Over the centuries, Europe’s forests have been optimised for timber production. The result is often very orderly, uniform stands lacking old, decaying trees or natural clearings.

This monotony can pose a problem for biodiversity. Researchers from the Biocenter at Julius-Maximilians-Universität Würzburg (JMU) in Bavaria, Germany, together with researchers from the Universities of Marburg and Munich and the Bavarian Forest National Park, have now investigated in the BETA FOR project how targeted interventions to restore a more varied forest structure affect the diversity of bats and birds.

The results have been published in the journal Current Biology. The team led by forest ecologist Professor Jörg Müller shows that when humans create gaps in the forest canopy and leave deadwood lying on the forest floor, this increases the diversity of both species groups at the forest landscape level. However, birds and bats react differently to changes in their habitat.

Birds as homebodies, bats as commuters

Birds behave like homebodies: they occupy fixed territories if they find everything they need there – from nesting sites to food. They benefit when their established forest area has a structure that is as complex as possible, with deadwood and gaps.

Bats, on the other hand, are like commuters: in a single night they cover long distances, visiting various ‘specialist shops’ along the way. Sometimes they hunt insects in a dark, dense corner of the forest; at other times they use light-filled gaps as entry corridors. For them, it is important that the individual sections of forest differ spatially from one another.

Which species appeared

The Würzburg study shows how diversity increases through gaps in the canopy and deadwood. In the case of bats, an average of two new species were found in more disordered forests. “That sounds like a small number, but it is actually significant because there are only 25 bat species in Germany in total,” says PhD student Clara Wild, the study’s first author.

The more structurally diverse forests attracted species such as the northern bat or the parti-coloured bat, for example. Both otherwise prefer open terrain and are rather rare in dense, uniform forests.
Birds benefited particularly strongly from local interventions, such as artificially created forest gaps with deadwood. In their case, it was primarily so-called functional diversity that increased – that is, species with very distinct lifestyles were added – such as deadwood specialists like various endangered woodpecker species.

The experiment: 234 forest areas in six regions

The researchers conducted the study in six regions in Germany: near Lübeck, in the Saarland, in the University of Würzburg’s forest, near Passau, in the Hunsrück-Hochwald National Park and in the Bavarian Forest National Park.

In total, they studied 234 precisely defined forest plots measuring 50 by 50 metres. There, they deliberately manipulated the forest to create more diverse structures: in some areas they created gaps in the canopy, in others they placed deadwood such as tree stumps or fallen trunks. They then studied how species diversity changed over the following four to seven years.

Acoustic monitoring of calls and songs

To find out which animals live in the forest plots, the researchers used acoustic monitoring. At times when the animals are most active, recorders captured their calls or songs. These invisible spies monitored the forest over a three-month period without the animals being disturbed by human presence. In this way, the research team identified a total of 17 bat and 72 bird species.

“Our results show that we can promote biodiversity even in previous monotonous, species-poor forests,” explains Clara Wild. “Through small interventions that increase structural diversity, we can create valuable niches. This makes the forest more diverse and attracts pest controllers such as birds and bats alike.”

Forestry should have the courage to leave gaps

The new findings provide further guidance for the forestry sector. “A structurally rich forest is much more resilient to climate change thanks to its diversity,” says Jörg Müller. For forestry operations, this means having the courage to leave gaps: “Leaving deadwood in the forest may cost some timber yield in the short term, but it ensures the long-term stability of the entire ecosystem.”