Understory plants expand following a spruce beetle outbreak
A forest understory with minimal understory vegetation. Photo was taken near the onset of a spruce beetle outbreak.
Spruce beetles have caused widespread Engelmann spruce mortality in subalpine forests across western North America. Although the effects of spruce beetle outbreaks on the forest overstory have been well-documented, we know comparatively little about how the understory responds to the increases in resource availability brought about by spruce mortality, or about what mechanisms drive understory response. Filling this knowledge gap is important because the understory contains most of the plant diversity in subalpine forests, and it plays a key role in soil stabilization, wildlife habitat provision, tree regeneration, and other ecosystem functions.
To address this knowledge gap, scientists took advantage of a spruce beetle outbreak at the Glacier Lakes Ecosystem Experiments Site (GLEES), a USDA Forest Service research site on the Medicine Bow National Forest in southeastern Wyoming. They measured understory plant cover and richness (the number of unique species present) in 75 permanent plots during and 10 years after a spruce beetle outbreak. They also measured overstory mortality, as well as aboveground and belowground functional traits (morphological, physiological, and phenological characteristics of a species that relate to its functions) for 56 common understory plant species.
A forest understory with extensive vegetation cover. Photo was taken ten years after the onset of a spruce beetle outbreak.
The scientists found that total understory plant cover nearly doubled in the ten years following the spruce beetle outbreak and increased more where spruce mortality was more severe. Similarly, total understory plant richness also doubled and showed a weak positive trend with spruce mortality. The understory plant species that were more frequent across the landscape at the time of the spruce beetle outbreak had larger increases in cover, suggesting that pre-disturbance legacies were a major driver of post-disturbance communities. Additionally, taller species had larger increases in cover, suggesting that these species were better able to compete for the increase in light availability caused by spruce mortality. Interestingly, belowground traits did not meaningfully explain understory plant species responses, which could indicate that competition for water, soil nutrients, and other belowground resources was not as important as competition for light in this system.
The photographs at the right highlight a particularly dramatic understory plant response following a spruce beetle outbreak at the Glacier Lakes Ecosystem Experiments Site, Wyoming. The top photograph (courtesy of Paula Fornwalt) was taken in 2010 at the onset of the outbreak, and the bottom photograph (courtesy of Trevor Carter) was taken in 2020, ten years later.