Scat happens: Local influences of elk on primary succession, Mount St. Helens, WA

Background/Question/Methods

The 1980 eruption of Mount St. Helens devastated ~200 km² of forest. Nearly 30 years later, elk (Cervus elaphus roosevelti) are common in much of the blast zone and have distinct influences on succession. In order to explore how elk are modifying vegetation, I started several studies to answer the question: How do elk modify successional trajectories in a newly reforming landscape? Elk scat may promote seedling establishment or growth of certain species. Trampling by elk may alter successional trajectories locally. Through greenhouse and field studies, I show that elk activity in sites of primary succession can produce alternate stable patches.  

Results/Conclusions

Survey data from exclosures (analyzed with Detrended Correspondence Analysis) revealed that successional trajectories within exclosures differ from control sites; elk activity appears to reduce native species diversity and local tree cover. In greenhouse trials, shrubs (Alnus viridis, Vaccinium membranaceum), grasses (Elymus elymoides, Poa secunda) and an introduced forb (Rumex acetosella) all grew significantly taller when sown with elk scat than without. The herbs also accumulated significantly more biomass when allowed to establish near elk scat than when isolated. In contrast, DCA showed that infrequent trampling by elk did not alter succession in the field. Thus, the positive effects of scat on succession appear to be greater than the negative effects of trampling.