Replacing Missing Species

Native forest that would have once naturally occurred in an area can be recreated by actively replanting the area with the right plants, or by allowing natural succession and regeneration to occur.  To restore an area successfully takes planning, effort and time. 

A successful restoration planting should create conditions where native plants can regenerate themselves, so that eventually the planting can become self sustaining like a natural forest.  The right selection of plants will attract and provide habitat for native birds and other wildlife.

Bush layers

Healthy bush has a structure of many "layers", from tiny seedlings on the forest floor up to the tallest trees towering over the forest canopy. 

Layers within NZ native forest.

Layers within NZ native forest.  [Source: DOC]

The layers start with a deep covering of leaves on the ground (leaf litter) which, together with rotting logs and branches, provide ideal growing conditions for a mosaic of ferns, mosses, lichens and emerging seedlings.

Above the forest floor there should be an understory and sub-canopy layer, making it difficult to see far into the bush before there is a "wall" of vegetation.

Understory plants such as coprosmas, kawakawa, mingimingi and tree ferns occupy the layer from a height of approximately 20cm to 2.5m.  Above this, the taller sub-canopy trees (e.g. mahoe, pate, putaputaweta and pigeonwood) form a layer between the understory and the canopy.

Below the canopy you will also find saplings of various ages waiting to be "released" in the gaps left by older trees as they die and / or fall.

A healthy canopy will be almost continuous (except for tree-fall gaps) and will usually comprise a variety of species, with different trees dominating over time as they compete for space in the canopy.  Common canopy trees are tawa, titoki and pukatea, and on hillsides, kohekohe and hinau.  When looking up into the canopy you should see climbing vines (such as rata and supplejack) and different species of epiphyte e.g. Astelia and filmy ferns.

The final layer, the "emergents", are particularly tall trees that tower above the surrounding canopy e.g. kahikatea, totara and rimu.

Bush restoration

First-stage regeneration species – those that grow naturally under gorse, and which also are the species we have been planting in the Green Firebreaks – have mainly got there by means of bird droppings.  That's how these species regenerate, and these are the main kinds of trees that will regenerate by natural means.

There are some other slower growing, mid-canopy species such as tawa, kohekohe, kanuka, etc, which presently form obvious remnants in the valley – Cannons Creek Conservation Covenant being clearly dominated by the former two, while the Takapu (riparian) Conservation Covenant is more mixed, has more kanuka and manuka, also nikau.  Some of these species are spread by birds also.

So, how come there are no rewarewa, northern rata, rimu, kahikatea, miro, and only one or two pukatea, puriri, totara, maire, etc, occurring naturally in the valley?
Answer: because these were valuable timber trees which were milled out by the early settlers' timber mills at Tawa and Pauatahanui.

The settlers cleared the Belmont Hills of all native forest on the north side and the tops, to make farms.  Only little patches of native forest were left in awkward gullies.

Once the timber trees were taken out completely, the remaining trees were cut, dried out, and burnt.

This practice killed any seeds of podocarps which might have regenerated.

Some of the above mentioned species are not regenerated by birds as their cones are in some cases indigestible.

The 1993 booklet "Native Forest Restoration", Appendix II p.175 provides a list of "Bird distributed native plants".  That list does not include any of the following species: rimu, kahikatea, rewarewa, northern rata.

Such trees mainly rely, for regeneration, on letting the big old tree die and fall.  When it falls, it (a) opens a gap in the forest, which energises the seeds the tree has dropped directly below it throughout its lifetime; and (b) as the tree rots down, nutrients and bacteria that are needed to feed those seeds as they sprout, are released into the soil. 

Hence the Maori saying in relation to a great person's death: "A mighty totara has fallen" – this is not just a death announcement, nor just a tribute to the greatness of the deceased – it indicates that new life will spring from that old tree in that very place and will grow up to fulfil the promise of the old.

This is the process we have to imitate for certain species, if the forest in our valley is to be restored to its former completeness.  It will not happen otherwise.  Hence the need to locate the right sites – specific places we call micro-sites - to plant the species that are missing from our valley, the ones we know as emergents or forest dominants.