Impact of Hemlock Woolly Adelgid on Canopy Throughfall

• Investigators: Richard Cobb, Thomas Muller, David Orwig, Bernhard Stadler
• Contact: David Orwig
• Start date: 2002-05-01
• End date: 2002-10-31
• Location: South central Connecticut and Harvard Forest
• Latitude: +41.40 to +42.55
• Longitude: -72.40 to -72.16
• Elevation:
• Taxa: Adelges tsugae (hemlock woolly adelgid), Betula lenta (black birch), Tsuga canadensis (eastern hemlock)
• Keywords: carbon-nitrogen dynamics, epiphytic microorganisms, forest ecosystems, forest pests, hemlock woolly adelgid, litter, throughfall chemistry
• Release date: 2006
• EML version: knb-lter-hfr.84.2
• Revisions:
• Abstract:

  Exotic insect pests may strongly disrupt forest ecosystems and trigger major shifts on nutrient cycling, structure and composition. Although the immediate impact of these pests are frequently examined as physical disturbances (i.e., defoliation, decline in leaf area, and tree mortality) that initiate changes in ecosystem function, the insects often generate fundamental biochemical and trophic changes in tree canopies that may be equally important in altering ecosystem dynamics. Consequently, investigation of the linkages between canopy-level, ecosystem and environmental impacts may be critical for a thorough understanding of functional, structural, and compositional changes resulting from pest infestation. We sought to establish a better understanding of these linkages for the hemlock woolly adelgid (HWA), which is devastating hemlock forests in an expanding region across eastern North American and has the potential to eliminate this long-lived and extremely shade-tolerant species across much of its range. We examined the impact of the adelgid on hemlock needle chemistry and epiphytic microorganisms, litter production, and shoot growth in stands differing in their levels of infestation and linked these to shifts in canopy nutrient cycling and stand and landscape dynamics.

  HWA initiated major changes in canopy biomass and distribution. Whereas uninfested trees exhibit a decline in canopy biomass from the center to the periphery and a positive correlation between total needle litter and estimated biomass, infested trees support predominantly woody biomass, have significantly less total canopy biomass, produce less new foliage and exhibit no correlation between litter and canopy biomass. Foliar %N was strongly influenced by needle age and the level of infestation and was highest in young foliage supporting the highest densities of HWA. Foliar %C was unaffected by HWA or foliar age. Epiphytic microorganisms on hemlock needles exhibited little variation in abundance within canopies, but bacteria, yeasts and filamentous fungi were significantly more abundant on medium and heavily infested than uninfested trees.

  Throughfall chemistry, quantity, and spatial pattern were strongly altered by HWA. Beneath uninfested trees throughfall exhibits a strong gradient, decreasing in volumes from the canopy periphery to the trunk. Beneath infested trees the amount of throughfall is greatly increased, spatially unpatterned and characterized by higher concentrations of nitrogen compounds, carbon and cations.

  Across the southern New England landscape there is a strong south to north gradient of decreasing tree and sapling mortality and understory compositional change that correspond to the duration of infestation. Regionally, black birch, a nitrogen demanding species that is competitively enhanced by increasing nitrogen availability is profiting most from hemlock decline.

• Methods:

  At each site three throughfall samplers were placed beneath each tree, one close to the trunk, one at the periphery and one midway, to capture throughfall across the canopy spatial gradient. Each throughfall sampler covered 162.86 cm2 and remained in place through the entire experiment. Throughfall was collected every two weeks, from May 20th to August 31st. To determine the nitrogen content of needles on infested/uninfested shoots and egg mass biomass, needles and egg masses were detached with tweezers and composited for each shoot and determined with a Fisons dry combustion CHN autoanalyzer. At three sampling dates (May 22, July 1, and August 31, 2002) shoots from eastern hemlock trees were sampled for microbial analysis. Four shoots with needle age classes ranging from 0-3 years were cut with sterile scissors, transferred into a sterile Stomacher bag, stored in a cooler, and transported to the laboratory where they were frozen until microbial analyses could be completed.

• Use:

  This dataset is released to the public and may be freely downloaded. Please keep the designated Contact person informed of any plans to use the dataset. Consultation or collaboration with the original investigators is strongly encouraged. Publications and data products that make use of the dataset must include proper acknowledgement. For more information on LTER Network data access and use policies, please see: http://www.lternet.edu/data/netpolicy.html.

• Related datasets: HF021