Picea mariana Growth, Leaf N Concentration and Assimilation in a Bog Exposed to Nitrogen Treatments, 2013-2015

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). In September 2013-2015, we measured
annual leader extension of Picea mariana on four
trees (0.5-2.5 m tall) per plot. Using a visually estimated P. mariana density at Mariana Lakes Bog of 1 tree
m -2 , and N concentrations in P.
mariana needles, we calculated annual N assimilation
attributable to new needle production. Annual leader extension and
tree growth increased with increasing N input, with the response
becoming more pronounced from 2013 through 2015. Picea
mariana needle N concentrations also increased with
increasing N input, at a modest, but significant rate that was
consistent across all years while water addition alone had no
significant effect on P. mariana leader
extension, growth, needle N concentration, or N assimilation in any of
the years of measurement (p >= 0.54). At Mariana Lakes Bog, the
effect of N addition on P. mariana NPP became
more pronounced over time. We cannot determine the extent to which the
progressively steeper P. mariana growth response
to N deposition represents a cumulative effect of added N or is
related to interannual differences in temporal climatic variables.