In the first period of fieldwork, the mowing plots were not sampled, as they were in the process of being mowed, which would have compromised the reliability of the measurements. The placement of the second period of fieldwork allowed for the sampling of the mowing plots, in order to compare with the other treatments.  
The total sample size for the two periods of fieldwork were therefore; 6 blocks x 4 treatment plots = 24 sample surfaces in the 1st period and 9 blocks x 5 treatment plots = 45 sample surfaces in the 2nd period.