Seventeen different vegetation communities were identified in the study area and their distribution is depicted on Figure 1. The species composition and structure of these communities are shown on Table 1.
All of the site has been influenced by past livestock grazing and/or cultivation which has had a pronounced effect on vegetation. Most of the existing vegetation has succeeded from open pasture within the past several decades. The surrounding lands have been under intensive cultivation for a long time, consequently the surrounding seed source for native plant species to originate from was quite limited.
The creek has a prominent influence on much of the vegetation. The high water table on the floodplain (although most is not wetland) and the deposition of alluvium favours riparian associations. Woodlands dominated by Manitoba Maple and Crack Willow, are typical of impacted riparian zones in southern Ontario. Some of the Crack Willows here are very old (as evidenced by 1926 air photos). As the trees get old, they split apart, but the reclining limbs continue to grow. The younger riparian meadow is composed of plants whose seeds are spread in the alluvium such as Reed Canary Grass, asters, Spotted Jewelweed and Jerusalem Artichoke.
The steep slopes on east and west sides of the floodplain are covered with woody vegetation. Part of the east side contains a Black Locust plantation likely planted to stabilize the slopes. The rest is a dense thicket woodland of Common Buckthorn with a grove of mature deciduous trees (mostly Sugar Maple and American Basswood) above slope. These trees formed a hedgerow that has been there a century or more. Part of the west slope consists of a monoculture stand of White Cedar. The rest is an old Crack Willow stand that has been invaded by Manitoba Maple and Common Buckthorn.
An area of upland meadow occupies the tableland in the southwest portion of the site. The northern part of this community has a high amount of Red Cedar, which is an uncommon plant in Durham Region, consequently this is a locally rare vegetation type. The meadow seems to be regenerating very slowly, perhaps because of past grazing intensity and the tight clay loam soils. The presence of Red Cedar is a good indication of alkaline conditions.
A block of adjacent mixed forest dominated by White Cedar occurs on the floodplain immediately upstream (northwest) of the study area. It is the nearest area of forest to the site and may be the seed source for many of the native plants which have colonized the site since it was pastureland.
A stormwater management pond was constructed on the floodplain at the south east corner of the study area. Although artificially formed, the pond provides an open water habitat dominated by submersed aquatic plants such as Canada Waterweed and Sago Pondweed. A narrow fringe of emergent marsh vegetation of cattail, Great Burreed, Broad-leaved Arrowhead and Mud-plantain surrounds the pond. The East Lynde Creek provides aquatic habitat and patches of Curly Pondweed occur along some stretches.
The existing woody vegetation in the study area is overwhelmingly dominated by non-native plant species. In particular, Common Buckthorn is abundant in nearly every vegetation community, both floodplain and tableland, and is dominant in several. Because the site succeeded from open pasture, a native seed bank was not present. Buckthorn was able to establish and dominate.
Invasive non-native plants are undesirable because they outcompete native plants or prevent them from re-establishing. They also provide poor habitat for indigenous wildlife species. Common Buckthorn forms pure stands where even the ground layer is dominated by buckthorn seedlings. In some very mature, closed canopy buckthorn stands we found a more open understorey with some seedlings of Sugar Maple and basswood present. In dense, young to medium-aged buckthorn stands there was no native regeneration.
Riparian vegetation communities are particularly susceptible to invasion by non-natives. The Crack Willow - Manitoba Maple association is not native. More recently, Black Alder has become established along Lynde Creek (including the study area) and it is spreading rapidly. Seepage areas along the creek support stands of Himalayan Balsam which is replacing the related native Spotted Jewelweed. Several patches of Purple Loosestrife, much publicized as a problem species, have been noted but it is not yet abundant here.
Dog-strangling Vine which can be a serious invader of upland habitats, was also recorded but not in significant numbers. Autumn Olive was also found as scattered individuals. This species a recent colonist in southern Ontario, but its rapid spread suggests that it may soon become as serious a problem as Common Buckthorn. Norway Maple is common in the understorey of the adjacent woodlot to the northwest and spreading into the study area. It is known to dominate and displace native species in urban woodlots.
Through the course of field investigations, 212 species of vascular plant species were recorded. Of these, 95 species (45%) are non-native. Generally, locations where more than one third of the flora is exotic can be considered highly impacted. Seven native species which do not otherwise occur on site were planted in the vicinity of the storm pond. The compliment of the flora is a mix of upland field, riparian meadow and thicket with a small number of forest and wetland species. All recorded species are listed in Appendix A along with the general habitat in which they were encountered.
No provincially significant plant species (Oldham 1996) were found. Two species recognized as rare within the Regional Municipality of Durham (Varga 1999) were found in the study area and are described below:
Cow Parsnip (Heracleum maximum) - This is a tall plant that is adapted to rich alluvial soils of floodplains. Small groupings of Cow Parsnip were noted growing near the creek at several locations in the central part of the study area.
Gray Dogwood (Cornus foemina) - Several shrubs growing in the Manitoba Maple thicket at the north end of the property. This is a successional old field species.