Convergence in Perceptions of Ecosystem Services Supports Green Infrastructure Decision-making in a Semi-arid City.

Effective management of cities using ecosystem services from green infrastructure (GI) requires explicit consideration of the linkages between provision of services and ecosystem service demands (i.e., governance priorities). Identification of stakeholder knowledge and objectives in GI decision-making contexts with respect to ecosystem services may improve urban planning; yet this information is rarely explicit in local contexts and cases. We address this gap by surveying environmental stakeholders and practitioners to investigate how perceptions of ecosystem services influence GI practice in Tucson, AZ. Results indicate that the semi-arid environment and urban design led to prioritizations that focus on water sustainability and urban heat mitigation. We found strong agreement in environmental perceptions between different management sectors. We observed matches (as well as mismatches) between the ecosystem service priorities and important environmental issues. Ecosystem services prioritization revealed a unique classification of ecosystem services that reflects stakeholder priorities. Our findings suggest the study of ecosystem services supply and demand can inform local urban management. These findings from a semi-arid city further suggest that understanding stakeholder knowledge, perceptions, and priorities should be important for cities in other regions where GI is being implemented as an environmental solution to provide ecosystem services.

Spatial cover and carbon fluxes of urbanized Sonoran Desert biological soil crusts.

Biological soil crusts (BSC) are important contributors to nutrient cycling in arid environments such as the Sonoran Desert. BSC at an urban (University Indian Ruins) and at a non-urban site (Santa Rita Experimental Range) were compared to determine if their structure or function was influenced by proximity to an urban environment. The Step Point method was used in the field to determine ground cover; which was found to be similar between sites. However, the spatial distribution of the BSCs was significantly different, such that more BSCs were found under plants at the non-urban site (P < 0.05). Relative gross photosynthesis was measured in the lab by addition of a watering event. Gross photosynthesis was found to be higher in the non-urban BSCs (P < 0.001), indicating lowered productivity in urban BSCs due to effects caused by proximity to urban environments. This study provides evidence that BSCs at urbanized sites are affected functionally, and therefore may be contributing differently to carbon and nitrogen cycling in these ecosystems.

Transpiration rates of red maple (Acer rubrum L.) differ between management contexts in urban forests of Maryland, USA.

The hydrological functioning of urban trees can reduce stormwater runoff, mitigate the risk of flood, and improve water quality in developed areas. Tree canopies intercept rainfall and return water to the atmosphere through transpiration, while roots increase infiltration and storage in the soil. Despite this, the amount of stormwater that trees remove through these functions in urban settings is not well characterized, limiting the use of urban forests as practical stormwater management strategies. To address this gap, we use ecohydrological approaches to assess the transpiration rates of urban trees in different management settings. Our research questions are: Do transpiration rates of trees of the same species vary among different management contexts? Do relationships between environmental drivers and transpiration change among management contexts? These management settings included single trees over turfgrass and a cluster of trees over turfgrass in Montgomery County, MD, and closed canopy forest with a leaf litter layer in Baltimore, MD. We used sap flux sensors installed in 18 mature red maple (Acer rubrum L.) trees to characterize transpiration rates during the growing season. We also measured soil volumetric water content, air temperature, relative humidity, and precipitation at each site. In agreement with our initial hypothesis, we found that single trees had nearly three times the daily sum of sap flux density (JS) of closed canopy trees. When averaged over the entire measurement period, JS was approximately 260, 195, and 91 g H2O cm-2 day-1 for single trees, cluster trees and closed canopy trees, respectively. Additionally, single trees were more responsive to VPD than closed canopy and cluster trees. These results provide a better understanding of the influence of management context on urban tree transpiration and can help to identify targets to better manage urban forest settings to reduce urban stormwater runoff.

Soil microbial composition and carbon mineralization are associated with vegetation type and temperature regime in mesocosms of a semiarid ecosystem.

