Nitrogen-fixing tree species have been recognized for decades as particularly damaging to invaded ecosystems. The resultant elevated soil nitrogen availability might persist long after the causative mature individual trees have been removed. The altered soil chemistry can facilitate secondary invasions by plant species able to exploit the increases in nutrients, which can then dominate post-control succession.
In Hawai`i one of the most widespread and damaging N2-fixing tree species is albizia (Falcataria falcata) [formerly Falcataria moluccana]. This fast-growing species has aggressively invaded across the Archipelago, transforming composition, structure, and function of remnant lowland wet forests. There are an estimated four million F. falcata trees across the Islands. Albizia is currently widespread on Kauaʻi, Oʻahu, Maui, and Hawaiʻi Island, especially in the Puna and Hilo districts. Small populations occur on Lanaʻi; it is not known on Molokaʻi (Hawaii Invasive Species Council fact sheet.)
In addition to replacing native forests, albizia also threatens residential communities and agricultural lands. An estimated 720,000 trees tower more than 30 m (150 feet) high. Their spreading canopies are composed of brittle branches that fall during windstorms, crushing structures across the species’ wide invasive range.
Regeneration of albizia is restricted in stands with contiguous overstory by the severe reduction in light availability (to 20% or less of ambient levels). However, the tree boasts an abundant and persistent seedbank, so it sprouts promiscuously after mature individuals are removed.
To counter this rebound, control efforts have relied largely on clearing the land using large machinery (e.g., bulldozers). This is expensive and – worse – not very effective because the magnitude of disturbance to the soil disturbance often leads to explosive germination of the tree’sseeds.
A new strategy has been tried recently: killing the mature albizia through application of a target-specific herbicide (aminopyralid). The tree species succumbs at low doses. The herbicide-killed F. falcata quickly drop their leaves, thereby increasing litter inputs of N and P to the soil. Extant understory vegetation (non-native grasses and forbs) exploit the enhanced nutrient availability to form a continuous layer that severely limits germination of F. falcata seeds. In the study by Hughes et al. (2025), plots the number of albizia saplings per ha2 after three years was only 18, despite the presence of perhaps 8 million seeds!
So the early successional pioneer species, F. falcata was unable to re-establish. However, these aggressive non-native plants also prevent restoration of native Hawaiian species – which are vanishingly rare in these forests, both before and after the albizia trees were killed.
Hughes et al. (2025) suggest manipulating the succession trajectory by planting desired species either before or immediately following herbicide treatment of the canopy-sized F. falcata stands. These preferred species – probably species that have cultural importance to native Hawaiians or desired agricultural crops – could take advantage of the increased resources (i.e., light and nutrients). The result would be either restoration of native forests or productive agroforestry plantations across huge areas.
Without such intense intervention, restoration of native species is not expected due to their scarcity in the area and the significant disturbance (i.e., fire, &/or conversion to agriculture) before F. falcata stands established.
Of course, populations of the native tree ʻōhiʻa lehua (Metrosideros polymorpha) have been severely reduced by the newly introduced disease, rapid ʻōhiʻa death (ROD). The disease was first detected in the Puna District on Hawai`i Island – the area where Hughes et al. (2025) carried out their study. This occurrence must affect the chances for success of the proposed actions, although I have seen no speculation about this.
The proposed approach has not been applied over large areas, so success is not assured. Scientists would need to monitor such a project over the long term to see whether the planted vegetation can sustainably suppress F. falcata re-establishment. Albizia seeds retain 70 – 90% viability during storage for 18 months, possibly longer. Also, low numbers of saplings did still establish in the experimental plots. The non-native grasses might deplete the soil nitrogen levels, thus providing opportunities for secondary invasion by N2-fixing trees. Hughes et al. (2025) expect that land managers will have to carry out additional management actions, including targeted herbicide treatment to remove F. falcata seedlings before they become mature, full-sized trees.
Hughes et al. (2025) note that developing a successful large-scale method for managing this large, fast-growing, and disruptive NIS IAS tree would benefit not just Hawai`i but other parts of the Pacific basin.
SOURCE
Hughes, R.F., C. Morrison, E. Bufil, J. Leary. 2025. Ecosystem response to management of an invasive N-fixing tree in Hawai`i. Trees, Forests and People 21 (2025) 100932
Posted by Faith Campbell
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For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm