Seed and seedling ecology of tree species in neotropical secondary forests: Management implications

In spite of the growing importance of neotropical secondary forests as sour ces of timber and environmental services, the baseline information needed t o develop silvicultural options is still limited. In this paper I describe interspecific patterns of seed longevity in the soil, germination, and surv ival and growth of transplanted seedlings under closed canopy of nine tree species that are common in secondary forest stands in wet, lowland Costa Ri ca and most of which are timber species in the region: Cordia alliodora, Ha mpea appendiculata, Jacaranda copaia, Laetia procera, Rollinia microsepala, Simarouba amara, Stryphnodendron microstachyum, Trichospermum grewiifolium , and Vochysia ferruginea, Many of these species also occur throughout the lowland neotropics. Experiments were carried out in three replicate seconda ry-forest stands (20-30 yr old after pasture abandonment) located at La Sel va Biological Station in northeastern Costa Rica. Longevity of experimental seed cohorts differed markedly among species, from <3 mo (Cordia, Hampea, Simarouba, Vochysia), to >1 yr (Stryphnodendron). Similarly, germination of recently dispersed seeds in the understory ranged from 0% in Laetin to >75 % in Cordia and Vochysia. In contrast, seedling survival was uniformly low (<10% survival one year after transplanting except for Stryphnodendron, whi ch showed similar to 20% survival). The implications of these findings for the management of secondary forest s tands for timber production are varied and depend on the species of interes t. First, all study species appear to require nearly complete canopy openin g to regenerate as they show limited capacity either to germinate or to sur vive as seedlings in the understory. Second, some species that can germinat e at high levels in the shade can be managed at the seedling stage by openi ng up the canopy a few months after germination (e.g., Cordia, Simarouba, V ochysia). Third, species that show little or no germination under closed ca nopy (e.g., Jacaranda, Laetin, Rollinia) will need canopy removal simply to germinate in adequate amounts. Due to rapid declines in seed viability and seedling survivorship, however, any canopy manipulation must be performed not beyond 6 mo in order to guarantee adequate levels of soil-stored seeds or seedlings for future stand development. Site-preparation techniques may need implementation, given the potential of competing vegetation to interfe re with seedlings after canopy opening, as suggested by the high abundance of herbs and shrubs present in the soil seed bank in the study stands. This ecological scenario is likely to occur in secondary stands elsewhere in th e region as secondary stands are usually located within agricultural land. Finally, the results of this study suggest that ecological classifications of trees solely based on light preferences for stem growth may fail to acco unt for important differences among species in their regeneration mode. Thi s is of particular importance for refining silvicultural guidelines in neot ropical secondary forests.