Ariel E. Lugo

International Institute of Tropical Forestry

USDA Forest Service

1201 Ceiba Street, Jardín Botánico Sur, Río Piedras, PR 00926-1119

The forests of Puerto Rico are tropical in the sense that they do not experience frost, but in the Holdridge (1967) life zone system they are categorized as subtropical because of the relatively uniform and high biotemperature conditions on the island. The rainfall of these forests ranges from about 700 to over 5,000 mm per year (Daley et al. 2003). Rainfall and biotemperature conditions define six subtropical forest life zones (dry, moist, wet, rain, lower montane wet, and lower montane rain) on the island, which coupled to six geologic formations (karst, volcanic, ultramaphic, granitic, sandy, and alluvial), form 28 geoclimatic zones since not all possible combinations of climate and geology occur (Lugo 2005). The island soils are diverse, comprising 165 soil series of all 10 global soil orders (Roberts 1942). The island’s topography is steep with 63 percent of the land over 100 m elevation. In spite of the small area of the island (8,800 km²), it contains 550 native tree species (Little et al. 1974), and over 225 alien tree species (Francis and Liogier 1991). Naturalized alien species--both plant and animal--comprise an important fraction of the entire island’s biota. Overall, Puerto Rico’s forests and other terrestrial ecosystems contain 3,000 plant, 12,000 fungal, and 11,410 animal species, including 1,441 vertebrate and 9,969 invertebrate species (Joglar 2005). The level of plant endemism in Puerto Rico is 9.9 percent (Liogier and Martorell 1982).

The major types of forests on the island are (Lugo 2005): dry forests on karst and volcanic substrate, moist and wet forests on karst substrate, moist and wet forests on volcanic substrate, moist alluvial forests, moist and wet forests on ultramaphic substrate, and the following wetland forest types: mangroves, Pterocarpus officinalis forests, Prestoea montana forests, Cyrilla racemiflora forests, and elfin woodlands. All but the mangroves and Pterocarpus forests are wetlands on slopes as opposed to coastal, although the Pterocarpus forests also occur at higher elevation. The forests of Puerto Rico range from 1 to 30 m-tall, have smooth canopies when they mature (canopies are ragged after a hurricane), are characterized by high dominance of tree species, and contain about 60 tree species per hectare. These characteristics are typical of Caribbean forests in general and are attributed to persistent trade winds and recurrent hurricanes (Lugo and Scatena 1995, Lugo 2005).

Most forests of Puerto Rico are new forests sensu Lugo and Helmer (2004). New forests contain novel species composition or a mix of alien and native tree species (Hobbs et al. 2006). Pockets of primary and mature native forests remain scattered throughout the island but are concentrated in the Luquillo Experimental Forest/Caribbean National Forest (Lugo 1994). The success of naturalized alien species in Puerto Rico is partially due to the long history of human habitation, including some 200 years of intensive land uses that deforested over 95 percent of the island (Lugo 2002). This history of anthropogenic impact has surprisingly led to low levels of species extinctions (Lugo 1988) and to enrichment of forests as a result of the establishment of naturalized alien species (Lugo and Brandeis 2005). The species area curves for new forests reaches a higher plateau than that of native forests although the initial slope of the species area curve of native forests is steeper than that of the new forests (Lugo and Brandeis 2005).

The primary productivity of Puerto Rican forests is generally high, from coastal riverine mangroves to wet and moist forests inland. Lower rates of net primary productivity occur in dwarf mangrove, elfin woodlands, and dry forests. However, because those forests that have lower primary productivity usually also have a lower aboveground biomass, their turnover rates are similar and their resilience high (Lugo et al. 2002). This allows native forests to recover biomass in the average time between passages of high intensity hurricanes (60 yr; Scatena and Larsen 1991). After the passage of Hurricane Hugo, Scatena et al (1996) measured the highest net primary productivity rates in the Luquillo Mountains. The five-year mean rate of 21.6 Mg/ha.yr with annual peaks of as high as 41 Mg/ha.yr exceed those measured in high yielding plantations (Francis 1995, Scatena et al. 1996).

Associated with the flux of organic matter through forests in Puerto Rico is a large flux of elements such as N, P, K, Ca, etc. (Frangi and Lugo 1985, Lugo 1992, Scatena et al. 1996). Forests do not appear to be nitrogen limited, although a new N flux has been found in the anaerobic soils of upland forested wetlands (dissimilatory nitrogen reduction to amonium, Silver et al. 2001). Nutrient use efficiencies for nitrogen flux in litter and rates of nitrogen retranslocation are usually low in comparison with nitrogen-limited forests (Lugo 1992). Phosphorus is limiting in dry forests (Lugo and Murphy 1986), dwarf mangroves, and moist karst forests (E. Medina et al., research in progress) as evidenced in high nutrient use efficiencies in litter fluxes and high rates of retranslocation in leaves. Phosphorus cycling is also tight in tabonuco forest (Sanford et al. 1991).

Forest successions are longer in the lowland forests compared to upland forests (Lugo and Scatena 1995). In native undisturbed forests, forest successions tend to be predictable. For example, there were no significant changes in species importance values in watersheds with native vegetation impacted by the force of Hurricane Hugo (Fu et al. 1996). In contrast, succession after abandonment of agricultural use is less predictable and dependent on the type of agricultural use and level of degradation of the site (Thompson et al. 2002, Molina Colón 1998). These successions include alien tree species in combination with native tree species and form new forest types (Helmer and Lugo 2004). These new forests reach maturity but appear to maintain a mixture of native and alien species in the late stages of succession (Lugo 2004).

Almost 45 percent of Puerto Rico is forested (Helmer 2004). Seven and one half percent of the island is under legal protection (Gould et al. 2006). The rest of the lands (93 percent) are under private ownership or not protected if under government control. The forests most impacted by human activity are lowland moist karst and lowland moist alluvial forests. Those best conserved are wet and rain volcanic and moist and wet ultramaphic forests (Helmer 2004).

Acknowledgments

This work was done in cooperation with the University of Puerto Rico. I thank Frank Wadsworth, Jean Lodge, and Mildred Alayón for reviewing the manuscript.

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Puerto Rico V2

21 April 2007