作 者 ：祁承经,曹福祥,曹受金*
期 刊 ：生态学报 2010年 30卷 23期 页码：6613~6623
Keywords:tropical forest, carbon sink, carbon resource, climate change,
摘 要 ：热带森林生产和储存有世界40%的生物量碳，这一碳汇的存在对于全球碳循环和人类的生态安全是极其重要的。由于气候变化热带森林碳储存量已经发生了一系列的变化。一种观点认为自20世纪80年代后期起热带森林的基面积（生物量）、茎个体密度、茎个体周转率均呈现显著增长，并归结为热带森林结构和动态协调一致的变化，同时将生物量的增加归功于高浓度CO2施肥。进而推断热带森林现今和今后数十年，它仍然是一个中等的碳汇。另一种观点认为热带森林生物量并无增加，其森林碳汇已沦落为碳源。在实验室中设置高浓度CO2条件栽培热带植物进行观测的多数结果是，无结构碳水化合物增加，而生物量并无增加。同时，随着CO2浓度升高，高温和干旱对热带森林将产生一系列更严重的负面影响，如森林生长量下降、死亡率以及森林火险增加；厄尔尼诺事件将会更加剧旱情和火灾，致使树木出现枯梢和死亡高峰。未来人类开发森林及林地利用改变将日益加剧，在自然和人为的综合影响下，不论是对热带森林生物量增加持肯定立场的生态学家，或对此持反对立场的生态学家，双方都一致认为未来退化的热带森林系统碳汇必然转变为碳源，甚至是一大规模的碳源。
Abstract:Tropical forests cover approximately 7% of the earth′s surface. They, however, are of global importance since they house one-half of all species, approximately 70% to 80% of all tree species, and store and process 40% of all biomass carbon found in the world. The tropical biome is therefore a critical component in the global carbon cycle. Despite this, less attention is given to tropical forests in relation to global climate change compared to high latitude regions. Since the mid-1970s a synchronous warming of 0.26℃ per decade has occurred in tropical forest regions around the world. IPCC has predicted a subsequent warming increase from 1℃ to 4℃ in all tropical forest regions by the twenty-first century. A series of prominent changes have already occurred in the structure and dynamics of tropical old-growth forests and their related biodiversity composition during the last decades of the twentieth century. Based upon numerous monitoring data taken from a long-term study plot within Amazonia, Phillips O. L., Lewis S. L., Baker T. R., and Malhi Y. et al. have shown that considerable increases have taken place with respect to stem density and basal area as well as an obvious acceleration in stem turnover (recruitment and mortality) in tropical forests since the 1980s. Phillips and Gentry and Plillips et al. provided the credible original data concerning growth and dynamic increases for pantropical trees. Subsequently, Baker proved that the diameter biomass (≥10 cm) of trees in Amazonian forests have increased by 1.22 Mg hm-2a-1 and converted the data to C stocks (=0.62 Mg C hm-2a-1). Moreover, Lewis et al. measured and analyzed stand-level changes within monitoring plots across South America applying six key ecosystem processes that spanned the years 1971 to 2002. These key ecosystem processes were stem recruitment, mortality, turnover, basal area growth, loss, and turnover. They found a significant increase of 0.10 m2 hm-2a-1 in basal area and 0.94 hm-2a-1 in stem density between the first and second monitoring periods. The above data, originating from varied and long-term study sites, shows the wide-ranging increases with more reliability and facility. Phillips et al. established a total twentieth century neotropical forest sink estimate of 0.79Pg C a-1. After taking Baker’s Amazonian forest data of 0.62 Mg C hm-2a-1 into account, the combined twentieth century tropical forest sink total they came up with was 1.60 Pg C a-1. They also predicted that tropical forest systems will remain at a level of a moderate carbon sink at the present time and in the decades to come, attributing these increases to the fertilization of elevated CO2 concentrations. Conversely, Clark and Wright cited the observational data taken from long-term plots in Panama, Costa Rica, etc., and concluded that little change has occurred in the growth of tropical forest systems. In addition, numerous experimental studies were carried out in laboratories and in the field by Krner and Würth et al to examine the impact of elevated CO2 on the growth of tropical forest plants. In most cases results showed no increase in biomass and only slight increases in leaf and branch TNC (total nonstructural carbohydrate) under elevated CO2 concentrations. As CO2 concentrations within the atmosphere increase, higher temperatures and droughts will lead to more serious negative impacts on tropical forest systems such as an overall decrease in forest growth and an increase in plant mortality as well as an increase in forest fire severity. El Ni o events worsen droughts and forest fires and lead to a dieback in forest systems and a peak in tree mortality. Meanwhile, human introduced changes in forest-land utilization and deforestation will intensify over time. Under the combined natural and anthropogenic effects taking place, the carbon sink of tropical forests will inevitably transform into a carbon source, potentially a vast carbon source. Phillips and Lewis et al. have provided a predication of future tropical sinks as follows: Mature Amazonian forests will either (i) continue to be a carbon sink for decades to come or (ii) soon became a neutral or a small carbon source or (iii) became a mega-carbon source in the near future. A switch from a moderate carbon sink to a neutral or a moderate carbon source would have repercussions on the global climate system and, consequently, human welfare given that a 0.4% annual increase in Amazonian forest biomass roughly compensates for the entire fossil fuel emissions of Western Europe (or the deforestation in Amazonia). An approximate 0.4% annual sink represents the difference between two much larger values: stand-level growth (averaging approximately 2%) and mortality (averaging approximately 1.6%). As a result, either a small decrease in growth or a small increase in mortality could shut a sink down. The future of tropical forest C sinks will therefore depend upon changes in land-use, extended forest fragmentation, and an increase or decrease in forest fire activity.
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