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School of Geography

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The Cause of Molinia caerulea (purple moor grass) Dominance in Peatlands. 

PI: Graeme Swindles

Overview: 1. Many peatland ecosystems in Europe have become degraded owing to the effects of drainage, burning, pollution, and climate change. There is a need to understand the drivers of peatland degradation as management and restoration have implications for the natural ecohydrological dynamics of such sensitive environments, and also attract substantial restoration costs.

2. A feature of many degraded peatlands is the presence of Molinia caerulea (Purple Moor-grass), which has taken over at the expense of other typical peatland plants, including sedges (Cyperaceae) and Sphagnum mosses. Here we present a comprehensive, replicated multiproxy dataset from Swarth Moor, a raised bog in the Yorkshire Dales National Park, to investigate the recent state of the site in a long-term context, and to determine the drivers of change.

3. The proximity of Dry Rigg Quarry to Swarth Moor and dust-loading from these workings has been suggested as being the primary instigator for the colonisation of M. caerulea. We test a series of potential factors for the shift to M. caerulea dominance at the site using proxy evidence, including: hydrological changes due to drainage/climate variability, pollution and dust-loading, burning and ecological dynamics. The records are dated using spheroidal carbonaceous fly-ash particles (SCPs - age equivalent stratigraphic markers for the last ~150 years) and AMS 14C dating. We also use historic and recent aerial photographs to illustrate the expanding footprint of Dry Rigg Quarry.

4. Multiproxy data show that there has been a long history of human impacts at Swarth Moor which have led to its current unfavourable condition. This includes peat cutting during the Iron Age, and in the medieval and post-medieval periods, and also more recently in the late eighteenth-nineteenth century.

5. The rise of Molinia caerulea occurs in the latter half of the twentieth century, dated between ~ AD 1951-1974 that corresponds with the post-war expansion of Dry Rigg Quarry (i.e. intensified dust-loading), but also correlates with rises in: i) micro-charcoal (burning), ii) fly-ash deposition and airborne pollution, and iii) contamination from soil dust and agrochemicals.

6. Our data indicate the current state of Swarth Moor is the product of a long history of human activity. However, the appearance of M. caerulea occurs coevally with an unprecedented variety of recent anthropogenic impacts, all of which have contributed to providing a suitable environment for its rise to dominance at the site.

7. Our research also indicates that although the peatland ecosystem at Swarth Moor is unfavourable there is evidence to suggest, due to the recent increase in Sphagnum papillosum (Papillose Bog-moss), that the peatland is beginning to recover. The presence of SCPs in the top ~10-20cm of eleven peat cores peat from across the site clearly indicates that peat has been accumulating in the recent past (last ~150 years).

8. Our work demonstrates the importance of palaeoecology for understanding the trajectories of peatlands, and how they respond and recover from past human-environment impacts. This is fundamental for determining whether it is better to manage and restore peatlands, or to allow them to recover naturally. As there are plans to re-wet the site, the multiproxy data provided by this study should be taken into account prior to the implementation of the proposed restoration strategy.

Start date: November 2014

End date: November 2015

Funder: Lafarge