Point Frame Measurements of 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 . Bogs, being ombrotrophic, may be
especially susceptible to increasing N deposition. To examine
responses to N deposition, over five years, we experimentally applied
N (as NH 4 NO 3 ) to a bog
near Mariana Lakes, 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 decreased frequency of S.
fuscum in the third through fifth year of N addition (Fig.
10A) and an increased frequency of S.
magellanicum in the second through fifth year of N addition
Dominant shrubs, all shrubs, and all vascular plant species
frequencies generally increased with increasing N input, with those
increases all significant in 2013-2015 (Fig. 10 G-I). Water addition
alone had no significant effect on the frequency of vascular plant
species or groups in any of the five years (p >= 0.12). At Mariana
Lakes Bog, we observed changes in vegetation even at low experimental
N loadings, with vegetation progressively changing over the 5 years of
the study (Figs. 10,11). 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.