Flowering phenology offers a sensitive and reliable biological indicator of climate change because plants use climatic and other environmental cues to initiate flower production. Drylands are the largest terrestrial biome, but with unpredictable precipitation patterns and infertile soils, they are particularly vulnerable to climate change. There is a need to increase our comprehension of how dryland plants might respond and adapt to environmental changes. I conducted a meta-analysis on the flowering phenology of dryland plants and showed that some species responded to climate change through accelerated flowering, while others delayed their flowering dates. Dryland plants advanced their mean flowering dates by 2.12 days decade-1, 2.83 days °C-1 and 2.91 days mm-1, respectively, responding to time series, temperature, and precipitation. Flowering phenology responses varied across taxonomic and functional groups, with the grass family Poaceae (-3.91 days decade1) and bulb forming Amaryllidaceae (-0.82 days decade1) showing the highest and lowest time series responses respectively, while Brassicaceae was not responsive. Analysis from herbarium specimens collected across Namibian drylands, spanning 26 species and six families, revealed that plants in hyper-arid to arid regions have lower phenological sensitivity to temperature (-9 days °C-1) and greater phenological responsiveness to precipitation (-0.56 days mm-1) than those in arid to semi-arid regions (-17 days °C-1, -0.35 days mm-1). The flowering phenology of serotinous plants showed greater sensitivity to both temperature and precipitation than that of non-serotinous plants. I used rainout shelters to reduce rainfall in a field experiment and showed that drought treatment advanced the vegetative and reproductive phenology of Cleome gynandra, a highly nutritional and medicinal semi-wild vegetable species. The peak leaf length date, peak number of leaves date, and peak flowering date of Cleome gynandra advanced by six, 10 and seven days, respectively. Lastly, I simulated drought and flood in a greenhouse experiment and found that flooding conditions resulted in higher germination percentage of C. gynandra than drought. My study found that the vegetative, and flowering phenology of dryland plants is responsive to climate change, with differential responses across taxonomic and functional groups, and aridity zones, which could alter the structure and function of these systems.

The study of organismal adaptations oftentimes focuses on specific, constant conditions, but environmental parameters are characterized by more or less marked levels of variability, rather than constancy. This is important in environments like soils where microbial activity follows pulses of water availability driven by precipitation. Nowhere are these pulses more variable and unpredictable than in arid soils. Pulses constitute stressful conditions for bacteria because they cause direct cellular damage that must be repaired and they force cells to toggle between dormancy and active physiological states, which is energetically taxing. I hypothesize that arid soil microorganisms are adapted to the variability in wet/dry cycles itself, as determined by the frequency and duration of hydration pulses. To test this, I subjected soil microbiomes from the Chihuahuan Desert to controlled incubations for a total common growth period of 60 hours, but separated into treatments in which the total active time was reached with hydration pulses of different length with intervening periods of desiccation, so as to isolate pulse length and frequency as the varying factors in the experiment. Using 16S rRNA amplicon data, I characterized changes in microbiome growth, diversity, and species composition, and tracked the individual responses to treatment intensity in the 447 most common bacterial species (phylotypes) in the soil. Considering knowledge of extremophile microbiology, I hypothesized that growth yield and diversity would decline with shorter pulses. I found that microbial diversity was indeed a direct function of pulse length, but surprisingly, total yield was an inverse function of it. Pulse regime treatments resulted in progressively more significant differences in community composition with increasing pulse length, as differently adapted phylotypes became more prominent. In fact, more than 30% of the most common bacterial phylotypes demonstrated statistically significant population growth responses to pulse length. Most responsive phylotypes were apparently best adapted to short pulse regimes (known in the literature as Nimble Responders or NIRs), while fewer did better under long pulse regimes (known as TORs or Torpid Responders). Examples of extreme NIRs and TORs could be found among bacteria from different phyla, indicating that these adaptations have occurred multiple times during evolution.

