12.0 ENVIRONMENTAL IMPACT OF LAND-USE
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Key points
- Levels of land slipping under forest over six years of age are five to ten times less than under pasture.
- Soil compaction is a recognised risk in forestry systems. There is no clear evidence to suggest more or less site degradation by one or other land-use.
- Agriculture is ranked nationally as the largest source of adverse effects on water quality. While site disturbance in forestry operations also presents a risk to water quality, the quality of water from pastoral catchments planted to forest can be expected to improve.
- Total water yield from a planted catchment will decline, largely by a reduction in quick flow. Low flow levels are also likely to decline although the local evidence is contradictory.
- Retaining or reestablishing riparian zones in pastoral or forestry systems will improve biodiversity. Forest companies are developing practices and standards in this area which are a significant improvement over past practice and over most pastoral environments.
- Weed and pest control are important issues regardless of land-use. Regulatory controls apply to all land-users equally.
- There is little evidence to suggest that afforestation at current rates will influence the local climate. Planting forests is promoted as a mechanism for reducing net greenhouse gas emissions.
- Introduced threats to the plantation resource are real, and prevention and control of introductions are the subject of substantial Government and private expenditure.
- Both land-uses generate potential health and safety risks. These are generally well understood and systems to manage them are improving.
- Commercial liability risk can increase for plantation forest neighbours. Fire is less commonly used as a management tool in forestry than in the past.
- Public awareness of aesthetic and landscape values appears to be increasing. Further research is needed to substantiate planning criteria based on these values.
12.1 Data Sources
This chapter is a review of literature available in this field, with
particular reference to the Wairoa District where possible and to the East Coast and
Hawke's Bay in general. Issues discussed are those raised by respondents to mail
questionnaires sent to all businesses and farmers (see Chapter 6) or raised in discussion
with the authors at public meetings.
Major environmental issues in the District are (a) erosion and its impact on hillside soil
loss and river aggradation, and (b) water quality and yield as a result of land management
practices, sediment loadings and effluent discharge into rivers and lakes. This review
will treat individual effects separately, although it should be recognised that many of
the processes are interrelated and are occurring simultaneously on the same land units.
12.2 Soil Loss
12.2.1 Mass movement erosion (slipping and earth flow)
Erosion is by nature episodic. On any given site the risk or occurrence
of erosion can be quantified in terms of frequency and of magnitude. The two
are interrelated, but only one can be influenced with any certainty. The frequency of
erosion is a function of external forces (strong wind, intense rainstorm, prolonged wet
periods, earthquakes etc.). The magnitude of erosion is a function of site
characteristics, i.e. geological structure, soil properties and vegetation cover.
Land-use, vegetative cover and soil conservation techniques do not change the frequency of
erosion but strongly effect its magnitude (Clough & Hicks 1992).
The mass movement processes that are most common in the Wairoa District
are shallow slip and earthflow erosion (Page 1988). There is considerable evidence for
frequent shallow slips under natural conditions (Clough & Hicks 1992). Slipping is
induced by short, intense rainstorms or prolonged wet weather. The magnitude of slipping
in any event is complex. Slipping under pasture occurs more intensely than under forest
for storm events of the same magnitude. As a result, average levels of slipping under
pasture are between two and ten times greater than under indigenous forest or scrub
(Clough & Hicks 1992). Harmsworth et al.(1987) identified land class VIIe2
(banded mudstone) as having the greatest incidence of slipping (4.5 landslides/ha) in a
study of storm damage at Otoi. Classes VIe6 (Taupo tephra) (1.3 landslides/ha) and VIIe9
(sandstone) (1.9 landslides/ha) also had severe rates of slipping.
Research on the East Coast after Cyclone Bola assessed the relative
magnitudes of loss from shallow mass movement and deep-seated earthflows. Studies showed
that slopes with pine plantations <6 year old had a mean landslide density of 0.62
landslides/ha, which is similar to that for adjacent slopes in pasture (0.68
landslides/ha) (Phillips et al. 1990). This corresponded to increases exceeding
300% over pre-cyclone landslide density for both scenarios. Mean surface erosion volumes,
calculated from eroded area caused by landsliding for this one event, were 790 m3/ha
for pine plantations <6 years old and 916 m3/ha for pastured slopes (Phillips et al.
1990). This suggests that reforestation only slowly enhances slope stability or reduces
surface erosion until six years after establishment (approximate stage of canopy closure).
Changes in planting density, timing of thinning and seedling genetic quality all influence
when canopy closure occurs and so are likely to affect the rate at which trees contribute
to site stability.
