Keywords
Cattle, Costa Rica, grazing, Hyparrhenia rufa, management, Palo Verde National Park, tropical dry forest.
Tropical dry forests are among the most endangered ecosystems in the world with less than 0.1% of the original dry forests of Pacific Mesoamerica under protection (Murphy and Lugo 1986, Janzen 1988 ). Deforestation for the development of cattle pastures has been the principal cause of tropical dry forest conversion throughout Mexico and Central America (Quesada 1974 , Toledo 1992 , Maass 1995 , Quesada and Stoner 2003 ). The conversion of tropical dry forests lo pasture lands results in a reduction in biodiversity (Lugo 1986 , Toledo 1988 ). Although most tropical dry forests are less biologically diverse compared to tropical humid or wet forests, their threatened status and high endemism elevate their conservation value (Toledo 1988 , Ceballos 1995 ). In addition lo reduction in species diversity (Guevara et al. 1992 ), other ecological consequences of tropical forest conversion include: (1) reduction in vegetative cover, (2) disruption of the water cycle, (3) changes in soil nutrients and status, and (4) soil compaction ( Buschbacher et al. 1988 , Uhl et al. 1988 , García-Oliva et al. 1994 , Maass 1995 , Johnson and Wedin 1997 ).
lt has been emphasized recently that despite Costa Rica's renowned conservation efforts of establishing protected areas (Boza 1993 ), the agriculture, cattle, and timber industries still threaten the biodiversity of this small country (Hunter 1994 , Vandermeer and Perfecto 1995 , Sánchez Azofeifa et al. 2001 , Quesada and Stoner 2003 , Stoner and Timm 2003 ). Although the establishment of extensive pastures for cattle grazing has been one of the major threats to tropical dry forests in Costa Rica, cattle are being used as a management tool within some protected areas of this country (Stoner and Timm 2003 , Quesada and Stoner 2003 ). The management plan implemented by the Tempisque Conservation Area, Guanacaste, includes the use of cattle within the boundaries of Parque Nacional Palo Verde and neighboring Reserva Biológica Lomas Barbudal ( Vaughan et al. 1995 ). The decision to use cattle to control fires and to promote regeneration of the forest was based on a survey that concluded that cattle grazing reduce tires by controlling the expansion of "African star-grass" (Hyparrhenia rufa) and favor the fast succession of dry forest ( Barboza 1995 , 1996 ). A review of cattle grazing at Palo Verde and its impact on wildlife is provided by Stoner and Timm (2003) .
Whether or not cattle significantly reduce the fire hazard remains questionable since this grass species is not consumed by cows during the dry season when it is tall, dry, and low in nutritional value (Daubenmire 1972 ). In the tropical dry forest of Brazil it has been demonstrated that as much as 60% of cattle's diet comes from browsing on seedlings, trees and shrubs during the dry season (Thomas et al. 1983 ). In Guanacaste, Costa Rica, cattle browse on at least 16 native tree and shrub species common to this tropical dry forest (Conklin 1987 ). Regardless if cattle are consuming H. rufa, it is important to document the effect that they may have on the structure and composition of the tropical dry forest before this practice is adopted in other protected areas within the Neotropics.
The objective of our research was to compare the vegetation within an ungrazed area of tropical dry forest with an area that has experienced intermittant cattle grazing to determine the short-term effects of cattle grazing on the vegetation. We compare species diversity, floristic composition, and vegetation structure in nearby sites with no grazing and with intermittent cattle grazing within a national park.
