Point Frame Measurements of Poor Fen Moss and Vascular Frequencies Under Increasing N Deposition Over Five Years, 2011-2015

Development of the oil sands has led to increasing atmospheric N
deposition, with values as high as 17 kg N ha-1 yr-1; regional
background levels <2 kg N ha-1 yr-1. To examine responses to N
deposition, over five years, we experimentally applied N (as NH4NO3)
to a poor fen near Mariana Lake, Alberta, at rates of 0, 5, 10, 15,
20, and 25 kg N ha-1 yr-1, plus controls (no water or N addition).
Anchored point frames were established at each plot and measure at 30
points along the frame each year in July. We used absolute frequencies
of Sphagna and vascular plants to examine species and community
responses to N treatment. Increasing N input led to led to decreased
abundance of S. fuscum and increased abundance of S. angustifolium, S.
magellanicum, Andromeda polifolia, Vaccinium oxycoccos, and of
vascular plants in general. For dominant shrubs and all vascular
species combined, N addition had no effect in the first two years, but
frequencies increased with increasing N addition in 2013-2015.
Pairwise year comparisons indicated a gradual change in plant species
composition as the experiment progressed. Of the 21 pairwise
comparisons between nominal N treatments, 18 were significant,
indicating a clear influence of N addition on plant community
composition. These results suggest that bogs that have persisted under
very low ambient N deposition may be especially sensitive to
increasing N deposition, in terms of plant species’ relative
abundances and plant community composition.