The Ecological Life Zones of Puerto Rico and the U
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RESUMEN La mayoria de 10s paises neotropicales, except0 Mexico y Brasil, han sido delineados en mapas por el sistema Holdridge de zonas de vida natural, Para poder comparar con e&as areas, se hizo un mapa de Puerto Rico y las Islas Virgenes, el que in&rye una description de cada una de sus zonas de vida. El texto en Ingles contiene balances hidricos, una description de la teoria Holdridge sobre biotemperatura, y muchos m;;is detalles en general. La corta versibn en Espaiiol se ofrece coma manual de campo para ser usada con el mapa. En estas islas hay seis zonas de vida, las cuales existen tambien en Centro y Sur America, representando alli Qreas extensas. Con este mapa ser6 posible aplicar directamente en estas Breaa investigaciones hechas en Puerto Rico y las Islas Virgenes y, a la inversa, prdcticas satisfa~tor~as llevadas a cabo en dichas areas se& aplicables en las zonas de vida correspondientes en Puerto Rico y las Islas Virgenes. SUMMARY Most of the neotropical nations except Mexico and Brazil are mapped according to the Moldridge system of ecological life zones. In order to have comparability with these areas, Puerto Rico and the U.S. Virgin Islands were mapped and a discussion and a description of each life zone > were written. The manuscript in English offers water balances, a description of the Holdridge theory of biotemperature, and greater detail in general. The shortened version in Spanish is intended as a field manual to be used with the map. Six life zones are present on these islands and are present also in South and Central America, representing large areas there. Research done on the islands will be directly applicable to those areas and conversely, successful practices done there will be applicable in corresponding life zones on the islands. THE ECOLOGICAL LIFE ZONES OF PUERTO RICO AND THE U. S. VIRGIN ISLANDS L/ John J. Ewe? and Jacob L. Whitmore’ INTRODUCTION The ecosystems of the Commonwealth of Puerto Rico and the U. S. Virgin Islands are extremely important for two reasons. First, they form a substantial component of the life-support system for nearly three million people. Second, environmental changes due to man’s impact show up quickly on densely populated islands: this region can, therefore, indicate to other tropical and subtropical countries the kinds of changes that they might expect in the near future. During the past 100 years Puerto Rico and the U. S. Virgin islands have gone through a cycle consisting of massive degradation of their natural ecosystems (see, for example, Wadsworth, 1950) followed by an increasing general awareness, within the past five years, of the importance and unique and fragile nature of the plant-animal communities of this region. Ecosystem modification in the past was primarily due to agricultural activities, while today the issues are highways, transmission lines, ports, refineries, mines, power plants, and urbanization. Although ecological considerations now constitute an integral part of most land use planning, much remains to be done. This life zone map, tied to a world-wide system, was prepared to facilitate comparisons and the flow of related ecological information into Puerto Rico and the U. S. Virgin Islands from countries which encompass similar environments. Likewise, these countries, most of which have not yet undergone the degree of ecological modification that Puerto Rico and the U. S. Virgin Islands have, may benefit from the experiences of these islands. In Puerto Rico and the Virgin Islands the ecosystems have been better studied than in most tropical regions. Ecological studies of the area prior to 1900, most of which were qualitative observations, have been summarized by Wadsworth ( 1950). Murphy ( 19 16) was one of the first to provide detailed descriptions of Puerto Rico’s forests after the turn of the century. His work was followed by the classical study (involving less than four months of field work!) by Gleason and Cook (1926) (see also, Cook and Gleason, 1928), which is still a standard reference for descriptions of Puerto Rican plant communities. About 20 years after the Gleason and Cook study, J. S. Beard began a series of investigations throughout the Caribbean, summarized in his monograph on the vegetation of the Windward and Leeward Islands (Beard, 1949). Dansereau (1966) published physiognomic and floristic descriptions of more than 100 vegetation types in Puerto Rico, but did not include a detailed map of his units. A companion article by Buell and Dansereau (1966) did, however, contain maps of the vegetation of the Roosevelt Roads military base. Williams (1967) described several vegetation types in Puerto Rico from a physiognomic and -‘1 In cooperation with the University of Puerto Rico 21 Department of Botany, University of