DEVELOPMENT OF A POPULUS-BALSAMIFERA CLONE IN SUB-ARCTIC QUEBEC RECONSTRUCTED FROM SPATIAL ANALYSES

1 The age, diameter and position of all living (n = 750) and dead (n = 286) stems of a northern clone of Populus balsamifera n were recorded to study spatial dispersion patterns and to reconstruct clonal develo pment. Position, age and diameter of all Picea spp. stems found within the study area were also recorded. Spatial autocorrelation analyses w ere performed on the data using age and diameter as variables. 2 Resul ts from spatial analyses suggest that P. balsamifera a stems could be classified into three age groups, each one with a distinct spatial pat tern: younger stems (< 20 years old) have a significant anisotropic pa ttern in a south-easterly direction for the first three distance class es (up to 21.6 m); older trees (greater than or equal to 40 years old) also show significant anisotropy in a south-easterly direction, but o nly for the first distance class (i.e. 0-7.2 m); and middle-aged indiv iduals (20-39 years old) have no significant iso- or anisotropic patte rn. 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 similar to 15-20 years apart: a first phase of postfire colonization, a second of consolidation, and a more recen t one of directional expansion. The physical environment (e.g. elevati on) and the presence of potentially competing shrubs are factors which probably have limited and may possibly still restrict clonal expansio n. 3 There were numerous dead stems less than or equal to 5 cm in diam eter and these were contagiously dispersed; mortality was density-depe ndent for these individuals, as suggested by the positive correlation between live and dead stem numbers. The most likely density-dependent mechanism involved here is intracohort competition. The presence of Pi cea 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 furthe r expansion as evidenced by the large number of new suckers produced a t 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.