论滇南西双版纳的森林植被分类

本文基于多年研究成果的总结,对西双版纳森林植被的分类、主要植被类型及其特征进行了系统归纳,并讨论了它们与世界类似热带森林植被的关系。以群落的生态外貌与结构、种类组成和生境特征相结合作为植被分类的原则和依据,可以将西双版纳的热带森林植被分类为热带雨林、热带季节性湿润林、热带季雨林和热带山地常绿阔叶林四个主要的植被型,包括有至少二十个群系。热带雨林包括热带季节雨林和热带山地(低山)雨林二个植被亚型。热带季节雨林具有与赤道低地热带雨林几乎一样的群落结构和生态外貌特征,是亚洲热带雨林的一个类型,但由于发生在季风热带北缘纬度和海拔的极限条件下,受到季节性干旱和热量不足的影响,在其林冠层中有一定比例的落叶树种存在,大高位芽植物和附生植物较逊色而藤本植物和在叶级谱上的小叶型植物更丰富,这些特征又有别于赤道低地的热带雨林。热带山地雨林是热带雨林的山地亚型,是该地区热带山地较湿润生境的一种森林类型,它在植物区系组成和生态外貌特征上类似于热带亚洲的低山雨林,隶属于广义热带雨林植被型下的低山雨林亚型。热带季节性湿润林分布在石灰岩山坡中、上部,在群落外貌上类似热带山地常绿阔叶林但在植物区系组成上与后者不同,它是石灰岩山地垂直带上的一种植被类型。热带季雨林是分布在该地区开阔河谷盆地及河岸受季风影响强烈的生境的一种热带落叶森林,是介于热带雨林与萨王纳之间的植被类型。热带山地常绿阔叶林(季风常绿阔叶林)是西双版纳的主要山地植被类型,它分布在热带季节雨林带之上偏干的山地生境。它在植物区系组成上不同于该地区的热带季节雨林,在生态外貌特征上亦不同于热带山地雨林,是发育在受地区性季风气候强烈影响的热带山地的一种森林植被类型。     

滇南热带季雨林的一些问题讨论

通过分析云南南部的水、热条件及植被分布,讨论了季雨林植被类型及其特征,认为云南南部的季雨林是介于热带雨林与萨王纳之间的,在干季基本上是落叶的一种森林植被类型,符合Schimper (1903)定义的热带季雨林植被,并考虑它是一种生态学意义上的经向地带性植被,与该地区的纬向地带性植被热带季节雨林一起共同构成云南南部的水平地带性植被。在云南南部的石灰岩山坡分布的过去被认为是季雨林的森林植被,尽管也受到季节性干旱的影响而不同程度地具有落叶成分,但它在群落外貌上与典型的季雨林不相同,在植物区系组成上也明显不同于该地区非石灰岩山地的季风常绿阔叶林,在分布上亦是在石灰岩低山沟谷的热带季节性雨林水平地带性植被带之上,根据其生态外貌、植物区系组成和生境特点,我们建议用“热带季节性湿润林”来称呼这类石灰岩山地森林类型,在性质上属东南亚热带北缘石灰岩山地垂直带上的一种植被类型。     

中国热带雨林的群落特征、研究现状及问题

中国的热带雨林主要分布在西藏东南部、云南南部、广西南部、台湾南部和海南岛,它们具有与亚洲热带季风气候地区其他热带雨林类似的群落结构、生态外貌和物种多样性,是亚洲热带雨林的北缘类型。由于发生在热带季风气候地区北缘,受到季节性干旱和热量不足的影响,它们在植物区系组成上缺少典型的热带分布属种;在生态外貌上,低地的热带雨林的林冠层中具有一定比例的落叶树种,大高位芽植物和附生植物比例相对较低,而藤本植物和叶级谱上的小叶植物更为丰富;与热带亚洲非季节性气候地区的低地热带湿润雨林有一定区别,被称为热带季节性雨林。中国的热带季节性雨林在西南部与热带山地的常绿阔叶林交错过渡,在南部与亚热带常绿阔叶林交错过渡。通过比较,云南与广西的热带季节性雨林在群落结构和生态外貌特征上最接近;云南含龙脑香科植物的热带季节性雨林尽管分布海拔偏高(可分布到海拔1 100m,最高可达1 300m),但体现了低地热带雨林的群落结构特征,并在植物区系组成上具有最高比例的热带亚洲分布属种。中国不同地区对热带雨林研究的文献在对其界定、分类、描述及生态外貌特征和树种丰富度等方面的记载都不尽相同。该文简要评述了中国热带雨林的群落特征、研...     

