Corbett, A. L. 1998. Root responses to nutrient heterogeneity: a comparison of dominant and subordinate species from old fields. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

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Many potential mechanisms have been proposed to explain coexistence among plant species. Spatial heterogeneity in soil resources at scales smaller than individual plants may promote coexistence by forcing a trade-off between the efficient locating of nutrient patches (scale foraging) and the efficient exploiting of nutrient patches (precision foraging). Evidence from Campbell et al. (1991) suggests that dominant species use a scale foraging strategy whereas subordinate species use a precision foraging strategy.

To test whether dominant and subordinate species differed in foraging strategy under heterogeneous soil resource conditions, I conducted a series of greenhouse experiments in which I varied patch size and intensity, using two dominant (Bromus inermis and Solidago canadensis) and three subordinate (Achillea millefolium, Rumex acetosella, and Silene latifolia) herbaceous perennial species commonly found in local old fields. One of the dominant species (Bromus) consistently had high tissue nitrogen content, but had lower nitrogen contents as patch size and nutrient intensity decreased. This performance pattern was consistent with that expected for a scale forager. the other dominant species (Solidago) and the three subordinate species had lower nitrogen contents overall, but more consistent nitrogen contents across patch sizes and nutrient intensities. This performance pattern was consistent with that expected for a precision forager. There appeared to be a trade-off between the ability to acquire a large amount of nitrogen and the ability to maintain constant acquisition levels in the plant as nutrient patch sizes changed.

I also predicted that precision and scale-foraging species would differ in the mechanisms they used to respond to nutrient heterogeneity. Although I found differences among species with respect to which response mechanisms they used, the differences were not related to precision vs. scale foraging strategies. Two of the four precision-foraging species, Achillea and Solidago, ubcreased their branching density in nutrient patches. All species were able to selectively allocate root biomass to nutrient enriched pot quadrants. None of these species changed root system topology in response to nutrient patches. Rumex and Silene, two of the precision-foraging species, adjusted nitrogen uptake so that root nitrogen concentrations were constant across patch sizes. Bromus (a scale forager), Achillea and Solidago (precision foragers) adjusted nitrogen uptake so that root concentrations in enriched pot quadrants were higher than in background quadrants. These five species did not differ in the patch size or nutrient intensity at which they used a response mechanism.

To fully understand the role that nutrient heterogeneity may play in determining plant community structure, we must also know the size, intensity and frequency of small scale patches in the environment. Spatial sampling and geostatistical analysis techniques are useful tools for quantifying spatial variation of nutrients in the field, but it is unclear what spatial sampling pattern will be most sensitive to multiple scales of heterogeneity. Using computer simulated data and semivariance analysis, I determined that a stratified-nested grid sampling regime would be more sensitive than a random or stratified grid sampling regime when determining the magnitude and scale of environmental heterogeneity across several sites.

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