Materials and methods
The region of Palo Verde (more area than is presently park lands) was the Stewart family cattle ranch from 1923 until 1979 and has contained cattle under different densities throughout this time (D.A. Stewart, pers. com.). Approximately 18 000 cattle were rotated within an area of 1 00 000 ha including parts of the present-day national park (i.e. Laguna Palo Verde) from 1923-1949. These records also indicate a decline to an average of 7 000 head of livestock per year on 23 000 hectares from 1963-1979. During this period, the cattle were regularly rotated from uplands outside of the Palo Verde area during the rainy season to the Laguna Palo Verde and adjacent pastures during the dry season (D.A. Stewart, pers. com.). In 1975, 18 000 ha were expropriated from the Stewart family by the Costa Rica government as part of a settlement program (Mozo 1995 ). In 1977 part of the lands expropriated were designated as a national wildlife refuge (Executive Decree No. 6942-A, April 18, 1977) and in 1979 all cattle were removed from the newly created protected arca (D.A. Stewart, pers. com.). In 1990 the wildlife refuge and the adjacent arca were fused into a single park, Parque Nacional Palo Verde. In 1991, cattle were introduced to some areas of Palo Verde including the wetlands and flat lowlands. This was a result of the Tempisque Conservation Arca deciding to utilize cattle as a "management tool" to control fires and to manage the wetlands ( McCoy 1994 , Mozo 1995 ).
Our study sites were chosen based on aerial photographs of Parque Nacional Palo Verde from 1956, 1970, and 1986 (Instituto Geográfico Nacional, Costa Rica), and interpreted Landsat TM satellite images from 1992 (Maldonado et al. 1995 ). These photographs and images indicate that the lowlands and the hills between the ridges in the arca between Cerro La Carreta and Cerro El Jocote were covered with forest in 1970 and 1986. At the end of the 1980's severas fires went through this region of the park eliminating much of the forest cover originally present in the 1970 and 1986 aerial photographs ( Vaughan et al. 1995 ). In 1991, with the implementation of the management program in Palo Verde, cattle were introduced into the park and allowed to graze within a 200 ha fenced-in arca called La Carreta. Cattle were maintained within this arca from May until January and the density varied between 1.0 and 1.4 head per ha; occasionally cattle were removed when there was not sufficient food for them to eat within the arca (Barboza 1995 ).
Vegetation surveys were conducted in two sites (treatments) that were located near each other: (a) intermittently grazed; and (b) ungrazed. The arca of La Carreta where cattle were regularly introduced between 1990-1996 and allowed to graze was considered vegetation influenced by intermittent cattle grazing. This arca contained Hyparrenia rufa interspersed with shrubs and small trees. The arca between Cerro La Carreta and Cerro El Jocote where cattle were never observed nor were there signs such as tracks, manure, or trampled or grazed seedlings, was considered ungrazed.
H. ruffa was not observed in this arca. Interviews with park personnel and one of the former owners of the Palo Verde ranch (D.A. Stewart) indicate that cattle were not found in this arca either when it was a ranch, or more recently when the cattle program was reinitiated within Palo Verde. Since the aerial photographs showed both our grazed and ungrazed habitats to consist of forest in 1970 and 1986 and fires have passed through this entire arca in subsequent years, we assume that any differences in the vegetation between the two treatments are due to the effects of cattle grazing.
Vegetative sampling protocol: Data were collected in July, 1996. The method of variable arca sampling was used to compare floristic diversity, composition, and vegetation structure (Stern 1998 ) between the intermittently grazed and ungrazed sites. This vegetation rapid sampling method was developed for and used by the Conservation International Rapid Assessment Program (web site: http://www.conservation.org ). Sampled stems were divided into the following three size classes (diameter at breast height, 1.3 m; dbh): (a) 1.0-2.5 cm, (b) >2.5-5 cm, and (c) >5 cm. Six transects two meters wide were conducted within each type of habitat and all stems greater than 1.0 cm dbh were identified to species. A relatively narrow transect width of 2 m was selected to minimize oversampling spatially-clumped stems often of a single species. Each transect was continued until 50 stems were recorded for each of the three size classes, for a total of 150 stems (each stem was considered an individual). In two transects, both at the intermittently grazed site, 50 stems of the large size elass (>5.0 cm) were not found in which case the transect was discontinued after a distance of 1 km; these two cases both included > 40 stems. To compare size classes between the two sites, we recognized category (c) as >5-10 cm and later extrapolated a fourth category (d) of >10 cm based on field data. Plant species determinations were made in the field by local experts and with the aid of Janzen and Leisner (1980) and Gentry (1993) .
