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Harvard Forest Symposium Abstract 2014

  • Title: Nitrogen saturation leads to increased weevil infestation of red oak acorns
  • Primary Author: Michal Bogdziewicz (Adam Mickiewicz University)
  • Additional Authors: Elizabeth Crone (Tufts University); Michael Steele (Wilkes University); Rafa Zwolak (Adam Mickiewicz University)
  • Abstract:

    The increase of anthropogenic inputs of reactive nitrogen caused by fossil fuel combustion is a recent stress for both terrestrial and aquatic ecosystems all over the world. The Chronic N Amendment Study at Harvard Forest was designed to study the effects of nitrogen fertilization in oak dominated hardwood forests of North America. Oaks produce large seed crops in an intermittent fashion that is called mast seeding. A recent study by Callahan et al. (2008) conducted at Harvard Forest showed that nitrogen saturation can increase seed production in red oaks, especially in non-mast years. This can have wide ecological consequences that can alter ecosystem functioning in several ways. One of effects of mast seeding is predator control. Scarce seed production in non-mast years can keep predator numbers low and then large seed crop in mast year satiate predators and allows seeds to escape predation. However, sustained seed production in non-mast years caused by N fertilization may allow predators to persist in higher numbers and this can interfere with predator satiation.

    In October 2013 we collected mature red oak acorns from the ground under the canopy of 12 red oak trees from 16-fold N increase (high N) plot (152 acorns in total), 20 oak trees from 8-fold N increase (low N) plot (305 acorns), 9 oak trees from theadjacent control plot that did not received fertilizer and 11 trees from another Prospect Hill red oak stand that was also historically nitrogen limited (455 acorns). For each acorn we measured mass, dimensions, basal scar width, basal scar thickness, number of weevil larvae in the top and bottom half of an acorn and number of Dipteran larvae.

    We found strong differences in weevil abundance (number of weevils larvae per acorn) and prevalence (percent of infested acorns) between treatments. Weevils numbers were highest in acorns from high N plot (0.35 ± 0.15 SD) intermediate in low N plot (0.23 ± 0.16) and lowest in control acorns (0.10 ± 0.04). The same pattern was found in prevalence as 70 % (± 0.16 SD) of high N acorns, 59 % (± 0.26) of low N acorns and 41% (± 0.2) of control acorns were infested. Again, differences between all treatments were significant (p<0.05). On the other hand, we found no differences in mass and size between acorns from different plots.

    Our results indicate that N saturation leads to increased weevil infestation of red oak acorns. This increase can be caused by increased production of seeds by N fertilized oaks that leads to increased weevil numbers (lack of bottom-up predator control like in an ordinary masting system). As an alternative (but not mutually exclusive) hypothesis, nitrogen deposition could alter chemical composition of acorns, increasing their food quality for weevils. We will test these possibilities in our future work. This investigation is a part of a broader study on masting and plant-animal interactions conducted as a collaboration between Harvard Forest and Adam Mickiewicz University in Poznań, Poland.

  • Research Category: Large Experiments and Permanent Plot Studies, Physiological Ecology, Population Dynamics, and Species Interactions