Gap formation and cyclical change in heathland vegetation
The phasic, cyclical model of Calluna-dominated vegetation dynamics, proposed by A.S. Watt, is evaluated by investigation of the gap in the degenerate plant. Succession in the gap is analysed by Markov models and found to be non-Markovian, with a number of processes occurring simultaneously. Vegetation changes are better interpreted in terms of certain ecological attributes of the species concerned. The inter-relationships between three important moss species are investigated further. Regeneration of Calluna is very variable. Seedling establishment requires a safe site and sufficient moisture, and depends on wet summers. Vegetative layering occurs more frequently but varies between parent plants and substrate types. The presence of soil micro-organisms appears necessary for adequate adventitious root production. An outbreak of heather beetle was monitored and contrasted with outbreaks in the Netherlands. The population was reduced by a parasitoid which acted density independently. At these low densities, a mosaic of gaps is formed in the vegetation by spatial heterogeneity of heather beetle attacks. These gaps behave in a similar fashion to degenerate gaps, and most Calluna regeneration occurs by layering. The value and limitation of Watt's model is discussed, especially by reference to forest gap-dynamics theory.