The battle to hold the hills
Denis Hocking
New Zealand Tree
Grower February 2006
Erosion is an inevitable feature of the New Zealand landscape. Well it
certainly has been over the last 20 million years or so when New
Zealand has been pushed up and buckled around by tectonic forces.
Rapidly rising land forms, large areas of soft rocks derived from
marine sediment and periodic intense rainfall are a recipe for erosion.
Of course, you can also add earthquakes and volcanic eruptions to the
mix. This is geology on amphetamines.
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| Erosion on Wanganui sandstone –
surface slipping is ten times greater under pasture. |
A natural phenomenon
If erosion is a natural phenomenon for New Zealand, the present rate of
erosion in our softer sedimentary hill country soils is not natural. In
the 80 to 150 years since much of our hill country was converted from
forest to pasture cover, rates of soil loss have increased about ten
fold, especially from surface slipping, gully erosion and earth flows.
The results are numerous and often spectacular.
- Rapid thinning of the depth of topsoil from perhaps a metre under
indigenous forest to 10 to 20 cm or less under pasture. This means a
loss of nutrients which can, and generally have been, replaced ‘out of
the bag’. But inevitably there is also an irreplaceable loss of
moisture holding capacity and a sustained decline in pasture
productivity on old slip scars.
- Damage from slipping to farm and public infrastructure such as
fences, tracks, roads and culverts.
- Increased sediment loads in many of our rivers, with a loss of
channel capacity, often a raising of river beds, extra silting on flood
plains and a huge dump of soft sediment out on to the near shore sea
beds.
- There is also a significant loss of sequestered soil carbon,
significant amounts of which will be converted to the greenhouse gas
methane from fermentation in anaerobic sediments.
Serious environmental problem
Soil erosion has to be one of New Zealand’s most serious environmental
problems and has been for many years. It is over 80 years since Guthrie
Smith wrote about the problems of deforestation and soil erosion in his
classic Tutira. Catchment
Boards were established in the 1940s and
1950s, prompted in large part by the 1938 Esk flood and we did see some
response to cyclone Bola in the form of the 1992 East Coast Forestry
Project.
| Summary
map of landsliding in the February storms 2004 |
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There have also been numerous studies on the impacts of different
floods and storms, including the 1977 Wairarapa storms, cyclone Bola,
and the 1992 winter and 2004 flood in Manawatu and Wanganui.
All these studies came to the same conclusion – closed canopy forest
cover, be it indigenous, exotic plantation or even taller scrub cover,
reduces soil erosion on unstable slopes by around 90% compared to
pasture cover. The 1992 study also looked at the efficacy of spaced
poplars and concluded that well maintained, spaced poplars reduced soil
erosion by about 60% to 70%, while poorly maintained poplar plantings
had minimal effect. In other words, there is still three or four times
as much erosion under good spaced, poplar plantings than
under closed canopy forest.
Yet for all this experience and research, it is difficult to avoid the
feeling that over the last 10 to 15 years, soil conservation has
dropped below the political radar and off the national consciousness.
Central government has washed its hands of the business and it has been
relegated to the recesses of the regional councils.
How trees reduce soil loss
Landcare Research scientist Mike Marden seemed to come to rather
similar conclusions in an excellent paper in the November 2004 NZ
Journal of Forestry. Anyone interested in the issues of soil
erosion
and land use should read this paper, which is one of the best reviews
of the subject that I have seen.
As Mike Marden’s paper emphasises, we know a lot about the processes of
soil erosion and how trees and forests act to reduce soil loss. There
are several mechanisms. The canopy reduces the damage that is done
directly from falling rain by interception and significant
re-evaporation, transpiration reduces excess moisture in the soil, but
the key effect is the mechanical reinforcement of the soil provided by
tree roots. The article
by Leith Knowles explains this effect and the requirement for
about 30 tonnes of radiata pine root mass per hectare to ensure
effective protection. Poplar and Douglas fir have less root mass but
higher tensile strength, resulting in similar levels of protection per
tree.
There is more debate about whether forest cover reduces the speed and
scale of flooding during major storms. It probably does, but according
to a recent FAO report, the effect is relatively minor compared to the
role of forest cover in stabilising soil cover. However, local
anecdotal information suggests that this effect can be quite
significant.
 Why so
much erosion prone hills?
If we know about the problem and understand the solution, why do we
still have millions of hectares – variously estimated at two
to five million hectares – of erosion prone hill country in pasture
rather than forest? We can only conclude that despite the talk of
clean, green and sustainable, many hill country landowners are quite
happy to settle for brown, scoured and a quick buck. A Kyoto related
meeting in Wanganui in September gave a rather disconcerting insight
into the thinking of a group of high profile hill country farmers –
dedicated to eradicating scrub, singing the praises of subsidies to
this end especially the old Land Development Encouragement Loans of the
1980s, and overwhelmingly opposed to plantation forestry. But, as noted
above, we have also had changes in local and central government over
the last 20 years that have shifted the focus. With
regional councils succeeding and subsuming the old Catchment Boards in
1989, and being delegated many other resource and environment related
functions, soil conservation seems to have taken a distant back seat in
most cases. Meanwhile central government, which had played key roles in
soil and water issues through the Ministry of Works and the National
Water and Soil Conservation Authority, dispensed with these
institutions in 1988, washed their hands of soil and water issues and
devolved all responsibilities to regional councils.
