Development of a Populus Balsamifera Clone in Subarctic Quebec Reconstructed from Spatial Analyses

Abstract

1 The age, diameter and position of all living (n = 750) and dead (n = 286) stems of a northern clone of Populus balsamifera were recorded to study spatial dispersion patterns and to reconstruct clonal development. Position, age and diameter of all Picea spp. stems found within the study area were also recorded. Spatial autocorrelation analyses were performed on the data using age and diameter as variables. 2 Results from spatial analyses suggest that P. balsamifera stems could be classified into three age groups, each one with a distinct spatial pattern: younger stems ( 40 years old) also show significant anisotropy in a south-easterly direction, but only for the first distance class (i.e. 0-7.2 m); and middle-aged individuals (20-39 years old) have no significant isoor anisotropic pattern. Results from partial Mantel tests comparing the spatial occupation of the site by stems from the three age groups suggest that the clone developed in three phases 15-20 years apart: a first phase of postfire colonization, a second of consolidation, and a more recent one of directional expansion. The physical environment (e.g. elevation) and the presence of potentially competing shrubs are factors which probably have limited and may possibly still restrict clonal expansion. 3 There were numerous dead stems < 5 cm in diameter and these were contagiously dispersed; mortality was density-dependent for these individuals, as suggested by the positive correlation between live and dead stem numbers. The most likely densitydependent mechanism involved here is intracohort competition. The presence of Picea spp. does not appear to have significantly affected P. balsamifera mortality, but it may have limited clonal expansion (i.e. ramet birth). 4 The clone studied may still be in a phase of growth and of further expansion as evidenced by the large number of new suckers produced at the edge of the area already occupied. Invasion of the clone by the more shade-tolerant Picea spp. could occur in the future, if there is no disturbance (e.g. by fire), but the recent low recruitment of Picea spp. and the production of new Populus suckers where older stems have died in the central part of the clone suggest that this is unlikely.