Data analysis: A chi-square test was used to determine differences between the intermittently grazed and ungrazed sites in terms of: (a) number of plant species, (b) number of stems in the small (1.0-2.5 cm dbh) and large (>5 cm dbh) size class of the fifteen most common species, and (c) number of stems in the small/medium (here lumped, 1.0-5.0 cm dbh) and large (>5 cm dbh) size class of the 20 species recognized by Hartshorn (1983) as representative of mature dry forest at Palo Verde.
Fisher's alpha was the algorithm used lo compare plant species diversity between the intermittently grazed and ungrazed samples; this test, based on species presence and not relative abundance, is appropriate and reliable for comparative purposes when sample sizes are relatively small (Condit et al. 1998 ). A Mann-Whitney U test was used to compare differences in Fisher's alpha between the six transects of the intermittently grazed and ungrazed sites. Fisher's alpha was also calculated for larger (>10 cm dbh) trees only, and for Hartshorn's (1983) tree data from Palo Verde mature forest. In 1970, Hartshorn (1983) established a 4 ha permanent plot in Palo Verde and all trees (>10 cm dbh) were identified. According to Hartshorn this forest had experienced little human intervention and cattle grazing had been minimal. The data obtained by Hartshorn (1983) represents the only plant species database of original mature forest available for this tropical dry forest. Finally, a Shannon index of diversity using the 6 replicates of each treatment was calculated lo compare diversity between the grazed and ungrazed sites and the Shannon t-test was used lo test for differences between these habitats ( Magurran 1988 ).
Results
When the fifteen most common species at each site were compared and divided into size classes, floristic differences between the two habitats were even greater (Fig. 1 ). Nine of the fifteen most common species in ungrazed habitat were not found in the fifteen most common species in the grazed area. Furthermore, the size class structure between the two habitats was significantly different (X 2 = 5.7, p < 0.016, df =1), with greater representation of small stems (1.0-2.5 cm) in the intermittently grazed area and a greater representation of large stems (>10 cm) in the ungrazed area ( Fig. 1 ).
Further evidence of the impact that grazing has on the floristic composition and structure of the Palo Verde tropical dry forest was demonstrated by comparing Fisher's alpha values for large trees only from the grazed and ungrazed sites with Hartshorn's (1983) permanent tree plot data in Palo Verde mature forest ( Table 2 ). The ungrazed area is more similar to the forest in the Hartshorn plot than to the nearby intermittently grazed site. lt should be noted that the Hartshorn plot is located approximately 20 km from our sampling area and data were collected in 1970.
Finally, there was a significant difference in the number of small stems (1.0-5 cm) and large stems and trees (> 5 cm) in the grazed and ungrazed treatments when comparing only the 20 species most commonly found in Hartshorn's mature forest (X2 = 27.3, p < 0.0001, df = 1; Table 3 ). While the intermittently grazed area had a greater abundance of small stems (of the 20 species considered) than the ungrazed area (75 versus 30 stems, respectively), this ratio changed as the cohort aged (large stems, 30 versus 80 stems, respectively) as many small stems were possibly eaten by cattle, trampled, or killed by other factors.
Discussion
Impact of cattle grazing on vegetation structure and composition: We demonstrate that the floristic structure and composition of the intermittently grazed sites sampled were different from the sites sampled with no cattle grazing. The analysis of floristic composition showed significantly lower species diversity at the sites where intermittent cattle grazing occurred than at the sites with no grazing ( Table 1 ). Although the habitat with intermittent grazing was less diverse at the species level, 14 species were unique to this area thus contributing to the overall diversity of the landscape; however, the area with no grazing contributed more to the overall species diversity with 42 species unique to this habitat.
