Today we are looking at the spatial variations of vegetation that exists in
a volcanic landscape. Our case study is the magnificent landscape of the
Tongariro National Park in New Zealand. This landscape has numerous volcanoes
including Mount Tongariro, Mount Ngaruahoe and Mount Ruapehu.
The first pattern I want to cover is that of altitudinal zonation. This diagram
shows the zones of different forest types found in New Zealand as altitude
increases. At high altitudes vegetation types are generally shorter and sparser.
Plant life gets shorter and sparser until eventually only ice and snow is found
on the bare rocks. At lower altitudes the climate is generally warmer so the
vegetation is characterized by large forest trees. This is because air cools as
it rises, so areas of higher altitude have colder temperatures. We call the
change of temperature as air rises the Adiabatic lapse rate. Generally average
daily temperatures fall about 0.6 degrees for each 100 vertical metres.
Let's take a closer look at the interactions vegetation has with soil,
relief and climate to produce these patterns.
At high altitudes wind speeds have high velocities where the temperatures are cold.
Parent volcanic material dominates. Further, rates of condensation increase as temperatures fall.
The rainfall also increases with altitude. Only the hardiest organisms can survive here
such as this Moss located at 1650 metres on Mount Ruapehu.
Mountainous areas with steep slopes have high rates of erosion.
Any loose material is quickly carried down slope. Thus soils are thin.
Even at this altitude there are signs that plants are clinging
to the bare rock. Just in the crack here we've got some grass and there's some
very thin soil that's formed in between the cracks of this volcanic rock.
At an altitude of 1,200 meters soils are thin but are still deeper than they are
higher up the mountain. Here there numerous species of small plants growing
such as mountain Inaka
Snow Tototara
Bog Pine
tussock grass
Mountain Toatoa
and flowering plants such as the Mountain Daisy. In the summer there is a spectacular
display of alpine flowers.
These plants trap silt and ash that have been
transported by wind, mountain streams or melting ice and themselves add organic matter.
They act to further protect the surface from erosion.
Mountain Beech is also found here. However at this altitude their growth is stunted.
The Mountain Beech grows from the 700 meter mark up to the tree line. It can grow to 15 meters tall.
This is the smallest of the beech species in New Zealand
Another interesting tree found here is the conical shaped Pähautea or the New Zealand Cedar.
This tree can be found even at altitudes of 1,200 metres and
they can live as long as 1,000 years.
Around 900 meters beech forest dominates. Beech trees are evergreen broadleaf
trees with small rounded leaves, as you can observe here on the forest floor.
Adverse altitude temperatures are still cold but a much more acceptable and
soils are becoming deeper.
Towns such as Waiouru are at a similar altitude to this.
New Zealand has five species of beech tree: The Red Beech,
the Silver Beech,
Black Beach, Hard Beech and Mountain Beech.
At this site we find Mountain Beech and the larger Silver Beech which can grow to 25 metres.
Silver Beech enjoys the cool temperatures and wetter climates that exists at this altitude.
One tree that must be avoided here is the Bushman's Lawyer. Many a hiker has been caught by its barbs.
(Grunts) Oh, oh my goodness. Look at that cut. That was done by this.
This evil plant is the Bush Lawyer. Beware of this plant it will cut you.
You can also find here the Rangiora tree, more more affectionately known as the
bushman's friend. You can imagine from these broad leaves why many hiker has found it useful.
Mountain cabbage trees
flaxes
Hall's Totara and other species are also found here.
The Red Beech is the largest beech tree.
It grows to 35 metres but prefers the warmer temperatures at lower altitudes.
At lower altitudes from sea level to 650 metres Podocarp forest dominates.
Podocarps are conifers that are distinguished from pines because they
bear small fruit instead of cones.
These forests are stratified with emergent
podocarps over 30 metres tall dominating.
Podocarps include some of New Zealand's largest tree species such as the Tototara
the Matai or black pine
the Miro or brown pine
the Kahikatea or white pine
the Maire and
the Rimu.
These trees live to be hundreds of years old. Rimu is the dominant podocarp of the Central North Island forests.
Kahikatea is New Zealand's tallest species with one tree
reputedly to have been up to 68 metres tall.
It prefers swampy soils and is often found growing on the floodplains of rivers.
Many of these trees have epiphytes growing on them.
Epiphytes are plants that grow on other plants such as these ones here.
The next layer is the canopy which
has trees that grow to about 20 metres.
Examples of this include the Hinau,
the Rewawera and the Pukatea. The Pukatea is unique as it is
the only New Zealand native that has buttress roots
which is similar to the species that usually frequent tropical rainforests.
Below this is the sub canopy which includes species such as the Mahoe or Whitey Wood.
Tree ferns such as the Ponga, Mamuka and Wheke,
the Lancewood,
and Cabbage Trees.
Below this is an assortment of small species no taller than 5 meters such as the Horopito.
The Horopito is a shrub that is also known as Pepperwood because of its
reddish leaves which when eaten, they leave a peppery taste in the mouth.
This little beauty is called the Horopito tree. You can identify it by its reddish leaves.
In Maori (language) it's known as the Horopito and the
English name for this is called the Pepperwood and the reason why it's called
this is because when you chew it it's got a very peppery taste, and quite frankly it
is more like eating a chili. I'll give you a little demonstration here.
All right, this one here. (Mumbles with food in mouth) It's very environmentally friendly. But um, my goodness! Don't swallow this this okay.
Don't do this at home. Now when you finish this ... yuk! Gosh I'm going to spit this out.
Many juvenile trees such as the young Miro that you can see here growing in
close proximity to its mother tree are growing in this layer.
They're waiting for a gap to open up so they can compete for the sunlight.
On the forest floor there are mainly mosses and ferns such as Hounds Tongue Fern
the Crown Fern
and Kidney Fern.
There is a lot less sunlight penetrating down here.
These lowland forests are home to numerous native bird species
including the Kiwi
the Tui,
bell bird
the Keruru or Native Pigeon
Fantails
the North Island Tomtit,
Silver Eyes
and the North Island Robin.
On the flatter land at lower altitudes erosion rates are much slower
and deposition of alluvium by rivers and flood occurs.
Thus soils are often deeper and more fertile than higher up.
In this volcanic environment there may be alternating layers of ash laid down from historic volcanic eruptions.
On the forest floor
there is a thick layer of decaying plant and animal matter.
We call this humus. Below that is the topsoil. You have to dig a lot deeper
before you get to the bedrock below.
While the vegetation patterns associated with altitudinal 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 sterilized seeds and 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 vegetation of an area
is 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 which have started 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 metres from the parent tree.
For example, after an eruption there will be just a 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 forests 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.
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 be colder climate and thinner soils.
2. Volcanic eruptions and human impact interrupt the stages of plant succession in this landscape.
3. Vegetation also varies in this landscape due to be varying climatic conditions that exist in this region
Thank you for watching.