In regions with large agricultural fields that are far from sources of propa-gules, windbreaks and remnant trees in pastures and agricultural fields may be important reservoirs of native tree species (Guindon 1996; Holl 1998; Harvey and Haber 1999; Harvey 2000; Fig. 6.19). For example, the effects of planted windbreaks on seed deposition patterns were examined on dairy farms in Monteverde, Costa Rica, by Harvey (2000). The windbreaks were planted strips of trees about 5 m wide and 9 m tall and were 7-8 years old at the time of the study. Trees and rows within the windbreaks were spaced at 1.5 m. The most common species were Cupressus lusitanica, Croton niveus, Casuarina equisetifolia (all exotic species), and Montanoa guatemalensis (na-
tive). Windbreaks were found to receive significantly greater densities and species richness of seeds of tree and shrub species than pastures. Windbreaks received an average of 39 times as many tree seeds and 67 times as many shrub seeds as pastures. In addition, windbreaks received an average of two times as many tree species and more than two times as many shrub species as pastures. The differences in the seeds entering windbreaks vs. pastures appeared to be due almost entirely to the enhanced activity of birds in windbreaks. Bird-dispersed seeds occurred in greater densities (about 100 times greater) and the number of bird-dispersed species was three times greater in windbreaks than in pastures. The high densities of bird-dispersed seeds within windbreaks suggest that windbreaks increase forest seed recruitment by serving as habitat and/or movement corridors for seed-dispersing birds (Harvey 2000).
Windbreaks may serve as sources of woody colonists if the agricultural lands are later abandoned. Positioning of windbreaks within the landscape may affect seed deposition patterns by influencing the movements of seed-dispersing birds. Tree recruitment may be higher in windbreaks that are connected to forests. Windbreaks could be made more attractive to birds by including native, fruit-producing trees, by increasing their species and structural complexity, and by positioning them between forest patches to facilitate bird movement (Harvey 2000).
Remnant trees in pastures or agricultural fields may play an important role in conserving biodiversity within agricultural systems because they provide habitat and resources that are otherwise absent from agricultural landscapes (Harvey and Haber 1999). For example, in a survey of 237 ha of pastures in Monteverde, Costa Rica, Harvey and Haber (1999) found over 5,000 trees of almost 200 species, with a mean density of 25 trees ha"1. Primary forest trees accounted for over half of the species and over one-third of the individuals. More than 90% of the species were known to provide food for forest birds or other animals. In addition, many of the species were important as sources of timber, firewood, or fence posts for farmers. Reasons for leaving trees in the pastures included using them for shade for cattle, timber, fruits for birds, and fence posts. Results of surveys among farmers suggested that farmers in the region would be receptive to programs promoting the conservation of forest trees in pastures if these programs would fit the particular requirements of shade management for cattle and if they allowed farmers to use a small proportion of their trees for timber, fuelwood, or fence posts. The conservation of pasture trees must be part of larger conservation initiatives that include the conservation of large forest tracts, key habitats, forest fragments, migration routes, and corridors (Harvey and Haber 1999).
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