Diversification of Molinia-dominated blanket bogs using Sphagnum propagules

This dataset contains vegetation and water table data from a trial described in the paper "Pilkington, M.G., Walker, J., Fry, C., Eades, P., Meade, R., Pollett, N., Rogers, T., Helliwell, T., Chandler, D., Fawcett, E. and Keatley T. (2021). Diversification of Molinia-dominated blanket bogs using Sphagnum propagules. Ecological Solutions and Evidence. Article DOI: 10.1002/2688-8319.12113

The trial investigates the establishment and growth of Sphagnum colonies arsing from planting Sphagnum propagules ("plugs") amongst purple moor grass, Molinia caerulea (‘Molinia’), a vascular plant which can dominate upland blanket bog of the UK. Increasing dominance by Molinia on these globally rare and protected bogs is a growing threat to diversity and carbon storage, as well as increasing the risk of wildfire.

Establishment and growth of plug-derived Sphagnum was measured by collecting visually-assessed data on cover (%) within 2 m x 2 m quadrats. The cover of blanket bog "indicator species" was calculated according to the definition of this group as stated in Common Standards Monitoring and the concept of Favourable Condition. Water table data were collected manually from monthly campaigns.

The following key results were found:

There was an initial rapid increase in cover of plug-derived Sphagnum (PDS) which suggested an advantage over that of naturally-occurring Sphagnum colonies, the latter remaining below 1% cover throughout; subsequent plateauing of PDS cover in areas of moderate Molinia cover (< 80%) was linked with drought stress, whereas declining cover in Molinia-dense areas (> 80%) suggested additional competition for light. The cover of Molinia was only weakly reduced by, and then completely recovered from, a baseline flailing treatment. Increasing cover of PDS in all of the treatments had no clear effect on the cover of Molinia. The cover of naturally-occurring indicator species was strongly reduced by a baseline flailing treatment; subsequent recovery was not complete, even with contributions by PDS. There was a negative linear relationship between Molinia cover and indicator species cover, over all ranges of Molinia cover. Water table depths were lowered by PDS during the first three years of the trial, likely due to facilitated capillary conduction of water though the buried plug tissues.

Methods
Vegetation cover within each fixed quadrat was meaured by visual assessment of indivdual species (e.g. Molinia caerulea, Sphagnum cuspidatum) and groups (e.g. blanket bog indicator species, pleurocarpous mosses) consistent with UK Common Standards Monitoring and the concept of Favourable Condition, and expressed as a percentage of the total area of each fixed qaudrat (4 m2). Total cover can exceed 100% due to overlapping layers of vegetation. 

Water table depth was meaured as the distance between gound level and the surface of the water table in dipwells which were situated within the fixed vegetation quadrats.

All vegetation data and water table data were shown as raw data and also as trajectories of change over time, by expressing the data relative to the control and normalising to a common zero starting point. Statisitical tests mainly involved Mann-Whitney U as the data did not fulfil the ANOVA criteria of a normal distribution. Trend lines describing the relationship between covers of Molinia, plug-derived Sphagnum (PDS) and indicator species were fitted using linear/polynomial regression models, with Molinia and plug-derived Sphagnum/indicator species cover as the predicting and responding variables, respectively.