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

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 in a
greenhouse over two 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.  Several phenotypic traits were measured at the
start and end of the two-year experiment, with additional measures at the
end.  Initial and final morphological measures were used to
assess the retention of taxon-diagnostic traits at the two stages and the
related measure of plasticity after two years of growing under contrasting
environmental conditions. Variation in relative growth rates and
survivorship was examined across taxa, between glabrous and pubescent
groups, and across relative elevations within Ko`olau Volcano (1-4: low to
high).  Finally, Pianka's niche overlap index was estimated
for all taxon pairs.

近缘类群间的生态位分化（niche partitioning）模式，例如处于初期适应辐射（adaptive radiation）的类群，可为揭示生态位在生态与进化时间尺度上的形成、分化与整合机制提供关键线索。夏威夷铁心木属（Metrosideros）是一个形成于约300万年前的树木适应辐射类群，它们在夏威夷各岛形成连续林分并占据群落优势，为研究适应辐射早期阶段的生态位分化提供了理想研究体系。本研究以采自瓦胡岛库劳火山沿海拔梯度分布的8个铁心木类群的开放授粉果实培育的实生苗为材料，在温室中开展为期两年的生长与存活监测实验，设置了四种光照与磷素养分组合处理。实验选取4个无毛类群（代表混合支系）与4个被短柔毛类群（代表单一的瓦胡岛特有支系），旨在检验类群间以及无毛与被柔毛类群间的分化假说。本研究在两年实验的初始与结束阶段分别测定了多项表型性状，并在实验结束时补充测定了部分性状。通过对比初始与结束阶段的形态学测定结果，我们可评估类群诊断性状在两个阶段的保留情况，以及在不同环境条件下生长两年后的表型可塑性（phenotypic plasticity）水平。我们还分析了不同类群、无毛与被柔毛类群间以及库劳火山沿相对海拔梯度（1-4：由低至高）的相对生长速率与存活率的变异情况。最后，我们计算了所有类群对的皮安卡生态位重叠指数（Pianka's niche overlap index）。