Transition from historic grasslands to woody plants in semiarid regions has led to questions about impacts on soil functioning, where microorganisms play a primary role. Understanding the relationship between microbes, plant diversity and soil functioning is relevant to assess such impacts. We evaluate the effect that plant type change in semiarid ecosystems has for microbial diversity and composition, and how this is related to carbon mineralization (CMIN) as a proxy for soil functioning. We followed a mesocosm experiment during 2 years within the Biosphere 2 facility in Oracle, AZ, USA. Two temperature regimes were established with two types of plants (grass or mesquite). Soil samples were analyzed for physicochemical and functional parameters, as well as microbial community composition using 16S rRNA amplicon metagenomics (Illumina MiSeq). Our results show the combined role of plant type and temperature regime in CMIN, where CMIN in grass has lower values at elevated temperatures compared with the opposite trend in mesquite. We also found a strong correlation of microbial composition with plant type but not with temperature regime. Overall, we provide evidence of the major effect of plant type in the specific composition of microbial communities as a potential result of the shrub encroachment.

Author Correction: Biotic soil-plant interaction processes explain most of hysteretic soil CO2 efflux response to temperature in cross-factorial mesocosm experiment.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Biotic soil-plant interaction processes explain most of hysteric soil CO2 efflux response to temperature in cross-factorial mesocosm experiment.

Ecosystem carbon flux partitioning is strongly influenced by poorly constrained soil CO2 efflux (Fsoil). Simple model applications (Arrhenius and Q10) do not account for observed diel hysteresis between Fsoil and soil temperature. How this hysteresis emerges and how it will respond to variation in vegetation or soil moisture remains unknown. We used an ecosystem-level experimental system to independently control potential abiotic and biotic drivers of the Fsoil-T hysteresis. We hypothesized a principally biological cause for the hysteresis. Alternatively, Fsoil hysteresis is primarily driven by thermal convection through the soil profile. We conducted experiments under normal, fluctuating diurnal soil temperatures and under conditions where we held soil temperature near constant. We found (i) significant and nearly equal amplitudes of hysteresis regardless of soil temperature regime, and (ii) the amplitude of hysteresis was most closely tied to baseline rates of Fsoil, which were mostly driven by photosynthetic rates. Together, these findings suggest a more biologically-driven mechanism associated with photosynthate transport in yielding the observed patterns of soil CO2 efflux being out of sync with soil temperature. These findings should be considered on future partitioning models of ecosystem respiration.

The capacity of urban forest patches to infiltrate stormwater is influenced by soil physical properties and soil moisture.

Forest patches in developed landscapes perform ecohydrological functions that can reduce urban stormwater flows. However, urban forest patch contributions to runoff mitigation are not well understood due to a lack of performance data. In this study, we focus on the potential of urban forest patch soils to infiltrate rainfall by characterizing rates of unsaturated hydraulic conductivity (K) in 21 forest patches in Baltimore, Maryland. Soil bulk density, organic matter, soil moisture, percent of coarse fragments (≥2 mm), and texture were evaluated at the same locations to assess drivers of K. The K was significantly higher in soils with high sand content and related positively with the percent of coarse fragment material in the soil. Forest patch size did not impact K. We estimate that 68 percent of historic rainfall could be infiltrated by urban forest patch soils at the measured K rates. Continuous monitoring at one forest patch also showed that K is dynamic in time and influenced by antecedent soil moisture conditions. We conservatively estimate that unsaturated urban forest patch soils alone are capable of infiltrating most rain events of low to moderate intensities that fell within these forest patches in the Baltimore region. Considering this ecohydrologic function, the protection and expansion of forest patches can make substantial contributions to stormwater mitigation.

Combination of DGT and fluorescence spectroscopy for improved understanding of metal behaviour in mangrove wetland.

