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In this paper we outline how geomorphological understanding has underpinned forest management in New Zealand's erodible steeplands, where it contributes to current forest management, and suggest where it will be of value in the future. We focus on the highly erodible soft-rock hill country of the East Coast region of North Island, but cover other parts of New Zealand where appropriate. We conclude that forestry will continue to make a significant contribution to New Zealand's economy, but several issues need to be addressed. The most pressing concerns are the incidence of post-harvest, storm-initiated landslides and debris flows arising from steepland forests following timber harvesting. There are three areas where geomorphological information and understanding are required to support the forest industry — development of an improved national erosion susceptibility classification to support a new national standard for plantation forestry; terrain analysis to support improved hazard and risk assessment at detailed operational scales; and understanding of post-harvest shallow landslide-debris flows, including their prediction and management.
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My evidence will cover erosion susceptibility and sediment yield in Otago (including sediment yield from plantation forests in relation to other land uses), the current state of knowledge of forestry influences on erosion (derived from studies largely elsewhere in New Zealand – including erosion processes and sediment yields), how erosion is mitigated by forest management practices, and concludes with comments on changes suggested in the Officers report based on submissions to the original proposal.
Soil loss, vegetation recovery, and sediment delivery to streams following plantation harvesting, Coromandel
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The objectives of this research were to assess vegetation recovery and quantify sediment generation rates from hauler logging at Whangapoua Forest, and to assess the relative significance of sediment delivery from the various sources to the stream channels. Finally, the sediment delivery ratio, i.e. the ratio between what is generated and what is exported as sediment yield, will be calculated. This paper reports results on sediment generation and vegetation recovery and catchment sediment yield.
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This report presents the work undertaken on the Waiapu River Catchment Study, the purpose of which was to investigate the geophysical, social, cultural, and economic dimensions of the erosion problem in the Waiapu River catchment in order to inform future policy decisions with respect to the catchment in the context of the Deed of Settlement between Ngāti Porou and the Crown. The report includes:
1) An outline of existing knowledge related to the (i) geophysical and land use aspects and (ii) social, cultural, and economic aspects of erosion and land use change within the Waiapu River catchment and the wider East Coast region.
2) Identification of gaps in the existing knowledge apparent to the research team, and recommendations for addressing any gaps identified.
3) A description of a baseline from which future progress in the Waiapu catchment may be measured (benchmarking) for the (i) geophysical and land use aspects and (ii) social, cultural, and economic aspects of erosion against the values of Ngāti Porou for the catchment and well-being of the people.
4) An outline of the project team‘s interpretation of a desired state for Ngāti Porou and consideration of possible options for addressing the erosion problem.
5) Assessments of the scope (size and scale) of the erosion problem in the catchment and critical evaluations of the effectiveness of erosion mitigation measures for the (i) geophysical aspects and (ii) social, cultural, and economic aspects of erosion.
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This is a summary of recent literature on the positive and negative environmental impacts of planted forests in New Zealand. Key results are as follows: Biodiversity - plantations provide a habitat suitable for a wide range of indigenous forest species, both aquatic and terrestrial. Water yield - afforestation can reduce peak catchment flood flows by up to 50%. Water quality and leaching - planted forests have the lowest potential for nitrate and phosphorous leaching with levels similar to indigenous forests. Sediment yield - afforestation of whole catchments can reduce sediment load to waterways by 50-90%. Soil erosion - recent research provides further support that plantations mitigate soil erosion. Impacts on soil nutrients the high level of soil organic N present in established pastures decreases markedly when the pasture is planted in pine forest. This information is relevant to forest managers needing to manage the environmental effects of their trees, and provides the baseline for further research. For the Forest and Environment FRST programme the information will contribute to planning research on indicators of sustainability.
Quantification of the Flood and Erosion Reduction Benefits, and Costs, of Climate Change Mitigation Measures in New Zealand
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The research brief involves three tasks:
1. (a) to define the circumstances under which tree establishment, as a climate change mitigation measure in New Zealand, will have significant flood and landslip reduction benefits (or costs), and then (b) to survey and summarise work that provides a credible quantitative estimate of the physical landslip or flood reduction benefit from tree establishment
2. to provide national estimates, where available, of physical co-benefits of climate change mitigation measures or advice on alternative methodology where such information is not available
3. to provide national estimates, where available, of monetary co-benefits of climate change mitigation measures; or advice on alternative methodology where such information is not available. This report first discusses the available information sources. It then presents the detailed information about estimates of benefits and costs, in three chapters.
