Nalini Nadkarni. Monteverde, Costa Rica.



MONTEVERDE: Ecology and Conservation of a Tropical Cloud Forest. An extract.
Nalini M. Nadkarni and Nathaniel T. Wheelwright, editors.
Photography by Javier Martín
	Dear Javier: I am happy to hear from you and know you are working on disseminating information about Monteverde to others. Buena suerte! Give my "abrazos" to the big trees of Monteverde! Nalini
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INTRODUCTION Nalini M. Nadkarni and Nathaniel T. Wheelwright Monteverde has been variously described as a virgin tropical cloud forest, a Quaker dairy community, an artists' commune, a haven for those seeking spirituality, a model for tropical rain forest conservation, and a "forest in the clouds" where the sound of the bellbird's call and images of mist-enshrouded trees long linger in visitors' minds. The environment of Monteverde is typical of many tropical montane cloud forest regions, but Monteverde provides a unique setting because of its biogeographic, human, and conservation history. This book was created to fulfill three objectives: to compile what we know about Monteverde's natural history, ecology, and conservation; to identify areas where information is lacking; and to facilitate communication among those who carry out research, education, and conservation. Contributors include a wide range of people with expertise from many different fields, levels of training, and approaches to understanding the natural world and they have communicated in many modes, ranging from the objective style of scientific prose, statistics, and tables to the more reflective descriptions of personal experiences. In addition to academic scientists, we have invited the voices of farmers, natural history guides, anthropologists, educators, and homemakers, all of whom have important insights into Monteverde's biology and conservation.


Historical Overview In 1951, a band of fewer than 50 North American Quakers bought land and settled in Monteverde (Essay 1.1; Fig. 1.1). By 1977, visitors to the area were still relatively uncommon. Now, just two decades later, nearly 50,000 visitors walk the trails of Monteverde each year to catch sight of a quetzal or absorb the peaceful outlook of the community. Growth of ecotourism has been phenomenal, eclipsing the small single-family farm as the region's economic mainstay (Fig. 1.2). New agricultural methods, a changing local, regional, and global economy, and the sheer number of visitors have changed Monteverde and surrounding communities. In the early 1960s, biologists first became aware of Monteverde. Welcomed by the small farming community and working largely independently or in small groups, they overcame the lack of scientific facilities and infrastructure to document the diverse tropical biota. A surprising number of these biologists have taken up residence in Monteverde, weaving their work into the rich tapestry of the human community of Monteverde. Although much of the research from Monteverde has been published as primary scientific literature, it has never before been synthesized. Only one symposium (Association for Tropical Biology 1984) has brought together Monteverde biologists as a group.

A major reason for producing this book is that research from Monteverde has not previously been integrated into forms that can be readily channeled into education and conservation. For two decades, Monteverde has been a mecca for student groups to observe and study tropical montane landscapes (Fig. 1.3). Foremost among these have been the graduate courses offered by the Organization for Tropical Studies (OTS). Many preliminary studies led to dissertation theses and long-term research. Numerous undergraduate groups from North American colleges and universities have also conducted research in Monteverde. Little background material has been available for their research projects and the scattered nature of the available information has hampered useful input to the scientific record from these student groups (Essay 1.2). The mixture of biologists, educators, and ecotourists in Monteverde has produced an opportunity for conservation. Monteverde has been viewed as a model for conservation at grassroots level. Funds have come from government agencies, foundations, non-governmental organizations, and individuals to support land acquisition, native tree nurseries, and environmental education. Strong links between conservation and biology are needed to maintain high-quality conservation practices. Compiling and synthesizing existing information is an important first step in forging these connections.

Chapter 2: THE PHYSICAL ENVIRONMENT Kenneth L. Clark, Robert O. Lawton, and Paul R. Butler 2.I. INTRODUCTION Because biological diversity is directly related to diversity of the physical environment, a clear picture of the physical setting of the Cordillera is crucial to understand its ecology and conservation. The physical setting of Monteverde and the Cordillera de Tilarán encompasses a wide range of environmental conditions. The size, position across the trade windflow, geology, erosional dissection, and hydrology of the Cordillera interact to produce extraordinary physical diversity, which parallels its great biological diversity. A major difference between tropical montane and lowland regions is the way biological diversity is distributed across the landscape. Montane regions are usually less diverse at the scale of 0.01-0.1 km2, but are as species-rich as nearby lowland areas at scales of 10-100 km2. We have two goals in this chapter. First, we review what is known of the climate and weather, geology and geologic history, geomorphology, soils, and hydrology of Monteverde. Our account will focus on higher elevations in Monteverde and wetter areas on the Caribbean slope, with less attention to the drier environments on the lower Pacific slope. Second, we point out areas where our knowledge is incomplete and suggest promising lines of future research. Although the geology and geomorphology of Monteverde are moderately well-known, our knowledge of the rates of many geomorphic processes, particularly erosion, is poor. We also lack information on soils and hydrology, particularly of wind-driven cloud and precipitation inputs, evapotranspiration, and stream outputs from forests and other land-use types in Monteverde. Quantitative information on how variability in the physical environment interacts with biotic processes at the population, community, and ecosystem levels is scant.

