Revegetation dynamics of cliff faces in abandoned limestone quarries.
Abstract
Revegetation dynamics on vertical cliff faces in abandoned limestone quarries were examined at 18 sites in southern Ontario, Canada, abandoned between 1901 and 1976. Sites varied greatly in depth and size (area) and surrounding vegetation cover. Two of the older smaller sites were adjacent to natural escarpments and were concealed by dense Thuja occidentalis forest; others were completely open and exposed with little adjacent vegetation cover, and some were filled with water. The greatest distance between sites was 350 km. A wide range of biotic (vegetation sampling on the quarry walls, tops and floors) and abiotic (age, size, latitude, distance from the Niagara escarpment, geological formations present, aspect, presence of ledges, substrate instability) variables were measured in randomly positioned plots at each site. Quarry walls ranged in age from 17 to 92 yr since abandonment and this known chronosequence was used to investigate successional patterns in the vegetation. Site age and the density of trees adjacent to the quarry walls had the strongest influence on vegetation community composition and abundance. During the first 70 yr after abandonment, species richness was high and variable. Community composition was also extremely variable during this period. After 70 yr, species richness on quarry walls dropped suddenly and the variation in vegetation composition was reduced. This period of sudden change correlates with the growth of neighbouring trees on the quarry floor that start to shade the quarry walls at about 60 yr since abandonment. The final form of the vegetation community on the vertical cliffs in long abandoned quarries is similar to the naturally occurring community on cliffs of the Niagara Escarpment. The results of this study suggest that natural successional processes can be used to rehabilitate the vertical cliff faces that are produced in limestone quarry operations. The benefits of allowing natural processes to rehabilitate the walls of abandoned limestone quarries are: the recruitment of an uncommon, but native component of biodiversity; a reduced initial cost; and a reduced long-term maintenance cost. It is recommended that abandoned limestone quarries should not be rehabilitated by backfilling of exposed quarry walls.