OBSERVATIONS ON THE IMPACT OF PRODUCTION FORESTRY ON MARGINAL LAND INLAND OF KAWAKAWA

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1 OBSERVATIONS ON THE IMPACT OF PRODUCTION FORESTRY ON MARGINAL LAND INLAND OF KAWAKAWA Bob Cathcart Environmental and Land Management Consultant AgFirst Northland 4 April 2017 The following observations are based on land resource inventory/land use capability surveys of the Kawakawa River catchment conducted by Brian Burridge and Bob Cathcart in 1967 (1) and almost 50 years later by Bob over some of the same land in In each case the surveys followed the procedures now documented in the Land Use Capability Survey Handbook, so the inventory resource inventory data that lies behind these observations is directly comparable. THE LAND The land surveyed in detail (1:7,500) in 2015 is managed by a Maori Forestry Trust, one of several forest owners with pine forests on land within the Kawakawa River catchment, extending over into the Waitangi River catchment in the north and the Kaikou-Mangakahia River catchment in the south. Included in the recommendations following the 1967 (1:63,360) soil conservation survey, 1965 for the Mangakahia River catchment, was that the severely eroded gumland hill country should be afforested to control sheet and gully erosion, the argument being that productionprotection forestry is a more sustainable land use on this class of land. Extensive areas were planted to pine forests in the early 1980s, much of it under joint-venture schemes. [These soils/land resource inventory units were assessed as Class 4e, 6e, 6s, and 7e (with patches of 4s) in the NCC 1967 survey. The same surveys, updated to provide for changes in inventory data (vegetation) and in how the inventory data was presented, were incorporated into the first MWD Land Use Capability Worksheets (NZLRI) (2) in Harmsworth (3) describes them as Northland NZLRI units Class 4e12, 4w4, 4s4, 6e19, 6s5, and 7e8 respectively.] CLAIMED EFFECTS OF PINE FORESTS The Forestry Trust is under pressure from some of its shareholders to not re-plant pine trees as it is claimed pines poison the soil, water and air, and have detrimental effects on human health and indigenous ecosystems. It is understood financial arrangements around forest establishment have resulted in low returns to shareholders, insufficient to enable replanting and management. Opposition to production forestry is further exacerbated by the impact of logging trucks on narrow, unsealed roads and on residents living alongside these roads.

2 Much of the land involved is described as gumland hill country, podzolised soils on sandstone and mudstone, and, in particular, on siliceous shales. Ferrar (4) described this high-silica shale as the Ngatuturi Beds, now known to be part of the Northland Allochthon and outcropping from just north of Mangamuka on the upper, northern side of the Hokianga Harbour, near Horeke on the southern shore, between Ngawha and Maromaku, and between Pipiwai and Titoki. Like other rocks with a high silica content, these shales podzolise more quickly than other sedimentary and volcanic parent materials. Taylor and Sutherland (5) recorded only a hill soil phase of Pokopu gravelly silt loam. More stable sites carried even more podzolised Hukerenui silt loam or the mature podzol, Wharekohe silt loam. These soils are particularly prone to sheet erosion and to large, amphitheatre-shaped gullies. The shale rock is in places extremely acid with gully erosion exposing sulphides which oxidise to produce acid sulphates. A ph of 1.6 was recorded in 1965 in water oozing from a gully in this rock type near Kaikohe (6). When surveyed in 1967, much of this land was clothed in short gumland scrub - stunted Manuka, Dracophyllum, Gleichenia, prickly Hakea and similar low-fertility heathland plants. It was frequently burned, with fires clearing thousands of hectares at a time. The burned land was not developed to pasture or grass, just allowed to regenerate to gumland scrub. The fires, over time, removed most, if not all of the peaty O horizon typical of podzols, exposing the white silica sand or silt A horizon. Little would grow on and protect the exposed silica surface, resulting in accelerated sheet erosion. Typical gumland scrub vegetation (Photo by Geoff Heaps, but not within the area surveyed) Drainage channels, old logging roads and farm tracks developed into gullies, some 30 metres or more deep and discharging large volumes of gravel. Pockets of indigenous forest, which eventually grew in these old gullies, escaped the many of these fires but shrunk in area as fires singed their edges. The age of the remnant forest in some of these gullies suggests the surrounding land has been in gumland scrub and prone to these erosion forms since long before European and even Maori settlement.