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Evidence will cover the erosion susceptibility of the Coromandel area and the relative importance of mechanisms of sediment generation and delivery to streams. I will also comment on the history relating to conditions of “catchment constraints” as they pertain to erosion and comment on the various appealed consent conditions as they pertain to the issue of erosion, sediment mitigation and sediment yield.
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Characteristics of the suspended sediment load in the Waipaoa River, New Zealand, and in two of its tributaries (Mangatu and Te Arai Rivers) are examined for evidence of different dominant erosion processes in the basins upstream. Different shapes of the suspended sediment concentration-water discharge relations lead to differences in
long-term average yield, event-yield magnitude-frequency relations, and relative importance of large flows and rare events. In the Mangatu River, frequent runoff events are relatively more important to the long-term yield (11,540 t km-2 yr-1 ), half of the long term average load is transported by events with return periods less than 1 year, and there is little evidence of an erosion threshold limitation on sediment supply. This is consistent with the predominance in the Mangatu basin of hillslope erosion processes that involve scour by surface and channelized runoff, particularly gully erosion. This contrasts with the Te Arai River where sediment concentration tends to be much lower at low and moderate flows, frequent runoff events transport less of the long-term yield (4600t km- 2yr-1) than do rarer, large-magnitude flood events, event sediment yields are an order of magnitude lower during events with sub annual return periods, and half of the long-term average load is transported during events with a return period of >2 years. Some of these characteristics appear to result in part from two populations of runoff events in the Te Arai basin; nonetheless, they are consistent with field evidence that most of the sediment Supplied to the Te Arai stream network is generated by shallow landslides which are activated once a rainfall threshold is exceeded.
Slopewash erosion following plantation harvesting in pumice terrain and its contribution to stream sedimentation, Pokairoa catchment, North Island, New Zealand
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Post-harvest sediment generation by slopewash erosion was measured from sites of soil disturbance on hauler-logged areas in pumice terrain; Pokairoa catchment, Central North Island (38.19°S, 166.45°E), New Zealand. Slopewash and a storm-initiated landslide were the principal sediment-generating processes. Slopewash and vegetation recovery rates were measured at field plots on sites with shallow and deep disturbance, and a regression relationship was established between sediment loss (g day-1 mm rain-1 m-2) and percent vegetation cover for both plot types. At the plot scale, in the first post-harvest year, deep-disturbance sites generated 85% of total slopewash produced from both disturbance sites combined. At 3.8 t ha-1, this was ~5 times more than that generated from shallow-disturbance sites. In year 2, slopewash declined, with deep-disturbance plots generating 1.2 t ha-1, twice that from shallow-disturbance plots. By 21 months, when groundcover occupied 80% of plot area, sediment generation had declined to almost zero. At the scale of forest management areas (compartments), 75% of the total slopewash generated from both disturbance classes combined, during the 2-year post-harvest study period, was generated in the first post-harvest year. Of this, 63% was generated within the first 7 months following harvesting and before the application of desiccant. In the second post-harvest year further sediment loss from deep-disturbance sites was limited because most readily available sediment has already been removed, whereas the minimal decrease on uncompacted shallow-disturbance sites was more a function of infiltration rates exceeding rainfall than a consequence of site recolonisation. Rates of surface lowering on sites of shallow- and deep-disturbance on pumice terrain in Pokairoa differ by an order of magnitude. The relative contribution of sediment delivered to the Poumako Stream by slopewash and landslides was highly dependent on their connectivity with stream channels. During the period of this study, a single storm-initiated landslide was the most important hillslope process, contributing the equivalent of ~6000 times more sediment to Poumako Stream than was delivered by slopewash from 38 ha (excluding roads and landings) of clearfelled forest.
Economic costs of hill country erosion and benefits of mitigation in New Zealand: Review and recommendation of approach
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The available literature relating to the economic costs of hill country soil erosion and the benefits of its control in New Zealand and overseas was reviewed and described. Based on the findings of the review, an economic approach to the assessment of the economic costs of erosion is recommended and an analytical framework for the prioritisation of erosion control tree planting was developed and described. The key findings of the study were:
1. The impacts of hill country soil erosion and sedimentation have both on- and off-site implications in terms of economic costs such as productivity declines and increased incidence of downstream flooding damage.