广东季雨林的几个问题

 季雨林是分布在具有明显干湿季节变化热带地区,在干季或多或少,甚至全部落叶的森林植被;是介于热带雨林向热带稀疏林过渡的居间类型,而不是由热带雨林向亚热带常绿阔叶林过渡的植被类型。应归属于经向地带性植被,而非纬度地带性植被。并且,在广东南部沿海地区的气候条件下,也不会有季雨林发育。广东南部沿海地区的榕树+香蒲桃+野苹婆(Ficus microcarpa+Syzygium odoratum+Sterculia lanceolata)群落等类型不是季雨林,而是热带季节雨林,属广义的热带雨林范畴。     

中国西南地区热带季节雨林及山地常绿阔叶林热值及能量分配格局

能量分配格局是研究生态系统能量流动的基础,但是由于热带森林结构的高度复杂性和物种多样性,对它的热值和能量分配格局的全面研究还很少。该文研究的热带季节雨林位于西双版纳,是分布于热带亚洲北缘的一种森林类型;山地常绿阔叶林位于云南省中部的哀牢山,属于我国西部亚热带地区的山地常绿阔叶林。该研究的目的是探讨这两种重要森林类型的热值和能量分配格局,验证Golley(1961,1969)提出的世界范围内植被的热值由低纬度向高纬度、由低海拔向高海拔升高的规律。热值的测定采用SDCM-Ⅲa氧弹测量仪。两个森林样地面积都是1 hm²,能量分配格局及年固定量根据生物量和生物量增量计算。研究结果表明,热带季节雨林样地的热值低于山地常绿阔叶林,乔木层的热值>灌木层>草本层,所有器官中叶片的热值较高。由于以前种植砂仁(Amomum villosum)的影响,热带季节雨林样地的能量现存量小于山地常绿阔叶林,但是因为地处高温高湿、光照充足的地区,热带季节雨林的能量年固定量高于山地常绿阔叶林。对于热带季节雨林样地来说,97%的能量储存在乔木层中;山地常绿阔叶林样地的乔木层储存了88%的能量,可见乔木层是维持森林能量结构的关键层。研究结果为Golley的结论提供了更加丰富的实验证据。     

西双版纳不同海拔热带雨林凋落量变化研究

在3个海拔梯度(600、1 100和1 600 m)选取8块热带雨林样地,研究了印度季风环境下西双版纳热带季节雨林和山地雨林的凋落动态随海拔的变化及其与气候的关系。在3个梯度上,年平均温度分别为22.1、20.1和16.6 ℃,年平均温度递减率为-0.005 3 ℃·m^-1。随海拔增加,年平均降雨量增加(分别为1 532、1 659和2 011 mm),但旱季的降雨量基本相同(282~295 mm);年蒸发量变化较小(分别为1 369、1 374和1 330 mm);年平均空气相对湿度降低(分别为86%、81%和84%),旱季后期湿度降低更明显;样地土壤含水显著增加。热带季节雨林凋落量(1 072~1 285 g·m^-2·a^-1)显著高于热带山地雨林凋落量(718~1 014 g·m^-2·a^-1)。凋落量和凋落进程变异系数与海拔之间存在线性显著负相关,凋落量与温度线性显著正相关而与降雨量显著负相关。旱季凋落高峰受到空气相对湿度和土壤含水量影响,随海拔增加空气相对湿度降低使得海拔1 105~1 720 m的凋落高峰提前,但土壤含水量继续增加又会使凋落高峰推后。研究结果得出:1)热带季节雨林凋落量与东南亚热带潮湿雨林相近;2)旱季水分限制随海拔增加而变化,影响凋落高峰出现时间;3)随海拔增加,热带山地雨林凋落年进程由季节性向平稳过渡。     

