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Browsing by Subject "Herbivory"

Now showing 1 - 4 of 4
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    Consequences of Biodiversity in Tree Diversity Experiments
    (2018-05) Grossman, Jacob
    This dissertation reports on four studies that explore the consequences of changes in tree biodiversity for three ecosystem processes (growth, leaf herbivory and disease, and leaf decomposition) in tree-dominated ecosystems in eastern Minnesota, USA. In Chapter 1, I present the Forests and Biodiversity (FAB) experiment and assess the role of taxonomic, phylogenetic, and functional diversity in this experiment on stem growth of 12 species. Over the third to fifth years of the experiment, trees with more diverse neighborhoods produced more biomass than trees in less diverse neighborhoods. This complementary overyielding effect was associated with species richness (taxonomic diversity) and was better predicted by tree diversity at larger (12 m2) rather than smaller (0.25 m2) spatial scales. I also measured three forms of invertebrate herbivore damage and two forms of disease damage on leaves of nine FAB species; results from this study are presented in Chapter 2. I assessed the consequences of diversity for damage across four spatial scales. Herbivory and disease responded to a variety of metrics of community diversity and these effects were species-specific. Damage, regardless of what kind, was better predicted by community structure and diversity at small spatial scales (1-4 m2) than large scales (9-16 m2). Chapter 3 consists of the presentation of results from the Biodiversity in Willows and Poplars (BiWaP) experiment, in which both the genetic diversity and species diversity of three Salicaceous species was manipulated, and tree growth and herbivory were measured. Diversity did not have a consistent effect on productivity because one dominant species suppressed hetrospecific neighbors. In contrast, specialist gall formation was best predicted by genetic identity and genotypic diversity suppressed leaf mining. Finally, through a separate litterbag decomposition experiment designed in parallel to FAB and presented in Chapter 4, I measured the consequences of leaf chemical diversity for decomposition over two years. When litter from multiple species were mixed, it did not lose mass, cellulose, or lignin differently than would be expected based on monoculture. But more labile carbon fractions (soluble contents and hemicellulose) decomposed more slowly in more functionally diverse litter mixes.
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    Effects of herbivores on ground flora and nutrient dynamics along Lake Superior's North Shore Highlands
    (2012-12) McGarry, Elizabeth Anne
    Impacts due to ungulate herbivores such as moose and white-tailed deer have been of interest to researchers and resource managers for years. Recent studies have begun to suggest the presence of alternative stable states in these systems where the existence of hysteresis could result in significant changes to the understory community that would be costly and time consuming to undo. However, before such assertions can be made, the presence of different states needs to be established. Our study investigated changes to the understory community and nutrient dynamics of twelve paired moose and white-tailed deer exclosure and control plots located along a 95-mile section of the Lake Superior North Shore Highlands. Exclosure ages ranged from 11 to 24 years old. During the summer of 2010, we measured understory and shrub species percent cover, sapling densities, litter depth, and bulk soil composition inside and outside of exclosures at each site. Nitrogen availability was also measured at three sites using resin bags. Browsing significantly affected understory vegetation at all sites except one, as well as ammonium (NH4) availability and percent of nitrogen and carbon in the mineral soil. The directions of these effects varied by site and seemed to be related to differences in canopy composition, age of exclosure, and relative ungulate population size. Future studies should monitor long-term changes through time, as some trends may be important early on in forest recovery while others do not appear until years later. Although our study design was not able to detect evidence of hysteresis or affirm the presence of alternative stable states, we believe it does suggest evidence of instability and non-linear ecosystem impacts of ungulate herbivory.
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    Temporal changes in spatial patterns of moose browse, causes and consequences.
    (2010-03) Hodgson, Angela Lynne
    Ecologists determine mechanisms by observing spatial and temporal patterns of abundance and distribution in natural systems. While there has been a long history of research on techniques for describing temporal patterns of abundance, and their causes and consequences, there is still a need for ecological research to focus on the causes and consequences of spatial patterns. Progress on this goal, though, has been hindered by the lack of long-term data on spatial patterns in natural ecosystems. I present findings from one of the first long-term studies of changes in spatial patterns of plants in response to herbivory, and discuss causes and consequences. My research was conducted in the southern boreal forest on Isle Royale, Michigan, and focused on temporal changes in the spatial pattern of woody browse species that are consumed by a large herbivore (moose). I concluded that browsed woody sapling biomass is aggregated within moose feeding stations and the degree of aggregation has changed over a 20-year period. The cause of this fluctuating pattern of aggregation is due to the competing influences of inverse density dependent browsing by moose, causing an increase in aggregation, and inverse density dependent growth, causing a decrease in aggregation. Annual changes in aggregation are determined by the relative contribution of consumption and growth to changes in spatial pattern in any given year. Next, I developed a simulation model to determine the consequence of aggregation of browse on the intake rate of large herbivores. I found that the spatial aggregation of browse within feeding stations can decrease the intake rate up to 30% for herbivores feeding on low density browse (<15 g/m2). Both density and the spatial arrangement of browse, therefore, is important to consider when determining the functional response of large herbivores. Finally, I used long-term data on consumption and spatial pattern of browse to test whether a mechanistic modified contingency model could predict observed moose diet selection. This model assumes that moose select their diet in order to maximize short-term intake rate. Model predictions were consistent with observed diet selection during both summer and winter in two study sites.
