Volcanic plugs

As one drives along the main highway from Auckland to Dargaville, two large volcanic 'rocks' rise abruptly from the gently rolling, rural landscape. Tokatoka and Maungaraho Rock sit like islands within the surrounding countryside, and due to their steep, rocky terrain act as refuges for native vegetation. These towers of rock are volcanic plugs - the extremely hard former cores of volcanoes (comprising hardened magma), around which the previous volcanoes have eroded away.

I have visited several volcanic plugs from around the North Island (including Paritutu Rock at New Plymouth and the Pinnacles on the Coromandel Peninsula), to observe the distinctive plants that one often finds in these habitats - which are generally windshorn and have significant areas with shallow soils. Skeletal soils and clefts on Maungaraho Rock (one face of which is pictured above, left) are home to a critically-endangered Hebe (H. saxicola), which is restricted to the upper parts of this landmark.

Whilst walking to the top of Maungaraho Rock to see Hebe saxicola (which is pictured above), we also encountered a particularly beautiful grouping of a fellow rupestral, Astelia solandri, through which long bambusiform canes of one of our most beautiful native orchids, Dendrobium cunninghamii, tumble.

Volcanic plugs lend a distinctive appearance to the landscape on the northern side of Whangarei Heads, where several interesting plant species occupy the dramatic peaks of Mt Manaia and Bream Head. One of these species, Pomaderris paniculosa ssp. novaezelandiae, is capable of growing in the most exposed situations on rocky outcrops of Mt Manaia (as shown above). This bronze-tinted shrub (which is a fine plant for difficult situations within gardens in the north of the country) is a common element of the windshorn vegetation near Mt Manaia's summit, where it cascades down rock faces, through low shrubland and over the edge of tracks (as pictured below, left).

Another inhabitant of the same environments, Pimelea acra, is practically confined to peaks around Whangarei Heads. When growing amongst other low shrubs (as it is in the images above, right, and below, left), this attractive species of 'native daphne' takes on a somewhat upright, bushy form, whilst it assumes a much lower, mounded habit when growing in more exposed aspects, such as the cliff pictured below on the right.

Pimelea acra is one of several recently-described species of 'native daphne', whose recognition as distinct entities gives a better impression of the full diversity within this genus. One of the major characteristics that marks it out from similar species is its bushy growth form (as opposed to the flat growth habit of species like Pimelea prostrata and Pimelea urvilleana); an appearance that it shares with two other recently-described species of the north, Pimelea orthia and Pimelea eremitica (the latter being restricted to Maunganui Bluff on Northland's western coast).

After Mt Manaia, we moved on to Bream Head, where I was particularly interested in seeing two plants that we use on a relatively regular basis within gardens. The first of these is the Whangarei Heads form of Hebe parviflora (pictured below), a species that is mostly found further south from this part of the country (from the eastern Bay of Plenty south). The form that we generally plant within gardens (when working in the north) was originally sourced from this population, and has longer, more willow-like leaves than other commercially available forms (which are generally sourced from the Wellington region), as well as possessing distinctly yellowish stems.

On the way up to see the main subject of our interest on this occasion, Pseudowintera insperata, we also came across robust clumps of Dianella latissima - a recently-described species of turutu that has a particularly wide leaf and rhizomatous growth form, making it an excellent plant for cultivation (notably as a tidier, much more compact alternative to coastal flax).

On this trip to the north, the one plant that I really wanted to find was the Northland horopito (Pseudowintera insperata), a species that we have planted with success in a variety of situations (from urban plantings to gardens). This beautiful species (which is pictured below, in several different stages of growth) is critically endangered in the wild, as (other than one plant near Wellsford that is in poor health) it only occurs on Mt Manaia and Bream Head, and is only represented by about 50 to 60 adults in the wild.

Considering that I had mostly seen P. insperata as compact shrubs within relatively open settings in cultivation (such as at the University of Auckland, from where the species was originally recognised as distinct), it presented a very different face in the wild. Where we experienced it, it grows within cool forest in a limited area of the mountain, where it forms part of a low forest canopy. Although we saw several seedlings and a number of juveniles within the surrounding area of the adult plants, I left with an acute sense of the threats that face this extremely rare resident of Whangarei Heads' volcanic plugs.

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