We would have thought that the heavy machinery (see below figure) would have caused compaction of the surface peat layers - we actually noticed an about 15-20 cm downward push of our plastic tube dipwells during mowing in 2013 and relocated plots with a metal detector (above picture)! However, the peat bounced back up (and so did the rhizon DOC water samplers at our monitoring plots). We therefore compared the peat depth one year after mowing (2014) to that recorded before (2012) together with determining any bulk density changes by repeated coring at mown, burnt and uncut plots; this specifically tested if there were any peat surface impacts an overall peat depth measurement might not detect (due to general peat depth measurement uncertainties).
Mossdale after burning
Mossdale after mowing
Whitendale uncut plot within mown area
Below graphs: Surprisingly, there were no differences in bulk densities for only the mown treatments over the top 50 cm at either sites (the below graph shows data for the three sites: D1 0-5 cm; D2 10-15 cm; D3 20-25 cm; D4 40-45 cm). In fact, Nidderdale and Mossdale showed a general increase in bulk densities for burnt and uncut plots at the surface as well as for mown plots. These differences might represent sampling artifacts (i.e. defining the peat surface is difficult) or, most likely, different moisture regimes at sampling time and thus shrinkage/expansion issues (see Morton & Heinemeyer, 2019). However, Whitendale was virtually unchanged:
Below graphs: Comparison of peat depth across all three sites as control (C) burnt vs. mown treatment (T) and per treatment (T1-T5) and per block (1-4); DN is the uncut comparison. This test also did not show any detectable peat compaction after mowing:
Below graphs: Although we could not detect any peat compaction due to mowing with heavy machinery (BD or depth) we did detect impacts of mowing on the plot level micro-topography (i.e. mowing reduced the micro-topography by 'chopping off' the tops of tussocks and hummocks, mainly of sedges). We measured about 50 grid points across each 5 x 5 m plot for their offset against the mean peat surface (outside the plot) and compared the STDEV between treatments. The STDEV comparison showed that burnt (C) and uncut (DN) plots were similar whereas mown plots showed a reduced mean STDEV. In addition, a blox plot (shown for Whitendale) revealed the same pattern, note particularly the larger offset ranges (quartiles) for C and DN versus all mown treatments (M: with or without brash removed (BR) or Sphagnum (Sp) added), and M treatments all showing a lower median offset (reduced hummocks). The similarity between C and DN treatments support this reflects a real mowing effect and not differences in plot location:
Below graphs: Absolute offsets in relation to the average peat surface. Note that an offset was applied to all data per site (for further information see Heinemeyer, Sloan & Berry, 2019) to allow data to be displayed as a range from positive to negative (i.e. above and below mean peat level, respectively). Again, any of the mowing plots showed a clear tendency of a negative offset compared to the burnt (FI; with the exception of Whitendale) and/or uncut (DN) plots: