Manual de Plantas de Costa Rica 155 
the Santa Elena Peninsula have been continually exposed for approximately 85 million 
years, fueling the observation that this is the oldest biotic neighborhood (longest con- 
tinually above water) in Central America (Janzen, 1998). This antiquity, high tempera- 
tures, and soils made up mostly of serpentine rocks have given rise to a particular, semi- 
desert and isolated flora. The vegetation, in general, is dominated by herbs and shrubs 
with a few scattered small trees, mainly Byrsonima crassifolia, Cochlospermum viti- 
folium, Curatella americana, and Roupala montana, all of which are common through- 
out the region (see, e.g., Elizondo C. & Jiménez M., 1988; Vargas U., 2001). The sa- 
vannas of the Santa Elena Peninsula are characterized by such grasses as Hyparrhenia 
rufa, Paspalum pectinatum, and Trachypogon plumosus, as well as by a number of other 
herbaceous or shrubby species, especially Agave seemanniana, Bulbostylis paradoxa, 
Eriosema crinitum, E. diffusum, Krameria ixine, Rhynchospora barbata, Schoeno- 
caulon officinale, Sida ciliaris, Simsia santarosensis, Stylosanthes humilis, S. viscosa, 
and Turnera diffusa. Shrubs frequently found on exposed ridges and dry slopes include 
Bursera graveolens, B. schlechtendalii, B. tomentosa, Calliandra rubescens, Colub- 
rina triflora, Cordia guanacastensis, Coursetia elliptica, Croton niveus, Diphysa hu- 
milis, Erythroxylum rotundifolium, Euphorbia schlechtendalii, Helicteres guazumifo- 
lia, Lippia cardiostegia, L. graveolens, L. oxyphyllaria, Mimosa tricephala, Piptadenia 
flava, and Trixis inula. Patches of forest in flat areas are made up primarily of tree spe- 
cies such as Bursera simaruba, Dalbergia retusa, Enterolobium cyclocarpum, Gua- 
zuma ulmifolia, Quercus oleoides, Rehdera trinervis, Samanea saman, and Tabebuia 
ochracea. The vegetation along watercourses, known also as gallery forest, is the home 
of such important species as Astronium graveolens, Brosimum alicastrum, Cordia 
gerascanthus, Guaiacum sanctum, Hymenaea courbaril, Manilkara chicle, Steno- 
cereus aragonii, Sterculia apetala, Swietenia macrophylla, and Terminalia oblonga; 
these forest patches are the only refuge for some of these species. The coastal vegeta- 
tion is varied, but some conspicuous elements are Acacia riparia, Agave seemanniana, 
Amphipterygium adstringens, Bursera glabra, Caesalpinia bonduc, C. coriaria, C. eri- 
ostachys, C. platyloba, Haematoxylum brasiletto, Jouvea pilosa, Melocactus curvispi- 
nus, Opuntia guatemalensis, Phragmites australis, Physalis minuta, Prosopis juliflora, 
Sesuvium portulacastrum, Tephrosia cinerea, and Uniola pittieri (some of which are 
found in Costa Rica only here). 
The climate of the Santa Elena Peninsula is one of the driest in Costa Rica. Based 
on indicator species, it would appear that Santa Elena receives between 100 and 300 
mm less annual precipitation than the average (1528 mm) recorded for Santa Rosa Na- 
tional Park (Janzen, 1998). Obviously, the climate and other factors already mentioned 
contribute to the special character of the vegetation here, which comprises dry, decid- 
uous forest, with a few patches of evergreen or gallery forest in river basins or other 
places where sufficient water is close to the surface. 
The vegetation here is dominated by species in families (listed here in order of 
abundance of individuals) such as Fabaceae (especially the subfamilies Faboideae and 
