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The pedunculate oaks typical of Leipzig's floodplain forest and other German oak forests are struggling to regenerate in the understory due to a lack of light. One reason for this is the absence of flooding in floodplain forests.

In a two-year oak experiment in Leipzig's floodplain forest, researchers from Leipzig University and the German Center for Integrative Biodiversity Research (iDiv) found that the current tree dieback—caused by drought and pest outbreaks—combined with the thinning of certain understory species, can actually support oak regeneration.

Fittingly, today (16 April) is the day when the city of Leipzig celebrates its floodplain forest. Their findings have been in the journal Forest Ecology and Management.

"This is a promising new approach to forest dynamics, which have been fundamentally altered by climate change," says Annalena Lenk from the Institute of Biology at Leipzig University, lead author of the paper, adding that the lack of oak regeneration and the poor hydrological condition of many floodplains are widespread problems across Central Europe.

In Leipzig, for example, urban expansion into the floodplain and the channeling of rivers have led to the loss of the typical flood dynamics characteristic of such habitats. She explains that this has encouraged the spread of flood-intolerant tree species such as sycamore and Norway maple. Their dense canopy casts deep shade over the forest floor, making it much harder for light-demanding species like pedunculate oak to regenerate.

According to Lenk, this species plays a key role in biodiversity conservation, as it provides a habitat for a wide range of organisms, including insects, fungi, birds, bats and lichens.

At present, however, a new structural shift in forest composition is becoming apparent. The droughts of recent years and the spread of pests in many parts of Europe—including Leipzig—have led to a sharp rise in tree mortality. As a result, forest stands have opened up significantly. This could, on the one hand, promote the natural regeneration of light-demanding species such as pedunculate oak. On the other hand, it may also allow more dominant species, such as maples, to spread further.

The researchers investigated how these recent structural changes in the floodplain forest affect the regeneration of pedunculate oak and how they can be combined with forest management measures. They analyzed the impact of different silvicultural interventions—and the associated availability of light—on the stand microclimate and the establishment of pedunculate oak.

To do so, they set up eight research plots in areas of Leipzig's floodplain forest with a relatively healthy upper canopy and eight more in areas with high levels of dead or diseased trees. On two subplots of each research plot, they selectively removed flood-intolerant tree species such as sycamore and Norway maple, as well as elder, from the understory. The other two subplots were left untreated.

In addition to these plots, the researchers also included eight existing patch clearings from Leipzig's forest management program—areas ranging from 0.13 to 0.72 hectares that had already been partially opened up and planted with young oaks. In early 2022, the team planted a total of 1,200 one-year-old oaks protected by browsing guards, along with 80 approximately five-year-old oaks, and monitored their development over a two-year period.

"At the end of 2022, we replaced oaks that had already died with new plantings. In both years, we measured crown diameter in summer and height and root collar diameter in winter to calculate growth," explains Lenk, describing the methodology. The oaks were also tested for drought stress using leaf samples.

In their oak experiment, the researchers found that "forest areas with high tree mortality in the upper canopy can be effectively used to promote the regeneration of pedunculate oak—but only if maple is removed from the understory. Maple is widespread, because it responds more quickly to growing conditions, it overtops and shades the oaks," says co-author Christian Wirth.

Without the flood-intolerant species, the oaks were able to benefit from the increased light levels and develop well, as the canopy had been thinned by tree dieback.

"Compared to the patch clearings, the plantings on the thinned forest plots showed fewer signs of drought stress, which was mainly due to higher air humidity," the researchers found. "While oaks benefit from increased light availability, this positive effect is weakened by drought stress," explains Lenk.

The light conditions and microclimatic environment on plots with high tree mortality in the upper canopy and selective thinning of specific species in the understory appear to offer a balanced combination of sufficient light and a stable forest microclimate—even in a drought year—that supports oak regeneration.

The research findings could be considered in practice as part of climate-adapted forest management strategies aimed at promoting biodiversity—including as a supplementary measure alongside future flooding events. The research plots have been established as long-term observation sites, allowing data collection to continue in the coming years. For example, a master's thesis is planned for next year that will examine the effects of different management methods—and the resulting light conditions—on the herb layer.

The influence of climate change on vegetation processes, and in turn the impact of vegetation on the climate, is a central focus of Leipzig University's proposed Cluster of Excellence, Breathing Nature. Leipzig's floodplain forest is an important research platform in this collaborative project.