Stephen J. Mitchell

Ecological mitigation of hillslope instability: ten key issues facing researchers and practitioners

BackgroundPlants alter their environment in a number of ways. With correct management, plant communities can positively impact soil degradation processes such as surface erosion and shallow landslides. However, there are major gaps in our understanding of physical and ecological processes on hillslopes, and the application of research to restoration and engineering projects.ScopeTo identify the key issues of concern to researchers and practitioners involved in designing and implementing projects to mitigate hillslope instability, we organized a discussion during the Third International Conference on Soil Bio- and Eco-Engineering: The Use of Vegetation to Improve Slope Stability, Vancouver, Canada, July 2012. The facilitators asked delegates to answer three questions: (i) what do practitioners need from science? (ii) what are some of the key knowledge gaps? (iii) what ideas do you have for future collaborative research projects between practitioners and researchers? From this discussion, ten key issues were identified, considered as the kernel of future studies concerning the impact of vegetation on slope stability and erosion processes. Each issue is described and a discussion at the end of this paper addresses how we can augment the use of ecological engineering techniques for mitigating slope instability.ConclusionsWe show that through fundamental and applied research in related fields (e.g., soil formation and biogeochemistry, hydrology and microbial ecology), reliable data can be obtained for use by practitioners seeking adapted solutions for a given site. Through fieldwork, accessible databases, modelling and collaborative projects, awareness and acceptance of the use of plant material in slope restoration projects should increase significantly, particularly in the civil and geotechnical communities.

Stem growth responses in Douglas-fir and Sitka spruce following thinning: implications for assessing wind-firmness

Abstract Diagnosing the stand hazard component of windthrow risk requires evaluation of the ‘acclimation’ of trees to wind loads. Height–diameter ratio is a commonly used indicator of relative wind-firmness. A sample of coastal Sitka spruce ( Picea sitchensis ) and interior Douglas-fir ( Pseudotsuga menziesii ) trees, representing a range of initial slenderness, were sampled from stands which had very high densities prior to thinning. Annual height increment, radial increment, allocation of radial increment along the bole, and height–diameter ratio were reconstructed using stem analysis. Temporarily reduced height increment, increased radial increment and increased basal allocation contributed to a reduction in height–diameter ratio. This reduction was most pronounced in trees which were initially more slender. The reverse-S pattern of height–diameter ratio adjustment and the patterns of growth allocation suggest a period of acclimative growth during which trees re-equilibrate with post-thinning wind loads. Observing the rate of stem form adjustment could be useful in diagnosing wind-firmness when scheduling multiple thinning entries in high-density stands.

Extreme weather and landslide initiation in coastal British Columbia

Abstract More frequent more intense storms predicted by climate models for the Pacific Northwest of North America could increase the regional landslide hazard. The impacts of one such storm are examined on Vancouver Island, British Columbia, during which 626 mapped landslides occurred, encompassing >5 km2 total area and generating >1.5 × 106 m3 of sediment. The relationship between rainfall intensity, air temperature and wind speed obtained from mesoscale numerical weather modelling is examined relative to landslide incidence within steep terrain. A critical onset of rainfall intensity between 80 and 100 mm in 24 h that results in a rapid increase in landslides with increasing precipitation is demonstrated. The argument is presented that this result is more useful for landslide management decisions than a minimum threshold. The component of wind-driven rain was calculated, and results indicated that wind caused increased concentrations of rainfall associated with the occurrence of landslides. Approximately half the landslides studied were not related to rainfall alone, but to rain on snow, and we argue that wind played a crucial role. This often neglected component of hydrological analysis remains a major challenge as the role of snow transition zones and a warming climate in coastal mountain watersheds is considered.

Overturning resistance of western redcedar and western hemlock in mixed-species stands in coastal British Columbia

Specific information about the applied forces that cause trees to fail is required to validate mechanistic models of windthrow in different forest types. Static tree-pulling tests were conducted to examine the overturning resistance of western redcedar (Thuja plicata Donn ex D. Don) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) in a mixed species second-growth stand in coastal British Columbia. Although widely used, tree-pulling techniques are not standardized. Data from three inclinometers were used to estimate stem deflection, which was found to increase with tree slenderness. Differing methods of fitting stem curvature had a small effect on estimates of self-loading at failure. The distance of the pivot point from the centre of the stem base increased with tree diameter. Accounting for the correct self-loading at failure produced a small difference in the overall turning moment regressions but did not improve the fit of these regressions. However, this difference increased with tree size and wa...

