Ministry of Environment
Environmental Stewardship Division
Riparian ecosystems were unevenly distributed between the different sub-units (Figure 31). They were at their maximum in the Comox Sub-unit but declined in extent southwards; the Islands Sub-unit contained only six polygons. Riparian ecosystems cumulatively occupied 1.6% of the study area (Table 2).
Figure 31: Proportion of sub-units containing riparian structural stages
Some riparian polygons were dominated by the early seral stages1 of sparsely vegetated or shrubby gravel bars, but most of them were mapped as structural stages 4 and 5—young stands of deciduous trees (usually red alder or cottonwood)—or young coniferous forests (Figure 32). Only a small proportion of the total represented mature or old growth forests of structural stages 6 and 7. Old floodplain forests of structural stage 7 were very rare, having been identified for only 12 polygons in total and accounting for only 110 ha; 17 additional sites contained stage 7 as a secondary component. Mature stands in stage 6 occurred more often with 1,500 ha mapped in 177 polygons and another 118 polygons contained mature riparian forest stage 6 as a secondary component. Gullies rarely occurred as the dominant or primary sub-class with only 7 units mapped; however, they occurred as secondary components in 116 polygons, the highest number occurring in Cowichan (43 sites) and Nanaimo (37 sites) sub-units.
Figure 32: Occurrence of Riparian Structural Stages as pure or dominant components
The large mountains of the Vancouver Island Ranges to the west of the three more northerly sub-units provide a large2 catchment with relatively abundant rainfall for the headwaters of a number of major river systems. These rivers drain to the east, collecting additional water along the way. They then cross a relatively broad band of the Nanaimo Lowland where they widen and have the opportunity to form large floodplains. Consequently some major river systems have developed in the three most northerly sub-units, such as the Campbell, Tsolum and Oyster rivers in the Comox Sub-unit, the Englishman, Nanaimo, Qualicum and Little Qualicum rivers in the Nanaimo Sub-unit, and the Cowichan and Chemainus rivers in the Cowichan Sub-unit. These rivers and their tributaries are where the major concentrations of riparian ecosystems occurred.
In the Capital Sub-unit there is far less opportunity for large river systems to have developed; as the mountains are lower and less extensive, there is generally less rainfall, and the hummocky and irregular nature of the bedrock has not permitted the development of large catchments. Some small systems have developed, mainly in the western portion where there are more rugged hills and higher rainfall. In the eastern half on the much lower elevations of the Saanich Peninsula and around Victoria, riparian areas had very little representation. On the Gulf Islands, the low relief, generally tiny catchment areas, and low rainfall are even less conducive to riparian development, and this was reflected in the results, which illustrated riparian areas in the Gulf Islands are minimal.
Dominant tree species were recorded for a few riparian ecosystem
polygons only. Western hemlock, western red cedar, black cottonwood,
bigleaf maple and Douglas-fir (in various combinations) were
the most commonly identified trees in stages 5, 6 and 7. However,
in earlier stages, red alder was frequently the predominant species.
Riparian polygons usually occurred as ‘pure’ units
(Figure 33). Secondary ecosystems were rarely identified, although
occasionally wetlands were indicated within a predominantly riparian
polygon.
Field checking of riparian units was conducted in 30% of the polygons in all, including three of the 12 structural stage 7 stands.
Figure 33: Occurrence of RI as 'pure' or 'complexed' polygons
Woodland ecosystems were identified for 0.6% of the study area (Table 2). Garry oak-dominated woodlands accounted for nearly half of the woodland polygons. The majority of these occurred in the Capital and Islands sub-units then diminished further north (Figure 34). Very few woodlands were identified in the most northerly part of the area. This is consistent with the range of the oaks, which were historically predominant only in the southern part of the study area.
Figure 34: Woodland ecosystems by sub-unit
Arbutus-dominated stands were identified in 221 polygons. In most cases, arbutus was found mixed with Douglas-fir. Only a very small proportion of polygons showed arbutus as the dominant tree species, combined with either Garry oak or a mix of Garry oak and Douglas-fir. Other units showed Douglas-fir as the dominant species, but variously mixed with oak, arbutus, and in a single case, Sitka spruce. In the southern third of the study area, oak and arbutus woodland ecosystems were frequently complexed with either coastal bluff or terrestrial herbaceous ecosystems, and less often with older second growth forest.
