Orokonui trustee Kelvin Lloyd reports on regeneration at the ecosanctuary.
Orokonui Valley's history over the past 150 years includes extensive farming use.
You can find not only old earth fences and stone buildings from historic farming operations, but relatively recent fences incorporating tanalised posts and wire of sufficient quality to be retrieved and reused operationally.
So how did the ecosanctuary get to be covered by forest?
The answer lies in the ecological processes of regeneration and vegetation succession.
Kanuka (Kunzea ericoides) is an excellent pioneer and being unpalatable to grazing animals, would have been able to regenerate to some extent in the presence of grazing stock within farmed parts of the valley.
It also regenerates in response to fire, and much of the current kanuka canopy is formed of approximately 100-year-old trees that would have established as seedlings after the 1907 fire.
Other stands of kanuka forest, mostly on the western side of the valley, regenerated more recently, particularly after farm stock were excluded from the valley to enable its clean water to be used for the adjacent former Orokonui hospital.
The exclusion of farm stock would have allowed palatable native trees such as mahoe (Melicytus ramiflorus), broadleaf (Griselinia littoralis), kohuhu (Pittosporum tenuifolium), tarata (P. eugenioides), and mapou (Myrsine australis) to regenerate beneath the kanuka canopy.
Broadleaved trees such as these form extensive subcanopy stands under much of the current kanuka canopy, and via a process called vegetation succession are destined to replace the kanuka.
Kanuka has a lifespan of approximately 120 years and does not regenerate beneath the shade of a broadleaved canopy, so within 70 years both the older stands on the eastern side and the younger stands on the western side will have substantially made way for broadleaved forest.
Fifty baseline vegetation plots were measured in the ecosanctuary prior to fence construction and pest eradication, and the same plots were re-measured last summer to gain some insights on regeneration and succession in the absence of pest animals.
Analysis of the plot data is not simple, as vegetation changes due to successional processes would have occurred even in the presence of pest animals.
So the analyses will have to take account of the decline in kanuka abundance and increasing abundance of broadleaved trees that would have happened anyway.
A preliminary analysis shows sapling densities are higher within the ecosanctuary than in control plots outside.
Further work will look at how the nutritional composition of the forest has changed between the two surveys, by looking at nutrients in foliage and fruit.
Succession to broadleaved forest is not a bad thing, as while insectivorous birds such as rifleman, grey warbler and brown creeper seem to enjoy kanuka habitat, most broadleaved tree species have fleshy fruits that provide good annual food crops for fruit-eating birds such as bellbird, tui, kereru and silvereye.
Kaka also seem to prefer broadleaved forest, where they remove strips of bark to tap the sugary phloem tissue as well as nest in broadleaved tree hollows which do not form in kanuka forest.
And what of the podocarps?
Within the ecosanctuary there are several patches of original forest with emergent trees of rimu (Dacrydium cupressinum), miro (Prumnopitys ferruginea), Hall's totara (Podocarpus cunninghamii), and pokaka (Elaeocarpus hookerianus).
These are valuable both for their fruit crops for birds and as seed sources to re-establish emergent trees across the remainder of the ecosanctuary.
This process is definitely occurring, with sapling rimu trees now approaching the canopy and considerable regeneration of saplings of Hall's totara, miro and rare matai (Prumnopitys taxifolia).
The trust land on which the visitor centre now sits has quite a different history.
In the recent past the bulk of it was a forestry plantation, and when the trust bought the land, it was covered by numerous young pine and macrocarpa trees emerging from gorse and what appeared to be scattered native trees.
A stand of mature macrocarpas was on the site of the current visitor centre, and was felled and milled, with timber being used in ecosanctuary structures.
With the help of Forest & Bird's wilding tree group, the younger pines and macrocarpas were cut down to reveal an encouraging layer of young wineberry (Aristotelia serrata) and kohuhu.
Additional and ongoing gorse control by volunteers and planting of grassy gaps means native regeneration is occurring apace on the trust land, and will continue to grow into forest.
An unusually formed cabbage tree in front of the visitor centre entrance once grew within the corridor of tall macrocarpas, and while stripped of branches during their felling, now stands as a very visible sign of regeneration at the ecosanctuary.
• Kelvin Lloyd is an ecologist with Wildland Consultants and set up and recently remeasured 50 permanent vegetation plots within the ecosanctuary to assess the regeneration response after eradication of pest animals.
