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Title: Soil Moisture as a Mediator for Nitrogen Competition : Implications for Species Invasion and Climate Change
Author: Everard, Katherine Anne
ISNI:       0000 0004 2681 0765
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2009
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Simple models of competition for one and two resources have been wellinvestigated for both nitrogen and water. Ecosystem models of nitrogen andwater, including their interactions are also well known, but competition andecosystem processes have rarely been considered together. Using a simpleecosystem model of the nitrogen and water cycles including the dependenceof the nitrogen cycle on soil moisture I examine the outcome of competitionfor nitrogen. This model shows that when there are species specific effectson soil moisture, the competitive outcome for nitrogen can be coexistenceor alternative stable states. The last century has seen widespread invasion of Californian grasslands byannual grasses and forbs from the southern Mediterranean region. Both nitrogenand water have been shown to be limiting in this system. The nativeperennial grasses deplete the soil moisture and nitrogen to a lower levelthan the invasive annual grasses, suggesting that natives should be bettercompetitors. Parameterising my model for a Californian grassland systemI show that in fact the invasive annual grasses are the superior competitorfor nitrogen at lower soil moisture when nitrogen competition is mediatedby soil moisture. The competitive outcome at current rainfall levels in Californiais limited to competitive exclusion by the annuals or coexistence. Climate change caused by anthropogenic emissions of CO2 is already affectingcommunities and ecosystems. Global climate models predict changes inboth mean amount and distribution of precipitation. I investigate the effectof changing precipitation distributions on both ecosystem and communityprocesses in nitrogen limited systems. There is wide variation in ecosystemresponse to increased variation in rainfall. The outcome of N competitionwas more predictable, with the best competitor at lower soil moisture beingconsistently favoured with increasing rainfall variability. This might proveanother problem in restoring native grasses in Californian grasslands.
Supervisor: de Mazancourt, Claire ; Crawley, Mick Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral