The sensitivity of urban heat and urban vegetation to regional and global climate phenomena such as drought, heat waves, and climate change is a key concern in climate policy, urban planning, public health, and water and energy use. These signals are often difficult to resolve as urban climates vary at fine spatiotemporal scales and have complex interactions with land cover, surface morphology, and background climates. In this dissertation, I used multi-source airborne and satellite imagery from optical and thermal sensors to evaluate urban drought impacts on urban vegetation and climate and to investigate the efficacy of a novel spaceborne thermal sensor for fine-scale 24-hour monitoring of urban heat and its relationships with environmental and morphological drivers over the course of a day. In Chapter 1, I explored interactions between urban vegetation cover and urban heat during the 2012 to 2016 California megadrought in urbanized Los Angeles County, CA, USA using an 8-year time series of optical and thermal imagery. Over the course of the drought, I found strong spatiotemporally variant losses in green vegetation cover and complex coupling between losses of green cover and urban heat. In Chapter 2, I exploited the unique orbital characteristics of the NASA ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) sensor to develop composited 24-hour urban land surface temperature (LST) imagery for diurnal analysis of urban heat. I found that ECOSTRESS composites were suitable for resolving fine-scale (in both space and time) interactions between environmental drivers and urban LST over a full diurnal cycle as well as higher order features such as variability in heating and cooling rates based on land cover type. Finally, in Chapter 3, I used fine-scale urban surface morphology data from LiDAR with a parameterization of subpixel sun-surface-sensor geometry to extract facet-scale urban LSTs (e.g., wall, roof, road) from a 2.5-year time series of ECOSTRESS imagery. I then tracked the diurnal course of thermal anisotropy in urbanized Los Angeles County and New York City, NY, USA finding strong spatiotemporal variability in angular effects on urban LST as a function of surface-sensor geometry and surface morphology as well as separation in facet-scale LST based on facet orientation and time of day. These findings highlight the sensitivity of urban vegetation and climate to drought and potential tradeoffs between efforts to increase urban green cover and water conservation needs under a warming and drying climate. In addition, this work suggests that satellite platforms with atypical sampling regimes (e.g., from a precessing orbit) can provide data that is unique and appropriate for characterizing otherwise undersampled dimensions of urban thermal climates.

Fine roots and rhizomorphs have important implications for the global carbon balance, but the processes underlying these carbon sinks are not well understood. In this study, we coupled continuous minirhizotron observations with an array of solid-state CO2 sensors. We calculated soil respiration using a gradient flux method. Using a Kaplan-Maier survival analysis, we determined a median longevity of 347 d for fine roots and of 400 d for rhizomorphs. Radiocarbon (14C) analysis suggested an age of 7 yr for fine roots <1 mm and 17 yr for roots of 1 mm in diameter. We found rapid changes in root length (maximum of 38.1 cm m–2 d–1) and rhizomorph length (maximum of 105.4 cm m–2 d–1) during sampling of four consecutive days. Changes in rhizomorph length were more variable than root length, and rhizomorphs were negatively correlated with daily changes in soil moisture. The variation in root length may be associated to prior environmental conditions. Fine root length was correlated with daily CO2 production, and variation in daily fine root length could contribute up to 4680 g C ha–1 d–1. We observed a clockwise diurnal hysteresis effect in soil respiration with soil temperature that changed in amplitude and shape along the year. Our results show the importance of shorter intervals of minirhizotron measurements to understand rapid fine roots and rhizomorphs variation. Furthermore, continuous minirhizotron measurements should be couple with continuous measurements of multiple sensor arrays to explain biophysical factors that regulate belowground carbon dynamics.

A minority of exotic plant species undergo differentiation in vigor following introduction, leading to an explosion in population sizes and aggressive range expansion. Investigations into the mechanisms that determine successful invasion historically emphasized phenotypic traits in hopes of identifying ecological predictors and subsequent control mechanisms. Yet, it is now recognized that post-introductory evolution of invasiveness is common in many systems, frustrating efforts to identify ecological predictors. This suggests that evolutionary mechanisms ought to be given increased consideration. But this does not mean that regional differences in ecological interactions are unimportant. Many investigations demonstrate that invasive plant species experience facilitation in introduced relative to native range soils. My objective was to integrate these two promising fields of study in order to obtain a more holistic view of the mechanisms underlying invasion. Here I utilized seed and soils from native and introduced regions of the locally abundant grass species Bromus rubens L. (Pavlick and Anderson 2007, = B. madritensis ssp. rubens, Fortune et al. 2008), also known as Red brome. B. rubens is a winter annual common in the Mediterranean (native range) and Southwestern United States (introduced range). I examined the complexities of potential evolutionary and ecological factors leading to the invasion success of this species by concentrating on 1) patterns and promoters of regional differentiation, 2) the impacts of differentiation on competitive ability, and 3) the contribution of multiple ecological factors to plant-soil interactions. I found that introduced populations showed a strong signal for diversifying selection toward more aggressive growth. In a competitive environment introduced genotypes demonstrated greater reproductive fitness relative to native genotypes, regardless of competitor’s genotypic or region of origin. Finally, a plant-soil interaction growth assay suggested that increased resource availability coupled with decreased interactions with both antagonistic and beneficial soil fungi in introduced soils contributed to the invasion success in B. rubens. Together these patterns indicate that the occurrence of post-introductory evolution is of major importance to the development of invasive characters and increased competitive ability, and that ecological interactions among hosts and respective soil communities greatly contributed to the dynamics observed in this system.