The effect of short day treatments on containerized Douglas-fir morphology, physiology and phenology

The effect of short day treatments (‘blackout’) on Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) container seedlings at the time of lift and following cold storage was investigated. Variables measured included height, root collar diameter (RCD), root growth capacity (RGC), photosynthetic efficiency after −18 °C freezing (PEF), and days to terminal bud break (DBB). From one to four blackout dormancy induction treatments were started on three dates (July 12, July 26, and August 10) with 10 or 20 d between multiple blackouts. Increasing the number of blackout treatments resulted in lower RCD, lower DBB in the late winter/early spring, and higher PEF in the early fall. Later blackout start dates decreased PEF in the early fall, and increased overall height and late fall RGC as compared to earlier blackout start dates. Nurseries growing Douglas-fir seedlings from coastal Pacific Northwest provenances should be aware that blackout regimes can decrease RGC in the late fall, and cause quicker dormancy release in the early spring. Coastal Douglas-fir can be lifted and planted in the early fall, when RGC and DBB are relatively high. If planting between February and April is necessary, seedlings given blackout should be cold stored in January to maintain an adequate level of dormancy, RGC and PEF.

Editorial: Mechanical Signaling in Plants: From Perception to Consequences for Growth and Morphogenesis (Thigmomorphogenesis) and Ecological Significance

Plant morphogenesis and its regulation have fascinated researchers for more than two centuries. Among determinants of morphogenesis mechanical signals appear as an important cue. The fact that plants respond to mechanical stimuli was reported by Darwin in the 1850’s. As described by Iida in this research topic, mechanical stimuli were used in traditional agriculture practices like mugifumi. In the past 40 years, the study of mechanical signaling in plants has regained interest because of its implication in fundamental processes of organo- and morphogenesis and their potential as an innovative means of controlling plant growth.

Modeling Windthrow at Stand and Landscape Scales

Windthrow results when the wind loads acting on individual trees exceed their stem or anchorage resistance, leading to stem breakage or uprooting. While recurrent windthrow is an important natural disturbance process, it also affects management planning, and can pose a threat to human safety and infrastructure. Empirical approaches to windthrow susceptibility modeling can represent the full range of site, stand, and management conditions across windy landscapes, but provide limited insights into associated mechanisms and are of unknown accuracy in locations or scenarios not represented in the underlying observational data sets. Hybrid empirical–mechanistic approaches provide useful frameworks for integrating knowledge of component processes, but are simplified representations of the biology and mechanics of windthrow and of the range of site and stand conditions within which windthrow occurs. Both empirical and hybrid-mechanistic models have been integrated with forest inventories and growth and yield models within geographic information system (GIS)-based decision support systems that enable researchers to evaluate contrasting management and climate scenarios. However, these models are not yet available in formats that enable easy use by practitioners. Hybrid-mechanistic models could be improved by better representation of the interaction between wind and canopies and among trees, and the process of damage propagation during storms. As well, the models need to be validated for sites that better reflect the range of stand conditions and management regimes in forested landscapes. Wide availability of high-resolution satellite images for change detection, high-resolution spatial weather data sets, and LiDAR for characterizing terrain, soil drainage, and stand structure set the stage for rapid improvement in landscape-scale modeling of windthrow with both empirical and hybrid-mechanistic approaches.

Assessing and promoting windfirmness in conifers in British Columbia

Windthrow is a natural disturbance agent which disrupts forest and stand level plans in British Columbia (BC). The objectives of this thesis were to: i) review the factors contributing to windthrow risk in BC, ii) investigate patterns of stem growth in conifers following thinning and consider their use for diagnosing windfirmness; and iii) present a diagnostic method of assessing windthrow risk suited to the heterogeneity of BCs forests. Stem analysis was used to reconstruct the post-thinning stem growth patterns for 25 Sitka spruce (Picea sitchensis (Bong.) Carr.) from a coastal stand which was thinned in 1980, and 45 Douglas-fir {Pseudotsuga menziesii var. glauca (Beissn.) Franco) from a stand on the interior plateau with 1978 and 1984 thinned portions. Both stands had very high initial densities and sample trees represented a range of stem slenderness in the thinned and control portions. Following thinning, height increment was temporarily reduced, radial increment increased and the allocation of radial increment became more basal in each of the three thinning treatments. The post-thinning stem slenderness curves were reverse-S in shape with initially more slender trees showing the greatest decline. Critical turning moments of 36 Douglas-fir trees winched to failure were related to tree size. With a uniform wind profile over the crown length, critical wind speed declined with slenderness. The relationship between safety factor and slenderness varied between treatments depending on the attenuation of the within-canopy wind profile. The short term growth responses, their relationship with initial stem slenderness and the shape of the slendemess adjustment curves suggested a pattern of form re-equilibration following thinning. It appears that observation of the magnitude and duration of post-thinning stem form adjustment is a useful diagnostic tool for scheduling subsequent thinning entries in stands with high initial slendemess, and for identifying more vulnerable trees. Forest managers in BC use a system of site and stand diagnosis during the preparation of stand level prescriptions. A diagnostic framework for windthrow risk is presented which can be incorporated into the prescription process. In this framework, the principle of acclimative growth is used in assessing the stand hazard and treatment risk components of windthrow risk. Recommendations are made for a comprehensive program of windthrow management.