Species that were abundant in both intermittently grazed and ungrazed areas, included Acacia collinsii, Dalbergia retusa, and Tabebuia ochraceae. Some of the most abundant species in the intermittently grazed area included severas early successional species such as Tabebuia rosea, Cassia pallida, and Piscidia carthagenensis. In contrast, severas of the most common species in the ungrazed area included species characteristic of mature tropical dry forest ( Hartshorn, 1983 ) such as Luehea candida, Calycophyllum candidissimum, Jacquinia spinosa, and Pachira quinata. The significant difference in size class distributions between intermittently grazed and ungrazed areas emphasizes the impact that cattle have on the structure of tropical dry forest ( Fig. 1 ). There were significantly more small stems in the intermittently grazed area and significantly fewer large stems. The low abundance of even the smallest size class of species that are characteristic of mature dry forest in the intermittently grazed area suggests that cattle grazing have a detrimental effect on the regeneration of tropical dry forest, and that the impact of cattle may direct a change in the specíes composition of the forest as the small class matures.
Of the twenty most common tree species in the Hartshorn plot from mature forest within Palo Verde, six species were not found in any of our transects. Twelve of the 20 most common species from the Hartshorn plot were found in our transects from both intermittently grazed and ungrazed sites. The ungrazed sites contained 12 common mature forest species in greater abundance's and from larger size classes than in transects from the intermittently grazed sites. The species diversity (Fisher's Alpha values) of the Hartshom plot was intermediate between the diversity of our ungrazed and intermittently grazed sites. A possible explanado for the highest diversity in the ungrazed area is that this habitat represents a mosaic of both pioneer and mature forest species because this area is regenerating from fires.
Since aereal photographs indicate that both our intermittently grazed and ungrazed areas consistes of forest in 1970 and 1986, and fires probably affected both of these areas equally since they are adjacent lo each other, the subsequent changes in vegetation are most likely due to the effect of cattle grazing. Alternatively, severas other factors undoubtedly have contributed to the differences observed in these two habitats. The presence of H. rufa in the intermittently grazed area may prevent the establishment of other plants since grasses often inhibit the establishement of woody and herbaceous species in open areas ( Guariguata et al. 1995 , Zahawi and Augspurger 1999 ). Furthermore, grazing promotes the horizontal expansion of H. rufa, allowing this aggressive grass to outeompete other woody colonizers ( Parsons 1972 , McNaughton 1985 , Milchunas et al. 1988 ). Finally, abiotic factors encountered by seeds in the more open areas in the intermittently grazed habitat likely decrease the survival of small seedlings in this area (Nepstad et al. 1990 , Gerhardt 1993 , Nespstad et al. 1996 ).
Many studies in western North America have demonstrated that grazing by domestic livestock in native ecosystems results in changes in plant species composition, disruption of ecosystem function, alteration of ecosystem structure, and changes in faunal diversity (for a review see Tables 1-4 in Fleischner 1994 ). In general, these studies show a decrease in plant species diversity accompanied by a change in floristic composition and structure within areas grazed by domestic livestock (Cottam and Evans 1945 , Gardner 1950 , Rummell 1951 , Blydenstein et al. 1957 , Winegar 1977 , Webb and Stielstra 1979 , Reynolds and Trost 1980 , Szaro and Pase 1983 , Bock et al. 1984 , Brady et al. 1989 , Schulz and Leiniger 1990 ). Furthermore, numerous researchers have documented the negative impacts of cattle grazing on native vertebrate populations (Berry 1978 , Duff 1979 , Reynolds and Trost 1980 , Jones 1981 , 1988 , Crouch 1982 , Mosconi and Hutto 1982 , Szaro et al. 1985 , Taylor 1986 , Taylor and Littlefield 1986 , Campbell 1988 , Medin and Clary 1989 , Bowen and Kruse 1993 , Stoner and Timm 2003 ).
