Intraspecific divergence and evolution of a life-history trade-off along a successional gradient in Hawaii's Metrosideros polymorpha

The importance of environmental gradients in the diversification of long-lived tree species is poorly understood. Two morphologically distinct varieties of the endemic Hawaiian tree, 'hi'a lehua (Metrosideros polymorpha), are the canopy dominants at alternate extremes of a successional gradient formed by the recurring disturbance of lava flows on east Hawai'i Island. The maintenance of these varieties despite hybridization may be due to disruptive selection at either end of the successional gradient. To test this hypothesis, seeds from three, replicate monotypic stands of each variety on east Hawai'i Island were germinated and the resulting seedlings grown under four combinations of light and nitrogen levels in a greenhouse, and at early- and late-successional field sites. Growth and survivorship measures revealed differential fitness of these varieties in high- and low-light environments in the greenhouse with corresponding differential fitness in early- and late-successional field sites. Unique light-by-nitrogen interaction effects on growth were observed in each variety, and only the late-successional variety appeared to be nitrogen limited. These two varieties exhibit the classic plant life-history trade-off between fast growth in high light and high survivorship in shade, but notably within a single tree species. These findings strongly implicate a role for Hawaii's striking environmental heterogeneity in the emergence of at least two endemic forms of this woody genus.