Robert N. Schaeffer, Claire M. Wilson, Laura Radville, Mauri Barrett, Elizabeth Whitney, Sofia Roitman, Esther R. Miller, Benjamin E. Wolfe, Carol S. Thornber, Colin M. Orians, Evan L. Preisser

Forest pests, exotic and native, are pervasive agents of tree stress and mortality worldwide. Their impacts are mediated through disruption of host functional traits that are linked to nutrient acquisition, growth, and reproduction. Our understanding of the magnitude of these impacts however has been limited due to the fact that most studies tend to focus only on a single pest, as well as aboveground functional traits.

Here, using a long-term common garden, we experimentally tested the impact of multiple pests on the long-lived conifer Eastern hemlock (Tsuga canadensis). Hemlock is a foundation species of eastern forests of the United States and has suffered severe declines due to attack by two exotic herbivores, the hemlock woolly adelgid (HWA: Adelges tsugae), and to a lesser degree, the elongate hemlock scale (EHS: Fiorina externa). In this study, we experimentally infested trees with HWA and EHS individually, or in combination, for periods of two to four years and assessed effects on belowground functioning by excavating whole-root systems and measuring belowground root biomass, architecture, and interactions with mutualistic ectomycorrhizal fungi (EMF).

We found that trees infested with HWA had a greater root to total biomass ratio, however feeding had no observable effects on root architecture or resource allocation. HWA feeding significantly reduced EMF colonization of hemlock fine roots, though surprisingly, EMF diversity was unaffected. In contrast to HWA, EHS (alone or in conjunction with HWA) feeding had no observable effect on belowground traits or EMF colonization alone; however, its presence mediated the strength of HWA effects when trees were co-infested.

Taken together, our results suggest that HWA-induced loss of aboveground biomass and functioning, coupled with loss of belowground foraging through interactions with EMF, might accelerate eastern hemlock stress and mortality through nutrient limitation. However, the presence of EHS either accentuates or ameliorates these effects, with the nature of these non-additive effects being dependent on the timing of herbivore arrival. Thus, our results demonstrate that it is important to take a community wide approach to gain insights into how trees respond to pressures experienced in increasingly disturbed forests.

Photo provided by authors.

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