by John Walsby June 1994
All around the world,
wherever there has been wholesale clearance of native forest for
agricultural development, subsequent problems have arisen with
land stability and with rivers flooding or changing course. Wherever
there is no forest cover to absorb heavy rainfall or delay run-off,
water gushes down cleared uplands, stripping away topsoil and
starting landslips. River channels, unable to contain sudden torrential
flows, burst their banks and the lowlands developed for pasture
or cropping become flooded. Frequently they are left blanketed
with thick layers of silt.
Tropical countries
like Bangladesh, are particularly vulnerable to these so-called
"natural disasters" (which are actually not natural
but the result of human interference) because they are subject
to long periods of extremely heavy rain during the monsoon season.
The consequences are often catastrophic but even in New Zealand
major flood and landslip damage has occurred regularly since European
settlers started large scale bush clearance. In the top half of
the North Island, the river banks along the lower tidal reaches
are protected by fringing mangrove forest but these trees cannot
survive in freshwater upstream or in colder regions further south.
To stabilise river
banks which had earlier been stripped of native vegetation, pioneer
farmers started planting European willows; trees with long, wiry,
slender branches and narrow leaves. In winter most lose their
leaves and the orangy-brown bark of the most recent growth gives
the bare branches a golden hue on sunny days. They have subsequently
become so widespread that willow flanked waterways are now a part
of the character of the New Zealand countryside that we take for
granted. But it has not always been this way.
The first species introduced
to New Zealand was the weeping willow, one of many species of
tree willows which usually have single trunks, broad crowns and
long lance shaped leaves with serrated margins. The first couple
of weeping willow cuttings were planted about 150 years ago. When
they grew to sizable trees they in turn became the source of cuttings
of many other weeping willows throughout the country.
Tree willows (distinct
from osier and shrub willows) are well suited for river bank stabilisation
because their extensive root systems form dense fibrous mats above
and below the waterline. These protect soft muddy banks from being
eroded when rivers are in full spate.
The crack willow, Salix
fragilis, was originally the most popular species as it was easy
to propagate. Its scientific name alludes to the ease with which
young shoots break off and its common name, to the sharp sound
made when they are removed. Propagation involved nothing more
than pushing freshly removed shoots into muddy ground where they
quickly rooted. This made it easy to plant up long stretches of
slumping river banks but it also resulted in widespread accidental
plantings.
When twigs are snapped
off by cattle, possums, wind or rushing flood water, they take
root easily wherever they become stranded. In places this has
led to serious problems when the interlaced roots and branches
of maturing, self-planted crack willows have completely choked
small streams.
When travel to New
Zealand was only possible by sea, all willows introduced here
were brought as cuttings because willow seed is only viable for
a short time. Today many of the common species can still only
be propagated from cuttings because they are unable to set seed
here. All willows are single sexed trees and only the osier and
grey willows, which have both male and female plants in New Zealand,
can set seed. By chance, there are only female plants of golden,
bitter and weeping willows, and only male plants of the crack
and golden-weeping willows.