]]> Studies evaluating the impact of domestic grazing on tropical ecosystems also have documented changes in vegetation structure and composition. In the Western Ghats, Kamataka, India, dry forests expanded into former pasture/savanna habitats when protected from human impacts of fire and grazing (Puyravaud et al. 1994 ). In the eastern Amazonian rainforest of Pará, Brazil, the success of forest regeneration in pasture lands depended on the intensity of pasture use. Furthermore, the composition and structure of recovered mature forest was not always similar to the mature forest prior to land clearing ( Buschbacher et al. 1988 , Uhl et al. 1988 ). A study in Curagao, Netherlands Antilles, documented that vegetation composition and structure was significantly different between rock vegetation with livestock grazing compared to rock vegetation with no livestock ( Debrot and Freitas 1993 ).Implications for management: The management objectives of the Tempisque Conservation Arca for the cattle project in Parque Nacional Palo Verde and Reserva Biológica Lomas Barbudal include the following: (1) reduce the risk of forest fires by managing the amount of combustible material, (2) minimize the impact of fire on biodiversity, (3) facilitate ecological restoration by using controlled grazing, and (4) generate a socio-economic benefit for participating cattle owners ( Mozo 1995 ). A parameter that was not considered in the management objectives of the Tempisque Conservation Arca is the impact that cattle have on the regeneration of tropical dry forest.
The management plan that is currently being implemented, claims that the Palo Verde area has been exposed to disturbance caused by cattle grazing during the last 400 years (Vaughan et al. 1995 ), however, no data is presented to confirm this statement. McCoy (1994) and Vaughan et al. (1995) argue that this disturbance has maintained a high diversity of plant species in dry forest communities and interpret this phenomenon as a result of the intermediate disturbance hypothesis proposed by Connell (1978) . Therefore, according to the management plan, cattle are essential for the regeneration of tropical dry forest communities and must be maintained in these areas perpetually (Vaughan et al. 1995 ).
In contrast to the management plan, the earliest records of cattle in the Palo Verde region come from 1923 when the Stewart family began a cattle ranch in this area (D.A. Stewart, pers. com.). Furthermore, it is difficult to interpret the disturbance of cattle at Palo Verde in the context of Connell's intermediate disturbance hypothesis as suggested by the management plan. This hypothesis suggests that highest diversity will be observed at intermediate levels of frequency and intensity of natural disturbances (Connell 1978 ). Cattle are not native animals of tropical dry forests of Mesoamerica and they do not represent a natural disturbance. Their presence within an area creates a constant disturbance in terms of both frequency and intensity that is determined by human intervention. The intensity of land use (i.e. disturbance by cows) depends largely upon the density of animals in a given area and the availability of forage. These two variables are controlled by human interests. For example, the density of cattle in the Palo Verde region when it was managed as a cattle ranch from 1923-1979 varied from 0.18-0.30 head of cattle per hectare. Since the implementation of the cattle program in Palo Verde and Lomas Barbudal the density has increased to 0.7-1.0 head of cattle per hectare (Mozo 1995 ), for a total of approximately 4000 cattle in 1998. In contrast to the cattle rotation schedule utilized by the Stewart family when Palo Verde was a ranch, the current management plan assigns specific sites to the participating cattle owners that result in areas with high animal density and no rotation. This practice may cause an incalculable damage to the structure and dynamics of this tropical dry forest.
An analysis of change in land-use of the area of Palo Verde and Lomas Barbudal was obtained using interpretad images of aerial photographs from 1956 and compared to satellite images interpretad in 1993 (Maldonado et al. 1995 ). This analysis shows that the area covered with pastures increased from 5% in 1956 to 12% in 1993 within Palo Verde and Lomas Barbudal. In contrast, Parque Nacional Guanacaste, another tropical dry forest also located in northwestem Costa Rica, has experienced an overall decrease in pasture land from 1979 to 1985. In only 6 years, 28% of the pasture land became successional deciduous or evergreen forest (Kramer 1997 ). Restoration of pastures to forest in Parque Nacional Guanacaste depends upon the distance to the seed sources, the seed dispersal mode of the plant species, the proximity to the forest patches, and the number and species of animals involved (Janzen 1986 ). Cattle, which were formerly present in this park, have been eliminated from this area and are not considered to be a viable management tool for controlling tires.
