Stand-level Approach Research Articles

Precision thinning – a comparison of optimal stand-level and pixel-level thinning

ABSTRACT Precision forestry allows decision-making on tree level or pixel level, as compared to stand-level data. However, little is known about the importance of precision in thinning decisions and its long-term effects on within-stand variation, stand economy and growth. In this study, silviculture was optimized for Net Present Value (NPV) in 20 conifer-dominated forest stands in hemi-boreal southern Sweden. The precision-thinning approach, Precision Thinning (PT), is compared with a stand-level approach, Stand Level Thinning (SLT) that is optimized for the same criteria but based on stand-level data. The results suggest no substantial long-term benefit or drawback in implementing thinning decisions based on pixel-level data as compared to stand-level data when optimizing stand economy. The result variables NPV and Mean annual increment of living stem volume (MAInet) were not higher for PT than for SLT. The within-stand variation in basal area (m2/ha−1) was lower at the end of the rotation compared to the start of the simulation for both SLT and PT. At the end of the rotation, SLT had higher variation in basal area compared to PT. However, pixel-level information enables adapting the silviculture to the within-stand variation which may favour other forest management goals than strictly financial goals.

Modelling aboveground biomass and fuel load components at stand level in shrub communities in NW Spain

Shrub-dominated ecosystems cover large areas globally and play essential roles in ecological processes. Aboveground biomass expressed on an area basis (AGB) is central to many of the ecological processes and services provided by shrublands and is important as the main fuel source for wildfires. Hence, its accurate estimation in shrublands is crucial for ecologists and land managers. This is especially relevant in fire-prone regions such as NW Spain, where shrublands are an important part of the landscape, providing multiple services, but are severely impacted by wildfires. Although biomass models are available for numerous shrub species at the individual plant level, operational models based directly on easily measured shrub stand attributes are scarce. In this study, equations for estimating AGB and loads of different fuel components by size and condition (live and dead) from stand biometric variables were developed for the nine most prevalent shrub communities in NW Spain. Non-linear iterative seemingly unrelated regression was used to fit compatible systems of equations for estimating fuel loads, with shrub stand height and cover and litter depth as predictors for individual shrub communities and all data combined. In general, the goodness-of-fit statistics indicated that the estimates were reasonably accurate for all communities (grouped and ungrouped). The best results were obtained for AGB and total fuel load, including litter, whereas the poorest results were obtained for standing live and dead fine fuel load. Model performance was reduced when height was the only independent variable, although the reduction was small for most fuel categories, except litter load for which the variability was adequately explained by the litter depth. These results illustrate the feasibility of the stand level approach for constructing operational models of shrub fuel load that are accurate for most of fuel components, while also highlighting the ongoing challenges in live and dead fine fuel modelling. The equations developed represent an appreciable advance in shrubland biomass assessment in the region and areas with similar characteristics and may be instrumental in generating fuel maps, fire management improvement and better C storage assessment by vegetation, among other many uses.

Decision Support Tools and Strategies to Simulate Forest Landscape Evolutions Integrating Forest Owner Behaviour: A Review from the Case Studies of the European Project, INTEGRAL

For forest sustainability and vulnerability assessment, the landscape scale is considered to be more and more relevant as the stand level approaches its known limitations. This review, which describes the main forest landscape simulation tools used in the 20 European case studies of the European project “Future-oriented integrated management of European forest landscapes” (INTEGRAL), gives an update on existing decision support tools to run landscape simulation from Mediterranean to boreal ecosystems. The main growth models and software available in Europe are described, and the strengths and weaknesses of different approaches are discussed. Trades-offs between input efforts and output are illustrated. Recommendations for the selection of a forest landscape simulator are given. The paper concludes by describing the need to have tools that are able to cope with climate change and the need to build more robust indicators for assessment of forest landscape sustainability and vulnerability.

Species proportions by area in mixtures of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.)

Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) dominate many of the European forest stands. Also, mixtures of European beech and Scots pine more or less occur over all European countries, but have been scarcely investigated. The area occupied by each species is of high relevance, especially for growth evaluation and comparison of different species in mixed and monospecific stands. Thus, we studied different methods to describe species proportions and their definition as proportion by area. 25 triplets consisting of mixed and monospecific stands were established across Europe ranging from Lithuania to Spain in northern to southern direction and from Bulgaria to Belgium in eastern to western direction. On stand level, the conclusive method for estimating the species proportion as a fraction of the stand area relates the observed density (tree number or basal area) to its potential. This stand-level estimation makes use of the potential from comparable neighboring monospecific stands or from maximum density lines derived from other data, e.g. forest inventories or permanent observations plots. At tree level, the fraction of the stand area occupied by a species can be derived from the proportions of their crown projection area or of their leaf area. The estimates of the potentials obtained from neighboring monospecific stands, especially in older stands, were poorer than those from the maximum density line depending on the Martonne aridity index. Therefore, the stand-level method in combination with the Martonne aridity index for potential densities can be highly recommended. The species’ proportions estimated with this method are best approximated by the proportions of the species’ leaf areas. In forest practice, the most commonly applied method is an ocular estimation of the proportions by crown projection area. Even though the proportions of pine were calculated here by measuring crown projection areas in the field, we found this method to underestimate the proportion by 25% compared to the stand-level approach.

Development and evaluation of individual tree- and stand-level approaches for predicting spruce-fir response to commercial thinning in Maine, USA

Commercial thinning is a widely used silvicultural method in the spruce-fir (Picea-Abies) forests of the northeastern USA, but limited work has been done on predicting both short- and long-term individual tree- and stand-level responses to this treatment. Based on annual inventories of permanent plots from a replicated thinning experiment in Maine, USA, stand- and individual tree-level treatment response functions for spruce-fir stands were developed and evaluated. The developed thinning modifiers were shown to significantly improve predictions of annual stand-level basal area growth and mortality as well as species-specific, individual tree-level annual diameter increment, height to crown base increment, and mortality. For these response variables, duration and magnitude of the response functions were significantly influenced by thinning intensity and to a lesser extent by thinning method. In contrast, modifiers for stand-level dominant height increment as well as tree-level height increment did not show significant improvement when compared to the baseline model. When the developed modifiers were included in stand- and individual-tree growth models, a significant improvement in prediction over the baseline models was achieved, but the individual-tree approach was superior to predicting long-term response to various alternative thinning treatments. Overall, the study highlights the advantage of including thinning modifiers in growth and yield models, even for relatively shade-tolerant species, and the effectiveness of an individual-tree approach in structurally diverse and mixed-species stands.

Leaf area index estimation in mountain even-aged Pinus silvestris L. stands from hemispherical photographs

In this study a stand level approach is proposed for estimating leaf area index (LAI) in even-aged Scots pine stands using gap fraction data derived from hemispherical photographs. The approach includes both the effect of the spatial distribution of the foliage elements as well as the slope of the terrain. Two chronosequences consisting of 11 plots were established in two Scots pine ( Pinus sylvestris L.) forests in the Central Mountain Range of Spain. 12 hemispherical photographs were taken in each plot and variables related to the stage of development, density, relative spacing and site index were calculated. The gap frequency Poisson model was used to estimate the leaf area index. A new function is proposed to assess the effect of the foliage inclination on light transmittance for Scots pine, based on the assumption that the foliage elements show circular horizontal cross-sections. A new model was also developed to restrict the clumping effect on the vertical component in the Poisson model. The effect of the slope on LAI estimates was determined and included in the model. The results indicate that both the clumping effect correction and the slope correction improve the fitting of the Poisson model to the gap frequency experimental curves, and that the function proposed for the inclination angle correction may provide an acceptable approach for some coniferous species. The relationship between the parameters of the gap fraction model (the foliage inclination angle and the clumping coefficient) and stand level variables, that is, the stage of development, density, spacing of the stand and the site index, is analysed.

