Ecological Disturbances May Cause Willow Shrub Expansion
Following recent reports of canopy cover increases in Alaska, northern Quebec, and in northern Russia, Myers-Smith et al. studied willow expansion on Herschel Island, a land mass of about 100 km2 reaching an elevation of 183m above sea level. The island’s soils are “composed of glacial and marine deposits, underlain by ice-rich permafrost” (611) that support lowland tundra flora. Animal species on the island include “musk oxen, caribou, collared lemmings, and rock ptarmigan” (611); herbivores that will feed on willows if their preferred food sources (lichens and other shrubs) are less abundant.
Isla H. Myers-Smith et al. quantified shrub expansion and identified factors that could be affecting their expansion. The researchers collected repeat photography data, data from the International Tundra Experiment (ITEX), vegetation surveys, and growth ring data to measure the extent of willow spread and its height increase in the study site. Using statistical analysis, the researchers determined that the change in the area of coverage and growth in the height of shrubs was significant.
Their results indicated “patches have increased in size and height, and the cover has transitioned from discrete patches to nearly continuous cover” (614). The rate of growth varied between the different shrub species, with S. pulchra plants being shorter than other willow species. Growth was not limited by herbivory (being eaten by herbivores). The researchers suggest that ecological disturbances such as permafrost thaw and flooding may contribute more towards shrub expansion and growth. These ecological disturbances can be linked to climate change.
The report provides evidence of “increases in canopy cover and height of canopy forming willows on Herschel Island in the Western Canadian Arctic” (621). They suggest further monitoring of “long-term vegetation plots will improve… estimates of shrub change and rates of patch expansion” (621). The researchers also suggest future research to focus on the effects of this growth pattern on the tundra ecosystem.
This is a summary article written by Imtihan Ahmed. For more information, please access the full document:
Expansion of Canopy-Forming Willows Over the Twentieth Century on Herschel Island, Yukon Territory, Canada
Isla H. Myers-Smith, David S. Hik, Catherine Kennedy, Dorothy Cooley, Jill F. Johnstone, Alice J. Kenney, and Charles J. Krebs
AMBIO: A Journal of the Human Environment 2011 40 (6), 610-623