2. Some of the component costs associated with hill country erosion are inherently difficult to disaggregate, quantify, and assess.
3. Tree planting can be an effective means of erosion control, and in some areas, radiata pine woodlot planting may be a relatively profitable alternative to pastoral grazing.
4. A cost-benefit analysis approach, with support from non-market valuation techniques, to the assessment of erosion costs is generally recommended.
5. In order to identify the locations where erosion control would be most effective, it is recommended that economic value at risk be defined in a spatial sense and that this
information be used in association with information on the physical susceptibility to erosion and sedimentation (an analytic framework for this was developed).
Declining soil loss with increasing age of plantation forests in the Uawa Catchment, East Coast Region, North Island, New Zealand
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After Cyclone Bola (1988), landslide damage to Pinus radiata plantations and farmland on Class VI-VIII land in Uawa catchment, East Coast Region, New Zealand, was assessed. Landslide area in plantations was measured photogrammetrically for each age class between 1 and 8 years old and for stands >8 years old. Soil-loss calculations were based on a mean landslide depth of 0.96 m measured from 547 landslides. Soil losses from forested and farmed land within Uawa catchment were 33% and 37% greater, respectively, than yields from similar hill-country terrain elsewhere in the East Coast Region and were in the order of 11.6 and 46.6 million m^3, respectively. Afforestation was effective in reducing soil loss. Within forested areas the highest soil loss (2019 m^3/ha) was was derived from stands less than 1 year old. This loss halved to 1002 m^3/ha from stands 2-8 years old and declined further to 217 m^3/ha from stands 8 years of age and older. This trend represents a 10-fold reduction in soil loss within 8 years of forest establishment. Soil loss on a per hectare basis from farmland (1631 m^3/ha) was 43% greater than that from adjacent areas of mixed-aged (1-8 year old) plantation forest (935 m^3/ha).
Sediment sources and delivery following plantation harvesting in a weathered volcanic terrain, Coromandel Peninsula, North Island, New Zealand
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Sediment generation and vegetation recovery was measured over a 2-year post-harvest period in a 36-ha catchment of exotic forest located in andesitic terrain, Whangapoua Forest (36.46◦S, 175.36◦E), Coromandel, New Zealand. Slopewash, soil scraping (on-slope removal of the regolith by the repeated dragging of logs), and storm-initiated landsliding were identified as the principal sediment-generating processes. Slopewash and vegetation recovery rates were measured using field-based plots located on sites of shallow- and deep-disturbance and a regression relationship was established between sedimentation rate (accumulation (g)/day.mm rain.m2) and per cent vegetation cover for both plot types. At the basin scale, slopewash was calculated using the plot-based rates times the total area of deep- and shallow-disturbance sites as identified from a ground-based, transect survey and using sequential aerial photography. Sediment production, by soil scraping and landsliding, was determined by multiplying mean scar depth by the total affected area. In the first post-harvest year deep-disturbance sites generated 92% of total slopewash produced from both disturbance classes combined, and in year 2, slopewash halved. Half of the first post-harvest year’s slopewash-derived sediment was generated within the first 7 months following the completion of harvesting and before the application of desiccant. Thereafter, on deep-disturbance sites, slopewash rates declined further as sites became hardened against the generation of new sediment (i.e. sites became sediment limited). In contrast, during both the initial post-harvest recovery period and the post-desiccation period, the decline in sediment production on shallow-disturbance sites was more a consequence of site recolonisation. Sediment generated and redistributed by scalping and by landsliding occurred at the time of the respective events and coincided with the early part of the first post-harvest year. Collectively, soil scraping, slopewash, and landslides generated 1864 t (52 t/ha) of sediment, 88% of which remained on-slope. Of the sediment delivered to streams(228 t), landslides contributed 72%, soil scraping 26%, and slopewash 2%. For this harvested basin a single, storm initiated, landsliding event was the most important hillslope process responsible for the generation of sediment and its delivery to streams, and slopewash was the least important.
