UVM Theses and Dissertations
Format:
Print
Author:
Schwenk, W. Scott
Dept./Program:
Natural Resources
Year:
2008
Degree:
PhD
Abstract:
Regeneration and land protection have restored forests to much of the landscape of northern New England and New York, yet forests of this area continue to face many potential threats. These range from changing climate to acid deposition to invasive diseases. One perspective for better understanding how these forests function, and may respond to current and future stressors, is to consider how major components of the forest food web interact. I have taken this approach in examining the interactions among trees, herbivorous insects, and insect-eating birds, and the environment at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. Following a review of the current status of understanding about trophic cascades - a special case of interactions among predators, herbivores, and plants - I present the results of the studies I conducted.
In one experiment, my colleagues and I studied the effects of birds on insects and tree sapling growth from 2004 to 2007 at three elevations across a range of climatic conditions. By constructing frames covered with netting around saplings, we monitored insect abundance and tree growth in the absence of insect-feeding birds. Overall, arthropods were 18% more abundant where birds were excluded. However, birds did not substantially affect sapling growth. We suggest that the relatively high understory vegetation density relative to abundance of birds limited their ability to deplete arthropod biomass. Climate effects operated at multiple scales, resulting in a complex interplay of interactions among the food web. These patterns resulted in a mosaic of interaction strengths across time and space, suggesting an overall pattern of limited top-down control, possibly interspersed with hotspots of trophic cascades.
Additionally, we compared patterns of insect and moose herbivory on saplings during the experiment. Most moose browse occurred during the winter and was patchy over space and time. Browsed trees often suffered a drastic loss of buds. Saplings responded by dramatically increasing shoot growth, but their growth did not fully compensate for moose browse, and they produced fewer leaves than in the prior year. By contrast, herbivory by leaf-chewing insects (mainly caterpillars) occurred during the summer and was more widespread and uniform. It also was less intense (average 5% leaf area removed) than browse, and depressed rather than increased shoot growth. In one year, the biomass of leaf-chewers and the damage they caused were greater on browsed than unbrowsed trees, suggesting that browsing may improve leaf quality for herbivores.
In another study, my colleagues and I investigated how trees, herbivorous insects, and birds responded to the addition of calcium to the soil of a forested watershed. This addition was intended to replenish Ca depleted from forest soil in part by acid deposition. We found that Ca levels were much higher in the leaves of three tree species in the treated watershed than in a nearby reference area. While we found partial support for the hypothesis that trees where Ca had been added were more resistant to insect herbivores, overall the effects of Ca on insects were limited or inconsistent. We also did not observe strong effects of Ca addition on a bird that feeds heavily on herbivorous insects, the Black-throated Blue Warbler. To date, it does not appear that the Ca treatment has caused a cascade of changes in the herbivorous insect or bird communities.
In one experiment, my colleagues and I studied the effects of birds on insects and tree sapling growth from 2004 to 2007 at three elevations across a range of climatic conditions. By constructing frames covered with netting around saplings, we monitored insect abundance and tree growth in the absence of insect-feeding birds. Overall, arthropods were 18% more abundant where birds were excluded. However, birds did not substantially affect sapling growth. We suggest that the relatively high understory vegetation density relative to abundance of birds limited their ability to deplete arthropod biomass. Climate effects operated at multiple scales, resulting in a complex interplay of interactions among the food web. These patterns resulted in a mosaic of interaction strengths across time and space, suggesting an overall pattern of limited top-down control, possibly interspersed with hotspots of trophic cascades.
Additionally, we compared patterns of insect and moose herbivory on saplings during the experiment. Most moose browse occurred during the winter and was patchy over space and time. Browsed trees often suffered a drastic loss of buds. Saplings responded by dramatically increasing shoot growth, but their growth did not fully compensate for moose browse, and they produced fewer leaves than in the prior year. By contrast, herbivory by leaf-chewing insects (mainly caterpillars) occurred during the summer and was more widespread and uniform. It also was less intense (average 5% leaf area removed) than browse, and depressed rather than increased shoot growth. In one year, the biomass of leaf-chewers and the damage they caused were greater on browsed than unbrowsed trees, suggesting that browsing may improve leaf quality for herbivores.
In another study, my colleagues and I investigated how trees, herbivorous insects, and birds responded to the addition of calcium to the soil of a forested watershed. This addition was intended to replenish Ca depleted from forest soil in part by acid deposition. We found that Ca levels were much higher in the leaves of three tree species in the treated watershed than in a nearby reference area. While we found partial support for the hypothesis that trees where Ca had been added were more resistant to insect herbivores, overall the effects of Ca on insects were limited or inconsistent. We also did not observe strong effects of Ca addition on a bird that feeds heavily on herbivorous insects, the Black-throated Blue Warbler. To date, it does not appear that the Ca treatment has caused a cascade of changes in the herbivorous insect or bird communities.