Heli Peltola

Comparison of two models for predicting the critical wind speeds required to damage coniferous trees

Two independently developed mathematical models (GALES and HWIND) for predicting the critical wind speed and turning moment needed to uproot and break the stems of coniferous trees were compared and the results tested against field data on the forces experienced by forest trees and the wind speeds required to damage them. The GALES model calculates the aerodynamic roughness and zero-plane displacement of a forest stand. The aerodynamic roughness provides a measure of the stress (force/unit area) imposed on the canopy as a function of wind speed and the zero-plane displacement provides a measure of the average height on the tree at which the wind acts. Together they allow a calculation of the bending moment imposed on the tree for any wind speed. Data from almost 2000 trees uprooted during pulling experiments and destructive sampling of green wood then allow the model to make predictions of the wind speed at which the tree will be overturned and at which the tree will break for a number of coniferous species. The model assumes a linear relationship between tree stem weight and the maximum resistive moment that can be provided by the root system and it assumes that the stress in the outer fibres of the stem induced by the wind is constant with height. In the HWIND model the turning moment arising from the wind drag on the crown is calculated assuming a logarithmic upwind profile. Together with the contribution from the overhanging weight of the stem and branches caused by bending of the stem this provides the total bending moment. The angle of stem bend is explicitly calculated from the stiffness of the stem. The breaking strength of the stem and the support given by the root-soil plate are calculated from previous experiments on timber strength, and tree resistance to overturning by using root-soil plate mass to derive the resistive moment. This allows calculation of the wind speed required to break and overturn the tree. Model comparisons were performed for individual Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L.) with varying tree height and stem taper (dbh/height). Tree location was at the forest stand edge on a podzolic soil. Model comparisons gave good agreement for the critical wind speeds at the forest edge required to break and overturn trees with a maximum difference in prediction of 26%. Slightly better agreement was obtained for Norway spruce (mean difference of 10.8%) than Scots pine (mean difference of 12.3%) and the best agreement was for trees with a taper of 100. At higher taper the GALES model generally predicted higher critical wind speeds than the HWIND model whereas at lower taper the reverse applied.

Sensitivity of managed boreal forests in Finland to climate change, with implications for adaptive management

This study investigated the sensitivity of managed boreal forests to climate change, with consequent needs to adapt the management to climate change. Model simulations representing the Finnish territory between 60 and 70° N showed that climate change may substantially change the dynamics of managed boreal forests in northern Europe. This is especially probable at the northern and southern edges of this forest zone. In the north, forest growth may increase, but the special features of northern forests may be diminished. In the south, climate change may create a suboptimal environment for Norway spruce. Dominance of Scots pine may increase on less fertile sites currently occupied by Norway spruce. Birches may compete with Scots pine even in these sites and the dominance of birches may increase. These changes may reduce the total forest growth locally but, over the whole of Finland, total forest growth may increase by 44%, with an increase of 82% in the potential cutting drain. The choice of appropriate species and reduced rotation length may sustain the productivity of forest land under climate change.

Modelling the short-term effects of climate change on the productivity of selected tree species in Nordic countries

A boreal version of the process-based simulation model, BIOMASS, was used to quantify the effect of increased temperature and CO2-concentrations on net primary production (NPP). Simulations were performed for both coniferous (Pinus sylvestris, Picea abies) and deciduous broad-leaves stands (Fagus sylvatica, Populus trichocarpa), growing in different Nordic countries (Denmark, Finland, Iceland, Norway and Sweden), representing a climatic gradient from a continental climate in Finland and Sweden to a maritime in Denmark, Norway and Iceland. Simulations with elevated temperature increased NPP by ca. 5–27% for the coniferous stands, being less for a Scots pine stand growing in a maritime climate (Norway) compared with a continental (central Sweden, eastern Finland). The increase in NPP could largely be ascribed to the earlier start of the growing season and more rapid recovery of the winter-damaged photosynthetic apparatus, but temperature-driven increases in respiration reduced carbon gain. The effect of elevated temperature on NPP was similar in the P. trichocarpa stand on Iceland, mainly caused by an earlier budbreak and a more rapid leaf development in spring. Increased temperature reduced, however, NPP for the F. sylvatica stand in Denmark, since elevated temperature had no effect on budbreak but increased the water deficit and water demand during the summer and lowered photosynthesis. Increased CO2-concentrations had an additional effect on NPP by 25–40% for the conifers and beech, which originated from increased photosynthesis, through enhanced carboxylation efficiency in summer and improved water use efficiency (beech). The effect of elevated CO2 on NPP was somewhat less for the P. trichocarpa by 13%. # 2003 Elsevier Science B.V. All rights reserved.

