黄土高原塬区的植被属性探讨

Vegetation attributes over tableland areas in the Loess Plateau

塬区作为黄土高原上引人注目的重要人工植被区,有关森林的问题多有不同的论述。为厘清事实而达到明辨,在分析植被地带性环境的基础上,以人工林直接证据获取为重点,采取直接证据与间接证据相结合的方法进行考证。着重于显域生境中龄林、成熟林,尤其成熟林的渭北塬地人工林调查,由区别于疏林的林冠郁闭程度(多数在0.6以上)、持续稳定生长的过程(如从幼龄林起始终维持着较高的郁闭度)、特殊森林环境的形成(以林下特征灌木茅莓(Rubus parvifolius)以及层间植物杠柳(Periploca sepium)等的出现为反映)、大径材生产(林分的平均胸径可达14.36 cm)、成材利用和良好更新(采伐后第2代萌芽林的发育)等正常立地反应,表明了塬地人工林的森林特点。尽管广为种植的树种刺槐(Robinia pseudoacacia)在阳坡条件下的生长及成林状况较差(郁闭度0.27),但耐旱树种油松(Pinus tabulaeformis)、侧柏(Platycladus orientalis)却相对较好。典型显域生境下的这种情况也符合森林地带森林生长分布的规律。除此直接说明塬地森林属性外,主要根据关联程度高的现有天然林与塬面的空间关系以及土壤类型(褐土、粘黑垆土、灰钙土等)分布进行间接推证。并非所有塬面都是草原性的,森林地带的塬面适合发育森林,不能说因长期农作就不适宜。20—30多年来苹果(Malus pumila)等经济林的发展亦表明森林地带塬面森林潜在发育的普遍性。

Loess tableland areas are the important artificial vegetation ones in the Loess Plateau.There are different discussions about their forest attribute.In this study,we emphatically took direct proofs and also combined indirect proofs with them to clarify some facts of forest attribute based on analyzing vegetation zonal environments.The forest plantation investigation stressed middle-aged and mature plantations,especially mature plantations in the Weibei tableland,which showed that the tableland plantation enjoyed forest characteristics through normal site condition.The result indicated that steadily persisting growth and canopy closure were different from the sparse woodland.Production of timber with large diameter,and utilization logging after growing into useful timber and then fine regeneration formed distinct forest environment.Although the plantation of Robinia pseudoacacia was poor in the sunny slope,drought-tolerant Pinus tabulaeformis and Platycladus orientalis were relatively well under the same condition.This case belongs to the typical zonal habitat according with the regularity of forest growth and distribution in the forest zone of the Loess Plateau.Besides the direct explanation of the tableland forest attribute,the reliable indirect inference was mainly based on the spatial relationship among the present natural forest,the tableland top surface,and the distribution of soil types.Those results showed that not all the tableland was dominated by steppe.The tableland top surface in the forest zone of southeastern Loess Plateau was suitable for the development of the forest.Thinking about unsuitability for forest growth owing to long-term crop cultivation in history could be wrong.Over last twenty to thirty years or more,wide expansion of the economic forest largely consisting of apple trees has showed that there would be the potential universality that the forest might grow up on any tableland top surface in the forest zone of the Loess Plateau.