March in the forest with Chris Horne and Barbara Mitcalfe
Metrosideros robusta, Northern rātā
Te rātā whakaruruhau, “rātā, the giver of shelter”, describes the reverence which Māori felt for these huge, long-lived trees that sheltered a heavy load of other plants perching in their crowns.
Northern rātā is one of five endemic NZ tree rātā species, all of which can become towering forest giants, emergent at 20-30m in height, with trunks measuring several metres in diameter – we measured the diameter of one in the Akatarawa Forest at 16 tramping boot lengths! Northern rātā leaves have an intra-marginal vein encircling the leaf just inside the outer edge, a characteristic of the metrosideros genus. Dark green, neatly elliptic in shape, the leaves measure about 30mm x 20mm, but it's that tiny notch at the tip that is the clinching clue, so remember: “n” for notch = “n” for northern.
Trampers in tall forest may not notice northern rātā in the canopy, but the pale, flaky bark of the trunk is a giveaway, and bright crimson leaves on the forest floor can indicate its presence. Masses of scarlet, staminate flowers adorn the crown from November to January and their nectar produces a distinctive honey. The inner bark of rātā was used extensively for dressing wounds and as a treatment for ringworm. Because of its extraordinary strength, heaviness and durability, rātā, “ironwood”, was often chosen to make logging rails, bogies, and even cogs in the milling machines.
Northern rātā used to be numerous in coastal to lower montane (up to 700m a.s.l.) forest from North Cape to Westland. Imagine Te Ahumairangi / Tinakori Hill when it was covered with thousands of scarlet-flowering northern rātā, delighting nectar-feeding tūī and korimako. When these rātā were fired to create space for pasture, they burnt for months on end. (“You would think we had set the world on fire”. wrote Albert Kilmister, a 19th-century Karori farmer).
Northern rātā sometimes germinates on the ground, as most trees do, but often starts life as a wind-blown seed in the crown of a large, mature tree such as a rimu. It is not parasitic, just epiphytic. Over many 100s of years, its roots snake down towards earth, some even encircling the supporting tree's trunk. Eventually these roots fuse together like a trunk, by which time the old, supporting tree has declined and rotted away. This process, misunderstood, was responsible for some people regarding rātā as a strangler.
Possums can kill a huge, mature rātā in two years by preferentially eating its leaves, buds and flowers. Decline in pollinators such as bees, lizards, bats and nectar-feeding birds also threatens the survival of the species. A further threat in and near urban areas is the presence of overwhelming numbers of pōhutukawa, Metrosideros excelsa, with which it hybridises. Northern rātā, Metrosideros robusta, is a naturally-occurring Wellington species, once very numerous here. Pōhutukawa is not a naturally-occurring Wellington species. Humans introducing and planting it so extensively here may have risked the eventual loss of northern rātā as a separate species.