Patterns of primary succession of native and introduced plants in lowland wet forests in Eastern Hawai'i.
The majority of Hawaii's lowland wet forests no longer exist, with many of the last remaining patches found on the eastern, windward sides of the largest islands. To better understand successional patterns and invasion in these native systems, we quantified basal area (BA) and densities of woody species and understory cover at nine sites in the Puna district on the Island of Hawai'i, representing age gradients of native stand development on both 'a'ā and pāhoehoe lava flows. On both flow types, BA of native species increased (from 5 to 50 m2/ha) and stem densities decreased (from 3700 to 2600 stems/ha) with increasing stand/flow age. Both native and introduced species compositions diverged between substrate types on older flows. We found that lowland wet native forests remain at least partially intact in several locations, but their functional and compositional integrity is increasingly compromised by invasion of nonnative species, such as Psidium cattleianum and Melastoma candidum, which become more common at sites greater than 300-yr old. This time period may represent a threshold, after which abiotic environmental conditions no longer constrain recruitment of introduced species. On older flows, nonnative stem densities swamped those of native species by an order of magnitude, with nonnative stems (height >1.3 m) achieving densities as high as 18,000 stems/ha. In addition, all stands lacked recruitment of native woody species in the understory, suggesting that without management, the native components of these forests may soon no longer be self-sustaining.