Monteverde : Ecology and Conservation of a Tropical Cloud Forest

land and montane sites in terms of ambient air tem-

perature, and transitional between the Caribbean and

Pacific sides of Costa Rica in terms of incident solar

radiation and amounts and seasonality of precipita-

tion (Coen 1983, Herrera 1985, Vargas 1994). Costa

Rica is a relatively small landmass between the Ca-

ribbean Sea and Pacific Ocean, so continental low pres-

sures are not generated during any season. Rather,

the migration of the Intertropical Convergence Zone

(ITCZ), a zone of low pressure associated with in-

tense solar radiation and heating that follows the

seasonal migration of the sun, largely controls the

seasonality of cloud cover and precipitation (Riehl

1979). Weather systems that affect Monteverde are

regional to global in scale and include polar cold

fronts, tropical storms, and hurricanes. At a smaller

scale, topographic position and exposure to trade-

wind-driven clouds and precipitation play major

roles in controlling microclimate across Monteverde

(Lawton and Dryer 1980).

2.1.1. Daylength, Solar Angle, and

Solar Radiation

Daylength at Monteverde oscillates seasonally be-

tween 11 hr 32 min on 22 December and 12 hr 42

min on 23 June (Fig. 2.1). Solar angle is 90° above

horizontal at noon on 23 April and 23 August and

reaches a minimum of 56.6° on 22 December (Fig.

2.2). Calculated clear-sky, instantaneous shortwave

radiation at noon varies between 875 and 1085W/m^2

(assuming a value of 0.7 for atmospheric transmit-

tance), but reflectance and absorption of solar radia-

tion by clouds strongly affect both daily and seasonal

patterns of incident solar radiation. For example,

instantaneous noontime incident solar radiation

measured at a leeward forest site in the MCFP from

October 1991 to September 1992 showed seasonal

attenuation by clouds, particularly by cumulus and

stratocumulus clouds in July and August, when com-

pared to calculated clear-sky incident solar radiation

(Fig. 2.3).

The seasonality of cloud types (Sec. 2.1.3) also has

an effect on incident solar radiation, which poten-

tially results in seasonal differences on east- and west-

facing slopes (Fig. 2.4). Combined with the seasonal

variation in solar angle and daylength, changes in

cloud cover and type affect incident radiation, which

likely has major effects on ecosystem processes, such

as evapotranspiration, primary production, and nu-

trient cycling, and may also cue phenological changes

and other processes in plants and animals. The link-

age of this type of abiotic data with information on

biotic processes is needed.

16 The Physical Environment

Figure 2.1. (top left) Calculated mean monthly daylength
(hours:minutes) at the latitude of Monteverde (10° N).
Figure 2.2. (top right) Calculated mean monthly solar
angle above horizontal (degrees) at noon at the latitude of
Monteverde (10° N). Figure 2.3. (at right) Mean monthly
instantaneous shortwave radiation (W/m^2 ) at noon
calculated for clear-sky conditions in Monteverde p), and
measured at a leeward cloud forest site in the MCFP from
October 1991 to September 1992 (•; mean ± 1 S.D.).