This was compared with the mean landslide density for slopes under
stands of pine 6-8 years old (0.21 landslides/ha) which was one-third that of younger
stands on similar terrain. Mature pine plantations (>8 years) had less than 0.06
landslides/ha and, on an area basis, showed a 0.6% increase in landslide area after
Cyclone Bola. This was one-tenth of the landslide density for slopes under pines <6
years old. Surface erosion owing to landsliding was reduced from a mean of 790 m3/ha
in <6 year-old stand to 48 m3/ha under older stands (Phillips et al.
1990).
Much of the sediment generated by this erosion ends up in watercourses
and on floodplains. Page et al. (1994 ) showed that sedimentation rates in Lake
Tutira, a closed catchment, increased between five and seven times after deforestation and
conversion to pasture. They estimated that approximately 57% of eroded material made its
way into watercourses in the catchment during Cyclone Bola.
Studies by Watson (1990) on root development and biomass at Mangatu
Forest indicate that Pinus radiata root biomass increases at rates of 1-2
t/ha/year from the time of planting to age 8-10 years, and then at 3-4 t/ha/year up to at
least 25 years. The increase in root biomass from about age 8-10 and increased rainfall
interception by the closed canopy enhance slope stability and reduce the likelihood of
shallow landsliding during storm events. At age 8, root systems have developed strong
lateral structural roots which extend up to 4 m from the root bole and vertical 'sinker'
roots which penetrate to 2 m depth. The interlocking of adjacent tree-root networks
reinforces the upper soil horizons and prevents the widespread development of shallow
landsliding. Slope stability improves until about age 15 years. Further improvement in
slope stability as stands age is thought to be negligible, though the influence of
understorey vegetation is likely to increase.
Table 12.1 Examples of the effect of land-use on shallow slipping
| Land-use / vegetative cover | Black, unpubl. - Hawke's Bay, Cyclone Bola (slip scars/ha) | Phillips et al. 1990 - East Coast, Cyclone Bola (m3 soil loss/ha) |
| Forest Scrub Pasture Pines (<6 years) Pines (6-8 years) Pines (>8 years) Pines (>30 years) |
0.12 0.3-0.5 2.9 2.9 0.5-0.76 0 |
916 790 370 48 |
Although not extensive in area, earthflows are locally significant erosion events.
There has been no work in the District specifically addressing earthflows, but data from
the Gisborne District on similar rock types is likely to be reasonably representative.
Hick's work at Waihora (Hicks 1989) suggests that tree cover can reduce the incidence of
flow initiation and movement. Pearce et al. (1987) showed that although drier soil
profiles and tree root networks affect only the upper 25-30% of an earth flow, their
combined effects retard earth flow movement and sliding on basal surfaces several metres
below the base of the rooting zone. Tree root systems influence the movement of earth
flows substantially thicker than the rooting zone by creating a reinforced upper soil
layer 1-1.5 m thick that has significant tensional strength and higher shear strength than
the underlying flow material.
Table 12.2 Effect of land-use on earthflow movement
| Vegetative cover | Pearce et al 1987 - Mangatu (movement in m/yr/earthflow) | Hicks 1989 - Waihora (unstable surface %) |
| Stable pasture Unstable pasture Poor plantings Effective plantings |
3.5-10 0.2-0.5 |
1.6 11.3 9.1 2.7 |
Channelised mass movement (debris avalanches and flows) occur in the very steep
mountainous areas under intense rainfall events, and may contribute 1500-4500 m3/km2/yr
of sediment to watercourses (O'Loughlin & Pearce 1982, Grant 1982). Owing to
difficulty of measurement data is scarce, and much of it is indirectly derived from
measurements of river sediment transport. Almost all of this erosion, and the most
susceptible areas, are in the forested ranges in the west of the District and not in
productive use.
Clearly afforestation can reduce the magnitude of mass movement erosion, but there are
periods of susceptibility to erosion in a plantation rotation. These occur between
planting and canopy closure at age 6-8 years. In subsequent rotations this period of risk
is perhaps reduced as roots of harvested trees can provide residual but declining
cohesiveness over the first 2-3 years of the subsequent rotation.
Slipping also has a significant impact on pasture productivity in the short-term and
potential productivity in the long-term. The study by Douglas et al. (1986) of
pasture production in the Wairoa District indicates a minimum of 15 years to recover
pasture productivity to a steady state after slipping. Maximum recovered productivity
under normal management is likely to be at least 20% below original. This is largely a
potential loss, and can be masked by additional management inputs, e.g. fertiliser,
improved grasses, subdivision.