Understanding bioavailable metal behaviour in situ is critical for pollution evaluation and contaminant management in mangrove wetland. Here, the diffusive gradients in thin films technique (DGT) was used for characterizing the (bio)available portions of Cr, Zn, Pb, Cu, Fe and Mn in two mangrove wetlands affected by industrial waste discharges (Jiulong Estuary) and domestic discharges (Zhangjiang Estuary), in Fujian Province, China. In addition, fluorescence excitation emission matrices-parallel factor analysis (EEM-PARAFAC) was applied for characterizing the occurrence and behaviour of dissolved organic matter (DOM) in soil solution, as well as their feasibility for assessing behaviours of metals. The results demonstrated that the combination of DGT and EEM, which are well suitable for studying DOM impacted metal behaviors in mangrove sediments. Discharge of difference wastewater into the mangrove wetlands of Jiulong and Zhangjiang Estuaries, gave rise to the DGT-labile metal concentration of Zn > Cu > Cr > Pb and Zn > Cr > Pb > Cu, respectively. A variety of humic-like fluorescent components was characterized here, providing valuable insights into the chemical composition of DOM in rhizosphere and bulk sediment. Terrestrial humic-like compounds indicated a different binding affinity for heavy metals in mangrove sediments. These findings are useful for the future understanding of the metal speciation and molecular binding mechanisms in such mangrove wetlands.

Identifying the key catastrophic variables of urban social-environmental resilience and early warning signal.

Pursuit of sustainability requires a systematic approach to understand a system's specific dynamics to adapt and enhance from disturbances in social-environmental systems. We developed a systematic resilience assessment of social-environmental systems by connecting catastrophe theory and probability distribution equilibrium. Catastrophe models were used to calculate resilience shifts between slow and fast variables; afterwards, two resilience transition modes ("Less resilient" or "More resilient") were addressed by using probability distribution equilibrium analysis. A tipping point that occurs in "Less resilient" system suggests that the critical resilience transition can be an early warning signal of approaching threshold. Catastrophic shifts were explored between the interacting social-environmental sub-systems of land use and energy (fast variables) and environmental pollution (slow variables), which also identifies the critical factors in maintaining the integrated social-environmental resilience. Furthermore, the early warning signals enable the adaptability of urban systems and their resilience to perturbations, and provide guidelines for urban social-environmental management.

Nematode Community Response to Green Infrastructure Design in a Semiarid City.

Urbanization affects ecosystem function and environmental quality through shifts in ecosystem fluxes that are brought on by features of the built environment. Green infrastructure (GI) has been suggested as a best management practice (BMP) to address urban hydrologic and ecological impacts of the built environment, but GI practice has only been studied from a limited set of climatic conditions and disciplinary approaches. Here, we evaluate GI features in a semiarid city from the perspective of soil ecology through the application of soil nematode community analysis. This study was conducted to investigate soil ecological interactions in small-scale GI as a means of assessing curb-cut rain garden basin design in a semiarid city. We looked at the choice of mulching approaches (organic vs. rock) and how this design choice affects the soil ecology of rain basins in Tucson, AZ. We sampled soils during the monsoon rain season and assessed the soil nematode community as a bioindicator of soil quality and biogeochemical processes. We found that the use of organic mulch in GI basins promotes enhanced soil organic matter contents and larger nematode populations. Nematode community indices point to enhanced food web structure in streetscape rain garden basins that are mulched with organic material. Results from this study suggest that soil management practices for GI can help promote ecological interactions and ecosystem services in urban ecosystems.

The Photovoltaic Heat Island Effect: Larger solar power plants increase local temperatures.

While photovoltaic (PV) renewable energy production has surged, concerns remain about whether or not PV power plants induce a "heat island" (PVHI) effect, much like the increase in ambient temperatures relative to wildlands generates an Urban Heat Island effect in cities. Transitions to PV plants alter the way that incoming energy is reflected back to the atmosphere or absorbed, stored, and reradiated because PV plants change the albedo, vegetation, and structure of the terrain. Prior work on the PVHI has been mostly theoretical or based upon simulated models. Furthermore, past empirical work has been limited in scope to a single biome. Because there are still large uncertainties surrounding the potential for a PHVI effect, we examined the PVHI empirically with experiments that spanned three biomes. We found temperatures over a PV plant were regularly 3-4 °C warmer than wildlands at night, which is in direct contrast to other studies based on models that suggested that PV systems should decrease ambient temperatures. Deducing the underlying cause and scale of the PVHI effect and identifying mitigation strategies are key in supporting decision-making regarding PV development, particularly in semiarid landscapes, which are among the most likely for large-scale PV installations.