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Together, the heat island effect and air pollution pose a threat to human health and well-being in urban settings. Nature-based solutions such as planting trees are a mitigation strategy to improve outdoor temperatures (thermal comfort) and enhance air quality in urban areas. In this study, outdoor thermal comfort, and particulate matter levels were compared between treeless and treed areas to provide a better understanding of how street trees improve thermal comfort and air quality. Street trees decreased the physiological equivalent temperature from 46.3 to 44.2 °C in summer but increased it from 36.4 to 37.5 °C in autumn. Air temperature and relative humidity contributed more in summer while wind speed contributed more in autumn. Particulate matter concentrations were negatively correlated with physiological equivalent temperature in summer but not in autumn. The presence of trees decreased concentrations of fine particulate matter in hot summer conditions but increased in hot autumn conditions. The presence of trees increased coarse particulate matter in very hot summer conditions in summer and in hot autumn conditions. Overall, the layout of trees in urban street canyons should consider the trade-off between outdoor thermal comfort and air quality improvement.

Increased nitrogen and phosphorus pollution causes eutrophication in water bodies. Using aquatic plants to remove nutrients from water is an attractive phytoremediation. It is a cost-effective, environment-friendly, and efficient way that reduces water body eutrophication by the plant. It is important to choose suitable macrophytes to remove excess N and P under different nutrient conditions. In this study, six macrophyte species (Polygonum orientale, Juncus effuses, Iris pseudocorus, Phragmites australis, Iris sanguinea, Typha orientalis) were tested. Simulation experiment was conducted under five N and P levels. The removal rate, relative growth rate, and the dynamic nutrition concentration of cultivated solution were investigated. Of all the treatment, a 23-95% reduction in N removal and a 29-92% reduction in P removal were recorded. The results showed I. sanguinea is a promising species to treat various eutrophic waters and the other five species can be used specifically to treat certain types of water. The data provided a theoretical guidance to plant species selection for phytoremediation of polluted water bodies for the purpose of water quality improvement around the different reservoir in northern China.

The net influence of trees on local air quality in urban street canyons, negative or positive, has rarely been evaluated by field investigations. To quantify the influence of street trees on particulate matter (PM) and total suspended particle (TSP) concentration near ground level, a seasonal field investigation was carried out in a street canyon with an aspect ratio of 0.52 in Shenyang, China. The greening tree species in this street were Sophora japonica and Ulmus pumila. We monitored PM1, PM2.5, PM4, PM10, TSP concentration and the microclimatic conditions at locations with or without trees from before the leaf expansion season until the end of the defoliation season. Concentration of PM2.5 was higher at the tree-stand location, whereas concentration of PM10 and TSP were higher at the tree-free location during the leaf-on season. Street trees were beneficial to coarse particle removal during the leaf-off and leaf-expansion season, but PM pollution increased during defoliation. Wind direction was more uniform at the tree-stand location. Street trees weakened the impact of the wind on PM dispersion near ground. This indicates that street trees might pose a health threat to pedestrians because fine PM can enter the human respiratory system more easily than coarse PM.

Livestock grazing is a crucial cause of vegetation degradation and desertification in sandy lands. The sparse-elm grassland of Horqin Sandy Land, China has suffered severe degradation of biodiversity and ecosystem services. Management to exclude grazing is often necessary for ecological restoration, especially in arid and semi-arid regions. We report effects on vegetation and soils in a 10-year experiment to exclude livestock, completely or seasonally, in comparison with a continuously grazed area in Horqin. Complete exclusion of grazing and restriction of grazing to summer both led to significantly increased plant cover and density relative to the grazed control. Species richness increased, reflected in higher Shannon-Wiener indices; only complete exclusion increased the Simpson diversity index, whereas Pielou evenness was significantly lowest under seasonal grazing. Exclosure treatments were also associated with improved soil texture, and increased water retention, available nitrogen, total nitrogen, total carbon and total phosphorus. Soil pH and C/N ratio were highest under the seasonal grazing regime. The results indicated that exclosure management indeed improved biodiversity and ecosystem services in an erosion-prone region. Although total exclosure was most effective in restoration of degraded sparse-elm grassland, seasonal grazing management was highly beneficial and represented a good compromise with resource utilization and economic development.

