Plant community response to elk activity in the Mount St. Helens blast zone

Background/Question/Methods - Thirty years after the 1980 eruption of Mount St. Helens, elk (Cervus elaphus roosevelti) are common on the recovering landscape, impacting vegetational dynamics across multiple temporal and spatial scales.  In 1991 an exclosure (ungrazed) and adjacent control (grazed) area were constructed in the blast zone near Coldwater Lake to compare plant community response to the presence and absence of elk activity.  Quadrat and line intercept surveys of vegetation were performed in 1991 and 2009. 

Results/Conclusions - Initial quadrat surveys revealed that grazed and ungrazed vegetation were similar in species composition, although mean richness, cover and diversity were all greater in the ungrazed area than in the grazed area.  By 2009, mean richness, cover, and diversity had increased in both areas, but more so in the grazed area.  Native cover doubled in both areas, but exotic cover declined by nearly half in the ungrazed area (U) while remaining constant in the grazed area (G).  Species composition of the two plots was more similar to each other within sample years (Sørenson’s community coefficient, G91↔U91 = 0.828, G09↔U09 = 0.825) than to themselves between sample years (G91↔G09 = 0.475, U91↔U09 = 0.48).  Comparisons of both areas over both sample years revealed that graminoid cover increased six-fold in the grazed area, but remained constant in the ungrazed area; woody plant cover doubled in both areas, and forb cover increased by 26% in the grazed area and 42% in the ungrazed area.  Line intercept surveys over that same interval, however, showed that woody plant cover in the ungrazed area increased by 25%, but declined slightly in the grazed area, revealing that differences in scale of measurement reflect differences in scale of cover.  We conclude that while succession is occurring in both areas, elk are altering the trajectory of succession.  They are increasing overall diversity in the system, slowing (but not stopping) the spread of woody plant cover, promoting rapid expansion of graminoid cover, and facilitating the persistance of exotic species.  The trends observed in plant community response to elk activity in this early successional landscape are similar to those reported in mature ecosystems.