, monthly precipitation usually does not exceed 100mm. During the lowest rainfall months (February–April), water may become limiting and considerable defoliation takes place, especially in the north and west. Other factors controlling forest structure and composition include topography, twentieth-century land-use history, and hurricanes (Brokaw and Walker 1991; Cooper-Ellis et al. 1999).
Solar forcing of early Holocene droughts on the Yucatán peninsula
