Title

Seedling growth and survivorship of native tree species in response to light heterogeneity in wet second-growth forests of northeastern Costa Rica

Date of Completion

January 2001

Keywords

Biology, Ecology|Agriculture, Forestry and Wildlife|Biology, Plant Physiology

Degree

Ph.D.

Abstract

I examined the effect of light heterogeneity on seedling survival and growth responses of four canopy tree species in three second-growth forests (15–18 yr. old) in Northeastern Costa Rica. Dipteryx panamensis (Fabaceae/Papilionideae), Vochysia guatemalensis (Vochysiaceae) and Virola koschnyi (Myristicaceae) are shade tolerant species but benefit from gaps, whereas Hyeronima alchomeoides (Euphorbiaceae) is a fastgrowing species in open habitats and is rarely found in understory conditions. ^ Light conditions were experimentally created to produce four treatments: small gap, large gap, shrub-removal, and control. I transplanted 15 seedlings of each species into each treatment. I also transplanted seedlings of each species in an open pasture. Light availability ranged from 5.8 to 54.9% full sun in the forest sites, and from 32.2 to 87.9% in pasture. Seedlings grew under these conditions for 15 months. ^ All species showed high seedling survival and growth in the large gap or at light levels >20% full sun, and slow growth in the shaded understory and gap edge microsites. Species differed in their survival responses at low light availabilities (5–20% full sun). At low light, Hyeronima showed the lowest survival rates and the steepest survival Increments. Dipteryx and Virola showed high survival, and Vochysia showed intermediate Survival responses. Seedling survival and growth responses to light varied among sites and species. Initial height was an important covariate affecting seedling survival and growth. ^ Virola seedlings had lower leaf area ratios and produced more stem per shoot mass across light microsites than the other species. There was evidence of different degrees of phenotypic plasticity among study species in terms of growth characteristics. ^ I discuss growth and morphology species responses to a light gradient in the context of species shade tolerance and regeneration requirements. My results suggest trade-offs between maximum growth in high light and seedling mortality in the shade for Hyeronima and Virola, but not for Dipteryx and Vochysia, whose survival in the shade might be compromised by other ecophysiological factors. ^