Better off dead: A dangerous tree may make a valuable mulch

Joanna Norton and Rebecca Ostertag, University of Hawai‘i at Hilo



Wood chipping machine deposits a pile of albizia mulch next to fence as a horse looks onAlbizia trees are considered invasive and dangerous in Hawai‘i, but they represent a ready source of nitrogen-rich mulch.

East Hawai‘i is known for its lush vegetative growth, but its fastest growing tree is not native to the islands. Falcataria moluccana, known locally as albizia, was originally introduced to Hawai‘i to quickly reforest areas that were eroding due to poor land management. Unfortunately, but true to expectations, the tree quickly spread, and now grows across large swaths of the state. Because the species can grow to 150 feet tall, at a rate of up to 15 feet per year, albizia easily outcompetes slower-growing native species, literally casting them into the shade. Albizia's weak, brittle wood, large size, and abundance also makes it a menace to people and infrastructure due to the tendency to lose branches or fall on houses, roads, and electrical lines during storms. Removal for safety reasons is often necessary but expensive, and the wood has little commercial value. However, many of the areas with heavy albizia growth lie on relatively recent lava flows or on degraded agricultural lands so have soils lacking in organic material, and the trees do have the ability to fix nitrogen, making them potentially good as fertilizer.

Image of several spreading albiziaSpindly albizia trees tower over a building. (Image: Forest & Kim Starr/CC BY 4.0)

This project, in collaboration with the US Forest Service, the Big Island Invasive Species Council, and the PI-CASC Manager Climate Corps program, investigates the potential benefits of composted albizia mulch applied to agricultural land in East Hawai‘i Island. Because the mulch contains a large amount of carbon and nitrogen, a management approach that takes unwanted biomass from albizia trees and applies it to cropland could replace or supplement synthetic fertilizers while stimulating albizia removal efforts. To determine if the albizia mulch will affect plant health and yield, we are growing cassava and corn plants in agricultural plots, and will analyze yields, plant tissue nutrients, soil nutrients, and water-holding capacity to assess performance of the composted albizia mulch compared to status quo fertilization. If this approach is found to be economically viable, climate change mitigation and resilience could be a byproduct of agricultural activities, and could potentially decrease the use of synthetic fertilizers by replacing them with a widely available local product. This kind of climate-smart agriculture can simultaneously benefit farmers and the environment, lowering costs while improving safety and local ecosystems.

Our hope with this work is to show that using composted albizia mulch not only provides a benefit by removing the trees, but actually improves soil or plant health. Albizia wood is available in virtually unlimited quantities, and should improve the overall long-term health and function of the soil. In this way, a troublesome tree could be put to good use, providing an ecological and economic benefit instead of causing damage.



Quick Summary


      Falcataria moluccana, also known as albizia, is a fast-growing invasive tree that causes both ecological and economic damage. The carbon- and nitrogen-rich mulch created from removed albizia could be used for local agriculture, but there is scant information on the effects of composted albizia mulch on plant and soil health and yield.
      Our work compares composted albizia mulch with commercial standard fertilizers by applying each to plantings of cassava and corn, and determining if plant yield, soil nutrient and water levels, and plant nutrient levels are affected.
      Our goal is to provide information about the agricultural effects of using composted albizia chips as mulch, so that farmers and other land managers can integrate climate-smart agricultural uses into their albizia management plans.




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This project was supported by the Pacific Islands Climate Adaptation Science Center (Cooperative Agreement #G15AP00059 from the US Geological Survey). For more information, please contact Joanna Norton (jnorton@hawaii.edu) or Rebecca Ostertag (ostertag@hawaii.edu). To learn more about climate science at PI-CASC, contact David Helweg at dhelweg@usgs.gov or visit: https://nccwsc.usgs.gov/pacificislandscsc.