Changes in canopy structure and ant assemblages affect soil ecosystem variables as a foundation species declines

Authors

  • Joseph A. Kendrick,

    1. Bennington College, Bennington, Vermont 05201 USA
    2. Harvard University, Harvard Forest, 324 North Main Street, Petersham, Massachusetts 01366 USA
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    • Present address: Brown University, Department of Ecology and Evolutionary Biology, Providence, Rhode Island 02912 USA.

  • Relena R. Ribbons,

    1. University of Tennessee, Department of Ecology and Evolutionary Biology, Knoxville, Tennessee 37996 USA
    2. Bangor University, School of the Environment, Natural Resources, and Geography, Bangor, Wales LL57 2UW United Kingdom
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  • Aimée T. Classen,

    1. University of Tennessee, Department of Ecology and Evolutionary Biology, Knoxville, Tennessee 37996 USA
    2. The Natural History Museum of Denmark, University of Copenhagen, Universitetsparken 15, 2100 Copenhagen Ø, Denmark
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  • Aaron M. Ellison

    Corresponding author
    1. Harvard University, Harvard Forest, 324 North Main Street, Petersham, Massachusetts 01366 USA
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  • Corresponding Editor: D. P. C. Peters.

Abstract

The decline of Tsuga canadensis (eastern hemlock)—a foundation tree species—due to infestation by Adelges tsugae (hemlock woolly adelgid) or its complete removal from a stand by salvage logging dramatically affects associated faunal assemblages. Among these assemblages, species composition (richness and abundance) of ants increases rapidly as T. canadensis is lost from the stands. Because ants live and forage at the litter-soil interface, we hypothesized that environmental changes caused by hemlock loss (e.g., increased light and warmth at the forest floor, increased soil pH) and shifts in ant species composition would interact to alter soil ecosystem variables. In the Harvard Forest Hemlock Removal Experiment (HF-HeRE), established in 2003, T. canadensis in large plots were killed in place or logged and removed to mimic adelgid infestation or salvage harvesting, respectively. In 2006, we built ant exclosure subplots within all of the canopy manipulation plots to examine direct and interactive effects of canopy change and ant assemblage composition on soil and litter variables. Throughout HF-HeRE, T. canadensis was colonized by the adelgid in 2009, and the infested trees are now declining. The experimental removal of T. canadensis from the canopy was associated with an increase in the rate of cellulose decomposition by >50%, and exclosure of ants from subplots directly reduced their soil nitrate availability by 56%. Partial least squares path models revealed sequential interactive effects prior to adelgid infestation: canopy change (as a proxy for associated environmental changes) altered both decomposition and ant assemblage structure; changes in ant assemblage structure and decomposition rates altered nitrogen availability. The results illustrate that biotic changes directly associated with decline of T. canadensis can have cascading effects on ecosystem nutrient availability and cycling.

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