Hannu Väisänen

Modelling the dynamics of the forest ecosystem for climate change studies in the boreal conditions

Abstract This paper summarizes a forest ecosystem model developed for assessing the effects of climate change on the functioning and structure of boreal coniferous forests under the assumption that temperature and precipitation are the basic dimensions of the niche occupied by any one tree species. Special attention is paid to specifying weather patterns to a level representing the time constant of different physiological and ecological processes relevant to the regeneration, growth and death of trees. The long-term dynamics of the forest ecosystem have been coupled with climatic factors at the level of mechanisms, e.g., photosynthesis and respiration, in terms of the energy flow through the ecosystem. Furthermore, hydrological and nutrient cycles couple the dynamics of the forest ecosystem with climate change through soil processes representing the thermal and hydraulic properties of the soil and the decomposition of litter and humus with the mineralization of nutrients. Simulations for southern Finland (62°N) and northern Finland (66°N) indicated that a transient increase in temperature by 4°C over a period of 100 years could substantially increase soil temperature and reduce soil moisture in forest ecosystems dominated by Scots pine. At the same time, the temperature increase could enhance photosynthetic production and consequent stemwood growth in southern Finland by about 8% and in northern Finland by about 19%. Given the current temperature but elevating CO2 concentration, the increase in photosynthesis in southern Finland could be about 23% and in northern Finland about 21%, but the concurrent elevation in temperature and CO2 concentration increased photosynthesis by about 32% in southern Finland and by about 40% in northern Finland. Transpiration decreased by as much as 10–20% under the changing climate with the consequence that water-use efficiency increased by as much as 25–45%, the higher values representing southern Finland.

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.

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.

More timber from boreal forests under changing climate

Abstract The effects of increases in temperature, precipitation and atmospheric CO 2 concentration on timber yields from stands of Scots pine ( Pinus sylvestris L.) in southern Finland (61°N) are addressed. The assessment is based on simulations using a process-based model in which temperature, precipitation, and atmospheric CO 2 are among the main drivers linking the dynamics of the tree stands directly and indirectly with the changing climate. These factors control photosynthesis, respiration, transpiration and the uptake of nitrogen and water, with consequent effects on the growth and development of tree stands. The timing of thinnings and the length of the rotation were related to the dynamics of the tree stand in compliance with the thinning rules applied in practical forestry. The simulations indicated that an increase in precipitation of 9 mm per decade alone did not affect timber yields. However, a temperature increase of 0.4°C per decade, and the combination of temperature and precipitation increases would increase timber yields by 10% during one rotation. An elevation in the concentration of atmospheric CO 2 by 33 μmol mol −1 per decade alone would increase removals of timber by 20%, and a combination of increases in temperature, precipitation and CO 2 concentration would increase removals by 30%. A rise in precipitation did not have any effect on the length of the rotation, but the other combinations shortened the rotation; by 9 years in the case of elevating temperature, by 17 years in the case of elevating atmospheric CO 2 concentration, and by 23 years in the case of the combined elevation of temperature, precipitation, and CO 2 concentration due to more rapid tree growth and development. These changes can be expected to affect the supply of timber and also the profitability of forestry.

A procedure for generating synthetic weather records in conjunction of climatic scenario for modelling of ecological impacts of changing climate in boreal conditions

Abstract A simulation procedure for the calculation of temperature, cloudiness, radiation, precipitation, air humidity, windiness and atmospheric carbon is presented. The procedure generates the hourly or daily values of the weather factors based on long-term weather statistics. The basic factor behind the weather pattern indicated by different weather factors is air temperature, which modifies the cloudiness produced by a stochastic process in other respects. Consequently, the radiation and precipitation coming onto a site and the air humidity are also modified by temperature. This facilitates the future weather patterns to be calculated on the basis of the temperature increase allowing to introduce the effect of the suggested climatic change also into the other weather factors, assuming that the basic variability of the weather factors remains unchanged. The future windiness is calculated as random process without correlation to other weather factors.

Structural development of Pinus sylvestrís stands with varying initial density: A preliminary model for quality of sawn timber as affected by silvicultural measures

The effects of stand density, thinning and pruning on the quality of sawn timber of Scots pine (Pinus sylvestris L.) were studied based on a computer model. Procedures for the tree growth and crown structure produce a tree, whose properties in terms of location, dimensions and quality (living, dead) of branches are known for any moment of the selected rotation. Consequently, the size, quality (living, dead) and location of knots in sawn timber are recognizable allowing the grading of sawn pieces. The model computations showed that the natural dynamics of the crown system in narrowly‐spaced stands could yield sawn timber nearly of the same quality as resulted from pruning in widely‐spaced stands. Thinning increased the branch growth and the branchiness of the wood yielding lower grade for sawn pieces, respectively.

