Hugging Hemlocks

In the early hours of most summer mornings, our team of researchers trudges through the New England forest along a well-worn path, shaded by the young, light-green leaves of the oak, maple and birch trees above us. Equipped with backpacks overloaded with gear including measuring tapes, hard hats, clipboards, and hammers, we chat about our families, our hometowns, and our career goals; our conversations are often punctuated by exclamations and discussions about the funky invertebrates and fungi we encounter along the path. As we walk, the forest thickens around us. The ground becomes noticeably spongier, quilted with reddish-brown pine and hemlock needles. The light filtered through the trees above is reduced and the forest gets quieter, characteristics typical of a hemlock forest.

Hemlock forests are remarkable for their shadiness, low temperatures, and stillness. In stark contrast to the brightness allowed by the leaves of a hardwood forest, the needles and branches of hemlock trees are arranged in horizontal, overlapping patterns, absorbing 99% of available light before it reaches the ground. By blocking the light from seedlings growing on the forest floor, the shade-tolerant hemlock ensures a forest in which it alone dominates: few other species of trees can tolerate such dim conditions. The incredibly unique habitat that hemlocks provide for a number of species (including red-backed salamanders, soil arthropods, web-building spiders, hemlock trout, migrating birds, barred owls, porcupines, fisher weasels, snowshoe hares, and moose) underscore the important role hemlock forests play in adding to the diversity of ecosystems on a regional scale.

Sadly, hemlocks all over the eastern United States are dying. They are being colonized by an invasive, Asian, aphid-like insect accidentally introduced to North America in 1951: the hemlock wooly adelgid. The adelgid has since spread across 18 eastern states and is responsible for the deaths of millions of hemlock trees. It is extremely unlikely that the adelgid infestation will be controlled before the vast majority of hemlocks are dead; thus, in anticipation of the invasion of the adelgid, many landowners have pre-emptively decided to harvest the hemlocks on their land in order to salvage the wood before it dies and rots. This raises several questions: what are the effects on the forest community when a hemlock dies due to adelgid colonization? Due to logging? Is one death "preferable" to the other?

In order to understand the consequences of hemlock loss—both by colonization of the adelgid and by logging—the Harvard Forest initiated a long-term research project (the Harvard Forest Hemlock Removal Experiment) that simulates hemlock loss in each scenario. Researchers delineated study plots within still non-infested hemlock forests in which hemlocks were either logged or girdled (a process in which the bark and vascular tissue are cut at the base of the trunk to prevent nutrient flow, thus simulating the slow death caused by the adelgid). This initial treatment was carried out in 2005; since then, the plots have exploded with the growth of a new generation of plants, marking the beginning of a new type of forest that will gradually replace the hemlocks as they die off.

This summer, our team is responsible for continuing the long-term monitoring of the Hemlock Removal Experiment by meticulously surveying all plots for the first time in five years. We measure every tree, find and mark every new tree, and account for every tree that has died and/or toppled over since the last monitoring was done in 2009. By the end of the summer, we will likely have monitored six or seven thousand trees.

Though this may sound incredibly tedious, and in fact can be at times, I feel so lucky to have such an upbeat, energetic, intelligent, and fun-loving crew to work with. It's incredible to have a job where I can spend most of my day outside, witnessing first-hand how a forest recovers from a major disturbance. And the best part: when I wrap my arms around a tree to get the measuring tape in place, I even get to be a true tree-hugger!