Balancing Pastoral and Plantation Forestry Options in New Zealand and the Role of Agroforestry

Pastoral agriculture and forestry enterprises are key features of New Zealand’s landscape and are very important economically. They are competing landuses, particularly on moderate to steep hill country. Agroforestry involving spaced trees of Pinus radiata on pasture was developed in the 1970s to provide dual incomes from livestock enterprises and the later tree crop. In contrast, wide-spaced trees of predominantly Populus and Salix spp. are planted mainly for erosion control. Characteristics of pastoralism and plantation forestry are reviewed, including trends in conversions between these landuses. Roles, challenges and opportunities with wide-spaced trees are presented, together with current and future research initiatives. Pastoralism and forestry will continue to compete strongly for hill country sites and at present there is an increasing trend of converting previously forested areas to pasture, particularly in the central North Island. Agroforestry involving Pinus radiata has virtually ceased because of adverse effects on wood quality, pasture production and animal performance. There are millions of wide-spaced trees of Populus and Salix spp. on hill country and their planting is expected to continue unabated because they are the most practical and efficient means of enabling pastoralism on erodible slopes and they provide multiple ecosystem services. The species have significant advantages compared to other woody species but many older trees have grown very large because they have received negligible or no silviculture. This is an increasing problem, requiring development and implementation of appropriate management strategies. There is growing interest by landowners in the environmental outcomes of spaced-tree plantings

Source-tracking cadmium in New Zealand agricultural soils: a stable isotope approach

Cadmium (Cd) is a toxic heavy metal, which is accumulated by plants and animals and therefore enters the human food chain. In New Zealand (NZ), where Cd mainly originates from the application of phosphate fertilisers, stable isotopes can be used to trace the fate of Cd in soils and potentially the wider environment due to the limited number of sources in this setting. Prior to 1997, extraneous Cd added to soils in P fertilisers was essentially limited to a single source, the small pacific island of Nauru. Analysis of Cd isotope ratios (ɛ114/110Cd) in Nauru rock phosphate, pre-1997 superphosphate fertilisers, and Canterbury (Lismore Stony Silt Loam) topsoils (Winchmore Research Farm) has demonstrated their close similarity with respect to ɛ114/110Cd. We report a consistent ɛ114/110Cd signature in fertiliser-derived Cd throughout the latter twentieth century. This finding is useful because it allows the application of mixing models to determine the proportions of fertiliser-derived Cd in the wider environment. We believe this approach has good potential because we also found the ɛ114/110Cd in fertilisers to be distinct from unfertilised Canterbury subsoils. In our analysis of the Winchmore topsoil series (1949-2015), the ɛ114/110Cd remained quite constant following the change from Nauru to other rock phosphate sources in 1997, despite a corresponding shift in fertiliser ɛ114/110Cd at this time. We can conclude that to the present day, the Cd in topsoil at Winchmore still mainly originates from historical phosphate fertilisers. One implication of this finding is that the current applications of P fertiliser are not resulting in further Cd accumulation. We aim to continue our research into Cd fate, mobility and transformations in the NZ environment by applying Cd isotopes in soils and aquatic environments across the country

Modelling of root reinforcement and erosion control by ‘Veronese’ poplar on pastoral hill country in New Zealand