A variance-covariance structure to take into account repeated measurements and heteroscedasticity in growth modeling

This study proposes a within-subject variance-covariance (VC) structure to take into account repeated measurements and heteroscedasticity in a context of growth modeling. The VC structure integrates a variance function and a continuous autoregressive covariance structure. It was tested on a nonlinear growth model parameterized with data from permanent sample plots. Using a stand-level approach, basal area growth was independently modeled for red spruce (Picea rubens Sarg.) and balsam fir [Abies balsamea (L.) Mill.] in mixed stands. For both species, the implementation of the VC structure significantly improved the maximum likelihood of the model. In both cases, it efficiently accounted for heteroscedasticity and autocorrelation, since the normalized residuals no longer exhibited departures from the assumptions of independent error terms with homogeneous variances. Moreover, compared with traditional nonlinear least squares (NLS) models, models parameterized with this VC structure may generate more accurate predictions when prior information is available. This case study demonstrates that the implementation of a VC structure may provide parameter estimates that are consistent with asymptotically unbiased variances in a context of nonlinear growth modeling using a stand-level approach. Since the variances are no longer biased, the hypothesis tests performed on the estimates are valid when the number of observations is large.

A review of stand basal area growth models

Growth and yield modeling has a long history in forestry. The methods of measuring the growth of stand basal area have evolved from those developed in the U.S.A. and Germany during the last century. Stand basal area modeling has progressed rapidly since the first widely used model was published by the U.S. Forest Service. Over the years, a variety of models have been developed for predicting the growth and yield of uneven/even-aged stands using stand-level approaches. The modeling methodology has not only moved from an empirical approach to a more ecological process-based approach but also accommodated a variety of techniques such as: 1) simultaneous equation methods, 2) difference models, 3) artificial neural network techniques, 4) linear/nonlinear regression models, and 5) matrix models. Empirical models using statistical methods were developed to reproduce accurately and precisely field observations. In contrast, process models have a shorter history, developed originally as research and education tools with the aim of increasing the understanding of cause and effect relationships. Empirical and process models can be married into hybrid models in which the shortcomings of both component approaches can, to some extent, be overcome. Algebraic difference forms of stand basal area models which consist of stand age, stand density and site quality can fully describe stand growth dynamics. This paper reviews the current literature regarding stand basal area models, discusses the basic types of models and their merits and outlines recent progress in modeling growth and dynamics of stand basal area. Future trends involving algebraic difference forms, good fitting variables and model types into stand basal area modeling strategies are discussed.

Ecologically-based forest planning and management for aquatic ecosystems in the Duck Mountains, Manitoba

Current forest management guidelines require the extensive use of stand level approaches to minimize impacts on aquatic ecosystems. As forest management practices in Canada evolve from a sustained yield, timber-based focus to a more sustainable approach, the need has been recognized for the addition of landscape or watershed-based management strategies to address the cumulative effects of management practices on aquatic systems. Additional concerns expressed by regulatory agencies have resulted in the need to restrict cumulative harvest rates and monitor harvest levels for a forest management operation in Manitoba. Present riparian management practices and watershed analysis procedures are reviewed for the operations of Louisiana-Pacific Canada in Manitoba. Requirements are outlined for multi-scale indicators and research to support the development of a watershed-based planning approach for use in forest management planning and monitoring. The development of this watershed-based approach is linked to the Forest Watershed and Riparian Disturbance (FORWARD) project. Key words: riparian management, watershed management, natural disturbance, forest harvest.

Resistance and elasticity: promising concepts for the management of protection forests in the European Alps

Many mountain forests in the European Alps protect people and assets against natural hazards such as snow avalanches. These forests are mainly managed to ensure continuous and effective protection. Silvicultural operations should make protection forests as resistant as possible to natural disturbing agents such as wind that have the potential to impair effective protection, and they should increase their elasticity, i.e. speed of recovery once disturbances have impaired the protective effect. However, current management approaches do not yet consistently focus on enhancing resistance and elasticity in the face of multiple disturbances. Moreover, they use a stand level approach instead of an ecosystem-based approach. These shortcomings can be overcome by explicitly integrating resistance and elasticity into forest management. In this paper, the meaning of resistance and elasticity is clarified and illustrated with examples. A procedure to integrate resistance and elasticity into forest management is presented, with the five steps: Identification of disturbances and slow undesirable changes, identification of characteristics relevant to the resistance and elasticity of a forest to disturbances, identification of variables for monitoring these characteristics, establishment of target values for each variable, and implementation including monitoring. Suggestions to quantify resistance and elasticity are made.

