Elevational clines predict genetically determined variation in tropical forest seedling performance in Borneo: implications for seed sourcing to support reforestation

While much research has focused on genetic variation in plants in relation to abiotic clines in temperate and boreal forests, few studies have examined similar relationships in tropical forests. Genetic variation in desirable performance traits of trees, such as drought tolerance, fast-growth, and carbon sequestration rates, is widely used to improve reforestation efforts in nontropical systems. However, evolutionary processes such as local adaptation are poorly understood in tropical forests making it difficult to locate desired phenotypes. To test for genetic variation in growth rate in relationship to climatic clines, we conducted a common garden study over 18 months in a nursery using four dipterocarp tree species, represented by 9-12 half-sib families, sourced across an elevational gradient ranging from lowland to hill forests (circa 130-470 m above sea-level) in Malaysian Borneo. We found genetic variation in growth for all four species with fast-growing half-sib families growing 42-88% faster than poorly performing half-sib families. Furthermore, in three species we found that elevation of seedling origin predicted seedling performance; in Shorea fallax and S. johorensis, half-sib families originating from low elevations performed the best. In S. argentifolia half-sib families' seedlings from low elevations grew slowly. Because elevation is a good proxy for climate, the finding of elevational clines predicting genetic variation in growth provides evidence of evolution affecting the function of tropical tree species. Our research highlights opportunities to better understand evolutionary processes in tropical forests and to use such information to improve seed source selection in reforestation.