Abstract: Pathogens, pests, invasive species, climate change and other drivers have led to declines in oaks (Quercus spp.) across North America. Oaks dominate many temperate hardwood forests, making it important to understand the consequences of their loss. The primary goal of this study is to establish a mechanistic understanding of how oak loss affects forest soundscapes. Specifically, we ask: How does oak loss affect forest soundscapes through its impact on physical structure, biotic structure, and ecosystem functioning? We hypothesize that: (1) oak loss will reduce structural complexity, (2) oak loss will positively influence biotic structure due to compensatory growth by non-oaks, and (3) these changes will reduce soundscape diversity and complexity.
This study is based at the Black Rock Forest Consortium’s Future of Oak Forests project, which consists of a series of adjacent, 75m x 75m experimental plots on the north slope of Black Rock Mountain, NY, representing four treatments: 100% non-oaks girdled, 100% oaks girdled, 50% oaks girdled, and an unaltered control. Each treatment is replicated three times, creating a 3 x 4 grid of forest plots.
Data was collected over the course of multiple 2-week campaigns between April 2021 and February 2022. Campaigns were scheduled as to capture seasonal variation in forest structure and soundscapes. For each campaign, we: 1.) quantified forest physical structure via ground-based LiDAR; 2.) completed biotic inventorying to census changes in biotic structure, soil moisture, and temperature; 3.) used dendrometers to measure ecosystem function; and 4.) used bioacoustic sensors to quantify soundscape properties (e.g., indices of bioacoustic complexity and diversity). A total of five campaigns were completed, each returning the described data across a gradient of oak loss. Data were analyzed in R using linear models, PCA data reduction, and structural equation models to test our three hypothesized effects of oak loss.
Our statistical analyses suggest that oak loss leads to increases in forest structural openness and decreases in stand density, which are both associated with declines in acoustic diversity. However, these structural metrics explained only 14% and 13% of the variation in acoustic diversity between study plots, suggesting a relatively limited role of forest structure in shaping forest soundscapes. In conclusion, our data suggest that the widespread decline of oaks across North America will reduce the complexity of forest physical and biotic structure and impact ecosystem functioning, but that such impacts may not be associated with significant changes in forest soundscapes.