Hou Lin, Hou Sijia
College of Forestry, Northwest A&F University, Yangling, Shaanxi, China.
College of Transportation, Southeast University, Nanjing, Jiangsu, China.
PeerJ. 2017 Sep 4;5:e3764. doi: 10.7717/peerj.3764. eCollection 2017.
Restoration of degraded forest ecosystem is crucial for regional sustainable development. To protect the country's fragile and fragmented environment, the Chinese government initiated an ecological engineering project, the Natural Forest Protection Program, in seventeen provinces in China beginning in 1998. Fully hillside-closed forest protection (vegetation restoration naturally without any artificial disturbance) was one of vital measures of the Natural Forest Protection Program applied nation wide. Whether plant diversity, biomass and age structure of dominant tree species and soil nutrients in protected stands may become better with increase of protected period are still open problems.
We investigated community diversity, biomass of dominant tree species, age structures, and analyzed soil chemical properties of a population at protected sites representing different protected ages at Huanglongshan Forest Bureau on the Loess Plateau, Shaanxi, China.
Plant species richness of community was significantly affected ( < 0.05) by forest protection and the effect attenuated with protection age. Shannon evenness index of plant species generally increased with protection age. Stands protected for 45 years had the highest tree biomass and considerable natural regeneration capacity. Contents of organic carbon, available phosphorus and available potassium in top soil increased in protected stands less than 45 years, however decreased significantly thereafter. Long-term forest protection also decreased the content of mineral nitrogen in top soil.
We found that the richness of shrubs and herbs was significantly affected by forest protection, and evenness indices of tree, shrub and herb increased inconsistently with protected ages. Forest protection created more complex age structures and tree densities with increasing age of protection. Content of soil mineral nitrogen at 0-20 cm soil depth showed a decreasing trend in stands of up to 30 years. Soil available phosphorus and potassium contents were higher in stands with greater proportions of big and medium trees. Long-term protection (>45 years) of stands in southeast Loess Plateau, China, may be associated with decreasing plant species richness, proportion of medium to large trees, dominant biomass of and soil nutrients.
退化森林生态系统的恢复对区域可持续发展至关重要。为保护国家脆弱且破碎的环境，中国政府于1998年起在中国17个省份启动了一项生态工程项目——天然林保护工程。全山坡封禁森林保护（自然植被恢复，无任何人为干扰）是在全国范围内实施的天然林保护工程的重要措施之一。受保护林分中植物多样性、优势树种的生物量和年龄结构以及土壤养分是否会随着保护期的增加而变得更好仍是未解决的问题。
我们调查了中国陕西省黄土高原黄龙山林局不同保护年限的受保护地点的群落多样性、优势树种生物量、年龄结构，并分析了土壤化学性质。
森林保护对群落植物物种丰富度有显著影响（P<0.05），且这种影响随保护年限的增加而减弱。植物物种的香农均匀度指数一般随保护年限增加而增加。保护45年的林分树木生物量最高，且具有相当的自然更新能力。小于45年的受保护林分表层土壤中有机碳、有效磷和有效钾含量增加，但此后显著下降。长期森林保护也降低了表层土壤中矿质氮的含量。
我们发现灌木和草本植物的丰富度受森林保护影响显著，乔木、灌木和草本植物的均匀度指数随保护年限增加的变化不一致。随着保护年限的增加，森林保护创造了更复杂的年龄结构和树木密度。在0-20厘米土壤深度处，林分年龄达30年时土壤矿质氮含量呈下降趋势。中大径级树木比例较高的林分土壤有效磷和钾含量较高。在中国黄土高原东南部，对林分进行长期保护（>45年）可能会导致植物物种丰富度、中大树比例、优势生物量和土壤养分下降。
