Stability among cyclic change in an Antipodean pond and bolster heath system 1983–2017

Abstract

A long-term study of a bolster heath–pond system at Newdegate Pass, Mount Field, Tasmania, was designed to test a theory of cyclical dynamics and to determine why there was constancy in the proportions of species in stable mosaic bolster heath, while their individual distributions changed. Groups of permanent plots were monitored approximately once every 5 years between 1983 and 2017, in apparently stable, apparently colonising, apparently recovering and apparently degrading bolster heath. The putatively stable sites exhibited an average of 37% spatial turnover in the three early 5-year periods, with reciprocal replacements between many pairs of species; however, there was little evidence of long-term directional change except for expansion of a gymnosperm shrub across the top of the bolster mosaic at one site. This pattern suggests a cyclical succession of species in the stable mosaic; however, the mechanism driving cyclicity remains unclear, with facilitation, competition and stochastic processes being implicated at different times and locations. These cyclic changes were likely to have been largely endogenous, given the variability and autocorrelations in relative growth rates of the stable mosaic over short horizontal distances, conforming with neutral theory. The process of dam vegetation loss after draining was rapid, as was vegetation colonisation of the floor of one dam and recovery from fire. On a putatively colonising site on rocky ground, changes were subdued but directionally consistent with the previously proposed model, which was consistent with all long-term observations.