2.II. CLIMATE AND WEATHER OF MONTEVERDE Most of the climate and weather data were collected at 1450 m at the Pensión (1956-1971), at 1520 m at John Campbell's residence (1972-present), and intermittently throughout or near the Monteverde Cloud Forest Preserve (MCFP) (Lawton and Dryer 1980, Crump et al. 1992, Clark 1994, Bohlman et al. 1995, W. Calvert and A. Nelson, unpubl. data). The climate of Monteverde is transitional between lowland and montane sites in terms of ambient air temperature, and transitional between the Caribbean and Pacific sides of Costa Rica in terms of incident solar radiation and amounts and seasonality of precipitation (Coen 1983, Herrera 1985, Vargas 1994). Costa Rica is a relatively small landmass between the Caribbean Sea and Pacific Ocean so continental low pressures are not generated during any season. Rather, the migration of the Intertropical Convergence Zone (ITCZ), a zone of low pressure associated with intense 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 tradewind-driven clouds and precipitation play major roles in controlling microclimate across Monteverde (Lawton and Dryer 1980).

2.II.A. Daylength, Solar Angle, and Solar Radiation Daylength at Monteverde oscillates seasonally between 11 hr and 32 min on 22 December and 12 hr and 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 1085 W m-2 (assuming a value of 0.7 for atmospheric transmittance), but reflectance and absorption of solar radiation by clouds strongly affects 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-September 1992 showed seasonal attenuation by clouds, particularly by cumulus and strato-cumulus clouds in July and August, when compared to calculated clear-sky incident solar radiation (Fig. 2.3). The seasonality of cloud types (Sect. 2.II.C) also has an effect on incident solar radiation, which potentially results in seasonal differences on east- and west-facing slopes (Fig. 2.4). Combined with the seasonal variation in solar angle and daylength, the effects of cloud cover and type on incident radiation likely have major effects on ecosystem processes such as evapotranspiration, primary production, and nutrient cycling, and may also cue phenological changes and other processes in plants and animals. The linkage of this type of abiotic data with information on biotic processes is needed.

2.II.B. Temperature The reduction in ambient air temperature with increasing altitude (adiabatic cooling) causes air temperatures to be lower at Monteverde compared to lowland sites, but higher than at montane sites in Costa Rica such as Villa Mills (3000 m) and Volcán Irazú (3400 m) (Table 2.1; Fig. 2.5). Mean annual temperature at Monteverde measured at 1460 m from 1956-1995 was 18.5°C, with a minimum of 9.0°C and a maximum of 27.0°C (J. Campbell, unpubl. data). Mean monthly minimum and maximum temperatures during the same period ranged from 14.0-17.6°C in December, and from 16.5-21.2°C in June, respectively (Fig. 2.5). The coolest air temperatures are associated with outbreaks of polar air, which typically originate in North America (Sect. 2.II.D). Air temperatures at lower elevations on the Caribbean and Pacific slopes of the Monteverde area are higher, but no long-term records exist.

2.II.C. Cloud Water and Precipitation Mean annual precipitation depth measured at 1460 m at Monteverde from 1956-1995 was 2519 mm. Minimum and maximum annual precipitation during that time were 1715 mm (1959) and 3240 mm (1996), respectively (J. Campbell, unpubl. data; Table 2.1; Fig. 2.6). Reported precipitation depths are minimum estimates for the upper portions of Monteverde because standard raingauges substantially underestimate wind-driven cloud water and precipitation. For example, at a leeward cloud forest site (20 m higher in elevation and ca. 2.5 km east-south-east from the Monteverde weather station), annual precipitation depth collected in 1991-1992 with a standard rain gauge was 3191 mm; an additional 886 mm of wind-driven cloud water and precipitation was collected with a cloud water collector (Clark 1994). Additional wind-driven inputs represented 22% of total hydrologic inputs. In comparison, reported precipitation depth for the Monteverde weather station was 2223 mm for this period. Precipitation throughout the Cordillera varies spatially with elevation and exposure to the tradewinds. The migration of the ITCZ controls the seasonality of precipitation and the types of clouds and precipitation, particularly on upper slopes and ridges in Monteverde. In areas that are exposed to the tradewinds, moisture from clouds and wind-driven precipitation intercepted by the vegetation may represent a major hydrological input; the actual contribution to a forest is difficult to quantify (Stadtmüller 1987, Cavelier and Goldstein 1989, Bruijnzeel and Proctor 1993, Cavelier et al. 1996). Terms for cloud water and mist inputs include "occult precipitation" (Dollard et al. 1983), and "horizontal precipitation" (sensu Stadtmüller 1987, Bruijnzeel and Proctor 1993) when wind-driven precipitation is included. We define "convective precipitation" as precipitation that originates from cumulus or cumulo-nimbus clouds (mean windspeeds <2 m s-1), "wind-driven precipitation" as precipitation that originates from stratus or strato-cumulus clouds with minimal cloud immersion (mean windspeeds ³2 m s-1), "mist" as precipitation that originates from stratus clouds with cloud immersion, and "cloud water" as non-precipitating stratus cloud immersion.

