Prairies and Garry oak woodlands were once extensive in the lowlands of Washington and Oregon. Today less than 3% of these areas still exist and many are severely degraded. The extent and distribution of prairies, oak woodlands, and forests are important attributes of the cultural landscape of San Juan Islands National Historical Park in Washington, and as such are monitored by the National Park Service. Our monitoring objectives were to detect change in: 1) the extent of physiognomic cover types (herbaceous, shrub, tree, developed and unvegetated); 2) the proportion of areas dominated by exotic versus native species; and 3) the quality of herbaceous cover types – high (predominantly native with <10% exotic cover), medium (<50% exotic cover), and low.
Sampling was accomplished in 2007-2009 and 2012-2015 using line-intercept belt transects selected following a serially-alternating augmented panel design. Status, defined as the 2015 mean percent cover for a metric, was estimated as the weighted mean of the percent cover estimates at each transect. Transect data were weighted to account for the different length of transects to ensure inference to the park unit. For trend we fit a linear statistical mixed model for each of the cover types over all transects and years.
Results/Conclusions
Significant declining trends were detected for herbaceous cover, with an estimated trend of -0.52% per year (95% CI: -0.31, -0.74). The estimated trend in high-quality native herbaceous cover also decreased, at a rate of -0.12% per year (95% CI: -0.27, 0.03). This trend was significantly different from zero (p=0.0876) at alpha 0.10. The 2015 status estimate of herbaceous vegetation cover was 49.08% (95% CI: 40.59, 57.58). Shrub cover was 10.52% (95% CI: 6.91, 14.13), while tree cover was 35.95 (95% CI: 27.12, 44.77). The 2015 status of exotic herbaceous vegetation cover was estimated as 82.01% of herbaceous cover (95% CI: 76.18, 87.85). The 2015 cover of exotic shrubs was 13.79% of shrub cover (95% CI: 6.42, 21.15), while exotic tree cover was 0.39% of tree cover (95% CI: 0, 0.91). We could not sample all designated transects in some years, reducing our ability to adequately model or detect trends in all metrics. We expect that additional data collection will improve our ability to assess assumptions in future years. Results indicate the need for park restoration efforts to focus on mitigating shrub and tree encroachment, as well as managing exotic species abundance especially in native-dominated herbaceous communities.