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In this study,we selected eighteen species of common street trees along main roads in Guangzhou Municipality to observe and compare their leaf surface morphology using Scanning Electron Microscopy(SEM),and to evaluate how the contact angle of water droplet on leaf surfaces measured by goniometer influence the dust-retaining capability.It was found that the maximum leaf dust-retaining capabilities per unit area of the eighteen species of trees 26 days after rainfall were 0.066—1.831g/m2,indicating significant differences(up to 27 times) among different trees.The plants with network structure on leaf surface,having high stomatal density and large opening of stomata,such as Mangifera Indica,have strong dust-retaining capability;while those whose leaf surfaces are smooth and covered by epicuticular wax with stomata arranged regularly without obvious undulation,such as Bauhinia blakeana,Swietenia mahogani,Lagerstroemia indica Linn.and Scheffera arboricola,have poor dust-retaining capability.There was a significant negative relationship between leaf contact angle and dust-retaining capability(r=-0.614).Leaves with contact angle smaller than 90° demonstrate hydrophilicity.Their maximum dust-retaining capabilities 26 days after rainfall ranged from 1.0 to 1.831 g/m2.For leaves with rugged morphological structure,hook or ridge-shaped rumple,prominence and stomata density ranging from 20 to 60,the measured contact angles are smaller,making the contact area between leaf and dust larger.Therefore,it is not easy for dust on such trees as Mangifera Indica,Bischofia Polycarpa and Ficus altissima Bl.,to detach from leaf surface and these species have a strong dust-retaining capability.On the contrary,the special surface structure and hydrophobic wax of tree leaves with larger contact angle,such as Alstonia scholaris,Chukrasia tabularis,Lagerstroemia indica Linn.,Scheffera arboricola and Bauhinia blakeana,make it difficult for dust particles to attach to leaves,resulting in small dust-retaining capabilities of less than 1.0g/m2.This study has shown that the wax content of surface foliage,stomata density and contact angle of leaves are the three main factors controlling the dust-retaining capability.It is therefore advisable to consider choosing tree species whose leaf surface morphology enhances dust-retaining capability for urban greening and thus to improve the environmental effectiveness of urban vegetation.

Soil microbe is the most important decomposer in forest ecosystems,and plays a key role in material recycling and energy conversion.In this study,the spatial patterns of soil microbial biomass in typical Karst peak-cluster depressions were analyzed through both geo-statistical and traditional statistical methods.The results showed that large variation existed in soil microbial biomass.Soil microbial biomass carbon(Cmic),soil microbial biomass nitrogen(Nmic),and soil microbial biomass phosphorus(Pmic) varied from 44.29—5209.63,20.91—1894.37,0.34—77.06 mg/kg,respectively.The Cmic was significantly positively correlated with Nmic.The value of Cmic/Nmic is 4.78,which obviously lower than that in other ecosystems.Semivariance analysis revealed that the Gaussian model fitted best for Cmic and Nmic,the Spherical model fitted best for Pmic,and the Exponential model fitted best for Cmic/Nmic.The SH%(C/(C+C0)×100%) of soil microbial biomass exhibited moderate spatial autocorrelation,ranging from 25% to 75%.Therefore,the spatial patterns were affected by both random and structure factors.The autocorrelation distance of Cmic and Nmic were about 50 m.Moran′s I decreased gradually with the increase of separation distance,and showed a statistically negative correlation.Moran′s I of Pmic increased when the separation distance was larger than 70 m,which indicated positive correlation.Moran′s I of Cmic/Nmic was lower than others,ranging from-0.2 to 0.2 m.The spatial distribution of Cmic and Nmic were similar,and showed a convex-type distribution in which the contents were high in the middle and low on the two sides.In contrast,The Pmic was distributed with a pattern of high in the upper slope and depressions.The content of Cmic/Nmic showed a concave-type distribution with many small patches.We conclude that soil microbes exist in a certain spatial pattern,and that its contents sharply reduced after disturbing.As a result,the protection of Karst primary ecosystems against human disturbing should be strengthened.