Document: JAM-3-69-44

The effect of human disturbance on plant assemblages and soil crusts of the shrub steppe in southeastern Washington.

HAWKE, M.A.* and J.R.KARR

University of Washington, Seattle WA 98195 USA ¹

Abstract:
Disturbance of native terrestrial plant assemblages often leads to an increase in alien plant species at the expense of native species. This study documented that this response occurs in the shrub steppe system located in the cool desert of SE Washington. Endemic soil crusts (stable aggregates of mosses, lichens, algae, and microorganisms) play important ecological roles in this system, and they are also sensitive to disturbance. We used a combination of 100-m line transects, 20 by 50-cm quadrats, and 1-m wide belt transects to quantify attributes of shrubs, grasses, forbs, and soil crusts at nine, fourteen, and six sites in 1997, 1998, and 1999 respectively. Disturbed sites were altered by diverse human activities (including agriculture, remedial activities, construction, landfill) that occurred from one to more than 50 years ago. Reference sites had minimal disturbance. Our goal was to screen a suite of potential indicators of the health of the system by identifying those attributes that were consistently different between reference and disturbed sites during all three years. Attributes that had higher values in disturbed sites compared to reference sites included taxa richness of alien species, and relative abundance of annual taxa, especially alien annuals. Attributes that showed a reduction with disturbance included total taxa richness, and the taxa richness, density, relative abundance, and percent cover of native plant species. The occurrence of a low percent cover of native shrubs together with reduced cover of soil crust was highly indicative of disturbance. Out of approximately 60 attributes screened, about fourteen were able to significantly distinguish between reference and disturbed sites (p< 0.5) for at least two out of three years. Another twelve attributes showed a consistent response to disturbance, and were statistically significant (p<0.5) in one out of three years. The attributes that showed such consistent responses to disturbance over the course of this study could act as candidate metrics in the development of a multimetric index to assess the biological condition of terrestrial systems, akin to the index of biological integrity (IBI) used for aquatic systems.

Keywords: disturbance, shrub steppe, soil crust

This abstract is being presented at: 10:30 AM in session:
Poster Session #12: Disturbance Ecology.