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, 'ōhi'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 si...

学界对环境梯度在长寿树种物种分化过程中的重要性认知仍十分有限。夏威夷特有树种欧希亚勒胡花（'ōhi'a lehua，*Metrosideros polymorpha*）存在两个形态迥异的变种，二者分别作为对应生境的冠层优势种，占据夏威夷岛东部由反复熔岩流扰动形成的演替梯度的两个极端生境。尽管存在杂交现象，但这两个变种仍得以维持，其成因可能是演替梯度两端均存在歧化选择（disruptive selection）。为验证该假说，研究人员采集了夏威夷岛东部各变种的3个重复单优林分的种子，将其萌发后，将所得幼苗分别置于温室中4种光照与氮素水平组合的环境下，以及早、晚演替阶段的野外样地中栽培。通过测定生长与存活指标，研究人员发现两个变种在温室高、低光照环境中存在适合度差异，且该差异与早、晚演替野外样地中的适合度差异相对应，[原文未完结]