Bruce C. Nicoll

Towards developmental modelling of tree root systems

Abstract Knowledge of belowground structures and processes is essential for understanding and predicting ecosystem functioning, and consequently in the development of adaptive strategies to safeguard production from trees and woody plants into the future. In the past, research has mainly been concentrated on growth models for the prediction of agronomic or forest production. Newly emerging scientific challenges, e.g. climate change and sustainable development, call for new integrated predictive methods where root systems development will become a key element for understanding global biological systems. The types of input data available from the various branches of woody root research, including biomass allocation, architecture, biomechanics, water and nutrient supply, are discussed with a view to the possibility of incorporating them into a more generic developmental model. We discuss here the main focus of root system modelling to date, including a description of simple allometric biomass models, and biomechanical stress models, and then build in complexity through static growth models towards architecture models. The next progressive and logical step in developing an inclusive developmental model that integrates these modelling approaches is discussed.

Wind loading of trees: influence of tree size and competition

Wind damage to forests is an important ecological disturbance factor. At the same time, it can have serious economic consequences due to a reduction in timber production. Current models for predicting the risk of wind damage are useful, but generally only focus on the “mean” tree within uniform stands. This paper presents measurements made of wind loading on trees of different sizes within four forest stands of different structure and management history, but all well-acclimated to current wind conditions. Each tree demonstrated a linear relationship between the maximum hourly turning moment and the square of the average hourly wind speed at the canopy top; we defined this ratio (the gradient of the line Mmax vs. u2) as the turning moment coefficient (TC). TC was correlated with tree size, in a relationship that differed little between the four forest sites despite the differences between the stands. The relationship between TC and individual tree competition within each stand was investigated, using both distance-independent and distance-dependent competition indices. All sites showed decreasing TC with increasing competition. However, the relationships differed between sites and would also be expected to change through time for a single site. The distance-dependent indices offered no improvement over the simpler, non-spatial indices that required only a diameter distribution. We suggest how, subject to further work, the results presented could be applied to calculate the risk of wind damage to trees of different sizes within a forest stand, and how the risk of wind damage to individual trees might change in response to thinning.

Comparison and validation of three versions of a forest wind risk model

Predicting the probability of wind damage in both natural and managed forests is important for understanding forest ecosystem functioning, the environmental impact of storms and for forest risk management. We undertook a thorough validation of three versions of the hybrid-mechanistic wind risk model, ForestGALES, and a statistical logistic regression model, against observed damage in a Scottish upland conifer forest following a major storm. Statistical analysis demonstrated that increasing tree height and local wind speed during the storm were the main factors associated with increased damage levels. All models provided acceptable discrimination between damaged and undamaged forest stands but there were trade-offs between the accuracy of the mechanistic models and model bias. The two versions of the mechanistic model with the lowest bias gave very comparable overall results at the forest scale and could form part of a decision support system for managing forest wind damage risk. Comprehensive description and validation of three versions of a mechanistic wind risk model for even-aged forests.Analysis of the performance of mechanistic and logistic regression models against measured damage in a conifer forest.All models provided acceptable discrimination between damaged and undamaged forest stands.The two version of the mechanistic model with the lowest bias gave very comparable overall results at the forest scale.Statistical analysis showed that increasing tree height and local wind speed were the main factors associated with damage.

Are forest disturbances amplifying or canceling out climate change-induced productivity changes in European forests?

Recent studies projecting future climate change impacts on forests mainly consider either the effects of climate change on productivity or on disturbances. However, productivity and disturbances are intrinsically linked because 1) disturbances directly affect forest productivity (e.g. via a reduction in leaf area, growing stock or resource-use efficiency), and 2) disturbance susceptibility is often coupled to a certain development phase of the forest with productivity determining the time a forest is in this specific phase of susceptibility. The objective of this paper is to provide an overview of forest productivity changes in different forest regions in Europe under climate change, and partition these changes into effects induced by climate change alone and by climate change and disturbances. We present projections of climate change impacts on forest productivity from state-of-the-art forest models that dynamically simulate forest productivity and the effects of the main European disturbance agents (fire, storm, insects), driven by the same climate scenario in seven forest case studies along a large climatic gradient throughout Europe. Our study shows that, in most cases, including disturbances in the simulations exaggerate ongoing productivity declines or cancel out productivity gains in response to climate change. In fewer cases, disturbances also increase productivity or buffer climate-change induced productivity losses, e.g. because low severity fires can alleviate resource competition and increase fertilization. Even though our results cannot simply be extrapolated to other types of forests and disturbances, we argue that it is necessary to interpret climate change-induced productivity and disturbance changes jointly to capture the full range of climate change impacts on forests and to plan adaptation measures.