西双版纳热带季节雨林的研究

本文以多个样方的资料分析,从群落综合特征上研究了西双版纳热带季节雨林的特点和类型、物种组成、生活型谱、叶级谱等。根据上层标志树种、群落生态外貌特征和生境把西双版纳的热带季节雨林区分为低丘雨林和沟谷雨林二个群系组,各包括若干群系。低丘雨林中面积最大和最有代表性的群系是大药树、龙果林,而沟谷雨林中最有代表性的群系是番龙眼、千果榄仁林。二者相比,低丘雨林的物种多样性要小一些,附生植物相对少一些,小、矮高位芽植物和小叶、落叶树种比例稍高,在生态上向季雨林和热带山地的常绿阔叶林过渡,有更强的地方代表性。沟谷雨林则更接近典型的湿热带雨林。     

“西双版纳热带山地雨林的植物多样性研究”的一些问题讨论

该文针对“西双版纳热带山地雨林的植物多样性研究”论文中存在的一些问题进行了讨论。原文所依据的6个调查样地, 从其分布海拔、生境、群落的生态外貌特征、植物区系组成及单位面积植物种数的统计上反映出它们并非都属于同样的植被类型,即原文所称的热带山地雨林,而可能分别属于《云南植被》中所应用的热带季节雨林的次生林群落(样地I和II)、季风常绿阔叶林群落(样地V和VI)及类似于苔藓常绿阔叶林(样地III和IV)的群落类型。由于这些样地代表了不同的植被类型, 导致在对这些样地的植物多样性特征的比较上出现较大差异。对原文在资料分析和与其它森林群落植物多样性的比较上存在的一些问题也作了讨论。原文的研究结果显示了西双版纳不同海拔高度上森林植物群落的物种多样性特征,但并不能全部运用于该地区的热带山地雨林这一特定植被类型。     

云南热带森林植被分类纲要

热带森林是云南的重要植被类型,虽对云南热带森林植被的系统分类及研究在《云南植被》中有所体现,并发表了大量论文,但在植被类型的划分、命名及其解释上却不尽相同,特别是对群系的记录很不全面。该文总结已掌握的研究资料,参考世界类似热带森林植被的研究成果,系统地对云南热带森林植被的类型、分类、物种组成和群落特征等进行了归纳。结果表明:云南的热带森林植被包括3个主要的植被型,即热带雨林、季雨林、季节性湿润林。云南的热带雨林是在亚洲热带北缘季风气候下发育的在水分、热量和分布海拔上均到了极限条件的热带雨林类型,分为热带季节性雨林和其山地变型—热带山地雨林二个植被亚型,前者包括19个群系,后者包括12个群系。文中的热带季雨林是依据其原始定义指介于热带雨林与萨王纳之间的一种热带落叶至半常绿森林植被,其分布主要受水分条件制约,非纬向地带性植被;它们分为落叶季雨林和半常绿季雨林2个植被亚型,前者包括7个群系,后者包括3个群系。把分布于云南南部石灰岩山地的在一些植物学文献中称之为"石灰岩季雨林"的热带森林植被,考虑其常绿至半常绿的外貌特征、特殊的植物区系组成以及乔木层没有一个明显的无叶(落叶)期(其落叶现象可能主要由于局部生境的干燥和历史的原因,并非是由于地区性气候干旱所导致),它们与季雨林的原始定义不符;该文使用名称热带季节性湿润林来定义这类发生在石灰岩山中、上部,在热带季节雨林带之上的热带山地垂直带上的植被类型。季节性湿润林也分为季节性常绿湿润林及季节性半常绿湿润林2个植被亚型,各包括2个群系。     

