At Palmyra, native forests store more carbon than coconut plantations

There are lots of interesting things about native forests in the Islands, and The Nature Conservancy has just added an important one.

These forests do a better job of capturing carbon than planted forests, at least in the example of Palmyra Island, where the Conservancy and the U.S. Fish and Wildlife Service operate an atoll refuge a few hundred miles south of Hawai’i.

Native Hawaiian forests are excellent at capturing moisture and preventing aggressive, eroding runoff. That is in part due to the complexity of the Hawaiian forest. There are generally canopy trees, and smaller understory trees and shrubs and at the lowest level, ferns and mosses.

So when a heavy rain falls, its downward force is diminished by all the leaves and branches it encounters on the way down, and then the forest soaks it up like a sponge. So mountain streams in native forest areas run clear, not muddy. And they keep running even after weeks of dry weather, as the natural spongy forest floor lets the water seep out slowly.

By contrast, in a forest dominated by alien trees, like eucalyptus or Java plum, there is little understory growth. And heavy rains often lead to muddy runoff, eroded gullies, and sediment-filled streams. And shortly after the rain stops, the soils dry out.

(Image at right: Pisonia forest at Palmyra. Credit: Andrew Wright.)

A new study at Palmyra by The Nature Conservancy has shown that another benefit of native forests is that they also store more carbon than single-species forests like the coconuts that once dominated Palmyra’s coralline ground.

The paper was published in PLOS One by Kate Longley-Wood, Mary Engels, Kevin D. Lafferty, John P. McLaughlin and Alex Wegmann. The title: Transforming Palmyra Atoll to native-tree dominance will increase net carbon storage and reduce dissolved organic carbon reef runoff.

At Palmyra, the Conservancy has been replacing dense coconut stands, which are not native to the island, and were planted to promote a copra industry, with the native forest that once existed there.  They knew there would be impacts of this conversion, but it wasn’t entirely clear what they would be.

“To better understand how this landscape-level change will alter the atoll’s carbon dynamics, we used field sampling, remote sensing, and parameter estimates from the literature to model the total carbon accumulation potential of Palmyra’s forest before and after transformation,” the authors wrote.

Their research showed that the new forest increased carbon storage on the atoll’s land areas by nearly 12 percent, and also reduced the flow of dissolved organic carbon into the island’s lagoon. That, in turn, is expected to result in healthier corals and a strong community of the species reliant on the coral reefs.

“We’ve demonstrated that better stewardship of natural resources can increase their carbon capture ability,” said lead author Kate Longley-Wood, Ocean Mapping Coordinator with TNC’s Protect Oceans, Lands and Waters program. “That native tree species are better for carbon capture and ocean health is the icing on the cake.”

All that said, it takes time for the effect to be seen, and the story will likely change somewhat over time as the restored forest matures. There is a loss of carbon in the standing trees when the coconuts are cut down and allowed to be replaced by native Pisonia grandis (Pu’atea in Tahitian or cabbage tree in English), Heliotropium foertherianum (beach heliotrope), Pandanus tectorius (hala or screwpine) and other species native to the atoll.

The native species are considered to be superior habitats for native seabirds, they store more carbon, and they support a larger native ecosystem.  But the results don’t mean it’s appropriate to go around cutting down all the coconuts elsewhere, as they could be important parts of the human communities on some islands. The scientific name of coconut is Cocos nucifera, sometimes written C. nucifera.

“C. nucifera’s role in human migration and settlement throughout Oceania is notable, and control of C. nucifera to transform native forest should be balanced with the societal value provided by C. nucifera to Pacific Island communities,” the paper said.

© Jan TenBruggencate 2022