Root connections can trigger physiological responses to defoliation in nondefoliated aspen suckers

Abstract

In species such as aspen ( Populus tremuloides Michx.), trees are interconnected through their root system owing to their regeneration mode by root suckering. These root connections challenge classic notions of forest dynamics that consider trees as individuals competing for resources, because root connections allow trees to share water, minerals, and carbohydrates. The purpose of this study was to demonstrate that trees can directly influence the physiology of other nearby trees through root connections. In the summers of 2007 and 2008, pairs of aspen suckers (i.e., two suckers connected by a parental root) were selected and divided into three height classes and compared with each other (dominant, codominant, suppressed). Suckers distally positioned on the parental root were manually defoliated, and the effects of defoliation on photosynthesis, stomatal conductance, and specific leaf area (SLA) were measured on connected but nondefoliated suckers. Results showed that defoliation caused physiological responses in the interconnected trees in summer 2007, which was drier than summer 2008. Defoliation of a connected sucker had a greater effect on suppressed suckers, for which mean photosynthesis rate increased by 17% compared with that of controls. The effect was less important for codominant (14% increase) and dominant (12%) trees. SLA of suppressed suckers also increased, while the increase in SLA values was smaller for codominant and dominant suckers. In summer 2008, no defoliation effect was observed, probably owing to high moisture conditions that resulted in much higher stomatal conductance values compared with those in 2007 (+55%). Under high humidity conditions, leaf specific hydraulic conductance does not constrain stomatal conductance, so the rate of CO2assimilation was probably at its maximum capacity. This study demonstrated that trees could physiologically interact through root connections, and these interactions should thus be considered in studies of stand dynamics.