Domain B: mild, humid "nikau" belt

"Nature does nothing uselessly."

- Aristotle (384-322 BC).

 
Map of Domain B: mild, humid
Domain B: mild, humid "nikau" belt.
 

Subzones:

(i) Seaward: The "typical" environment of mild, humid climate, loess deposition, fast nutrient cycling growth;

(ii) Inland: This area shares most environmental features experienced in sub-zone (i) but with far less sedimentation deposition evident;

(iii) Paremata and Camborne Hills: In this area there were thin forest topsoils over firm, heavy clay subsoils of low fertility. Forest clearance and persistent grazing have further reduced the topsoils. Kowhai, totara and titoki are likely to have become more dominant than kohekohe and tawa on steeper banks. Kanuka dominates early succession (taking at least 80 years to develop into young podocarp-broadleaf forest). A strong visual feature today is the band of kanuka and kowhai on headlands around the inlets.

Surface run-off is significant in winter with implications for water quality in the inlet and the now semi-contained estuarine areas within Porirua Arm. Kanuka is the only native capable of bulk colonisation of the skeletal soil cover. Vegetation clearance or grazing compaction of topsoils compounds the extremes of drying and flooding experienced in this sub-zone;

(iv) Pukerua Bay: Wind funnels through this valley, increasing sediment deposition and creating an unusually high energy environment.

Numbered inset on the right is explained below.
 

Environmental character

Mid (relatively few or light frosts) and humid year round. Moderately seasonal rainfall with annual mean of 1019mm. Average January maximum temperature 21°C. Average minimum July temperature 5°C. Salt is not a dominant factor. These are great conditions for nikau, kohekohe and Streblus spp. (flowering plants in the Mulberry family).

Prevailing winds have deposited loess so contours are smooth, streams erode readily to broad, flat valleys, and soils are generally deep silty loess. Swamps form in the shallow valley floors but there is little entrapment of cold air.

Altitude range is generally 100-300m so domain can include upper reaches of coastal escarpment in places (lower boundary can be at lower altitudes where coast is in lee of Mana Island). Physical conditions are fairly homogenous with few extremes.

Relatively soft coastline with sandy bays separated by headlands. There were once areas of duneland and backswamp in Titahi Bay, Onehunga Bay, Mana, Paremata and Plimmerton but no unmodified duneland remains in Porirua City today (here are remnants of foredune only, in Titahi Bay, Tirau Bay and Ngatitoa Domain at Mana).

'Natural' ecological character

Domain B subsections

 
Drawing of Domain B subsections.
Domain B subsections.
 

1. Northerly aspects

Pioneer: manuka (Leptospermum scoparium), kanuka (Kunzea ericoides).

Secondary: kohekohe (Dysoxylum spectabile), kaikomako (Pennatia corymbosa), kawakawa (Macropiper excelsum) dominated two-tier forest; ngaio (Myoporum laetum) on dry spurs or where succession has experienced heavy grazing.

Mature: two-tier forest dominated by kohekohe (D.spectabile), with some tawa (Beilschmiedia tawa), matai (Prumnopitys taxifolia), totara (Podocarpus totara), rimu (Dacrydium cupressinum), hinau (Elaeocarpus dentaus), nikau (Palmae rhopalostylis), Streblus spp.

Although a vigorous tree, the dominant canopy species, kohekohe, is under long-term threat from a combination of possum browse, vigorous regrowth in exposed situations (at the expense of reproductive vigour) and rodent browse of seeds.

2. Southerly aspects

As for (1) but there is more likely to be tawa (B.tawa) in the canopy of mature forest.

Mature tawa is unlikely to survive wind exposure; regeneration is reduced by rodent browse of seeds, wind exposure and decrease in native pigeons/kereru distributing fruits.

3. Broad valley floors

Secondary: sedges (Cyperus spp.), toetoe (Cortaderia fulvida), rushes (Juncus spp.), swamp flax/harakeke (Phormium tenax), manuka (L.scoparium), five-finger (Pseudopanax arboreus).

Mature: three-tier forest with kahikatea (Dacrycarpus dacrydioides) and pukatea (Laurelia novae-zelandiae) emergent over tawa (B.tawa) and nikau (P.rhopalostylis).

Many of these valleys would have developed into wetlands. The largest example is the Taupo Stream and its tributary creeks.

4. Coastal foreshore

Minor saltmarsh and rushland occurs where flat bottomed creeks meet the sea (some of the now contained Porirua Arm areas are slowly becoming freshwater marshes) and on brackish benches raised by earthquakes, but sandy beaches dominate between headlands. There is generally only a narrow band of larger coastal trees fringing the shoreline (ngaio (M.laetum), karaka (Cornynocarpus laevigatus), taupata (Coprosma repens), akiraho (Olearia paniculata)).

Continue to Domain C or return to Porirua's Ecological Sites Inventory.