Sven Wagner

Estimating the fruit dispersion of anemochorous forest trees

Abstract This paper studies the problem of modelling fruit dispersal of anemochorous forest trees by the statistical analysis of fruit collections in traps. The statistical model of fruit dispersion and the choice of trap positions turn out to be very important. It is shown that the lognormal distribution of ‘fruit–tree’ distance seems to be of great practical interest. The trap positions have considerable influence on the precision of the estimates, particularly on the total number of fruits per tree.

Calibration of grey values of hemispherical photographs for image analysis

The hemispherical photography method is widely used for the estimation of solar radiation in forests or agricultural crops. However, calibration problems repeatedly arise concerning the grey-value information of the photographs. This article introduces a method which may help to solve these calibration problems. The method takes into consideration two information transport media (black-and-white film and digital image analysis). This method is based on the exposure of the photographs to the zenith luminance of an overcast sky and the determination of the gamma function of the film by means of an optical density wedge photograph. By taking these characteristics into account, it is possible to obtain reproducible and bias-free results from the diffuse site factor even with different film materials. Moreover, the modelling of penumbral effects is shown. Limitations are discussed.

Canopy effects on vegetation caused by harvesting and regeneration treatments

This paper reviews the primary effects of canopy on understorey vegetation to provide insight into the management of the canopy space to benefit tree regeneration. Site conditions, like nutrient and water availability, overstorey conditions, e.g. tree species, and canopy density, are important determinants of ground vegetation. An investigation of canopy effects is concerned about how the canopy trees modify site conditions. As canopy density may be deliberately modified during regeneration treatments, the effect of canopy density on individual species in the herbaceous layer and tree regeneration is important. This autecological perspective focuses on the successional traits of species to help understand species differences in fecundity, survival, density and growth. From a synecological perspective, the importance of successional traits for the outcome of competition between species arising from differences in canopy densities is highlighted. This review shows that moderately dense canopies may favour tree regeneration over aggressive shade-intolerant graminoids or forbs. This is particularly true for shade-tolerant and intermediate shade-tolerant tree species. To better understand and utilise this phenomenon, research should try to identify and isolate different canopy effects.

Relative radiance measurements and zenith angle dependent segmentation in hemispherical photography

Abstract When analysing hemispherical photographs taken from forest stands the grey value of the smallest picture unit — so-called ‘pixel’ — is taken as a measure of transmissivity. This derived information of transmission is based on the grey value of unobscured pixels which serves as a reference value. However, a particular grey value of unobscured pixels is actually not constant for all pixels of a photograph; but is influenced by (i) sky luminance heterogeneity and (ii) vignetting properties of lenses. A method to assess the two factors mentioned is proposed. This method takes into account the film-specific relation between received relative radiation energy and the films grey value. The relation is constant for a particular film and can be determined by photographing an optical density wedge on each film. The method is applied in vignetting measurements on two custom ‘fisheye’ lenses which reveal significant differences in the properties of the lenses as well as for particular aperture openings. Consecutive relative radiance measurements on photographs taken under overcast skies gave consistent sky luminance distributions with this method compared to standard distributions from literature. Moreover, the method gives reasonable relative radiance values from photographs of increasing degree of over-exposure taken under identical skies. An instruction for deriving zenith angle dependent reference values for unobscured sky within one single photograph taken on an overcast day is given which can also be applied to digital camera device.

Forest Management Approaches for Coping with the Uncertainty of Climate Change: Trade-Offs in Service Provisioning and Adaptability

The issue of rapid change in environmental conditions under which ecosystem processes and human interventions will take place in the future is relatively new to forestry, whereas the provision of ecosystem services, e.g., timber or fresh water, is at the very heart of the original concept of forest management. Forest managers have developed ambitious deterministic approaches to provide the services demanded, and thus the use of deterministic approaches for adapting to climate change seem to be a logical continuation. However, as uncertainty about the intensity of climate change is high, forest managers need to answer this uncertainty conceptually. One may envision an indeterministic approach to cope with this uncertainty; but how the services will be provided in such a concept remains unclear. This article aims to explore the fundamental aspects of both deterministic and indeterministic approaches used in forestry to cope with climate change, and thereby point out trade-offs in service provisioning and adaptability. A forest owner needs to be able to anticipate these trade-offs in order to make decisions towards sustainable forest management under climate change.