Land slipping in the District can also have a large immediate financial impact in terms
of replacing fences, reinstating tracks and oversowing damaged hillsides. Disaster
recovery grants on the East Coast after Cyclone Bola totalled approximately $23 million,
and on-farm damage estimates totalled $17 million, with an additional $5 million loss in
revenue from lost grazing in the year following the storm and $3 million/annum thereafter.
Payments to individual farmers averaged $83 000 for large farms (>800 ha) and $30 000
for small farms (150 ha to 800 ha) (Korte 1989).
12.1.2 Surface soil loss (sheet wash, rilling)
Wairoa District has a moderate to high susceptibility to insidious surface erosion
owing to the soft sedimentary rocks, ash cover on some sites, steep slopes and high
rainfall intensity. Erosion of this type moves surface soil particles downslope within the
pasture sward and also moves sediment from the scar edges and slip surface. There is,
however, very little data available to confirm relative rates of loss by this process.
Fortunately the most extensively reported measures, those of Campbell (1945), were
taken in Hawke's Bay. His data suggest orders of magnitude difference between land areas
under different vegetation cover types.
Table 12.3 Field Measurements of Surface Erosion Magnitude
| Vegetation cover type | Surface erosion magnitude (tonnes/ha/event) |
Forest Scrub Pasture (depending on slope, intensity of grazing and herbage removal) Devegetated, eroded or bare ground |
<0.01-0.29 0.11-0.29 0.07-4.42
5.60-14.57 |
The erosive potential under pasture is strongly dependent on slope and intensity of
grazing. At low slope angles and under ungrazed improved pasture levels were very low
(0.07 tonnes/ha /event). This increased with grazing (1.41 tonnes/ha/event) and where
pasture was depleted or overgrazed (4.12 tonnes/ha/event), and was greatest on steep
slopes (4.42 tonnes/ha/event at 33 degrees).
This data clearly indicates the increased background levels of loss under pastoral
farming when compared with scrub or indigenous forest, but also highlights the risks of
overgrazing, blanket vegetation removal when establishing or harvesting forests, and the
value of oversowing harvested sites in forestry or recently slipped sites under pasture.
12.2 Soil degradation
Both agricultural and plantation forestry affect physical, chemical and biological
characteristics of the soil. The significance of the changes can be assessed relative to
some alternative condition or in terms of trends in productivity under present use. Under
plantation forest, soil is subject to lower rainfall additions relative to pasture because
of interception and reevaporation in the canopy. Soil temperature fluctuations are
moderated, once again by the canopy. Macro and micro nutrient losses are determined by the
amount of soil loss and by the amount of foliage and small branches lost or transferred
during harvesting. Pastoral farming loses significantly greater amounts of macro and micro
nutrients through removal of animal products from the system and from volatilisation and
leaching of nitrogen. Soil loss is also a major cause of nutrient loss from the system.
Both land-uses can reduce site quality by compacting soils with heavy equipment at
harvesting, during tillage or in road or track construction.
12.2.1 Nutrient depletion
Data describing relative nutrient loss rates for pasture and forest crops is sparse.
The impact of trees on nutrient availability varies and is complex. Smith (1994),
reviewing work in this field overseas, suggests that trees in low-nutrient soils reduce
nutrient availability and those in high-nutrient soils increase it. He suggests that the
outcome for any site depends on the plantation's effect on the equilibrium between inputs
and outputs of organic matter. The majority of plantation forests in New Zealand are
planted on soils where nutrient levels are not limiting. Under these conditions, there is
some evidence to suggest that plantation forests increase the level of plant-available
nutrients in the top 25 cm of soil. Possible processes for this include mobilising
nutrients from deeper in the soil profile than small vascular plants or grasses can reach,
and increasing interception of airborne nutrients as sea salts or dust.
In low-fertility soils, where some nutrients are in marginal supply, removal of biomass
at harvesting - particularly foliage and small branches - may be expected to generate
deficiencies or alter nutrient balances. This occurs on some coastal sand sites, on
excessively leached sites and on older clay soils in the Auckland and Northland regions.
Negative effects are managed by determining the degree to which weathering and plant
litter inputs replace losses, by retaining organic matter and nutrients on site as far as
possible, by establishing leguminous herbs and by applications of appropriate fertiliser.
