The rata trees are flowering again, now that possums have been controlled in Pukaha Mount Bruce

The Pukaha Mount Bruce Forest is one of the largest and most accessible remnants of the "Seventy Mile Bush", an almost unbroken tract of forest that stretched from Opaki to Norsewood, before being largely cleared and the land converted to agriculture.

Generally, the forest is in good heart, with extensive regeneration of emergent and canopy trees and a vigorous and diverse shrub and ground flora. Animal control efforts already in place have halted any extensive damage and a continuation of these will aid the regenerative process.

It is one of New Zealand's great "vanished forests". Early accounts of the forest all mention the size of the trees and the luxuriance of the growth. One has it "so dark that you need to light a torch during the day to see the way ahead." These, along with the abundance of birdlife and the sheer immensity of the area covered are common and recurring themes.

In "A History of the Birds of New Zealand" W.L. Buller gives a description of the forest as seen from the road, and though he mentions kohekohe, which almost certainly was not present, the narrative serves as a good description of the type of forest that was the typical cover of this area.

Rimu is a dominant tree, with "just a sufficient admixture of kahikatea and rata to set off the peculiar softness of the former", while tree ferns, Cyathea dealbata and C. medullaris, along with Dicksonia fibrosa (as D. antarctica) are always present and the ground is "covered with ferns and mosses and cryptogams in amazing variety."

His description of the seasonal changes in the forest is by no means confined to the variation in birdlife. Rather, it is the changes in the plants as well which took his imagination, from the spring time sight of the Clematis "hanging in garlands round the trees, festooned in clumps among the lower vegetation on the roadside, and displaying its petals of snowy white in great profusion " and later, in the summer, having shed its flowers, "bunches of silky green tassels, containing the seed vessels of this plant and possessing a characteristic beauty of their own."

Kohia (Passiflora tetrandra) is mentioned as characteristic, "hanging from almost every branch", though, as yet, it has only recently been found in the Pukaha Mount Bruce forest. Of more interest is the mention of "a bunch of Loranthus ablaze with its crimson flowers and forming a picturesque object amidst its green surroundings." The crimson mistletoe, now Peraxilla colensoi, but formerly included in the genus Loranthus, has only recently been rediscovered in the Tararua Range, having not been seen for a number of years. Its inclusion as a plant of the 70-mile bush provides an interesting and somewhat enigmatic addition to the flora, given that its usual hosts are beech trees, and that these were hardly the dominant feature of this forest, though they were present.

Other early accounts of the 70-mile bush are as much concerned with its felling and conversion to agricultural land as they are with its composition, but all are united by reference to both the size and density of the trees and the abundance of the birdlife. A northern rata (Metrosideros robusta) in the Mangatainoka area was said to be 66 feet (31 metres) in circumference. By contrast large measured rata currently alive, including some at Mount Bruce, are between 9 and 12 metres in circumference. The soil fertility and "magnificent stands of matai, rimu, tawa and maire" are everywhere stressed.

Geology
Mount Bruce is an isolated elongated massif tending NNE to SSW. The forest occupies an outlier of greywacke emergent through beds of tertiary mudstone. The soils are skeletal and rubbly on the upper slopes, deeper on the lower slopes.

When viewed from a distance the high point (Bruce's Hill) and the associated ridge and gully system stand out distinctly from the nearby Tararua foothills and the low rounded hills of the northern Wairarapa.

Little, if any, geological work has been undertaken on Pukaha Mount Bruce itself, though there has been work done in the vicinity.

Rocks and geological formations in the wider area span Jurassic (190 - 135
MY) to Holocene (10,000 yr. before present), and cover a range of rock types including greywackes, limestones, sandstones and mudstones (papa). Limestone occurs on both sides of Pukaha Mount Bruce, literally over the road to the west. Limestone is also common to the north. The recent geology in the area to the south of Pukaha Mount Bruce is strongly influenced by the Ruamahanga River.

