How Energy and Forest Structure Affect Bird Diversity in Northwestern Forests

A new study reveals how energy levels and forest structure influence bird diversity in forests across the northwestern United States. Scientists looked at forests from the Pacific Coast to the Rocky Mountains and found that both the amount of available energy (like heat and sunlight) and the complexity of forest structure (like tree age and layers) play important roles in supporting different bird species.

The researchers found a surprising pattern: Bird diversity increases with energy levels up to a point but then decreases when at the highest levels of available energy. This pattern, called a hump-shaped relationship, means that in energy-limited landscapes, the most productive areas (valley bottoms, deep soils, greater precipitation) will often support more species. In energy-rich landscapes, the very most productive places may support fewer species.

In these productive landscapes, having a mix of young and old trees and different layers of plants helps increase bird diversity. Structural complexity, such as a variety of tree heights and dense shrub layers, allows more birds to find the habitats they need. In contrast, in low-energy landscapes, forest structure matters less, and small “hotspots” of higher energy are where most bird species are found.

The study showed that forests in energy-rich areas, like those west of the Cascade Mountains, may maximize biodiversity with regular disturbances including logging. These disturbances reduce competition among plants and create more varied habitats. In energy-poor areas, like those east of the Cascades, recovery from disturbances is slower, so managers need to carefully manage higher-energy biodiversity hotspots and allow more time for forests to regenerate.

Another important finding was that in lower-energy landscapes, bird diversity was highest in areas with some disturbance, but not too much. Cutting down too many trees in these areas can harm the slow-recovering forests and reduce biodiversity, but eliminating disturbance will also reduce biodiversity. The researchers suggest that logging should happen less frequently in these regions.

In energy-rich forests, managers can help bird diversity by creating patches of forests at different growth stages. This “mosaic” of young and old trees helps support a wide variety of bird species, including those that prefer open areas or dense forest edges. In some areas, bird diversity peaked right before tree canopies closed, highlighting the importance of early growth stages.

Overall, the study shows that managing forests for bird diversity requires understanding local conditions. In energy-rich areas, creating a mix of different forest structures works best. In energy-poor areas, protecting small high-energy spots and allowing longer recovery times between disturbances is key. By tailoring forest management to these differences, we can help protect the many bird species that call these forests home.