Nutrient depletion in agricultural systems is caused primarily by leaching, animal
transfer and soil loss owing to erosion (Hedley et al.1990). Maintenance
applications of key elements are essential components of clover-based pasture systems, to
maintain clover nitrogen fixation and growth of productive pasture species. The
significant increase in nitrogen flows in improved pasture systems inevitably results in
leakages from the system, particularly from dung and urine patches and from surface
erosion of particulate matter. Continuous external applications of phosphatic fertiliser
may build up levels of macronutrients in the soil profile. Soil loss through erosion then
may result in increased losses of these nutrients.
12.2.2 Structural deterioration
Structural deterioration in pasture causes declines of 10-15% in pasture growth on
light soils under pasture (Brown & Evans 1973). Trampling and pugging of pasture has always been a
problem for farmers, compacting the soil, increasing ponding in the winter and reducing
the availability of soil water to plant roots in the summer. Compaction is also a problem
in plantation forestry with the use of heavy machinery in harvesting and roading. Tree
growth is adversely effected on skidder-logged sites and on skid sites where logs are
aggregated, although reported effects are variable and not well quantified (Dyck &
Cole 1990). Natural alleviation of compaction is slow (Gameda et al. 1994), and
compacted soils can reduce soil biological activity, restrict plant root extension, reduce
water infiltration and increase surface runoff.
12.2.3 Acidification
There is evidence of acidification in both pasture and plantation forestry systems.
Acidification in pastoral systems owing to the removal of cations in livestock production
and addition of phosphatic fertilisers is overcome by applications of lime. Levels of
acidification are rarely monitored or modified artificially. Lower pH levels than on
comparable pasture sites lead to lower extractable aluminium and exchangeable calcium
levels (Hawke & O'Connor 1992, Giddens et al. 1995). The long-term impact on
productivity is yet to be assessed.
There is a common concern among pastoral farmers that land managed for production forestry will not easily be able to be returned to pasture production because of acidification. Work by Davis (1992), experience in Chile, where arable crops succeed a plantation rotation (P. MacLaren, pers. comm.) and in New Zealand with oversowing after harvesting (as a weed control measure) would suggest that this fear is groundless.
12.2.4 Contaminants / residues
There is growing evidence of historical heavy metal contamination through use of
superphosphate high in cadmium (Roberts et al. 1994). The magnitude of this effect
and its impact on accumulation in edible products is likely to be much higher in
agricultural systems than in plantation systems because of the application rates used.
Clough & Hicks (1992) argue that the few measurements within New Zealand, and more
extensive overseas tests, suggest that contamination effects of farm production are minor
in comparison with environmental and off-site effects such as pollution of water bodies,
disruption of ecosystems and risks to human health.
There is very little known about the extent of pesticide contamination of our rivers
and lakes, although past surveys have established the presence of DDT and other
organochlorine pesticides, use of which has long been discontinued (MAF 1993). The
evidence indicates that pesticides have no long-term detrimental effect on soil
productivity and crop growth if applied at the recommended rate, but that pesticides
moving out of the soil can have adverse impacts on wildlife and on the production from
aquatic ecosystems.
There are valid concerns over primary and secondary processing contamination effects in
both sectors, e.g. handling of timber treatment chemicals, acid leachates from log storage
areas, use of chlorine-based bleaching in pulp and paper manufacture, treatment and
disposal of meat processing effluent.
12.2.5 Biological activity
Replacing native forests and grasslands with pasture, crops and plantations has
resulted in both contraction of ranges and exploitation of modified habitats by native
soil fauna species, and both general and restricted dispersal of introduced species
(Yeates 1991). Earthworm populations decline significantly under pine plantations
established on pasture (Yeates 1988, Percival et al. 1984) indicating changes in
soil biological activity but the influence of that, given the concomitant change in soil
environment, is unclear. Mycorrhizal associations in plantation forests benefit soil
health particularly on poorer soils (Dyck et al. 1985, Davis 1992).
12.3 Water Quality
Water quality must be defined relative to a proposed use, e.g. human consumption,
supporting aquatic fauna or flora, recreational use etc. O'Loughlin (1994) identifies five
key parameters: (a) chemical water quality (concentrations of anions, cations, heavy
metals etc.); (b) physical parameters (pH, temperature and electrical conductivity); (c)
biochemical oxygen demand; (d) concentrations of suspended solids (affecting clarity and
turbidity); and (e) concentrations of microbial organisms (bacteria, faecal coliforms
etc).
