The English oaks typical of the Leipzig riparian forest and other German oak forests can hardly rejuvenate due to a lack of light in the undergrowth. One reason for this is the lack of flooding in alluvial forests. Researchers from Leipzig University and the German Centre for Integrative Biodiversity Research (iDiv) have discovered in a two-year oak experiment in Leipzig’s alluvial forest that the current tree mortality as a result of droughts and the spread of pests can be used in combination with a thinning out of certain species in the undergrowth for oak regeneration. They have published their study in the journal "Forest Ecology and Management". In keeping with this topic, today (April 16) is Leipzig’s Alluvial Forest Day.
The researchers investigated how these current structural changes in the alluvial forest affect English oak regeneration and how they can be combined with maintenance measures. They evaluated the influence of various forest management measures and the associated availability of light on the stand microclimate and the establishment of English oak.
They established eight research plots each on forest areas with a relatively healthy upper tree layer and on forest areas with many dead and diseased trees in the Leipzig riparian forest. The flood-intolerant tree species sycamore, Norway maple and elder were removed from the lower vegetation on a small scale in two subplots per research plot. The other two subplots were left untreated. At the beginning of 2022, the scientists planted a total of 1,200 one-year-old oaks in browsing protection grids and 80 approximately five-year-old oaks on these and eight existing femel plots (forest clearings of 0.13 to 0.72 hectares with young oak plantations) from the Leipzig forest management and observed their development over two years.
More light for young English oaks thanks to thinned treetops
"At the end of 2022, we replaced dead oaks with new plantings. In both years, we measured the crown diameter in summer and the height and root collar diameter of the oaks in winter in order to be able to calculate their growth," says Annalena Lenk, describing the methodology. The oaks were also examined for drought stress using leaf samples.
In their oak experiment, the researchers found that ,,Forest areas with high tree mortality in the overstorey can be used well to promote English oak regeneration - but only if the maple in the understorey is removed. The maple is present everywhere. Because its growth starts faster, it overgrows and shades out the oaks," says co-author Christian Wirth. Without the flood-intolerant species, the oaks were able to benefit from the stronger light irradiation and develop well, as the treetops were thinned out by the forest dieback.
compared to the femel areas, the plantations on the thinned forest areas were less affected by drought stress, which was mainly due to the higher humidity. Although the oaks benefit from increasing light availability, this positive correlation is weakened by drought stress," explains Annalena Lenk. The light conditions and microclimatic conditions on the plots with high tree mortality in the upper stand and species-specific thinning in the lower stand appear to be conducive to oak regeneration with a balanced combination of sufficient light and a stable forest microclimate, even in a drought year.
Adapting forest management to climate change
The research results can be considered in practice for forest management adapted to climate change to promote biodiversity, also as a complementary measure to future flooding. The research plots are designed as long-term observation plots so that data can continue to be recorded in the coming years. Next year, for example, a master’s thesis will be written on the effects of various management methods and thus light influences on the herb layer.
The influence of climate change on vegetation processes and, conversely, the effect of vegetation on the climate are the subject of Leipzig University’s Cluster of Excellence application "Breathing Nature". The Leipzig alluvial forest is an important research platform in this joint project.
Publication in "Forest Ecology and Management":
"Elevated tree mortality as a regeneration niche for oak? Testing different management approaches in a meliorated floodplain forest" , doi.org/10.1016/j.foreco.2025.122678
