Risto Sievänen

Effects of age and site quality on the distribution of biomass in Scots pine (Pinus sylvestris L.)

The distribution of the above-ground and below-ground biomass of Scots pine in southern Finland were investigated in trees of different ages (18–212 years) from two types of growth site. Secondly, some structural regularities were tested for their independence of age and growth site. Trees were sampled from dominant trees which could be expected to have a comparable position in stands of all ages. All stands were on sorted sediments. The biomass of the sample trees (18 trees) was divided into needles, branch sapwood and heartwood, stem sapwood and heartwood, stem bark, stump, large roots (diameter >20 cm), coarse roots (five classes) and fine roots. The amount of sapwood and heartwood was also estimated from the below-ground compartments. Trees on both types of growth site followed the same pattern of development of the relative shares of biomass compartments, although the growth rates were faster on the more fertile site. The relative amount of sapwood peaked after canopy closure, coinciding with the start of considerable heartwood accumulation. The relative amount of needles and fine roots decreased with age. The same was true of branches but to a lesser degree. The relative share of the below-ground section was independent of tree age. Foliage biomass and sapwood cross-sectional area were linearly correlated, but there were differences between the growth sites. Needle biomass was linearly correlated with crown surface area. The fine root to foliage biomass ratio showed an increasing trend with tree age.

LIGNUM: a model combining the structure and the functioning of trees

The model LIGNUM treats a tree as a collection of a large number of simple units that correspond to the organs of a tree. The model describes the three-dimensional structure of the tree crown and derives growth in terms of the metabolism taking place in these units. The time step is one year. The structural units are: tree segments, branching points and buds. Tree segments are separated by branching points. The buds produce new tree segments, branching points and buds. The tree segments contain wood, bark and foliage. A model tree consisting of simple elements translates conveniently to a list structure: the computer program implementing LIGNUM treats trees as a collection of lists. The annual growth of the tree is driven by the available photosynthetic products after accounting for respiration losses. The photosynthetic rate of foliage depends on the amount of intercepted light. The amount of photosynthates allocated to the growth of new tree segments is controlled by the light conditions and the amount of foliage of the mother tree segment. The biomass relationships of the tree parts follow, e.g. from a pipe model hypothesis. The orientation of new tree segments results from application of simple branching rules. LIGNUM has been parametrized for young Scots pines (Pinus sylvestris L.).

Predicting the decomposition of Scots pine, Norway spruce, and birch stems in Finland.

Models were developed for predicting the decomposition of dead wood for the main tree species in Finland, based on data collected from long-term thinning experiments in southern and central Finland. The decomposition rates were strongly related to the number of years after tree death. In contrast to previous studies, which have used the first-order exponential model, we found that the decomposition rate was not constant. Therefore, the Gompertz and Chapman-Richards functions were fitted to the data. The slow initial decomposition period was mainly due to the fact that most dead trees remained standing as snags after their death. The initial period was followed by a period of rapid decomposition and, finally, by a period of moderately slow decomposition. Birch stems decomposed more rapidly than Scots pine and Norway spruce stems. Decomposition rates of Norway spruce stems were somewhat lower than those of Scots pine. Because the carbon concentration of decaying boles was relatively stable (about 50%) the rate of carbon loss follows that of mass loss. Models were also developed for the probability that a dead tree remains standing as a snag. During the first years after death, the probability was high. Thereafter, it decreased rapidly, the decrease being faster for birch stems than for Scots pine and Norway spruce stems. Almost all stems had fallen down within 40 years after their death. In Scots pine and Norway spruce, most snags remained hard and belonged to decay class 1. In birch, a higher proportion of snags belonged to the more advanced decay classes. The models provide a framework for predicting dead wood dynamics in managed as well as dense unthinned stands. The models can be incorporated into forest management planning systems, thereby facilitating estimates of carbon dynamics.

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.

Improving the simulation of stand structure in a forest gap model

Abstract There is currently great interest in improving the applicability of forest gap models to changing environmental conditions, in order to facilitate the assessment of possible impacts of climatic change on forest ecosystems. Moreover, for the development of mitigation strategies, it is necessary to include forest management options in the models. Both the simulation of transient effects of climatic change and of forest management regimens require a realistic representation of stand structure in gap models, since tree species respond to variations in stand density in characteristic ways, depending on their ecological strategies. In this study, we compared the effect of five different height growth functions that are sensitive to stand density on simulated stand structure of the FORSKA forest gap model. We used long term observation data from a beech thinning trial at Fabrikschleichach, Bavaria, to test the alternative functions. First, we compared simulation results of the original FORSKA model with measured stand development from 1870 to 1990. Whereas simulated stand level variables (e.g. biomass, mean diameter and height) showed good correspondence with observations, individual tree dimensions and simulated stand structure were quite unrealistic. After calibrating parameters of the height growth functions with data from a lightly thinned plot at Fabrikschleichach, we ran the model with data from a heavily thinned plot for validation. All five functions considerably improved the simulation of height/diameter relationships and stand structure. However, there were distinct differences between functions. The best correspondence with measurements was shown by a function which uses the relative radiation intensity in the centre of a tree crown as an indicator of the competition status of the tree. This function is rather simple and needs only two growth parameters, which can be derived for different functional types of species, according to their shade tolerance. With the new, flexible height growth function it should be possible to extend the applicability of gap models to more realistic simulation experiments including forest management and natural disturbance. To our knowledge, this was the first attempt to employ long term forest observation data for the calibration and validation of a forest gap model. The results suggest that such data could be very useful in model testing and improvement.

