In this blog post, Pedro Mittelman and Bernado Araújo discuss how forest composition can affect plant–animal interactions without necessarily altering animal communities or population densities.
Forest mortality
We are walking through a dead forest. It is not yet winter, but as we move amid still standing Norway spruce (Picea abies) trunks along the Harz mountains, we find no leaves attached to them. Here, approximately 20,000 hectares of forest have died in the last four years, victims to the combined onslaught of heat, drought and bark beetles. All impacts connected to — and potentialized by — climate change.
Forest mortality due to our rapid changing climate has been on the rise worldwide. In Europe — and in Germany in particular — two forestry strategies have emerged as possible solutions to this problem: the use of non-native species and the planting of mixed forests. Both of them, however, come with their share of controversies.
Mixed forests
Mixed forests can potentially offer more resilience against changes in climate simply by containing trees with a more diverse array of vulnerabilities and needs than their monoculture counterparts. Some species might withstand long dry periods but fail to handle extreme heat very well. Others may be fine with rising temperatures, but vulnerable to storms. Whenever one calamity strikes, it’s less likely to hit the entire forest as hard if you have multiple species — with multiple strengths and weaknesses — on site.
There are also potential collective benefits to diversity. Mixed forests can invite more biodiversity by granting their inhabitants a wider range of habitat features, resources and micro-environmental conditions.
The study
And as far as conversations about wildlife management go, “exotic” is always a complicated word to drop. Promising as a non-native commercial species such as the Douglas-fir (Pseudotsuga menziesii) might appear at first glance — being, at least in theory, more resistant to droughts and pests than native conifers like the Norway spruce tree — there is always the risk that it’ll eventually bring unforeseen consequences to local biodiversity. We can’t know in advance if interactions between, let’s say, rodents and the seeds they are responsible for dispersing across the landscape will be affected by the newcomers.
That’s where our study comes in. We set out to understand how different forest compositions — that is, forests with different proportions of Douglas-fir, Norway spruce and European beech (Fagus sylvatica) — can influence how local rodents interact with seeds. Beeches are broadleaf native species that, under natural conditions, are dominant over the two conifers. If left unchecked, beeches will outcompete these species and slowly take over the landscape.
We investigated managed forests in northern Germany between 2021 and 2023 — We looked at beech seed production in these areas, and also at small terrestrial mammals, which turned out to be mainly yellow-necked mice (Apodemus flavicollis) and bank voles (Clethrionomys glareolus). These rodents love beech seeds, and in order to know if they were mostly eating or storing them for the winter, we attached stainless‐steel wires and numbered tags to the seeds.
The results
We believed that forest composition would affect rodent populations, but to our surprise, we found no clear difference in the populations and species present in forests with different trees. It’s not like composition didn’t matter, though. It did affect how rodents interact with seeds. In forests with fewer broadleaf trees, rodents harvested beech seeds faster and in greater quantities. These seeds also survived less in these forests, being more frequently eaten than cached. The deliciousness of beech seeds apparently protected conifer seeds from predation, which is an interesting thing to take note of when looking to safeguard conifer wood production.
The introduction of Douglas-firs also didn’t seem to alter small mammal communities or their interactions with seeds. In other words, mice and voles don’t seem to care if their neighbourhood conifers are Norway spruce trees or a newcomer from North America. This means that incorporating a mix of tree species, including non-native ones like the Douglas-fir, can support biodiversity without drastically changing the dynamics of small mammal populations and their seed interactions.
In short, rodents play a crucial role in managing the dominance of beech trees in mixed forests by preying on their seeds. This natural predation can help maintain a balanced forest composition, preventing any single species from becoming too dominant. And adding an exotic conifer to this mix didn’t seem to disturb small mammal communities or their feeding habits. In the end, rodents just love their beech seeds way too much to care about the conifer next door.
This work is part of the EnriCo project, which focuses on the effects of mixed forests and non-native Douglas-fir on multiple ecosystem functions.
Read the full article “Forest composition shapes seed-rodent interactions in a gradient of broadleaves and conifers” in Journal of Applied Ecology.