Blog by Dr. Sandra Müller & Taylor Shaw (WP1)
One of Dr. Forest’s research hypotheses is that acoustic diversity increases as tree species richness increases. We are investigating this hypothesis on two research platforms. The Exploratory Platform describes a network of forest plots, selected within mature forests to represent a gradient of tree species richness levels. The second platform (Experimental Platform) consists of experimental tree plantations that were specifically designed and planted to investigate the biodiversity-ecosystem function relationship[1]. That is, the effect tree species richness has on functions such as productivity and nutrient cycling, or ecosystem services such as carbon sequestration or water security. To date, there are 28 such tree diversity experiments around the globe, totaling 1,235,920 trees planted trees (Paquette et al., 2018). The advantage of such plantations is that researchers can experimentally control for other environmental factors such as soil properties, land-use history, climate factors, etc., that would additionally affect ecosystem functions and therefore potentially mask the effects of tree species richness.
Figure 1: Design of the Sardinilla planted forest: a) Location of the Sardinilla planted forest between Panama City and the city of Colón, north of the Panama Canal; b) Arial photo taken in 2000, before planting; c) Arrangement of plots ranging from monocultures to 6 species polycultures in the main experiment. Annual precipitation: 2661 mm, annual mean temperature: 26°C, dry season: January – May.
On the other hand, tree species richness likely has implications for associated biodiversity because “diversity begets diversity.” Trees provide habitat and food sources for other animals, so higher species richness can cascade to higher species richness in other trophic levels. While many studies support this hypothesis, concluding that forests with higher tree species richness provide more ecosystem services while being more resilient to disturbance, most re-forestations are still planted as monocultures (Lewis et al., 2019; Messier et al., 2021).
Figure 2: Images of two plots of the Sardinilla planted forest. Left: Example of a five-species mixture; right: example of a monoculture with Luehea seemannii. Photographs are courtesy of Matthias Kunz and were taken in the middle of each plot, on June 10th, 2017.
In a recent Dr. Forest-related publication, we showed that polycultures in the Sardinillia-planted forest in Panama (Figure 1 and Figure 2) have higher acoustic activity from orthopterans (Müller et al., 2022). Orthopterans respond to small-scale microclimatic changes that could be due to higher structural diversity in these polycultures. Insects are important indicator species for disturbances in tropical forests but are difficult to monitor with traditional methods. Orthopterans largely dominate the composition of tropical soundscapes, making their presence easy to detect using ecoacoustic methods, as we could show here. We look forward to sharing our Dr. FOREST results, which use a similar approach as the Panama project but are located in a temperate climate, so we expect birds instead of orthopterans to dominate the soundscape.
Figure 3: Spectrogram of a night recording on a 5-tree species plot, with the high vocal activity of orthopterans (recording from 22:10 on 09.04.2017). An audio example can be listened to below.
Figure 4: Spectrogram of a night recording on a monoculture plot, with the low vocal activity of orthopterans (recording from 22:10 on 09.04.2017). An audio example can be listened to below:
Figure 5: Diurnal pattern of High-Frequency Cover, an index of acoustic activity of orthopterans vocalizing between 8 – 24 kHz, in relation to tree species richness (SR), monocultures in grey, polycultures in yellow, orange, and red. Left: dry season, right: the rainy season.
This supports the hypothesis we are investigating in experimental forest plantations in Belgium and Germany as part of Dr.Forest. In contrast to the tropical soundscapes in Panama, birds dominate the soundscapes of temperate forests, and the temporal patterns are determined not only by day and night cycles but also by the change of the four seasons. This results in high acoustic activity in spring and comparatively quiet soundscapes during the other seasons (see 07/2021 blog post “A Year of Sound Recording in Dr. Forest Plots”). Using a similar approach as in the Sardinilla study, we would test whether acoustic diversity in these plots increases with increasing tree species richness and how this in turn affects human well-being.
Blog based on the article by Müller, Mitesser, Oschwald, Scherer-Lorenzen Michael & Potvin, 2022.
References
Lewis, S., Wheeler, C., Mitchard, E., and Koch, A. (2019). Regenerate natural forests to store carbon. Nature 568, 25–28. doi:10.1038/d41586-019-01026-8.
Messier, C., Bauhus, J., Sousa-Silva, R., Auge, H., Baeten, L., Barsoum, N., et al. (2021). For the sake of resilience and multifunctionality, let’s diversify planted forests! Conserv. Lett. n/a, e12829. doi:https://doi.org/10.1111/conl.12829.
Müller, S., Mitesser, O., Oschwald, L., Scherer-Lorenzen, M., and Potvin, C. (2022). Temporal Soundscape Patterns in a Panamanian Tree Diversity Experiment: Polycultures Show an Increase in High Frequency Cover. Front. Ecol. Evol. 10. doi:10.3389/fevo.2022.808589.
Paquette, A., Hector, A., Castagneyrol, B., Vanhellemont, M., Koricheva, J., Scherer-Lorenzen, M., et al. (2018). A million and more trees for science. Nat. Ecol. Evol. 2, 763–766. doi:10.1038/s41559-018-0544-0.