The effects of wind speed and direction on radial growth of structural roots

We used dendrochronological techniques to explore the relationships between tree wind loading and structural root annual radial growth. Study trees were shallow rooted 46-year-old Sitka spruce that had grown in an exposed upland plantation experiencing strong prevailing wind from the south west. We constructed growth ring chronologies using measurements of annual growth ring widths on stem sections and from the largest structural roots, and related them to wind climate data for the life of the trees. Separate root chronologies were constructed from samples from each quadrant around the trees relative to the prevailing wind direction, at 0.5 m, 0.75 m, 1.0 m and 1.25 m from the tree centre, i.e. 16 separate root chronologies. In this preliminary analysis, we correlated yearly growth data from the chronologies with mean hourly and maximum hourly wind speed in each month of the corresponding year. There were few significant correlations between wind speed and stem chronologies, but many positive correlations with the root chronologies. The analysis indicated an overall increase in allocation of assimilates to the structural roots in response to wind. Allocation to structural roots on the lee side was most strongly correlated with maximum wind speeds and this allocation appeared to be at the expense of roots further from the tree, that would have had less of a structural role. Development of windward roots was correlated well with maximum gusts in the corresponding years. There were, however, few significant correlations between wind maximum gusts and thickening of roots in the two sectors perpendicular to the wind direction. These results indicate that these adaptive changes in allocation within root systems resulted from flexing of structural roots in the vertical plane, rather than from twisting. We propose that the amount of secondary thickening in any part of the root system in any one year is proportional to the amount of bending stress experienced at that point. The time of year that the roots experience stress appears to be important: significant correlations between wind speed and root growth ring chronologies were predominantly from wind movement during January and February, and July to October. The January and February correlations could result from storage of assimilates in rays adjacent to the dormant cambium, in regions that experience the most stress. These stored assimilates would enhance radial growth when the cambium becomes active in the spring. There was little influence of wind on root development during spring and early summer when most assimilates are used in shoot growth and stem thickening.

DEVELOPMENT OF PRUNUS ROOT SYSTEMS IN A CITY STREET: PAVEMENT DAMAGE AND ROOT ARCHITECTURE

Summary We surveyed the pavement (sidewalk) damage around five 30-year-old cherry trees in a street in Sheffield and examined the architecture of their root systems. The tarmac and soil were removed, and the positions of the woody roots mapped. Most roots had been constrained by the road on one side and a wall on the other. Pavement damage had been caused not just by roots directly below the surface, but also by fast growing roots as deep as 0.4 m below the pavement. Almost all damage was caused by roots over 100 mm diameter. Large surface roots that had been chiselled down during previous pavement repairs had callused on both sides of the damaged area, increasing the area of contact and the risk of damage to the new pavement.

Autumn frost damage: clonal variation in Sitka spruce.

Current-year needles of Sitka spruce (Picea sitchensis) can be injured by frost after hardening of stem and buds in the autumn, and in Scotland trees of southern provenances are most susceptible to damage. In October 1993 a frost of -5°C caused needle damage to clonal plants of Alaskan, Queen Charlotte Islands, and Washington provenances of Sitka spruce that had been selected for variation in the onset of root dormancy. Visual damage assessment revealed significant differences among clones in the severity of damage, and this damage was positively correlated with the lateness of root dormancy. Measurement of electrolyte leakage from shoot samples confirmed the observed differences between clones. The large clonal differences in frost hardiness found in this study demonstrate a potential for selecting frost tolerant trees from within southern provenances of Sitka spruce.

Institutional factors and opportunities for adapting European forest management to climate change

Abstract Despite the fact that the institutional environment is acknowledged to influence the implementation of regional adaptations of forest management to climate change, there are few empirical studies addressing the institutional factors and opportunities of adaptation. Using Ostrom’s institutional analysis and development framework, we aimed to identify : (1) the critical and distinctive characteristics of the forest resource and institutional context that may determine how climate change-adaptive forest management measures are implemented and (2) the opportunities for implementing the planned adaptation measures. The analysis is performed on ten European case study regions which differed in many resource-dependent factors, policy arena factors and incentives for changes. The main factors influencing the adaptation are the ownership pattern, the level of policy formation and the nature of forest goods and services. Opportunities for adaptation are driven by the openness of the forest management planning processes to the stakeholders participation, the degree to which business as usual management is projected to be non-satisfactory in the future, and by the number and nature of obstacles to adaptation. Promoting local self-governance mechanisms and the participation of the external stakeholders in forest management planning or in the regional forest or climate change policy adaptation may be a way of overcoming path dependency, behavioural obstacles and potential policy failures in implementing adaptation. The study argues that both climate change belief systems and political participation are important to explain adaptation to climate change when multiple decision-making levels are at stake.

Hazard Assessment of Vegetated Slopes

The hazard assessment of vegetated slopes are reviewed and discussed in terms of the stability of the slope both with and without vegetation, soil erosion and the stability of the vegetated slope from windthrow and snow loading. Slope stability can be determined by using either limit equilibrium or finite element stability analysis methods. The limit equilibrium methods are extended to incorporate the vegetation parameters that are important for the stability of a vegetated slope. The factors that contribute to soil erosion are reviewed and the techniques for assessing and measuring the rate of soil erosion are presented. The assessment of windthrow hazards are comprehensively discussed and a mechanistic model called ForestGALES is introduced which has flexibility for testing many different forest management scenarios. The hazards presented by snow loading on forested slopes are briefly reviewed.