Changes in Canopy Processes Following Whole-Forest Canopy Nitrogen Fertilization of a Mature Spruce-Hemlock Forest

Experimental field estimation of organic nitrogen formation in tree canopies

“…The experimentally determined upper limit to the capacity for net organic N production in summer months in southern Scotland was similar for both conifer species, despite the differences in species, age, size and canopy structure, at up to 1.6 mmol N m -2 mth -1 , based on assumed limiting production rates in the presence of a large excess of inorganic N deposition to the canopy. This may be contrasted with results for a mature conifer forest in eastern North America (Gaige et al, 2007), where canopy retention of experimentally applied inorganic N was up to 80% or 20 mmol N m -2 mth -1 during the growing season. This very large % retention was ascribed to the canopy architecture and the means of delivery, as small droplets of 0.48M NH 4 NO 3 solution from a helicopter.…”

“…Although it is possible that the additional N measured in TF was cycled from the roots through the canopy, it is more likely that the higher N status of the canopy led to lower retention of wet-deposited N. Direct manipulations of forest canopies, as opposed to the soil, are even rarer. One recent study, using helicopter addition of NH 4 NO 3 to a mature conifer forest canopy (Gaige et al, 2007) showed conversion of both NH 4 + and NO 3 -to organic N in throughfall, by the use of 15 N isotopic labelling, in a forest where organic N made up 80% of the total N below canopy in untreated areas (i.e. where canopy transformation processes were apparently very active).…”

“…They can, however, be very useful in identifying pathways and processes. The use of stable isotopes to study the rate of exchange of material with plant canopies has already been mentioned (Dail et al, 2009, Gaige et al, 2007 in a study where a large input of N was applied to a forest with relatively low N deposition. This study demonstrated that exchange processes occurred rapidly, and generated significant transformation of inorganic to organic N in the process.…”

“…The Deepsyke experiment followed 5 years of treatment, so the surface epiphytes and microbial populations may have already adapted somewhat to the additional N inputs. Moreover, the N additions were made in small increments between May and November each year, more frequently and with much smaller solution concentrations than the 5 applications during the growing season in the helicopter experiment (Gaige et al, 2007). As observed at the AMBER site, albeit with a different tree species, the net amount of organic N measured below the canopy was a very small fraction of the total N applied to the canopy, again suggesting a finite capacity to transform additional inorganic N, in this case around 1 mmol m -2 mth -1 under the same summer conditions (the same year).…”

Effects of soil warming and nitrogen foliar applications on bud burst of black spruce

“…In addition, to overcome the limited availability of nitrogen in the soil, trees have recourse to the absorption of this element directly at the canopy level. With respect to fertilization to the soil, N foliar applications are readily absorbed with retention values higher than 70% (Gaige et al, 2007) and have a more rapid influence on plant physiology (Sparks, 2009). Changes in N availability may influence the physiology of plants and other related organisms, especially in the long term (D'Orangeville et al, 2013;Houle and Moore, 2008;Rossi et al, 2012). Several studies have analyzed the effect of N on bud burst (Fløistad and Kohmann, 2004;Kula et al, 2012;Lumme and Smolander, 1996;Murray et al, 1994;Sigurdsson, 2001;Thitithanakul et al, 2012), sometimes producing contrasting results probably related to the different treatment types and doses, or the age of the plants.…”