Adjustment of structure and function of Hawaiian Metrosideros polymorpha at high vs. low precipitation

1. Populations of Metrosideros polymorpha establish across a broad range of precipitation in Hawai‘i – from <400 to >10000 mm per year. To determine whether adjustment of hydraulic and photosynthetic traits could contribute to this success in both high and low rainfall, we sampled populations on the wet and the dry sides of Hawai‘i Island, replicated on two different-aged lava flows at similar elevation and mean annual temperature. 2. We quantified 24 stem and leaf traits in an integrated study of hydraulics, gas exchange, leaf chemistry and anatomy. 3. Values for traits associated with capacity for water transport through the plant and gas exchange per leaf area were higher at dry sites, including photosynthetic capacity, nitrogen per leaf area and hydraulic conductivity on both sapwood area and leaf area basis. These adjustments, due to plastic and/or heritable differentiation, would partially compensate for generally lower water availability. 4. Specific leaf area shifted towards lower values at the dry sites. However, several other traits associated directly with drought tolerance were inflexible across sites, including stem vulnerability to embolism and leaf cuticular conductance, indicating that the ability to persist through dry periods is sustained across the species’ range. 5. The ability of M. polymorpha to establish across a wide range of habitats is associated with, on one hand, adjustments in traits that would enable sustained growth across a dramatic range of moisture supply while simultaneously maintaining as fixed other traits that would contribute to survival through extended drought.