USING NATIVE HAWAIIAN PLANTS IN LANDSCAPING WILL PROMOTE AWARENESS AND CREATE NEW DEPOSITS OF NATIVE FLORA
Plant local! We know there’s debate about how strongly native Hawaiian species like `ohi`a lehua should be emphasized in local landscaping projects. Often plants are chosen based on availability, popularity, ease of growth and economics. Here’s another consideration that hits closer to home. Hawaii’s native plants face a multitude of threats in their natural environments (fueling our infamous title of “endangered species capitol of the world”). Use of native species in landscaping efforts will not only showcase and promote an awareness of the unique beauty of Hawaiian flora, but done wisely can also create “repositories” of genetic stock. However, given the findings of our research and related studies on the evolution and biogeography of Hawaii’s flora, we strongly urge the landscape industry to keep native species as local to their source as possible and leave genetic introductions to conservation managers.
Hawai`i is an unprecedented natural laboratory for experiments in local adaptation and speciation (the emergence of new species from existing ones), drawing scientists from all over the globe to study its flora and fauna. The islands are the most geographically isolated archipelago in the world and support a surprising diversity of environments; Hawai`i Island alone encompasses eleven of the world’s thirteen climate zones according to one classification system. The islands boast over a thousand native plant species that derive from an estimated 263 unlikely, accidental colonists. These colonists traversed the Pacific by wind, sea, or bird within the past several million years. Plants that managed to establish multiple populations and spread to new areas found themselves in a range of environments and sometimes even isolated on new islands. Exposure over thousands of generations to Hawaii’s diverse environments, along with isolation of populations on separate islands, have shaped the morphological and genetic diversity that we see today in Hawaii’s native plants.
`Ohi`a lehua (Metrosideros polymorpha) is Hawaii’s most abundant and variable tree and a striking example of diversification within and among islands. While variation in the lehua flower color is what most people notice, the more important variation within this species is in its vegetative and ecological characters. `Ohi`a is actually a member of a larger group, Hawaiian Metrosideros. According to Diana Percy and colleagues, Metrosideros arrived in the Hawaiian Islands very roughly four million years ago on the island of Kaua`i. Over its roughly 4-million-year history in Hawai`i, Metrosideros has colonized every high island and diversified into five named species, including the hypervariable `ohi`a lehua. The eight varieties of `ohi`a lehua differ in their vegetative characters, and they specialize in different habitats. Many are single-island endemics (occurring on only one island), while a few have broader distributions. Collectively, the ecological amplitude of these varieties helps to explain why `ohi`a can be found in bogs and deserts, new lava flows and old forests, and in a continuous stretch from the Puna coastline to 8,100 feet on Hawai`i Island. Because of its diversity, `ohi`a is an excellent model for those of us who are trying to understand precisely how a diverse environment can drive new species to emerge from an existing one.
Our lab group at the University of Hawai`i Hilo has been using `ohi`a to try to understand how speciation happens, how long it takes, and what genetic changes are involved. Our work ranges from tests of reproductive barriers (such as flowering time variation, pollen tube or embryo rejection, or hybrid inviability or infertility), to studies of morphological and genetic variation among populations. Our studies began with a test of cross-fertility between the two most common varieties at middle elevation on east Hawai`i Island. We wanted to know what barriers, if any, existed between the successional varieties, var. incana (colonizer of new lava flows with hairy leaves) and var. glaberrima (dominant on old substrates with naked leaves). We quickly discovered we were in a hybrid zone with abundant first-generation, and apparently less common second-generation hybrids. We found that hybridizing these two varieties is easily done, which was not surprising given the high frequency of first-generation hybrids at the study site. Our results also revealed, however, that these hybrids, especially the second-generation hybrids, had reduced fertility. These hybrids would not make good seed (or pollen) trees in forest restoration program! The late-acting reproductive isolation we observed between these two varieties of `ohi`a indicates that they are significantly genetically diverged from each other and part way along the speciation process. In a more recent greenhouse study of the seedling ecology of these two varieties, graduate student Keenan Morrison demonstrated differences not only between varieties in their responses to different levels of light and soil nitrogen, but also among same-variety populations taken from different elevations on east Hawai`i Island. For example, seedling mortality in response to light and nitrogen actually flip-flopped between two populations separated by just 500 feet of elevation. These results are highly consistent with other observations that identify elevation as a major driver of differentiation within Metrosideros. Taken together, these observations at both the adult and seedling stages indicate clearly that these two common varieties harbor a tremendous amount of functional genetic variation within just windward Hawai’i Island. This does not begin to consider the variation within populations on the older islands of Maui, Lana`i, Moloka`i, O`ahu, and Kaua`i, where these two varieties are also found. Importantly, these are the varieties being sold for landscaping throughout Hawai`i!
We’re just beginning to uncover the mysteries of this dominant species and its cousins, but all the signs we have to date indicate that local adaptation within Hawaii’s diverse landscape has played a significant role in the diversification of this group. By working with our environment and keeping native species local to their source, the landscape industry will help prevent homogenization of native species. So, just as we know it’s good for our health and the environment to “eat local,” it’s clearly just as important to “grow local and plant local!” Let’s work together to landscape Hawai`i natively and wisely!
Elizabeth Stacy is an Associate Professor of Biology and Tropical Conservation Biology and Environmental Science at the University of Hawai`i Hilo; Jennifer Johansen is Field Technician at UH Hilo and Founder of Ho`oulu Lehua “inspiring growth in the native forests and youth of Hawai`i”. Lab website: http://www2.hawaii.edu/~estacy/index.html