Pre- and post- reforestation gully developoment in Mangatu forest, East Coast, North Island, New Zealand
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Following clearance of the indigenous forest and conversion of the land to pasture early in the 20th century, gully erosion became a pervasive feature in the headwaters of the Waipaoa River basin, and was notably problematic in the 140-km2 area now covered by the Mangatu Forest. In this area, before reforestation in 1961, gully erosion affected c. 4% of the terrain. After a 24-year exotic reforestation programme the area affected by gullies was reduced to 1.5%, but of the eight gullies larger than 10 ha in 1960 none had stabilized by 1988, although four had at least halved in size. Estimates that a gully will stabilize or increase in size under a range of conditions suggest that in the case of gullies <1 ha in area, formed in terrain underlain by Cretaceous rock, there is a >80% probability of stabilization after one forest rotation (c. 24 years). For gullies between 1 and 5 ha in area the probability of stabilization is c. 60%. Gullies of 5 ha have an even chance of stabilizing over the time frame of a single rotation. The key determinant is gully size and shape at the time of planting and, within this size range, these relationships were stronger for linear than for amphitheatre-shaped gullies. Between 1939 and 1988 sediment production from gullies in the portion (76%) of the Mangatu Forest underlain by Cretaceous-aged rock was c. 22 000 t km ^2/yr, and during the period of maximum sediment production (1939–1960) they may have accounted for c. 17% of the Waipaoa River’s average annual suspended sediment load. Reforestation reduced the contribution to c. 8% in the period between 1970 and 1988. However, the off-site (downstream) impact of sediment generated by the remaining 420 active gullies in the Waipaoa River catchment is significant, not least on the capacity of the scheme that protects high-value agricultural land on the Poverty Bay Flats from flooding. A targeted reforestation programme may be an alternative to raising the height of the existing artificial levees. It is estimated that additional exotic plantings totalling c. 15 400 ha (c. 7% of the Waipaoa River basin area)would produce a >64% reduction in sediment production from gullies on pastoral hillslopes within one forest rotation (c. 24 yr).
Protective value of vegetation on tertiary terrain before and after Cyclone Bola, East Coast, North Island, New Zealand
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The effects of six vegetation types on landslide densities on Tertiary bedrock terrain were examined before and after Cyclone Bola struck the East Coast of the North Island of New Zealand in March 1988. Indigenous forest and exotic pine plantations more than 8 years old provided the best protection against the formation of shallow landslides, both before and during Cyclone Bola. Regenerating scrub and exotic pines6-8 years old provided an intermediate level of protection. Greatest damage occurred on pasture and in areas of young (<6 years old) exotic plantations where canopy cover was negligible and root development limited. Sites under older vegetation types with a closed canopy (indigenous forest and plantations of exotic pine >8 years old) were four times less susceptible to landsliding during Cyclone Bola than those under regenerating scrub and exotic pines 6-8 years old, and 16 times less susceptible than those under pasture and young exotic pines (<6 years old).
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Roots with under-bark diameters of between 1 and 4 mm from 11 New Zealand
indigenous riparian plant species — lacebark (Hoheria sexstylosa Col,), kowhai
(Sophora tetraptera J.S.Mill.), manuka (Leptospermum scoparium J.R. et G.Forst.),
fivefinger (Pseudopanax arboreus (Murr.) Philipson), kohuhu (Pittosporum
tenuifolium Sol. ex Gaertn.), rewarewa (Knightia excelsa R.Br.), cabbage tree
(Cordyline australis (Forst.f.) Endl.), ribbonwood (Plagianthus regius (Poit.) Hochr.),
lemonwood (Pittosporum eugenioides A.Cunn), tutu (Coriaria arborea Lindsay),
and karamu (Coprosma robusta Raoul) — were tested to determine their live rootwood
tensile strength using a Floor Model 1195 Instron Universal Testing Machine.
These results were coupled with those from earlier tests on the native tree species
southern rata (Metrosideros umbellata Cav.), red beech (Nothofagus fusca (Hook.f.)
Oerst.), hard beech (N. truncata (Colenso) Cockayne), mountain beech (N. solandri
var. cliffortioides (Hook.f.) Poole), manuka, kanuka (Kunzea ericoides var. ericoides
(A.Rich.) Joy Thomps.), and kamahi (Weinmannia racemosa L.f.), and the exotic
plantation species Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) and radiata
pine (Pinus radiata D.Don) to allow a wider-ranging comparison of live root-wood
tensile strengths of those trees and shrubs that can commonly be found growing in
potentially unstable slope and/or riparian environments throughout New Zealand.