Water quality is closely related to soil movement (erosion, earthworks) and nutrient
movement. For example, streams draining agricultural and recently disturbed exotic
forested watersheds have significantly higher concentrations of nitrogen and phosphorus
than those draining indigenous forested watersheds or mature undisturbed plantations
(O'Loughlin 1994). Careful management practices in both agriculture and plantation
forestry can significantly reduce impacts. High water quality from undisturbed forested
catchments is attributed to efficient nutrient cycling and lower incidence of erosion
events. This difference is accentuated in stormflow, where agricultural land and disturbed
forest land contribute high levels of sediment and nutrients from surface flow while
forested catchments tend to deliver excess water through subsurface processes (Fahey &
Rowe 1992). The risk of increased nitrate and phosphate loadings in streams draining
pastoral catchments is extremely high given the reliance on legume-based agriculture and
increased surface flow to streams in contrast to forests, and is exacerbated where stream
margins are grazed or accessible to stock (Cooper & Thomsen, 1988).
Wilcock (1986) concluded that the yield of potential water pollutants from
land surfaces is very dependent on land use; the data are summarised below.
| Pollutant levels | Pines | Pasture |
| Suspended solids (median kg/ha/year) | 700 (300-2000) | 1300 (600-2000) |
| Phosphorus (total P kg/ha/year) | 0.1-0.8 (mean 0.5) | 0.3-1.7 (mean 1.4) |
| Nitrogen (total N kg/ha/year) | 1-7 (mean 1) | 4-14 (mean 8) |
Although the extent of the adverse effects of pastoral agriculture (e.g. sedimentation, nutrient loading, faecal contamination, destruction of riparian vegetation) is poorly quantified, Regional Council officials from around the country rank agriculture as the biggest source of adverse effects on water quality, followed by human sewage disposal (MAF 1992). Forestry was cited as significant but less serious than other sources of water quality problems.
Production forestry involves a number of activities - earthworks, site preparation, establishment, tending, fertiliser use and harvesting - which may influence water quality. Of these activities, roading, site establishment, fertiliser application and skid sites and tracks at harvesting affect water quality most (Priest & Rennes 1979). Plantation forestry can be expected to improve the chemical quality of water relative to pastoral land, although burning of harvested sites for land preparation and direct application of fertiliser to watercourses can rapidly increase conductivity, total N and total P over base levels.
Of major agricultural impacts on water quality, sedimentation and nutrient loading of surface water bodies is rated the most serious; changes to physical characteristics and faecal contamination of surface waters are ranked as slightly less serious impacts, followed closely by nitrate contamination of groundwater (MAF 1992).
The Hawke’s Bay Regional Council monitors water quality in the Wairoa District, and has classified rivers into two types and five subgroups (Hooper 1994). The Waiau and Mohaka are classified as relatively unmodified rivers with mountainland catchments. They have low conductivity and nutrient concentrations, and temperature and pH are within acceptable levels for most uses, with the exception of domestic and some industrial uses. The limitations for these uses are in both rivers turbidity from suspended sediment generated primarily in unmodified native forest. Turbidity levels also limit the use of water from the Mohaka for irrigation, and can smother in-stream aquatic life.
The Wairoa River originates in a mountainland catchment but is modified by agricultural use. Ionic concentrations are higher than in the previous group because of agricultural nutrient inputs; water from the river is nonetheless suitable for most uses, apart from domestic or industrial consumption because of turbidity. There is particular concern over the impacts of wastewater discharge in the lower river from effluent and sewerage treatment systems. A decrease in oxygen concentrations and an increase in suspended solids, bacterial concentrations, ammonia nitrogen and biochemical oxygen demand have resulted in a conclusion that there are potential health risks in participating in contact water sports in the lower Wairoa river and estuary.
Lowland catchment rivers that are monitored are the Nuhaka, Waikari and Waihua. These typically are of lower quality than those from upland catchments, with higher conductivity, alkalinity and hardness. The Nuhaka River falls into the lowland, general group and is characterised by medium conductivities and ionic concentrations, with limitations for irrigation (ionic concentrations) and domestic and industrial use (turbidity and hardness). The Waikari and Waihua have higher sodium, chloride and sulphate ion concentrations resulting from pastoral land-use, and are limited similarly to the Nuhaka. Hardness levels in these rivers have exceeded that acceptable for potable water over 50% of the time.
Temperatures in all rivers were below levels limiting to trout for more than 75% of the time. Biochemical oxygen demand, dissolved oxygen and pH were also recorded at acceptable levels, with little incidence of excessive plant or algal growth. Turbidity levels increased in winter and spring as a result of increased rainfall and overland flow.
Relative to other rivers nationally the Mohaka and Waiau belong to cluster 3 in a 9-rank scale (Smith & Maasdam 1994) and the Wairoa to cluster 4. The remaining rivers do not fit well in the classification because of high alkalinity, conductivity and calcium concentrations (cluster 8 or 9) but do not have some of the other negative characteristics.