An approach for modelling the mean fine root biomass of Norway spruce stands

The applicability of a heuristic model for estimating mean fine-root biomass of Norway spruce stands based on the coordinates and the diameters at breast height (diameter at a height of 1.3 m, dbh) of their trees was tested. The model was developed based on the following assumptions which were derived from the literature: (1) the maximum distance the roots of a tree can be found depends on the dimension of the tree and exceeds the edges of the crown; (2) fine-root biomass decreases with increasing distance from the tree trunk; (3) fine-root biomass increases with the dbh; (4) maximum fine-root biomass of a tree is not allocated directly around the tree’s trunk but at some distance from the stem. On the basis of these assumptions the model calculates a relative fine-root biomass at a given point within a stand. Four different versions of the model were compared, with each version differing with respect to the assumed decrease in fine roots with decreasing dbh and the approaches used to calculate the contribution of a subject tree to the fine-root biomass at a given point within a stand (additive versus consumptive). Using regression analysis we parameterised each model type with the data of 70 soil cores from a 75-year-old Norway spruce stand in southern Germany (Bavaria). The relative fine-root biomass calculated by the four different model types accounted for 62–72% of the variation of the measured fine-root biomass. The parameterised models were used to predict the fine-root biomass of 60 given points of a second Norway spruce stand based on its dbhs and stem coordinates. The comparison of measured and predicted mean fine-root biomasses of the second stand revealed no significant differences between the measured mean and the means estimated by three of the four model types. Whereas with two of the model types we achieved means and medians, respectively, nearly identical to the measured average, none of the model types was able to predict values as high as the measured maximum. Constraints of the models and points that need to be considered regarding the minimum number of soil cores needed for a reliable parameterisation of the model are discussed.

Hemispheric image modeling and analysis techniques for solar radiation determination in forest ecosystems.

Hemispheric image processing with the goal of solar radiation determination from ground-based fisheye images is a valuable tool for silvicultural analysis in forest ecosystems. The basic idea of the technique is taking a hemispheric crown image with a camera equipped with a 180° fisheye lens, segmenting the image in order to identify solar radiation relevant open sky areas, and then merging the open sky area with a radiation and sun-path model in order to compute the total annual or seasonal solar radiation for a plant. The results of hemispheric image processing can be used to quantitatively evaluate the growth chances of ground vegetation (e.g., tree regeneration) in forest ecosystems. This paper shows steps towards the operationalization and optimization of the method. As a prerequisite to support geometric handling and georeferencing of hemispheric images, an equi-angular camera model is shown to describe the imaging geometry of fisheye lenses. The model is extended by a set of additional parameters to handle deviations from the ideal model. In practical tests, a precision potential of 0.1 pixels could be obtained with off-the-shelf fisheye lenses. In addition, a method for handling the effects of chromatic aberration, which may amount to several pixels in fisheye lens systems, is discussed. The central topic of the paper is the development of a versatile method for segmenting hemispheric forest crown images. The method is based on linear segmentoriented classification on radial profiles. It combines global thresholding techniques with local image analysis to ensure a reliable segmentation in different types of forest under various cloud conditions. Sub-pixel classification is incorporated to optimize the accuracy of the method. The performance of the developed method is validated in a number of practical tests.

Evaluation of different approaches for modelling individual tree seedling height growth

We compared different approaches for modelling height growth of individual beech seedlings in a controlled factorial experiment as well as in field data from naturally regenerated beech seedlings under the canopy of overstorey mature beech trees. Several competition indices, a model of overstorey fine root density, relative photosynthetically active radiation (PAR) values, and soil water values were used in these approaches. In the factorial experiment relative PAR and soil water content were measured and used for the prediction of seedlings height growth. In the field experiment this was done by using relative PAR and estimated fine root biomass as a surrogate for below ground resource availability. The latter approach was compared with a model where we used various competition indices representing the impact of overstorey trees on beech seedlings. Our results suggested that (1) models which combine resource based growth functions are suitable for the prediction of individual height growth of beech seedlings. Resource based models offer the opportunity to investigate on the independent multiplicative effect of irradiance and water supply and their interactions on tree seedlings. It was (2) shown that a combined model could be used not only to predict individual height growth of beech seedlings in a controlled experiment but also in the field. The model parameters of a pure light response function for the controlled factorial experiment are comparable to those obtained in the field study. The results showed (3) that the precision of predicting beech seedlings height growth is comparable between the model types tested within this study. Approximately half of the observed variation in seedlings relative height growth rate could be explained. However, the simple competition index approach provides no information on the environmental factors constraining tree seedlings growth; whereas the multiplicative combined models can be used to get a better understanding of growth dynamics in the field.

