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LETTER: Albizia versus native forest

Friday 16 July 2021 | Written by Supplied | Published in Letters to the Editor, Opinion

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LETTER: Albizia versus native forest

Dear Editor, Re: Letter to the Editor (July 15) stating that Albizia has replaced acres and acres of native forest. It went on to suggest that eventually the only native trees on Rarotonga will be those in our backyards.

When I arrived in 1980, many of the inland slopes and lower ridges were monocultural fernlands of Tangle Fern (Tuanu‘e), while many other lower-slopes had a broad band of Tree Hibiscus (‘Au). Some outer slopes supported native forest. Uphill of the Fernland and Hibiscus slopes there was a near pristine native forest.

Merlin (1985) did not mention Albizia (‘Ārapitia) among the exotic trees of the Fernland, but in 1995 McCormack mentioned Albizia making “pure stands in many former Fernland slopes.”  Its spread continued and now there are very few outer-slope fernlands left. Also, during the 1980s and ‘90s, the Ministry of Agriculture planted exotic trees, mainly Caribbean Pine and various Acacias, on many areas of the upper fernlands.

Despite its abundance in former fernlands, Albizia has shown very little tendency to invade native forest except where it has been seriously disturbed.

The inland native forest remains in relatively good condition although exotic trees such as Cecropia (Rau-māniota) and African Tulip (Kō‘ī‘ī /Pititī Vai) continue to spread and much of the understory is now dominated by exotic shrubs, especially Night-blooming Cestrum (Tiare Ariki-va‘ine) and Inkberry/Ardisia. Various invasive vines continue to spread, such as Balloon Vine, Red Passionfruit and Mile-a-minute.

However, the biggest threat to the native forest is the massive-leaved Peltate Morning-glory. Whether this vine is native or introduced is uncertain, but what is certain is that since the 1980s it has spread rapidly over the tops of many trees in many parts of the inland. In the 1980s I wrote that Grand Balloon Vine was the “green cancer” of our native forest, but today, I realise that the real “green cancer” of the native forest is Peltate Morning-glory.

Despite the gradual invasion of the inland native forest by exotic trees, shrubs and vines, it is still one of the most pristine native forests in the tropical South Pacific. I have visited about 70 islands between Fiji and Rapa Nui and the Rarotonga inland forest, upslope of the bands of Albizia and Tree Hibiscus, is still at the top of my list of pristine forests.

The reason the extensive native forest still exists is worth repeating. The very steep slopes and the lack of large-trunked millable trees kept those early European timber enthusiasts at bay. What a blessing.

Probably 50 per cent of the inland trees are Cook Islands Homalium (Mato), which has relatively small multiple trunks of very hard wood. Its extensive shallow roots are also a major anti-erosion device. For its anti-milling and anti-erosion efforts, the Cook Islands Homalium deserves much credit.

If the Albizia forest were removed, would native trees repopulate the old fernland slopes? It is extremely unlikely. The fernlands were very infertile, which is why they rarely supported regenerating native forest. Albizia improves soil fertility by the decomposition of its nitrogen-rich leaves and it also provides increased shade and moisture for regenerating shrubs. Unfortunately, the shrubs are mainly fast-growing invasives, rather than natives.

Leaving the Albizia forest undisturbed is probably the best policy, except for small areas where somebody is going to nurse transplanted native trees and shrubs for many years.

Gerald McCormack

Research Officer

Cook Islands Natural Heritage Trust

The problematic Albizia trees lining the mountains at Raemaru Park should be an easy fix.

The former MP that operates his business next door to Raemaru Park should attend to this invasive species, instead of cutting trees all over the island.

Just a thought.

Get Real

(Name and address supplied)