Report: Trees for steep slopes
Sustainable Forest Solutions
Reviewer: Mike Marden
The Joint NZ Farm Forestry Association/Forest Owners Association (NZFFA/FOA) Environment committee commissioned this report to consider and explore species and management options that might minimise soil erosion in highly erodible hill country while maintaining productivity of the land.
This report assembles information for growers and territorial authorities to be well informed on plantation forestry options for steep slopes and hill country that mitigate the risk of erosion. The focus is on alternative species and regimes that offer the opportunity to improve slope stability beyond what can be achieved with standard practice radiata clearfell practices. However, it should be noted that New Zealand-based research quantifying the effectiveness of alternative exotic forest species in mitigating erosion is scarce, as is evidence of the suitability and growth performance of these species when grown on steep (and generally less fertile) slopes, or that they will be profitable. Filling these information gaps would better inform land managers of the range of species best suited to different terrain types and most likely to fulfil environmental obligations under the National Environmental Standards for Plantation Forestry (NES-PF).
National Environmental Standards for Plantation Forestry (NES-PF)
The NES-PF specifies that erosion-prone (ESC red zone) land can now only be planted or replanted with a territorial authority resource consent and the application will be subject to a detailed risk assessment. Although the aim of the risk assessment is to mitigate significant adverse environmental effects, such as storm-initiated slope failures with the potential to form debris flows that could result in significant damage to downstream infrastructure and property, this requirement may potentially increase compliance costs, particularly for growers operating in steeper terrain.
Marginal or negative returns for forestry on steep slopes may lead to plantation forest activities being shifted away from the steeper landforms most in need of forest cover, onto more stable landforms. If this were to occur the overall environmental outcome would not improve because the steepest, most erosion-prone land would likely remain in pastoral use and at increasing risk to further erosion.
Standard-practice radiata pine clearfell regimes are no longer a permitted activity on steeper (red zoned) slopes greater than 2 hectares. In order to retain an erosion-mitigating forest cover, alternative regimes and/or species will be required, along with evidence that significant adverse environmental effects can be minimised. Of course, the other option is to retire land from productive use to become private conservation forest, but this will result in reduced wealth generated from the primary sector. An improved understanding of risk and erosion thresholds would therefore better inform decisions on future land use options, particularly for steep hill country, keeping in mind that ecosystem services function best when there is a revenue base from the activity to maintain them, which might not occur in the private conservation estate.
Erosion mitigation practices
A national erosion susceptibility classification (ESC) has been developed to support the NES-PF. Current limitations of the ESC include the scale of mapping, the quality of underlying data and misclassification of land (Phillips et al. 2017).
For red-zoned land, in the absence of evidence supporting risk mitigation measures, the adequacy of such measures would be at the discretion of territorial authorities. Although the risk level itself can be assumed from the ESC; setting conditions for consent to meet specific performance thresholds such as estimated sediment yields would need to somehow match the measure to the risk. This could potentially be achieved by factoring in:
- Tree stocking rates so as to shorten the duration of the 'window of vulnerability' (i.e. time for the roots of the new planting to replace the rotting roots from the previous rotation). Slower-growing species and sites with low site index would require a higher stocking;
- Rotation length for alternative commercially viable species;
- Measures that restrict the likelihood of landslide occurrence that could form debris flows.
Additional research is needed to quantify the erosion-control effectiveness of "alternative" species to radiata pine (exotic and indigenous). Furthermore, there is a need to better understand relationships between alternative species restocking rates on forest cutover, and their level of effectiveness in mitigating post-harvest sediment generation rates, relative to that measured for radiata pine planted at the same densities. Although detailed information on how different species reinforce soils (such as lateral root reinforcement versus vertical anchoring) would be useful within a comprehensive research strategy, the immediate priority could instead focus on simple models of species-specific, post-harvest rates of root decay and for juvenile plants to establish the growth rates and above-ground tree performance (tree height, canopy width, tree biomass) to be able to predict and define the time it takes for canopy closure to occur according to stocking rate. Managing effects and consequences of erosion could then be simplified to the relationship between the mitigation measures themselves, and for example reduced levels of sediment or reduced occurrence of debris flows that exit the forest boundary.
Estimating the effect that mitigation activities would likely have on consequences of erosion would offer immediate security in terms of the plantation forest industries social licence to operate in erodible steeplands. Additionally, it will provide more scientifically robust evidence in support of the effectiveness of alternative species or regimes in mitigating surface erosion processes, and in reducing the occurrence of shallow landslides during storm events, that will greatly assist territorial authorities during the consenting process. Fine-tuning trade-offs between economic and environmental outcomes may be possible as more empirical evidence becomes available over time. Benchmarking levels of pre- and post-harvest erosion against the status-quo pastoral land use when seeking territorial authority permission to afforest steeplands may also provide useful decision-making tools for both landowner and regulator.