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    Tree regeneration dynamics and drivers across the temperate-boreal transition zone.
    (2012-03) Fisichelli, Nicholas Anthony
    The upper Great Lakes region in central North America contains a forest transition zone where temperate and boreal tree species reach their northern and southern range limits, respectively. It is only within this narrow latitudinal band (~3 degrees), that relatively warm-adapted temperate and cold-adapted boreal tree species are found growing together in upland mesic sites. If climate is a main driver of forest dynamics within this region, recent and predicted climate change should result in major forest shifts, including the expansion of temperate species and range contraction of boreal species. Such changes should first be manifest in growth and abundance trends of tree species in the understory regeneration layers. In addition to climate, numerous other factors such as overstory composition, understory abiotic environment, competition with shrub and herbaceous layers, and browse pressure drive tree regeneration trends. Interrelationships and interactions among these drivers will ultimately determine the direction and rate of forest change. We explored these research topics through field studies of naturally established seedlings and saplings at 124 upland mesic forest sites across a three state (Minnesota, Wisconsin, and Michigan U.S.A) 170,000 km2 area of the temperate-boreal transition zone. Chapter 1 examined relative abundance shifts of temperate and boreal tree regeneration at two spatial scales: local ecotonal boundaries between temperate and boreal dominated stands and across the regional temperate-boreal transition zone. Because we compared understory performance across locally changing overstory composition, we calculated species regeneration success as the difference in relative abundance between the understory and overstory layers. At the local scale, both shade-tolerant temperate and boreal species exhibited positive tree regeneration success across ecotonal boundaries. However, across the region, regeneration performance varied with mean summer temperature and to a lesser extent mean annual precipitation. Changes in regeneration success were generally greatest at the warm end of the transition zone, with temperate broadleaf Acer saccharum, Fraxinus nigra, and Ostrya virginiana responding positively and boreal Abies balsamea showing significantly reduced performance. For the most frequent temperate species, Acer rubrum, regeneration success was greatest in boreal neighborhoods and at cool and dry sites. Other species did not exhibit detectable shifts in regeneration success, potentially due to traits such as shade-tolerance, palatability, and mode of reproduction. Overall we found that numerous tree species growing across the temperate-boreal transition zone are likely sensitive to climate at early stages of development, with observed shifts in regeneration success concomitant with the direction predicted in response to climate change. Chapter 2 assessed the relative importance of and interrelationships among explanatory variable sets in explaining the composition of the tree regeneration layer. We used redundancy analysis (RDA) and variation partitioning to quantify the unique, shared, and total explanatory power of four sets of drivers: climate, understory abiotic environment, overstory composition, and understory biota. The results showed that all four driver sets individually explained a significant portion of tree regeneration compositional variation and additionally that there were strong relationships among explanatory variables. Overstory composition, which directly influences seed availability and also was found to be closely associated with understory environmental conditions and biota, had approximately twice the explanatory power of any of the other three driver sets. Some of the strongest individual drivers were overstory Acer saccharum and Populus tremuloides, soil pH, mean summer temperature, and mean annual precipitation. Suites of associated drivers included cool, moist, sandy, and acidic conditions; overstory boreal broadleaf species, light availability, shrub abundance, and forb cover; and warm temperatures and graminoid cover. Due to the strong interrelationships among drivers, the direction and rate of forest change will likely depend on how the importance of drivers shifts with climate and, for the biotic drivers, on the rate and magnitude of their own responses to climate change. Chapter 3 investigated sapling height and radial growth rates of five temperate and boreal species. This study included over 1700 stems of naturally established, competing saplings growing at 14 sites across the temperate-boreal transition zone. Top performing linear mixed-effects models typically included two-way interactions among mean summer temperature, browse pressure, understory light levels, and initial sapling size. As hypothesized, temperate sapling growth increased and boreal growth decreased with increasing temperatures. However, the relative performance of competing species shifted depending on the level of browse pressure. Positive temperate growth responses to temperature were eliminated by heavy browse pressure, tilting growth rates in favor of less palatable boreal conifers at all but the warmest sites. Spatial variations in browse pressure levels across the region suggest that temperate expansion may proceed most rapidly in areas where browsing is least intense. Growth responses to temperature also varied with sapling size and, for the least shade-tolerant species in the study, Quercus rubra, light availability. Enhanced growth by temperate species in response to warmer temperatures was most detectable under favorable conditions including low browse pressure and high understory light availability, suggesting that any efforts to facilitate forest compositional changes will need to take into account these trends.