Does a reciprocal transplant experiment of neighboring Vachellia karroo populations demonstrate local adaptation?

Abstract

Vachellia karroo (Hayne) Banfi & Galasso (formerly Acacia karroo) is the most widespread tree in southern Africa, and exhibits wide morphological variation such that the phenotypes have been considered as separate species. We sought to determine whether local adaptation was occurring in two neighboring Vachellia karroo populations (ca. 15 km apart) in South Africa that show tremendous size variation as adults. We examined seedling and sapling performance of genotypes grown in reciprocal environments that differ in climatic and edaphic conditions. We used a field reciprocal transplant experiment at Richards Bay and Empangeni, and complemented this with a common garden study. We measured total height, stem diameter, thorn length, leaf area, leaf weight, plant aboveground dry weight and specific leaf area after 4 months and 16 months for common garden and field-grown plants, respectively. For greenhouse-grown plants, root dry weight was also measured. Phenotypes of seedlings and saplings from the two populations maintained their differences in the greenhouse and in the field. Greenhouse-grown plants showed lower proportional root allocation for Richards Bay than Empangeni (i.e. root mass fraction: 0.20 ± 0.01 vs 0.60 ± 0.02). In the field, there were significant genotype-by-environment interactions for two of the seven performance traits for reciprocally-transplanted populations. We found limited evidence for local adaptation that is attributed to variation in edaphic conditions. Local adaptation was evident in two leaf traits (leaf area and leaf dry weight) that showed a genotype-by-environment interaction. These two traits clearly have important fitness consequences for photosynthesis. However, plants from one population grew taller and had longer thorns at both sites. We conclude that the magnitude of divergent selection in the two populations is relatively low and may be accompanied by processes such as genetic drift, leading to limited local adaptation and genetic differentiation.