北方山区主要森林类型树木叶片氮、磷回收效率研究

养分回收是植物养分利用策略的重要驱动力,可减少养分流失,降低其对环境的依赖性,对植物种群和群落稳定性、生态系统养分循环都具有重要的生态学意义。本文在收集北方山区(黄土高原、太行山区、京北山区)天然林、人工林等各种森林成熟叶片和凋落物氮、磷养分含量等资料的基础上,根据群落生活型和管理方式将各类森林进行归并,对其养分回收效率进行了综合评估。结果表明,各类森林植被通过成熟叶片和落叶中氮、磷含量计算得到的叶片氮、磷回收效率分别是24.5%~71.3%和18.1%~75.4%,均值分别是45.5%和47.4%。北方山区天然林和人工林成熟叶片的平均氮含量分别是11.6 g·kg~(-1)和21.6 g·kg~(-1),人工林成熟叶片氮含量显著高于天然林,养分在人工林生长过程中可能基本处于消耗状态。不同地区天然林与人工林叶片磷含量存在差异:太行山区天然林成熟叶片和枯落叶片磷含量显著高于人工林,而京北山区人工林成熟叶片磷含量较高,枯落叶片磷含量较低。灌木成熟和枯落叶片中氮含量显著高于乔木,而两者间磷含量无显著差异;灌木叶片氮回收效率高于乔木。通过3个地区的比较发现,京北山区树木叶片氮、磷回收效率分别是62.1%和67.8%,高于其他两个地区。相关分析表明,北方山区森林枯落叶片氮含量是影响叶片氮、磷回收效率的重要因子。通过对不同树种养分回收效率的比较发现,针叶树种养分回收效率大于阔叶树种,这说明在土壤养分贫瘠山区针叶树种的养分回收效率更高,更能适应这种贫瘠的土壤环境。在山区进行人工建植时,针叶树种高的养分利用效率应该被考虑进来。

Nitrogen and phosphorus resorption efficiency of forests in North China

Nutrient cycling is one of the basic functions of forest ecosystems. As two of the main nutrition elements, nitrogen and phosphorus are critical for proper metabolism and growth processes of plants. In order to reduce the dependence on external nutrient, plants can resorb nutrients from senescing leaves prior to abscission and store them into other plant tissues for reuse. Resorption efficiency, defined as the percent nutrient reduction between live and senescent leaves, is used to quantify the resorption capacity of a plant. Nutrient resorption is a key process of plant nutrient utilization strategy. Through nutrient resorption, plants can reduce nutrient loss from the ecosystem and lower the dependence on the environment. This is crucial for the stability of plant population and community, and for ecosystem nutrient cycling. Based on published papers on forests in mountain regions of North China (including the Loess Plateau, Taihang Mountain, North Beijing Mountain Area, etc.), the systemization of nitrogen and phosphorus contents in mature and senescent leaves of forests was conducted depending on the life forms and management methods of forests. All in all, we collected data on nitrogen and phosphorus at 13 sites in the three regions and conducted a comprehensive assessment on nutrient resorption efficiencies. We compared nitrogen and phosphorus resorption efficiencies between trees and shrubs, as well as between natural and plantation forests. In order to explore the factors regulating nutrient resorption efficiency, we also analyzed the relationships between the contents of nitrogen and phosphorus against nutrient resorption efficiency. The results showed that the ranges of nitrogen and phosphorus resorption efficiencies for forests were respectively 24.5%-71.3% and 18.1%-75.4%, with averages of 45.5% and 47.4%, both slightly lower than global average. Average nitrogen content in mature leaves of plantation forests was 21.6 g·kg^-1, significantly higher than that of natural forests (11.6 g·kg^-1). This indicated that in the process of growth of plantation forests, there could be excessive consumption of nutrients. In Taihang Mountain, nitrogen content in mature and senescent leaves and nitrogen resorption efficiency for shrubs were significantly higher than those for trees. However, phosphorus resorption efficiency was relatively lower, demonstrating that plant growth in the region was mainly limited by nitrogen supply. Comparisons among different areas showed that nitrogen and phosphorus resorption efficiencies in North Beijing Mountain Area were higher than those in the Loess Plateau and Taihang Mountain. Correlation analysis showed that both nitrogen and phosphorus resorption efficiencies were significantly correlated with nitrogen content in senescent leaves, indicating that the nitrogen content of senescent forest leaves (also called nutrient resorption proficiency) in North China was the main limiting factor of nitrogen and phosphorus resorption efficiencies. Based on the comparison of nutrient resorption efficiencies of different vegetation species, nutrient resorption efficiencies were greater for conifer forests than that for broad-leaved species. This suggested that in nutrient-barren soils in mountain terrains, nutrient resorption in conifer forests was more efficient, and conifer trees were probably more adaptable to such barren soil environment. In the process of development of artificial plantations in mountain regions, it was recommended to consider high nutrient use efficiency of conifer forests.