Further, I investigated the effect of precipitation variation on functional diversity on the same experiment and found a positive response of diversity to increased interannual precipitation variance. Functional evenness showed a similar response resulting from large changes in plant-functional type relative abundance including decreased grass and increased shrub cover while functional richness showed non-significant response. Increased functional diversity ameliorated the direct negative effects of precipitation variation on ecosystem ANPP but did not control ecosystem stability where indirect effects through the dominant plant-functional type determined ecosystem stability.
Analyses of 80 long-term data sets, where I aggregated annual productivity and precipitation data into five-year temporal windows, showed that precipitation variance had a significant effect on aboveground net primary production that is modulated by mean precipitation. Productivity increased with precipitation variation at sites where mean annual precipitation is less than 339 mm but decreased at sites where precipitation is higher than 339 mm. Mechanisms proposed to explain patterns include: differential ANPP response to precipitation among sites, contrasting legacy effects and soil water distribution.
Finally, increased precipitation variance may impact global grasslands affecting plant-functional types in different ways that may lead to state changes, increased erosion and decreased stability that can in turn limit the services provided by these valuable ecosystems.
United States have led to significant modifications in its environment at local and
regional scales. Both local and regional climate changes are expected to have massive
impacts on the hydrology of Colorado River Basin (CRB), thereby accentuating the need
of study of hydro-climatic impacts on water resource management in this region. This
thesis is devoted to understanding the impact of land use and land cover (LULC) changes
on the local and regional hydroclimate, with the goal to address urban planning issues
and provide guidance for sustainable development.
In this study, three densely populated urban areas, viz. Phoenix, Las Vegas and
Denver in the CRB are selected to capture the various dimensions of the impacts of land
use changes on the regional hydroclimate in the entire CRB. Weather Research and
Forecast (WRF) model, incorporating the latest urban modeling system, is adopted for
regional climate modeling. Two major types of urban LULC changes are studied in this
Thesis: (1) incorporation of urban trees with their radiative cooling effect, tested in
Phoenix metropolitan, and (2) projected urban expansion in 2100 obtained from
Integrated Climate and Land Use Scenarios (ICLUS) developed by the US
Environmental Protection Agency for all three cities.
The results demonstrated prominent nocturnal cooling effect of due to radiative
shading effect of the urban trees for Phoenix reducing urban surface and air temperature
by about 2~9 °C and 1~5 °C respectively and increasing relative humidity by 10~20%
during an mean diurnal cycle. The simulations of urban growth in CRB demonstratedii
nocturnal warming of about 0.36 °C, 1.07 °C, and 0.94 °C 2m-air temperature and
comparatively insignificant change in daytime temperature, with the thermal environment
of Denver being the most sensitive the urban growth. The urban hydroclimatic study
carried out in the thesis assists in identifying both context specific and generalizable
relationships, patterns among the cities, and is expected to facilitate urban planning and
management in local (cities) and regional scales.
Constructed treatment wetlands (CTW) have been a cost-efficient technological solution to treat different types of wastewater but may also be sources of emitters of methane (CH4) and nitrous oxide (N2O). Thus, my objective for this dissertation was to investigate CH4 and N2O fluxes via multiple pathways from the Tres Rios CTW located in Phoenix, AZ, USA. I measured gas fluxes from the CTW along a whole-system gradient (from inflow to outflow) and a within-marsh gradient (shoreline, middle, and open water sites). I found higher diffusive CH4 release in the summer compared to spring and winter seasons. Along the whole-system gradient, I found greater CH4 and N2O emission fluxes near the inflow compared to near the outflow. Within the vegetated marsh, I found greater CH4 emission fluxes at the vegetated marsh subsites compared to the open water. In contrast, N2O emissions were greater at the marsh-open water locations compared to interior marsh. To study the plant-mediated pathway, I constructed small gas chambers fitted to Typha spp. leaves. I found plant-mediated CH4 fluxes were greater near the outflow than near the inflow and that CH4 fluxes were higher from lower sections of plants compared to higher sections. Overall, Typha spp. emitted a mean annual daily flux rate of 358.23 mg CH4 m-2 d-1. Third, using a 30-day mesocosm experiment I studied the effects of three different drydown treatments (2, 7, 14 days) on the fluxes of CH4 and N2O from flooded CTW soils. I found that CH4 fluxes were not significantly affected by soil drydown events. Soils that were dry for 7 days shifted from being N2O sources to sinks upon inundation. As a result, the 7-day drydown soils were sinks while the 14-day drydown soils showed significant N2O release. My results emphasize the importance of studying ecological processes in CTWs to improve their design and management strategies so we can better mitigate their greenhouse gas emissions.