MUDDY FEET - Auckland Regional Council

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construction and intensive farming, but A. marina is still found at considerable distances
upstream. In the Piako, there is measurable saltwater penetration 50 km upriver, and
mangroves extend 5-6 km inland (Young & Harvey, 1996).
The average height of A. marina at its southern limits is less than one metre, though there are
extensive groupings of mangroves throughout the Ramsar site made up of trees in the 2-3 m
range (particularly at the margins of the tidal creeks), and those lining the Piako River mouth
are mostly in the 3-6 m range. Many of the Ramsar Site trees of 1 m or less have trunks with
a diameter at the base of 80 mm or more, and a canopy of up to 5.5 m. Many stands of A.
marina in the Far North (Kerikeri to Parengarenga) contain trees that are predominantly in the
6-9 m range. The average height of established populations at the margins of tidal creeks in
Whangarei Harbour is five metres, while those inhabiting poorly drained (=low in oxygen) or
less tidally washed areas may have a mean height of less than 1 m.
B. Burns (personal communication 2001) reports that, “The difference in tree size and shape
both within and between populations is also related to differences in productivity". Tall
mangroves are more productive than stunted trees (Woodroffe 1982). Differences in tree
form are often spatially distributed in a population with tall mangroves of several metres
height occurring at the front edge of a mangrove forest or around tidal drainage channels, and
stunted mangroves often less than 1 metre tall occurring at the back of the forest and in
interfluves between drainage channels. There is often a gradation of tree form between these
extremes of size. De Lange and de Lange (1994) suggest that differences in tree form are
related to sediment textural differences, with taller trees growing on finer sediments (which
are richer in available phosphates than coarse sediments). Passioura et al (1992) hypothesise
that the difference in growth form is a result of differences in the build-up of salts as a byproduct
of transpiration, and that greater flushing of soil water near the drainage channels
limits the salinity of these areas, allowing taller mangroves to grow. In contrast, stunted
mangroves are formed in areas where flushing is inadequate to prevent salinity build-up.”
Very few other vascular plants occur amongst the mangroves, but in the better drained sites
(slightly higher ground) salt marsh ribbonwood (Plagianthus divaricatus), mingimingi
(Coprosma propinqua) and pohuehue (Muehlenbeckia complexa) may establish, mainly near
the essentially dry margins of the stopbanks, amongst the old (and stunted) mangroves.
There are some areas of “salt marsh savannah” within the extensive new mangrove forests
between the Waitakaruru and Waihou rivers where these 3 shrubs are well established
(especially mingimingi), usually in association with glasswort (Sarcocornia quinqueflora).
One such Sarcocornia-dominated savannah occurs as vast patches in the vicinity of the
Appletree Pump (midway between the Piako and the Waitakaruru), extending as much as 200
m from the stopbank to the edge of the dense mangrove forest between it and the sea (photo 1,
Figure 5.6). As little as a 5-10 cm difference in elevation in these areas can be the
determining factor as to whether the ground is occupied by stunted mangrove trees or
glasswort (and eventually by one or more of the saltmarsh shrubs).
Aerial photographs also show that these marginally higher, cleared (naturally, or by grazing)
areas amongst the mangrove may be invaded by the introduced and noxious Spartina grass
(Thomson 2000).
There is no competition from any other native plant that prefers muddy intertidal brackish
water habitats within the mangrove’s range in New Zealand, and there are no devastating
pests or diseases that place regular stresses upon them. Thus, A. marina is capable of
EcoQuest Education Foundation EQRS/1 12/04