In general terms, lakes in predominantly pasture-dominated catchments are often nutrient-enriched, many showing adverse signs of eutrophication (depleted oxygen levels, poor water clarity, scums, blooms of phytoplankton). Another factor influencing eutrophication is fertiliser application. Work by Howard-Williams et al. (1983) on the Tiniroto and Putere lakes confirm high nitrate and phosphate levels and frequent incidence of algal blooms associated with nutrient enrichment.
Management strategies which can reduce the impacts of land-use on water quality include restricting access and activity at watercourse margins, protecting tracks and earthworks from collapse and/or surface erosion, use of low-impact harvesting systems and careful management of fertiliser application.
12.4 Water yield
The volume of streamflow directly affects both water quality and the availability of water for use. A given amount of contamination, whether it be nutrients, pesticides, sediment or faecal coliforms, will have a greater impact on water quality when there is reduced flow.
Fahey & Rowe (1992) summarise land-use change impacts on the water balance of small catchments and the production of streamflow as follows.
| Land-use change | Observations |
| Forest/scrub pasture | Increased annual water yields; can cause streamflow to double, mostly through increased quickflow during small and medium storms |
| Native forest pines | Medium-to-high rainfall areas can cause water yield to increase by 75% in the first few years after clearfelling (as a result of small and medium-sized storms through higher quickflows); a return to pre-treatment levels is normally achieved after 5 years, and may be lower than that before disturbance; however, quickflows may remain higher on average, whereas delayed flows are smaller |
| Gorse/scrub pines | Similar to above; water yield may increase by up to 100% immediately after clearance, but quickly decline after planting, eventually by as much as 60% after canopy closure |
| Pasture pines | Annual yields may be reduced by 50% once the canopy closes; low flows may be reduced by as much as half, and peak discharges by 80%; changes may be beneficial as a means of flood protection, but may have serious consequences where downstream users are dependent on sustained flows for irrigation, hydroelectric power, municipal supplies |
| Pasture improvement | Improvement (oversowing, top dressing) of low-production pastures can reduce annual yields by up to 50%; peak discharges may fall 75% |
Priest & Rennes (1979) observe that when forest is harvested, water normally transpired is available for stream flow, hence large increases in the annual flow have been observed after clearcutting. Normal timber harvest may increase peak flows from small autumn and spring storms, but not the large midwinter events - this is because soils are saturated in both forested and non-forested areas by midwinter.
The area of least confidence is the impact of afforestation on low flows. Fahey & Rowe (1992) report declines of up to 50% from studies of small catchments in the South Island. Black (1993) analysed low-flow data comparing exotic forested catchments with a pastoral catchment near Patunamu forest, and showed that afforestation improves infiltration, storage and average low-flow levels if the rainfall is greater than the transpiration requirements. Black concluded that general low water yield in the District occurs regardless of vegetation cover, with a tendency for slightly higher flows in forested areas, and that a change from pastoral to forestry land-use would not cause a reduction of minimum flow. He acknowledged, however, that the effects on flow distribution and duration of low flow had not been addressed, and required further study.
Computer modelling of forest water balance may be able to calculate harvesting schedules to achieve desirable water yields. A spin-off of this would be better control of sediment yield and maintenance of water quality.
Pearce et al. (1977) found that the soil profile under forest stands on the East Coast is substantially drier for much of the year than it would have been under pasture cover: the winter period of high water content under forest stands is approximately 3-4 months, as against 6-8 months under pasture. They suggest that high rainfall interception rates by the forest canopy more than offset the lower annual transpiration rates from forest cover. Soil water contents under both vegetation types are normally recharged, and reach their highest levels during the winter period of high rainfall, when transpiration rates are low but interception losses are greatest in absolute terms. The generally lower water content after reforestation reduces both earth flow movement rates and the duration of high movement rates.
Agriculture directly affects water flows by extracting water for irrigation and stock watering (MAF 1992). Removal of riparian vegetation and wetlands increases flow variability, exacerbating both peak and low flows and therefore reducing the volume of water available to process or dilute contaminants during critical low-flow periods. Pasture management and intensity of grazing can have significant effect on pasture transpiration rates and pasture cover, resulting in significant variations in water yield potential.
Primary processing in both the agricultural and forestry processing sectors can use significant volumes of water and create effluent problems.
12.5 Biodiversity - vegetation and habitat degradation
Vegetation is important environmentally for a range of reasons. It reduces the erosive force of rain and running water, provides shade to waterways, food for in-stream and ex-stream fauna and habitat for larger animals, and reinforces soil and stream banks.
