Quantifying stand structure and structural complexity along a management gradient in temperate forests
von Melissa Stiers
Datum der mündl. Prüfung:2020-08-21
Erschienen:2020-10-01
Betreuer:Prof. Dr. Christian Ammer
Gutachter:Prof. Dr. Christian Ammer
Gutachter:Prof. Dr. Holger Kreft
Dateien
Name:eDiss_FINAL_OhneCV.pdf
Size:3.94Mb
Format:PDF
Zusammenfassung
Englisch
The structure and structural complexity of forests influence many important characteristics of forest ecosystems, as well as their functions and services, such as ecosystem stability, maintenance of biodiversity and carbon storage. Forest management affects the spatial structures of forests and thus has an impact on many of these services and functions offered by forest ecosystems. In this context, it is often discussed whether forest management has a reducing or promoting effect on the structural complexity of a forest stand. In order to answer this question, it is essential to gain a better understanding of the development, properties and dynamics of structural complexity in forests. This doctoral thesis will contribute to this by studying different aspects of structural complexity in forests using highly detailed, three-dimensional terrestrial laser scanning data. The first aim of this thesis was to quantify the structural complexity of forests along a gradient of management intensity in order to analyze the effects of forest management. In a first study (chapter 2), the structural complexity of traditionally and alternatively managed forests, lately unmanaged forests in German National Parks, and completely unmanaged primary forests of European beech (Fagus sylvatica L.) in the Western Carpathians was quantified using a stand structural complexity index (SSCI). It was found that structural complexity does not increase linearly with decreasing management intensity but that management can promote structural complexity. The lowest structural complexity was found in the lately unmanaged National Parks, while stands of younger developmental phases of traditional management do not differ significantly from the structural complexity found in one of the primary forests. Furthermore, differences in structural complexity could be identified between different phases of forest development. While the structural complexity in so-called “vault-like” forests, single-layered stands in the optimum phase, is minimal, it is increased by the multi-layered structures in, for example, thickets with overstory trees. The emergence of natural regeneration is decisive for the establishment of multiple stand layers and thus for the future structural development of a forest stand. Therefore, a second study (chapter 3) focused on the analysis of the structural complexity of natural regeneration of European beech and the identification of spatial distribution patterns of regeneration patches in dependence of canopy gap characteristics. Unravelling the mechanisms behind such spatial top-down-relationships between canopy gaps and natural regeneration is helpful to control and manage the regeneration’s composition and development. A significant positive relationship between gap size and the size of a regeneration patch was found in beech forests. However, no homogeneous, significant offset between the centers of the regeneration patch and the projected gap polygon could be identified, as was the case in literature for the regeneration of other, mostly light-demanding tree species. Furthermore, it could be shown that the mean regeneration height continuously decreases from positions within the projected gap polygon to positions under closed canopy in the adjacent stand. However, the largest plants were not located directly in the center of the gap polygon, but at the outer edges of the projected gap polygon. Furthermore, it was determined that natural regeneration of beech benefits from the higher amount of diffuse radiation outside the gap polygon, which is why it can be concluded that the emergence of natural regeneration is also promoted in the penumbral zone. Overall, these results once again confirmed the high shade-tolerance of beech. Therefore, we conclude that the effect of higher availability of direct or diffuse radiation in and around canopy gaps may be overruled by other factors, such as competition with mature trees. This thesis also aimed at identifying management systems that can lead to an increase in stand structural complexity. In a third study (chapter 4), the focus was therefore on quantifying the structural complexity of forests, which have been managed according to the guidelines of continuous cover forestry for several decades. We compared their structure with that of traditional age-class forests and completely unmanaged primary beech forests. Continuous cover forestry is of particular interest because it aims at a target state, which includes multi-layered, highly-structured forests, which fulfill both economic and social demands. In order to objectively quantify the structures of this target state, a new index for structural constancy (ISC) was developed. In addition, already established indices for the description of the spatial forest structure were calculated. The new ISC was able to distinguish continuous cover forests and even-aged age-class forests. However, we were not able to detect a significant difference between the continuous cover forests and the primary beech forests as natural reference. Overall, it could therefore be concluded that continuous cover forestry is capable of creating forest stands of high structural complexity. Finally, based on the results of the three studies presented here, we derived management recommendations, which intend to enable forest managers to promote structural complexity in forests. In order to generate a high degree of vertical and horizontal heterogeneity, management methods should be chosen that create differently sized and shaped canopy gaps to diversify growth conditions. To promote structural complexity not only at stand level, but also at larger, regional scale, structural heterogeneity between neighboring stands should also be increased. While both ceasing and intensifying forest management do not lead to a rapid increase in structural complexity, it was first noted that traditional forest management is capable of disrupting phases of low structural complexity during stand development and thus promoting structural complexity. Furthermore, the results of this thesis allow the conclusion that continuous cover forestry according to the principles of close-to-nature forest management is particularly suitable to produce sustainable forests with a high degree of multifunctionality and a stand structural complexity similar to primary forests. We therefore conclude that forest management does not necessarily lead to a simplification of the structural complexity, but that specific management systems and methods can increase structural complexity and thus also enhance the associated properties of the forest ecosystem.
Keywords: terrestrial laser scanning; stand structural complexity; Fagus sylvatica; Management intensity; primary forests