Combined effects of herbivory and drought on the distribution of tree species along a tropical rainfall gradient
2 Smithsonian Tropical Institute, Panama
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Tree species distributions associated with rainfall are among the most prominent patterns in diverse tropical forests. Beside direct effects of water availability, herbivory (damage due to herbivores and pathogens) has long been hypothesized to shape tree species distributions.
We tested the hypothesis that higher herbivory in wet forests prevents less defended dry origin species from growing in wet forests (pest pressure hypothesis), and that drought prevents less drought tolerant wet origin species from growing in dry forests.
In a reciprocal transplant experiment at a wet and a dry forest site in Panama, seeds of 26 species with contrasting origin were planted. Half of the seeds and the resulting seedlings were treated with an insecticide and a fungicide to exclude herbivory. Performance parameters of the seedlings were recorded during one wet and one dry season.
The herbivore exclusion significantly increased all the performance parameters. However, we did not find a higher effect in wet forests for dry origin species. As expected, survival of wet origin species at the dry site was reduced during the dry season, indicating that drought limits the establishment of wet origin species in dry forests.
In conclusion, although herbivory is an important factor for establishment success in tropical forests, our results did not support the pest pressure hypothesis for the seed and early seedling stage. Drought on the other hand limits distribution of wet origin species.