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University of Arizona (2021)

Plant-Plant Interactions During Arid Grassland-Shrubland State Transition

Pierce, Nathan

Titre : Plant-Plant Interactions During Arid Grassland-Shrubland State Transition

Auteur : Pierce, Nathan

Université de soutenance : University of Arizona

Grade : Doctor of Philosophy (PhD) 2021

Transitions from grassland to shrubland are synonymous with desertification in many arid rangeland systems. Traditional desertification models emphasize abiotic feedbacks that modify the physical environment in ways that promote shrub proliferation and impede grass survival. Inherent in this perspective is the assumption that biotic interactions between grasses and shrubs have little bearing on state transition dynamics. Furthermore, the extent to which density-dependent interactions among shrubs might determine the magnitude and pattern of their cover is unknown. I addressed these assumptions and knowledge gaps over 4 years using field-based selective removal experiments with shrubs (Prosopis glandulosa) and grasses (Bouteloua eriopoda) at the Jornada Experimental Range and long-term ecological research (LTER) site in the northern Chihuahuan Desert. Shrub-on-Shrub Interactions : Intraspecific interactions between shrubs were not evident in any year, supporting the assumption that abiotic variables, rather than competitive interactions, constrain maximum shrub cover. Grass-on-Shrub Interactions : In years with above-average growing season precipitation, ANPP of small shrubs increased when grasses were removed, a result not evident in dry years or for larger shrubs. Grasses may therefore slow the rate at which shrubs attain a physical stature that can modify the physical environment in self-promoting ways. Shrub-on-Grass Interactions : Following consecutive years of above-average precipitation, grass ANPP responded positively to shrub removal, and allocation to vegetative reproduction and grass patch area increased. These results demonstrate that biotic interference by shrubs upon grasses can reinforce and magnify abiotic feedbacks during grassland–shrubland transitions. A simple model that accounts for total shrub neighborhood canopy area explained as much variance in grass ANPP reductions as did more complex models that include shrub proximity, volume, and/or grass patch size. Results also identify that grass patches are most influenced by shrubs within 5 m, that grass defoliation effects may be magnified in the presence of shrubs, and that shrubs may facilitate grass production at low levels of shrub abundance. Results from these field experiments (i) provide insights on how shrub-grass interactions can amplify or dampen the abiotic drivers of desertification, (ii) help explain how woody plants can continue to proliferate despite low or reduced livestock grazing pressure, and (iii) generate hypotheses that can help refine experiments to address the mechanisms of belowground competition at play where grasses and shrubs co-occur in arid ecosystems.

Mots clés : Arid Grassland Desertification State Transition Vegetation Change


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