Vegetation Community Selection by Ungulates on the Isle of Rhum. I. Food Supply

Abstract

(1) The seasonal patterns of aerial biomass and biochemical composition of live graminoids, forbs, rushes, dwarf shrubs and dead plant material were examined at Harris on the Isle of Rhum, Scotland in 1982-83. (2) There was a marked seasonality in the biomass of live material of all plant growth forms, with high abundance in summer and low abundance in winter. The biomass of dead material showed the opposite trend. (3) The biochemical profiles and potential digestibility by ruminants of all plant growth forms varied seasonally. The graminoids, forbs and rushes had their lowest contents of cell-wall constituents and lignin in the spring and summer and their highest contents in the winter, whereas the dwarf shrubs had their lowest contents of cell-wall constituents in the summer. Overall, the graminoids had the lowest contents of lignin. The forbs and dwarf shrubs had the lowest contents of cell-wall constituents. The total potential digestibility of graminoids and forbs was greater than that of dwarf shrubs, whereas the potential digestibility of the intracellular material was greater in forbs and dwarf shrubs. (4) The eight main vegetation communities in the study area differed in their percentages of different plant growth forms. (5) Across vegetation communities, there was a negative relationship between the biomass of live plant material and the mean potential digestibility of the live material within the community. This was not the case for the relationship between the biomass of live material and mean cell-wall content of the live material within a community.