Species selection as a key factor in the afforestation of coastal salt-affected lands: Insights from pot and field experiments.

Soil salinization is a significant global issue that leads to land degradation and loss of ecological function. In coastal areas, salinization hampers vegetation growth, and forestation efforts can accelerate the recovery of ecological functions and enhance resilience to extreme climates. However, the salinity tolerance of tree species varies due to complex biological factors, and results between lab/greenhouse and field studies are often inconsistent. Moreover, in salinized areas affected by extreme climatic and human impacts, afforestation with indigenous species may face adaptability challenges. Therefore, it is crucial to select appropriate cross-species salinity tolerance indicators that have been validated in the field to enhance the success of afforestation and reforestation efforts. This study focuses on five native coastal tree species in Taiwan, conducting afforestation experiments on salt-affected soils mixed with construction and demolition waste. It integrates short-term controlled experiments with potted seedlings and long-term field observations to establish growth performance and physiological and biochemical parameters indicative of salinity tolerance. Results showed that Heritiera littoralis Dryand. exhibited the highest salinity tolerance, accumulating significant leaf proline under increased salinity. Conversely, Melia azedarach Linn. had the lowest tolerance, evidenced by complete defoliation and reduced biomass under salt stress. Generally, the field growth performance of these species aligns with the results of short-term pot experiments. Leaf malondialdehyde content from pot experiments proved to be a reliable cross-species salinity tolerance indicator, correlating negatively with field relative height growth and survival rates. Additionally, parameters related to the photosynthetic system or water status, measured using portable devices, also moderately indicated field survival, aiding in identifying potential salt-tolerant tree species. This study underscores the pivotal role of species selection in afforestation success, demonstrating that small-scale, short-term salinity control experiments coupled with appropriate assessment tools can effectively identify species suitable for highly saline and degraded environments. This approach not only increases the success of afforestation but also conserves resources needed for field replanting and maintenance, supporting sustainable development goals.

Effect of air quality improvement by urban parks on mitigating PM2.5 and its associated heavy metals: A mobile-monitoring field study.

Field mobile monitoring of PM2.5, equipped with a highly accurate device, was performed for two types of urban parks in Taiwan. Measurements were taken in the morning and evening rush hours, on certain weekdays and weekends, every month over a year. We designed six calculation schemes of the rate of PM2.5 mitigation by urban parks to comprehensively compare the average and maximum mitigation effects in relation to the vegetation barriers. The mitigation rate, from the lowest (2.51%) to the highest (35.57%) depended on the calculation schemes. The Taipei Botanical Garden (TBG) with a dense, multilevel forest has a stable PM2.5 mitigation effect and strong ability to improve air quality inside the park under severe PM2.5 pollution. In contrast, Zhonghe No.4 Park (ZHP), an open park with mostly a single-storied stand, has variable PM2.5 mitigation effect, leading to either quick dissipation or accumulation of PM2.5 inside the park. Furthermore, the dry deposition of PM and the associated heavy metals were investigated using camphor trees as bioaccumulators. Dry deposition flux of the leaf-deposited PM2.5 exhibited similar results in ZHP; whereas, noticeable higher results were observed inside TBG. In addition, most of the PM2.5 deposition flux from field estimations were similar to those in i-Tree Eco when considering the loss of mass due to the dissolution through water filtration, indicating that i-Tree Eco may be reliable to model the removal of PM2.5 in the parks in Taiwan. Moreover, we examined nine heavy metals' content in the deposited PM, and most of the detectable elements were significantly higher outside both parks, demonstrating the mitigation effects of urban parks in reducing not only the PM2.5 concentration but also the toxicity of the pollutant. This study provides direct evidence of the important ecosystem services, namely air quality improvement and biomonitoring effect, derived from urban parks.

Dry deposition of particulate matter and its associated soluble ions on five broadleaved species in Taichung, central Taiwan.

Many studies have estimated particulate matter (PM) removal by urban trees using dry deposition models; however, few studies have quantified the accuracy of their results. Thus, this study investigated the dry deposition of PM and its associated soluble ions in five broadleaved species in three districts of Taichung, central Taiwan, through field experiments. The total suspended particulate (TSP) dry deposition flux on leaf surfaces varied with sampling time, site, and tree species. By contrast, single-factor effects were observed for PM10 and PM2.5. The average dry deposition velocities of TSPs, PM10, and PM2.5 were 0.63, 0.062, and 0.028 cm s-1, respectively. Moreover, the dry deposition velocities of sulfate and nitrate were estimated to be 0.186 and 0.194 cm s-1, respectively. A significant relationship was observed between the ambient concentration and the dry deposition flux for all size fractions of PM. By contrast, weak and negative correlations were found between particle deposition velocity and wind speed. The measured PM2.5 dry deposition velocity was approximately equal to the dry deposition velocity obtained with the i-Tree model (0.03 cm s-1), which indicated the promising application potential of i-Tree in Taiwan. Compound and rough leaves, such as leaves of the Taiwan golden-rain tree, intercepted a high amount of PM2.5, whereas the pongam tree, which has thin leaves and wax surfaces, exhibited the lowest TSP interception. Species difference mostly occurred in the dry deposition flux of nitrate rather than sulfate; however, the interception of sulfate by trees revealed the possibility of the long-range transport of air pollutants. The results of this study elucidate the dry deposition of PM and its associated soluble ions in real-world situations.