Post-harvest seedling recruitment following mountain pine beetle infestation of Colorado lodgepole pine stands: a comparison using historic survey records

The extent and severity of overstory lodgepole pine (Pinus contorta var. latifolia Engelm. ex Wats.) mortality from mountain pine beetle (Dendroctonus ponderosae Hopkins) has created management concerns associated with forest re- generation, wildfire risk, human safety, and scenic, wildlife, and watershed resources in western North America. Owing to the unprecedented nature of the outbreak and associated management in the southern Rocky Mountains, it is unknown if the forests that regenerate after this current period of extensive change will differ from those that regenerated in the past. Here, we compare the density and species composition of post-harvest seedling recruits in pre-outbreak (1980-1996) and outbreak stands (2002-2007). Lodgepole pine accounted for more than 95% of post-harvest seedling recruitment and the density of seedlings colonizing clearcuts was equal during both the pre-outbreak and outbreak periods. Compared with harvested areas, the density of tree regeneration was 75% lower in uncut forests and was more evenly distributed among subalpine fir (Abies lasiocarpa (Hook.) Nutt.) and lodgepole pine. This comparison provides evidence that the density of seedling recruitment will be at least as high after extensive pine beetle caused mortality as under healthy, pre-outbreak conditions and that the species composition of stands regenerating after this outbreak will differ between treated and untreated areas. Resume´ : L'etendue et la severitede la mortalitedes tiges dominantes de pin tordu latifolie ´ (Pinus contorta var. latifolia Engelm. ex Wats.) causee par le dendroctone du pin ponderosa (Dendroctonus ponderosae Hopkins) a engendredes pro- blemes d'amenagement associesala regeneration forestiere, au risque de feux de foret, ala securitehumaine ainsi qu'aux ressources visuelle, faunique et hydrique dans l'ouest de l'Amerique du Nord. Acause du caractere inedit de l'epidemie et de l'amenagement qui en decoule dans la partie meridionale des Montagnes Rocheuses, nous ne savons pas si les forets qui se regenerent apres cette periode de changements intensifs seront differentes de celles qui se sont regenerees dans le passe ´. La presente etude compare la densiteet la composition en especes des semis recrutes apres la coupe en periodes pre ´-epidemique (1980-1996) et epidemique (2002-2007). Plus de 95 % des semis recrutes apres la coupe sont des semis de pin tordu latifolieet la densitedes semis qui ont coloniseles coupes ablanc en periode pre ´-epidemique est la meme qu'en periode epidemique. Comparativement aux aires recoltees, la densitede la regeneration arborescente est 75 % plus faible dans les forets non coupees et plus equitablement distribuee entre le sapin subalpin (Abies lasiocarpa (Hook.) Nutt.) et le pin tordu latifolie ´. Cette comparaison fournit la preuve que la densitedes semis recrutes sera au moins aussi forte apres une mortalitesevere causee par le dendroctone du pin ponderosa qu'elle l'etait dans les peuplements sains en periode pre ´-epidemique et que la composition en especes des peuplements qui se seront regeneres apres cette epidemie sera diffe ´- rente dans les aires coupees et non coupees. (Traduit par la Redaction)