Recent rise to dominance of Molinia caerulea in environmentally sensitive areas: new perspectives from palaeoecological data.
A characteristic of some heath and moorland areas in maritime north-west Europe is the widespread dominance of Molinia caerulea (purple moor grass). The overwhelming local supremacy of this species concerns farmers, owing to its relatively low palatability for grazing stock, and conservationists, owing to the monotonous, species-poor landscapes that often result under Molinietum. In some environmentally sensitive areas (ESAs) in England and Wales, UK, Molinietum is believed to have ousted Callunetum in recent decades; experiments sponsored to control the species have predicated its infiltration and replacement of heather-dominated stands. Experimental control of M. caerulea in ESAs on Exmoor, England, was paralleled by palaeoecological studies to verify its recent rise, assess its status in moorland, and test the utility of the techniques for such research. Peat profiles from two localities on Exmoor were sampled and subjected to recently developed techniques of plant macrofossil counting and to conventional pollen analysis. One locality was 'white moor', clearly dominated by M. caerulea; the other was 'grey moor' (an admixture of ericaceous shrubs) that had become invaded (allegedly recently) by M. caerulea. Dating of profiles employed a range of methods, including conventional radio-carbon dating, Accelerator Mass Spectrometry (AMS) dating and the counting of spheroidal carbonaceous particles, to attempt to delimit horizons of recent peat growth. The pollen and macrofossil data confirmed the recent ousting of Calluna vulgaris and rise to dominance of M. caerulea in the grey moor, but also provided evidence of an earlier unsuspected (pre-Callunetum) presence of M. caerulea. The overwhelming dominance of M. caerulea in the white moor was also a recent phenomenon, but was only partly at the expense of C. vulgaris. The palaeoecological data indicated a greater antiquity and former abundance of M. caerulea than is often appreciated and suggested that, over the past millennium, vegetation dominance has alternated between Callunetum and grass moor containing at least some M. caerulea, while the former C. vulgaris-dominated grey moor itself developed originally from grass moor.