Competition and facilitation between tree species change with stand development

Summary of the hypothesized interspecifi c interactions: positive ( ), neutral (0), or negative ( ) as compared with intraspecifi c interactions. mixed across the stand. Effect of/onSoil productivity ∗ Light availability Black spruce/trembling aspen 0/– In order to ensure all sites were of similar quality in each / Trembling aspen/black spruce 0/ – /0Trembling aspen/jack pine 0/0 0/0Jack pine/trembling aspen 0/ – 0/0Black spruce/jack pine 0/0 / ∗ early stage development/late stage development. patterns (Longpr e et al. 1994), the apparent neutral rela-tionships between black spruce and trembling aspen in even mixtures could be the result of a balance between positive and negative interactions, as hinted by the fact that aspen eff ect on spruce is positive when spruce is present in a much higher proportion than aspen (L e gar e et al. 2004) but nega-tive when aspen dominates the canopy (Cavard et al. 2010). Th e positive interactions have been hypothesized to be a result of improved soil conditions for spruce by nutrient-rich aspen litter (L e gar e et al. 2005, Lagani e re et al. 2009, 2010) and a lower interspecific competition than intraspecific competition for aspen, due to the smaller stature of black spruce. Th e negative interactions have been hypothesized to be the adverse eff ect of black spruce litter on soil productiv-ity (Prescott et al. 2000, Crawford et al. 2003), leading to less available soil resources for aspen, and for black spruce, to have a greater interspecifi c competition than intraspecifi c competition, due to the higher stature of aspen. nI this study, we investigated individual tree growth through stand development in single- and mixed-species stands to examine the dynamics of interspecifi c interactions in the same sampling plots that were used in a previous study (Cavard et al. 2010). Th e fact that both canopy stratifi cation and the positive or negative infl uences of litters on soil productiv-ity are not instant processes led us to the following hypoth-eses: 1) when mixed with trembling aspen, spruce trees may grow more slowly at the early stage of stand development and grow faster at the later stage of stand development; 2) conversely, when mixed with spruce, aspen may grow faster at fi rst, but more slowly in the later stages of stand develop-ment; 3) fi nally, relationships between shade-intolerant jack pine and trembling aspen are expected to be mostly neutral, with perhaps a slight decrease in aspen growth in the later development stage of mixed stands (Table 1).