The dynamics of rush circles in subalpine grassland

Abstract

Geometric patterning in vegetation is often the result of cyclic succession, although circular forms sometimes result from the clonal growth of individual plants. We investigated whether cyclic succession or clonal growth created patterns in Poa subalpine grassland with rush (Empodisma minus) circles on the Central Plateau of Tasmania, Australia. We mapped change in the number, size and reflectance of the rush circles from 1953 to 2011 using aerial photographs, tested whether the circles occurred in distinct microenvironments, and whether the circles were likely to be individuals and determined whether the smallest circles were associated with disturbance. The number and area of circles increased with time. As circles became larger they became more reflective, evenly across their extent. The circles were associated with flatter patches in the plain. Surface soil characteristics did not differ between an exemplar circle and adjacent grassland. The Empodisma foliage in each circle tended to have colour consistently distinct from that of other circles in its vicinity, indicating that the circular form was a product of vegetative expansion of a single individual at a much more rapid velocity than the expansion of woody plant rings. The smaller circles were sometimes associated with frost-damaged Astelia alpina mounds, although far from confined to them. We did not find any instances of Empodisma circles <0.1 m in diameter. We deduced that the probability of establishment of new individuals of Empodisma in the grassland was extremely low, or depended on an infrequent disturbance. Although the increasing reflectance of circles with size and the lack of differentiation of soils between circle and grassland indicated the possibility of cyclic succession, the area covered by circles increased at the decadal time scale. The increases in circle coverage and number may have resulted from changes in climate and grazing regimes on the eastern Central Plateau since the 1940s.