Responses of planted Populus tremuloides seedlings to grass competition during early establishment

Abstract

Root systems of aspen seedlings display limited architectural plasticity in response to below-ground competition, but seedlings compensate for restricted rooting space and reduced root system size, by optimizing water uptake. Below-ground competition with grasses often plays a critical role during tree seedling establishment, but many underlying mechanisms are not well understood. We used a controlled field experiment to study how trembling aspen (Populus tremuloides Michx.) seedlings compete with smooth brome grass (Bromus inermis L.) for space during the first 3 years of seedling establishment and how it affected aspen seedling development. Our study showed that competition with grasses had a limited impact on architectural plasticity of aspen seedlings. Seedlings faced with competition from smooth brome were overall smaller and most architectural parameters, with the exception of shoot height, appeared simply scaled down proportionally. Shoot height changed less than other parameters, because aspen competing with grass allocated relatively more carbon to shoots than roots and adopted a slender shoot morphology to quickly overtop the competition. Aspen growing with grass competition had significantly smaller root systems. Both lateral extent and maximum rooting depth were reduced by ~ 50%. In response to the restricted rooting space, roots of aspen seedlings faced with grass competition had a lower specific root length. Root carbohydrate reserves were not affected by competition; however, aspen roots growing with grass competition had higher soluble sugar concentrations which may be associated with the observed three times higher water uptake efficiency per unit root biomass. Our findings suggest that aspen seedlings have limited capacity for architectural plasticity in response to root competition, but are at least temporarily able to compensate for reduced root system size and rooting space, by optimizing water uptake efficiency.