Sunday, 24 July 2011

Scrub weeds

Yesterday was one of the coldest winter days in Wellington for many years, and one of the friends visiting us is as much of a weather nerd as I am, so in the afternoon we set off up nearby Johnston’s Hill to try to get a bit more altitude in the hope of seeing some snow.
A sleet shower approaches Karori from the south.
We were rewarded by a sleety shower on the way down, but the real cold set in overnight; however there has been little precipitation in Wellington so far.
The top of Johnston’s Hill is regenerating scrub on what once was sheep farming land.  Under the scrub are seedlings of native trees that one day will be a new forest canopy here.  Right now the scrub holds sway, and the best thing to promote forest on these hills is to try to make sure the scrub doesn’t get burned, which only sets the whole process back to the beginning.
The scrub is interesting, because unlike many places locally there are three species involved, all of similar abundance, but very different biology.

In this picture grey leaved tauhinu, yellow-flowered gorse, and orange-flowered Darwin’s barberry form a mosaic of textures and colours on Johnston’s Hill, with Wellington City in the background.
Tauhinu (Ozothamnus leptophyllus).
Tauhinu (Ozothamnus leptophyllus)  is the only native among this trio.  It’s a tall shrub, with small silvery leaves.  Being in the daisy family (Asteraceae) its seeds are dispersed by wind, each one in its own little fruit with a pappus attached that catches the wind.  I like it that tauhinu can hold its own against gorse and barberry; we even get seedlings coming up in the garden, testimony to the effectiveness of wind dispersal.
Darwin's barberry (Berberis darwinii)
Darwin’s barberry (Berberis darwinii) is a South American plant, first collected by Charles Darwin on the Beagle voyage.  It’s prickly, tall (it can be a small tree), and shade tolerant, so it can survive as the native canopy grows up around it.  Worse, it has fleshy fruit, so its seeds are dispersed long distances by birds.  It and its relatives are serious conservation weeds in parts of New Zealand, though they were originally introduced as ornamentals and hedge plants.
Gorse (Ulex europaeus)
Gorse (Ulex europaeus) is a familiar plant to New Zealanders and Europeans.  First introduced as a hedge plant here, it quickly spread and became established.  As a legume, it harbours nitrogen-fixing bacteria in root nodules, so it thrives under a regime of super-phosphate topdressing.  Sheep will eat the young shoots, but once the stems have hardened into prickles they’re inedible, so gorse can be hard to control.  Its seeds are dispersed explosively when the pods fly open on warm summer days, so they can disperse only 5–10m as a rule.  But those seeds are one of its real strengths because they can live in the soil for decades.  Our house was built in 1931, but whenever the ground is disturbed, gorse seedlings will pop up.  The big conservation value of gorse is that its litter breaks down to a nitrogen-rich mulch, it shelters regenerating natives, and eventually when the native trees or the gorse bushes grow tall it can’t regenerate in the shade.  Then the gorse is said to be a nurse crop for native forest regeneration, an attribute put to good use by Hugh Wilson at Hinewai on Banks Peninsula.

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