Landscape Management, part of Custodial and Grounds Services in the Physical Plant, has been responding to an apparent decline in the health of a number of trees across campus.
While last year’s drought is the likely culprit, it is important to understand the various factors that contribute to dieback in vegetation. Tree decline is a complex and gradual process, often brought on by a suite of stressors that may eventually result in mortality. As is typical for plant life in urban environments, our campus trees experience stressors that primarily include disease, insect pressure and construction damage. When subjected to drought conditions, trees are further stressed, their defense mechanisms are disrupted, and they may succumb to pest and other pressures.
For example, during a drought, a tree that exhibits some resistance to Dutch elm disease (DED) may contract the disease, which takes advantage of the tree’s weakened condition. This was true for the American elm (Ulmus americana ‘American Liberty,’ a resistant variety) recently removed from the corner of Thatcher Way and Eastman Lane.
Several years ago, the walkways in this area were reconfigured, and a significant portion of the tree’s root system was severed in the process. The elm initially rebounded following the disturbance, but sustained drought conditions placed enough stress on the tree that it succumbed to DED.
In this case, “sustained drought conditions” refers to the drought-like circumstances that occurred at the end of Summer 2015, compounded by low levels of winter precipitation in 2016, and eventually the official drought last summer.
Like the American elm described above, Scotch pine trees removed from an area near the Whitmore Administration Building experienced a similar fate. However, these particular trees had Diplodia, a common fungus that affects many species of pine. Diplodia is transmitted by bark beetles and causes Diplodia blight, which kills needles and may result in tree death after severe and repeated infections. The pines by Whitmore were formerly impacted by the installation of new curbing.
When a pine tree is stressed or in decline, it emits a pheromone that acts as an attractant for bark beetles. The bark beetles then bore into the tree, feeding on its tissue and introducing the Diplodia fungus carried on their bodies from the last infected host. The combined effect of the fungus and severing of the vascular tissue from the feeding beetles was exacerbated by the drought, and the Scotch pines did not survive.
The effects of last year’s drought can be seen throughout campus in the dieback of shrubbery and tree canopies, as well as tree loss. This year alone, Landscape Management removed 80 dead trees, which is in stark contrast to the usual dozen or so that are downed annually. In addition to the removal of standing trees, maintenance of dead wood from canopy dieback has been a priority for the department’s tree crew.
Another visible response to the drought is a reduction in the number of new flower plantings this growing season. Unsure of potential relief from the drought, Landscape Management significantly reduced new plantings, which require greater quantities of water for initial establishment. Instead, the department focused on completing projects in target areas of campus, such as the campus pond and terrace planters by Tobin Hall.
Although the drought took its toll, Landscape Management is hopeful that the campus grounds will rebound in response to ample rainfall this spring and summer. In the meantime, the department continues to encourage the surviving greenery through diligent maintenance.