Height growth strategies in open-grown trees

This paper analyses the height growth of open-grown trees as a compromise between reducing self-shading by decreasing foliage density, and reducing structural costs by controlling crown dimensions. Self-shading is described with an equation which accounts for the reduction of foliage-specific photosynthesis due to increased volume density of foliage. Tree growth is described using a differential equation based on the carbon balance, where allocation of growth between wood and foliage follows the pipe-model theory. Crown shape is constant and crown length is equal to tree height. In this model, the construction and maintenance costs of wood relative to photosynthetic production increase with decreasing foliage density in the crown. The main result is that optimal height growth for maximizing accumulated net production is a bell-shaped function of time and rather independent of parameter values. As a result of optimal height growth, foliage biomass follows an allometric function of tree height. The allometric exponent is in the range 2–3, depending on the environmental parameters. The model is most sensitive to the fertility of the growth site, such that fertile sites favour low values and poor sites high values of the exponent. The results are compared with Scots pine and red pine trees grown in the open. The empirical material is consistent with the model.

Belowground interspecific competition in mixed boreal forests: fine root and ectomycorrhiza characteristics along stand developmental stage and soil fertility gradients

We studied fine roots and ectomycorrhizas in relation to aboveground tree and stand characteristics in five mixed Betula pendula Roth, Picea abies (L.) H. Karst., and Pinus sylvestris L. stands in Southern Finland. The stands formed gradients of developmental stage (15-, 30-, and 50-year-old stands) in the stands of medium fertility, and of site fertility in the young stands (30-year-old fertile, medium fertile, and least fertile stands). The biomass of the external hyphae of ectomycorrhizas (ECM) was the highest, and the diversity of the fungal community the lowest, in the most fertile stand. The vertical distributions of fine roots of the three tree species were mostly overlapping, indicating high inter-specific belowground competition in the stands. We did not find any clear trends in the fine root biomass (FRB) or length across the stand developmental stages. The FRB of the conifers varied with site fertility, whereas in B. pendula it was almost constant. In contrast to the conifers, the specific root length (SRL) of B. pendula clearly increased from the most fertile to the least fertile stand. This indicates differences in the primary nutrient acquisition strategy between conifers and B. pendula.

A process‐based model for the dimensional growth of even‐aged stands

An individual tree, process‐based stand growth model is presented. It is based on the carbon balance, according to which tree growth depends on the activities of photosynthesis, respiration and senescence. A simple model is specified for each component of the carbon balance. Next, equations for the tree structure, in which e.g. pipe‐model theory is utilized, are presented. The growth model for dry‐weights of tree compartments based on the carbon balance is transformed using these equations to allow the expression of growth in terms of diameter and height. It is also possible to aggregate a number of physiological and biometrical coefficients into a small number of generalized coefficients of the dimensional growth model. Additional components, including the equations for recession of the crown base and tree survival that are necessary for a stand growth model, are specified. Comparison of the stand growth model with a yield table and a growth model for a sapling stand suggests that the model is capable of...

Crown architecture of grafted Stone pine (Pinus pinea L.): shoot growth and bud differentiation

The singular umbrella-like crown shape of Stone pine can be interpreted as a consequence of primary shoot-growth patterns and posterior axis differentiation due to differential secondary growth and down-bending of branches. This paper centres on the first aspect, analysing the growth, branching and flowering behaviour of about 5,000 individual shoots on 27 grafted Stone pines. The data measurement on standing trees allowed to study correlations of topologic and geometric variables in the shoot and their ancestors. The only significant correlations were found with parameters of the mother shoot formed the previous year and with the number of cones born 3 years before by the respective ancestor. The fitted relationships between geometric and topologic shoot and branch variables are the first step of a structural model construction that can be completed with functional components like a radiation and a carbon allocation submodel, stressing the importance of the heavy Stone pine cones as carbon sinks, with a total annual allocation similar to stem wood. In conclusion, the Stone pine crown shape emerges as consequence of the lack of initial vigour differentiation between stem and main-branch apical meristems that favour the generalized sylleptic reiteration in the open-grown trees.

The effect of apical dominance on the branching architecture of Arctostaphylos uva-ursi in four contrasting environments

Summary We studied horizontal spreading and axillary bud activation of colonizing branches of a clonal dwarf shrub, Arctostaphylos uva-ursi , in SW Finland in four habitats with varying pollution, nutrient, light and competition levels. A. uva-ursi showed high plasticity in the proportion of released buds, which changed according to the stress and resource level of the habitats. Apical dominance of lateral branching was the strongest in the resource-poor habitats (low soil nutrient or low light levels). However, when the apex of the parent shoot was terminated (due to mortality of an apical bud or the formation of an inflorescence) the disruption of apical dominance caused intensive branching in the poor habitats. Apical dominance of the dominant shoots was much weaker in the resource-rich habitats. When nutrient availability and light level were relatively high, branching frequency was high in both intact (expanded) and terminated parent shoots. Shoot mortality and the proportion of terminated shoot apices was the highest in the polluted habitat, as a result of soil toxicity. This caused intensive branching in all shoot hierarchies, which may enhance the plant’s survival in heavy metal polluted soil. We conclude that plastic branching enables this species to recover after damage, or to respond to changed resource levels according to the ‘reserve meristem hypothesis’. The branching response to different environmental conditions was simulated by means of an L-system architectural model based on the demographic and morphological parameters measured in each habitat.