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Pure and mixed larch (本研究以塞罕坝地区华北落叶松纯林、白桦纯林和华北落叶松与白桦混交林为对象,每种林分类型设置2个林分密度(200～340和880～1100株·hm

Litter is an important contribution to forest soil. Litter decomposition plays an important role in nutrient cycling of forest ecosystem. A field litterbag experiment was conducted to examine the dynamics of decomposition rate, nutrient release and enzyme activity during litter decomposition in the pure forests of森林凋落物是森林土壤的重要组成部分,凋落物分解在调控森林生态系统养分循环中发挥了关键作用。采用凋落物分解袋法,研究河北塞罕坝地区华北落叶松与白桦,华北落叶松与蒙古栎,华北落叶松、白桦和蒙古栎混合凋落叶及纯华北落叶松凋落叶分解过程中分解速率、养分释放和酶活性的变化。结果表明: 经过近2年的分解,混合凋落叶分解速率均显著高于纯华北落叶松凋落物叶;在所有处理中,华北落叶松与白桦混合凋落叶分解速率最高。在凋落叶分解过程中,不同处理养分含量变化一致,凋落叶N、P含量呈上升趋势,C、K含量和C/N呈下降趋势;相对纯华北落叶松凋落叶,各混合凋落叶分解可以促进凋落叶C、K的释放,但对N、P的释放有一定的抑制作用。在凋落叶分解过程中,不同处理凋落叶过氧化氢酶、脲酶、酸性磷酸酶活性呈上升趋势,蔗糖酶活性呈下降趋势;凋落叶分解速率与凋落叶过氧化氢酶、脲酶、酸性磷酸酶活性呈正相关,与蔗糖酶活性呈负相关。总体来看,华北落叶松和白桦、蒙古栎凋落叶混合可以促进华北落叶松凋落叶的分解,且凋落叶中酶活性动态变化与凋落叶的分解密切相关。.

Quantitative analysis of the relationships between the distribution of natural plant communities and environmental factors was studied using two-way indicator species analysis (TWINSPAN) and detrended canonical correspondence analysis (DCCA) methods, in the Saihanba National Nature Reserve in Hebei Province. The results showed that the natural plant communities in the Saihanba Reserve could be classified into eight types, i.e., Sanguisorba officinalis + Carex heterostachya + Trollius chinensis community, S. officinalis + Elymus sibiricus + Geranium platyanthum community, C. lanceolata + Ophiopogon chingii community, Spiraea pubescens - Tripolium vulgare community, Armeniaca sibirica + S. pubescens - T. vulgare + C. lanceolata community, Quercus mongolica + Betula costata - Lespedeza bicolor + S. pubescens - C. lanceolata community, Betula platyphylla + Populus davidiana - Corylus mandshurica + Rosa davurica - Phlomis umbrosa + C. lanceolata + Thalictrum aquilegifolium community and Picea asperata + B. platyphylla - Rosa davurica + Salix taishanensis var. hebeinica - C. lanceolata + Commelina diffusa community. DCCA analysis indicated that slope position and soil nutrients were the main influence factors for the spatial distribution of natural plant communities in the Saihanba National Nature Reserve. Topographic, spatial and soil factors accounted for 34.02% of the variation of species pattern. Of the variation, soil factors accounted for 15.67%, topographic factors 10.05% and spatial factors 6.00%. In the interactions among all factor combinations (2.30%), the interaction between soil and topographic factors was the largest (0.93%), and the interaction among soil, topographic and spatial factors was the least (0.16%). The unexplained portion accounted for 65.98% of spatial distribution, which might be due to anthropogenic disturbance or interspecific competition.运用双向指示种(TWINSPAN)和除趋势典范对应分析(DCCA)等方法定量分析了河北省塞罕坝保护区天然植物群落分布与环境因子的关系.结果表明: 塞罕坝保护区天然植物群落可划分为8种类型,即地榆+异穗苔草+金莲花群落、地榆+老芒麦+毛蕊老鹳草群落、披针叶苔草+铁丝草群落、柔毛绣线菊-铁杆蒿群落、山杏+柔毛绣线菊-铁杆蒿+披针叶苔草群落、蒙古栎+硕桦-胡枝子+柔毛绣线菊-披针叶苔草群落、白桦+山杨-毛榛+山刺玫-糙苏+披针叶苔草+唐松草群落、云杉+白桦-山刺玫+河北柳-披针叶苔草+节节草群落;DCCA分析表明,坡位和土壤养分是影响塞罕坝保护区天然植物群落空间分布的主要因子.地形、空间和土壤因子解释了物种格局变异的34.02%,其中,土壤因子占15.67%,地形因子占10.05%,空间因子占6.00%.在各类因子间的交互作用(2.30%)中,土壤和地形因子的交互作用最大(0.93%),土壤、地形和空间因子三者的交互作用最小(0.16%).65.98%未能解释的部分,可能归因于未被选取的因子如种间的协同和竞争作用或者人为干扰.