3 The recent survey took place after almost 70% of the first rotation of pines had been harvested. The changes noted were: Soil Erosion - The incidence of soil erosion has been dramatically reduced by afforestation and control of burning. Slip-shaped patches of native scrub and regenerating bush within the pine forest record storm-induced slip damage where cells of high intensity rainstorms within more widespread heavy rainfall events, a feature Northland weather systems, have tracked across the area. Forest managers are aware of the erodibility of these soils and their regolith and build and manage their infrastructure to prevent risk. Whereas the 1967 survey recorded severe to very severe sheet and gully erosion, 3 and 4 in the Land Use Capability Handbook (7), the recent survey recorded mainly Ө or 1, except for a narrow strip of land hit by a high intensity storm event in July Regeneration - While this land previously reverted to gumland scrub, the Pokapu soils have come away in dense broadleaf indigenous shrub, grass and sedge species after harvest. This suggests that the soil fertility status has been raised by the pine forest. Modern harvesting methods strip bark, needles and light branches, leaving them spread on the forest floor. This material contains most of the nutrients, in particular phosphorus and calcium taken up during the life of the pine trees, both from added fertiliser and drawn from deeper in the profile than the former heath vegetation could reach. Die-off of mycorrhiza and decay of vegetation following harvest releases nitrogen. Harvesting methods lightly cultivates surface layers of the soil and, together with the nutrients create an ideal seedbed for regenerating plants, helping to explain the proliferation of broadleaf indigenous species. Indigenous broadleaf species regenerating on Pokapu soils

4 As expected, the more podzolised Hukerenui and Wharekohe soils have regenerated to manuka and other gumland species but, as yet, there appears to be a lower incidence of prickly hakea. This suggests that on the Pokapu soils and possibly all but the mature Wharekohe podzols, gumland heath vegetation is fire-induced, an observation supported by Enwright(8) in surveys of heathland vegetation at Te Paki. That is, control over burning while the land has been under production forest management has encouraged the regeneration of species previously suppressed by frequent burning. Remnant bush, indigenous scrub and wetland vegetation have also recovered in the absence of fires. Tree species (totara, tanekaha, rimu, kauri, kahikatea) are emerging above mature manuka and the bush remnants are spreading into their unplanted margins. Healthy kuta, rush beds, wetland manuka, Gleichenia and sphagnum moss have re-established on the swampy valley floors. SUMMARY OF CONCLUSIONS ADVISED TO FORESTRY TRUST Included in a report back to the Trust is the following: s management of its pine forests is having a beneficial effect on its land, some of which previously could not support native forest. It has: 1. Controlled what was the worst erosion and land abuse (burning and erosion) in Northland; 2. Reduced the discharge of sediment to streams and improved water quality; 3. Allowed remnant patches of native bush and wetlands to recover to what is now a very healthy state; 4. Provided a maintained roading network that also gives access for pest control, weed control, servicing bee hives and fire-fighting; and is 5. Protecting and improving the land while longer-term land use options are being considered. Where to from here? There are some landowners who would like to eventually restore all of the Trust land back to native vegetation. The Trust is, however, being realistic and knows that the only sustainable use of much of this land in the meantime is pine forest. It sees its land in a mosaic of pine forest, native bush, wetlands, gumland scrub/manuka. It is also looking at eco-tourism and extraction of health products from indigenous plants. Almost 15% of the land is in established or regenerating bush and a similar area in gumland (predominantly manuka) scrub and wetlands. The area of production forest on the remaining land can be gradually reduced as difficult (to log) and low-yielding areas are retired, either to manuka or bush. Management of native species for timber, particularly totara, is a serious option. The pine forests and the absolute control of burning they demand will protect the land in the meantime and finance the existing infrastructure (including over 450 km of roading. Without productive pine forests at risk, a return to the spectacular fires of pre-1980 is a very real possibility.

5 REFERENCES 1. Burridge, B. E., Soil Conservation Survey of the Kawakawa River Catchment, Bay of Islands County. Northland Catchment Commission, July NZLRI (New Zealand Land Resource Inventory), Landcare Research - Manaaki Whenua, Lincoln, New Zealand [ 3. Harmsworth, G.R Land Use Capability classification of the Northland region. A report to accompany the second edition (1:50,000) NZLRI worksheets. Landcare Research Science Series 9. Lincoln, Manaaki Whenua Press, 269p. 4. Ferrar, HT and Others, The Geology of the Bay of Islands Subdivision, Kaipara Division. Bulletin 27, Geological survey Branch of Department of Mines, New Zealand Government Printer, Wellington. 5. Sutherland, C.F., Cox, J.E., Taylor N.H., Wright, A.C.S. 1980: Soil map of Mangakahia- Dargaville area (sheets P06/07), North Island, New Zealand. N.Z. Soil Bureau Map Department of Agriculture/Ministry of Works and Development Soil Conservation Service revegetation trials, Mangamutu Gullies, August (Internal Report - ph1.6 measured in Ngawha gullies, Mangamutu Stream) 7. Lynn IH, Manderson AK, Page MJ, Harmsworth GR, Eyles GO, Douglas GB, Mackay AD, Newsome PJF NZ Land Use Capability Survey Handbook a New Zealand handbook for the classification of land 3 rd Edition. Hamilton, AgResearch; Lincoln, Landcare Research; Lower Hutt, GNS Science Enwright, NA, Northland Heathland Vegetation, NZ Journal of Ecology, Vol 12, 1989.