Leucopis argenticollis (Zetterstedt) and Leucopis piniperda (Malloch) are known to feed on the lineage of Adelges tsugae Annand that is native to western North America, but it is not known if they will survive on the lineage that was introduced from Japan to the eastern USA. In 2014, western Leucopis spp. larvae were brought to the laboratory and placed on A. tsugae collected in either Washington (North American A. tsugae lineage) or Connecticut (Japanese lineage). There were no significant differences in survival or developmental times between flies reared on the two different adelgid lineages. In 2015 and 2016, western Leucopis spp. adults were released at two different densities onto enclosed branches of A. tsugae infested eastern hemlock (Tsuga canadensis (L.) Carr.) in Tennessee and New York. Cages were recovered and their contents examined 4 weeks after release at each location. Leucopis spp. larvae and puparia of the F1 generation were recovered at both release locations and adults of the F1 generation were collected at the Tennessee location. The number of Leucopis spp. offspring collected increased with increasing adelgid density, but did not differ by the number of adult flies released. Flies recovered from cages and flies collected from the source colony were identified as L.argenticollis and L. piniperda using DNA barcoding. These results demonstrate that Leucopis spp. from the Pacific Northwest are capable of feeding and developing to the adult stage on A. tsugae in the eastern USA and they are able to tolerate environmental conditions during late spring and early summer at the southern and northern extent of the area invaded by A. tsugae in the eastern USA.

We explored the middle-Holocene decline of Tsuga canadensis by measuring the diameters of pollen grains in two lake-sediment cores from New England. We hypothesized that a drop in pollen size at the time of the decline followed by an increase in pollen diameters as Tsuga recovered during the late Holocene might indicate reduced abundance of Tsuga in the vicinity of the lake during the decline, as smaller pollen grains travel farther than larger ones. To provide context for this hypothesis, we also measured the diameters of Tsuga pollen grains in the surface sediments of sites spanning the modern-day gradient of Tsuga in New England. Both fossil records exhibited a reduction in pollen size during the interval of the middle-Holocene decline, with diameters similar to those observed in the upper sediments of those sites, yet larger than Tsuga pollen grains in the surface sediments of coastal sites beyond the modern range of Tsuga. This pattern suggests that Tsuga persisted in scattered, low-density populations during the middle Holocene, as it has remained on the landscape since European settlement.

We investigated the effect of elevated CO2 on the growth and mycorrhizal colonization of three tree species native to north-eastern American forests (Betula papyrifera Marsh., Pinus strobus L. and Tsuga canadensis L. Carr). Saplings of the tree species were collected from Harvard Forest, Massachusetts, and grown in forest soil under ambient (c. 375 ppm) and elevated (700 ppm) atmospheric CO2 concentrations for 27–35 wk.

In all three species there was a trend to increasing whole-plant, total-root and fine-root biomass in elevated CO2, and a significant increase in the degree of ectomycorrhizal colonization in B. papyrifera and P. strobus, but not in T. canadensis. However, in T. canadensis the degree of colonization with arbuscular mycorrhizas increased significantly. In both the ambient and elevated environments, on the roots of B. papyrifera and P. strobus 12 distinct ectomycorrhizal morphotypes were identified. Distinct changes in the ectomycorrhizal morphotype assemblage of B. papyrifera were observed under CO2 enrichment. This change resulted in an increase in the frequency of ectomycorrhizas with a higher incidence of emanating hyphae and rhizomorphs, and resulted in a higher density of fungal hyphae in a root exclusion chamber.