|Forest structure is of functional importance for ecosystem stability and resilience, as well as for numerous other ecosystem functions and services. In this context, a high structural complexity can have a positive effect on diverse functions and services. Silvicultural management has the potential to negatively or positively affect forest structure and its complexity. In order to investigate the relationship between silvicultural management and forest structure, one first has to quantify forest structure and its complexity.
To do so, various 3D measures based on terrestrial laser scanning data were used to quantify forest structure and structural complexity along a management gradient. The main objective of the thesis was to investigate the impact of different management systems on the structure of European forests and to compare them with formerly managed and primary forests. As beech (Fagus sylvatica L.) is the natural dominating tree species in Europe, beech-dominated forests were selected for this study.
First, the understory complexity index (UCI) was introduced as a measure to describe the structural complexity of the forest understory (chapter 2). The UCI is based on the fractal dimension of a cross-sectional horizontal polygon, which represents the forest understory between 0.8 and 1.5 m. It is a density-dependent measure. Forest stands with advanced tree regeneration, as e.g. thickets, showed high UCI values compared to forest stands with a lower regeneration density. Comparing different management systems and forest types, the results revealed that the understory complexity was either large during the early phases of stand development or when the senescence of trees has largely proceeded, as found in beech primary forests.
The second research aim of the study was to investigate, how structural complexity express itself in terms of the spatial distribution and density of plant material and how forest management influences these structural attributes (chapter 3). For that purpose, the structural complexity, as well as the density and spatial distribution of plant material within different forest strata of differently managed and unmanaged beech forests were quantified. The results showed that forest strata with a rather high structural complexity were characterized by a rather high density and a random to regular distribution of plant material. This could be observed for the primary beech forests and uneven-aged beech stands. Forest strata with a low structural complexity, as found in formerly managed forests, showed in contrast a rather low density and a clustered distribution of plant material. It can be assumed that structural complexity increases with increasing density and increasing homogeneity of the spatial distribution of plant material within a forest stand.
Finally, the focus was on analyzing the influence of the understory complexity and vertical heterogeneity on the overall stand structural complexity of beech-dominated forests. The sub-study (chapter 4) on short-term dynamics of structural complexity revealed for uneven-aged stands that an increase of the understory complexity lead to an increase in the overall stand structural complexity. In this context, the initial canopy openness positively influenced the development of a complex understory and thus the overall structural complexity. Whereas in the younger, even-aged beech stands, an increase of vertical stratification positively influenced the stand structural complexity. In summary, short-term dynamics of stand structural complexity strongly depend on the developmental stage or canopy openness of the forest.
This thesis not only contributes to the understanding of structural complexity and its relationship with other structural attributes, but also provides information on structural characteristics of different forest types. Managed and primary beech forests can be quite similar in terms of structural complexity. Especially, single-tree selection cutting seems to be a useful management approach for creating a complex stand structure. Furthermore, the thesis particularly highlighted the structural importance of the forest understory for the development of a complex stand structure. Therefore, management interventions could especially focus on the creation of complex understory structures, especially in early developmental stages, if an enhancement of structural complexity is desired. Lastly, the study demonstrated the wide range of potential applications of terrestrial laser scanning data for forest structure analysis.