塔里木荒漠河岸林干扰状况与林隙特征

对塔里木河中游荒漠河岸林林隙基本特征和干扰状况进行了研究。结果表明：形成的林隙形状近似于椭圆形，椭圆长短轴比率在扩展林隙(EG)和冠空隙(CG)有所不同，平均分别为1.52和2.31；林隙密度约为62.5个?hm-2, EG和CG在塔里木荒漠河岸林景观中的面积比例分别为69.52%和29.03%，干扰频率分别为1.45%?a-1和0.61%?a-1,林隙干扰返回间隔期约为164a。林隙大小结构表现出以小林隙为主的偏正态分布，EG大小40—200m2，CG大小0—80m2。林隙形成速率为1.30个?hm-2a-1，20—30a前形成的林隙最多。林隙形成方式由树木折干 枯立形成的最为普遍,占形成木总数95.73%。林隙大多由2—5株形成木形成, 而由4株形成木创造的林隙最多，平均每个林隙拥有形成木4.1株。林隙形成木主要为森林建群种，林隙形成木分布最多的径级在5—25cm,高度在4—8m，每株形成木所能形成的EG面积为27.12m2, CG面积为11.32m2。边缘木的径级结构呈正态分布,而高度结构呈偏左的正态分布，平均每个林隙拥有8.375株边缘木，林隙边缘木平均胸径比形成木平均胸径高73.1%，表明荒漠河岸林林隙干扰十分频繁，地下水位的持续下降是林隙形成的驱动力。

Disturbance regimes and gaps characteristics of the desert riparian forest at the middle reaches of Tarim River

Desert riparian forests, widely distributed in the basins of inland rivers of the northwest arid regions of China, are dominated mainly by Populus euphratica Oliv. and Populus pruinosa Schrenk species. The largest and most typical desert riparian forest is found along the Tarim River Basin and occupies over 350000 km². This forest plays an active role in climatic regulation, desert expansion control, river banks protection, channels stabilization and oasis conservation. The riparian forest not only protects the fragile desert ecological system but also acts as an irreplaceable natural barrier. The Tarim River riparian forest is highly prized in the Tarim Basin for its great ecological, economical and social benefits. As a consequence of the increase in human population, an excessive exploitation of soil and water resources in upper and middle reaches of the Tarim River, induced recession, nonbearing trees and death of a large area of the desert riparian forest beginning with the 1960s. These effects directly endangered projects for development of sustainable oasis agriculture and for maintenance of an ecological balance. The objective of this study was to elucidate the characteristics of gaps in the riparian forest, the disturbance regimes and causal factors of gaps and predict a developmental trend as a theoretical base to manage and realistically protect the desert riparian forest. The P.euphratica forests of the Nature Reserve Region of Luntai County in the middle reaches of Tarim River, is the only remaining original desert forest and the center zone of distribution for P.euphratica. The research plots were located in Luntai (84°15'E, 41°09'N). The expanded gaps and canopy gaps were investigated in a 0.64hm² plot in 2009, by identifying all gap makers and gap border trees and measuring their stems. The results show that gaps in the desert riparian forest were nearly elliptical in horizontal shape, and the average ratio of long axis to short axis for each gap was about 1.52 for expanded gaps (EG) and 2.31 for canopy gaps (CG).Gap density was approx 62.5 per hectare, and the area occupied by EG and CG in desert riparian forest landscape was 69.52% and 29.03%, respectively. The results also show that the natural disturbance frequency was 1.45%/a for EG and 0.61%/a for CG, and the return interval of canopy gaps was about 164 years. For most EG and CG, whose size were in the range of 40-200m², and 0-80m², respectively. The gaps were formed within the last 50 years, most of them being 20 to 30 years old, and their formation rate was about 1.30 gap hm^-2a^-1. Most gaps were created by a combination of stem breakage and natural death (95.73% of total gaps). The majority of gaps consisted of 2 to 5 gap makers with the 4 gap makers category being the most abundant group and the average number of gap makers was 4.1 per gap. The diameter at breast height (DBH) of most gap makers ranged from 5 to 25cm, and their height varied from 4 to 8m. The average area of EG and CG formed by each gap maker was 27.12m² and 11.32m², respectively. The structure of DBH classes of the gap border trees (GBT) show a normal distribution, but tree height classes show a left-skewed distribution. The average numbers of GBTs was 8.375 per gap. The average DBH of GBT was 73.1% higher than that of gap makers, indicating the gap disturbance was a frequent occurrence in the desert riparian forest, and the continuously declining level of underground water was the driving force of gap formation.