Background The control of erosion processes is an important issue worldwide. In New Zealand, previous studies have shown the benefits of reforestation or bioengineering measures to control erosion. The impetus for this work focuses on linking recent research to the needs of practitioners by formulating quantitative guidelines for planning and evaluation of ground bioengineering stabilisation measures. Methods Two root distribution datasets of ‘Veronese’ poplar (Populus deltoides x nigra) were used to calibrate a root distribution model for application on single root systems and to interacting root systems at the hillslope scale. The root distribution model results were then used for slope stability calculations in order to quantitatively evaluate the mechanical stabilisation effects of spaced trees on pastoral hillslopes. Results This study shows that root distribution data are important inputs for quantifying root reinforcement at the hillslope scale, and that root distribution strongly depends on local environmental conditions and on the tree planting density. The results also show that the combination of soil mechanical properties (soil angle of internal friction and cohesion) and topographic conditions (slope inclination) are the major parameters to define how much root reinforcement is needed to stabilise a specific slope, and thus the spacing of the trees to achieve this. Conclusions For the worst scenarios, effective root reinforcement (>2 kPa) is reached for tree spacing ranging from 2500 stems per hectare (sph) for 0.1 m stem diameter at breast height (DBH) to 300 sph for 0.3 m stem DBH. In ideal growing conditions, tree spacing less than 100 sph is sufficient for stem DBH greater than 0.15 m. New quantitative information gained from this study can provide a basis for evaluating planting strategies using poplar trees for erosion control on pastoral hill country in New Zealand

Use of New Zealand native browse shrubs on sheep and beef hill country farms : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at the School of Agriculture and Environment, Massey University, Palmerston North, Manawatū, New Zealand

Sheep and beef cattle farming on hill country through the historic clearing of native vegetation for pasture has caused biodiversity loss and increased the risk of soil erosion. Exotic tree species such as poplar and radiata pine can be used to control erosion, but there is current interest in using native plants on the hill country for indigenous biodiversity restoration in addition to erosion control. However, there is limited information on the forage value, biomass, carbon stock, and potential economic impacts of native plants compared to exotics species. This thesis was aimed to address the lack of information available on native shrubs and their comparison to exotics trees and shrubs. The forage feeding value results revealed that native shrubs had consistent nutritional composition across seasons, higher metabolizable energy, and lower crude protein than the exotic shrub Salix schwerinii (Kinuyanagi). Findings on in vitro fermentation characteristics showed that native shrubs were highly digestible, yielded higher volatile fatty acids, microbial proteins, and greenhouse gases than S. schwerinii. Estimation of biomass revealed that the native shrubs were similar in aboveground biomass accumulation, but differed in allocation to foliage, branch, and stem. Melicytus ramiflorus (Mahoe) had lower foliage biomass while Coprosma robusta (Karamū) had lower branch biomass, among the evaluated shrubs. Estimated carbon stock accumulation was higher for Pittosporum crassifolium (Karo) due to a greater woodier portion (branch and stem) than M. ramiflorus and C. robusta but lower than exotic trees. The data from the native shrub studies was used in the bioeconomic model and showed that planting native shrubs or radiata pine on steep slopes equal to 10% of the farm area would reduce farm feed supply. This reduction would result in a decrease in sheep flock size and sheep flock net cashflow, particularly with higher planting rates and with of radiata pine. While radiata pine had a surplus overall farm net cashflow, native shrubs had negative cashflow due to high seedling costs and low carbon income, making their use on the farm currently unprofitable at the modelled prices. The study's findings suggest that replacing exotic trees with native shrubs can provide high-quality summer browse for livestock. The decision to plant native shrubs on steep hill country slopes would depend on the farmer’s financial situation and interest in biodiversity conservation and profits. However, reducing planting costs and increasing the carbon price would be necessary to make investing in native shrubs profitable and more attractive to farmers

Drivers and New Opportunities for Woody Vegetation Use in Erosion Management in Pastoral Hill Country in New Zealand

Increases in the magnitude and frequency of rainfall events in New Zealand due to climate change, coupled with existing concerns about sediment and nutrient contamination of waterways, are changing policy and practice around erosion management and land use. We describe the challenges around slope erosion reduction, cover current legislation and management practices, illustrate how modeling can inform erosion management and describe new opportunities, whereby native species can become a new active management tool for erosion control. Passive erosion management depending on natural revegetation by slow growing woody species is used on land retired from grazing but is much less effective than active erosion management in reducing shallow slope erosion. Active erosion management using exotic fast-growing poplar and willow trees strategically placed on hillslopes is effective in reducing erosion, but these trees can be hard to establish on drier upper slopes. An endemic woody tree, Kanuka, grows on drier slopes and is being tested as an erosion control tool. Kanuka seedlings have been successfully established on pastoral slopes, including drier slopes. A spatial decision support tool developed to identify pastoral hillslopes at high risk of erosion has improved decision-making when positioning appropriate trees on these slopes

