Invasive insect effects on nitrogen cycling and host physiology are not tightly linked

Impact of hemlock woolly adelgid (Adelges tsugae) infestation on xylem structure and function and leaf physiology in eastern hemlock (Tsuga canadensis)

Hemlock woolly adelgid (Adelges tsugae Annand) (HWA) is an invasive insect that feeds upon the foliage of eastern hemlock (Tsuga canadensis (L.) Carrière) trees, leading to a decline in health and often mortality. The exact mechanism leading to the demise of eastern hemlocks remains uncertain because little is known about how HWA infestation directly alters the host's physiology. To evaluate the physiological responses of eastern hemlock during early infestation of HWA, we measured needle loss, xylem hydraulic conductivity, vulnerability to cavitation, tracheid anatomy, leaf-level gas exchange, leaf water potential and foliar cation and nutrient levels on HWA-infested and noninfested even-aged trees in an experimental garden. HWA infestation resulted in higher xylem hydraulic conductivity correlated with an increase in average tracheid lumen area and no difference in vulnerability to cavitation, indicating that needle loss associated with HWA infestation could not be attributed to reduced xylem transport capacity. HWA-infested trees exhibited higher rates of net photosynthesis and significant changes in foliar nutrient partitioning, but showed no differences in branch increment growth rates compared with noninfested trees. This study suggests that HWA-induced decline in the health of eastern hemlock trees is not initially caused by compromised water relations or needle loss.

Defoliating Insect Mass Outbreak Affects Soil N Fluxes and Tree N Nutrition in Scots Pine Forests

Biotic stress by mass outbreaks of defoliating pest insects does not only affect tree performance by reducing its photosynthetic capacity, but also changes N cycling in the soil of forest ecosystems. However, how insect induced defoliation affects soil N fluxes and, in turn, tree N nutrition is not well-studied. In the present study, we quantified N input and output fluxes via dry matter input, throughfall, and soil leachates. Furthermore, we investigated the effects of mass insect herbivory on tree N acquisition (i.e., organic and inorganic 15N net uptake capacity of fine roots) as well as N pools in fine roots and needles in a Scots pine (Pinus sylvestris L.) forest over an entire vegetation period. Plots were either infested by the nun moth (Lymantria monacha L.) or served as controls. Our results show an increased N input by insect feces, litter, and throughfall at the infested plots compared to controls, as well as increased leaching of nitrate. However, the additional N input into the soil did not increase, but reduce inorganic and organic net N uptake capacity of Scots pine roots. N pools in the fine roots and needles of infested trees showed an accumulation of total N, amino acid-N, protein-N, and structural N in the roots and the remaining needles as a compensatory response triggered by defoliation. Thus, although soil N availability was increased via surplus N input, trees did not respond with an increased N acquisition, but rather invested resources into defense by accumulation of amino acid-N and protein-N as a survival strategy.