Mechanical site preparation and nurse plant facilitation for the restoration of subarctic forest ecosystems

Abstract

Sustainable forest management implies successful regeneration following disturbances. Tree regeneration in subarctic ecosystems can, however, be constrained by limitations to seedling establishment related to cold soils, slow decomposition rates, and competition by ericaceous species. We established a field trial at the northern limit of commercial forests in Québec, Canada, to evaluate to what extent mechanical site preparation (MSP) and planting of a nurse N2-fixing species could promote conifer establishment on a site burned in 2007. The experiment comprised four treatments applied in 2010: standard MSP (disc trenching), standard MSP plus planting of Alnus crispa, intensive MSP, with larger furrows than standard MSP, and a control. Main plots were divided and planted in 2011 with Picea mariana (Mill.) Britton, Stearns & Poggenb. or Pinus banksiana Lamb. We monitored seedling survival, growth, nutrition, and microsite over a 3-year period. Results revealed interactions between treatments and planted species. Mechanical site preparation resulted in higher conifer growth relative to the control conditions, and planting Alnus resulted in growth gains similar to those obtained from intensive MSP. We measured competitive interactions between Alnus and the conifers that might eventually cancel out the initial benefits derived from facilitation by planting the nurse species. Longer term monitoring of interspecific interactions is needed to unravel the mechanisms responsible for the facilitative effect and identify the best management practices.

Role of green alder in boreal conifer growth: competitor or facilitator?

“…In 2007, a wildfire burned the previous stand, which was 61-80 years old and composed of 12-17 m tall black spruce trees with a canopy cover of 41%-60%. Visual assessment conducted in July 2010 showed the abundance of charred organic material (Thiffault and Hébert 2017).…”

“…Our experiment builds from a subset of the experimental setup described by Thiffault and Hébert (2017). The establishment of three silviculture scenarios were completed in 2011; they were randomly applied within five blocks on 15 main plots:…”

“…Indeed, closed managed forests are progressively opening at their northern limit, mainly due to tree regeneration failure after natural or anthropogenic disturbances (Girard et al 2009;Oris et al 2014;Splawinski et al 2019). In subarctic ecosystems, tree regeneration failure is caused by the absence of advanced regeneration, insufficient post-disturbance seed rain, the lack of suitable seedbeds, and competition by ericaceous species (Thiffault and Hébert 2017;Payette and Delwaide 2018). The boreal forest is a carbon sink and an important source of fibre (Brandt et al 2013); regeneration failure thus threatens the sustainable provision of these ecosystem services, such as carbon sequestration (Kurz et al 2013).…”

“…We addressed these questions using a field trial established in northern Québec, Canada, on a site burned in 2007 (Thiffault and Hébert 2017). The experiment aimed at testing the effects of mechanical site preparation (MSP) and planting of a nurse N 2 -fixing species, American green alder (Alnus alnobetula subsp.…”

“…; considered a fast-growing, light-demanding, acquisitive-type species) under three silviculture scenarios: MSP, MSP with plantation of green alder, and a control treatment without MSP. Early results have evidenced a higher growth rate for conifers planted in plots treated with MSP, as well as with alder as a companion species, than in control plots (Thiffault and Hébert 2017). However, alder exerted significant competition for light, which could eventually decrease the initial benefits of this nurse species on planted conifers.…”