Yield and morphological responses of wild blueberry (vaccinium spp.) to forest harvesting and conifer release treatments

Abstract

This thesis synthesizes three papers on the effects of forestry practices on the growth and fruit production of lowbush (V. angustifolium) and velvet leaf (V. myrtilloides) blueberry in i) young jackpine, ii) boreal mixedwood and iii) lowland black spruce plantations in northwestern Ontario. The main objectives of the three papers were to investigate: i) the impacts of forest herbicide and alternative conifer release treatments on the growth and fruit production of Vaccinium spp.; ii) the phenology of Vaccinium spp. in order to determine an optimal spray time that might reduce susceptibility of blueberry to herbicide injury and iii) the morphological plasticity of velvet leaf blueberry bushes growing in clearcut, partial cut and uncut second-growth boreal mixedwood forests. i) It was shown that application of Vision® herbicide significantly affects the abundance, growth and reproductive performance of Vaccinium spp. in treated clearcuts. Compared with untreated areas, fruit productivity of Vaccinium spp. in Vision® treated plantations was reduced by as much as 58 % three years after disturbance. Reductions in berry production were attributed to toxic effects of the herbicide to stems and below-ground reproductive tissue. Conversely, percent cover and the number, dry weight and fresh weight of berries increased significantly after brushsaw cutting. ii) Patterns of leaf development in V. angustifolium and V. myrtilloides indicated that selective control of competing vegetation in plantations with reduced damage to Vaccinium spp. may be possible with herbicide application before active growth of new blueberry shoots (i.e. early May) or during leaf senescence and abscission (i.e. September to October). Foliage of blueberry turned colour in late August with about 30 % abscission by the last week of September. With most of the foliage lost by early autumn, application of foliar herbicides at this time may have limited effects upon blueberry growth and fruit production, since without leaves, little herbicide can be absorbed or translocated to below-ground vegetative organs. iii) V. myrtilloides was able to persist in both open and closed habitats in boreal mixedwood forests managed for commercial timber exploitation. Persistence under heavy shade conditions was attributed to plasticity in morphological and biomass allocation. Specific leaf area, individual leaf weight, number of berries, number of reproductive shoots and the proportion of total biomass in stems and foliage changed along a gradient in understory light ( % PPFD) from 0 % to 67 % PPFD in forests harvested by clearcutting and shelterwood logging. Reproductive performance of V. myrtilloides was best under the partial shade conditions associated with shelterwood cutting. The results of this thesis indicate that clearcut logging and silvicultural strategies of weed suppression such as herbicide application can adversely affect both the berry production and vegetative growth of Vaccinium spp. in northwestern Ontario. Conversely, partial cutting and conifer release with brushsaw cutting offer a silvicultural alternative that is less destructive to blueberry.