Canopy openness modifies tree seedling distributions along a tropical forest elevation gradient

Abstract

In theory, canopy openings can influence tree species establishment and resulting distributions over environmental gradients, but evidence concerning the magnitude and direction of such effects remain scarce. In this study we examine how canopy openings influence seedling persistence and growth and resulting elevation range limits. We transplanted 1360 seedlings of eight woody species (trees Trichilia habanensis, Persea caerulea, Cedrela tonduzii, Cornus disciflora, Quercus salicifolia, Q. bumelioides, Q. costaricensis and the shrub Hypericum irazuense) under the forest canopy and in open areas with understorey vegetation removed in a replicated design along a 2900 m elevation gradient in Costa Rica. We recorded survival, stem diameter and height over 18 months. We used hierarchical generalized additive models to examine relationships of seedling survival and growth with elevation and treatment (inside forest versus open area). We found a clear effect of open versus forest conditions on survival across the elevation gradient for six species. Species planted into open areas with full light exposure survived 100–1000 m below (~ 0.5–5°C warmer), and in one case 300 m above (~ 1.8°C colder) their observed ranges. Above 2900 m, survival of all species was markedly greater under the forest canopy versus open sites, suggesting facilitation from the forest, as frost occurred in open areas above this elevation. The resulting upper range increased between 200 and 1160 m (~ 1–6°C colder) compared to their observed range. At lower elevations, severe insect herbivory occurred on some species likely influencing lower range limits. Open areas influenced how seedling survival and growth varied with elevation for all the studied species. All species survived and grew outside the elevation ranges where they are typically observed. We suggest that the influence of forest gaps and clearings over extended gradients offers novel insights into tree range dynamics, limits and conservation.