Application of a gap model for the simulation of forest ground vegetation in boreal conditions

Abstract This paper describes an application of a gap model for the simulation of the succession of ground vegetation as controlled by the dynamics of tree stratum. Initial composition of the ground cover comprises grasses, herbs, dwarf shrubs, mosses and lichens occupying the site. The same species could survive throughout the successional period under study, but their abundance is modified by the availability of light and nitrogen in the forest floor, which is controlled by the gap dynamics. The successional pattern simulated by the model for the ground vegetation is close to the empirical measurements of the boreal conditions on mesic sites. The output of the computation can be obtained either in the form of graphs or a list of species.

Model computations on the effect of rising temperature on soil moisture and water availability in forest ecosystems dominated by scots pine in the boreal zone in Finland

Based on model calculations, the moisture of soil for sites with and without a cover of trees under the current and rising temperature was studied assuming a 5 °C increase in annual mean temperature over a period of 100 years. The calculation for southern Finland (61°N) showed that the soil moisture under elevated temperature could be reduced compared to that under current temperature conditions. This was also true for northern Finland (66°N), but there the reduction in soil moisture was less substantial. In particular, when trees were present, the soil moisture during the growing season was reduced due to enhanced evapotranspiration. In the presence of trees, the moisture content of the surface soil was only half that under the current temperature. In these conditions, reduced accumulation of snow and a thin humus layer allowed the soil to freeze to deep layers, thereby causing further reduction in soil moisture due to poor transfer of water deeper in the soil.

Model Computations on the Effects of Elevating Temperature and Atmospheric CO2 on the Regeneration of Scots Pine at the Timber Line in Finland

Based on model computations, the regeneration of Scots pine (Pinus sylvestris L.) was studied at the northern timber line in Finland (70°N) in relation to elevating temperature and atmospheric CO2. If a transient increase of 4°C was assumed during the next 100 years, the length of growing season increased from the current 110–120 days to 150–160 days. This was associated with ca. 5°C increase in the soil temperature over June–August with larger variability in temperature and deeper freezing of the soil due to the reduced depth and duration of the snow cover. At the same time, the moisture content of the surface soil decreased ca. 10% and was more variable, due to less infiltration of water into the soil as a consequence of the enhanced evapotranspiration and deeper freezing of the soil. The temperature elevation alone, or combined with elevating CO2, increased flowering and the subsequent seed crop of Scots pine with a decrease in the frequency of zero crops. In both cases, temperature elevation substantially increased the success of regeneration in terms of the number of seedlings produced after each seed crop. The increasing number of mature seeds was mainly responsible for the enhanced regeneration, but increasing soil temperature also increased the success of regeneration. The soil moisture was seldom limited for seed germination. In terms of the density of seedling stands, and the height and diameter growth of the seedlings, the establishment of a seedling stand was substantially improved under the combined elevation of temperature and CO2 in such a way that the temperature increased the number of mature seeds and enhanced germination of seeds and CO2 increased seedling growth. Even under the changing climatic conditions, however, the growth of the seedling stands was slow, which indicated that the northward advance of the timber line would probably be very slow, even though regeneration was no longer a limiting factor.

Impacts of climate change on primary production and carbon sequestration of boreal Norway spruce forests: Finland as a model

The aim of this study was to estimate the potential impacts of climate change on the spatial patterns of primary production and net carbon sequestration in relation to water availability in Norway spruce (Picea abies) dominated forests throughout Finland (N 60°–N 70°). The Finnish climatic scenarios (FINADAPT) based on the A2 emission scenario were used. According to the results, the changing climate increases the ratio of evapotranspiration to precipitation in southern Finland, while it slightly decreases the ratio in northern Finland, with regionally lower and higher soil water content in the south and north respectively. During the early simulation period of 2000–2030, the primary production and net carbon sequestration are higher under the changing climate in southern Finland, due to a moderate increase in temperature and atmospheric CO2. However, further elevated temperature and soil water stress reduces the primary production and net carbon sequestration from the middle period of 2030–2060 to the final period of 2060–2099, especially in the southernmost region. The opposite occurs in northern Finland, where the changing climate increases the primary production and net carbon sequestration over the 100-year simulation period due to higher water availability. The net carbon sequestration is probably further reduced by the stimulated ecosystem respiration (under climate warming) in southern Finland. The higher carbon loss of the ecosystem respiration probably also offset the increased primary production, resulting in the net carbon sequestration being less sensitive to the changing climate in northern Finland. Our findings suggest that future forest management should carefully consider the region-specific conditions of sites and adaptive practices to climate change for maintained or enhanced forest production and carbon sequestration.