Drylands cover over 40% of the Earth’s surface, account for one third of global carbon cycling, and are hotspots for climate change, with more frequent and severe droughts coupled with deluges of novel magnitude and frequency. Because of their large terrestrial extent, elucidating dryland ecosystem responses to changes in water availability is critical for a comprehensive understanding of controls on global aboveground net primary productivity (ANPP), an important ecosystem service. The focus of this dissertation is to investigate cause-effect mechanisms between altered water availability and ecosystem processes in dryland ecosystems. Across a network of experimental rainfall manipulations within a semiarid Chihuahuan Desert grassland, I examined short- and long-term dynamics of multiple ecosystem processes—from plant phenology to nitrogen cycling—in response to directional precipitation extremes. Aboveground, I found herbaceous plant phenology to be more sensitive in greenup timing compared to deep-rooted, woody shrubs, implying that precipitation extremes will disproportionately affect grass-dominated compared to woody ecosystems. Surprisingly, after 14 years of experimentally adding water and N, I observed no effect on ANPP. Belowground, bulk soil N dynamics remained stable with differing precipitation amounts. However, mineral associated organic N (MAOM-N) significantly increased under chronic N inputs, indicating potential for dryland soil N sequestration. Conversely, the difference between low- and high-N soil N content may increase a drawdown of N from all soil N pools under low-N conditions whereas plants source N from fertilizer input under high-N conditions. Finally, I considered ecosystem-level acclimation to climate change. I found that N availability decreased with annual precipitation in space across continents, but it posed initially increasing trends in response to rainfall extremes at the Jornada that decreased after 14 years. Mechanisms for the acclimation process are thus likely associated with differential lags to changes in precipitation between plants and microorganisms. Overall, my dissertation demonstrates that examining linkages between multiple ecosystem processes, from aboveground phenological cycles to belowground N cycling dynamics, can provide a more integrative understanding of dryland response to climate change. Because dryland range is potentially expanding globally, water limited systems provide a unique and critical focus area for future research that revisit and revise current ecological paradigms.

Desert organisms lead harsh lives owing to the extreme, often unpredictable environmental conditions they endure. Climate change will likely make their existence even harsher. Predicting the ecological consequences of future climate scenarios thus requires understanding how the biota will be affected by climatic shifts. Biological soil crusts (biocrusts) are an important ecosystem component in arid lands, one that covers large portions of the landscape, improving soil stability and fertility. Because cyanobacteria are biocrust’s preeminent primary producers, eking out an existence during short pulses of precipitation, they represent a relevant global change object of study. I assessed how climate scenarios predicted for the Southwestern United States (US) will affect biocrusts using long-term, rainfall-modifying experimental set-ups that imposed either more intense drought, a seasonally delayed monsoon season, or a shift to smaller but more frequent precipitation events. I expected drought to be detrimental, but not a delay in the monsoon season. Surprisingly, both treatments showed similar effects on cyanobacterial community composition and population size after four years. While successionally incipient biocrusts were unaffected, mature biocrusts lost biomass and diversity with treatment, especially among nitrogen-fixing cyanobacteria. In separate experiments, I assessed the effect of rainfall with modified event size and frequency after a decade of treatment. Small, frequent rainfall events surprisingly enhanced the diversity and biomass of bacteria and cyanobacteria, with clear winners and losers: nitrogen-fixing Scytonema sp. benefited, while Microcoleus vaginatus lost its dominance. As an additional finding, I could also show that water addition is not always beneficial to biocrusts, calling into question the notion that these are strictly water-limited systems.

Finally, results interpretation was severely hampered by a lack of appropriate systematic treatment for an important group of biocrust cyanobacteria, the “Microcoleus steenstrupii complex”. I characterized the complex using a polyphasic approach, leading to the formal description of a new family (Porphyrosiphonaceae) of desiccation resistant cyanobacteria that includes 11 genera, of which 5 had to be newly described. Under the new framework, the distribution and abundance of biocrust cyanobacteria with respect to environmental conditions can now be understood. This body of work contributes significantly to explain current distributional patterns of biocrust cyanobacteria and to predict their fate in the face of climate change.

Climate change is increasing global surface temperatures, intensifying droughts and increasing rainfall variation, particularly in drylands. Understanding how dryland plant communities respond to climate change-induced rainfall changes is crucial for implementing effective conservation strategies. Concurrent with climate change impacts on drylands is woody encroachment: an increase in abundance of woody plant species in areas formerly dominated by grasslands or savannahs. For example, the woody plant, Prosopis velutina (velvet mesquite), has encroached into grasslands regionally over the past century. From an agricultural perspective, P. velutina is an invasive weed that hinders cattle forage. Understanding how P. velutina will respond to climate change-induced rainfall changes can be useful for management and conservation efforts. Prosopis velutina was used to answer the following question: Is there a significant interactive effect of mean soil water moisture content and pulse frequency on woody seedling survival and growth in dryland ecosystems? There were 256 P. velutina seedlings sourced from the Santa Rita Experimental Range in southern Arizona grown under four watering treatments where mean and pulse frequency were manipulated over two months. Data were collected on mortality, stem height, number of leaves, instantaneous gas exchange, chlorophyll fluorescence, biomass, and the leaf carbon to nitrogen (C:N) ratio. Mortality was low across treatments. Pulse frequency had less impact across response variables than the mean amount of water received. This may indicate that P. velutina seedlings are relatively insensitive to rainfall timing and are more responsive to rainfall amount. Prosopis velutina in the low mean soil moisture treatments lost a majority of their leaves and had greater biomass allocation to roots. Prosopis velutina’s ability to survive in low soil moisture conditions and invest in root biomass can allow it to persist as drylands are further affected by climate change. Prosopis velutina could benefit ecosystems where native plants are at risk due to rainfall variation if P. velutina occupies a similar niche space. Due to conflicting viewpoints of P. velutina as an invasive species, it’s important to examine P. velutina from both agricultural and conservation perspectives. Further analysis on the benefits to P. velutina in these ecosystems is recommended.

