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Appalachian State University (2011)

Factors influencing restoration success of abandoned agricultural fields on the lower San Pedro River terraces, southeastern Arizona

Gray, Carla Mae

Titre : Factors influencing restoration success of abandoned agricultural fields on the lower San Pedro River terraces, southeastern Arizona

Auteur : Gray, Carla Mae

Université de soutenance : Appalachian State University

Grade : Master of Science 2011

Résumé
Abandonment of agricultural fields is a common occurrence worldwide. Natural recovery, or succession, often occurs following land abandonment. However, in arid regions succession may not occur and active restoration techniques are sometimes used to facilitate native vegetation growth. Along the lower San Pedro River in southeastern Arizona, restoration projects have resulted in mixed success. Though some fields have responded to restoration treatments, others have not. Abiotic and biotic factors likely affecting restoration outcomes in this arid environment include soil conditions, restoration treatments, and seed availability. Predictor variables were examined to identify which environmental factors most influenced vegetation characteristics of abandoned agricultural fields on river terraces. To do this, GIS-based site suitability analysis was used to identify abandoned agricultural fields, and field data were collected in 20 fields. Woody stem density, basal area, and elevation were recorded in three 100 m2 study plots in each field. Soil samples were collected and pH, particle size analysis, and electrical conductivity of saturated soil paste were performed. In addition, environmental variables were collected at the field scale within a GIS. These variables included field area, distance to upland and terrace vegetation, and field distance to the San Pedro River. Management data were collected from landowners and managers about each of the 20 field sites, including number of years each field was farmed, time since abandonment, and whether restoration treatments such as planting/seeding, irrigation, grazing, or mowing were employed. Analysis of woody vegetation showed field sites had low woody basal area and low woody stem density. Low woody species richness was also observed and Prosopis velutina was the dominant species at field sites. Field distance to terrace and upland vegetation was variable, but never more than 0.5 km away. Soil pH tests showed alkaline soil conditions, and electrical conductivity of soil samples revealed medium to high soil salinity levels. Soils were predominately of a sand and silt texture. Management information described a long history of cultivation at sites, involving a decade or more of agricultural use at each field site. Two regression models were created, one for woody basal area and one for woody stem density. All fourteen soil, field, and management/history variables were entered into each model. Significant predictors of woody basal area included distance to terrace vegetation, percent clay in the soil, and post planting and/or seeding treatment. Significant predictors of woody stem density included field area, years farmed, and time (years) since abandonment. ANOVAs determined that restoration management treatments were generally not associated with significant increases in woody stem density, woody basal area, or percent herbaceous cover. Ultimately, the factors limiting restoration were complex and interconnected. However, the findings of this project point toward lingering agricultural legacies impeding restoration efforts.

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