Comparison of a physiological model and a statistical model for prediction of growth and yield in boreal forests

The structural and functional properties of a physiological model (FinnFor) and a statistical model (Motti), developed independently, were analysed in order to assess whether the former would provide the same prediction capacity as the latter, which is based on a huge body of long-term inventory data. The predictions were compared in terms of (i) stand-level variables, (ii) analysis of volume growth graphs, and (iii) stand structure variables (diameter and height distributions). Both unmanaged and managed (thinned) stands of Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and silver birch (Betula pendula) growing on medium-fertility sites in central Finland were used for the comparison. In general, the outputs of the models agreed well in terms of relative growth rates regardless of tree species, with the implication that both predict competition within a stand and the effect of position on tree growth in a similar way. The statistical model was stable in its predictions, but not as sensitive to initial stand conditions and management as that based on physiological processes, but the two models agreed well in their dynamics and predictions. The process-based model may therefore be applied to practical management situations, in order to achieve more precise predictions under changing environmental conditions, as in the case of climate warming. On the other hand, some elements of process-model thinking could be incorporated into statistical models in order to make these responsive to changing conditions.

Mechanical stability of trees under static loads

Wind affects the structure and functioning of a forest ecosystem continuously and may cause significant economic loss in managed forests by reducing the yield of recoverable timber, increasing the cost of unscheduled thinning and clear-cuttings, and creating problems in forestry planning. Furthermore, broken and uprooted trees within the forest are subject to insect attack and may provide a suitable breeding substrate, endangering the remaining trees. Therefore, an improved understanding of the processes behind the occurrence of wind-induced damage is of interest to many forest ecologists, but may also help managers of forest resources to make appropriate management decisions related to risk management. Using fundamental physics, empirical experiments, and mechanistic model-based approaches in interaction, we can study the susceptibility of tree stands to wind damage as affected by the wind and site and tree/stand characteristics and management. Such studies are not possible based on statistical approaches alone, which are not able to define the causal links between tree parameters and susceptibility to wind damage. The aim of this paper is to review the recent work done related to tree-pulling and wind tunnel experiments and mechanistic modeling approaches to increase our understanding of the mechanical stability of trees under static loading.

MODEL COMPUTATIONS OF THE IMPACT OF CLIMATIC CHANGE ON THE WINDTHROW RISK OF TREES

The more humid, warmer weather pattern predicted for the future is expected to increase the windthrow risk of trees through reduced tree anchorage due to a decrease in soil freezing between late autumn and early spring, i.e during the most windy months of the year. In this context, the present study aimed at calculating how a potential increase of up to 4°C in mean annual temperature might modify the duration of soil frost and the depth of frozen soil in forests and consequently increase the risk of windthrow. The risk was evaluated by combining the simulated critical windspeeds needed to uproot Scots pines (Pinus sylvestris L.) under unfrozen soil conditions with the possible change in the frequency of these winds during the unfrozen period. The evaluation of the impacts of elevated temperature on the frequency of these winds at times of unfrozen and frozen soil conditions was based on monthly wind speed statistics for the years 1961–1990 (Meteorological Yearbooks of Finland, 1961–1990). Frost simulations in a Scots pine stand growing on a moraine sandy soil (height 20 m, stand density 800 stems ha−1) showed that the duration of soil frost will decrease from 4–5 months to 2–3 months per year in southern Finland and from 5–6 months to 4–5 months in northern Finland given a temperature elevation of 4°C. In addition, it could decrease substantially more in the deeper soil layers (40–60 cm) than near the surface (0–20 cm), particularly in southern Finland. Consequently, tree anchorage may lose much of the additional support gained at present from the frozen soil in winter, making Scots pines more liable to windthrow during winter and spring storms. Critical wind-speed simulations showed mean winds of 11–15 m s−1 to be enough to uproot Scots pines under unfrozen soil conditions, i.e. especially slender trees with a high height to breast height diameter ratio (taper of 1:120 and 1:100). In the future, as many as 80% of these mean winds of 11–15 m s−1 would occur during months when the soil is unfrozen in southern Finland, whereas the corresponding proportion at present is about 55%. In northern Finland, the percentage is 40% today and is expected to be 50% in the future. Thus, as the strongest winds usually occur between late autumn and early spring, climate change could increase the loss of standing timber through windthrow, especially in southern Finland.

Swaying of trees in response to wind and thinning in a stand of Scots pine

Observations have been made of the windspeed, wind direction, and tree movement at the edge and 20 m within a stand of Scots pine (Pinus sylvestris L.) close to 11 m in height. The spectra of windspeed near canopy top, together with the output of accelerometers and video observations of tree movement at mid-crown, were compared in the same stand prior and two years after first thinning. Furthermore, the transfer of wind energy into tree movement was investigated by calculating the mechanical transfer function (Hm2) between the wind spectrum (Suu) and the trees response (Syy), i.e. Hm2= Syy/Suu. Trees were found to behave like damped harmonic oscillators. They reacted to sudden increases in windspeed, reached their greatest displacement during the first cycle, and then returned to their rest position under the influence of damping. The spectral peak frequencies in Syy and in Hm2coincided with the estimated natural sway frequency of trees. Response in the second mode was, however, also evident, especially within the unthinned stand. The periodogram plots showed a consistent trend of a marked decrease in the response of the tree to increase in frequency. Almost no difference in the wind energy transfer, i.e. peak frequencies and peak width, and damping of the system was found between Scot pine at 2700 and 1500 stems per hectare. However, along the stand edge tree movement was greater than within the stand indicating greater wind energy transfer and damping of the system along the stand edge than within the stand.

