Jens Peter Skovsgaard

Evaluating forest growth models

Effective model evaluation is not a single, simple procedure, but comprises several interrelated steps that cannot be separated from each other or from the purpose and process of model construction. We draw attention to several statistical and graphical procedures that may be used both with data used for model calibration and with data used in the evaluation of the model. We emphasize that the validity of conclusions depends on the validity of assumptions. These principles should be kept in mind throughout model construction and evaluation.

Evaluating a growth model for forest management using continuous forest inventory data

Abstract Model evaluation should include qualitative as well as quantitative examinations of the model. The qualitative parts should comprise a critical appraisal of model logic as well as theoretical and biological realism of the model. The quantitative parts should comprise statistical tests and comparisons of predictions with observations independent of those used to fit the model. Comprehensive model evaluation requires several alternative approaches and criteria. Model evaluation is not one simple procedure, but consists of a number of interrelated steps that should not be separated from each other or from model construction. It is stressed that models can only be evaluated in relative terms, and their predictive value is always open to question. Thus, model evaluation is an ongoing process. A case study with the PBRAVO growth model for maritime pine (Pinus pinaster Ait.) in the Leiria forest, Portugal, illustrates the utility of selected criteria and graphical techniques. Based on theoretical examinations and tests with data from continuous forest inventories, we conclude that the Leiria version of the PBRAVO model does not adequately represent reality and that forecasts lack sufficient accuracy for forest management purposes.

Recommendations for Standardized Documentation and Further Development of Forest Growth Simulators

Summary:The present paper documents the practical applicability of growth simulators, gives an overview of current model approaches, defines standards for the description and evaluation of growth models and growth simulators and indicates research needs. The recommendations aim to give users of growth simulators confidence in the transition to modern prediction systems and to increase the level of acceptance of new information technologies. The recommendations provide developers with guidelines for model description, model evaluation and software development and thus contribute towards efficient co-operation.Zusammenfassung:Der Beitrag dokumentiert die praktische Relevanz von Wachstumssimulatoren, gibt einen Überblick über aktuelle Modellansätze, definiert Standards zur Beschreibung und Evaluierung von Wachstumsmodellen und -simulatoren und weist auf Forschungsbedarf in diesem Bereich hin. Die Empfehlungen sollen Anwendern von Wachstumsmodellen Vertrauen beim Übergang zu zeitgemäßen Prognosesystemen vermitteln und die Akzeptanz neuer Informationstechnologien erhöhen. Indem dieses Papier Modellentwicklern Leitlinien für die Modellbeschreibung, Modellevaluierung und Softwareentwicklung gibt, kann es zu einer effizienten Kooperation aller Beteiligten beitragen.

Distribution of biomass and carbon in even‐aged stands of Norway spruce (Picea abies (L.) Karst.): A case study on spacing and thinning effects in northern Denmark

The main objective of this case study was to explore the possible influence of forest management on the levels and distribution of biomass and carbon (C) in even-aged stands of Norway spruce [Picea abies (L.) Karst.] in Denmark. Data originated from a long-term thinning experiment and an adjacent spacing experiment at stand ages of 58 and 41 years, respectively. Biomass of 16 trees from different thinning and spacing treatments was measured or partly estimated, and soils were sampled for determination of C stocks. All trees in each plot were measured for stem diameter and some for total height, to allow for scaling-up results to stand-level estimates. For trees of similar size, foliage biomass tended to be higher in the spacing experiment, which was located on slightly more fertile land. Foliage biomass increased with increasing thinning grade, but the effect could not be separated from that of tree size. At stand level, foliage biomass tended to increase with increasing spacing as well as with increasing thinning grade. For branchwood, stems and roots (including below-ground stump), the biomass increased with increasing tree size and stand volume at tree and stand level, respectively, but no differences between stands, spacings or thinning grades were observed, apart from that expressed by tree size or stand volume. At stand level, C stocks of all biomass compartments decreased with increasing thinning grade, while the distribution between compartments was hardly influenced. The ratio between above-ground and stem biomass was about 1.21 at stand level, while the ratio between below- and above-ground biomass was about 0.17. Thinning influenced the C stock of the forest floor and mineral soil oppositely, resulting in no effect of thinning on total soil C.

Assessing the quality of permanent sample plot databases for growth modelling in forest plantations

Informed plantation management requires a good database, since the quality of information depends on the quality of data, growth models and other planning tools. There are several important questions concerning permanent plots: how many plots, where to put them, and how to manage them. Plot measurement procedures are also important. This paper illustrates graphical procedures to evaluate existing databases, to identify areas of weakness, and to plan remedial sampling. Two graphs, one of site index versus age, another with stocking versus tree size, may provide a good summary of the site and stand conditions represented in the database. However, it is important that these variables, especially site index, can be determined reliably. Where there is doubt about the efficacy of site index estimates, it is prudent to stratify the database according to geography, soil/geology or yield level (total basal area or volume production). Established permanent plot systems may sample a limited range of stand conditions, and clinal designs are an efficient way to supplement such data to provide a better basis for silvicultural inference. Procedures are illustrated with three data sets: teak plantations in Burma, Norway spruce in Denmark, and a clinal spacing experiment in India.

