Repositories of biodiversity: Arboreal seed banks in forest and pasture relict tree crowns in Monteverde, Costa Rica

Background/Question/Methods

When human activities shift closed canopy forests to landscapes of fragmented patches and isolated trees in pastures, seed reservoirs are reduced and the microenvironments that foster regeneration are altered. This habitat change reduces diversity and abundance of arboreal communities in secondary and plantation forests. However, canopy soils that originate in old forest trees and persist on branches of relict trees in pastures consist of decomposed epiphytes, intercepted atmospheric nutrients, and sloughed bark. They support an as yet undocumented repository of seeds that may enhance the reproductive capacity of agricultural landscapes. My objective was to document the diversity, abundance, and viability of seeds in arboreal soil mats in pasture relict trees and compare them to arboreal seed banks in adjacent closed forest tree canopies and in soil on the forest floor in Monteverde, Costa Rica.

Results/Conclusions Pasture relict canopy mats, closed forest canopy mats, and forest floor soils are distinct communities with respect to seed composition (Non-metric Multidimensional Scaling ordination; A = 0.2733, p < 0.0001). There was a greater density of seeds in the forest floor soil (272.1 g dm-3), than either the forest canopy (115.4) or the pasture canopy (193.2). Species richness and diversity was highest for the forest floor (55 species), exceeding both pasture (39) and forest canopy (33) arboreal mats. The growth forms in all three habitats were similar, dominated by trees (55% of all seeds), followed by shrubs (15%) and epiphytes (12%). Guild structure of pollinators was similar in all three communities, with dominance by insect (65%) and wind pollination (25%). Dispersal in all three habitats was dominated by birds (60%) and bats (25%), though pasture trees had a significantly higher proportion of wind-dispersed seeds (22%). Greenhouse experiments with pasture mats revealed that seeds in 32 taxa (trees, shrubs, epiphytes, and lianas) germinated and grew up to 16 cm in 2 months. Previous studies on epiphyte litterfall in Monteverde show that canopy soil and its accompanying seed banks fall from or "ride down" branches at an annual rate equivalent to 15% of total fine litterfall, which would bring seeds to the pasture floor and augment the limited amounts of seed brought in by wind and animals. Thus, in addition to providing perches for birds and shade to regenerating seedlings, pasture relict trees provide a hitherto overlooked source of "auxiliary" propagules that could enhance recovery and restoration of forest gaps and pastures