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Harvard Forest REU Symposium Abstract 2014

• Title: Beneath the Canopy: Understanding the effects of light availability on understory vegetation in T. canadensis forests after logging and simulated A. tsugae infestation
• Author: Jessica Robinson A Robinson (Knox College)
• Abstract:

  Eastern hemlock (Tsuga canadensis), a foundation species, is declining due to infestation by the invasive hemlock woolly adelgid (Adelges tsugae). Due to this the once heavily shaded understory of hemlock forests are experiencing increased light availability as all sizes of T. canadensis trees lose their needles and die. Understanding how light availability affects forest understory vegetation composition will help us to predict how these forests may regenerate after this major disturbance. I hypothesized that understory vegetation and tree sapling species richness and abundance is positively correlated with light availability, regardless of whether hemlocks died standing or were removed by logging. In 2004, a large experiment was established to study hemlock decline; treatments included girdling to mimic the slow death of T. canadensis due to A. tsugae infestation, and logging to mimic a common human response to hemlock decline, plus a control. Every year, canopy photos are taken within a 15m grid to document light availability in each of the 90 x90 m plots. I sampled understory vegetation composition and sapling abundance at the grid posts where photos were taken. Higher 2013 global site factor (GSF; proportion of reflected and absorbed light that reaches the camera) doesn’t necessarily cause increased response in species richness and abundance. The absence of correlation suggests that treatment, rather than GSF, has a greater effect on vegetation response variables. Understory vegetation responds to girdling versus logging loss of T. canadensis in a more complex way than a simple response to light availability.

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