Tuesday, 12 June 2012

Wednesday Wildflower: tree lucerne

When we lived in Okuti Valley on Banks Peninsula, there was a tree lucerne right outside the back door, and there were nearly always a couple of kererū (New Zealand wood pigeons) in it, feeding on the young shoots.  In spring they'd move to the poplar shelter belt (we counted 35 there one day) and in summer they'd be feeding on fruit, including the neighbor's plums.

Tree lucerne is a common woody weed in New Zealand, but it's one of those weeds that's not really much of a problem.  It was deliberately introduced (sometime before 1919) as a hedging plant and as livestock food, hence the New Zealand name, which is a reference to lucerne or alfalfa (Medicago sativa).  It comes from the Canary Islands, where it's known as tagasaste, a name which is sometimes also used in New Zealand. Now tree lucerne is naturalized pretty much throughout the country.  Its value as a fodder plant is also reflected in its value as a food for kererū.
Tree lucerne, Chamecytisus palmensis. A, a cluster of flowers; B, flowers, seen from the side and from beneath; C, seed pods developing; D, leaves: upper surface on left; E, seeds.
The flowers are typical of the legume subfamily Papilionoideae.  They have 5 petals: a large standard or flag petal, clearly seen in B above; opposite that is a keel that's formed from two petals joined together; and alongside the keel is a wing petal on each side.  The standard overlaps the wings and the wings overlap the keel.  The pale structures at the stalk ends of the seeds are strophioles; their function is associated with water uptake by the germinating seeds.

Tree lucerne, Chamaecytisus palmensis, near Northland Tunnel, Wellington, New Zealand.
Here in Wellington tree lucerne is common on roadside banks and cliffs, and can form a trunk a few metres tall and 150 mm diameter.  The white flowers form during winter and are a source of nectar for bees.

Like gorse and other legumes, it's a nitrogen-fixer, or at least it harbours bacteria that are nitrogen-fixers.  That's part of its value as a livestock food, and also might give it a role in eco-restoration, as a nurse crop for native forest regeneration.  If kererū are attracted to the leaves, they might bring seeds of native plants to regenerate under the shelter of the tree lucerne.

This is a plant with so many virtues, it's perhaps unkind and misleading to call it a weed, and you'll note I've renamed the series "Wednesday Wildflower".

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