Manual de Plantas de Costa Rica | The physical environment| 83 
Pacific slope is presumably responsible for the diminished diversity of certain groups 
of plants, even in areas with high annual rainfall. Especially affected are epiphytes, but 
also some terrestrial plants, as certain ferns. For example, more species of Araceae are 
recorded from the La Selva Biological Station (with ca. 4000 mm of rain annually, at 
35-140 m elevation) than from the entire Pacific slope of Costa Rica below 300 m el- 
evation (including the Osa Peninsula, also with ca. 4000 mm of annual rain). Also, ferns 
in general, even including terrestrial genera such as Diplazium, are markedly more di- 
verse on the Costa Rican Caribbean slope. 
Temporales, intervals of nearly continuous rain lasting for as long as four to six 
days, occur on both slopes, and are largely responsible for the peak rainfall periods of 
each regime. According to Coen (1983), less than ca. 15 rainy days per year usually ac- 
count for at least 70% of the total annual rainfall in much of Central America. The 
Caribbean temporales result from northern winter cold fronts (as discussed previously). 
Pacific temporales occur when moisture-laden winds blow in from the west, off the Pa- 
cific Ocean, mostly in September and October (Coen, 1983). Torrential rainfall, along 
with high winds and flooding, may also be caused by sporadic hurricanes impacting 
Costa Rica from the Caribbean side. However, unlike some other countries in the re- 
gion, Costa Rica has suffered very few direct hits by hurricanes (just one in more than 
100 years). 
The local distribution of rainfall within Costa Rica is profoundly affected by topo- 
graphic factors, particularly the height and orientation of mountain ranges. Maximal 
rainfall occurs on ridges and (especially) windward slopes, while leeward slopes are 
relatively dry (Coen, 1983). This rain-shadow effect can be seen dramatically, over 
short distances, in some parts of Costa Rica. Fila Retinto, on the windward slope of the 
Pacific Fila Costefia above Palmar Norte, is very wet and supports a flora essentially 
identical to that of the Osa Peninsula; but scarcely 14 km due east, the area about La- 
garto in the gorge of the Rio Grande de Térraba, on the lee side of the Fila Costefia, is 
markedly drier, with a flora featuring many disjuncts from the Guanacaste region. 
Rainfall on windward slopes in tropical regions tends to increase with elevation up 
to acertain altitude, then decrease. In the Costa Rican axial cordillera, the zone of max- 
imal rainfall is ca. 1200—2000 m (Herrera, 1985). Stations in this zone on the Carib- 
bean slopes of the Cordilleras Central and de Talamanca (especially the notoriously wet 
Tapanti region) may experience an annual average of 308—329 days with recorded rain- 
fall, with maxima as high as 344-359 days. The average annual total at these sites may 
exceed 7700 mm, with a maximum for one year of over 9500 mm (Herrera, 1985). Ex- 
ceptionally high rainfall (> 5000 mm per year) also occurs in the Pacific Fila Costefia, 
on the Pacific slope of the northern Cordillera de Talamanca (Rio Naranjo basin) and 
the Caribbean slope of the Cordillera de Tilaran, in the volcanic hills at the far north- 
eastern corner of the country (Cerro Coronel, etc.), and on Cocos Island. 
The lowest rainfall levels in Costa Rica occur on the floors of valleys (Coen, 1983), 
especially the Central Valley (vicinity of Cartago), the Rio Tempisque basin (vicinity 