海南霸王岭不同森林类型附生兰科植物的多样性和分布

附生兰科植物是热带林附生植物的主要类群之一,对于维持热带林生态系统的物种多样性及生态功能具有重要的作用。以海南岛霸王岭国家级自然保护区内的6种热带原始林类型(热带季雨林、低地雨林、热带针叶林、山地雨林、山地常绿林及山顶矮林)中的附生兰科植物为研究对象,通过样带调查(每个森林类型设置12个10m×50m的样带,记录每个样带内胸径(DBH)≥5cm的树木及藤本上附生兰科植物的物种名称、株数及附生位置)分析了附生兰科植物的物种多样性、附生位置及其在不同森林类型中的分布规律。结果表明:1)3.6hm2森林调查样带内共记录到附生兰科植物9634株,分属于26属60种;2)除趋势对应分析(DCA)结果表明,6种森林类型中的附生兰科植物可分成5组(其中,山地常绿林与山顶矮林内的附生兰科植物归为一组);3)分布海拔范围相临近的森林类型的附生兰科植物具有较高的相似性,山地常绿林和山顶矮林附生兰科植物的相似性最高(88.9%);4)6种森林类型中,较高海拔的3种森林类型(山地雨林、山地常绿林和山顶矮林)中,附生兰科植物的丰富度和多度均显著高于其在较低海拔的3种森林类型(热带季雨林、低地雨林和热带针叶林),其中,附生兰科植物在山地常绿林内的丰富度和多度均最高;5)热带季雨林、低地雨林、热带针叶林及山地雨林内,宿主冠区附生兰科植物的多度均高于干区;山地常绿林内两者之间无显著差异;而山顶矮林干区的附生兰科植物的多度高于冠区;6)调查木上附生兰科植物的发生率在高海拔森林类型均高于其在低海拔森林类型,各森林类型内附生兰科植物的多度及物种丰富度与宿主胸径均存在显著正相关关系。     

On the Classification of Forest Vegetation in Xishuangbanna,Southern Yunnnan

Xishuangbanna of southern Yunnan is a region of extremely interest to biologists and also a hotspot for biodiversity conservation . It is located in a transitional zone from tropical Southeast Asia to temperate East Asia biogeographically. The present paper reviewed vegetation types of Xishuangbanna and suggested a revised classification system based on theupdated study results over the last two decades . By combining physiognomic and floristic characteristics with ecological performances and habitats , the primary forest vegetation in Xishuangbanna can be organized into four main vegetation types: tropical rain forest, tropical seasonal moist forest, tropical montane evergreen broad-leaved forest and tropical monsoon forest. The tropical rain forest can be classified into two subtypes , i. e. tropical seasonal rain forest in the lowlands and tropical montane rain forest on higher elevations. The tropical seasonal rain forest in this region shows similar forest profile and physiognomic characteristics to those of equatorial lowland rain forests and is a type of world tropical rain forest. Because of conspicuous similarity on floristic composition , the tropical seasonal rain forest in Xishuangbanna is a type of tropical Asian rain forest . However , since the tropical seasonal rain forest occurs at the northern edge of tropical SE Asia, it differs from typical lowland rain forests in equatorial areas in maintaining some deciduous trees in the canopy layer , fewer megaphanerophytes and epiphytes but more abundant lianas and more plants with microphyll . It is a type of semi-evergreen rain forest at the northern edge of the tropical zone . The tropical montane rain forest occurs in wet montane habitats and is similar to the lower montane rain forests in equatorial Asia in floristic composition and physiognomy . It is a variety of lower montane rain forests at the northern tropical edges of tropical rain forests . The tropical seasonal moist forest occurs on middle and upper limestone mountains and is similar to the tropical montane evergreen broad-leaved forest of the region in physiognomy, but it differs from the latter in floristic composition. The monsoon forest in Xishuangbanna is a tropical deciduous forest under the influence of a strong monsoon climate and is considered to be a transitional vegetation type between tropical rain forest and savanna in physiognomy and distribution. The tropical montane evergreen broad- leaved forest is the main vegetation type in mountain areas . It is dominated by the tree species of Fagaceae , Euphorbiaceae , Theaceae and Lauraceae in majority. It differs from the tropical montane rain forests in lack of epiphytes and having more abundant lianas and plants with compound leaves . It is considered to be a distinct vegetation type in the northern margin of mainland southeastern Asia controlling by a strong monsoon climate, based on its floristic and physiognomic characteristics.     

Ecological and biogeographical studies on the tropical rain forest of south Yunnan,SW China with a special reference to its relation with rain forests of tropical Asia