A few trembling aspen sites were identified in the woodland category; however, only five of the polygons indicated aspen as a leading species either alone or mixed with bigleaf maple and red alder. Although more aspen stands certainly occurred in the study area, they were often very small. Most aspen sites in the study area were smaller than the SEI minimum polygon size of 0.5 ha. Only a few polygons were mapped, such as a site at Fort Rodd Hill in the Capital Sub-unit and at Buttertubs Marsh in the city of Nanaimo.
Whereas it was clear that woodlands were very fragmented and poorly represented in the area as a whole, the areal extent may have been even lower than indicated. Nearly half (294 sites) of all the WD-dominated polygons had secondary ecosystems mapped within them (Figure 35) so the actual area occupied by woodlands in these units was lower than indicated (see Section 4.1 for discussion on limits of analysis). This was countered to some extent by the fact that woodlands were also identified as secondary ecosystems in 285 polygons. Of the 579 complexed WD units, 359 occurred in combination with terrestrial herbaceous ecosystems and an additional 177 polygons occurred with coastal bulff ecosystems.
Figure 35: Woodland as 'pure' or 'complexed' units
Additionally, some polygons identified as woodland during air photo interpretation may not have represented oak or arbutus woodlands, but may in fact have been second growth disturbed areas. For example, in eight of the polygons, bigleaf maple was identified as the leading tree species, red alder as the leading species in two polygons, and in 27 polygons, arbutus alone is identified. Douglas-fir and arbutus together were indicated for another 172 polygons.
Wildflowers were often noted in woodland stands, but the condition of the sites was often poor, with trails, trampling and general disturbance frequently observed. Broom and other introduced species were found throughout. Because the condition of most woodland sites (especially those with Garry oak) was often poor, the few sites of good to excellent quality and condition are noteworthy. The Elkington property north of Duncan was a good example of the type of Garry oak meadow that was once common on southern Vancouver Island. Other examples were found at Mt. Tzuhalem in the Cowichan Sub-unit and Mt. Maxwell on Salt Spring Island.
Nearly one-quarter (23%) of these polygons were field checked; some were clearly on cool aspects, atypical for woodland units. Field checking these units would likely result in some polygons being excluded from the SEI database, as some may be heavily disturbed areas or logged and burned sites now dominated by young arbutus or arbutus/Douglas-fir mixes.
Forests older than 100 years comprised 2.6% of the study area (Table 2). Only 5% (529 ha) of this was identified as predominantly mixed older forest containing more than 15% of deciduous trees combined with the conifers. The remainder was mapped as predominantly coniferous forest (Figure 36).
The Capital Sub-unit had by far the greatest representation of older forest, the majority of which existed within areas protected under a variety of tenures—in regional and provincial parks, on federal lands, and within the CRD water supply area. The sub-units of Comox, Nanaimo and most notably Cowichan, had poor representation of older forests. In the Comox area, an extensive fire 60 years ago and in the Cowichan Valley, intensive harvesting in the last 60 years would explain this. In the Cowichan Valley, less than 1% of the land area remained in older stands.
Figure 36: Proportion of sub-units containing Older Forest ecosystems
Although older forests would once have predominated through much of the area, it was clear that they were now very fragmented. Through the whole study area, only 15 polygons individually exceeded 100 ha. Eleven of these occurred within the Capital Sub-unit; two were in Nanaimo Sub-unit, and two in the Comox Sub-unit. In general, the older forests largely consisted of numerous very small fragments scattered across the landscape.
The age class information in the SEI database points to relatively poor representation of forests in the older age classes. A small number of polygons exceeded 250 years and the rest appeared to be between 100 and 250 years old.
Figure 37 shows that there were almost as many complexed older forest units (287) as there were pure units. The most common combination was with older second growth forest and a few additional sites contained HT.
Figure 37: Occurrence of OF as 'pure' or 'complexed' polygons
Various tree species combinations were identified as the dominants in these polygons. Throughout the area, Douglas-fir was the most common leading species listed under dominants. The rest were primarily Douglas-fir mixed with western hemlock or Douglas-fir mixed with western red cedar. Other combinations of dominant trees were much less common. A few sites included red alder and grand fir in the dominants.