Hurricanes are infrequent disturbances that affect tropical ecosystems and cause obvious damage on aboveground variables, but their impact on soil respiration is unclear. This study is the first attempt to continuously measure soil CO2 concentrations in a seasonally dry tropical forest to describe diel patterns of soil CO2 and respiration. Here we used solid-state CO2 sensors and the gradient flux method to calculate soil respiration following the unexpected pass of hurricaneWilma at the Yucata´n Peninsula, Mexico. Mean annual soil respiration for the year following the hurricane was 10.4 mmol CO2m-2 s-1 with a cumulative soil respiration of 3821 g CO2 m-2 a-1. Soil CO2 concentrations returned to prehurricane levels after 6 months at 16 cm depth, however, soil CO2 remained higher at 2 and 16 cm depth 1 year after the event. Contrary to previous studies, our results showed that soil respiration was 18% higher 1 year after the hurricane than during the same dates before the event. Diel patterns of soil respiration were decoupled from soil temperature with higher values at night between 2400 and 0600 h. Between 67 and 70% of total soil respiration was produced in the 2–8 cm layer, and this contribution was relatively constant across the sampled year and before the hurricane.We found seasonal shifts in the amplitude and shape and of diel hysteresis effects of soil respiration suggesting that other biophysical mechanisms regulate daily patterns of soil respiration other than soil temperature alone in this tropical forest.

California deserts are faced with unprecedented anthropogenic change. Impact factors range from expanding urban centers and military bases, to potential significant habitat loss from solar and thermal power expansions (including ground water exploitation and depletion beyond recovery, land stripping for power generation units, and fragmentation from power and associated transportation corridors), and climate change. Together these factors threaten remaining suitable habitat for endangered and for other endemic desert species. Our goal here is to outline the scope of environmental changes that are underway, and to outline research needs necessary to provide long-term sustainability of federally- and state-listed species and their habitats, ensuring that energy developments are also fully compliant with the letter and intent of state and federal resource protection statutes. We identified several topic areas that are of concern to land managers and project developers in the California deserts. These represent topic areas badly in need of research using state-of-the-art techniques coupled with known expertise, tailored to the desert areas to be impacted by the proposed developments.

The recovery plan for the California red-legged frog was reviewed. There are two main elements for our region in southern California. The first is the designation of critical habitat for recovery of this species. We suggest that the criteria for designation of critical habitat should be explicitly defined and listed. The map boundaries should then result from those criteria. This would reduce the maps to those specific areas with habitat in which recovery efforts could be made. Requiring large regions to await evaluation on an individual project basis undermines the support for the ESA and creates controversy where likely none should exist. The second element is restoration of the populations. While we support this effort, we are concerned with the use of extensive substrate disturbance in the survey for exotic pest species and the restrictive use of a single “genotype” used in recovery. Instead of seining to remove pests, we recommend careful evaluation of the literature with targeted research toward the different pests. We further suggest that having both Baja and LA populations is probably more appropriate and provides a better chance for having a survivable population.

• Characterization of spatial and temporal variation of soil respiration coupled with fine root and rhizomorph dynamics is necessary to understand the mechanisms that regulate soil respiration.

• A dense wireless network array of soil CO2 sensors in combination with minirhizotron tubes was used to continuously measure soil respiration over 1 yr in a mixed conifer forest in California, USA, in two adjacent areas with different vegetation types: an area with woody vegetation (Wv) and an area with scattered herbaceous vegetation (Hv).

• Annual soil respiration rates and the lengths of fine roots and rhizomorphs were greater at Wv than at Hv. Soil respiration was positively correlated with fine roots and rhizomorphs at Wv but only with fine roots at Hv. Diel and seasonal soil respiration patterns were decoupled with soil temperature at Wv but not at Hv. When decoupled, higher soil respiration rates were observed at increasing temperatures, demonstrating a hysteresis effect. The diel hysteresis at Wv was explained by including the temperature-dependent component of soil respiration and the variation dependent on photosynthetically active radiation.

• The results show that vegetation type and fine root and rhizomorph dynamics influence soil respiration in addition to changes in light, temperature and moisture