Determining Hawaii’s most common tree requires distinguishing between native species and the introduced species that now dominate much of the landscape. Hawaii’s unique and isolated geography led to the evolution of a fragile ecosystem. However, the islands now host a high number of non-native plants that have successfully colonized large tracts of forest. These invasive trees have become so prevalent that they have fundamentally altered the structure of many island ecosystems.
Identifying the Most Abundant Species
The single most abundant native tree in Hawaii is the ‘ōhi’a lehua (Metrosideros polymorpha). This endemic species is widespread across the six largest islands, occupying diverse habitats from near sea level to 8,500 feet. The ‘ōhi’a is a foundational tree, often the first to colonize new lava flows, and it is the dominant tree in wet, high-elevation native forests, making up approximately 80% of the native forest cover.
However, the most common tree by sheer numbers is often an invasive, non-native species. The most widespread of these is the strawberry guava (Psidium cattleianum), which occupies over 350,000 acres. This aggressive invader forms dense, single-species thickets in moist, lowland, and submontane forests, often displacing the native ‘ōhi’a. Another non-native species claiming massive acreage is Albizia, known for its rapid growth and tendency to form monocultures in low-elevation disturbed forests.
Factors Contributing to Non-Native Dominance
The success of species like strawberry guava stems from a combination of biological traits and historical introductions. Strawberry guava was introduced in 1825 for its edible fruit and quickly escaped cultivation, aided by prolific fruit production. The small seeds are readily dispersed across vast distances by birds and, significantly, by feral pigs that consume the fallen fruit.
These invasive species thrive because they are released from the natural checks and balances of their native environments, such as specialized pests or diseases. They possess a rapid reproductive cycle and high tolerance for various soil and climate conditions, allowing them to colonize disturbed areas quickly. Non-native trees also have faster growth rates and can out-compete the slow-growing native trees, establishing a dominant presence in the forest understory and canopy.
The Ecological Ramifications of Widespread Trees
The dominance of non-native trees has severe implications for Hawaii’s delicate ecosystem, particularly concerning the island’s freshwater supply. Invasive trees often have a higher rate of evapotranspiration than native species, meaning they pull more water from the ground and release it into the atmosphere. This increased water use results in a significant reduction in the amount of water that recharges underground aquifers and streams.
These non-native forests fundamentally change the soil and light environment, creating conditions unsuitable for native plant regeneration. Strawberry guava, for example, forms dense canopies that block sunlight and produces a thick mat of surface roots and leaf litter that suppresses native seedlings. The resulting single-species forests lack the complex structure of native forests, leading to a loss of biodiversity and habitat for native fauna. The accumulation of biomass from some invasive species contributes to a higher fuel load, increasing the risk and intensity of wildfires.