Source-tracking cadmium in New Zealand agricultural soils: A stable isotope approach

Cadmium (Cd) is a toxic heavy metal, which is accumulated by plants and animals and therefore enters the human food chain. In New Zealand (NZ), where Cd mainly originates from the application of phosphate fertilisers, stable isotopes can be used to trace the fate of Cd in soils and potentially the wider environment due to the limited number of sources in this setting. Prior to 1997, extraneous Cd added to soils in P fertilisers was essentially limited to a single source, the small pacific island of Nauru. Analysis of Cd isotope ratios (ɛ114/110Cd) in Nauru rock phosphate, pre-1997 superphosphate fertilisers, and Canterbury (Lismore Stony Silt Loam) topsoils (Winchmore Research Farm) has demonstrated their close similarity with respect to ɛ114/110Cd. We report a consistent ɛ114/110Cd signature in fertiliser-derived Cd throughout the latter twentieth century. This finding is useful because it allows the application of mixing models to determine the proportions of fertiliser-derived Cd in the wider environment. We believe this approach has good potential because we also found the ɛ114/110Cd in fertilisers to be distinct from unfertilised Canterbury subsoils. In our analysis of the Winchmore topsoil series (1949-2015), the ɛ114/110Cd remained quite constant following the change from Nauru to other rock phosphate sources in 1997, despite a corresponding shift in fertiliser ɛ114/110Cd at this time. We can conclude that to the present day, the Cd in topsoil at Winchmore still mainly originates from historical phosphate fertilisers. One implication of this finding is that the current applications of P fertiliser are not resulting in further Cd accumulation. We aim to continue our research into Cd fate, mobility and transformations in the NZ environment by applying Cd isotopes in soils and aquatic environments across the country

Environmental factors influencing survival of poplar material planted for erosion control on hill country farms in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Agricultural Science at Massey University, Manawatū, New Zealand

