Vegetation ecology of rock outcrop ecosystems of the Gulf Islands in the Coastal Douglas-fir zone, British Columbia

Rock outcrop ecosystems of the Gulf Islands in the Coastal Douglas-fir (CDF) biogeoclimatic zone of British Columbia were investigated at multiple scales with the following objectives: (1) to refine distribution information for constituent species, (2) to investigate landscape (i.e. site-level) features that influence the patterning of native and introduced plant species of rock outcrop habitats, including (a) geographic position (latitude), (b) geology (rock type), and (c) grazing intensity, (3) to integrate vegetation patterns observed at each sampling scale (site, plot, microplot) to derive a classification scheme for rock outcrop vegetation, and (4) to interpret rock outcrop ecosystem dynamics and address conservation and management implications. A total of 311 plant species were identified from inventoried sites. The majority of plant taxa in rock outcrop ecosystems were herbs and bryophytes, and most of the rare species were mosses. Each landscape feature was associated with unique patterns of coverage and richness for different life form groups (studied by origin and rarity ranking). Uncommon bryophyte species richness was higher in southern sites where exotic vascular species coverage was highest. In contrast, the coverage and richness of native and uncommon graminoids was higher in northern sites. The overall richness of native herbs, and the richness of uncommon herbs was greatest in sedimentary rock sites, where exotic species also had greater coverage and richness. Ungrazed sites showed higher vascular plant species richness, whereas intensely grazed sites revealed higher bryophyte coverage, and greater richness of rare bryophytes. Sequential principal component analyses were used to classify vegetation and characterize scale-related vegetation-habitat relationships. Three major landscape categories were identified, based on the primary environmental gradients found to influence large-scale vegetation patterns: meta-igneous rock sites >49°N (META-N), meta-igneous rock sites <49°N (META-S), and moderately-grazed sedimentary rock sites (SED). Vegetation assemblages and fine-scale habitat relationships differed among categories, although there were some similar trends overall. The first gradient identified for each landscape category was related to microplot moisture. Within each category, the richness of native herbs (overall, and uncommon species) and the richness of bryophyte species was higher in microplots where seepage species had higher coverage. The second gradient identified for each landscape category was related to exposure and potential soil development. Within each category, bryophyte coverage and richness (and within META-N and SED landscapes, the richness of rare bryophytes) was highest in microplots with the greatest exposure and lowest potential for soil development. Conversely, the richness of native vascular plant life form groups (particularly graminoids) was correlated negatively with this gradient. Rock outcrop ecosystems showed higher richness of native herbs and native mosses per unit area than did regenerating and mature forests ([Less-Than or Equal to]5 years to [Greater-than or Equal to]90 years old) in CDF zone landscapes. Rock outcrops also supported a greater proportion of uncommon herb and rare moss species per unit area than forested CDF habitats. Results showed a weak, negative relationship between distance of plots from forest edges and the number of native species "shared" with sampled CDF forests, indicating that edge proximity may be important for the perpetuation of some rock outcrop taxa, particularly woody plants and bryophytes. The richness of native outcrop "exclusive" species was correlated more to site-specific landscape factors (i.e. rock type, geographic position, and grazing regime) than to island size or rock outcrop ecosystem polygon size. Rock outcrop ecosystems are not equilibrial; instead they represent a collection of assemblages in different stages of continuous primary succession. Within each landscape category, unique fine-scale successional trajectories were identified in association with overall soil and vegetation development. Rock outcrop ecosystems, and their constituent microhabitats and species, are perpetuated by different types of disturbances that act at different scales and with varying frequencies, the effects of which depend on geographic position, rock type, and land use history. This thesis has shown that patterns of coverage, richness, and rarity may differ among life form groups. Whereas past management strategies for species recovery have focused predominantly on vascular plants, results presented here indicate that bryophytes also deserve our attention. Conservation objectives must be considered carefully, not only for the species and life form groups of primary concern, but also for the environmental variables that influence those species and groups. Successful management strategies will rely on multi-faceted and multi-scale approaches to ensure the perpetuation of diverse rock outcrop ecosystems within the CDF zone.

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