The recent article “What’s killing our ohias besides rapid ohia death?” has a proposal that warrants viewing with caution. The author suggests that new horticultural species from Australia should be introduced to Hawaii due to native ‘ohi‘a die-offs.
For background, scientific studies show that ‘ohi‘a death and disease are due to a suite of human-introduced species and habitat destruction. Rapid ‘ohi‘a death (ROD) is caused by an introduced fungus, and trees become more susceptible to this fungus when they are damaged by humans (for example, weedwackers), introduced animals (for example, root damage from feral pigs) or strong winds. Trees without wounds are unlikely to become infected because the fungus has no point of entry.
Hawaii is estimated to have lost about one million ‘ohi‘a trees due to disease since 2013, only a small fraction of the 350 million ‘ohi‘a statewide. Preliminary research suggests that ‘ohi‘a within protected, fenced forests with few invasive plant and animal species show lower death rates, indicating that management actions can help prevent disease.
Though likely proposed with good intention, suggesting that Australian species would be a replacement for native Hawaiian ‘ohi‘a is problematic and could have unintended negative consequences. Arguing for the increased introduction of alien species due to the expected collapse of Hawaiian species is not new. As a consequence, highly invasive plants were introduced across the islands, including many Australian species that have become invasive in Hawaii, including bottle brush, melaleuca, Australian tree fern, octopus tree, and nearly 40 eucalyptus species.
Ornamental nurseries and botanical gardens are the primary source of invasive species globally because of the sheer number of cultivated species and a preference for plants that grow quickly, need little care, and have few local pests. Bringing in species within the same taxonomic family as ‘ohi‘a (Myrtaceae), as proposed by the author, may prove to be especially dangerous. With that fact in mind, the Hawaii Department of Agriculture has proposed an administrative rule to ban imports of plants in the Myrtaceae family because of the risk of introducing new fungal varieties, diseases, and insect pests.
Non-native plants are the foundation for local agriculture and horticulture, and therefore an important challenge is for policy makers and land managers to maintain the benefits provided by these species while preventing problematic weeds. Agroforestry systems that mix native and non-native non-invasive species have long been used in Hawaii and continue to be important.
However, many of the planted non-native species are Polynesian introductions that are food crops (ulu, for example) or have cultural significance (such as kukui). In addition, there is a big difference between implementing such strategies on lands already dominated by non-native species and introducing new Australian species into intact forests, especially those species whose invasive potential is unknown.
Various tools are publicly available that can address whether particular species are likely to become invasive.
The Hawaii Pacific Weed Risk Assessment, for example, has a screening tool designed to predict a plant species’ tendency to become invasive in Hawaii. The Plant Pono program (plantpono.org) uses this assessment to highlight low-risk plants to use and is freely available online.
A forest is much more than the sum of its parts. The various components that have evolved and grown together deliver important ecosystem services that could be disrupted if new and potentially invasive or disease-harboring species are brought in. While the author suggests that Australian species “can help us cut our landscape water bill,” science has shown that most introduced species use more, not less, water than ‘ohi‘a . The idea that ‘ohi‘a conserves or gathers water is not new.
In fact, the Hawaiian word ‘ohi‘a can further be broken down to ohi – ia, or to gather. ‘Ohi‘a gathers rainwater, soil, and people. The tree is the first to colonize a lava flow, catalyzing ecosystem formation. ‘ohi‘a is as ubiquitously important to the Hawaiian culture as it is to the ecosystem, which is evident in the many mo‘olelo, ‘olelo no‘eau, and oli mentioning ‘ohi‘a as well as the countless traditional uses of ‘ohi‘a wood, foliage, and flowers.
Many natives and locals consider ‘ohi‘a as a family member, and some can trace their ancestral lineage to the ‘ohi‘a people.
The central role of ‘ohi‘a trees in Hawaiian ecosystems and culture is the motivation for research, management, and community engagement that seeks to ensure this critical tree species continues to thrive. Planting and protecting native species and avoiding further introductions may be the best way for managers to continue to protect native forests.
This article was authored by Stephanie Yelenik, Helen Sofaer, Lucas Fortini, Kylle Roy and Rick Camp of the U.S. Geological Survey’s Pacific Island Ecosystems Research Center.