Payments for ecosystem services (PES) are transactions between landholders and the beneficiaries of the services their land provides. PES schemes are growing worldwide with annual transactions over ten billion dollars (Salzman et al., 2018). Much can be learned from looking at oldest and best funded PES schemes on working agricultural land. Initiated in 1985, the USDA’s Conservation Reserve Program (CRP) is the oldest private conservation PES program in the United States. CRP incentivizes farmers to put their land into conservation through an annual payment. In Iowa, CRP has been a source of extra income and a way for farmers to buffer the fluctuating costs of cash crops, such as corn and soy. The dominance of agriculture in Iowa poses many challenges for water quality. A potential solution to the problem, implemented through CRP, is the use of conservation practices to mitigate the negative effects of agricultural run-off.

This dissertation considers three aspects of the problem:

1. the relationship between changes in land cover due to CRP enrollment and changes in water quality, controlling for a range of factors known to have an effect on the filtering role of different land covers;

2. the inter-annual variability in water quality measures and enrollment in different CRP conservation practices to examine the cost-effectiveness of specific conservation practices in mitigating lake sedimentation and eutrophication;

3. discrete choice models to identify what characteristics drive the enrollment by farmers into specific conservation practices.

Results indicate that land cover and CRP have different impacts on different indicators of lake water quality. In addition, conservation practices that were cost-effective for one water quality variable tended to be cost-effective for the other water quality variables. Farmers are making decisions to enroll in CRP based on the opportunity cost of the land. Therefore, it is necessary to alter financial incentives to promote productive land being putting into CRP through continuous sign-up. The United States Department of Agriculture (USDA) needs a more effective way to calculate the payment level for practices in order to be competitive with the predicted value of major crops.

Biological soil crusts (biocrusts) are topsoil communities of organisms that contribute to soil fertility and erosion resistance in drylands. Anthropogenic disturbances can quickly damage these communities and their natural recovery can take decades. With the development of accelerated restoration strategies in mind, I studied physiological mechanisms controlling the establishment of cyanobacteria in biocrusts, since these photoautotrophs are not just the biocrust pioneer organisms, but also largely responsible for improving key soil attributes such as physical stability, nutrient content, water retention and albedo. I started by determining the cyanobacterial community composition of a variety of biocrust types from deserts in the Southwestern US. I then isolated a large number of cyanobacterial strains from these locations, pedigreed them based on their 16SrRNA gene sequences, and selective representatives that matched the most abundant cyanobacterial field populations. I then developed methodologies for large-scale growth of the selected isolates to produce location-specific and genetically autochthonous inoculum for restoration. I also developed and tested viable methodologies to physiologically harden this inoculum and improve its survival under harsh field conditions. My tests proved that in most cases good viability of the inoculum could be attained under field-like conditions. In parallel, I used molecular ecology approaches to show that the biocrust pioneer, Microcoleus vaginatus, shapes its surrounding heterotrophic microbiome, enriching for a compositionally-differentiated “cyanosphere” that concentrates the nitrogen-fixing function. I proposed that a mutualism based on carbon for nitrogen exchange between M. vaginatus and its cyanosphere creates a consortium that constitutes the true pioneer community enabling the colonization of nitrogen-poor, bare soils. Using the right mixture of photosynthetic and diazotrophic cultures will thus likely help in soil restoration. Additionally, using physiological assays and molecular meta-analyses, I demonstrated that the largest contributors to N2-fixation in late successional biocrusts (three genera of heterocystous cyanobacteria) partition their niche along temperature gradients, and that this can explain their geographic patterns of dominance within biocrusts worldwide. This finding can improve restoration strategies by incorporating climate-matched physiological types in inoculum formulations. In all, this dissertation resulted in the establishment of a comprehensive "cyanobacterial biocrust nursery", that includes a culture collection containing 101 strains, isolation and cultivation methods, inoculum design strategies as well as field conditioning protocols. It constitutes a new interdisciplinary application of microbiology in restoration ecology.

