MORPHOLOGICAL PLASTICITY AND REGENERATION STRATEGIES OF VELVET LEAF BLUEBERRY (VACCINIUM MYRTILLOIDES MICHX.) FOLLOWING CANOPY DISTURBANCE IN BOREAL MIXEDWOOD FORESTS

The effects of canopy disturbance on the abundance, growth, morphologi cal plasticity, biomass allocation and fruit production of velvet leaf blueberry (Vaccinium myrtilloides Michx.) were examined in 1996 in a second-growth boreal mixedwood forest near Nipigon, northwestern Ontar io that had been logged by either shelterwood cutting or clearcutting in 1993. We found that V. myrtilloides was able to persist in both ope n and closed canopy boreal mixedwood forests managed for commercial ti mber extraction. Persistence under heavy shade conditions was accompan ied by significant morphological and biomass allocation plasticity. Sp ecific leaf area, leaf area, individual leaf weight, and the proportio n of total biomass in stems and foliage changed along an understory li ght gradient from 0% to 67% percent photosynthetic photon flux density (% PPFD). The degree of above-ground morphological plasticity may exp lain blueberry's ability to survive under low Light conditions. Reprod uctive performance of V. myrtilloides was greatest under the partial s hade conditions associated with shelterwood cutting. Blueberry bushes growing in clearcuts overgrown with 3-year old aspen (Populous tremulo ides Michx.) saplings remained mostly vegetative whereas the number, f resh weight and dry weight of berries in shelterwood cuts was 94% grat er than that produced after clearcutting. We attributed the lower frui t yields in the clearcuts to heavy shading from regenerating hardwoods , and mechanical damage to above-ground biomass. The paucity of seedli ng regeneration as well as extensive mechanical damage to above-ground stems by logging equipment delayed vegetative regeneration of V. myrt illoides in large canopy openings of the clearcut blocks. Unlike other more aggressive ericaceous species (e.g. Kalmia angustifolia var. ang ustifolia L., Gaultheria shallon Pursh.), V. myrtilloides was unable t o resist invasion from faster growing hardwood species (e.g. P. tremul oides) and was rapidly overtopped. V. myrtilloides plants in the uncut control blocks received 3.9% of full sunlight, whereas those growing in the partial cut and clearcut blocks received an average of 25.3% an d 32.5% PPFD, respectively. Cover of vegetation over-topping blueberry plants was highest in the uncut forest (90.3%), but was not significa ntly different between the partial cut (45.5%) and clearcut (50.1%) tr eatment blocks. (C) 1998 Elsevier Science B.V.