Location and General Description
This relatively small ecoregion lies on the Pacific Slope, spanning the borders of northwestern Costa Rica and Nicaragua, between the crests of Costa Rica's central chain of volcanoes on the east and the Pacific Ocean on the west.
The rain shadow created by Tilaran Mountain Range gives this ecoregion's climate significant seasonal variability. For about five months, usually November through March, the nearly incessant easterly tradewinds bring moisture-laden air from the Caribbean Sea that is trapped on Central America's Atlantic slope by the tops of the mountains, depriving the Pacific side of the Isthmus precipitation. During the rest of the year, the tradewinds are absent or substituted by reversed movements of air created by low pressures from hurricanes in the Caribbean Sea, and orographic uplift and resultant cooling of air from the Pacific Ocean inundates the area with heavy rainfall. Approximate 225 mm falls monthly during this time of year, often referred to as the wet season it comprises around 90% of the 1,500 mm of annual precipitation, adapted from (Coen 1983).
The deciduous vegetation here distinguishes this ecoregion from wetter ecoregions to the south and east and quickly increases in dominance as one moves west from the cloud forest habitat of the mountain peaks. During the dry season, most canopy trees are leafless -- at the lower elevations only the riparian forests are evergreen (Stiles and Skutch 1989).
The flora of this seasonal moist forest ecoregion exhibits extremely high beta diversity, which is to say its species composition changes rapidly over short distances as one moves away from the adjacent highland cloud forests, down the mountains into areas increasingly impacted by the rain shadow. The colonizing human population preferred this ecoregion's drier climate and largely converted it to an agricultural landscape as much as 70-100 years ago. Thus, in the absence of protected areas, or even large intact blocks of habitat, taxonomists have poorly researched the area.
Perhaps the most outstanding characteristics of these Pacific Slope habitats is their tight ecological linkage to the wetter habitats of the adjacent mountain tops and Atlantic Slopes. Many species of invertebrates and birds migrate seasonally between these habitats (Stiles and Skutch 1989, Powell and Bjork 1994, Haber pers. comm.). Some of the most charismatic cloud forest species such as the resplendent quetzal (Pharomacrus mocinno) and three-wattled bellbird (Procnias tricarunculata) are equally dependent on the seasonal moist forests as they migrate annually to these moist forests at the completion of their breeding season. These birds apparently migrate into the moist forests to take advantage of delayed fruiting cycles of tree species, predominately of the Lauraceae family, that are endemic to the ecoregion.
The Costa Rican seasonally moist forests have been extensively altered by human intervention. Lowland areas have been cleared for cattle, while mountain slopes, regardless of their steepness, have been cleared to grow beans, corn, coffee, as well as to support dairy cattle; most of Costa Rica's population and a significant portion of Nicaragua's lives within this ecoregion. During the past 100 years, virtually the entire ecoregion has been stripped of its native vegetation, with only small forest fragments remaining, totaling less than 10% of the ecoregion's original forest cover (Palminteri pers. obs.). The lack of lower and middle-elevation Pacific Slope forests threatens both the species that reside in these habitats and the altitudinal migrants the breed in the neighboring highlands (Stiles and Skutch 1989; Powell and Bjork 1994). The small protected areas in the ecoregion total less than 30,000 ha, or around 3% of the ecoregion (Powell pers. comm.).
Types and Severity of Threats
In the past decade, there has been substantial regeneration of lower-elevation hillsides following the collapse of Costa Rica's cattle industry due to rising labor costs and decreasing yields after soils gave out from annual burning, over grazing, and resulting erosion. However, above 800 meters, the situation remains critical as a growing human population puts ever-increasing demands for living space and agricultural products that grow well in the moderate mid-elevation climate.
Justification of Ecoregion Delineation
These transitional forests mark a unique and graded ecotone between the moist forests and dry forests of Costa Rica and Nicaragua, and host endemic species as well as maintain unique processes such as elevational migrations of many species. This region is distinct from surrounding areas in both floral and faunal distributions. Delineation’s for the ecoregion were derived primarily from Holdridge (1962 and 1967) and Tosi (1969) to encompass the "humid tropical forest" life zones of the Pacific slopes of northern Costa Rica and southern Nicaragua. On the Nicoya Peninsula and along the slopes of the Cordillera Central, small aggregates of "very humid tropical forest" life zones were lumped for broader scale coverage. As well, this ecoregion was separated from the Caribbean slope portions of these same life zones on the basis of species distributions (Powell pers.comm.). Isla Ometelepe was on Lake Nicaragua and the volcanic mountains around the northern portion of the lake were also included due to species similarities (Powell pers. comm.). Portions of similar habitat south of the Central Valley of Costa Rica were not included on the basis of distinct floral and faunal distributions.
Coen, E. 1983. Climate. In D. H. Janzen, editor, Costa Rican Natural History. Chicago: University of Chicago Press.
Haber, W. A. 2000. Personal communication.
Holdridge, L.R. 1962. Mapa ecólogico de Nicaragua. Agencia para el Desarrollo Internacional de Gobierno de los Estados Unidos de America, Managua, Nicarauga.
Holdridge, L.R. 1967. Life zone ecology. Tropical Science Center, San Jose, Costa Rica.
Palminteri, S. 2000. Personal observation.
Powell, G. V. N. 2000. Personal communication.
Powell, G. V. N., and R. D. Bjork. 1994. Implications of altitudinal migration for conservation strategies to protect tropical biodiversity: a case study of the Resplendent Quetzal Pharomacrus mocinno at Monteverde, Costa Rica. Bird Conservation International (4).
Stiles, F. G., and A. F. Skutch. 1989. A guide to the birds of Costa Rica. New York: Cornell University Press.
Tosi Jr., J.A. 1969. Republica de Costa Rica: mapa ecológico. Map 1:750,000. Tropical Science Center, San Jose, Costa Rica.
Prepared by: George Powell, Sue Palminteri, and Jan Schipper
Reviewed by: In process