Biomass, basic density and biomass expansion factor functions for European beech (Fagus sylvatica L.) in Denmark

The objective of this study was to analyse the within-tree allocation of biomass and to develop biomass functions for above- and below-ground components of European beech in Denmark. Separate functions were developed for stem, branches, below-ground stump and root system, total above-ground biomass and total tree biomass. For each of these components or aggregate components, models were also developed for the average basic density of wood and bark. To enhance the versatility of the models, a function for estimating the biomass expansion factor (BEF) was also developed. The functions were based on 66 trees measured for total biomass. Model performance was evaluated based on 74 trees measured only for above-ground biomass. The trees were sampled in 18 different forest stands covering a wide range of tree sizes and stand treatments. Models were estimated using a linear mixed-effects procedure to account for within-stand correlations. The functions for biomass and BEFs included only diameter at breast height and total tree height for individual trees as predictor variables. Inclusion of additional variables reflecting site quality or stand density did not improve model performance. The functions for basic density included individual tree diameter, tree height and quadratic mean diameter as predictor variables, indicating an effect of stand density on the basic density of wood and bark.

Site-specific height growth models for six common tree species in Denmark.

Abstract Site-specific height growth models were developed for European beech, pedunculate and sessile oak, Norway spruce, Sitka spruce, European silver fir and Douglas fir in Denmark. The models were derived based on base-age invariant methods to fit the three-dimensional surface of stand age, stand top height and site index in a manner that directly estimated site indices as parameters from the fitting procedure. The estimated base-age invariant models represented the observed age–height patterns well across the range of site conditions. For all species the model explained more than 98% of the observed variation in stand height development and exhibited no apparent bias across the range of predicted site indices. Compared to historical site index curves, the growth patterns expressed by the site-specific height growth models were considerably different, indicating that the historical site index curves were biased. The pattern of deviation depended on tree species and site conditions.

Functions for biomass and basic density of stem, crown and root system of Norway spruce (Picea abies (L.) Karst.) in Denmark

Abstract Models for predicting the biomass of forest trees are becoming increasingly important for assessing forest resources and carbon sequestration in forests. We developed functions for predicting the biomass and basic density of above- and below-ground parts of Norway spruce (Picea abies (L.) Karst.) in Denmark. Separate models were developed for branches (including foliage), stem and the below-ground stump and root system as well as for the aggregate components of total above-ground biomass and total tree biomass. Trees were sampled in 14 forest stands, reflecting the range of growth conditions and thinning practises of Norway spruce in Denmark. Because of inclusion of experimental plots, data reflected a wider range of thinning practices than commonly used in forestry practice. The data included measurements of biomass and basic density from 114 trees, two of which were regarded as outliers and consequently excluded in the final model estimation. The final models reflected known properties of tree growth and allocation of biomass among different tree components of even-aged Norway spruce. The models were successful in predicting biomass, basic density and biomass expansion factors across a wide variety of tree sizes, stand treatments and growth conditions. The models are believed to substantially improve national estimates of carbon sequestration and biomass resources.

Precommercial thinning of pedunculate oak: Recreational preferences of the population of Denmark for different thinning practices in young stands

Abstract The objective of this study was to investigate how thinning practice influences the preferences of adult Danes for forest recreation in young stands of even-aged oak. The study was based on five pairs of colour photographs from each of five recently thinned plots in a 13-year old experiment. The plots included 7000 stems ha−1 (unthinned), 5300 ha−1 (traditional thinning), 1000 ha−1, 300 ha−1 and 100 ha−1. All cut trees were left on the ground. The study was carried out as a national survey based on postal questionnaires (n=243, response 73%). Interviewees ranked the photographs according to the criterion: “Which forest environment do you prefer as a visitor?” Visitor accessibility was interpreted as being more important than stand density per se. Thinning slash was pointed out as a major factor for the impression of accessibility. Low to mid-range densities (300–5300 ha−1) were preferred over very dense (7000 ha−1) or very open (100 ha−1) stands.

Management of oak forests: striking a balance between timber production, biodiversity and cultural services

ABSTRACT Identification of the ecosystem services provided by oak-dominated forests in southern Sweden is a prerequisite for ensuring their conservation and sustainable management. These forests seem well-suited for multiple-use forestry, but knowledge is limited regarding how to manage them for multiple uses. Management for the production of high-value timber species like oaks and management to conserve biodiversity, or for cultural services can be in conflict with each other. This study evaluates the capacity of three contrasting management regimes to provide societies with economic revenue from timber production, habitats for biodiversity and cultural services, and the study analyses associated trade-offs and synergies. The three regimes were: intensive oak timber production (A), combined management for both timber production and biodiversity (B) and biodiversity conservation without management intervention (C). We synthesized relevant scientific literature, governmental statistics and grey literature. Our assessments identified that Regime A provided the highest levels of economic returns and the lowest level of biodiversity. Regime C provided higher levels of habitat provision but at expense of wood production and cultural services. In contrast, Regime B provided a balanced delivery of timber production, biodiversity conservation and cultural services. We identified several stand-management options which provide comparatively synergistic outcomes in ecosystem services delivery. The use of these management options in combination with more traditional stand-management approaches may be a more effective means of achieving sustainable forest goods and services. EDITED BY Nicholas Brokaw