biomass, carbon, energy wood, evaluation, forestry modeling, MELA, optimization, simulation, tree-level model
In forestry modeling and analyses, the evaluation of the performance of the models is needed in order to analyze the applicability of the models as a part of a large modeling system. In previous studies, no systematic and detailed analyses of available biomass models to evaluate their applicability for the determination of carbon sequestered by trees and for the assessment of energy wood resources in a certain area were usually made. The aim of this study was to analyze the performance of available biomass models for estimation of biomass of trees in forestry modeling and analyses. This study was the first attempt to systematize the evaluation of the set of biomass models describing the different components of a tree. A set of Marklund’s (1988) and Hakkila’s (1972a, 1979, 1991) models were selected from the available biomass models for the estimation of the total biomass and its distribution into different components of Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and birches (Betula pendula, B. pubescens) growing on different sites throughout Finland.
Typically, the models are evaluated using statistical analyses. However, the statistical tests do not provide information about the performance of the models outside the testing data. In this study, due to the lack of representative empirical data, the biomass models were evaluated using other methods than statistical tests. Available models for the estimation of biomass at tree level were mapped using literature. The representativeness of the biomass models was evaluated by studying the structure of modeling data, capability of the models to describe different components of a tree and capability of used independent variables to describe the biomass of different components of a tree. The outputs of the selected Marklund’s (1988) and Hakkila’s (1972a, 1979, 1991) models were evaluated in relation to each other and compared with other studies. The comparisons were made on mineral soils and on peatlands, on fertile and on infertile mineral soils and in Southern, and in Northern Finland both. Furthermore, in a case study, the models were incorporated into the MELA forest planning system, and the applicability of the models for the large-scale calculations was analyzed. In the analyses, optimization was utilized to point out differences between the outputs calculated using different sets of models.
Marklund’s (1988) models proved to be more applicable than Hakkila’s (1972a, 1979, 1991) models for the estimation of biomass of different components of trees. The data behind Marklund’s (1988) models were the most representative compared to other models available for biomass calculations at tree level. Although the data were collected from Sweden, the variability in the data covers well the variability of the site conditions and structure of tree populations in Finland, too. Despite the wider range of growing conditions included into Marklund’s (1988) data, based on comparisons between the outputs of Marklund’s (1988) and Hakkila’s (1972a, 1979, 1991) models, Marklund’s (1988) models are also applicable in Finland. The models for different biomass components are derived from the same sample trees of pine and spruce for most of the components except the finest fraction of roots. Furthermore, Marklund (1988) has formulated models for different components of birch excluding stump, roots and leaves. In addition, from Marklund’s (1988) models for the biomass of above-ground components of pine and spruce it was possible to get full sets having both breast height diameter and height as independent variables. Excluding the living branches, Marklund (1988) had also models having both breast height diameter and height for the modeled above-ground components of birch. Marklund’s (1988) models provided acceptable estimates for the biomass of different components of trees over the whole diameter range irrespective of the tree species.
When the total above-ground biomass of trees is considered, Marklund’s (1988) models produce logical estimates throughout Finland. The models are at their best in regularly managed stands dominated by Scots pine. With regard to single components of a tree, the biomass models are the most applicable for the estimation of the biomass of the stem wood and that of stem bark. The uncertainty concerning the outputs of the models increases, when the deviation of the structure of the data used for the estimation of biomass of trees from the original modeling data increases. Based on this study, the performance of the models in terms of the biomass of stem wood and stem bark is realistic as regards the location and the fertility of site, but much more uncertainty is involved in the estimation of biomass of other components of a tree. All these results about the performance of Marklund’s (1988) models must also be taken into account, when the outputs of the models are used as a goal or a constraint in the optimization.
Based on the analyses made in this study, Marklund’s (1988) models are more applicable for the estimation of carbon sequestration of the above-ground components of trees than for the estimation of energy wood resources. Most of the above-ground components of trees consists of stem wood, for which Marklund’s (1988) models produced realistic outputs. The energy wood consists mostly of living branches, and the biomass estimates of the models for living branches were more unreliable.
As a conclusion, the evaluation of the models made in this study facilitated the determination of the model structure. Furthermore, it was possible to identify the special feature of the model performance, with an increase in understanding how the set of biomass models were functioning at different level of applicability (tree, stand, forest area). The evaluation produced information about the realism and generality of the model outputs, but the study of accuracy would have demanded empirical data. The lack of knowledge could be identified in order to direct the future studies to fill the gaps in the knowledge. Also the uncertainties in the model calculations could be identified. In this study, the methods were represented for the systematization of the evaluation of the set of biomass models for different components of a tree. The evaluation of the set of models for different components of a tree demanded a versatile study of the models in relation to each other. The methods used in this study were based on more efficient utilization of existing data and research results than usually have been made in the evaluation of the models. Although the statistical tests would not be possible for the evaluation of the models, this study showed that useful information about the performance of the models could be obtained using other evaluation methods.
Links: Media release 6.5.2005 (in Finnish), Centre of Excellence for Forest Ecology and Management, FIGARE Globaalimuutoksen tutkimusohjelma