A mechanistic model for assessing the risk of wind and snow damage to single trees and stands of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karst.), and birch (Betula spp.) is presented. The model predicts the critical turning moment and wind speed at which the trees will be uprooted or break at forest margins. The resistance to uprooting is predicted using the estimate of the root-soil plate weight to derive a resistive moment, while the resistance to stem breakage relies on values for the modulus of rupture determined for different species of timber. A tree is assumed to be uprooted if the total turning moment exceeds the support provided by the root-soil plate anchorage. Similarly, a tree is assumed to break if the breaking stress acting on the stem exceeds a critical value of the modulus of rupture. The model is in general quite sensitive to parameter changes, which partly results from the location in the forest to which it was designed to apply (the stand edge). The predictions of the critical turning moments needed to uproot and break trees nevertheless give a good agreement on average with the Finnish tree-pulling data for Scots pine, Norway spruce, and birch.

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This study was aimed at assessing the potential impacts of climate change on the depth and duration of soil frost under snow cover in forests growing at different geographical locations in Finland. Frost simulations using a process-based forest ecosystem model (FinnFor) were made for Scots pine Pinus sylvestris L. stands (height 17 m, stand density 1100 stems ha -1 ) growing on a moraine sandy soil. The climate change forecast used in the computations was based on the global ocean-atmosphere general circulation model HadCM2 that was dynamically downscaled to the regional level. The simulated climate warming during the winter months was about 4 to 5°C by the end of the 21st century. Frost simulations showed that the length of the soil frost period would lessen all over the country. Though winters will be warmer, the associated decrease in snow cover in southern Finland will increase the probability of frozen ground there in the middle of winter compared with the current climate. In central and northern Finland there will be so much snow, even in the future, that the maximum annual soil frost depth will decrease there.KEY WORDS: Climate change · Soil frost · Soil freezing · Snow cover · Hydraulic frost model · Scots pine Resale or republication not permitted without written consent of the publisherClim Res 17: [63][64][65][66][67][68][69][70][71][72] 2001 The modelling of frost in the soil profile under snowfree surfaces can be done with the help of the frost sum and soil properties (e.g. Saarelainen 1992, McCormick 1993, Venäläinen et al. 2001. The frost sum is the sum of below-0°C daily mean temperatures calculated from the beginning of the frost period. In Scandinavia the frost period typically starts in October and ends in May, in northern Lapland in June. If there is snow on the ground, the modelling of soil temperature becomes more complex. Models must include many variables describing both meteorological conditions, such as air temperature, short and long wave radiation, amount and type of snow, and soil characteristics, such as thermal conductivity and soil heat capacity (e.g. Bonan 1991, Cox et al. 1999. The influence of snow cover on temperature is illustrated in Fig. 1. The daily variation of air temperature in the case of late winter conditions can be more than 20°C, whereas at a depth of 80 cm below the snow surface the daily cycle is practically negligible.Jansson (1991) introduced a comprehensive soil model known as SOIL that includes the processes relevant for the calculation of soil temperature. Kellomäki & Väi-sänen (1997) have integrated this SOIL model into a process-based forest ecosystem model (FinnFor), which links ecosystem dynamics with climate through selected physiological processes. Peltola et al. (1999) used this model when they studied the consequences of climate warming on soil frost and on the windthrow risk for trees in different geographical locations in Finland. Peltola et al. (1999) used 2 options for climate warming: the increase of temperature was estimated to be...

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