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University of Alberta (2014)

Clipping and Watering Effects on Caespitose and Rhizomatous Grasses : Implications for Grazing Management

Broadbent, Tanner S

Titre : Clipping and Watering Effects on Caespitose and Rhizomatous Grasses : Implications for Grazing Management

Auteur : Broadbent, Tanner S

Université de soutenance : University of Alberta

Grade : Doctor of Philosophy (PhD) 2014

Résumé
Recent experimental evidence suggests that rotational grazing, despite strong perceptions to the contrary, does not promote plant community productivity relative to continuous grazing on rangelands. However, clipping studies from tame pastures of Alberta’s Aspen Parkland show clear plant community production benefits and compensatory yielding under defoliation regimes associated with rotational grazing (i.e., high intensity low frequency [HILF] defoliation). Unlike relatively mesic tame pastures that are often dominated by rhizomatous grasses, rangelands are generally semiarid native grasslands with a preponderance of caespitose grasses. This suggests that grass growth form may mediate plant community production potential. This study used a greenhouse and field experiment to compare growth dynamics of phylogenetically similar and co-occuring grasses of contrasting growth form (caespitose vs. rhizomatous) to test (1) whether caespitose grasses, compared to rhizomatous grasses, grow more determinately, and (2) if this in turn constrains compensatory yielding under some combination of defoliation frequency and intensity. Plant community productivity and composition were also assessed. Treatments included variable clipping and watering regimes, and the field experiment encompassed both a mesic lowland and drier upland mixedgrass prairie site. In the greenhouse, compensatory growth occurred in 3 rhizomatous grasses and 1 caespitose grass. However, this was not corroborated by the field experiment, where the caespitose grass (Hesperostipa comata) compensated under HILF defoliation and the rhizomatous grass (Pascopyrum smithii) did not—the opposite was observed in the greenhouse for both grasses. Although defoliation increased plant growth rates, compensatory growth was limited by declining tiller populations, especially in P. smithii. Compensatory yielding within the plant community at the mesic lowland site was similarly limited by declining P. smithii populations because this grass was the dominant contributor to yield. In contrast, compensatory yielding was common at the drier upland site where P. smithii was markedly less dominant. Results suggest that (1) determinate growth is not characteristic of caespitose grasses, but rather (2) compensatory responses depend on plant ability to maintain tiller populations under defoliation. Further, compensatory yielding under HILF defoliation within mixedgrass prairie plant communities may be limited to drier sites where more defoliation tolerant (but less productive) grasses are dominant.

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