Abstract
Ecologists have long classified Midwestern prairies based on compositional variation assumed to reflect local gradients in moisture availability. The best known classification is based on Curtis’ continuum index (CI), calculated using the presence of indicator species thought centered on different portions of an underlying moisture gradient. Direct evidence of the extent to which CI reflects differences in moisture availability has been lacking, however. Many factors that increase moisture availability (e.g., soil depth, silt content) also increase nutrient supply and decrease soil mechanical impedance; the ecological effects of the last have rarely been considered in any ecosystem. Decreased soil mechanical impedance should increase the availability of soil moisture and nutrients by reducing the root costs of retrieving both. Here we assess the relative importance of soil moisture, nutrient supply, and mechanical impedance in determining prairie composition and structure. We used leaf δ13C of C3 plants as a measure of growing-season moisture availability, cation exchange capacity (CEC) x soil depth as a measure of mineral nutrient availability, and penetrometer data as a measure of soil mechanical impedance. Community composition and structure were assessed in 17 remnant prairies in Wisconsin which vary little in annual precipitation. Ordination and regression analyses showed that δ13C increased with CI toward “drier” sites, and decreased with soil depth and % silt content. Variation in δ13C among remnants was 2.0‰, comparable to that along continental gradients from ca. 500–1500 mm annual rainfall. As predicted, LAI and average leaf height increased significantly toward “wetter” sites. CI accounted for 54% of compositional variance but δ13C accounted for only 6.2%, despite the strong relationships of δ13C to CI and CI to composition. Compositional variation reflects soil fertility and mechanical impedance more than moisture availability. This study is the first to quantify the effects of soil mechanical impedance on community ecology.
Highlights
Curtis [1], in his seminal work The Vegetation of Wisconsin, established a compositional gradient for prairies of the US Midwest by calculating a continuum index (CI: range 100 wet – 500 dry) based on the proportions of species present from five groups of indicator taxa assumed to be associated with different topographic positions or soil types [2,3,4]
Corbett and Anderson [10] found that variation in prairie composition in Illinois and Wisconsin was related to topographic position, soil texture, and soil chemistry
We identified a total of 212 vascular plant species, with an average of 47.1 ± 12.2 species per site
Summary
Curtis [1], in his seminal work The Vegetation of Wisconsin, established a compositional gradient for prairies of the US Midwest by calculating a continuum index (CI: range 100 wet – 500 dry) based on the proportions of species present from five groups of indicator taxa assumed to be associated with different topographic positions or soil types [2,3,4]. At Faville Prairie in southern Wisconsin, Partch [2] did find that direct, repeated measurements of % soil moisture through the growing season were related to local differences in composition which, in turn, appeared to be tied to local topography and depth to water table as one moved from dry and dry-mesic prairies to wet sloughs. He did not evaluate the relationship of species composition to soil moisture vis-à-vis other environmental factors. Almost all studies to date ascribe primacy to the effects of soil moisture, and none have quantified the relative importance of soil moisture vs. other factors in determining community composition and structure when both are included in predictive statistical models
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