| A city is a relatively large, central and densely populated urban area and is also an important place of human activities. Being "lungs of the city", urban forests play a significant functioning role in sustainable development of cities. Urban forests provide beautiful living environments, recreational areas for people and moderate local microclimates for human habitats. They purify air, sunlight and water, while they reduce stormwater runoff, energy costs and unhealthful substances. As a result, there has been a growing and wide-ranging demand for better understandings of the structure, function and services provided by urban forests in the past years.Guizhou Province in China is located in the center of the Southeast Asian karst region where karst landform is most developed in the world. The karst area within Guiyang, the capital city of Guizhou Province, accounts for 85% of the total urban area of the city. The Guiyang city, the first forested city in China, is also one of the best vegetated cities in karst areas across over the world, and is so-called a'wonder of green" in karst landform. In this study, the physical and chemical properties of soils, biomass production, stand productivity and carbon storage were investigated in main nine forest types in Guiyang city. The overall objective of the project is to study the basic structural characteristics of selected nine urban forest types developed in karst areas and provide scientific references for sustainable management of the unban forests.The main results showed:1. The soil in the study sites was acid and belongs to yellow earth with pH values of 4.68-5.33. The bulk density of soil ranged 0.92-1.16 g·cm-3; soil water content 22.9-40.9%; total soil porosity 56.2-65.2%, and soil organic matter content 25.1-33.0 g.kg-1. Overall, the forest soils in Guiyang city are characteristics of appropriate acid-basic scale, better structure and aggregation, adequate moisture and high organic matter content, which provide a suitable medium for vegetation growth.2. The species richness index was 2.140 in Poplar forests,1.528 in Black Locust (Robinia pseudoacacia L.) and Ovate Catalpa (Catalpa ovata G.Don.) mixed-forests and 1.905 in Scrub forests. The Shannon-Wiener diversity index was 2.361,2.285 and 1.118, simpson's Diversity Index 0.839, 0.888 and 1.000, evenness index 0.802,0.920 and 0.042, ecological dominance 0.161,0.111 and 0.086 for the three forest types, respectively.β-diversity index was in an order as Black Locust and Ovate Catalpa mixed-forests and Scrub forests (0.345)> Poplar forests and Scrub forests (0.286)> Poplar forests and Black Locust and Ovate Catalpa mixed-forests (0.200). In Poplar forests and Black Locust and Ovate Catalpa mixed-forests, a relative simple structure was found in tree layer with distinct dominate species, while high species diversity and evenness index and no apparent dominate species were found in understory (the shrub and herb) layer within these forests. In Scrub forests, woody vegetation species were richness with high diversity indexes, but there were less herb vegetation species with even distribution.