Koa Restoration in Gorse-Dominated Landscapes

Past ProjectsProjects

Koa Restoration in Gorse-Dominated Landscapes

Project Collaborators

  • Aaron Wehrman, Natural Resources and Environmental Management, University of Hawai‘i at Mānoa
  • Kualiʻi Camara, Department of Hawaiian Home Lands
  • JB Friday, Natural Resources and Environmental Management, University of Hawai‘i at Mānoa
  • Travis Idol, Natural Resources and Environmental Management, University of Hawai‘i at Mānoa
  • Owen Burney, Department of Plant and Environmental Sciences, New Mexico State University
  • Simon Landhäusser, Department of Renewable Resources, University of Alberta
  • Douglass F. Jacobs, Department of Forestry and Natural Resources, Purdue University
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Gorse (Ulex europaeus) covers thousands of acres of land on the slopes of Mauna Kea. This thorny shrub, capable of surviving subfreezing temperatures, has outcompeted most of the native plants and has blanketed the region. Much of this land is managed by the Department of Hawaiian Homelands (DHHL). Many attempts have been made to control gorse. This includes, outshading, spraying herbicide, and bulldozing. However, without constant upkeep, it is nearly impossible to truly get rid of gorse. This is why the DHHL and Dr. JB Friday with CTAHR Forestry Extension and others have planted koa (Acacia koa), sugi pine (Cryptomeria japonica), and redwood (Sequoia sempervirens) alongside the gorse to increase land productivity and biodiversity. A plan was implemented to crush rows into the field of gorse using a bulldozer. After this was done, and herbicide sprayed, the trees were planted in the middle. The hope is that these trees will establish themselves before gorse inevitably grows back in.

Acacia koa in a field of gorse.

One obstacle that may need to be overcome is frost damage in the koa seedlings. It is well documented that koa seedlings suffer from frost damage when exposed to sub-freezing temperatures. The plot of land where the reforestation is being done is at 2000 m above sea level and during cold, clear winter nights it gets below freezing. A previous study by TropHTIRC PhD student Dr. Kyle Rose found that decreasing canopy openness increases absolute minimum temperature and lessens the number of minutes spent below freezing. Unfortunately for this particular plot, there are no conifers to create canopy cover to protect from frost damage. Because of this, the seedlings were all planted in May 2022. This is so that the trees will be taller than the frost zone come winter time.

Temperature Sensor under solar shield

One problem with planting over the summer is the risk of drought. Should the summer be particularly dry, there is a high chance of mortality among the seedlings.Meaning, it would be extremely beneficial for the koa if these rows of gorse provide some sort of insulation.

A study led by Aaron Wehrman was put in place to set 20 Hobo MX2202 sensors in the gorse rows. These sensors take data once per minute between December and March to see whether or not the sensors in the rows of gorse are warmer than their counterparts placed in the open. Should the rows produce warmer temperatures, this method of crushing rows could be replicated on a large scale to reforest koa. If the sensors show sub-freezing temperatures, then precautions will need to be taken for future koa seedlings to avoid frost damage.

Healthy koa in gorse row
Healthy koa in gorse row

Currently, the koa planted in May are doing quite well. They have over an 80% survival rate and have grown ¾ of a meter between May and January. Over 400 trees are still going strong and it is hoped that a similar planting design can be implemented in more plots of gorse. The temperature data will start to come in very soon and we are extremely excited to show the data.