The Influence of Root Connections vs. Genotype on Growth of Aspen Trees

Trembling aspen (Populus tremuloides) is a clonal species and natural populations are composed of intermingled stems of the same or of different genotypes. Trees may also be connected (or not connected) by root grafts and/or root parental connections. It is commonly acknowledged that trees of the same genotype respond similarly to climate and environmental stress. However, our previous work has shown that root connections between trees could significantly influence growth of connected neighbours because they allow the direct sharing of resources and photosynthates. In this study we wanted to assess the relative contribution of genotype vs. root connection to tree growth in aspen stands of Quebec (Canada).We compared radial growth response of connected/non-connected trees of the same or of different genotype to a severe forest tent caterpillar (Malacosoma disstria) defoliation outbreak reported in the study area during 2000-2001 and to extreme environmental conditions that occurred during the last 20 years. Results show that the growth response to FTC outbreak and extreme environmental conditions between connected and non-connected trees was similar, but that non-connected trees exhibited a wider variability in growth response compared to connected trees. Moreover, mean ring width of connected trees from different genotypes was greater than that of trees of the same genotype but without root connections. Although this study showed that the sharing of resources through root connections did not affect radial growth of connected trees during stressful conditions, it showed that root connections lessens the significance of genotype-specific responses and represents an important ecological significance of root connections.