Hawaii has more native trees than any other state, but pines (Pinus spp.) do not make that list. Beginning in the 20th century, pine trees were introduced to the islands for lumber, but few of the species grew straight enough to make the experiment a success. But pines remain in Hawaii and have escaped cultivation, changing both the plant life above ground and, perhaps more disturbingly, the fungal communities below the soil's surface.
Pine Comes to Hawaii
Five different types of pine trees grow in Hawaii, all introduced from North America. Some have done remarkably well in the warm climate, like slash pine (Pinus elliottii). This stately conifer was brought into the 50th state to combat erosion and for use as lumber. The tree, which grows in U.S. Department of Agriculture plant hardiness zones 7 through 11, thrives in plantations in Kauai and Molokai and occasionally escapes cultivation. It has high drought tolerance and will grow in almost any soil, including clay and mud. The U.S. Forest Service calls the tree a prolific seed producer that "seeds itself into the landscape."
Other Successful Pines
Another tree introduced for lumber, the loblolly pine (Pinus taeda), which grows in USDA zones 6 through 9, grows rapidly in Hawaii and produces lumber for construction. It was planted and currently grows on Maui, Kauai, Molokai and Hawaii. Jelecote pine (Pinus patula), which grows in USDA zones 8 through 9, was not successful as a lumber tree. Although this pine is grown for lumber in South Africa and Swaziland, this is not the case in Hawaii, perhaps because the tree produces multiple leaders and steep branch angles in the tropical climate. However, the graceful jelecote, with its weeping branches, dark red bark and clustered cones, has been adapted as an ornamental tree in Hawaii, planted in parks, reserves and gardens.
Less Successful Immigrants
Not every pine introduced to Hawaii has thrived. The beautiful Monterey pine (Pinus radiata) can grow rapidly to 150 feet tall with a trunk diameter exceeding 4 feet. In Hawaii, these trees have been felled by sulfuric volcano fumes as well as by fungal disease and European pine adelgid. Cluster pine (Pinus pinaster) is another tall, straight pine introduced as potential lumber. Relatively slow-growing, the tree has also fallen victim to pests and diseases in Hawaii. Cluster pine regenerates profusely after fire, forming thick clumps that shade out native species. It has been nominated as one of the world's top 100 invaders. Both species grow in USDA zones 7 through 9.
No conifer is more invasive worldwide than pine, and the threat is greatest to tropical regions without native pine trees. Pines have a symbiotic relationship with certain fungi, called ectomycorrhizal fungi, and cannot survive unless they are present in the soil. So, for pines to colonize an area that does not have a native pine population, ectomycorrhizal fungi must also be introduced. As pines find their way out of plantations to wild areas of the state, native plants and the makeup of the soil organisms are under threat. This may make restoration projects in Hawaii impossible.