Three seasons are recognized in Monteverde on the basis of cloud and precipitation types: a) wet season (May-October), characterized by clear sky in the morning and cumulus cloud formation and convective precipitation in the afternoon and early evening; b) transition season (November-January), characterized by strong northeasterly tradewinds, stratus and strato-cumulus clouds, and wind-driven precipitation and mist during the day and night, and c) dry season (February-April), characterized by moderate tradewinds, stratus clouds or clear-sky conditions, and wind-driven mist and cloud water, particularly during the night (Fig. 2.6). During the transition and dry seasons, stratus cloud cover and wind-driven precipitation, mist, and cloud water depths typically increase with elevation and exposure to the trade winds. Maximum monthly precipitation in Monteverde occurs in the wet season in June, September, and October, when the ITCZ is directly over Costa Rica (Fig. 2.6). This occurs during maximum solar heating of land and sea surfaces, which results in high rates of heat (sensible heat) and water vapor release (latent heat) to the atmosphere (referred to as sensible and latent heat exchange, respectively). Absorbance of sensible heat by the atmosphere produces warm, buoyant air masses and generally unstable atmospheric conditions. Adiabatic cooling of these moist, ascending air masses causes water vapor to condense, and leads to the formation of cumulus and cumulo-nimbus clouds by the late morning and early afternoon. Cloud height may reach over 15,000 m. Clouds typically produce convective precipitation by the afternoon or early evening, often associated with intense lightning activity. When the ITCZ is at its northern boundary in late July and August, convective precipitation activity is typically reduced in Monteverde, a period referred to as the "veranillo" (little summer); light to moderate winds with mist and precipitation are interspersed with periods of convective precipitation. Cloud cover, however, may be greater at this time compared to other times of the year (Fig. 2.3). Ascending air masses at the ITCZ create a surface low pressure that must be replaced by air masses from regions to the north and south. This produces the surface tradewinds associated with global-scale Hadley cell circulation (Riehl 1979). When the ITCZ is located to the south of Costa Rica during the transition and dry seasons, northeasterly tradewinds deliver moist air from the Caribbean Sea to the lowlands on the Caribbean side of Costa Rica. Maximum mean wind velocity occurs from sea level up to 2800-3000 m, above which velocities decrease. When these air masses encounter the Cordilleras, they are forced to ascend (orographic uplift), and adiabatic cooling causes condensation and stratus or strato-cumulus cloud formation. Cloud base is typically at 1400-1700 m during these two seasons. Cloud immersion on the upper slopes and ridges along the continental divide (particularly on the Brillante Ridge and above Río Caño Negro) may approach 20-25% of the time, which is similar to the duration of cloud immersion in some montane forests in northeastern United States (Vong et al. 1991).

Compared to the wet season, cloud immersion and precipitation depths across Monteverde are more variable during the transition and dry seasons, and depend strongly upon topographic position and exposure to the tradewinds. Slopes and ridges on the east-facing (windward) side of the continental divide typically receive amounts of precipitation that are more characteristic of the Caribbean side of Costa Rica, primarily due to wind-driven precipitation inputs during the transition season (Essay 2.1; Fig. 2.6). Cloud immersion and wind-driven precipitation occur with decreasing frequency and duration on east-facing slopes and ridges as the dry season progresses. Slopes and valleys on the west-facing (leeward) side of the continental divide have precipitation regimes that are more similar to those on the northern and central Pacific side of Costa Rica (Fig. 2.6). The San Luis valley, for example, experiences relatively small amounts of wind-driven precipitation and minimal cloud immersion during the transition and dry seasons. Wind-driven cloud water and precipitation inputs represented a ca. 20% increase over annual precipitation input for the leeward cloud forest site in the MCFP that was immersed in cloud only ca. 7% of the time. Intermittent data collected at La Ventana on the Brillante, however, suggest that much higher inputs to windward cloud forests occur (Clark 1994; Essay 2.1). A similar pattern for cloud water and precipitation has been documented across a transect in northern Panama (Cavelier et al. 1996). The input of “mist” that augmented precipitation collected with standard rain gauges ranged between <200 mm (500 m elevation) and 2295 mm (on a ridge at 1270 m), which represented 2-60% of total hydrologic inputs. Other estimates of wind-driven cloud water and precipitation in Central American and northern South American montane forests range from 70-940 mm yr-1 (7-48% of total hydrologic inputs; Baynton 1968, Vogelmann 1973, Cavelier and Goldstein 1989, Asbury et al. 1994).

