Post disturbance dynamics of aspen mixed-woods : a comparison among fire, logging, and budworm outbreak

Abstract

Seventy-one aspen (Populus tremuloidies Michx.) mixedwood stands disturbed in the 1980’s were sampled in southeastern Manitoba, Canada to examine the response of arboreal and understory communities to natural and management-induced disturbances. Thirty stands were disturbed by severe spruce budworm (Choristoneura fumiferana Clem.) outbreaks, 21 by crown fire, and 20 by conifer-selective logging. All sampled stands were on well drained upland sites with similar pre-disturbance vegetation. For the understory community, species diversity, richness, and presence did not differ significantly between the disturbance types. Significant differences, however, were found in species evenness with the understory of logged stands being dominated by shrubs. Coefficient of community was consistently greater than percent similarity, indicating that differences between disturbance types occur primarily in species abundance. Comparisons were made with mature aspen (< 25 % and conifer) and mature mixed (> 25 and < 75 % conifer) stands. Logged stands were found to be more closely associated with mature aspen stands while budworm and fire disturbed stands were intermediate between the two mature stand types. Implications for management are discussed. Due primarily to differential canopy removal both fire and logging disturbed stands were dominated by post-disturbance aspen regeneration and contained little conifer recruitment while spruce budworm disturbed stands had abundant conifer recruitment and significantly less aspen regeneration. Seed availability and abundance was the primary factor affecting post disturbance conifer recruitment with seedbed condition and vegetative competition playing a secondary role. Timing of post-disturbance tree recruitment differed among the disturbances. Aspen was recruited more abundantly in the first five years following fire and logging and had significantly more recruitment after five years following end of budworm outbreak. White spruce (Picea glauca (Moench) Voss) and black spruce (Picea mariana (Mill.) B.S.P.) showed continual recruitment in fire and budworm disturbed stands but not in logged stands where recruitment was more abundant in the first five years. Balsam fir {Abies balsamea (L.) Mill.) showed continual recruitment following fire and logging, and recruited more abundantly after five years following budworm disturbance.