Florida, Gainesville 31 U. S. Forest Service, Institute of Tropical Forestry, Rio Piedras, Puerto Rico floristic viewpoint and compared them to forests in Southeast Asia and Texas. He used the map of forest types and forest regions prepared by Little and Wadsworth (1966). Several in-depth studies of specific forests have recently been carried out in Puerto Rico, most notably the studies by Odum and co-workers (Odum, 1970a) and by Howard and co-workers (Howard, 1968, plus sequential articles in same j’ournal) in two vegetation types within the Luquillo Experimental Forest. The basis of classification used for this map was the life zone system devised by Holdridge (1947, 1967). Its desirable features include: I .) it is simple enough to permit classification based on data likely to be available or easily obtained; 2.) it encompasses the variation inherent over the earth’s surface, yet without creating a huge number of units which differ only slightly among themselves; 3.) it is reproducible, such that individuals working independently classify similar landscape units in the same way; 4.) it is hierarchical, permitting the subdivision of primary units into locally important sub-units; and 5.) its units seem to bear a close relationship to natural and biologically meaningful landscapes, i.e. the units produced a recognizable order from the apparent chaos of nature. The life zones mapped are broad bioclimatic units, each of which may encompass a variety of soils, vegetation, microclimates, and land use patterns. The Holdridge system of life zone classification has now been applied to enough areas (Table 1) so that it has become a useful tool for comparing widely separated locations. Some of the maps (e.g. Peru, Colombia, Venezuela, Ecuador, Dominican Republic, Panama) have been accompanied by monographs containing descriptions of many of the life zones, their vegetation, and the prevalent land use patterns. THE HOLDRIDGE MODEL Holdridge first published his classification of world plant formations (now more correctly termed “life zones”) in 1947. In the ensuing 26 years various minor refinements and modifications were made in that original scheme-primarily changes in nomenclature-and these have been incorporated into the presently used diagram, Figure 1. Life Zones Each life zone lies within: I.) a Latitudinal Region (left side of Figure 1); 2.) an Altitudinal Belt (right side of Figure 1); and 3.) a Humidity Province (bottom of Figure 1). The variables used to delineate any given life zone are mean annual precipitation and mean annual biotemperature. Mean biotemperature is mean air temperature modified by substituting zero for values outside the range of 0 to 30 C, as discussed in Appendix A. The mean biotemperature scale appears along the sides of Figure 1. The values decrease geometrically from 30 at the bottom to 1.5 C at the top. The Latitudinal Region at any given location is determined by increasing the mean biotemperature to the value it would have at sea level. For making this conversion a lapse rate of 6 C per 1000 m is commonly used; most areas of the world fall in the range of 5.5 to 6.5 C per 1000 m. The mean biotemperature not corrected for elevation at any place determines the Altitudinal Belt. In naming life zones the name of the Altitudinal Belt is dropped from the lowest Altitudinal Belt within a given Latitudinal Region. Thus, we refer to Subtropical Wet Forest, rather than Subtropical Premontane Wet Forest. Table 1. Countries which have been mapped according to Holdridge’s life zone system. MAP COUNTRY SCALE REFERENCE* Colombia 1: 1 ,ooo,ooo Espinal et al. ( 1963) Costa Rica 1: 750,000 Tosi ( 1969) Dominican Republic 1:250,000 Tasaico ( 1967) Ecuador 1: 1 ,ooo,ooo Vivanco de la Torre, et al. (no da k) El Salvador 1: 1 ,ooo,ooo Holdridge ( 1959a) Guatemala 1: 1 ,ooo,ooo Holdridge ( 19591~) Haiti 1: 500,000 OAS ( 1972) Honduras I: 1 ,ooo,ooo Holdridge ( 1962b) Jamaica** 1: 500,000 Gray and Symes ( 1969) Nicaragua 1: 1 ,ooo,ooo Holdridge ( 1962~) Nigeria* * 1: 3 ,ooo,ooo Tosi ( 1968) Panama 1:500,000 FAO(l971) Peru 1: 1 ,ooo,ooo Tosi ( 1960) Thailand (seven sites) 1: 500,000 Holdridge et al. ( 193 1) United States, East of the Rockies** 1: 26,800,OOO Sawyer and Lindsey (1964) Venezuela 1:2,000,000 Ewe1 et al. ( 1968) * For countries which have been mapped more than once only the most recent work is cited. ** From climatological and topographic data. Meon Annuo/ Biofemperoiure in Degrees Cenfigrode Mean annual precipitation is the second variable used to define life zones. Precipitation lines traverse Figure 1 from the bottom upward and to the right at a 60” angle. The values shown increase from a low of 61.5 mm per year at thr left-hand side of Figure 1 to values in excess of 8000 mm per year at the lower right-hand corner of tile chart. As with biotemperature, the scale of mean annual precipitation is geometric.