Modifications in indigenous vegetation to support productive use have important environmental impacts on (a) water yield and quality, and therefore on aquatic flora and fauna, (b) diversity of plant species, reducing ecological resilience and habitat for animals and (c) weed introduction and spread.
Removal of riparian vegetation, particularly in pastoral systems but also in plantation forestry, results in significant impacts. Riparian zones have a number of functions, but generally they act as filters for sediment and nutrient-laden water, and provide shade and food for the aquatic communities living in the streams. Riparian zones can also provide corridors for wildlife, providing access to water and routes for migration.
Pastoral farming commonly removes most riparian vegetation and some wetlands. This leads to substantial modifications in stream environments (temperature, nutrient levels, flow levels, sediment levels, water holding and filtration capacity of wetlands), particularly where livestock enter streams, which can disrupt aquatic ecosystems, increase flow variability and flooding and increase nutrient loading of waterways (MAF 1992). Such changes can increase susceptibility to invasion by aquatic weeds e.g. water net (MAF 1993).
Some logging practices also have negative impacts on riparian vegetation by destroying it or by concentrating slash in watercourses. Slash entering waterways results from two main causes: (a) trees with an outward lean cannot be felled away form the stream, and (b) hauler operations tend to comb slash off high points and deposit it in depressions (including stream courses).
Clearance of native vegetation for planting exotic species can lead to a loss of biodiversity and of habitats for indigenous plant and animal species. The New Zealand Forest Accord, signed in 1991 between conservation and major forest companies, made a commitment that native forest, including regeneration, would not be cleared for plantation planting. The Forests Act 1993 limits private owners’ ability to clear native forest, but scrub clearance continues in both farm and forest development.
Plantation forestry can harbour exotic invasive weeds or involve species that are invasive in themselves. Weeds associated with pine plantations include gorse, broom, buddleia, pampas, Yorkshire fog, browntop and herbaceous broadleaves. These species have been demonstrated to reduce plantation growth and survival and, with others that are not commercially important to forest growers, may represent a threat to neighbouring landowners.
Weed infestation in plantations has led to intensive vegetation management practices. Historical emphasis was on the use of fire and herbicide, particularly in the establishment phase. With costs of weed control increasing and increased pressure against herbicide use alternative techniques - e.g. oversowing - are now being introduced, with economic and environmental benefits.
Particular weed pests in the Wairoa District include gorse, blackberry and Australian sedge. Goats have long been used to control gorse and blackberry, and this is the main reason for wandering herds of goats being retained or tolerated. Localised problems also occur with thistles and old man’s beard.
12.6 Animal pests
Plantations and pasture can harbour species considered by other land-users to be pests, e.g. goats, possums. The crucial issue is the investment in pest control by land managers. Both sectors and regional authorities invest significant sums in pest control. Coordination by the Regional Council provides an opportunity for a fair and equitable pest control programme to be established that benefits all landowners.
Pest control methods continue to be an area of considerable debate, with the use of sodium monofluoroacetate (1080) of particular concern. Drench resistance is also an issue, particularly where uncontrolled goats can act as a vector for parasites, as is the risk of cross infection of tuberculosis from possums to cattle.
Bovine tuberculosis is not present in the District; the nearest areas of threat are at Taupo and Tutira. The occasional individual reactor occurs, but this is not believed to be owing to a feral pest vector because there has been no clustering of such incidents. It is considered more likely to be a result of importing stock. No herds are currently under movement control.
Feral goats are a significant pasture and forest pest but are tolerated by many farmers. Feral rabbits are also a problem in localised areas.
12.7 Climate change
It is generally agreed by climatologists that the global increase in mean temperature of 0.3-0.70 C during the last 100 years is consistent with the effects of an increase in greenhouse gases. On current trends, an increase in mean temperature of up to 3.0 ± 1.50C at New Zealand latitudes can be expected by the year 2050, with rather less certain changes in rainfall and changes in the frequency and severity of extreme climatic events (Salinger & Pittock 1991).
Trees and tree crops are vulnerable to climate change, since the time scale for the projected increase in temperature is short in relation to their life span. The ability of present genotypes to acclimatise to change is another consideration (Whitehead et al. 1993).
Plantation forests are being promoted as an effective means of reducing net greenhouse gas emissions. Scientists studying the Greenhouse Effect have calculated that a hectare of pasture converted to plantation forest will store at least 100 tonnes of elemental carbon (Maclaren et al. 1993). The benefits of plantation forestry are derived by establishing forests on land currently not in forest cover, i.e. developing a high-carbon-density use from a low carbon-density use. A plantation forest is essentially a storehouse of carbon. The benefit to New Zealand’s net carbon emission rate is only long-term if new areas continue to be planted and if the net wood products pool from those forests is increasing (Maclaren & Wakelin 1991).
