Patterns of flowering on continuously-grazed dune and machair on Colonsay

摘要

The pattern of flowering was investigated in 5x5m plots on dune and adjacent machair, both continuously grazed, on the Isle of Colonsay. During the study period twenty six species were in bloom in the machair plots (pooled) and twenty one in the dune plots. Approximately half the total number of species in bloom (thirty two) occurred in both the dune and the machair plots. A greater proportion of flowering records were from insect pollinated species (cf. wind pollinated) on the machair compared with the dunes. Sward composition was studied using lx 1 m and 2x2m quadrats. Machair quadrats showed greater species richness; there was no overlap with dune quadrat values. For the species flowering in both stand types, the average value of the parameter 'plots-per-species' (treating each species as equivalent to every other; see Tables 3 & 4 for details) was always greater for machair than dune, though the differences were not large. However for these species which flowered on both dune and machair, the total number of species in bloom, pooled over all plots, was greater on the dunes on three dates (Table 4). In addition, considering all species, the average number of species in bloom per plot was greater on the dunes on two dates (Table 3). Several species had a relatively long flowering period; some were in bloom throughout the full 3.5 months of the study period. A greater proportion of the total set of records (n=635) was due to species which flowered throughout the study period on the machair compared with the dunes. This feature is important for nectar and pollen feeding insects and for human visitors to the dune and machair. Continuous, heavy grazing has prevented the upward growth of flowering stems. There was a repeated production of flowers low to the ground in some species which appears to have extended the flowering season. Throughout the study period the vegetation height in the machair plots was 1 cm. This low uniform sward, with an absence of less-palatable taller plants e.g. meadow buttercup, and in which the grassland 'weed' ragwort is extremely rare, is one of the many forms that the machair can take. In the dune plots the vegetation was made up of three layers, the upper layer being marram plants. Despite the many differences between the two stand types, both belonged to the same National Vegetation Classification plant community.