Using a regional hydrologic balance method, Zadroga (1981) estimated cloud water and precipitation inputs of up to 9000 mm for the headwaters of Río Peñas Blancas. He divided the total annual runoff by the total annual rainfall for streams in the San Carlos drainage basin (which includes the Río Peñas Blancas) on the Atlantic slope, and compared these ratios with streams in the Bebedero basin, which flows from the Pacific slope. For the San Carlos basin on the Atlantic slope, the ratio of runoff to precipitation was 102%, which indicates that more water ran off than fell as direct precipitation. Runoff exceeded monthly precipitation, which was attributed to either an underestimation of rainfall due to an insufficient number of precipitation gauges, or to an underestimation of the total precipitation depth due to inadequate sampling of cloud water and wind-driven precipitation (Zadroga 1981). An average annual cloud water and precipitation depth of up to 9000 mm was calculated for the upper Peñas Blancas watershed. In contrast, the ratio of runoff to rainfall was 34.5% for the Bebedero basin on the Pacific slope. Runoff exceeded precipitation only in the dry season months (January-April) when the river flows were maintained by water released from bank storage (Zadroga 1981). Wind-driven hydrologic inputs to tropical cloud forests may be greater than those to temperate montane and coastal forests. Studies in the United States suggest that although cloud water deposition equals approximately 20-30% of total hydrologic inputs, absolute depths are less than in tropical cloud forests (Bruijnzeel and Proctor 1993, Dingman 1994, Cavelier et al. 1996). Tropical cloud forests likely receive greater wind-driven hydrologic inputs due to relatively higher tradewind velocities and "wetter" cloud events, although cloud immersion may be of similar duration (Vong et al. 1991, Bruijnzeel and Proctor 1993, Asbury et al. 1994). Quantification of wind-driven cloud water and precipitation inputs in Monteverde is a major future challenge. A transect of cloud water and precipitation collectors from the Peñas Blancas valley to the San Luis valley and measurements of other meteorological variables is needed to estimate hydrologic inputs. A larger network of collectors along the continental divide is necessary to estimate maximum amounts of cloud water and precipitation inputs to the region. Micrometeorological techniques to measure cloud water inputs to forests are also promising, but they require complex instrumentation (Gallagher et al. 1992, Vong and Kowalski 1995).

2.II.D. Weather Systems Weather systems that affect Monteverde are regional to global in scale and can be classified into three categories: a) temporales del norte, b) temporales del Pacífico, and c) hurricanes. Temporales del norte are the result of outbreaks of cold, dry, polar air that originate in the North Pacific, occuring most frequently from December-February. As these strong cold fronts pass over the Gulf of Mexico and the Caribbean Sea, warm, moist air masses are forced to ascend above cooler, denser air. Adiabatic cooling forms stratus and strato-cumulus clouds, and the tradewinds force these air masses over the Cordilleras. Orographic uplift and further adiabatic cooling results in intense wind-driven precipitation and mist in Monteverde. Temporales del norte typically have the longest duration of the three types of weather systems at Monteverde, lasting up to 14 days with continuous precipitation (J. Campbell, pers. comm.). Temporales del Pacífico are the result of tropical low pressure systems in the Caribbean basin, occurr frequently from August-October, and correspond with the hurricane season in the Caribbean. They can reverse surface winds so that warm moist air is drawn over Monteverde from the Pacific Ocean. Orographic uplift and adiabatic cooling of these air masses, combined with high rates of sensible and latent heat exchange with land surfaces, produces stratus and strato-cumulus clouds which results in cloud immersion and precipitation throughout the day and night. Although they are typically shorter in duration than temporales del norte, they may result in high precipitation. The maximum daily precipitation depth recorded during a temporal del Pacífico was 160 mm (J. Campbell, pers. comm.). Hurricanes are relatively rare in Monteverde. In the last century, only one hurricane hit Costa Rica directly (Hurricane Martha, 21-25 November, 1969). However, high rainfall reported for September and October reflects the indirect effects of tropical depressions, some of which form hurricanes as they travel northward. For example, precipitation depth from Hurricane Gilbert (October, 1988) totaled 240 mm in 30 hours (J. Campbell, pers. comm.).