β-diversity index was relative low in all studied forests, meaning there was a relative high special heterogeneity among different urban forest types.3. Average biomass of individual trees was 7.99 kg in Camphor trees (Cinnamomum camphora (L.) Presl) (16 year-old),189.49 kg in Poplar trees (25 year-old),43.97 kg in Ovate Catalpa trees (38 year-old),58.82 kg in Black Locust trees (38 year-old),9.54 kg in Betula luminifera trees (Betula luminifera H. Winkl.) (10 year-old),7.03 kg in Italian Poplar trees (Populus euramevicana cv.'I-214') (10 year-old),5.56 kg in Masson pine trees (Pinus massoniana Lamb) (8 year-old),173.21 kg in Sawtooth Oak trees (Quercus acutissima Carr.) (38 year-old),1.98 kg in Red Maple trees(Acer rubrum L.) (11 year-old), and 5.13 kg in Cherry Blossom trees (Prunus yedoensis Matsum.) (5 year-old).The mean stand biomass was 15.15 t·hm-2 in Camphor forests,105.36 t-hm"2 in Poplar forests,83.65 t·hm-2 in Black Locust and Ovate Catalpa mixed-forests,4.03 t·hm-2 in Betula luminifera and Italian Poplar mixed-forests,2.60 t·hm-2 in Masson pine forests,129.04 t·hm-2 in Sawtooth Oak forests,4.95 t·hm-2 in Red Maple forests, and 12.82 t·hm-2 in Cherry Blossom forests. Within the six forest types, biomass in shrub layer ranged 1288.56-7264.14 kg·hm-2, of which aboveground biomass accounted for 54.8-83.5% and belowground for 16.5-39.7% of the total biomass. Biomass was 556.49-9494.98 kg·hm-2 and 946.17-9672.29 kg·hm-2 in herbivorous layer and dead litter layer, respectively.Stand productivity was 2.86 t·hm-2·a-1 in Camphor forests,12.12 t·hm-2·a-1 in Poplar forests,11.55 t·hm-2·a-1 in Black Locust and Ovate Catalpa mixed-forests,0.99 t·hm-2·a-1 in Betula luminifera and Italian Poplar mixed-forests,0.33 t·hm-2·a-1 in Masson pine forests,9.38 t·hm-2·a-1 in Sawtooth Oak forests,1.70 t·hm-2·a-1 in Red Maple forests, and 5.53 t·hm-2·a-1 in Cherry Blossom forests.4. The mean biomass in the Scrub forests was 12461.00 kg·hm-2, of which aboveground biomass accounted for 67.6%(8420.27 kg·hm-2) and belowground biomass accounted for 32.4%(4040.74 kg·hm-2). Only six herbivorous species were found with the Scrub forests with a mean biomass of 3745.14 kg·hm-2, which accounted for 23.1% of the total biomass in the forests. The Scrub forest was a specific forest type in the karst regions.5. The mean concentration of carbon in the main tree species in the karst urban forests was 585.72 g·kg-1 in Camphor trees,533.90 g·kg-1 in Poplar trees,483.46 g·kg-1 in Masson pine trees,544.54 g·kg-1 in Sawtooth Oak trees,547.71 g·kg-1 in Ovate Catalpa trees,521.21 g·kg-1 in Black Locust trees,549.07 g-kg"1 in Betula luminifera trees,553.43 g·kg-1 in Italian Poplar trees,492.54 g·kg-1 in Red Maple trees, and 524.94 g·kg-1 in Cherry Blossom trees.On average, the concentration of carbon in the shrub layer was 538.80 g·kg-1 in Camphor forests,528.08 g·kg-1 in Poplar forests,519.55 g·kg-1 in Masson pine forests,516.44 g·kg-1 in Black Locust and Ovate Catalpa mixed-forests,520.27 g·kg-1 in Sawtooth Oak forests,522.90 g·kg-1 in Betula luminifera and Italian Poplar mixed-forests, and 512.31 g·kg-1 in Scrub forests.The mean concentration of carbon in the herbivorous layer was 388.07 g·kg-1 in Camphor forests,335.14 g·kg-1 in Poplar forests,432.09 g·kg-1 in Masson pine forests,451.63 g·kg-1 in Sawtooth Oak forests,408.84 g·kg-1 in Black Locust and Ovate Catalpa mixed-forests,444.06 g·kg-1 in Betula luminifera and Italian Poplar mixed-forests, and 465.59 g·kg-1 in Scrub forests.The average concentration of carbon in dead litter layer was 518.98, 562.17,410.78,376.26,451.67,394.97, and 370.43 g·kg-1 in the Camphor forests, Poplar forests, Masson pine forests, Sawtooth Oak forests, Black Locust and Ovate Catalpa mixed-forests, Betula luminifera and Italian Poplar mixed-forests, and Scrub forests, respectively.6. Carbon storage in the tree layer was 8.94 C t·hm-2 in Camphor forests,59.84 C t·hm-2 in Poplar forests,1.27 C t·hm-2 in Masson pine forests,71.60 C t-hm-2 in Sawtooth Oak forests,46.36 C t·hm-2 in Black Locust and Ovate Catalpa mixed-forests,2.26 C t·hm-2 in