Explaining the coexistence of species in mosaic cushion heath

Cushion plants can occur in two-dimensional mosaics of species, which have been shown to be stable in their proportions of species, but unstable in their patterns (Gibson and Kirkpatrick 1992). Possible causes of this state of quasi-stability were explored for a cushion mosaic composed of four principal species, Pferygopappus lawrencii, Abrotanella forsteroides, Dracophyllum minimum and Oreobolus pumilio at Mount Field National Park, Tasmania. An area of a cushion mosaic was selected, the topography measured, temperature readings taken under different synoptic conditions and the ability of the two most common species, Pferygopappus lawrencii and Abrotanella forsteroides to repel moisture measured. Slope, aspect and radiation input in clear sky conditions were calculated for the 418 grid points at which topography and temperature were measured and the species noted. The differences between species in their distributions at the study site were related to slope, aspect, elevation, clear sky radiation input and temperature adjusted for radiation and slope. Chi-squared was used to determine the significance of differences between species for misting and droplet treatments. O. pumilio was associated with steep slopes and lower radiation inputs. P. lawrencii was at lower elevations than A. forsteroides. Water beaded on the surface of P. lawrencii at a higher rate than on A. forsteroides, regardless of drop size. Temperature alone was found to be a poor indicator of species occurrence. Species switched their temperature rankings in different weather conditions. To conclude, the data most closely fit a model of temporal environmental fluctuations with continuous competition. Differential cushion growth appears to lead to variation in topography and therefore differences in interception of radiation. This negative feedback mechanism may be contributing to the perpetuation of coexistence.