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No description available.

Shifting patterns of precipitation associated with climate change may affect water-limited ecosystems to a greater degree than atmospheric CO2 or temperature changes, yet we lack a mechanistic understanding of the effects of water in these ecosystems. In water-limited ecosystems, annual net primary productivity correlates strongly with total annual precipitation. However, precipitation in these ecosystems arrives in episodic events, suggesting that biophysical investigations should focus on the implications of discrete precipitation events. Further, examining dynamics of ecosystem processes over a period of days or weeks promises to link our leaf-level mechanistic understandings with larger scale patterns and temporal dynamics of ecosystem photosynthetic CO2 uptake, respiration and evapotranspiration.The objectives of this dissertation were to quantify: (1) the influence of biotic and abiotic features of an ecosystem (e.g., species composition and soil physical characteristics) on short-term patterns of resilience and resistance to a precipitation pulse; (2) the role of antecedent climatic conditions and the seasonal timing of rainfall in limiting ecosystem carbon exchange in response to precipitation events; and (3) the effect of changes in woody plant abundance on seasonal ecosystem carbon dynamics in relation to the North American Monsoon.Major findings and contributions of this research include defining the concepts of ecosystem functional resistance and resilience and their implications in the presence of a dominant nonnative bunchgrass in semi-arid grasslands (Appendix A); a better understanding of the influence of warm-season precipitation variability and the seasonal timing of rainfall on ecosystem carbon dynamics in a semi-arid grassland (Appendix B); the use of flux duration analysis, a novel approach to analyzing ecosystem carbon and water flux time-series data to distinguish between "pulse-driven" or "steady-state" ecosystems (Appendix C); and, finally, the application of flux duration analysis to quantify the sensitivity of ecosystem carbon exchange in response to seasonal rainfall in a riparian grassland and shrubland and the role that plant functional type diversity may play in constraining carbon exchange sensitivity (Appendix D).

Gray hawks became established in the San Pedro River (SPR) valley in the mid-1900s following landscape changes that created habitat for them. The population of gray hawks along the SPR is at the northern edge of the species’ range, and its growth has been documented periodically since the 1970s. A study in the 1990s quantified gray hawk diet and habitat use in this area, and found that gray hawks hunt primarily in mesquite, eat mostly lizards, and that their productivity is positively correlated with the percentage of mesquite in their territories. The gray hawk population along the SPR has nearly doubled since the initial study was conducted, and pairs now nest in areas that contain little or no mesquite. Our main objectives were to determine whether: a) diet and habitat requirements have changed for gray hawks along the SPR since the population has as expanded, and b) productivity has declined as the population has expanded into habitats of potentially lower quality. We used nest cameras to document prey deliveries, and ESRI ArcGIS to quantify vegetation types within estimated home ranges of gray hawks. We compared productivity of gray hawk pairs in the 1990s and the 2010s, as well as the current productivity of pairs in territories that had been occupied by gray hawks in the 1990s (original territories) and those that only became occupied after the original study was completed (new territories). We found that that gray hawks used a wider variety of vegetation types, such as nest trees surrounded by grasslands, and consumed a wider variety of prey than they did in the 1990s, and that productivity remained constant over time. Like many populations at the edge of their range, the gray hawks that initially settled in the San Pedro River valley likely had access to only a portion of the resources that are common at the center of the species’ range, and therefore appeared to have a narrower set of diet and habitat requirements than the species as a whole. Areas that are currently being used by gray hawks for nesting (e.g., nest trees surrounded by grasslands) were likely unsuitable in the 1990’s because they were being used for agriculture and grazing. The two chapters of this thesis will be submitted to journals for publication and therefore contain overlapping information.

Many hydrologic basins in the southwest have seen their perennial streamflows turn to ephemeral, their riparian communities disappear or be jeopardized, and their aquifers suffer from severe overdrafts. Under -management of ground -water exploitation and of conjunctive use of surface and ground waters are the main reasons for these events.

Landscapes are changing environments. Conservation of the amenities associated with landscapes must take into account the tendency of a landscape to change over time. Change is considered to be influenced by both cultural land use practices and natural resource processes which act on the landscape. A technique is developed which demonstrates an approach to measure the stability of landscapes. This approach also provides a means to qualify the importance of the elements which make up the landscape, thus defining the matrix of the landscape. A case study of the San Pedro National Conservation Area is used to demonstrate the technique. Sampled at three intervals 1935, 1973 and 1986 the landscape is shown to be stable, identifying this area with intrinsic value for conservation. Landscape scale assessments are shown to be inappropriate for ecosystem scale changes.

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From the Proceedings of the 1976 Meetings of the Arizona Section - American Water Resources Assn. and the Hydrology Section - Arizona Academy of Science - April 29-May 1, 1976, Tucson, Arizona / The case study of an Upper San Pedro River watershed is developed to show how a multiple objective approach to decision-making may be used in watershed management. The effects of various land treatments and management practices on water runoff, sediment, recreation,, wildlife levels, and commercial potential of a study area are investigated while observing constraints' on available land and capital. The example involves the optimization of five objective functions subject to eighteen constraints. In an iterative manner, the decision-maker proceeds from one noninferior solution to another, comparing sets of land management activities for reaching specified goals, and evaluating trade-offs between individual objective functions. This technique, which involves the formulation of a surrogate objective function and the use of the cutting plane method to solve the general nonlinear problem, hopefully provides a compromise between oversimplified and computationally intractable approaches to multiobjective watershed management.

From the Proceedings of the 1978 Meetings of the Arizona Section - American Water Resources Assn. and the Hydrology Section - Arizona Academy of Science - April 14-15, 1978, Flagstaff, Arizona / Discontinuous water quality data for the San Pedro River in southeastern Arizona is analyzed to illustrate the nature of water quality problems of ephemeral flow. The San Pedro drains a northerly-trending basin of 4,483 square miles, of which 696 are in Mexico and 3,787 in Arizona. Several questions arise in the consideration of a rational management plan: what is the necessity for protection of ephemeral flow quality when the channel consists of a dry wash much of the year, where there is little aquatic or wildlife to protect, and where occasional flow during flood conditions is put to little use by humans; and where and how do we use the ephemeral flow it is indeed decided to utilize it. Such questions as these form the basis of this discussion in an effort to bring out the point that water quality problems of ephemeral flow in arid areas differ from those in the humid zone. It is argued that in between the extremes of prohibiting or treating all runoff or eliminating all sources of pollution, there is actually little that can be done to control all sources of pollution in this typical arid stream, despite the fact that standards, for the most part unattainable, have been set for this flow.