Figures are reused with permission.New Zealand has naturally high erosion rates, and space planting trees (most commonly poplars and willows species) in hill country landscapes is a widely used mitigation tool (Hicks & Anthony, 2001; Basher, 2013; Marden & Rowan, 1993; Hancox & Wright, 2005; Marden, 2012). The effectiveness of planting is, foremost, dependent on the survival and establishment of the trees. Much existing information on tree survival and the factors that influence survival is unpublished, with few studies having quantitively investigated survival rates and multiple potential explanatory variables. Moreover, there has not been a study to understand how different planting practitioners view the importance of survival and what influences their decisions of where, how, and what to plant in different conditions. To explore these issues, this research involved a set of interviews which captured the current knowledge and views about tree planting and survival within a cross-section of New Zealand’s North Island regional and district councils and scientists (Chapter 4). The interviews revealed a general consensus on the importance of trees in erosion mitigation, but mixed views regarding the importance of certain survival variables (e.g., where on the stems the poles are cut, duration of time between harvest and planting, or wind exposure). Differences likely reflect the different regional climate conditions and practitioner experience. To enrich the quantitative data on tree survival in New Zealand hill country, a poplar planting trial was performed on three hill country farms in the North Island (Chapter 4). With climate considered to be one of the most important factors to affect survival, the trial tested the impact of different climatic conditions on plant survival over one planting season. Data from on-site climate stations and nearby NIWA (CliFlo) climate monitoring stations were used to analyse rainfall, temperature and solar radiation. The trial also tested the impact of morphometric variables (e.g., slope aspect, profile curvature, slope gradient and topographic position index), considered to affect the microclimate and soil conditions. At each farm, a north- and a south-facing slope susceptible to landslide erosion, was selected and each planted with 90 trees made up of three plant types; 30 3-m poplar poles planted using a pole driver, and 30 unrooted and 30 rooted poplar wands planted using a spade. Overall survival across all three plant types (poles, unrooted wands, rooted wands) at all trial sites combined was high (90.4%). The highest survival across all plant types occurred at the Hawke’s Bay site (93%) and the lowest occurred at the Taumarunui site (89%). South-facing aspects had slightly higher survival than north-facing aspects, of 90% and 89% respectively. The Wairarapa had no pole deaths which was attributed to experienced pole planting contractors, but the lowest rooted wand survival (76.5%). The Hawke’s Bay site had the highest unrooted wand (98.5%) and rooted wand survival (93%). South-facing aspects had higher pole survival (98%) than north-facing aspects (92.3%), but lower unrooted and rooted wand survival (90% and 84.3% respectively) than north-facing slopes (91% and 89% respectively). The results suggest there was no significant relationship between poplar deaths and morphometric or climatic variables. However, there was a statistically significant difference in growth rates between the north and south facing aspects, suggesting that microclimate may affect long-term survival. It was concluded that higher rainfall throughout the trial period potentially contributed to higher survival. Therefore, suggesting that in years of higher rainfall (i.e., East Coast during La Niña phases of the El Niño Southern Oscillation (ENSO) and positive Southern Annular Mode (SAM)), targeting planting in areas of higher non-survival risk should be completed to increase survival rates. Further research under more typical seasonal conditions is necessary to fully explore the morphometric or climate variable impacts. This research has captured the opinions and knowledge of experienced industry prractitioners which highlights the importance of several aligned variables that influence survival. It has provided a valuable systematic analysis of the cause of death of newly planted poplars and quantitatively assessed early establishment growth. These findings will be valuable to regional council planners, land management advisors and farmers and will potentially lead to more positive planting and reduced soil erosion outcomes for New Zealand hill country

Bioeconomic Modelling to Assess the Impacts of Using Native Shrubs on the Marginal Portions of the Sheep and Beef Hill Country Farms in New Zealand (article)

New Zealand hill country sheep and beef farms contain land of various slope classes. The steepest slopes have the lowest pasture productivity and livestock carrying capacity and are the most vulnerable to soil mass movements. A potential management option for these areas of a farm is the planting of native shrubs which are browsable and provide erosion control, biodiversity, and a source of carbon credits. A bioeconomic whole farm model was developed by adding a native shrub sub-model to an existing hill country sheep and beef enterprise model to assess the impacts on feed supply, flock dynamics, and farm economics of converting 10% (56.4 hectares) of the entire farm, focusing on the steep slope areas, to native shrubs over a 50-year period. Two native shrub planting rates of 10% and 20% per year of the allocated area were compared to the status quo of no (0%) native shrub plantings. Mean annual feed supply dropped by 6.6% and 7.1% causing a reduction in flock size by 10.9% and 11.6% for the 10% and 20% planting rates, respectively, relative to 0% native shrub over the 50 years. Native shrub expenses exceeded carbon income for both planting rates and, together with reduced income from sheep flock, resulted in lower mean annual discounted total sheep enterprise cash operating surplus for the 10% (New Zealand Dollar (NZD) 20,522) and 20% (NZD 19,532) planting scenarios compared to 0% native shrubs (NZD 22,270). All planting scenarios had positive Net Present Value (NPV) and was highest for the 0% native shrubs compared to planting rates. Break-even carbon price was higher than the modelled carbon price (NZD 32/ New Zealand Emission Unit (NZU)) for both planting rates. Combined, this data indicates planting native shrubs on 10% of the farm at the modelled planting rates and carbon price would result in a reduction in farm sheep enterprise income. It can be concluded from the study that a higher carbon price above the break-even can make native shrubs attractive in the farming system

