Adelgid infestation increases foliar water content in eastern hemlock

Background/Question/Methods

The invasive hemlock woolly adelgid, Adelges tsugae, infests and rapidly kills eastern hemlock (Tsuga canadensis) trees. The exact cause of death is unknown, but current research suggests that the adelgid depletes eastern hemlock of nutrients and induces water stress. To determine if the adelgid induces water stress in hemlocks, we measured percent relative water content (RWC) as a water status indicator in eight uninfested trees (controls), six trees that had been adelgid-infested for two years, and three trees that had been adelgid-infested for four years. Following recommended procedures, all samples were taken pre-dawn between 4:30 and 6:30 am on October 15^th, 2010. We collected ten needles of old growth (produced in spring 2009) and ten needles of new growth (produced in spring 2010) from branches in each of the four cardinal directions, collecting from the bud scar toward the terminal end of the branch. RWC was calculated separately for old- and new-growth needles as (fresh foliage weight – dry foliage weight) divided by fresh foliage weight and multiplied by 100.

Results/Conclusions

Old-growth needles had a lower RWC than new-growth needles in all three treatments. Foliar RWC in both of the adelgid treatments was significantly higher than foliar RWC in the control treatment; however, RWC in the four- and two-year adelgid treatments did not differ significantly. The result was unexpected because previous studies suggest that HWA increase water stress in eastern hemlocks. The higher RWC in adelgid-infested plants may be explained by increased stomatal closure following adelgid attack.