Research On Understory Vegetation Characteristics And Its Influencing Factors In Aerially-seeded Pinus Massoniana Plantations

In the process of restoring and reconstructing the degraded forest ecosystem, understory vegetation restoration has important ecological significance. In the region of southern Jiangxi Province, a large area of aerially-seeded pinus massoniana afforestation activities were organized in more than 10 counties or cities where the vegetation was damaged badly and the soil erosion was serious from early 60 s to 90 s in the 20 th century. After decades of restoration, soil erosion in this region has been significantly improved. But understory vegetation was sparse in most of the aerially-seeded Pinus massoniana plantations which was still a more prominent problem. Restoration of the understory vegetation was an urgent problem to be solved in aerially-seeded Pinus massoniana plantations’ management. So it had a very important theoretical and practical significance to explore the relationship between understory vegetation and the influencing factors in aerially-seeded Pinus massoniana plantations which can provide guidance of the restoration of understory vegetation in the whole south of Jiangxi Province and provide scientific basis for the mechanism of understory vegetation restoration in other serious degradation areas.This study chose Xingguo county as the research area which was representative in the afforestation of pinus massoniana by aerial seeding in south of Jiangxi Province and taken the aerially-seeded Pinus massoniana plantations as the research object. Through field investigation and laboratory measurement, the characteristics of understory vegetation and its influencing factors in aerially-seeded Pinus massoniana plantations were analyzed by. The main results and conclusions were as follows:(1) There were 52 species in the aerially-seeded Pinus massoniana plantations, which belonged to 50 genera and 33 families. The 52 species included 25 shurb plants, 24 herbaceous plants and 3 bryophyte plants. The floristic analysis showed that the 50 genera contained 11 floristic types in which the tropic type was dominant and the temperate type was relatively rich as a whole. According to the importance values of plants, Lespedeza formosa was the dominant shurb and Dicranopteris linearis was the dominant herbaceous. There were 6 shurb- herbaceous association types which were“Rhododendron simsiiIschaemum ciliare”, “Rhododendron simsii-Dicranopteris linearis”, “Lespedeza formosaIschaemum ciliare”, “Loropetalum chinense-Ischaemum ciliare”, “Glochidion puberumIschaemum ciliare”and “Syzygium grijsii-Lophatherum gracile” and 3 herbaceous association type which were “Ischaemum ciliare”, “Dicranopteris linearis” and “Lophatherum gracile”.(2) The average of species richness, Shannon-Weiner diversity index, Pielou evenness index and Simpson dominance index of shurb were 6、1.04、0.72 and 0.50, and the herbaceous were 4、0.63、0.52 and 0.32, which indicated that the species diversity of understory vegetation was not high. Canopy density, average diameter at breast height, total nitrogen, urease activity, noncapillary poropsity and slope position had significant conditional term effects on the understory vegetation presence. These six influencing factors can explain the 37.2% of the variation of understory vegetation presence. According to species location in the ordination space, most of the understory vegetation were present on the environment that average diameter at breast height was greater, and canopy density, soil total nitrogen was higher and position was in the downhill, while Dicranopteris linearis, Eriachne pallescens, Miscanthus floridulus and so on were more present on the severe environment that average diameter at breast height was smaller, and canopy density, total nitrogen was lower and position was in the uphill. The species riches of shurb significantly correlated with average diameter at breast height, canopy density, slope position, total nitrogen, available nitrogen, available phosphorus, organic materia and urease activity. The Shannon-Weiner diversity index of shurb significantly correlated with average diameter at breast height, canopy density, slope position, available nitrogen, organic materia and urease activity. The Pielou evenness index of shurb significantly correlated with average diameter at breast height, canopy density and total nitrogen, and the Simpson dominance index of shurb significantly correlated with average diameter at breast height and urease activity. The species riches of herbaceous significantly correlated with average diameter at breast height, canopy density, slope position, total nitrogen, available nitrogen, organic materia and urease activity. The Shannon-Weiner diversity index and Simpson dominance index of herbaceous significantly correlated with average diameter at breast height, canopy density, slope position, total nitrogen, available nitrogen, available phosphorus, organic materia and urease activity. The Pielou evenness index of herbaceous significantly correlated with canopy density only.