The mean live root-wood tensile strengths of these plant species ranged from 8 to
History and distribution of steepland gullies in response to land use change, East Coast Region, North Island, New Zealand
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Following the clearance of indigenous forest (~1880s to 1920s) for pastoral use, actively eroding gullies had by the late 1950s become a pervasive form of erosion occupying 0.7% of the 7468 km2 of pastoral hill country within the East Coast Region, North Island, New Zealand. Commencing in the early 1960s the primary strategy used to stabilise gully and other associated forms of erosion on degraded pastoral hill country was to establish exotic forest. We compare the status of gully erosion before reforestation commenced with that at the end of a ~40 year reforestation period (1957–1997) during which ~1350 km2 of exotic forest was planted. Trends in gully area and distribution by land cover (vegetation), but particularly in response to exotic reforestation, are examined for two contrasting geologic terrains. Over this ~40 year period, the incidence and extent of gully erosion in areas of hill country that have remained in pastoral use has declined. This was primarily due to the retirement and conversion of extensive areas of gully-prone farmland to exotic forest. Where implemented, the reforestation of this degraded pastoral hill country has proven to be efficient and successful in stabilising existing gullies. However, with the initiation of new gullies and growth of gullies that have remained untreated, the extent of land affected by gully erosion has increased by ~27% to ~0.9% of the regions' hill country area. To reverse this trend, further areas of severely eroding pastoral land will need to be retired with the aim of escalating the rate at which remaining gullies are treated and to prevent new ones developing, primarily through reforestation with exotic tree species or indigenous reversion.
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The onset of gully erosion following deforestation was mapped for the three largest and heaviest sediment-laden rivers within the East Coast Region, North Island, New Zealand. Gullies were then re-mapped after a ~40-year reforestation period (~1957–1997) and sediment production from gullies was calculated from these data bases using a degradation rate based on DEMs of gullies at differing stages of development in each of two different geologic/tectonic terrains. At the end of the measurement period the total composite gully area for the Waipaoa, Waiapu and Uawa catchments was 5%, 33% and 39% greater than before reforestation, and for the study period, gullies in both terrains collectively contributed the equivalent of 43%, 49% and 54% of the average annual suspended sediment yield from just 0.8%, 2.4% and 1.7% of hill country areas in these respective river systems. A potentially significant reduction in sediment production and yield at catchment-scale could be achieved through a more targeted approach to reforestation, particularly of gullies in the most highly erodible and unstable pastoral hill country areas of Waiapu catchment.
Gully degradation, stabilisation and effectiveness of reforestation in reducing gully-derived sediment, East Coast region, North Island, New Zealand
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Gully stabilisation was modelled by measuring the change in ‘active’ gully area before planting with exotic pines and at the end of a ~ 40-year reforestation period. A degradation model based on DEMs of gullies at differing stages of development was used to calculate sediment production from reforested gullies in both Cretaceous and Tertiary geological terrains. The total volume of gully-derived sediment was calculated at catchment-scale by combining the gully degradation and stabilisation models with GIS-based mapped distributions of gullies in 1957 and 1997, and then expressed as an equivalent percentage of the average annual suspended sediment yield for each of the three largest and heaviest sediment-laden rivers within the East Coast region, North Island. The modelling of gully-derived sediment yields before and after a 40-year reforestation period (1957–1997) provided a measure of the effectiveness of past reforestation efforts in reducing gully-derived sediment yield. These models were then used to forecast potential reductions in gully-derived sediment for future reforestation scenarios. The probability of gullies stabilising (a measured reduction in ‘active’ gully area following planting with pines) was strongly associated with gully size and the number of years since planting. The probability of gullies of equivalent size stabilising in response to planting is similar in both the Cretaceous and Tertiary geological terrain. Past reforestation has reduced sediment yield by 33%, 17% and 20% in the Waipaoa, Waiapu and Uawa catchments, nonetheless during the measurement period forested gullies in both terrains collectively contributed 55%, 23% and 54% of the total gully-derived sediment yield in these respective catchments. At catchment scale, gully-derived sediment yield in each of the three major catchments could be halved by 2030 and remain constant thereafter if all remaining untreated gullies were reforested before 2020 and no new gullies were initiated during this period. The greatest number of untreated gullies occurs in the Waiapu catchment, where it is expected that sediment yield would decline by 11Mt/year by the end of the modelling period.
