Persche et al., 2024 (Oecologia)
RESEARCH BRIEF #50
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MANAGEMENT IMPLICATIONS
Restored Midwest oak woodlands have greater breeding bird abundance and richness, and greater arthropod biomass than degraded sites.
Restored woodlands have higher soundscape saturation and acoustic complexity than degraded ones.
Bioacoustic monitoring is a promising tool for documenting oak woodland restoration outcomes.
In this study, the authors used multiple sampling techniques, including bioacoustic monitoring (recording sound to study animals and their behavior), to evaluate the effects of oak woodland restoration on birds, arthropods, and soundscapes during both day and night. Bioacoustics is increasingly used to monitor biodiversity and assess ecosystem function and habitat quality.
Before European colonization, vast North American woodlands were maintained by periodic groundfires from natural ignition and cultural burning. Midwestern US oak woodlands are characterized by open canopies and understories; they support a high diversity of folivorous (leaf feeding) arthropods and other species adapted to the high-light conditions. Many remaining Midwest oak woodlands have been degraded by fire suppression and mesophication, resulting in dense shrub cover and litter that limit oak regeneration and understory plant diversity. Prescribed fire and mechanical tree thinning have been successfully used to increase oak regeneration and restore understory plant diversity in degraded sites. However, detecting restoration effects on other taxa can be more complex; for example, the calling behavior of vocalizing species such as birds, amphibians, and invertebrates can be influenced by vegetation structural characteristics. Bird richness and abundance are often lower in degraded Midwestern US woodlands compared to those with intact fire regimes.
Study sites included ten 7 ha (17.3 ac) sites in five restored and five degraded woodlands in the Baraboo Range in Sauk County, southern Wisconsin. Sites were located on historically oak-dominated blufftops within or adjacent to several thousand forested acres. Degraded sites had oak-hickory overstories and dense understories; restored sites had predominantly oak overstories with sparse mid- and understories. Degraded sites had experienced decades of fire suppression; restored sites had undergone repeated rounds of tree thinning over 2–10 years and 1–5 controlled burns per site. Sites were paired based on landscape position and bedrock geology. Vegetation assessment included canopy cover, percent oak canopy, and herbaceous groundcover; arthropods were captured in traps and weighed to determine biomass. Birds were surveyed using point counts, and soundscapes were recorded continuously for four to five days during every15-day period from May to August 2022, which incorporates the breeding season for insectivorous birds in southern Wisconsin. The authors calculated two acoustic indices: Soundscape Saturation, which includes all acoustically active species, and Acoustic Complexity Index (ACI), which focuses on birds. Generalized additive models were used to predict both indices based on Julian date, time of day and level of habitat degradation. For this study, the authors hypothesized that the increased structural complexity of restored woodlands compared to fire-excluded ones would result in more saturated and complex soundscapes, due to a greater biodiversity of vocalizing species supported by increased resources and niche availability.
Restored woodlands had a higher percentage of oak trees in the canopy (45.4%) compared to degraded sites (24.5%), and canopy cover was significantly lower in restored sites (52.8%) than in degraded sites (79.7%). Mean herbaceous plant richness was higher in restored sites (25.6 species per point) compared to degraded sites (13.7 species per point). Arthropod biomass was greater in restored sites (2.03 mg/trap/day) than in degraded sites (1.42 mg/trap/day), with greatest differences in early June and early August. Both restored and degraded sites exhibited biomass peaks in early June, corresponding with nesting season. Mean avian species richness was greater in restored sites (36.8) than in degraded sites (29.2), as was avian abundance (25.1 territories/ha, or 10.1 territories/ac) compared to (17.5 territories/ha, or 7.1 territories/ac), respectively.
Compared to degraded sites, restored woodlands had greater diurnal (30 minutes before sunrise, and after sunset) soundscape saturation (31.6% vs. 25.9%) and diurnal acoustic complexity(0.482 vs. 0.469). Additionally, restored sites exhibited more pronounced seasonal peaks in both acoustic complexity and soundscape saturation (see Figure) compared to degraded sites. These peaks occurred in mid-June during nesting season and in early July during post-fledging season. Diurnal avian acoustic activity peaked around 07:00 am, decreased through the day, and exhibited a smaller peak around 7:00 pm, coinciding with dawn and dusk choruses.
Surprising the authors, nocturnal soundscapes showed no difference in saturation between restored and degraded sites, and nocturnal acoustic complexity was lower in restored sites compared to degraded sites. The authors suggest that denser understory vegetation and more humid conditions in degraded woodlands may provide better habitat for nocturnal animals such as tree frogs (Hyla ssp.) and nocturnal insects, or that lower bird abundance contributes to lower insect predation in degraded sites.
The authors conclude that their observations of richer vegetation and avian communities in restored sites are consistent with longer term responses in woodland restoration and that bioacoustic monitoring can capture nuanced differences that presence-absence metrics may fail to detect. Acoustic indices can be useful for monitoring restoration progress, especially when monitoring individual species is not practical.
Maia E. Persche, H. S. Sathya Chandra Sagar, Zuzana Burivalova, and Anna M. Pidgeon
