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"TROPHIC CASCADES" ON YELLOWSTONE'S NORTHERN RANGE. WHY ARE THE WILLOWS GETTING TALLER FOR THE FIRST TIME IN OVER 80 YEARS?
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Working in cooperation with Colorado State University, the US Geological Survey, and the National Park Service,
we have been examining differences between sites in which willows are getting taller and those in which they are still only waist height.
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When wolves were re-introduced to Yellowstone in 1995-1996, willows, aspen and cottonwoods began growing taller for the first time since the 1920s, when wolves were extirpated from the region. Several recent studies have explained this change as an example of a "top-down trophic cascade." This means that wolves, which are top predators, scare elk out of some of the areas in which willows grow, preventing them from browsing the plants down to bushy stumps. In other words, the top-down theory states that the effects of wolves have cascaded down from the top of the food chain to elk and then to plants. If this theory is correct, then wolves might make even more waves in the ecosystem. Once the willows are taller, beaver, which have been sparse in Yellowstone for the last 80 years, will be able re-colonize streams. Beaver dams, in turn, will trap sediment and change the water flow patterns, making changes in soil and stream chemistry that encourage even more willow growth.
But there is more to the story. Yellowstone's climate has been significantly warmer in the last 10 years.
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| The growing season is longer, and the seasonal distribution of precipitation has changed, concentrating more rain fall in fewer weeks every year (see the graphs). It seems likely that climate change has created "bottom-up" cascades that affect landscape-level vegetation patterns at least as much as the wolves.
As part of a three year project, we are currently studying the ways in which temperature, water availability, snow depth, and nutrient levels (such as nitrogen) control whether willows grow taller.
This study has important management implications. If our hypotheses are supported and we conclude that climate change is responsible for the recent willow growth, then re-introducing wolves to Yellowstone did not restore the ecosystem to a "naturally regulating state," as has been claimed. Instead, more proactive management will be required to counteract the widespread, landscape level changes that are likely to occur as global warming becomes more apparent in coming decades. 
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Graph data taken from USGS records and Snotel weather stations. Analysis conducted using factorial ANOVA in SPSS.
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