Vegetation recovery and indicative sediment generation rates by sheetwash erosion from hauler logged settings at Mangatu Forest
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Soil disturbance, vegetation recovery and sediment generation by sheetwash erosion on hauler-logged settings were evaluated at Mangatu Forest. Three disturbance indices were recognised: undisturbed, shallow disturbance and deep disturbance. The latter is here referred to as areas of exposed mineral soil (EMS). Groundcover vegetation recovery was slowest on sites of EMS. Based on plot-sized areas (9 m2), EMS per cent cover averaged 31 %, two years after cleafelling. Setting-based measurements showed that overall groundcover recovery was rapid, and resulted in an 83% reduction in areas of EMS within two years of clearfelling. Mean rate of sediment generation from areas of EMS averaged 11 (SE 3.5) kg/m2 for the first year and 4 (SE 2.4) kg/m2 during the second year after logging. Annual rates of sediment generation from areas of EMS on logged setting were 8 (SE 2.5) tonnes/ha in year one and one (SE 0.5) tonne/ha in year two. Most sediment mobilised from areas of EMS was relocated on slopes immediately downslope of source areas, where it was effectively entrapped by surviving groundcover vegetation and slash and in microtopographic hollows. Only those riparian areas of EMS with a direct connection to a watercourse contributed much sediment to stream channels, from which 2 (SE 0.6) tonnes of sediment per hectare of logged setting was generated in the first year after logging, declining to 0.2 (SE 0.1) tonnes/ha within two years. Sediment generated by sheetwash erosion from logged settings one year after clearfelling was at least several orders of magnitude less than that generated from non-forestry-related sources, especially gullies; it was not there-fore particularly significant, given the high background rates of sediment production within Mangatu Forest. Similar amounts of forest-related sediment entering streams where background rates of sediment production are low would, however, be regarded as unacceptable. It would be advantageous for the forest industry to minimise soil disturbance and consequent sediment production from hauler-logged settings, particularly within 10 m of stream channels.
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The impacts on slope stability of Cyclone Bole in 1988 and the February storms in Wanganui-Manawatu in February 2004 provided strong evidence that treeless steepland is very vulnerable to mass wasting during large cyclonic storms whereas forested slopes are relatively resistant to failure. The evidence form numerous studies indicates it is the tree root systems which provide most of the additional resistance. This paper briefly reviews the influence of trees on shallow landslides with most of the emphasis on radiata pine. nThe relatively rapid development of radiata pine root systems enables this species to provide good soil reinforcement after about 10 years of growth but within a few years of harvesting, most of the root reinforcement is lost. The various approaches that can be adopted to manage forests for soil protection, and identify where the most landslide-vulnerable slopes or parts of slopes are located, are outlined. Modern computer models, in combination with GIS, provide very useful predictions of instability.
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Experienced scientists in Landcare Research’s Erosion and Sediment Processes team – Chris Phillips, Michael Marden and Les Basher – expose the gaps in our knowledge regarding landscape responses to forest harvesting in New Zealand.
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Suspended sediment yields have been monitored since 1995 in two catchments in the erodible hill country of coastal Hawke's Bay, one in pasture (the Tamingimingi, 7.95 km^2), and the other initially in mature Pinus Radiata plantation forest (the Pakuratahi, 3.45 km^2). The latter was harvested by skyline hauler (85%) and skidder (15%) between December 1997 and October 1999. Post-harvest preparation in early 2000 included over sowing with grass and legumes, and ripping of hauler pads before replanting. Suspended sediment yields were calculated for 50 events at the Pakuratahi and 30 at the Tamingimingi. The relationship between storm sediment yields and corresponding peak flows was used to estimate yields for all unsampled storms exceeding 20 L/s/km^2 in both catchments. Data for the sampled storms and those calculated for the unsampled ones were then summed to estimate annual suspended sediment yields and totals for given periods during the forest rotation. During the pre-harvesting period (January 1995 to June 1997) total suspended sediment yield for the pasture catchmnent was three time higher than for the catchment in mature pines. In the logging phase of the harvesting period (January 1998 to December 1999) the situation was reversed, with the total yield for the harvested catchment was twice that of the one in pasture. Despite the removal of the vegetation cover during harvesting, slope disturbance was minimal, with the increase in yields thought to have come predominantly from road sidecast, landslides, and channel bed scouring. Yields from the harvested catchment declined markedly after over sowing and replanting, and in 2001 were substantially less than those from the pasture catchment. This reduction is attributed as much to a decline in forest-related activities as it is to a steadily increasing grass cover across all disturbed sites. Over the seven years of record (1995-2001) the catchment originally planted in pines and subsequently harvested has yielded only 20% more suspended sediment than the one in pasture. This suggests that, with average weather conditions during and immediately after harvesting, sediment yields from catchments in plantation forestry over a full forest rotation in this type of terrain should be less than from catchments left in pasture.
