LIGHT THRESHOLD FOR OPTIMAL BLACK SPRUCE (PICEA-MARIANA) SEEDLING GROWTH AND DEVELOPMENT UNDER BRUSH COMPETITION

Light threshold is suggested as a method for quantifying brush competi tion in black spruce (Picea mariana (Mill.) B.S.P.) plantations and pr edicting losses in tree growth as measured by the relative growth rate (RGR). The severity of the competing vegetation (expressed in terms o f density and height) around 300 planted tree seedlings, and the growt h status of the seedlings (expressed in terms df total height and curr ent height growth increment), were analyzed simultaneously with multiv ariate analysis of variance (MANOVA), with one level of light quantity (photosynthetically active radiation) reaching the upper one-half of the tree seedlings as the predictor variable. This study showed that i t is possible to establish a competitive status based on light interce ption, thus allowing a grouping of seedlings with similar growth chara cteristics and severity of competition. Canonical analysis showed that 60% of full sunlight reaching the upper one half of spruce seedlings can be defined as a threshold to significantly discriminate between st ressed and unstressed tree seedlings, based on the above-mentioned var iables and also based on spruce basal stem diameter. For spruce total height, current height increment, and basal stem diameter, significant differences were found between plots above and below this threshold, averaging 18.5, 44.7, and 23.2%, respectively. Measuring the intensity of light reaching the upper one half of the spruce seedlings also sig nificantly explains spruce relative growth rates, expressed in terms o f height growth increment and basal stem diameter growth increment ove r the following two growing seasons. The light threshold used in the M ANOVA and the ANOVA showed significant differences between plots above and below the threshold, averaging 70 and 58% for spruce height and b asal stem diameter RGRs, respectively. Instead of measuring competing vegetation variables and relative height of a crop species to infer li ght interception, this study demonstrated that a direct measure of lig ht attenuation at the tree seedling level can be used to assess the co mpetitive status and predict losses in tree growth for the period of t ime required by the seedlings to emerge from the Vegetation cover. Mor eover, this study demonstrated that canonical relations, rather than l inear relations, explain the pattern of competition for light between young spruce seedlings and brushlike vegetation. The light threshold s uggested in this study has potential for assessing interspecific compe tition problems in young black spruce plantations and is proposed as a tool to support a containment strategy of vegetation management.