Biomass growth response to spatial pattern of variable‐retention harvesting in a northern Minnesota pine ecosystem

Abstract

Variable‐retention harvesting (VRH) is an approach for sustaining complex structure in managed forests. A criticism of VRH is that ecological benefits may come at a cost of reduced growth of regeneration, due to competition with residual trees. However, the spatial pattern of retention, i.e., dispersed or aggregated, in VRH systems can be manipulated to minimize suppression of regeneration, and resource limitation to regeneration might be mitigated by reduction of woody shrubs. Continued growth of the residual cohort will compensate for growth reduction of regeneration, although this may differ with retention pattern. We examined aboveground whole‐stand biomass growth of trees in a VRH experiment in Pinus resinosa forest in Minnesota, USA. Treatments included dispersed retention, aggregated retention, and an uncut control, as well as a shrub treatment (reduced density or ambient). We addressed the following hypotheses: (1) biomass growth of a cohort of planted pine seedlings will be highest with aggregated rather than dispersed retention, (2) biomass growth of the planted seedlings will increase with shrub reduction, and (3) biomass growth of the residual overstory will be higher with dispersed rather than aggregated retention. Aboveground biomass growth of the planted pines ranged from 0.4 kg·ha−1·yr−1 in the overstory‐control–ambient‐shrub treatment to 23 kg·ha−1·yr−1 in the aggregated‐retention–shrub‐reduction treatment. The difference between the control and the retention treatments was significant (P < 0.0001), but not between dispersed and aggregated retention (P = 0.97). Thus, our first hypothesis was not supported. In all treatments, biomass growth was significantly higher (>100% increase) with shrub reduction (P = 0.001), supporting our second hypothesis. Biomass growth of residual trees ranged from 2404 kg·ha−1·yr−1 in the uncut‐control–ambient‐shrub treatment to 1043 kg·ha−1·yr−1 in the aggregated‐retention–shrub‐reduction treatment. Differences were significant between the control and retention treatments (P = 0.003), and marginally higher with dispersed vs. aggregated retention (P = 0.09), lending support to our third hypothesis. Our results suggest that managers have flexibility in application of VRH and can expect similar stand‐level biomass growth of planted regeneration regardless of retention pattern, but somewhat higher stand‐level biomass growth of retained trees with dispersed retention.

Long-Term Regeneration Responses to Overstory Retention and Understory Vegetation Treatments in the Northern Rocky Mountains

“…Previous research in stands harvested with partially retained overstories has demonstrated a negative correlation between regeneration growth and understory shrub cover (e.g., Harrington 2006, Mitchell et al 2007, Urgenson et al 2013, Palik et al 2014. Our results seem to support this argument, because we found a negative correlation of current regenerated tree density to both shrub biomass 2 yr and shrub biomass 2-10 yr .…”

“…One concern over VRH is that the retained overstory trees can hinder regeneration growth (Palik et al 2014). A number of studies have argued that retention trees reduce the new cohort's growth through competition for limited resources (e.g., Birch and Johnson 1992, Long and Roberts 1992, Rose and Muir 1997, Elfving and Jakobsson 2006, Temesgen et al 2006.…”

“…However, new cohort responses should differ, depending on the spatial arrangement of overstory trees (Coates 2000). Considering the highly flexible attribute of VRH (variable numbers, sizes, and spatial patterns of retention trees), it may be possible to ameliorate reduced regeneration growth by manipulating overstory retention spatial patterns to reduce competition impacts, or offsetting them by controlling understory vegetation (Palik et al 2014). Little is known about retention level effects, spatial pattern effects, and postharvest understory vegetation treatment effects (and their interactions) in many forest types; thus, decision support for forest managers charged with site-specific VRH prescription development is deficient .…”

Long‐term impacts of variable retention harvesting on ground‐layer plant communities in Pinus resinosa forests

“…Basal area of the study area averaged 32 m 2 ha −1 prior to harvest, with a moderately open canopy, and dominant trees averaging 27 m in height (Palik et al . ). Historically, fire was the primary natural disturbance in this ecosystem; however, fire has been excluded from these areas resulting in greater levels of understorey shrubs than documented in natural forest systems.…”

“…19 m 2 ha −1 ) than the other harvest treatments (Palik et al . ). Residual trees were retained evenly throughout the dispersed retention treatment.…”