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Production, botanical composition, and some soil fertility parameters of pastures on seasonally dry Wairarapa hill country subject to soil slip erosion were measured over 3 years, beginning in 1979. Measurement sites were on sunny and shady aspects of 'uneroded' areas of hillslopes, and slip scars of 4 ages (mean dates of slipping: 1977, 1961, 1941,and 1906). Average annual pasture production on uneroded areas was 8391 kg DM/ha. Pasture yields on the 1961, 1941, and 1906 sites, and the 1977 sites were about 77 and 20% of this level respectively. Uneroded sites were generally less steep than eroded sites, and this complicated interpretation of data. Correction for this slope discrepancy would increase the 77 and 201170 values slightly. The results of this study suggested that pasture production on hill soils formed under pastoral agriculture was unlikely to return to the production levels supported by soils formed under the original forest vegetation. Botanical composition of pastures was influenced by the time elapsed since erosion. Grass dominance increased, and legume dominance decreased, with increasing length of time since slipping. Uneroded sites were the most grass dominant. N and C contents of soil increased with time since slipping, but were much lower for soils on all slip scars than soils on uneroded sites.
Future proofing erosion prone hill country against soil degradation and loss during large storm events: have past lessons been heeded?
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The two damaging storms that struck the Manawatu and Bay of Plenty regions in 2004 serve to remind us of the occurrence of similarly damaging events in 1988 (East Coast) and 1977 (Wairarapa), both of which had severe and long term economic and social consequences. The most significant physical impacts common to storm events of this magnitude (approximate 100 year return interval) are the on-site damage caused by erosion, in particular soil loss from pastoral hill country, and the resultant off-site impact of sediment on the downstream infrastructure. Research has shown that a closed-canopy forest cover is effective in reducing the on-site risk of erosion during these large-magnitude events. This article presents examples where reforestation of previously eroded pastoral land has successfully mollified those factors that contribute to risk of landsliding, gully erosion, and earthflow movement and poses the question: why have we not future-proofed more of our hill country against inevitable soil degradation and soil loss in future storms?
Sediment yield following plantation forest harvesting, Coromandel Peninsula, North Island, New Zealand
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The suspended sediment yield following logging of a 36-ha plantation forest catchment on the Coromandel Peninsula was determined over a 30-month period (October 2000 to March 2003), as part of a wider investigation of sediment generation and delivery to streams. Streamflow was measured with a capacitance probe water-level recorder, and in situ turbidity and suspended sediment concentrations were monitored with a Greenspan turbidity probe and automated pump water sampler, respectively. Rainfall was measured at three locations in the catchment, using 0.1-mm resolution tippingbucket raingauges. A regression relationship was determined from a relationship between the concentrations of water samples collected in the stage-triggered autosampler and the simultaneous turbidity probe trace (r2 = 0.85). This relationship was used to calculate the suspended sediment yield for all fl ow events
using a combined sedigraph and hydrograph approach. Total suspended sediment yield for the catchment for the whole period was 73.2 t. On an annual basis this ranged
between 59 and 116 t km-2. Two storms with annual to 2-year return periods in April 2001 and February 2002 contributed 37% of the total sediment yield. Most landslides occurred during the April 2001 event. Suspended sediment yields determined from this study are in the range of those of previous forest harvesting studies in New Zealand.
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Chapter 5 is a brief description of the potential physical, economic and social impacts of the project. This includes effects on erosion, soil quality, water quality and quantity, biodiversity, climate change and greenhouse gases, and factors involved in agriculture-forestry changes.