Bucheckern-Voraussaat unter Fichtenschirm – Ergebnisse eines Versuchs des Deutschen Verbandes Forstlicher Forschungsanstalten/Sektion Waldbau

Zusammenfassung:Innerhalb der Sektion Waldbau des Deutschen Verbandes Forstlicher Forschungsanstalten wurde ein Bucheckern-Voraussaat-Versuch (Fagus sylvatica L.) unter Fichtenschirm (Picea abies [L.] Karst.) angelegt. Die Ergebnisse auf drei Versuchsstandorten (Arnsberg/Nordrhein-Westfalen; Tharandt/Sachsen und Freising/Bayern) werden dargestellt und analysiert. Während der ersten drei Vegetationsperioden werden auf den nach gleichem Versuchsdesign angelegten Versuchsflächen der Einfluss der Grundflächenhaltung des Fichtenschirms und einer durchgeführten Plätze-Kalkung auf die Etablierung und weitere Entwicklung der gesäten Buchen dokumentiert. Der Einfluss der Grundflächenhaltung des Oberbestandes auf die Pflanzenzahl je Platz ist im Bereich des Grundflächenintervalls 35,0 – 55,0 m2 durch eine Abnahme der Pflanzenzahlen während der drei Beobachtungsjahre gekennzeichnet. Die Versuchsstandorte unterscheiden sich signifikant in diesem Trend. Eine gleichzeitig oder kurz vor der Saat durchgeführte Kalkung beeinflusst den Verlauf der Pflanzenzahlabnahme nicht. Die auftretende Varianz der Buchenlängen werden durch die unterschiedliche Grundflächenhaltung des Oberbestandes der Versuchsparzellen erklärt. Weder die Kalkung noch der Versuchsstandort haben einen absicherbaren Effekt auf die Buchenlängen. Aus den bisher vorliegenden Ergebnissen und Erfahrungen bei der Versuchsanlage werden Schlussfolgerungen für die waldbauliche Praxis bei der Planung und Durchführung von Buchensaaten abgeleitet.Summary:An experiment on direct seeding of European beech (Fagus sylvatica L.) under the canopy of old Norway spruce stands (Picea abies [L.] Karst.) was initiated by the German Union of Forest Research Organizations (Silviculture Division). The experiment consists of three study sites located in different parts of Germany (Arnsberg/North Rhine-Westphalia, Tharandt/Saxonia, Freising/Bavaria). Based on a nearly identical experimental design the study tried to identify the impact of the basal area of the spruce shelter and the effect of liming on survival and length growth of the seedlings over a period of 3 years. During the three years of observation a canopy covering an area between 35.0 and 55.0 m2 caused a decrease of the number of beech tree specimens per plot. The experimental plots show significant differences in regard to this trend. The plant reduction trend is not affected by the liming of the plots prior to or simultaneously with seeding. The different areas of coverage of the tree canopy do have an affect on the variation of beech length on the different experimental plots. Neither liming nor the experimental sites themselves have a detectable effect on beech length. The results and experiences with experimental design contribute to planing and realization of beech seeding in the silvicultural.

Spatial optimization for dispersion of remnant trees in seed-tree cuttings and retention-tree stands of Scots pine

Abstract In this study, optimal spatial patterns for seed-tree cuttings and green-tree retention in Scots pine stands were investigated. The optimization approach is based on functions that describe the effects of single, retained trees on Scots pine regeneration. Two spatial optimization techniques were applied to this intrastand-level problem. The model approach assumes smooth, non-convex optimization problems with a convex set of feasible solutions. These problems were solved with a deterministic optimization algorithm with 500 initial tree position sets, also taking into account different forest management objectives such as high, homogeneous seed and seedling density. This required the use of multicriteria optimization procedures. The best spatial patterns identified differed significantly according to the objective functions considered. This was demonstrated by aggregation index values of stem maps and from visualization. Thus, the objective functions must be defined carefully to obtain best solutions for the management aims. A small degree of clumping in seed-tree cuttings did not appear detrimental to seedling density objectives. Therefore, short-term retention trees should be selected primarily by phenotype, not solely by a specific distribution scheme.