Model computations on the critical combination of snow loading and windspeed for snow damage of scots pine, Norway spruce and Birch sp. at stand edge

Abstract Model computations were made on the critical combination of snow loading and windspeed for snow damage of Scots pine ( Pinus sylvestris L.), Norway spruce ( Picea abies Karst.) and birch sp. ( Betula sp.) at the newly formed stand edge with varying tree height and stem taper using the model developed by H. Peltola, S. Kellomaki and H. Vaisanen (1996, HWIND: A Mechanistic Model for Wind and Snow Damage of Scotts Pine, Norway Spruce and Birch sp.) for the mechanism of wind and snow damage. In the computations, the total turning moment arising from the wind and snow load and from the bending of stem and crown was calculated along with the breaking stress of the stem and root anchorage. Windspeed variation within the crown and the vertical distribution of snow, stem and crown weight were also taken into account. According to computations, the critical combination of snow and wind loading for stem breakage and uprooting of trees was caused mainly by accumulation of snow on tree crowns, rather than by wind, which did, however, increase the risk of damage. The risk of damage increased along with stem taper decrease or tree height increase for all tree species studied. However, Scots pine and Norway spruce were found much more susceptible to snow damage than birch, which (being leafless) had much less crown area for snow attachment and wind loading. The trees most likely to suffer stem breakage were slightly tapering Scots pines and Norway spruces with tapers of 1:120 for varying tree heights of 12–20 m under short-term snow loading of 60 kg m −2 , i.e. they would have suffered stem breakage under windspeeds of less than 9 m s −1 above the tree canopy top. Respectively, even Scots pine and Norway spruce with tapers of 1:100 were at risk of stem breakage through sustained snow loading of 60 kg m −2 . In addition, even snow loads of 20–40 kg m −2 were found big enough to cause stem breakage of these trees with stem tapers of 1:120 during sustained snow loading. Correspondingly, similar pines and spruces with stem tapers of 1:120 were found to even more liable to be uprooted during conditions of unfrozen soil than of having their stem broken by short-term snow loading of 20–60 kg m −2 , i.e. less windspeed was needed to cause uprooting. However, pines and spruces with tapers of 1:80 were not at risk for stem breakage and uprooting. This was because snow would have more probably been dislodged from the tree crowns by windspeeds greater than 9 ms −1 which are needed to worsen the damage. Nor would very slender birch without leaves have suffered stem breakage or uprooting under any circumstances with windspeeds of less than 9 ms −1 .

Modelling the structural growth of Scots pine with implications for wood quality

Abstract A model for simulating the growth and development of individual Scots pines ( Pinus sylvestris L.) is presented in terms of three-dimensional structure of the tree as determined by the influence of local light conditions on branch growth, with implications for the properties of the wood. The basic computational unit for structural growth is the shoot. Each shoot produces new shoots with dimensions related to the amount of direct and diffuse radiation intercepted in the parent shoot and supplied by other shoots in the crown. The calculation procedure utilises the spatial distribution of shoots produced by the growth process (location, azimuth, inclination), and the structure of the shoots determined in terms of the density, angle and length of the needles. The allocation of tree biomass amongst the needles, branches and stem is basically related to the allocation of the growth of forming shoots between the shoot axis and the needles under the control of the hierarchical position of the parent shoot, branch age and the height of the tree. The growth, death and pruning-off of shoots and branches are modelled over the life span of the tree using time step of 1 year, taking special care to identify the location of green and dead knots in the wood. The properties of the stem are further described in terms of the stem form, wood density and heartwood, which are modelled in the context of the overall growth of the tree. The computation produces the three-dimensional distribution of ring widths, density and heartwood in the stem, i.e. from the pith to stem surface and from the stem base to tree top. The model produces quite a realistic crown and stem structure for simulated Scots pines regardless of the life span of the tree. Furthermore, the model is capable of calculating quite accurately, e.g. average wood density values for any section of wood within a tree.

A GIS-based decision support system for risk assessment of wind damage in forest management

In this study a GIS-based decision support system (DSS) was built for assessing the short- and long-term risk of wind damage in boreal forests. This was done by integrating a forest growth model SIMA and a mechanistic wind damage model HWIND into geographical information system software (ArcGIS 8.2) as a toolbar (DLL) using ArcObjects in ArcGIS and Visual Basic 6. In this DSS complex problems are solved within program so that forest gaps, edge stands and edges are automatically tracked when the forest structure changes over time as a result of forest growth dynamics and management. This DSS can be used to assess the risk of wind damage to Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and birch (Betula spp.) stands, regarding the number of stands and area at risk and length of vulnerable edges of these risk stands at certain critical wind speed classes (i.e. corresponding the maximum wind speed a tree/stand can resist). This DSS can helps forest managers to analyse and visualise (charts, maps) the possible effects of forest management, such as clear-cuts, on both the immediate and long-term risks of wind damage at both stand and regional level.