Abstract. Ecological and biogeographic analyses of the tropical rain forest in south Yunnan were made using data from seventeen sample plots and floristic inventories of about 1000 species of seed plants. The rain forest is shown to be a type of true tropical rain forest because it has almost the same profile, physiognomic characteristics, species richness per unit area, numbers of individuals in each tree species and diameter classes of trees as classic lowland tropical rain forests. As the area is at the northern margin of monsoonal tropics, the rain forest differs from equatorial lowland rain forests in having some deciduous trees in the canopy layer, fewer megaphanaerophytes and epiphytes but more species of lianas as well as more species of microphylls. In its floristic composition, about 80% of total families. 94% of total genera and more than 90% of total species are tropical, of which about 38% of genera and 74% of species are tropical Asian. Furthermore, the rain forest has not only almost the same families and genera, but also the same families rank in the top ten both in species richness and in dominance of stems, as lowland forests in southeast Asia. It is indisputable that the flora of the rain forest is part of the tropical Asian flora. However, most of the tropical families and genera have their northern limits in south Yunnan and most have their centre of species diversity in Malesia. More strictly tropical families and genera have relatively lower species richness and importance compared with lowland rain forests in tropical southeast Asia. Thus, the flora also shows characteristics of being at the margin of the tropics. Based mainly on physiognomy and floristic composition the tropical rain forest of Yunnan is classified into two types, i.e. seasonal rain forest and wet seasonal rain forest, the latter is further divided into two subtypes, i.e. mixed rain forest and dipterocarp rain forest. From analysis of geographic elements it is also shown that the tropical rain forest of Yunnan occurs at a geographical nexus with its flora coming mainly from four sources, i.e. Malesia, south Himalayas, Indochina and China.     

云南热带雨林:特征、生物地理起源与演化北大核心CSCD

云南热带雨林具有与东南亚低地热带雨林类似的群落结构、生态外貌特征和物种多样性,是亚洲热带雨林的一个类型。它的植物区系组成中有90%的属和多于80%的种为热带分布成分,其中约40%的属和70%的种为热带亚洲分布型,它含属种较多的优势科和在群落中重要值较大的科也与亚洲热带雨林相似,是亚洲热带雨林和植物区系的热带北缘类型。云南西南部、南部与东南部的热带雨林在群落结构和生态外貌上类似,但在南部与东南部之间有明显的植物区系分异,它们经历了不同的起源背景和演化历程。云南的热带雨林在很大程度上由西南季风维持。喜马拉雅隆升导致西南季风气候形成和加强,在云南热带局部地区产生了湿润气候,发育了热带雨林植被。现在的云南热带雨林里或其分布地区有落叶物种或热带落叶林存在,这不仅是季节性气候的影响,推测在晚第三纪或第四纪更新世云南热带地区曾经历了干旱气候。云南热带雨林的分布主要受制于局部生境,并非地区性气候条件。     

Geological History,Flora, and Vegetation of Xishuangbanna,Southern Yunnan,China1

Xishuangbanna of southern Yunnan is biogeographically located at a transitional zone from tropical Southeast (SE) Asia to subtropical East Asia, and is at the junction of the Indian and Burmese plates of Gondwana and the Eurasian plate of Laurasia. The flora of the region consists of a recorded 3336 native seed plant species, belonging to 1140 genera in 197 families, among which 83.5 percent are tropical genera and 32.8 percent are endemic to tropical Asia, suggesting a strong affinity to tropical Asian flora. The vegetation of Xishuangbanna is organized into four forest types: tropical rain forest, tropical seasonal moist forest, tropical montane evergreen broad‐leaved forest, and tropical monsoon forest. The tropical rain forest in Xishuangbanna has the same floristic composition of families and genera as some lowland equatorial rain forests in SE Asia, and is dominated (with a few exceptions) by the same families both in species richness and stem dominance. The exceptions include some deciduous trees in the canopy layer, fewer megaphanerophytes and epiphytes, and a higher abundance of lianas and microphyllic plants. We consider the tropical rain forest of Xishuangbanna as a type of tropical Asian rain forest, based on their conspicuous similarities in ecological and floristic characteristics.     