The rock type on Pukaha Mount Bruce itself is described as a part of the Tararua formation of the Torlesse supergroup. This rock is a greywacke, which dates from the Jurassic age and was deposited when New Zealand was situated near Antarctica. Most of the forested area remaining on Pukaha Mount Bruce grows on this rock, the only major exception being a small area near Bruce Stream which grows on the recently deposited alluvium associated with the stream itself.

Near the Ruamahanga River is the Mangaoranga formation which accumulated during the Miocene (25 - 7 MY) as a result of marine transgression. This formation contains both marine and terrestrial fossils dating back to the upper Miocene. The transgression, which was at its peak in the early Pliocene (7 - 2 MY), was the last major marine transgression to occur in the New Zealand region, inundating much of the present lowlands.

Fossils found in the Mangaorangan formation comprise a wide range of plants from estuarine species and those of warm temperate forests, including some which failed to survive colder times during more recent glacial episodes, to an assemblage typical of cool upland vegetation.

Included in these were the now extinct beeches of the "brassi" group. The relatively poor dispersal ability of these trees and their inability to disperse even relatively short distances over salt water would indicate that the forest from which they were derived was long established and not of recent origin, as would be expected had the forest colonised a landmass recently exposed by a falling sea level.

Rocks near Mauriceville, to the northeast of the Mangaorangan site, also contain a diverse fossil flora derived from both a coastal area and an elevated landmass.

Fossils from these sites date from a time long before the Tararua Ranges began their recent and rapid uplift and indicate that in the area of Mount Bruce there existed an extensive series of plant associations, covering a wide altitudinal band.

It could well be that Pukaha Mount Bruce is not only one of the remnants of the 70-mile bush, but is also one of its parents.

Forest plants represent a remarkably homogenous group of species within broad forest types, and, as would be expected, most of the species present on Pukaha Mount Bruce are also found in nearby remnants and in the Tararua Ranges. The major exceptions appear to be those species which prefer dry conditions, e.g., the small herb Acaena juvenca, which grows nearby in forest associated with the Ruamahanga River, and shrubs such as the matagouri (Discaria toumatou). Also absent are some species characteristic of lowland forest, such as the liana Fuchsia perscandens.

Forest typing maps show 9 broad vegetation types for the Pukaha Mount Bruce forest.

These are: -
F3 (rimu/kamahi)
F6 (rimu/kamahi)
I 16 (podocarp/red beech/kamahi)
N2, ND15 (logged rata-rimu/tawa/kamahi)
PF3 (logged rimu/tawa)
PF6 (logged rimu/tawa)
RF (regenerating forest)
R2 (mahoe shrubland, shrub hardwoods, general scrubland)
TI 16 (logged podocarp/red beech/kamahi)

Along with these, there is another comprised of red beech (Nothofagus fusca) over a well lit understorey containing Leucopogon fasciculatus and Metrosideros fulgens, along with terrestrial growth of normally epiphytic plants, particularly the orchids Earina mucronata, and Earina autumnalis, along with the kidney fern (Cardiomanes reniforme). This type has so far been seen in two small stands on the tops of leading ridges rising from the valley to the east of the National Wildlife Centre.

Japanese honeysuckle (Lonicera japonica) is present in low numbers near the western entrance to the Pukaha Mount Bruce track. Most of these plants have been hand pulled and regeneration from fallen vines is being dealt with as it is encountered

Apart from that, most weed species are those to be expected on and near a disturbed forest margin. Gorse (Ulex europaeus) has been reported from the vicinity of oneold logging track and though Himalayan honeysuckle (Leycesteria formosana) is prominent along some tracksides, it is only the continual disturbance involved with keeping the tracks open that allows it to persist. A shrubby Cotoneaster (C. franchetii) is present on the western side, near the logging site.

Pasture grasses and low herbaceous plants will all be displaced by the gradual return of their favoured open sites to shrubs, then forest.