作 者 ：张林, 罗天祥
期 刊 ：植物生态学报 2004年 28卷 6期 页码：844-852
关键词：叶寿命；比叶面积；叶氮含量；适应策略；常绿植被；落叶植被；生态系统；
Keywords:Human impacts, Harvesting practices Forest degradation, Forest restoration, Nutrient dynamics, Pinus massoniana, Dinghushan, Subtropics,
摘 要 ：科学家早已注意到，具有长叶寿命的植物通常生长于营养和（或）水分较为缺乏的环境，而具短叶寿命的植物一般生长在具有较高的营养可利用性地带。国外大量的实验研究结果表明，单位重量的叶氮含量（Nmass）与叶的最大光合速率存在密切正相关，而比叶面积（单位干重量的叶面积）与植物生产单位叶面积的物质成本呈负相关，二者又随叶寿命的增加而降低,这种相互关系几乎在所有植物种群和群落中都普遍存在，反映了植物对环境适应的趋同进化特征，是进一步理解生态系统行为特征的基础。松属(Pinus)及云杉(Picea)、冷杉(Abies)等常绿针叶树种的叶寿命一般随海拔的升高而增大，其相关叶性状也普遍存在明显的垂直分布格局：随着海拔升高，单位面积的叶氮含量（Narea）及最大光合速率增加，而比叶面积则减少。一般认为，长的叶寿命是对高寒及养分、水分贫乏等胁迫环境的适应，而短的叶寿命和（或）落叶性被认为是植物为了快速生长以及对干旱或寒冬等季节性胁迫环境的适应结果。根据成本-效益分析理论，在特定环境条件下叶寿命大小取决于形成并维护单位叶面积所需要的物质消耗与叶片碳收获的平衡，与常绿/落叶森林植被纬向/垂直地带性分布存在一种内在联系机理。因此，叶寿命及其相关叶性状成为将叶片水平上的生理生态实验数据扩展到整个群落冠层乃至生物地理群区的关键因子。加强叶寿命及其相关叶性状的格局分析研究，可为现有的区域生物地球化学循环模型与植被地理模型的连接（即区域植被动态模型）提供科学依据，这将有助于从机理上解释区域植被对全球变化的适应与响应机制。但是，以往植物生理生态研究大多注重于个体叶片水平的测定，仍不清楚如何实现这些叶性状在不同尺度间的转换，尤其是从叶片水平上升至整个群落冠层。国际上至今仍缺乏在生态系统水平上的大量野外观测数据来定量描述这些叶性状与群落特征、气候因子的数量关系；国内相关研究基本尚属空白。
Abstract:Over the past two decades, society has become increasingly aware of problems of forest degradation. The effects of forest degradation transcend individual countries and now impact global processes. Research on the restoration of degraded forests has become a key issue in global ecology. Of particular concern are the effects of human activities on forest productivity and site fertility, especially as the demands for fuel and timber from tropical forests increase. Removal and burning of biomass causes nutrient losses and changes to the soil’s physical and chemical characteristics. The amount of nutrient loss depends on the intensity of the activities, local environmental factors, and the type and successional state of the forest. If nutrient losses cannot be recovered during regrowth, forests often become degraded through time. Thus, it is important that the nutrient dynamics of human-impacted forests are well understood in order to develop plans for restoration of degraded forests and for sustainable forest management. Most of the primary tropical forests in southern China have been degraded by human activities during the past several hundred years. Factors leading to their degradation include: timber harvesting, unsustainable agriculture, overgrazing by domestic animals, and intensive harvesting for fuel. In extreme cases, the land has become completely denuded. Attempts to reverse this process of land degradation have been initiated in this region of southern China. Over the last few decades, large areas have been reforested with a native pine species, Pinus massoniana, to prevent further degradation to the landscape. Cutting of trees is now prohibited, but harvesting of the understory and collection of litter is still allowed to satisfy local fuel needs. Compared with whole-tree harvests, this practice removes less biomass from the forests; however, as the understory and litter are relatively nutrient-rich, this practice may slow or prevent the recovery of soil fertility and productivity of these forest ecosystems. The objective of this study was to determine the effects of harvesting understory plant species and litter on nutrient accumulation dynamics in a Pinus massoniana forest of subtropical China. The results are used to address the following questions: 1) How are nutrients distributed in plants of this pine forest; 2) What quantity of nutrients are removed annually from the ecosystem by the practice; 3) Is this harvesting practice sustainable or not; 4) What alternative management options are available for continued use to meet fuel needs while at the same time improving site fertility, productivity and regeneration processes; and, 5) How do stressed ecosystems respond when the stressors are removed, that is, how would the forest respond if the harvesting practice was stopped. In order to achieve the objectives above, an experiment was established in a MAB reserve of subtropical China in May 1990. The experiment was a paired-plot design with 20 replicates. Each pair consisted of a treatment (continued harvest) and control (no harvest) plot, 10 m×10 m in size and surrounded by a 10 m wide buffer strip. In the treatment plots, local people continued to harvest litter and understory plants according to local practice (about 2－3 times a year) from the beginning of the experiment in May 1990. The harvesting according to local practice occurred during the period of 1990 to 1995 and was stopped after this time. Control plots were protected from any harvesting. Each set of paired plots was similar in soil, slope, aspect, and elevation. In this paper we report only the results of nutrient dynamics and its responses to human impacts over a ten-year period from 1990 to 2000.