Betula luminifera and Italian Poplar mixed-forests,2.47 C t·hm-2 in Red Maple forests, and 6.78 C t·hm-2 in Cherry Blossom forests.Carbon storage in the shrub layer was 0.72,3.93,0.79,1.06,2.41,3.11, and 6.37 C t·hm-2 in the Camphor forests, Poplar forests, Masson pine forests, Sawtooth Oak forests, Black Locust and Ovate Catalpa mixed-forests, Betula luminifera and Italian Poplar mixed-forests, and Scrub forests, respectively.Carbon storage in the herbivorous layer was 0.32,0.19,1.26,1.07, 4.32,1.59, and 1.73 C t·hm-2 in the Camphor forests, Poplar forests, Masson pine forests, Sawtooth Oak forests, Black Locust and Ovate Catalpa mixed-forests, Betula luminifera and Italian Poplar mixed-forests, and Scrub forests, respectively.Carbon storage in the dead litter layer was 3.27,0.50,3.90,1.96,1.15, 1.05, and 2.56 C t·hm-2 in in the Camphor forests, Poplar forests, Masson pine forests, Sawtooth Oak forests, Black Locust and Ovate Catalpa mixed-forests, Betula luminifera and Italian Poplar mixed-forests, and Scrub forests, respectively.Carbon storage in the soils was 121.26,115.37,122.69,74.91,77.68, 111.00, and 112.15 C t·hm-2 in the Camphor forests, Poplar forests, Masson pine forests, Sawtooth Oak forests, Scrub forests, Black Locust and Ovate Catalpa mixed-forests, Betula luminifera and Italian Poplar mixed-forests, respectively.7. At ecosystem levels, carbon storage was 125.85,188.49,129.91, 150.60,165.24,120.168, and 8.34 C t·hm-2 in the Camphor forests, Poplar forests, Masson pine forests, Sawtooth Oak forests, Black Locust and Ovate Catalpa mixed-forests, Betula luminifera and Italian Poplar mixed-forests, and Scrub forests ecosystems, respectively.Carbon storage was mainly accumulated in the vegetation, dead litter and soil components in the forest ecosystems. The carbon storage in different components was in an order soil> vegetation> dead litter.Annual carbon sequestration in the eight forest types in the Guiyang city was 23.866 C t·hm-2·a-1, of which 7.02% was in Camphor forests (1.676 C t·hm-2·a-1),27.12% in Poplar forests (6.473 C t·hm-2·a-1),0.66% in Masson pine forests (0.157 C t·hm-2·a-1),21.40% in Sawtooth Oak forests (5.107 C t·hm-2·a-1),3.50% in Red Maple forests (0.835 C t·hm-2·a-1), 12.16% in Cherry Blossom forests (2.901 C t·hm-2·a-1),25.86% in Black Locust and Ovate Catalpa mixed-forests (6.172 C t·hm-2·a-1) and 2.28% in Betula luminifera and Italian Poplar mixed-forests (0.545 C t·hm-2·a-1). Annual carbon sequestration capacity was closely related to stand ages, stand productivities and the biological features of the tree species.8. The forested area was estimated to be about 187228.80 hm2 in the karst areas in the Guiyang city (excluding economic and fruit orchards in the karst areas). Therefore, the total carbon storage in the forested area was estimated to be 2243.59×10-4 C t, of which 10.31×10-4 C t was in Camphor forests (accounting for 0.46% of the total carbon storage),20.53×10-4 C t in Poplar forests (0.92%),1073.03×10-4 C t in Masson pine forests (47.82%),91.06×10-4 C t in Sawtooth Oak forests (4.06%),371.59×10-4 C t in Black Locust and Ovate Catalpa mixed-forests (16.5%),81.93×10-4 C t in Betula luminifera and Italian Poplar mixed-forests (3.65%), and 595.14×10-4 C t in Scrub forests (26.53%).The results from the study indicated that the afforestation projects had been successful in the karst areas in Guiyang city. Our results also demonstrate that urban forests in the karst regions play an important role in terms of carbon sequestration and improvement of local environments.
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