滇南勐宋热带山地雨林的物种多样性与生态学特征

 研究了鲜为人知的滇南勐宋地区的原始山地雨林植被, 根据分布生境、群落结构和种类组成特征,可将该山地雨林区分为沟谷和山坡两个类型, 分别定义为八蕊单室茱萸(Mastixia euonymoides)-大萼楠(Phoebe megacalyx)林和云南拟单性木兰(Parachmeria yunnanensis)-云南裸花(Gymnanthes remota)林。该山地雨林的外貌仍以单叶、革质、全缘、中叶为主的常绿中、小高位芽植物组成为特征,层间木质藤本植物仍较丰富,草本高位芽植物和附生植物丰富,但板根和茎花现象少见,属于热带山地垂直带上低山雨林或山地雨林植被类型。与该地区的典型热带季节雨林和赤道热带雨林相比, 勐宋的山地雨林群落中的大、中高位芽植物和藤本高位芽植物比例相对减少, 小、矮高位芽植物和草本高位芽植物比例相对增加,单叶、革质、非全缘叶和小叶比例相对增加,板根现象少见。与中国热带北缘-南亚热带地区（季风）常绿阔叶林比较, 勐宋的山地雨林有较多的附生植物和草本高位芽植物,相对较少的小高位芽植物和矮高位芽植物,小叶比例亦较少,非全缘叶和革质叶比例相对较低。故勐宋山地雨林是滇南热带北缘山地的一种较湿润生境的植被类型, 与所谓的季风常绿阔叶林不同。在物种多样性上,勐宋热带山地雨林在单位面积植物种数上并不比该地区的热带季节雨林低, 物种多样性指数与低丘季节雨林相当,比沟谷季节雨林低, 明显高于季风常绿阔叶林。     

西双版纳热带雨林与海南热带雨林的比较研究

西双版纳的热带雨林与海南低地热带雨林和热带季雨林有基本一致的植物区系组成,群落中优势科无论在种数百分比还是重要值排名上均较接近,显然属于同样性质的植物区系。在生态特征上,西双版纳热带雨林群落高大,分层不明显,B层为林冠层,散生巨树常见,在生活型谱上以高位芽植物占绝对优势,大、中高位芽植物相对较多,落叶树种比例小;以中叶、纸质,全缘和复叶比例较高为特征,具有最接近海南低地湿润雨林的群落垂直结构和生态外貌,其雨林特点虽不如湿润雨林浓厚,但明显强于海南的热带常绿季雨林和山地雨林,海南常绿季雨林群落高度明显较矮,小叶比例通常较高,革质叶比例亦较高,群落具遥明显的旱生特点,海南的山地雨林群落高度较矮,A层连续,成为林冠,无散生巨树,分层明显,在生活型谱上大高位芽植物比例减少,附生植物丰富,并具有相当比例的地面芽植物;叶级虽以中叶占优势,但通常革质,非全缘和单叶比例较高,明显由于热量不足的影响而带有亚热带森林特色,在物种多样性上,西双版纳热带雨林的乔木物种多样性指数似乎与海南的低地热带雨林相当,低于海南的山地雨林群落,海南的热带雨林群落种类丰富度不同人研究的结果差异较大,如果这些用于比较的数据可靠和具有可比性的话,西双版纳热带雨林的物种多样性要比海南的山地雨林低。     

Structural comparison of tropical montane rain forests along latitudinal and altitudinal gradients in south and east Asia

Geographical patterns of altitudinal zonation, floristic composition, and structural features of tropical montane rain forests were examined along latitudinal gradients in south and east Asia. On equatorial mountains, the tropical montane rain forests occur above 1000 m. Toward middle latitudes, they come farther down and reach sea level at c. 35° N. Thus, the forests are equivalent to the subtropical rain forests of the latitudinal, horizontal zonation series. They exhibit gradual changes in floristic composition and structure along both altitudinal and latitudinal gradients. On equatorial mountains, they are divided into three types, i.e. tropical lower montane, upper montane, and subalpine forests. The three tree regeneration types, having emergent, sporadic and inverse-J type stem-diameter class frequency distributions, coexist in the lower montane forests, but the upper and subalpine forests display only the inverse-J type species with a few species of the sporadic type. Toward the northern latitudinal limit, the distinction between the three tropical montane forest zones in equatorial mountains becomes less clear. This can be explained by temperature conditions: on equatorial mountains, a temperature sum of 85° C months which controls the upper limit of the lower montane forests, and a coldest month mean temperature of-1° C which controls the evergreen broad-leaved trees, appear at c. 2500 and c. 4000 m respectively. The altitudinal range between 2500 m and 3800 m, which is the upper forest limit, is covered by upper montane and subalpine forests. On the other hand, at the latitudinal northern limit, the tropical upper montane and subalpine forests cannot exist because the above mentioned two temperature conditions occur at nearly the same point. Thus, at the northern latitudinal limit of the tropical montane forests, the three zones of equatorial mountains amalgamate into a single subtropical lowland forest community. This is due to the seasonal temperature climate in middle latitudes in, e.g., central Japan and central China.A part of this paper was presented as an oral presentation at the Vth International Congress of Ecology, Yokohama 23–30.8.1990.