Forest pests and pathogens drive tree mortality through disruption of functional traits linked to nutrient acquisition, growth, and reproduction. Though pervasive and of significant ecological and economic consequence, tree mortality and the physiological processes underlying declines remain unclear for many tree species. To gain predictive insight into how tree health responds to antagonistic pressures, it is imperative that links be drawn between above- and below-ground functional traits. The contribution of root functional traits, however, to overall tree health and function and root response to attack has received far less attention than aboveground traits. To address this gap, we excavated whole-root systems from eastern hemlock (Tsuga canadensis) individuals that had been experimentally infested with hemlock woolly adelgid (HWA: Adelges tsugae) and elongate hemlock scale (EHS: Fiorina externa) individually or in combination for periods of 2-4 years. Belowground root biomass, functional traits, and storage nutrients were measured to assess impacts of herbivory exerted by these two exotics. We also quantified ectomycorrhizal (EM) colonization of fine roots and used culture-independent methods to examine fungal diversity.
Results/Conclusions -
Contrasting effects on belowground traits were observed in response to HWA and EHS feeding, either alone or in combination. Trees infested with HWA for 2-4 years had a significantly greater percentage of their total biomass comprised of belowground tissue, resulting from shifts away from aboveground biomass production. Surprisingly, HWA feeding had no observable effects on root functional traits (e.g., specific root length) or on resource allocation to roots, indicated through measures of carbon, nitrogen, and starch. HWA feeding did significantly reduce EM colonization of hemlock fine roots. Two and four years of continuous infestation reduced colonization by 5 and 14% respectively. In contrast, EHS feeding had no observable effect on belowground traits or EM colonization alone; however its presence mediated HWA effects when trees were co-infested. Simultaneous infestation within the same year yielded significant reductions in colonization, though this was only observed after two years of infestation and not four. Trees treated with both herbivores for two years had a 22% reduction in EM colonization compared to uninfested trees. Taken together, our results suggest that HWA attack causes stronger effects on below-ground functionality (through the extended phenotype of EM colonization), and this may accelerate eastern hemlock stress and mortality.