Effectiveness of Silvicultural Options in Renewal of Trembling Aspen–Jack Pine Mixedwood Stands, 21 Years After Treatment

Regenerating conifers after harvest through planting and postharvest broadcast application of herbicide is effective in ensuring the survival and growth of seedlings, but faces challenges in meeting broad social and ecological objectives of forest management. This study reports the effectiveness of alternative options in regenerating jack pine (Pinus banksiana Lamb.), 21 years after harvest of trembling aspen (Populus tremuloides Michx.)-dominated boreal mixedwood stands. The treatment options included (i) preharvest spray—aerial broadcast spray prior to harvest, (ii) postharvest partial spray—ground herbicide application in strips, (iii) partial harvest in strips, (iv) postharvest aerial broadcast, and (v) uncut reference. Twenty-one years after treatments, the four harvest treatments were similar in overstory density (4000 stems/ha) and basal area (BA, 20 m2/ha), but differed in composition and structure. The preharvest spray had an intimate mixture of aspen and jack pine (22% and 57% by BA, respectively), compared to spatial mosaics of aspen and pine corridors in the partial spray (36% and 41%), and aspen and maple corridors in the partial cut (21% and 31%). While the postharvest broadcast was pine-dominated (74% by BA) as expected, uncut and partial cut were similar in pine composition (10% by BA), which is inadequate for aspen and pine mixedwood stands. The early positive effects of preharvest spray and partial harvest on understory species abundance and diversity became neutral 21 years postharvest. The implications of these findings are discussed with respect to stand conditions before harvest, postharvest regeneration dynamics, and treatment objectives for the renewal of trembling aspen and jack pine mixedwood stands after harvest.