Growth and yield models for uneven-aged stands: past, present and future

Growth and yield modeling has a long history in forestry. Methods of measuring the growth of uneven-aged forest stands have evolved from those developed in France and Switzerland during the last century. Furthermore, uneven-aged growth and yield modeling has progressed rapidly since the first models were pioneered by Moser and Hall (1969) (Moser Jr., J.W., Hall, O.F., 1969. For. Sci. 15, 183–188). Over the years, a variety of models have been developed for predicting the growth and yield of uneven-aged stands using both individual and stand-level approaches. Modeling methodology not only has moved from an empirical approach to a more ecological process-based mechanistic approach, but also has incorporated a variety of techniques, such as, (1) systems of equations, (2) nonlinear stand table projections, (3) Markov chains, (4) matrix models, and (5) artificial neural network models. However, modeling the growth and yield of uneven-aged stands has received much less attention than that of even-aged stands. This paper reviews the current literature regarding growth and yield models for uneven-aged stands, discusses basic types of models and their merits, and reports recent progress in modeling the growth and dynamics of uneven-aged stands. Furthermore, future trends involving integration of new computer technologies (object-oriented programming and user-friendly interfaces), tree visualization techniques, and the spatially explicit application of Geographical Information Systems (GIS) into uneven-aged modeling strategies are discussed.

The UMF-index: an indicator to compare silvicultural practices at the forest or forest estate level

For more than two centuries forests in Europe have become increasingly shaped by a silviculture favouring pure, even-aged stands. Originally this was introduced to safeguard and restore wood production from remnants of a forest resource marked by heavy, unregulated exploitation. In response to developments in society, in particular regarding contemporary ideas on the sustainable use of natural resources, the current trend in forestry in Europe is towards a forest management based on natural processes. The silvicultural challenge is to create and maintain forest types that favour both production and environmental targets, i.e. yielding a high and valuable production, while simultaneously encouraging natural or close-to-natural biological processes. It is believed that such forest types, as a side effect, consider recreational, cultural and amenity values better than does ‘traditional’ plantation-oriented silviculture. In the temperate deciduous forest zone a more or less mixed broadleaved forest is considered the potential natural vegetation. Here, silviculture should move away from pure, even-aged stands managed under the clearcutting system with concentrated regeneration, restricted to a well-defined period and often occurring simultaneously over a relatively large area, towards a mixed, unevenaged forest with regeneration diffuse in space and time, occurring in minor forest gaps continuously throughout the rotation period. In terms of harvesting criteria this is a change from a stand level approach towards a single-tree philosophy. In time this development will lead to a forest that differs profoundly from the well known ‘normal forest’. As part of this undertaking, one of the challenges for forest research will be to deal with and synthesize findings from large-scale experiments and offer overall appraisals of management alternatives. In Denmark, such projects were initiated around 1995 (Skovsgaard et al., 1997). So, while accurate and detailed stand descriptions remain relevant, it is becoming increasingly important to characterize forest types and silvicultural practices at an aggregate level comprising a whole forest or forest estate. In this paper a simple indicator for this purpose is proposed and discussed.

Changements de productivit� dans quatre for�ts de ch�nes sessiles depuis 1930 : une approche au niveau du peuplement

Productivity changes in four Sessile Oak forests since 1930: a stand-level approach. We addressed the topic of long- term growth trends by using stand-level data gained in a network of permanent plots measured since 1930 in 4 Oak forests of north- ern France. These forests spread along a climatic gradient, from the atlantic sites (Normandie) to the more continental sites (Lorraine). In each forest, 10 plots have been observed, in stands of various ages, and submitted to different silvicultures. We analysed dominant height and stand basal area increments. The modeling of height-age curves revealed different curve shapes between forests. In 2 forests out of 4, the residuals exhibit a very strong structure with date: growth rate has steadily increased since the 1930s. Basal area increment was modelled with much more details, taking into account the effects of age, stand density, periodic fluctuations and different growth levels between plots. These effects are combined with a smooth trend function of date, the ampli- tude of which was fitted on a per forest basis. Several models were compared and we discussed the sensitivity of trends with regard to the underlying model. Whatever the model, we could estimate trends of considerable amplitude: most of them are increases of basal area growth of +25 to +50% between 1930 and 1990. Our results confirm very closely those gained by dendrochronological methods. Curiously, Oak stands from Normandie have a more complex trend: decreasing productivity between 1930 and 1960, then increase until now. Quercus petraea(Matt.) Liebl. / growth trend / global change / forest stand yield / growth model