This finding highlights the complicating fact that height of young aspen is both a cause and an effect of reduced elk browsing. "This means that the tallest young aspen grow faster because they are taller, not because wolves reduce elk browsing," said MacNulty. As the leader stem (main trunk) of a young aspen grows past the shoulder height of adult elk, it is decreasingly likely to be eaten as it grows taller, said MacNulty. They tend to eat plants at shoulder height for which they don't have to crane their necks. "The traditional method of sampling by only using the tallest young aspen plants to measure growth - which most research currently relies on - doesn't capture the entire picture."įor one, elk are picky about the aspen they consume. "These are extremely complex systems, and understanding them is a major challenge because they are difficult to properly sample," said Brice. But this is not the case - sampling only the tallest young aspen estimated a rate of recovery that was significantly faster than was estimated by random sampling of all young aspen within the stand, according to the research. The reasoning was that the tallest young aspen trees represent a 'leading edge' indicator of the future recovery of the entire aspen population. Previous studies evaluated aspen recovery in Yellowstone by measuring the five tallest young aspen within a stand. Previous research showed strong positive growth in young aspen as the elk populations decreased - a welcome result, as aspen forests have been vanishing from the northern Yellowstone landscape for the last century.īut new research from Elaine Brice and Dan MacNulty, from Utah State University's Department of Wildland Resources and Ecology Center, and Eric Larsen, from the University of Wisconsin Stevens Point's Department of Geography and Geology, shows that the effect of wolves on the recovery of aspen has been exaggerated by how it was measured.
When wolves were reintroduced into the Yellowstone food chain, they helped to reduce numbers of elk, which had been consuming young aspen trees. The Yellowstone story is a textbook example ofa trophic cascade, in which predators help plants grow by eating or scaring away herbivores that eat the plants.
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