While the vegetation patterns associated with altitude zonation can clearly be
seen on the southern and western slopes of Mount Ruapehu,
in most places in this region this is not so. This is because there are other
factors that influence vegetation patterns in this diverse landscape.
In this region forests have often been destroyed by volcanic eruptions as hot
ash, lahars or lava flows have buried plant communities.
To the east of Mount Ruapehu lies the Rangipo Desert.
2,000 years ago the area by the Desert Road was beech forest and mixed broad leaf forests.
But these were destroyed when pyroclastic flows from the Lake Taupo
eruption burned and buried everything in their path.
Today you find sparse vegetation on this landscape, mainly consisting of tussocks.
The Taupo flows (lava) sterilized seeds in the ash dunes have very poor soil quality.
Thus the forests have not replaced themselves.
Lake Taupo is actually a giant caldera. The last time it erupted was around
232 AD and it is believed to have been the largest eruption anywhere in the world in the last 2,000 years.
Further the climate here is tough.
Although the rainfall is over a thousand millimeters a year, strong drying winds
whip through the Tama Saddle and flow down the eastern slopes of Mount Ruapehu.
Summer heat parches the dry sandy surface
and winter winds bring heavy snows.
There's frost on the ground here over 100 days a year.
In other places plants have been carried away by rock slides.
Human activities like the building of roads and walking tracks
recreation such as this golf course at Whakapapa,
farming or military activities in the region also interrupt plant communities.
As plants are destroyed, so are the soils on which they grew.
As a consequence the entire sequence of vegetation succession processes has to start all over again.
Vegetation succession refers to the gradual stages by which a mature vegetation association develops.
The climax of vegetation of an area as that which is best suited to the climate, soil and drainage conditions found there.
This area here was once cleared possibly for
some sort of road that didn't end up happening.
But as you can see, the process of vegetation succession is starting all over again
because behind me you can see the climax beech forest.
Around me here however is just small ... small plants
that have started (growing) all over again.
At high altitudes Mountain Beech is the climax vegetation.
However beech forest can also be slow to regenerate as the beech seeds often don't fall further than 200 meters from the parent tree.
For example. after an eruption there will be just the rocky lava flow.
The soils of the volcanic plateau are examples of azonal soils.
That is young soils.
Because of the recently deposited tephra and alluvium
there's been little time for weathering processes to have occurred.
Over time lichens will grow on the rocks.
As soil develops they will give way to
colonizing plants which in turn will give way to tussocks and other alpine plants.
After perhaps hundreds of years the climax beech forest may dominate again.
On the western side of Mount Ruapehu the beech forest is rather patchy due to the
intermittent lava flows that have interrupted the natural plant succession over the centuries.
Introduced plant species like Purple Heather have had a
big impact on vegetation succession processes in this National Park.
This valley would once have been dominated by the yellow of the native tussock grass.
However the Europeans introduced Purple Heather which is native to Britain into this National Park.
Purple Heather has since replaced tussock as the dominant species in plant succession here.
The final factor I want to discuss is the impact on vegetation
that varying climatic conditions in different parts of this region have had.
For example, the direction that a mountain slope is facing,
known as aspect, creates its own micro climates.
With the prevailing winds coming off the Tasman Sea being westerly,
the western side of the ranges receives a higher rainfall than the east.
For this reason lowland forests that were
destroyed west of Ruapehu and Taupo, recovered much faster from the eruptions than on the eastern side.
Southerly and easterly winds tend to be colder and drier
bringing snow rather than rain.
Thus the southern slopes of Mount Ruapehu's Turoa
ski field gets more snow than the western slopes at a similar altitude because they're facing south.
So in summary we have learnt:
1. The higher the altitude the vegetation will get shorter and sparser due to the colder climate and thinner soils.
2. Volcanic eruptions and human impact interrupt the stages of plant succession in this landscape.
3. (and finally) Vegetation also varies in this landscape due to the varying climatic conditions that exist in this region.
Thank you for watching