Archipelago-wide demography of Hawaiʻi’s most abundant, but declining, native tree, Metrosideros polymorpha
As climate change threatens natural ecosystems, much of the focus of conservation biology is on minimizing effects for the rarest species. Yet, the majority of individual plants in natural ecosystems come from a few common species. While common species might be at lower risk of extinction, they engineer environments, are involved in many biotic interactions, and contribute disproportionately to ecosystem function. Thus, declines in their abundance and distribution have broad ramifications for other species and for ecosystem functioning. On each of the six largest Hawaiian Islands, the dominant tree species is Metrosideros polymorpha (‘ōhi‘a lehua), and ‘ōhi‘a has experienced a dramatic shift in its abundance over the past several decades. At present, the few studies that have been conducted on the population ecology, growth, and decline of ‘ōhi‘a have been limited to a few locations and life stages, and are thus insufficient for predicting how this foundation species will respond to future climate change.
With support from PI-CASC and the National Geographic Society, we have collected ‘ōhi‘a population data for 37 sites across the islands of Hawai‘i, O‘ahu, Maui, Kaua‘i, and Molokai. Within each site, permanent transects were established along which all woody plants were georeferenced, measured, and identified to species (Fig. 1, for example). Additional site data were collected for ground vegetation and seedling abundances. In order to review the population dynamics of ‘ōhi‘a across its range, sites were specifically selected to span gradients of substrate age, elevation, precipitation, and habitat degradation. Data are currently being analyzed, but initial results reveal alarming consistency across these gradients in population structure. Specifically, while adult trees are persisting despite various threats, a widespread lack of recruitment is evidenced by low abundances of sub-canopy juvenile trees and seedlings. Because all woody plants were censused, current analyses are focusing on the characterization of species with high rates of recruitment in order to model future turn-over in community composition as mature ‘ōhi‘a trees die. In addition to the establishment of permanent transects, a large manipulated experiment was conducted on O‘ahu to test whether seedlings are adapted to local precipitation levels. Twenty-seven populations of ‘ōhi‘a were identified in sites that ranged from 1,000 mm to over 4,500 mm annual precipitation. Seeds were collected from all 27 sites and then outplanted into all 27 sites to form a massive reciprocal transplant experiment. Seedling emergence and survival over the following 6 months (Jan – June, 2015) were quantified for all populations at all sites. Data are currently being analyzed, and preliminary results show that O‘ahu populations of ‘ōhi‘a differ significantly in ways that suggest seedlings are adapted to local rainfall patterns. These results will shed light on predictions for future species distributions under climate change of increasing drought.
Associate Professor of Botany, UH Mānoa