Species-area relationships in wet prairie remnants and restorations of the Willamette Valley, OR

Background/Question/Methods

Natural vegetation in wetlands is often patchy, appearing homogeneous at fine scales (within-patch), heterogeneous at meso-scales (between patches), and with repeating macro-scale patterns of patches within a site that produce a characteristic appearance and texture of vegetation. The success of efforts to restore native plant diversity in wetland restorations is often evaluated using metrics such as native species richness, percent cover, community composition, or various indices of diversity. However, it is less common to compare other types of patterns exhibited by vegetation. Examination of such patterns could reveal differences between sites in the degree of homogeneity of the vegetation, and help assess not only whether the restorations contain similar numbers of species or sets of species, but also whether the spatial pattern of species occurrence within the site is similar to that of reference sites. Including such analyses of pattern in our descriptions of remnant native vegetation can improve our understanding of the nature of the plant communities we hope to restore, and help identify plot size and number of plots required to characterize vegetation communities. Such analyses could also guide efforts to develop appropriate levels of within-site spatial heterogeneity, which may be important aspects of habitat.

Results/Conclusions

Species richness of wet prairie wetland vegetation in wetland remnants was compared to that of restored sites in the Willamette Valley, Oregon with 1 m²  micro-plots and 100 m² meso-scale plots, to determine whether sites could be distinguished based on native species richness alone, and if so, at what spatial scale the differences emerged. We also asked whether spatial patterns of cumulative species richness differed between sites in patterns that could help identify differences within and between patches using modified Whittaker nested plots of 1, 10, 100, and 1000 m². Total species richness (S) of the 1 m² microplots did not differ significantly among sites (average S=10 spp.), however, low native species richness distinguished restorations in which native vegetation was poorly established even at this scale.  Differences between sites in native species richness were significant at the 100 m² plot scale. Comparison of these data to published data from intensive sampling of two of the sites which included a complete floristic survey of the sites as well as fine-grain sampling in 0.5 m x 0.5 m plots (n=270 and n=450) were used to characterize spatial patterns and within- and between- patch differences at those sites.