Manual de Plantas de Costa Rica | The physical environment| 87 
Also of importance to plants is the amount of available sunshine. This parameter is 
correlated inversely with cloudiness and precipitation. Sunshine is therefore maxi- 
mized in dry climates and seasons, with the highest annual daily averages for Costa 
Rica all from the northern Pacific slope, and the lowest from montane and Caribbean- 
slope sites. Santa Rosa National Park averages 7.1 hours of sunshine per day (peaking 
from December through April, with daily averages as high as 9.7 hours), while Chirrip6é 
National Park averages just 3.8, and Tortuguero National Park 4.4 (Coen, 1983). July 
is the least sunny month at both of the last two stations, with daily averages of 2.4 and 
2.5 hours, respectively. Of course, the amount of solar radiation available for photo- 
synthesis depends directly on hours of daylight, but not necessarily on hours of sun- 
shine. Many other factors come into play, including time of day and year, elevation, 
slope, and atmospheric pollutants (Coen, 1983). Furthermore, solar radiation is trans- 
mitted through clouds to a certain extent (Gdmez P., 1986). 
Cocos Island is so far south (5°32) that it is not subject to the same climatic 
regimes as mainland Costa Rica. The climate of the island features abundant cloudiness 
and elevated precipitation year-round. The annual average maximum temperature at 
sea level is 27.6°C. Average annual rainfall ranges from 5000 to 7000 mm, with the 
highest levels occurring near the summit of the highest point, Cerro Iglesias (634 m). 
The driest months are January through March (Herrera, 1985). 
Climatic conditions in Costa Rica may be far from uniform even over very small 
areas. These so-called microclimatic phenomena are exceedingly varied, and largely 
beyond the scope of this account. A few examples should serve to make the point. 
During the dry season, a station in a Guanacaste riparian forest had soil temperatures 
6.5°C cooler, air temperatures 5.5°C cooler, and relative humidity 20 percentage 
points higher than a station just 62 m away in a deciduous forest (Janzen, 1976). Ac- 
cording to Janzen (1976), two stations in Costa Rica (of one forest type) would have 
to be separated by about 1000 m in elevation to display an equivalent difference in 
mean annual temperature. In evergreen forests, conditions in the canopy and along the 
edges are harsher (hotter and drier) than in the understory. To appreciate this, one has 
only to examine the adaptations of many canopy epiphytes (thickened leaves, pseudo- 
bulbs, etc.), or to contemplate the quick and sad fate of understory herbs and shrubs 
after the trees are removed. This last point also illustrates that alterations in the vege- 
tation cover can have reciprocal effects on the local climate. These effects may be 
rather far-reaching, as suggested by a recent study demonstrating that deforestation in 
tropical lowlands may have serious negative impacts on adjacent cloud forests, forc- 
ing them upward (Lawton et al., 2001). 
Climate and weather affect not only vegetation type and the distribution of plant 
species, but also the periodicity (phenology) of leaf-flushing, flowering, and fruiting. 
The most well-known pattern involves deciduous-forest trees that flower conspicu- 
ously and abundantly during the dry season, while leafless. However, dry-season flow- 
ering is by no means universal in Costa Rica. For example, species of Araceae and other 
