Towards a better understanding of tree seedling establishment and its use for forest rejuvenation in Central European forests

Rejuvenation of forests is one of the most important ecological and economic challenges in central Europe. In an existing large-scale experiment in southern German forests, c. 500 thousand c. 2 years old tree seedlings of Beech, Douglas fir, Silver, Oak have been planted in rows. However, our knowledge of how potential small-scale factors like tree stumps and tree mixture mechanistically enhance tree establishment after planting is limited.

We focused on 6 plots (c. 2500 seedlings) of Douglas fir, planted on a comparably dry site without mature trees. We observed that those trees differed in height, 3 years after planting. Moreover, trees of comparably greater height cluster together on a small spatial scale of 3 m radius. However, conventional tree planting methods (in rows) neglect those beneficial small-scale sites for tree establishment.

Using an app we programmed (Shiny package in R) for quickly finding trees clustered by e.g. height, clusters of 3 m radius of well- and poorly-established trees were identified. A combination of multispectral drone-derived optical parameters, morphological analyses of twigs and ¹³C analyses of tree needles was then used to provide insight into the factors driving the trees’ height differences.

First results showing the positive effects on the establishment of young trees are presented, i.e. incorporating spatial proximity to tree stumps and using a tree mixture in the planting method. Further steps to gain a better understanding of the mechanisms driving tree seedling establishment are discussed.