Review of Sediment Generation Processes in Exotic Forestry and the Applicability of Using Compound-Specific Isotopes to Understand Sediment Movement
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The objectives of this report are to:
• Review (in the relevant New Zealand literature, including grey literature) the processes of sediment generation within an exotic forest rotation and other land uses and sediment contribution to estuaries and harbours in the North Auckland and Coromandel regions.
• Provide an overview of factors such as soil type, topography, and climate and how these influence sediment generation with specific attention on the Coromandel (Whangapoua,Wharekawa) and North Auckland (Mahurangi) landscapes.
• Provide a linkage to illustrate how sediment generated enters the harbour.
• Review three reports by Dr Max Gibbs (NIWA) on the use of compound-specific isotopes (CSI) studies for sediment tracing. Establish the relevance and applicability of
the data to current forest operations.
• Provide responses to a set of specific questions raised following preparation of a draft of this report.
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Increasingly, in the East Coast region of the North Island of New Zealand stands of indigenous regenerating Kanuka are being felled and replanted with Pinus radiata. Conversion has occurred predominantly on erosion-prone hillslopes where storm-generated landsliding has been widespread and severe, but data on the relative effectiveness of these two forest species in enhancing slope stabilities are rare. For Kanuka and Pinus radiata, shear stress-displacement curves for their corresponding potential shear planes were measured at two site by in situ direct shear tests on soil with and without roots. The contribution of roots to soil strength was first estimated by calculating the difference between the maximum shear stress of the shear stress-displacement curves obtained for soil with and without roots. Results suggested that for individual trees the contribution from the roots to soil strength on a root cross-sectional area per unit shear area basis was independent of species for the two tree species tested. There were, however, significant differences in stand density between shear stress and shear displacement at the peak of the shear stress-displacement curve, and between the cross-sectional area of roots per unit shear area. Taking the shear strength of the combined soil-root system as the peak value of the shear stress-displacement curve produced from the in situ direct shear tests, a limited equilibrium slope stability analysis was used to derive the safety factors. A simple model developed using the relationship between the shear strength of the soil-root system, the specific root cross-section area, and slope angle was then used to determine safety factors for typical stand densities of naturally regenerating Kanuka for comparison with different Pinus radiate management regimes at equivalent stages of growth.
The model predicted that safety factors for stands of Pinus radiata in the forest 8 years after establishment would be lower than for the equivalent-aged stands of fully-stocked regenerating Kanuka under similar conditions. However, after 16 years the safety factor for a stand of Kanuka would be lower than that for Pinus radiata at final stocking densities typical of framing and biomass regimes.
In areas where vegetation plays a major role in soil conservation and erosion control, the model can be used to compare the stability of forested slopes with different species and stand densities. However, the model does not take into account the effect of buttressing by mature tree roots.
Effectiveness of reforestation in erosion mitigation and implications for future sediment yields, East Coast catchments, New Zealand: A review
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This paper argues that reforestation of unstable and degraded parts of the East Coast region, predominantly with exotic pines, has successfully stabilised existing erosion forms and prevented the initiation of new ones. Effectiveness is often assessed by capturing erosion scars on aerial photography taken before and after reforestation and any change in scar size is measured in a geographic information system (GIS). Combined with forest age data, modelled reforestation scenarios indicate that sediment generation from earth flows and shallow landslides would be negligible within eight to 10 years after planting, and if all remaining gullies were to be reforested before 2020, sediment yield could halve by 2030 and remain constant thereafter. In a similar time frame, the risk to infrastructure by sedimentation and flooding would likely decrease.
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Researchers compared pastoral farming with forestry over 12 years and found that a forest produces less sediment, uses slightly more water, reduces soil erosion, has a more positive effect on stream environments, and makes no real difference to water quality. Begun in 1993, the land use study was a response to public concern about the environmental effects of forestry on Hawke's Bay hill country. It was completed in 2005.
A paired catchment study approach was used, with one catchment in forest (Pakuratahi) and the other in pasture, farmed with sheep and beef, as a control (Tamingimingi). The catchments represented North Island hill country. Researchers compared the environmental effects of commercial forestry and pastoral farming through various stages of the forest rotation. The sequence of pre-harvest, harvest, replanting, and canopy closure covered time of major environmental change.