Niche conservatism and sympatric parallel evolution may help to maintain eight nascent tree taxa along a sharp elevation gradient

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Abstract Patterns of niche partitioning among closely related taxa, such as those in incipient adaptive radiations, can provide clues into how niches are created, partitioned, and integrated over ecological and evolutionary time scales. Hawaiian Metrosideros is a ~3‐MYO adaptive radiation of trees that dominates the Hawaiian Islands in continuous stands within islands and allows examination of niche partitioning at the early stages of adaptive radiation. We monitored growth and survivorship over 2 years of seedlings derived from open‐pollinated fruits of eight Metrosideros taxa distributed along the elevation gradient of Koʻolau Volcano, Oʻahu, under four combinations of light and phosphorus. We included four glabrous taxa, representing mixed lineages and four pubescent taxa, representing a single, Oʻahu‐endemic lineage and tested hypotheses of differentiation among taxa and between glabrous and pubescent groups. Variation among taxa in response to light and phosphorus was modest, with high‐elevation taxa showing relatively greater niche partitioning (per Pianka's Index). Patterns of growth rate variation (plasticity) along the elevation gradient in response to light and phosphorus were similar between the glabrous and pubescent groups, with low‐elevation taxa showing high and low plasticity to light and phosphorus, respectively. Patterns of survivorship across light and phosphorus treatments were highly concordant within the recently evolved pubescent lineage. Both growth rates and survivorship rates suggested the conservation of contrasting light niches within glabrous and pubescent groups. Finally, the retention of taxon‐diagnostic phenotypes despite 2 years of growth under contrasting environments implied significant heritable differentiation among the eight taxa, despite the likely presence of hybrids in the open‐pollinated seedlings. The coexistence of eight predominantly infraspecific tree taxa within the confines of Metrosideros forest on Koʻolau Volcano appears to result from a combination of niche conservatism within the glabrous and pubescent groups and sympatric parallel evolution between these groups along elevation gradients. The modest niche partitioning among taxa by light and phosphorus, especially at middle elevations, suggests that additional environmental factors contribute to local adaptation in these trees. Results of this study suggest that both stabilizing and diversifying processes contribute to the early stages of adaptive radiation in trees. Read the free Plain Language Summary for this article on the Journal blog.

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