Foliar nutrient resorption and litterfall production with stand age, overstory composition, and disturbance origin in boreal forests
Brant, Amber Nadine
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Nutrient resorption (NuR) prior to developmental senescence allows for nutrient conservation in plants, driven primarily by genetics, development stage, climate and soil fertility, and litterfall is the vector for the influx of nutrients to the soil. Despite their importance, changes in NuR and litterfall through stand development, under different overstory compositions, and following different disturbance scenarios in boreal forests are not fully understood. My objectives were to examine the effects of time since stand-replacing fire, overstory composition and disturbance origin on leaf NuR efficiency (NuRE) and litterfall production in a boreal forest. I used an age chronosequence with 7 to 209 years since fire to examine the effects of stand age and overstory composition (broadleaf or mixed with conifer) on the resorption efficiency of nitrogen (NRE), phosphorus (PRE) and potassium (KRE) in the leaves of trembling aspen (Populus tremuloides) and white birch (Betula papyrifera). I then compared NRE, PRE and KRE between stands of three disturbance origins (fire alone, full-tree logging alone, or full- tree logging followed by fire) in 7-year-old Populus tremuloides in pure stands or mixed with jack pine (Pinus banksiana). A similar age chronosequence was used to study the effects of stand age and overstory composition (broadleaf, mixedwood, and conifer) on litterfall production. Within this chronosequence, I examined how litterfall production varies with disturbance origin (fire and logging) for 7, 15 and 33 years since disturbance. In Populus tremuloides and Betula papyrifera, PRE and KRE decreased with increasing stand age. No overstory effects were found for Betula papyrifera, but PRE and KRE were lower in Populus tremuloides when mixed with conifer trees. Full-tree logging followed by fire resulted in a marginally lower PRE in Populus tremuloides in mixedwood stands when compared with fire and full-tree logging alone. Total, foliar and other litterfall production increased to 33 years since fire then reached a steady rate. Total and foliar litterfall were higher in broadleaf stands than conifer and mixedwood stands. Arboreal epiphytic lichen fall was highest in the 209- year-old conifer stands and 146-year-old mixedwood stands. Total and foliar litterfall peaked in October, other litter peaked in June and October, and epiphyte fall reached its maximum in June. Litterfall production did not vary significantly between fire and logging origin. The results of this study suggest that NuR in trees can be influenced by stand development stage since fire and that species composition in the overstory can play an interactive role. This study also indicates that, although fire and logging can result in variable effects on the soil environment, NuR is mostly conserved shortly after disturbance. Although it has been shown that overall forest productivity declines with age, this study demonstrates that litterfall can be maintained over time in boreal forests, regardless of tree species composition, but that broadleaf stands produce more litterfall than conifer and mixedwood stands. Disturbance origin may not be a significant driver in litter production. The implications of this study can fit into a larger framework of research on productivity and nutrient flux in northern boreal environments.