Research is also required that quantifies productivity gains from innovative practices by foresters who have recognised problems and produced solutions to these. Although satisfied with their solutions in terms of their own operation, other operators may not be aware of such innovative practices or paradigms. Although a "no worker on the slope, no hand on the chainsaw" policy is a worthy vision of Forest Growers Research for larger-scale harvesting operations, at the smaller scale sustainable harvesting of steeplands might best be optimised by innovations that involve less mechanisation and improve efficiency only at that scale. Resources are required to support and encourage innovation at the smaller scale and in particular continuous cover harvesting or small coupe harvesting efficiency.
For growers to deploy a species and regime at any scale, models would also be required that estimate returns according to rotation length or harvest regime, taking into account initial stocking and thinning practices that mitigate erosion. Extending the rotation length is desirable for reducing erosion over time, but it has negative impacts on economic rates of return. Slower growing species that yield high-value timber may offer an appropriate trade-off that satisfies both growers and territorial authorities. Alternatively, faster growing species managed as a continuous cover forest may prove to be profitable and environmentally sustainable on steeper slopes.
Estimating profitability should take place at the same time as quantifying the level of erosion mitigation for that regime.
Tree species and regimes
Rate of root decay is species dependent, as is growth rate. Increasing replant density for radiata pine on steeper slopes narrows the window of vulnerability time period, therefore models are required that define planting density according to risk, along with restocking rate on a species by species basis (Phillips et al. 2012).
Some timber species are far better suited to continuous cover management than radiata pine, primarily because they are more shade tolerant. Continuous cover forestry is clearly the "gold standard" for mitigating erosion resulting from harvesting. However, measures available to reduce erosion in a clearfell regime include:
- undertaking only best-practice earthworks (e.g. as per NZ forest road engineering manual);
- minimising soil disturbance and compaction when harvesting;
- managing slash to minimise risk for entrainment in debris flows;
- providing buffers between productive areas and water courses that act as slash traps;
- Identifying areas with excessive risk of erosion and retiring these from productive use.
Replanting or planting at high initial stocking rates and reinstating vegetative cover as soon as possible after harvesting are also important practices that mitigate erosion after clearfell harvesting.
Some timber species appear well-suited to preventing erosion because they coppice from the stump, so their soil-reinforcing roots stay alive. The roots of other species decay at a slow rate and continue to reinforce the soil for longer than low durability species such as radiata, allowing the next crop to establish before those roots lose their soil-reinforcing properties. This window of vulnerability for each species is therefore based on species root decay rate along with the time it takes for the next crop to attain canopy closure. The duration (number of years after planting) of this window of vulnerability is influenced by both the stocking rate (planting density) and species growth rate, thus a stand established at a high tree stocking using a fast growing species will attain canopy closure sooner than a stand established at a low stocking or with a slow-growing species.
Some specialty timber species could potentially be profitably grown on long rotations because of their high timber value. High-value species can also be managed as a continuous cover with supply matched to demand.
Unfortunately, detailed estimates of returns or timber values according to species or regime are not currently available. Indeed crystal ball-gazing may give better predictions of future timber values according to specific applications the timber may be used for, than thoroughly-researched predictions based on current timber values and speculative assumptions, because timber markets and values change in unpredictable ways.
Erosion risk will always remain to some degree unpredictable. Clearfell harvesting does have environmental consequences on steeplands. Managing erosion levels resulting from harvesting involves trade-offs between the grower's bottom line and environmental sustainability, with single-tree extraction methods being the most environmentally benign but also the highest cost.
All tree species require correct siting. Selection of species would principally consider suitability for the site. However, in general terms, by provisionally (in the absence of comprehensive data) rating early growth rate, suitability for continuous cover, root decay rate, productivity, timber value and ability to coppice, the four most suitable "alternative" species for steeplands are:
- Eucalyptus (stringybark/ash)
However, it should be noted that poplar and redwood are limited to sites that have reasonable shelter and fertility, generally lower slopes.
Totara shows the most promise among the native species in terms of overall potential as a profitable and productive plantation species. There are a number of other species available, including native species, that produce high-value timber on longer rotations than radiata pine, so offer options that could potentially reduce the erosion risk while still providing an acceptable level of return.
Disclaimer: The opinions and information provided in this report have been provided in good faith and on the basis that every endeavour has been made to be accurate and not misleading and to exercise reasonable care, skill and judgement in providing such opinions and information. The Author and NZFFA will not be responsible if information is inaccurate or not up to date, nor will we be responsible if you use or rely on the information in any way.