Ecological resilience theory describes dynamics of change and stasis in complex systems, but it is rarely applied as a concept in agroecosystem analysis due to the fundamental differences in managed and unmanaged ecosystems. Using ecological resilience as a framing concept, I explored the role agroecosystem complexity plays in resilience to climatic stresses at various scales. I used integrated crop-livestock systems (ICLS) as a model to test the effects of increasing spatial, temporal, and/or biological complexity on agroecosystem processes and functionality. ICLS refer to systems that intentionally leverage synergies from the coupled production of animal and crop commodities in an attempt to close nutrient and energy loops. Because they theoretically substitute improved agroecosystem functionality for increased anthropogenic inputs, ICLS represent an approach for the ecological intensification of agriculture. In large-scale, commercial production systems, ICLS are also a pathway to mitigating the numerous environmental, social, and economic externalities of highly specialized and concentrated agriculture. The addition of a higher trophic level to plant-based production systems, especially when livestock are integrated in space or time, creates new linkages among soil, plant, and animal components of the agroecosystem. These linkages and interactions are possible reservoirs for emergent properties such as resilience.
Focusing on an integrated beef-soybean production system in southern Brazil, I asked how the addition of a grazing animal to an otherwise monocultural crop production system could change agroecosystem processes ultimately affecting productivity and resilience to sub-optimal climate conditions. First, I explored the concept of ecological resilience as it applies to agroecosystems, with emphasis on productive functions, sources of system regulation and disturbance, and cross-scale interactions (Chapter 1). Second, I used meta-analysis to examine ICLS collectively – across management strategies, crop and livestock species, regions, and climates – for their performance in crop production relative to unintegrated systems (Chapter 2). I found that despite considerable variability among systems and contexts, crops grown in integrated systems consistently out-yielded crops grown in unintegrated systems, even in years with abnormally low precipitation. Third, I used an experimental approach to characterize soil water dynamics and plant physiological indices during the soybean growing season of the Brazilian beef/soybean ICLS (Chapter 3), showing that although soil water content metrics were consistently lower, crop phenology was delayed, and crop canopies used light less efficiently, soybeans in plots where cover crops had been grazed the previous winter produced equivalent yields to soybeans grown in control plots with ungrazed winter cover crops. Tradeoffs between soil water consumption by grazed cover crops during the winter and improved soil water availability in grazed plots during subsequent growing seasons were presumed to play a role in this outcome. Finally, using the information on soil water parameters collected in Brazil as well as a 16-year dataset for that site, I employed a process-based modeling approach to simulate for the first time the probable long-term productive and economic outcomes under historical and future climate conditions for this ICLS (Chapter 4). I found that even under hotter and wetter future climate conditions, and even where the inclusion of grazing animals resulted in yield costs for soybeans, system-wide productivity (including livestock gains from consuming forage cover crops) was greater in the integrated system in 55% of simulated years. Taken together, these findings suggest that the added ecological complexity of ICLS can contribute to beneficial resilience of crop production in both the short term and long term, and at both the field scale and system scale.
|Advisor:||Gaudin, Amelie C. M.|
|Commitee:||Roche, Leslie M., Eviner, Valerie|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 81/3(E), Dissertation Abstracts International|
|Subjects:||Agriculture, Plant sciences|
|Keywords:||Agroecology, Agroecosystems analysis, Cover crops, Diversified systems, Grazing, Sustainable intensification|
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