Effect of Variable Retention Harvesting and Stand Type on Soil Nitrogen Availability in Boreal Mixedwood Forests
Jerabkova, L.. 2006. Ph.D. Dissertation. Faculty of Forestry, University of British Columbia, Vancouver, BC
Abstract
Clear- cut harvesting has been shown to increase the nitrification rate and lead to elevated
levels of soil nitrate in some forests. This change in nitrogen (N) cycling and availability
could negatively affect regeneration, future site productivity as well as surrounding
ecosystems. Variable- retention ( VR) harvesting has been proposed as a more
environmentally acceptable alternative to clearcutting. VR harvesting retains a portion of
live trees in the cutblocks and it is assumed that this will create conditions more similar to
uncut forests and mitigate the negative effects of clearcutting on the N cycle. In boreal
mixedwood forests, maintenance of a deciduous component and management of mixed
stands is currently encouraged, partly based on the assumption that deciduous trees
facilitate faster N cycling and higher N availability. The evidence of this phenomenon is,
however, equivocal . In boreal mixedwood forests in northern Alberta, I assessed whether
the uncut deciduous - dominated forests had higher N availability and faster N cycling than
coniferous- dominated forests,and whether N availability was related to the proportion of
deciduous trees. I also assessed the effects of clearcutting and VR harvesting on soil N
availability in boreal mixedwood forests and characterized pathway sand rates of
nitrification. Uncut stands of deciduous forests had higher availability of ammonium but
did not have faster N cycling and higher nitrate levels as hypothesized. Clearcutting had
little effect on soil N availability in all forest types. Net and gross rates of N
mineralization and nitrification were not altered and N availability was not elevated by
clearcutting. VR - harvested sites did not differ either from clearcuts or uncut stands.
Despite very low soil nitrate concentrations, nitrification was an active process at all
sites, even in uncut coniferous stands. Nitrification was carried out mainly by
heterotrophs and both nitrate production and consumption increased with labile C
addition. Clearcutting in these boreal forests did not create a soil environment dominated
by nitrate at any site.VR harvesting of western boreal mixedwood forests may not be
justified based on nutritional concerns as the effects of clearcutting on N availability were
minimal.