To explore the changes of vegetation landscape pattern and its driving mechanism in Saihanba Nature Reserve, we analyzed vegetation type changes from 1989 to 2013 and the driving factors using random forest and Logistic regression models in conjunction with land dynamic degree indicator, based on three Landsat TM imageries obtained in 1989, 2000 and 2013. The results showed that the proportion of shrubland was always small in this area from 1989 to 2013. During 1989-2013, the proportion of shrubland rapidly decreased and plantation area significantly increased, while the area of grassland and natural secondary forest slightly changed. Key driving factors for the vegetation dynamics were dependent on vegetation type and time. The change of each vegetation type from 1989 to 2000 was significantly influenced by social factors, i.e. distance to road and total investment of afforestation. Since the implementation of the Natural Forest Protection Project and establishment of Saihanba Nature Reserve, the role of natural factors including elevation and aspect gradually became more important during the 2000-2013. The vegetation landscape dynamics were primarily determined by social activities, while the distribution patterns of vegetation types were probably controlled by natural factors in the study area.为探讨塞罕坝自然保护区植被景观动态变化规律及其驱动机制,本文基于3期Landsat TM影像数据(1989、2000和2013年),应用随机森林分类算法和Logistic回归模型,借助土地动态度指标,分析了1989—2013年研究区各植被类型的变化及引起其变化的驱动因素.结果表明: 1989—2013年间,灌木林所占比例较小;研究区植被景观变化的主要特征是灌木林的迅速减少和人工林的持续增加,草地和天然次生林变化不明显;不同时期影响各植被类型变化的驱动因素不同,1989—2000年,社会因素如距道路距离、造林总投资是影响各植被类型变化的主要因素,2000—2013年,随着“天保工程”的实施以及自然保护区的成立,自然因素如高程、坡向的影响逐渐加强;社会活动主导了研究区的植被景观动态,而自然因素在一定程度上决定了植被类型的分布格局.

Based on the 23 sheets of remote sensing images from 1976 to 2009, in combining with the water and sediment data from Lijin station and the annual precipitation data of Yellow River Basin from 1976 to 2008, this paper quantitatively analyzed the features of water and sediment discharge from Yellow River, and the evolution process of Yellow River Delta and related driving mechanisms. In 1976-2008, the annual runoff and the annual sediment discharge into sea changed largely and frequently, but overall, presented a decreasing trend. Since the course of the Yellow River changed its direction to Qingshui channel in 1976, the Delta coastline and area were generally in a silting-up state. The evolution process of the Delta could be approximately divided into three stages, i.e., 1976-1985, 1986-1995, and 1996-2009, and the increasing rate of the Delta decreased with the stages. The coastline and area of the Delta were significantly exponentially correlated to the sediment accumulated at Lijin station, and the inter-annual variation of the precipitation of the Yellow River Basin had a strong correlation with that of the sediment at Lijin station, suggesting that the annual variation of the precipitation in Yellow River Basin was the main factor affecting the runoff and sediment discharge into sea.