Succession in conifer-dominated boreal, sub-boreal and montane forests often seems uniform and without dramatic changes in species representation over time. Forests are often initiated as even-aged stands after forest fires, but are now increasingly being exposed to longer fire seasons and new or larger-scale insect outbreaks (by bark beetles and budworms, for example) as northern winters become shorter and milder. Several lines of research now indicate considerable diversity in the patterns of disturbance and recovery within landscape-level wildfires and insect outbreaks, apparently reflecting slight differences in pre-disturbance vegetation and in disturbance severity. A series of long-term monitoring plots were installed in even-aged forest stands dominated by lodgepole pine (Pinus contorta var. latifolia) trees in central British Columbia. All stands were in the process of being attacked by the mountain pine beetle (Dendroctonus ponderosae) at the time of plot establishment approximately 12 years ago, and some were subsequently treated with experimental or prescribed forest fires. Forest regeneration tallies (from 50 m2 circular plots) and plant species cover (from 10 m line transects) data collected over multiple years were evaluated to test the relative importance of pre-existing vegetation and local disturbance severity in determining subsequent plant community composition.
Results/Conclusions
Great microsite variability in pre-disturbance vegetation and in disturbance severity was observed in both beetle-attacked and burned forests. Small lodgepole pine trees and clusters of advance regeneration (mostly Abies lasiocarpa and Picea spp.), collectively referred to as “secondary structure,” remained alive at some spots in the wake of the mountain pine beetle outbreak, while other spots were dominated by shrubs or moss with no living trees. Where dominant, feathermoss carpets did not support much new vascular vegetation or new tree seedlings even six years after most trees died in the overstory. While secondary structure released, the greatest vegetation change over time in beetle-killed stands was the gradual deterioration of red-stemmed feathermoss (Pleurozium schreberi) cover. Lightly and moderately burned sites showed strong resilience in understory composition, with species such as Spiraea betulifolia, Vaccinium membranaceum, Arnica cordifolia and Calamagrostis canadensis resprouting vigorously, with trees represented by a large proportion of Picea seedlings. Severely burned microsites supported more Epilobium angustifolium, Polytrichum juniperinum, Ceratodon purpureus, and Pinus contorta tree seedlings, plus some native species not found before fire. The interplay of pre-existing vegetation, disturbance type and disturbance severity has already set trajectories of successional re-initiation, accelerated succession, and arrested succession across the forest landscape.