Today I think it is fair to say that there is renewed interest in soil
and water issues in Wellington, but it is not clear where any
responsibilities fits. And have you noticed how old most soil
conservators seem to be these days?
Lessons learned from the field day
What are the lessons we took away from the field day described below
(see NZ tree grower, February 2006 pg 20)? I fear that the first, and
perhaps most significant, was the complete lack of interest by
surrounding landowners. It would appear that most of our hill country
farmers are just not interested in trees and sustainability and we do
not have the institutions in place to educate, enthuse or enforce
farmers over land use issues. Various farm forestry branches may do
their best, but we desperately need a vigorous, well funded education,
extension, technology transfer programme from regional councils and
central government. I think the monitor farm programme in particular
needs to put a lot more emphasis on land use and sustainability issues.
The second point is that on the Frecklington property, virtually all
the plantations were on comparatively low productivity, pastoral
country. And if it is not low productivity now, it will be after the
next slip. Nor are the plantation areas devoid of grass. This property
illustrates very well the common finding that afforestation of the most
erosion prone areas does not result in dramatic losses of stock
carrying capacity. However I should add that in my experience, sheep
tend to find shaded plantation pasture unpalatable, and it is generally
better used for out of season cow maintenance. Further to this point we
are not talking blanket afforestation in any but a few extreme cases.
The Landcare Research group explained that their work on the East Coast
typically gave a 50% reduction in soil loss with 20% forest cover in a
number of problematic catchments.
Pastoral farming and forestry can co-exist to mutual advantage,
something we farm foresters have never doubted.
The need is for trees, not necessarily radiata pine. Blackwood, cypress
and eucalypts were all doing a very effective job at Rewa and there are
other options such as fodder trees that might be worth exploring.
Spaced poplars have their place on lower erosion risk country, but are
definitely much less effective than closed canopy forest. One of the
lessons learnt from the 2004 storm was the need for more fastigate form
poplars, tall and skinny like the old Lombardy poplars with less sail
area and shading. Several new clones that are in the system fit this
bill.
Loss of institutional knowledge
Perhaps one of the most worrying features with this whole problem is
the loss of institutional knowledge and experience within the remaining
institutions dealing with soil conservation. As outlined above, central
government washed its hands of soil and water issues almost 20 years
ago and the obvious repository for such issues and expertise, the
Ministry for the Environment, has neither the funding nor staff to
handle it. In addition, some regional councils seem to have lost, or
even actively dispensed with, soil and water expertise.
It is not too late to rescue much of this expertise, but we must move
quickly. We also need to move very quickly to entice capable, new
entrants into the field, train them and then offer them attractive
career paths. Soil conservators, or whatever name you want to burden
them with, need to have a status beside, or preferably above, farm
consultants.
Funding and subsidies
Funding is another issue we have to tackle. Who pays, and how much, for
soil conservation services from plantations or reversion to indigenous
forest? In National Water and Soil Conservation Authority and Catchment
Board days significant subsidy funding flowed through from central
government to Catchment Boards and from there to landowners with
Catchment Board schemes. But many of these were wasteful – Rolls Royce
standard schemes where only a Suzuki standard was required – and
economically inefficient.
Inevitably, direct subsidy schemes tend to reward mediocrity and
devalue the efforts of good operators who do the job themselves. Brian
Connell, then National Party spokesman on forestry, made it clear at
the Frecklington field day that they had reservations about direct
subsidies. Would joint ventures, with government bodies sharing the
funding and the proceeds, be more acceptable?
The question of responsibility
Underlying this debate is the question of responsibility for soil.
Under the Resource Management Act, soil is in a different category from
air and water and in essence belongs to the land owner. But should the
land owner have a responsibility to keep this topsoil on the hills and
out of the waterways, or are soil conservation plantings a public
service justifying tax payer or rate payer funds? It is noteworthy that
Horizons is trying to pursue a big, tax payer subsidy approach to soil
conservation.
I am undecided about these issues. In a perfect world, landowners take
responsibility for their land and act accordingly. In the real world
this obviously does not work, or it does not seem to work very often,
but I think we should be questioning the extent to which
landowners should have their shortcomings covered by tax payers?
The one argument that is unassailable in my opinion is the need to need
to bombard these landowners with the technical and educational
information on the role of tree cover in soil conservation and the
range of possibilities available. It is all old hat to farm foresters
and there was once a time when we thought we were world leaders in this
area. But how effective has the message and the messenger been in
recent years? I still feel the real battle is for the hearts and minds
of hill country landowners and there is no doubt
in my mind that the whole country needs to join in that battle.
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