Our study shows that cattle grazing changes the floristic composition and decreases the structural complexity of the vegetation in tropical dry forest. Although the ultimase long-term impact of cattle grazing on tropical dry forest ecosystems is still unknown, there is no doubt that cattle are a risk factor to the maintenance of biological diversity of dry tropical forest vegetation. The long-term ecological cost of preventing fires with the use of cattle will be much greater than the short-term economic cost of investing in alternative fire manamenent programs. The effectiveness of alternative fire prevention measures should be evaluated, such as: (1) hand-cutting H. rufa with machetes, (2) the creation of more frequent and larger fire breaks, and (3) the development of "green" fire breaks consisting of fast-growing fire-resistant native plant species. The economic cost of cach of these alternatives should be evaluated in the context of available resources and infrastructure. Concurrently, community-based environmental education programs that promote safe burning practices should be continued and expanded. Finally, future studies should determine the effect of fire on the regeneration of this ecosystem to better understand the interaction between grazing and fire. The strategy of utilizing cattle to "manage" natural ecosystems in the rangelands of western North America has been criticized previously as a large uncontrolled experiment (Bock et al. 1993, Noss 1994). it now appears that the Tempisque Conservation Area in Guanacaste, Costa Rica, is similarly initiating an uncontrolled experiment within one of the few remnants of tropical dry forests in Central America.
Acknowledgments
We thank the student participants of OTS 96-9 (Tropical Plant Systematics) for help with field work. The help of N. Zamora, U. Chavarría, and R. Espinoza for field assistance and plant identifications is also gratefully acknowledged. We thank J. Stanford, R. M. Timm, and an anonymous reviewer for helpful comments on an earlier draft of this manuscript.
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ResumenEn el noroeste de Costa Rica se utiliza ganado como una "herramienta de manejo" para reducir la cantidad de material combustible, principalmente Hyparrhenia rufa, un zacate que proviene de Africa. Este proyecto se desarrolla dentro del Parque Nacional Palo Verde y la Reserva Biológica Lomas Barbudal, ambos forman parte de los últimos remanentes de bosque tropical seco de Mesoamérica. Con el fin de determinar los efectos a corto plazo del pastoreo de ganado vacuno sobre la vegetación natural, realizamos un muestreo dentro de Palo Verde para comparar la composición florística de un área de pastoreo intermitente con un área sin pastoreo. Encontramos significativamente menos especies de plantas en el área con pastoreo intermitente comparado con el área sin pastoreo. La composición florística de estos dos hábitats fue significativamente diferente basada en una comparación del alfa de Fisher y el índice de Shannon de diversidad; ambos fueron significativamente más altos en el área sin pastoreo. El área no pastoreado tuvo mayor número de especies y mayor semejanza con bosque maduro. La estructura de la vegetación fue significativamente diferente entre las dos áreas pastoreadas y no pastoreadas, con mayor número de individuos de diámetros pequeños (1-2.5 cm) y menor número de individuos de diámetros grandes (>5 cm) en el área con pastoreo intermitente. Estos resultados sugieren que el ganado reduce la abundancia relativa y la densidad de las especies de árboles y cambia la composición florística y estructura del bosque tropical seco. El plan de manejo actual de Palo Verde y Lomas Barbudal no es apropriado debido al impacto que el ganado tiene sobre la estructura de la vegetación natural y no debe ser considerado como una alternativa viable para otras áreas protegidas del neotropico. Sugirimos que se evalúen métodos alternativos para prevenir incendios, como el corte de jaragua (Hyparrhenia rufa) con machetes, la creación de rompefuegos en mayor cantidad y magnitud, y rompefuegos verdes.
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1 . Institute of Economic Botany, T'he New York Botanical Garden, Bronx, New York, 10458-5126, U.S.A. ]]>
2 . Instituto de Ecología, Departamento de Ecología de los Recursos Naturales, Universidad Nacional Autónoma de México, Apartado Postal 27-3 (Xangari) Morelia, Michoacán México 58089. Fax: 52 (443) 322-2712. mquesada@oikos.unam.mx ; kstoner@oikos.unam.mx