Rangelands are an extensive land cover type that cover about 40% of earth’s ice-free surface, expanding into many biomes. Moreover, managing rangelands is crucial for long-term sustainability of the vital ecosystem services they provide including carbon (C) storage via soil organic carbon (SOC) and animal agriculture. Arid rangelands are particularly susceptible to dramatic shifts in vegetation cover, physical and chemical soil properties, and erosion due to grazing pressure. Many studies have documented these effects, but studies focusing on grazing impacts on soil properties, namely SOC, are less common. Furthermore, studies testing effects of different levels of grazing intensities on SOC pools and distribution yield mixed results with little alignment. The primary objective of this thesis was to have a better understanding of the role of grazing intensity on arid rangeland soil C storage. I conducted research in long established pastures in Jornada Experimental Range (JER). I established a 1500m transect in three pastures originating at water points and analyzed vegetation cover and SOC on points along these transects to see the effect of grazing on C storage on a grazing gradient. I used the line-point intercept method to measure and categorize vegetation into grass, bare, and shrub. Since soil adjacent to each of these three cover types will likely contain differing SOC content, I then used this vegetation cover data to calculate the contribution of each cover type to SOC. I found shrub cover and total vegetation cover to decrease, while grass and bare cover increased with decreasing proximity to the water source. I found areal (g/m2) and percent (go SOC to be highest in the first 200m of the transects when accounting for the contribution of the three vegetation cover types. I concluded that SOC is being redistributed toward the water source via foraging and defecation and foraging, due to a negative trend of both total vegetation cover and percent SOC (g/g). With the decreasing trends of vegetation cover and SOC further from pasture water sources, my thesis research contributes to the understanding of storage and distribution of SOC stocks in arid rangelands.

Aboveground net primary production (ANPP) and belowground net primary production (BNPP) may not be influenced equally by the same factors in arid grasslands. Precipitation is known to affect ANPP and BNPP, while soil fauna such as nematodes affect the BNPP through herbivory and predation. This study on black grama grass (Bouteloua eriopoda) in the Chihuahuan Desert investigates the effects of precipitation and nematode presence or absence on net primary production (NPP) as well as the partitioning between the aboveground and belowground components, in this case, the fraction of total net primary production occurring belowground (fBNPP). I used a factorial experiment to investigate the effects of both precipitation and nematode presence on the components of NPP. I used rainout shelters and an irrigation system to alter precipitation totals, while I used defaunated and re-inoculated soil for the nematode treatments. Precipitation treatment and seasonal soil moisture had no effect on the BNPP and a nonsignificant positive effect on the ANPP. The fBNPP decreased with increasing precipitation and seasonal soil moisture, though without a significant effect. No predator nematodes were found in any of the microcosms at the end of the experiment, though other functional groups of nematodes, including herbivores, were found in the microcosms. Total nematode numbers did not vary significantly between nematode treatments, indicating that the inoculation process did not last for the whole experiment or that nematodes had little plant material to eat and resulted in low population density. Nematode presence did not affect the BNPP, ANPP, or the fBNPP. There were no significant interactions between precipitation and nematode treatment. The results are inconclusive, possibly as a result of ecosystem trends during an unusually high precipitation year, as well as the very low NPP values in the experiment that correlated with low nematode community numbers.

Cities are hubs for economic and social development, but they are increasingly becoming hotspots of environmental problems and socio-economic inequalities. Because cities result from complex interactions among ecological, social and economic factors, environmental problems and socio-economic inequalities are often spatially interconnected, generating emergent environmental inequity issues due to the unfair distribution of environmental quality among socioeconomic groups. Since urban environmental quality is tightly related to the capacity of urban landscapes to provide ecosystem services, optimizing the allocation of ecosystem services within cities is a main goal for moving towards more equitable and sustainable cities. Nevertheless, we often lack the empirical data and specific methods for planning urban landscapes to optimize the provision of ecosystem services. Therefore, the development of knowledge and methods to optimize the provision of ecosystem services is essential for tackling urban environmental problems, reducing environmental inequities, and promoting sustainable cities. The main goal of this dissertation is to generate actionable knowledge for helping decision-makers to optimize the allocation of urban vegetation for reducing environmental inequities through the provision of ecosystem services. The research uses the city of Santiago de Chile as a case study from a Latin-American city. To achieve this goal, I framed my dissertation in four linked research chapters, each of them providing methodological approaches to help link environmental inequity problems with the development of urban planning interventions promoting an equitable provision of urban ecosystem services. These chapters are specifically aimed at providing actionable knowledge for: (1) Identifying the level, distribution, and spatial scales at which environmental inequities are more relevant; (2) Identifying the areas and administrative units where environmental inequities interventions should be prioritized; (3) Identifying optimal areas to allocate vegetation for increasing the provision of urban ecosystem services; (4) Evaluating the role that planned urban vegetation may have in the long-term provision of ecosystem services by natural remnants within the urban landscape. Thus, this dissertation contributes to urban sustainability science by proposing methods and frameworks to address urban environmental inequities through the provision of ecosystem services, but it also provides place-based information that can be readily used for planning urban vegetation in Santiago.