人为干扰对浑善达克沙地榆树疏林的影响

IMPACTS OF HUMAN DISTURBANCE ON ELMS-MOTTE-VELDT IN HUNSHANDAK SANDLAND

榆树 (Ulmuspumila)疏林灌丛草原是浑善达克沙地植被演替的顶极群落。但近年来 ,榆树数量减少、树龄老化严重。 2 0 0 1～ 2 0 0 2年 ,在浑善达克沙地榆树疏林集中分布地区 ,选取了轻度 (LD)、中度 (MD)和过度 (HD) 3个不同干扰强度的地点进行实地调查。分析了浑善达克沙地榆树数量减少、树龄老化与人类经济活动干扰的关系。结果表明 :1)当地居民用于修建围栏而砍伐了当地大约 30 %的成年榆树 ,这些榆树的树龄多在 30～ 70年之间 (胸径10～ 30cm) ,致使当地该树龄段的榆树大量消失 ;2 )牲畜啃食导致榆树幼苗或幼树失去生长的机会 ,造成榆树幼苗和幼树的密度下降 ,由轻度干扰地段 (LD)的大于 6 0 0株·hm-2 下降到干扰过度 (HD)的不足 10 0株·hm-2 ;3)由于人类经济活动的过度干扰 ,植被盖度降低 ,榆树种子失去了萌发所需要的位置稳定性 ,使成苗概率减小。为了保护和恢复榆树疏林在浑善达克沙地生态系统中的重要地位 ,通过分析 。

There are many elm (Ulmus pumila) distributed on the fixed sand dune in Hunshandak Sandland. Elms_motte_veldt is the climax community of vegetation succession in this region. Recently, the quantity of elms decreased sharply, especially for the young elms and seedlings, and the elms_motte_veldt vegetation was severely destroyed. In order to reveal the reason for these changes, three typical sites in the centre of Hunshandak Sandland containing elms were investigated during August 2001 and August 2002. These sites represented 3 levels of human disturbance: light disturbance (LD) in a ley of no pasturing for ten years during the plant growth season, moderate disturbance (MD) in a ley of no pasturing for two years, and heavy disturbance (HD) in a ley of over_pasturing for 50 years. The relationship between age structure of elms_motte_veldt and the degree of disturbance was analyzed to evaluate the impacts of different factors. The results indicated that: 1) about 30% of adult elms had been removed by the inhabitants to enclose their grassland, and those elms were 30-70 years old, which is the reason why the adult elms decreased sharply; 2) there were no enough fodder for livestock in the heavily disturbed grassland and they had to graze seedlings and young elm trees, resulting in the sharp decrease in the quantity of seedlings and young elm trees. The density of seedlings and young elm trees decreased from more than 600 plants·hm -2 (LD) to less than 100 plants·hm -2 (HD); 3) because of heavy human disturbance, the vegetation coverage decreased sharply, and the elm seeds lost the stability for sprouting, so almost no seeds could germinate. It could be concluded that: the human disturbance rather than climate disturbance caused the retrogression of elms_motte_veldt vegetation. Based on our analyses, it was unadvisable to plant exotic trees, such as Populus×xiaozhuanica and Pinus sylvestris var. mongolica Litv in Hunshandak Sandland to prevent dust storms. On the contrary, the native trees of U. pumila should be protected and planted during vegetation restoration of Hunshandak Sandland.