As a native tree species, Ulmus pumila var. sabulosa (sandy elm) is widely distributed in the Horqin Sandy Land, China. However, seedlings of this species have to withstand various depths of sand burial after emergence because of increasing soil degradation, which is mainly caused by overgrazing, climate change, and wind erosion. An experiment was conducted to evaluate the changes in its survivorship, morphological traits, and biomass allocation when seedlings were buried at different burial depths: unburied controls and seedlings buried vertically up to 33, 67, 100, or 133 % of their initial mean seedling height. The results showed that partial sand burial treatments (i.e., less than 67 % burial) did not reduce seedling survivorship, which still reached 100 %. However, seedling mortality increased when sand burial was equal to or greater than 100 %. In comparison with the control treatment, seedling height and stem diameter increased at least by 6 and 14 % with partial burial, respectively. In the meantime, seedling taproot length, total biomass, and relative mass growth rates were at least enhanced by 10, 15.6, and 27.6 %, respectively, with the partial sand burial treatment. Furthermore, sand burial decreased total leaf area and changed biomass allocation in seedlings, partitioning more biomass to aboveground organs (e.g., leaves) and less to belowground parts (roots). Complete sand burial after seedling emergence inhibited its re-emergence and growth, even leading to death. Our findings indicated that seedlings of sandy elm showed some resistance to partial sand burial and were adapted to sandy environments from an evolutionary perspective. The negative effect of excessive sand burial after seedling emergence might help in understanding failures in recruitments of sparse elm in the study region.

Exposure to near-ground air pollution in urban street canyons poses a serious threat to pedestrian health. To develop design guidelines for air quality improvement, correlations among PM1, PM2.5, PM4, PM10, TSP, street canyons morphology and climatic conditions were investigated by field experimental studies. The results showed that street canyons morphology and air humidity were two of the most important factors affecting suspended particulate matter concentration in urban street canyons. The concentrations of PM1 (9.18 ± 0.17 μg/m3), PM2.5 (18.12 ± 0.43 μg/m3), PM4 (31.66 ± 0.82 μg/m3), PM10 (77.57 ± 2.04 μg/m3) and TSP (104.06 ± 2.85 μg/m3) in deep canyons were significantly lower than those in medium and wide canyons. The pollutants distributed in E-W or N-S street directions were at a lower concentration compared with those in NE-SW or NW-SE street directions. Moreover, the pollutants showed significantly lower concentration in high-rise building areas than in multilayer building areas. This study provides a reference for future urban planning to reduce suspended particulate matter concentration in street environments.

The sky view factor (SVF) is the ratio of the visible sky area of a point in space to the total sky area. It provides the relationship between the visible sky area and covered surroundings, such as by buildings or street trees. The SVF has been widely used as a key parameter in urban climate research and urban planning practices. Significant research has taken place in the past decades on methods of calculating/estimating SVFs to improve their accuracy and efficiency. This review lists the methods used to calculate/estimate SVFs including geometric methods, fish-eye photographical method, Global Positioning System methods, simulation methods based on 3D city models or digital surface models, and big data approaches using street view images. We stress the principles, input data, application, accuracy and efficiency of each method. This review is meaningful for climatologists in solar radiation modeling and energy balance modeling fields, as well as for urban planners in the development of design guidelines to improve outdoor thermal comfort in the urban environment.

Spatial variability has been recognized as a critical issue in vegetation restoration, and this condition is favorable for understanding underlying ecological processes or mechanisms. To assess the potential effects of spatial scales and environmental factors on vegetation patterns on active sand dunes, a 0.25 ha (50 m × 50 m) sampling plot was selected on the windward slope of an active sand dune to analyze the spatial variability of annual psammophyte Agriophyllum squarrosum across scales and along the slope. The average density, coverage, and height of A. squarrosum seedlings at five grain sizes (from 1 m × 1 m to 25 m × 25 m) as well as along the slope were measured. In A. squarrosum community, density, coverage, and height had the highest values at fine scales, but the lowest values at coarse scales. The highest value decreased gradually as scale increased, which means spatial variability gradually decreased as grain size increased. The correlations between density and height as well as between densi...