(3) Through the contributions of various influencing factors to the coveage of four understory vegetation functional groups which were shurb type, Dicranopteris type, graminoid type and bryophyta type, we came to the conclusions that the selected influencing factors totally explained 74.2% variation of the four understory vegetation functional groups coverage. Stand characteristics, soil property and topography factor were respectively explained 55.0%(including 29.1% for separateness and 25.9% for interaction with other factors), 38.9%(including 12.1% for separateness and 26.8% for interaction with other factors) and 9.0%(including 5.6% for separateness and 3.4% for interaction with other factors) of the total variation. At low altitude, stand characteristics were the key factor to influence the 4 understory vegetation functional groups coverage. The relationships between the 4 understory vegetation functional groups coverage and influencing factors were showed that: average diameter at breast height and canopy density affected mostly and positively correlated with the coverages of bryophyta, graminoid and shurb types at significant level. Noncapillary porosity and soil water content showed a highly significant positive correlation to dicranopteris coverage, whereas average diameter at breast height, canopy density, soil nutrients and enzyme activity significantly negatively correlated with it. The relationships among 4 understory vegetation functional groups coverage were showed that: The coverages of graminoid, bryophyta and shurb types showed the positive correlations, which indicated the three groups could promote each other, while dicranopteris coverage had significant negative correlation with the three groups mentioned above.(4) The average of biomass of shurb and herbaceous in the aerially-seeded Pinus massoniana plantations were 1.87 t/hm2 and 2.32 t/hm2 respectively. The mian influencing factors of the shurb biomass and herbaceous biomass were not the same. Average diameter at breast height, soil total nitrogen, canopy density and slope position affected mostly and correlated with the shurb biomass at significant level, with the relative influence of 39.9%、35.5%、8.0% and 3.8% respectively. Soil water content, soil total nitrogen, canopy density and average diameter at breast height had great effects on herbaceous biomass and correlated with it at significant level, with the relative influence of 61.8%、17.1%、5.4% and 3.9%. According to the fitted functions between shurb biomass and influencing factors, when average diameter at breast height was 8 cm, or soil total nitrogen was 0.10 g/kg, or canopy density was 0.6, the shurb biomass changed greatly. As to the slope position, the downhill affected shrub biomass most obviously. According to the fitted functions between herbaceous biomass and influencing factors, when soil water content was 8.0%, or soil total nitrogen was 0.07 g/kg, or canopy density was 0.5, or average diameter at breast height was 8 cm, the herbaceous biomass changed greatly.(5) The existing understory vegetation were classified three types that were shurb type, Dicranopteris type and graminoid type in the aerially-seeded Pinus massoniana plantations based on the actual situation. There were 25 shurb and herbaceous species in the soil seed bank, which belonged to 24 genera and 17 families. The average seed density of the soil seed bank was 11.5×102 seeds /m2. Shurb type and graminoid type were significantly higher than Dicranopteris type,while shurb type and graminoid type had no significant differences. The species riches of soil seed bank of graminoid type was highest than the other two type, while there were no significant differences among the three types. The species composition of the soil seed bank had significant differences among the three types which were mainly due to the reason that the dominant species of different type were not quite the same. The species composition of the soil seed bank and the existing understory vegetation was significantly different, and the average of Sorensen similarity coefficient was 0.17. Poa annua, Ischaemum ciliare, Loropetalum chinense, Miscanthus floridulus and Lespedeza formosa were the first five species causing the differences, with the contribution of 16.8% 、11.0%、7.7% 、7.3% and 4.7% respectively. The similarity coefficient of species composition between soil seed bank and the existing understory vegetation of shurb type was highest, while the Dicranopteris type was lowest. Soil properties had some effect on seed density of the soil seed bank. Soil bulk density, available nitrogen, pH and urease activity significantly correlated with the seed density of the soil seed bank.