What happens to the remaining native plants when you remove alien invaders?
Perhaps not what you expected.
For instance, mature natives like the 'ohi'a (seedling image above right) don't suddenly experience growth spurts.
But opening up the canopy can let dormant native seeds sprout—seeds that might not have sprouted in the shade of dense alien undergrowth.
The researchers who conducted research on this include Rebecca Ostertag, Jodie R. Schulten, Keiko M. Publico, and Jaime H. Enoka, all of the University of Hawai'i at Hilo's Department of Botany, along with, Susan Cordell and Colleen Cole of the USDA's Institute of Pacific Islands Forestry, and Jene´ Michaud of the University of Hawai'i at Hilo's Department of Geology.
Their report, “Ecosystem and Restoration Consequences of Invasive Woody Species Removal in Hawaiian Lowland Wet Forest,” was published in the Springer journal, Ecosystems. (Ecosystems (2009) 12: 503–515 DOI: 10.1007/s10021-009-9239-3)
The team used several wet forest plots in the Keaukaha Military Reservation south of Hilo Airport. The native forest was dominated by ’ohi’a (Metrosideros polymorpha) and lama (Diospyros sandwicensis, along with extensive native undergrowth, but had been invaded by multiple species, including strawberry guava, macaranga, albizia, clidemia and the melastome Melastoma
The non-natives were cleared from a series of test plots, which were then compared over a three-year period with uncleared areas. In the removal plots, where more sun was now able to reach the ground, air temperatures and soil temperatures were higher, humidity was lower, less total leaf litter fell and it decomposed more slowly. Most of that was predictable.
Among the interesting findings was that the native trees, with their competition removed, did not respond by suddenly growing faster. They found no significant difference in the diameter of native trees in their removal plots compared to test plots where no weeds had been removed.
“Despite major environmental changes in the removal plots, native species’ diameter growth and litterfall productivity were not significantly greater after removal, testifying to the slow response capabilities of native Hawaiian trees,” the authors wrote.
“Our results are consistent with the expectation that native species are conservative in regards to resource use and may not strongly respond to canopy removal, at least at the adult stage.”
They did find that while 'ohi'a and some other native trees did not regenerate in the dense shade of alien-dominated forests, their seedlings did sprout in the more open forest created after clearing.
“We conclude that canopy opening is critical to avoid complete conversion of these forests to exotic-dominated systems,” they wrote.
And that's critical to saving the forest for native birds and other species that rely on native habitats.
© Jan TenBruggencate 2009
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