Species richness and abundance are two important species diversity variables that have attracted par- ticular attention because of their significance in determining present and future species composition conditions. This paper aims to explain the qualitative and quantitative relationships between species diversity pattern and grain size (i.e. size of the sampling unit), and species diversity pattern and sampling area, and to analyze species diversity variability on active sand dunes in the Horqin Sandy Land, northeastern Inner Mongolia, China. A 50 m×50 m sampling plot was selected on the windward slope, where the dominant species was annual herb Agriophyllum squarrosum. Species composition and abundance at five grain sizes were recorded, and the species-area curves were produced for thirteen grain sizes. The range of values for species abundance tended to increase with in- creasing grain size in the study area, whereas, generally, species richness did not follow this rule because of poor species richness on the windward slope of active sand dunes. However, the homogeneity of species richness in- creased significantly. With the increase in sampling area, species abundance increased linearly, but richness in- creased logarithmically. Furthermore, variograms showed that species diversity on the windward slope of active sand dunes was weakly anisotropic and the distribution pattern was random, according to the Moran Coefficient. The results also showed that species richness was low, with a random distribution pattern. This conflicts with the results of previous studies that showed spatial aggregation in lower richness in a sampling area within a community and inferred that the physical processes play a more important role in species diversity than distribution pattern on active sand dunes. Further research into different diversity patterns and mechanisms between active sand dunes and interdune lowlands should be conducted to better understand biodiversity conservation in sand dune fields.

Street trees provide critical ecosystem services and economic benefits that are often disregarded, due to their unknown monetary value. This study analyzed the structural characteristics of Dalian’s street trees and estimated the monetary value of structural and functional benefits by i-Tree Streets. Dalian’s street trees encompassed 28 species and were dominated by Ginkgo biloba, Platanus acerifolia and Sophora japonica, comprising 64.1% of a total of 57,699 trees. The age structure of street trees was distributed somewhat unevenly, with 18% young trees, 56% maturing trees, 25% mature trees and 1% old trees. These trees provide annual functional benefits valued at US$4.9 million and delivered a benefit-cost ratio of 3.2:1. The largest values associated with energy savings and property value were $1.7 million ($29/tree) and $1.5 million ($25/tree), respectively. The net carbon reduction benefits were valued at $935,205 ($16/tree). Smaller benefits resulted from air quality improvement ($381,088 or $7/tree) and stormwater runoff ($459,457 or $8/tree). The structural benefits were valued at $130 million, with the value of $4.5 million for carbon storage. These findings suggested that the benefits produced by street trees were worth the management costs. Our results provide a thorough understanding of the benefits produced by street trees to policy-makers and managers, and help them make informed policies to maximize and sustain the flow of benefits.

We investigated the effects of seed burial depth and soil water content on seedling emergence and growth of Ulmus pumila var. sabulosa (sandy elm), an important native tree species distributed over the European-Asian steppe. Experimental sand burial depths in the soil were 0.5, 1.0, 1.5, 2.0 and 2.5 cm, and soil water contents were 4%, 8%, 12% and 16% of field capacity. All two-way ANOVA (five sand burial depths and four soil water contents) results showed that seed burial depths, soil water content and their interactions significantly affected all the studied plant variables. Most of the times, seedling emergence conditions were greater at the lower sand burial depths (less than 1.0 cm) than at the higher (more than 1.0 cm) seed burial depths, and at the lower water content (less than 12%) than at the higher soil water content. However, high seed burial depths (more than 1.5 cm) or low soil water content (less than 12%) reduced seedling growth or change in the root/shoot biomass ratios. In conclusion, the most suitable range of sand burial was from 0.5 to 1.0 cm soil depth and soil water content was about 12%, respectively, for the processes of seedling emergence and growth. These findings indicate that seeds of the sandy elm should be kept at rather shallow soil depths, and water should be added up to 12% of soil capacity when conducting elm planting and management. Our findings could help to create a more appropriate sandy elm cultivation and understand sparse elm woodland recruitment failures in arid and semi-arid regions.