鼎湖山马尾松林植物养分积累动态及其对人为干扰的响应

 通过处理（根据当地习惯收割凋落物和林下层植物）和保护（无任何人为干扰）样地的比较试验，在10年时间里（1990～2000年）研究了鼎湖山生物圈保护区马尾松 (Pinus massoniana) 林群落植物养分积累动态及其对人为干扰的响应，在此基础上深入和较系统地分析讨论了不同的经营措施对马尾松林可持续性的影响，为我国目前大面积的退化马尾松林恢复和马尾松林可持续性管理提供理论依据。结果表明：1990～1995年，5年时间里由于人为干扰活动而直接从处理样地取走的各元素养分量，在林下层为（kg·hm－2）：132.72 (N)、4.72 (P)、63.32 (K)、23.51 (Ca)和7.00 (Mg)，在地表凋落物为（kg·hm－2）：48.93 (N)、1.85 (P)、17.28 (K)、19.25 (Ca)和2.92 (Mg)。1990～2000年，保护样地林下层和地表凋落物各元素养分贮量分别以39%～41%和37%～38%的年平均增长速率逐年提高，至1995年达到高峰，之后各元素贮量在林下层和地表凋落物均以14%的年平均速率下降。在处理样地，1990～1995年期间各元素贮量在林下层年平均积累速率为17%，之后（1995～2000年期间）则为26%；与此同时，各元素贮量在地表凋落物年平均积累速率为22%～23%，之后（1995～2000年期间）则为28%。在整个试验过程，马尾松林乔木层养分元素总贮量随时间而增加，但其增加的速率随时间和样地不同而异。1990～1995年，保护样地乔木层养分元素总贮量增加了34.9%～38.1%，较处理样地（收获林下层和凋落物）总贮量增加的百分比（29.3%～33.5%）高。然而，1995～2000年，保护样地乔木层养分元素总贮量增加的百分比为26.3%～28.9%，较处理样地（1995～2000年也停止人为干扰）总贮量增加的百分比（28.8%～32.1%）低。可见，1990～1995年，人为干扰活动导致处理样地马尾松林乔木层养分元素年平均积累量降低约1.58%～1.72%，即年平均增长量约减少0.12～2.39 kg·hm－2（2.39 (N)、0.12 (P)、0.77 (K)、1.98 (Ca)、0.29 (Mg)），这些量约相当于每年通过林下层和凋落物收割活动而直接从林地中取走的养分总量的6%～19%。正是由于长期以来受收割林下层和凋落物这种人为干扰的影响，鼎湖山马尾松林乔木层养分贮量较低。这种利用方式不仅直接从林地中取走大量的养分而且还对林地肥力产生间接的负面影响，其结果使该退化林地不能恢复或继续退化。作者建议的森林利用方法代替目前收割林下层和凋落物方式，既可以满足当地居民燃料的需求还有利于马尾松林的自然恢复。

NUTRIENT DYNAMICS IN RESPONSE TO HARVESTING PRACTICES IN A PINE FOREST OF SUBTROPICAL CHINA

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.