El Tercio

El Tercio Nature Reserve is a protected area located in the Usulután department of eastern El Salvador, encompassing forested terrain on the lower flanks of the volcanic highlands that transition toward the Pacific coastal plain. The reserve protects subtropical dry and humid forest ecosystems in a region characterized by extensive sugar cane cultivation, cattle ranching, and agricultural development that has severely fragmented natural habitat. Usulután department contains El Salvador's most extensive wetland systems along its Pacific coast, and El Tercio's upland forests contribute to the broader ecological matrix supporting biodiversity connectivity between coastal mangrove and wetland systems and highland forest reserves. The reserve provides critical watershed services and erosion control for agricultural lands in the surrounding lowlands.

El Tercio's forest supports a wildlife community adapted to the drier subtropical conditions of eastern El Salvador's lowland-highland transition zone. White-tailed deer, coyotes, gray foxes, nine-banded armadillos, and raccoons are common larger mammals. The reserve's bird community reflects the transition between dry forest and more humid highland zones, with dry forest specialists such as the turquoise-browed motmot (El Salvador's national bird), white-fronted parrots, and various flycatchers and wrens present alongside humid forest species. Migratory raptors use thermal columns above the reserve during spring and fall migrations, with concentrations of broad-winged and Swainson's hawks sometimes numbering in the thousands during peak migration. Reptiles including basilisk lizards, iguanas, and various snake species are abundant in the warmer, drier habitats.

The vegetation at El Tercio reflects the drier conditions of eastern El Salvador's lowland zone. Tropical dry forest dominates in areas of seasonal moisture deficit, characterized by deciduous tree species that drop their leaves during the extended dry season to reduce water loss. Dominant tree species include jocote (Spondias purpurea), nance (Byrsonima crassifolia), guácimo (Guazuma ulmifolia), and various Bursera species that are characteristic of Central American dry forests. Gallery forest along seasonal stream courses maintains semi-evergreen vegetation year-round, providing important dry season refugia for wildlife. Cacti and drought-adapted shrubs colonize rocky outcrops. The transition to more humid forest on north-facing slopes and at higher elevations introduces additional tree species and increases overall floristic diversity.

El Tercio lies at the geological transition between the volcanic highlands of central El Salvador and the sedimentary lowlands of the Pacific coastal plain. The underlying geology includes volcanic rocks from the central Salvadoran volcanic arc at higher elevations, transitioning to alluvial and sedimentary deposits in the lowland zones. The landscape has been shaped by both volcanic processes—including ash deposition from the numerous active volcanoes in the broader region—and fluvial erosion by rivers descending from the highlands to the Pacific coast. Soils in the reserve range from rich volcanic loams in upland areas to heavier clay soils in lower-lying terrain. The topography creates a mosaic of environmental conditions that supports the diverse vegetation types present in the reserve.

The climate at El Tercio is characterized by a pronounced tropical dry season from November through April, during which rainfall nearly ceases completely and deciduous trees lose their leaves, creating an arid, sun-exposed landscape. The wet season from May through October brings daily afternoon thunderstorms and occasional heavy rain events associated with Pacific tropical systems. Annual rainfall ranges from 1,200 to 1,800 millimeters, lower than highland areas of El Salvador. Temperatures are warm year-round, averaging 25–30°C in the lowland zones, with higher temperatures during the dry season. The landscape can experience extreme heat events during March and April immediately before the rains begin. Coastal influence from the nearby Pacific Ocean moderates temperature extremes somewhat and contributes moisture that supports the transition forest types.

The Usulután lowlands have been inhabited since pre-Columbian times, with evidence of settlement by indigenous peoples who developed agricultural systems adapted to the region's marked seasonal climate. The Lenca people occupied much of eastern El Salvador prior to Spanish contact, with some areas also inhabited by Pipil communities. Sugar cane, cotton, and indigo became important colonial-era crops in the Usulután lowlands, drawing on indigenous and enslaved African labor. The nineteenth and twentieth centuries saw intensive agricultural expansion, including major sugar cane plantations that converted large areas of lowland forest and wetland to cultivation. The 1980s civil war severely disrupted rural communities throughout the department, with displacement, land reform controversies, and population movements shaping the contemporary social landscape.

El Tercio's designation as a nature reserve reflects El Salvador's efforts to protect remaining upland forest fragments that contribute to watershed regulation and biodiversity conservation in the heavily cultivated Usulután lowlands. MARN established the reserve as part of a national network designed to maintain ecological connectivity between coastal wetland systems—which are internationally recognized for their importance to migratory birds and marine turtle nesting—and the volcanic highland reserves further inland. Management has focused on controlling illegal logging, preventing agricultural encroachment, and engaging local communities in compatible land uses. The reserve contributes to the broader conservation landscape of eastern El Salvador, where coastal mangroves, wetlands, and upland forests together support exceptional biodiversity.

El Tercio offers hiking through subtropical dry and transitional forest, with birdwatching as the primary naturalist attraction. The spectacle of raptor migration during spring and fall seasons can be extraordinary, with thousands of broad-winged and Swainson's hawks visible from elevated vantage points during peak movement periods. The transition between dry forest and gallery forest along stream courses provides diverse habitats to explore on a single walk. Scenic views across the Usulután lowlands to the Pacific coast are available from forest ridges. The broader Usulután department offers complementary attractions including the Bahía de Jiquilisco—one of the largest mangrove estuaries in Central America and an important sea turtle nesting area—and the volcanic landscape of the Islas del Golfo de Fonseca.

El Tercio is accessed from Usulután city, the departmental capital of Usulután department, located approximately 110 kilometers from San Salvador via the Litoral Highway (CA-2). Regular bus services connect Usulután with the capital and with smaller communities in the surrounding area. Local guide services for reserve visits may be available through community organizations or MARN's regional office. Visitor facilities within the reserve are minimal, so visitors should bring food, water, and appropriate gear. Usulután city offers accommodation and restaurants. The reserve is most conveniently combined with visits to other natural attractions in the department, including the Bahía de Jiquilisco Biosphere Reserve and the coastal zone.

Conservation at El Tercio faces intense pressure from agricultural expansion, particularly sugarcane cultivation and cattle ranching in surrounding lowland areas. Watershed protection is the most immediately valued service, as deforestation increases runoff and erosion, threatening water supply for downstream communities. Illegal logging for firewood and construction materials is a persistent challenge requiring community engagement and alternative livelihood programs. Fire management during the dry season requires coordination with neighboring farmers who use burning to prepare agricultural fields. MARN promotes agroforestry and silvopastoral systems as compatible land uses in buffer zones. Long-term conservation success depends on demonstrating the economic value of forest ecosystem services—water, erosion control, and wildlife—to local landowners and communities.