Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Abstract

Influence of topography on long-term successional trajectories in canyon grasslands.

Abstract

Question Some of the last remaining grassland communities of the Pacific Northwest Bunchgrass Region (PNWBR) in North America are found in canyon grasslands, primarily because the rugged topography has left many of these associations untouched by agricultural development. However, there have been no prior examinations into how canyon grassland communities change over the long term or how topography relates to these changes. In this study, we examined how successional stages changed over a 33-yr time period and which topographical variables were most related to these long-term outcomes (elevation, aspect or slope). We predicted that areas with more gentle slopes (<20%) would have changed the most because they have seen a higher concentration of historical land uses (e.g. livestock grazing, wildlife herbivory, and cultivation). Location: Canyon grasslands of Hells Canyon National Recreation Area, northwest USA. Methods: We looked at how the canyon grasslands have changed over the long term by repeating a vegetation survey of plant species cover and frequency that was completed in 1981 and comparing the results after a 33-yr time period. Results: We found that 58% of our plots stayed in the same seral stage or transitioned into later ones. As predicted, the plots that remained in early seral condition or transitioned from later to early seral stages were found on more shallow slopes. In addition, drier aspects within all plant associations appeared to be less resistant and resilient to change, particularly invasion by exotic species. Conclusions: The results of this resurvey suggest that topographic patterns may not only be related to the distribution of plant associations across the canyon grasslands, but may also influence which sites change most over time. Our findings are consistent with other studies of the relationships between topography and land uses through time in other grasslands, and are highly relevant to conservation management where unique vegetation communities develop within heterogeneous topographic areas.