It takes a community to maintain a tree hollow

Ross Wetherbee, Tone Birkemoe, Johan Asplund, Marek Renčo, Anne Sverdrup-Thygeson

This is a plain language summary of a Functional Ecology research article which can be found here.

Large, old trees were once common in forest throughout the world, but are now becoming increasingly rare, primarily due to human activities. When a tree grows old it develops a complex structure with many ‘microhabitats’, and the global decline of old trees threatens the many species that have evolved to use these microhabitats. The hollow is perhaps the most important microhabitat provided by old trees because it is protected from the outside world, and provides a stable and nutrient rich environment. We can think of the species that use the tree hollow as a community. Most of these species are small and can be split into three groups: macrofauna (mostly insects), mesofauna (small worms and insect-like invertebrates that can barely be seen without a microscope) and the microbes (fungi and bacteria). Some of the species in the community are specialized on hollows, while others can live in a variety of habitats. Very little is known about how these groups and species interact or about their roles in the processes going on inside tree hollows.

Large wooden boxes were used to simulate tree hollows. Overall, we found that beetles and other large invertebrates specialized on tree hollows are especially important because they influence the smaller species, speed up the process of decomposition, and increase access to nutrients (credit: Ross Wetherbee)

To gain a better understanding of this, we conducted a field experiment where we simulated tree hollows with large wooden boxes, and manipulated the numbers and types of species in the boxes. The boxes were built from oak boards and looked like large bird houses, but were filled with tree hollow material. We transported these ‘artificial hollows’ into the forest and hung them from hollow trees (see Figure), where they remained for 28 months (May 2017 to October 2019) so that natural processes could occur. After the field experiment, macrofauna and mesofauna were collected from the boxes and identified. We also determined the mass loss and nutrient concentrations in the boxes.

Overall, we found that beetles and other large invertebrates specialized on tree hollows are especially important because they influence the smaller species, speed up the process of decomposition, and increase access to nutrients. These findings show that complex communities are important in tree hollows, and suggest that there may be large impacts on ecosystems if specialized species are lost.

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