Abstract
Forested areas are increasing across Europe, driven by both reforestation programs and farmland abandonment. While tree planting remains the standard reforestation strategy, there is increased interest in spontaneous regeneration as a cost-effective method with equal or potentially greater benefits. Furthermore, expanding areas of already established forests are left for passive rewilding to promote biodiversity conservation. Effective and objective methods are needed for monitoring and analyzing the development of forest structure under these management scenarios, with airborne laser scanning (lidar: light detection and ranging) being a promising methodology. Here, we assess the structural characteristics and development of unmanaged forests and 28- to 78-year old spontaneously regenerated forests on former agricultural land, relative to managed forests of similar age in Denmark, using 25 lidar-derived metrics in 10- and 30-m grid cells. We analyzed the lidar-derived cell values in a principal component analysis (PCA) and interpreted the axes ecologically, in conjunction with pairwise tests of median and variance of PCA-values for each forest. Spontaneously regenerated forest in general had increased structural heterogeneity compared to planted and managed forests. Furthermore, structural heterogeneity kept increasing in spontaneously regenerated forest across the maximal 78-year timespan investigated. Natural disturbances showed strong impacts on vegetation structure, leading to both structural homogeneity and heterogeneity. The results illustrate the utility of passive rewilding for generating structurally heterogeneous forested nature areas, and the utility of lidar surveys for monitoring and interpreting structural development of such forests.
Highlights
Forest structure is important for biodiversity, notably via providing heterogeneous environments in terms of available resources and structural composition (Simonson, Allen & Coomes, 2014; Stein, Gerstner & Kreft, 2014; Tews et al, 2004)
And intensively managed forests often show less structural heterogeneity compared to unmanaged forests due to establishment strategy, and due to prevention of natural dynamics (Christensen & Emborg, 1996; Nielsen & Jensen, 2007; Richnau, Wistrom & Nielsen, 2012)
The presented lidar-based evaluation of vegetation structure resulting from different afforestation strategies and management actions, successfully performs statistical tests of pairwise structural similarities and differences between forest stands; it points out key structural attributes and, at least to some extent, provides interpretable axes
Summary
Forest structure is important for biodiversity, notably via providing heterogeneous environments in terms of available resources and structural composition (Simonson, Allen & Coomes, 2014; Stein, Gerstner & Kreft, 2014; Tews et al, 2004). And intensively managed forests often show less structural heterogeneity compared to unmanaged forests due to establishment strategy (planted as even-aged monocultures or naturally regenerated as fenced stands without gaps), and due to prevention of natural dynamics (harvesting the biologically young trees, by drainage, thinning and pest control) (Christensen & Emborg, 1996; Nielsen & Jensen, 2007; Richnau, Wistrom & Nielsen, 2012). Passive rewilding, defined as spontaneous ecological dynamics without management (Svenning et al, 2016), is one restoration strategy that is thought to lead to more heterogeneous forests compared to managed forests. Pathways toward forests under passive rewilding can be: (1) ceasing the silvicultural management of existing forests and (2) natural regeneration of new forest areas, for example, on abandoned farmland (Schnitzler, 2014)
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