One of the concerns raised by farmers is the potential for local changes in climate as a result of tree planting. While there is evidence to support the impact of extremely large contiguous areas of forest on climate patterns (e.g. the Amazon basin) there is no firm evidence to support changes in local weather patterns through regional afforestation or forest removal.
12.8 Widespread Loss as a result of introduced pests
There is widespread concern that monocultural forest plantations are more vulnerable to pest and disease attack than more diverse ecosystems, especially when under nutrient stress or where they are derived from limited genetic material. There is also debate as to whether the long-term health of the radiata pine resource in New Zealand has declined. Opposing arguments state that the genetic base within the species has been intentionally maintained at a broad level, and that this confers diversity and resilience. Threats from new pests and diseases are real, and are the focus of an expanding health surveillance programme on forest and at ports of entry.
At an official level historical costs of pest introductions and remedial control, the current costs of protection and the risk of new introductions are weighed against the costs of widespread establishment of alternative or mixed species stands (Handiside 1994). At present there appears to be no economic incentive to move away from reliance on predominantly a single species. Arguably the risks inherent in this approach are built into investors’ decision making, and they continue to invest in forestry.
12.9 Aesthetics/landscape
Large areas of even-aged plantations of single species create a homogeneous landscape substantially different from that of pastoral farming. This is often criticised as monotonous and visually unappealing. Pastoral farming similarly presents a relatively homogeneous visual landscape with limited diversity, but it is extremely familiar to most New Zealanders. It is also promoted as a typical New Zealand landscape in much of our international promotion. The relative merits of either landscape are a matter of individual preference, which is strongly determined by experience and expectation.
General principles of landscape preference, however, can be identified. A landscape that displays a sense of naturalism, coherence, diversity and mystery has been suggested as preferable (Thompson 1984). Work by Kilvert & Hartsough (1993) demonstrated a general response to plantation forests as unappealing, ordered and disruptive to the landscape. The results showed a preference for a high proportion of vegetation in a landscape and dislike of cleared areas, e.g. roads, harvested sites.
The planting of small special-purpose woodlots that relate to both the ecological and visual attributes of a diverse landscape is promoted by some landscape architects as enhancing the beauty and character of rural landscapes. Transition zones from one land-use to another are visually important. The natural transition from forest to grassland is usually complex, with a variety of species taking advantage of niche conditions. Modifying forest boundary management has been used in the past by various forest growers to ease the visual impact of production forests at the margin.
12.10 Health
Health issues in agriculture generally relate to the impact of product quality rather than the impact of the production system (Clough & Hicks 1992). There is potential, however, for system factors, e.g.faecal contamination of waterways or aquifers, to generate health risks. In the case of waterway contamination, water treatment costs are incresed and/or constraints are necessary on the use of rivers, lakes and seashore for contact recreation, domestic or industrial water consumption, and food production.
The most serious health issues in forestry relate to worker safety during harvesting and processing of wood products. Worker safety is also an issue in agriculture. Use of toxic chemicals (herbicides, pesticides) and dangerous or heavy equipment (chainsaws, logging machinery, tractors) are recognised as health and safety issues and are covered by legislative regulations.
Some contributors to this report have raised pollen from timber plantations as a potential respiratory health issue. No data on the potential for pollen from plantation softwoods as a health risk has been found.
12.11 Fire
Large-scale burning used to be carried out routinely after harvesting forests to reduce the amount of slash and facilitate planting. It is now less widely used, because research has indicated that long-term productivity and sustainability of forests is aided by retaining slash for nutrient recycling. Burning had negative impacts on water quality, nutrient retention and sediment yields by promoting quicker runoff until vegetation re-established (Dyck et al. 1981).
Plantation forestry is at greater risk of commercial loss by fire than pastoral land-uses, so regulatory control of some activities in and adjoining forests is common. This risk is transferred as a liability to neighbouring land-users without compensation, and is of widespread concern to farmers. Recreational use of forests is a potential benefit to a District, but can create a major fire hazard, especially where trees are close to residential developments.
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Contact for Enquiries
Rural Affairs Coordinator
Sector Performance Policy
MAF Policy
Ministry of Agriculture and Forestry
PO Box 2526
Wellington
NEW ZEALAND
Phone: +64 4 894 0675
Fax: +64 4 4 894 0745
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