Invasive species are non-native organisms that negatively impact their ecosystem. When introduced to a new ecosystem, these organisms face no native predators, and therefore outcompete the native species. Invasive species contribute to biodiversity loss, habitat destruction, and overall ecosystem degradation.
The hemlock woolly adelgid (HWA) is an invasive, aphid-like insect native to Asia. HWA feed by sucking the sap of hemlock trees and use the underside of branches and needles to lay their eggs. Their activity is indicated by the white egg casings found on the underside of needles. As HWA inhabits and feeds on hemlock trees, the tree begins to die, losing its needles from the bottom up. Trees typically die within 4-10 years of infestation. In 2015, 90% of the geographic range of eastern hemlock in North America was affected by HWA.
Eastern hemlock trees play an important role in ecosystems within the temperate deciduous forest biome. Growing in moist, shady groves typically found on north-facing slopes, eastern hemlocks create unique environmental conditions. Their dense canopies prevent sunlight from reaching the forest floor, resulting in a cooler, shady environment. Many species that live in hemlock forest ecosystems rely on the trees for food and shelter. Overall, eastern hemlock trees contribute to plant and animal biodiversity, benefitting the health of the forest ecosystem.
Some ecosystem services related to eastern hemlocks include stream flow regulation, nutrient retention, microclimate maintenance, provision of habitat for animals including brook trout, birds, mammals, and salamanders. Others include recreation and property value benefits, educational and research opportunities, and finally, the simple fact that eastern hemlocks are an iconic tree in forests in the eastern United States.
This study observes the impact of HWA on the canopy cover and resulting effect on light intensity, air temperature, and soil temperature in two different hemlock forest plots. One plot, located at Wilson Mountain in Dedham, MA, has a low infestation of HWA. The other plot, located at Blue Hills Reservation in Milton, MA, has a high infestation of HWA.
By determining the impact of hemlock woolly adelgid on different environmental factors in the understory of hemlock forests, we can more accurately predict the behavior of secondary succession. Secondary succession occurs when the dominant overstory trees (in this case hemlocks) die, making space for new species to grow. Secondary succession relates to this study because typically, without the presence of HWA, hemlock trees provide dense canopies for decades, allowing only shade-tolerant tree species (such as american beech and sugar maple) to grow in the understory; however, with the presence of HWA, these dense canopies disappear and the hemlocks soon die off, allowing more shade-intolerant trees (such as white pine and black birch).
Qualitatively, we can see the impact of HWA on secondary succession at my field site. In the low-infestation plot, the understory is mostly bare. In the high-infestation plot, on the other hand, the understory is full (this became quickly apparent to me, as I had to ninja my way through branches while collecting data). In fact, unsurprisingly, the two dominant understory species in the high-infestation plot are both shade-intolerant species: white pine and black birch. This information can be useful for foresters dealing with the effects of HWA as they predict the growth of future trees, as well as for tree health experts as they determine which HWA treatments will have the greatest long-term benefit for the forest.