A study on the biology of four Tasmanian cushion species

Donatia novae-zelandiae, Abrotanella forsteroides, Dracophyllum minimum and Phyllachne colensoi are four superficially similar bolster species found in the Tasmanian alpine vegetation. The ecological differentiation of the ranges of these species, the dynamics of the formation and persistence of complex mosaics of two or more of these species and the functional significance of the bolster form were areas in which there were many hypothesis but few data. Growth studies undertaken in Tasmania showed significant seasonal differences in the competitive ranking of cushion species. Annual shoot production ranged from 426-709 g/m2 for A. forsteroides; 359-477 g/m2 for D. minimum; 322-572 g/m2 for D. novae-zelandiae; and 282-285 g/m2 for P. colensoi. These figures are greater than those reported from cushion communities in New Zealand and from physiognomically similar communities in the northern hemisphere, probably reflecting the maritime climate and the longer growing season of the Tasmanian alpine zone. Seed production was highly variable between sites with D. novae-zelandiae producing between 2,370-38,900/m2 and D. minimum between 0- 17,140/m2 over the 1982/83 summer. Estimates of the lateral growth rate of A. forsteroides in open situations on peat substrates ranged from 6.7-14.7 mm diameter increase/year. D. minimum seedlings on rocky and well drained mineral soils only achieved lateral growth rates of between 0-7 mm/year. Data from the growth studies showed that cushion distribution at Mt. Field could not be explained by either soil moisture or soil nutrient preferences. Growth was found to be highly variable from year to year and season to season both between species and within species over their altitudinal ranges. It was concluded that interspecific competition between bolster species plays little role in determining their overall distribution patterns. Climatic modelling suggests that distribution of the species is primarily controlled by the temperature (this is presumed to relate to competition from taller growing shrubs and graminoids) and past climatic history. Donatia ilovae-zelandiae, Abrotanella forsteroidel, and Dracophyllum minimum may still be expanding their ranges following the climatic amelioration since the height of the Last Glacial. Investigations into the dynamics of a bolster heath modified and improved the model proposed by Jackson (1981). Pathways of succession due to changes in water table appear to result from complex interactions between the water table level, propagule source and species already present on the site. Data from a peat core indicate that once bolster communities have become established, they can persist for very long periods. In the succession from bare ground to closed bolster communities there is a general lack of pioneering species. Results from the growth study, the patterns of reinvasion of drained tarns and studies of permanent photopoints are consistent with the view that succession in bolster communities is best described by inhibition or tolerance models (Connell and Slatyer 1977). Competition appears to play an important role at the establishment phase in mature bolster communities but once establishment has occurred it is much less important. Competition also appears to be of little importance in the building phase of bolster Communities in highly stressed environments. Attempts to elucidate dynamics by studying pattern in mosaic bolster communities using spectral analysis failed due to the inability of this technique to separate patch size and inter-patch distance. The roots of all cushion species freeze at temperatures between -1° and -5° C. Nonetheless the insulating properties of cushion peals are so effective that it is unlikely that root freezing ever occurs with adult plants in the field, at least under the present climatic conditions. Similarly the thermal characteristics of peat soils are such that seedling death due to root freezing would also be an extremely rare event. Short interval temperature measurements showed A. forsteroides capable of achieving cushion surface temperatures 10° C above ambient temperatures. It is suggested that this feature of cushion plants may allow them to undertake photosynthesis under otherwise limiting conditions and may significantly extend their growing season. Anatomical investigation of the four cushion species showed a highly variable internal structure. This diversity is difficult to interpret. The scleromorphic nature of most of the species may be related to nutrition. The xeromorphic features exhibited by the four species and the thermal characteristics of the cushion form are inconsistent with the hypothesis that these species suffer significant periods of water deficit.