Balsam fir self-thinning relationship and its constancy among different ecological regions

The constancy of balsam fir (Abies balsamea (L.) Mill.) self-thinning relationship has been investigated among four study areas located in different ecological regions of Quebec's humid boreal forest. These four study areas contained respectively 348, 252, 146, and 55 observations (plots x measures) sampled over a period of up to 40 years. A self-thinning fitting method was developed to position objectively the self-thinning lines but, moreover, to allow comparisons among the different study areas. This method relies on principal component analysis to estimate the self-thinning line parameters and on the "jackknife" procedure to provide a standard error of these estimates. Results demonstrate a concordance for the slope (p = 0.136) and the intercept (p = 0.148) among self-thinning relationships of those study areas. The combination of these four study areas in one large data set, to provide a general estimation of balsam fir self-thinning line, has given a slope of -1.441 with a 4.114 intercept which is in agreement with the -3/2 power law of self-thinning. In this study, this law was able to describe the size-density relationship of stands of various ages and growing within different conditions as expressed by the different ecological regions.