Field checking was conducted in 138 polygons (29%). In these older forests, large fire-scarred trees were often observed and attested to the fire history of the area. The condition of these ecosystems, although often disturbed, was generally better than most of the more open units. However, some introduced species such as spurge laurel and European ivy were common in these forests, even abundant in some sites in the Capital Sub-unit in particular.
The following two ecosystem types were not categorized as sensitive but were included in the SEI inventory because they are valuable for their contributions to biodiversity and their importance to wildlife.
4.2.9 Older Second Growth Forest (SG)
A very large proportion of the total area of ecosystems mapped for this project—over 56%3 (44,890.5 ha)—was classified as older second growth forest, reflecting the extensive logging history of the area. Maturing stands of second growth forest are significant in that they are potential replacements for the diminished representation of older forests.
Within the older second growth forests, dominant trees varied. The most common dominant was Douglas-fir, either alone or combined with western hemlock, grand fir and occasionally red cedar or red alder, but a range of species mixes occurred. Mixed forests (with >15% deciduous component) accounted for about one third of the area of second growth forest; the remainder comprised coniferous stands with minimal deciduous tree cover.
SG primarily occurred in combination with other ecosystem types (Figure 38), usually as a secondary component. The most common combinations were with older forests in 209 sites and terrestrial herbaceous ecosystems in 183 sites. 19% of the SG polygons were visited.
Figure 38: Occurrence of SG as 'pure' or 'complexed' polygons
4.2.10 Seasonally Flooded Agricultural Field (FS)
Flooded fields covered 0.7% of the study area (Table 2). Greatest representation was within the Nanaimo and Cowichan sub-units, whereas Comox, Capital, and Islands areas had more limited occurrences (Figure 39). There were a few large and significant flooded fields in each sub-unit.
The average size was just over 10 ha. Only four polygons exceeded 100 ha—these were at Courtenay Flats, the farms south of Comox Airport on Richards Trail in the Cowichan Valley, and at Martindale Flats in the Capital Sub-unit. Another 10 polygons were between 50 and 100 ha. Field checking in this category was high with 45% of the polygons observed in the field; again this was partly due to the additional sampling of the CWS CVWI.
Figure 39: FS ecosystems by sub-unit
Most polygons classified as seasonally flooded fields occurred as pure units (Figure 40). Where complexed, FS occurred with wetlands in all but one site.
Figure 40: Occurrence of FS as 'pure' or 'complexed' polygons
4.2.11 Additional General Observations
The condition of the SEI polygons was found to be highly variable. In all areas, one of the most commonly encountered impacts upon the remaining natural ecosystems was the invasion of non-native plant species. Excluding flooded fields, which contained typically non-native vegetation, the ecosystems most affected by introduced species were the relatively dry and open habitats. Thus many of the sparsely vegetated, terrestrial herbaceous, coastal bluff and woodland polygons were the most extensively impacted. For example, terrestrial herbaceous ecosystems on the Gulf Islands were usually dominated by more than 50% cover of introduced grasses.
At many sites, shrubs were the most immediately apparent introductions. Broom was the most widespread and was widely distributed throughout the southern two thirds of the study area. The spread of this European species across the landscape has been greatly assisted by linear developments including hydro lines and roads, some of which were actively seeded with broom for erosion control. European gorse is another introduction that was dominant in a number of open sites in the south of the area. In woodlands and forests, shrubs such as spurge laurel and European ivy were common.
Whereas invasive species were most obvious in areas where there had been some past surface disturbance, even apparently undisturbed polygons often supported substantial non-native plant communities. On coastal bluffs, for example, non-native species occurred almost everywhere to some degree. More details of introduced species are provided in Appendix 10.
1 See Appendix 2 for description of these structural stages
2 Although these river systems are large relative to Vancouver
Island, they are small relative to other areas of British Columbia.
The SEI study area is characterized by relatively low rainfall
and dry conditions throughout the growing season.
3 The actual total will be higher than this, as only larger second
growth stands were mapped (Section 2.3.2).
Section 4.2.2 - 4.2.5 | Section 4.3 - 4.3.10
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