Quantification of vegetation effects on shallow landslide probability at regional scales

Shallow landslides are the cause of considerable direct and indirect losses to individuals, enterprises and society as a whole. An increase in shallow landslide occurrence is often related to the loss of the protective effect of trees by deforestation. Methods of targeted reforestation, such as silvopastoralism, can in turn reduce the risk associated with shallow landslides considerably. The effective implementation of such systems can benefit from guidance in terms of tree placement, planting density, planting pattern, tree species selection and tree size development prediction. For this guidance, computational models can help greatly, in addition to expert knowledge, especially on a regional scale. In this work a quantitative, physically-based, tool to assess the influence of vegetation on shallow landslide occurrence is presented. The focus is specifically on rainfall-induced translational shallow landslides on a regional scale. The results of this tool are ideally incorporated into holistic analyses of site-specific tree benefits and co-benefits. The tool as developed in this thesis is called SlideforMAP. It incorporates a probabilistic approach since the regional scale often poses challenges regarding heterogeneity in a range of parameters. Explicitly including this as uncertainty in the model can improve the model performance. Slope stability is computed by the limit equilibrium approach for a large number of randomly placed hypothetical landslide. The fraction of these hypothetical slides that are unstable, corresponds to local shallow landslide probability. The hydrological module is based on a topographic index and assumes a steady state sub-surface flux. All mechanical influences of vegetation are included with basal and lateral root reinforcement, being incorporated using the Root Bundle Model weibull approach on a single-tree basis. The model is calibrated and applied to three study areas (0.5 - 7.5 km2) in Switzerland. Tree density is 33 to 73 trees/ha on average, but all study areas consist of distinct forested and non-forested sections. Soil thickness, soil cohesion, and the ratio between precipitation intensity and hydraulic saturated conductivity are found to be the most sensitive model parameters. Performance is measured against a 2005 landslide inventory and the Receiver Operator Curve analysis. Area Under the Curve values are between 0.64 and 0.93. It is proven that the single-tree based inclusion of mechanical effects significantly improves model performance, as compared to a forest stand approach. To assess generalizability, SlideforMAP is applied in New Zealand silvopastoral systems in two small study areas (1.4 and 3.5 km2). Over these areas a full calibration and validation of SlideforMAP and an improved version of SlideforMAP is performed. The improved version includes interception, passive earth pressure, root compression, a non-steady state approach to the lateral flux and a runoff coefficient. The runoff coefficient is dependent on cumulative precipitation and relates tree presence to increased macropore presence and subsequently to an increased runoff coefficient. The availability of RADAR-based precipitation data enables a more realistic precipitation input by finding a representative rainfall intensity. The non-steady state approach, runoff coefficients, representative rainfall intensity and novelties. The inclusion of such novelties did not improve model performance, which could be due to flaws in its methodology, the performance measure or site-specific reasons. To place the application of SlideforMAP in context, it is compared to a statistical approach using binary logistical regression to shallow landslide susceptibility in New Zealand silvopastoral systems. For this a 2010 landslide inventory and a 2005 rainfall event are used. The model outputs are in agreement in approximately 70% of the study areas. Accountable for the remaining 30% are methodological differences, the heterogeneous input data in SlideforMAP and the difference in tree influence. SlideforMAP includes mechanical effects, where the statistical method implicitly includes all tree effects, but averaged over all trees. Practitioners are advised to prioritize tree planting on areas where both models are in agreement and predict high shallow landslide susceptiblity. SlideforMAP stands among other state of the art models focussed on vegetation effects on shallow landslide activity. This is due to the inclusion of single tree based lateral and basal root reinforcement and the tailoring to specific rainfall events. Further developments in SlideforMAP have enabled the application on large scales and for the analyses of specific components. All in all, land managers can benefit greatly by applying SlideforMAP to find the ideal targeted planting sites and stabilize the soil, as efficient as possible from a cost-benefit standpoint