| It has become a well-known fact that the global water resource is in extreme shortage and with severely uneven distribution. Due to the uneven distribution of water resources as well as the constraint of geographic factors, there appears an arid region of more than 30 million square kilometers all around the world. It approximately occupies 20% of the total global land area. Studies have shown that the area of arid regions in China (not including the area of semi-arid regions) largely occupies about 25% of the total land area. The arid regions in our country are relatively unique for they are largely located at the inner continent or northwestern part of China. Comparing with the arid regions in Africa, western Asia and Australia, these regions in our country are mostly surrounded by huge mountain chains. Therefore, it is very difficult for the wet vapor from sea to reach and correspondingly the climate is rather dry. Vegetation is rare in the arid plains and there essentially grows a kind of desert plant. Plant coverage is below 10% and it can reach 20% to 40% only on the sides of rivers and groundwater-suitable regions, where can grow lots of plant. But the vegetation species is limited. The relationship between plant and moisture is always a research focus in the ecological study of arid regions. Moreover, research for the growing status and morphology as well as physiology of plant, which are all subject to water, is a hot and difficult study issue in this field. This is also a basic must-face problem for the protection and reconstruction of the ecological environment in arid regions.Heihe is a typical inland river in the northwestern part of our country, the upper reaches of which is covered with forest grass, the middle reaches and lower reaches are respectively scattered by oasis farmland and oasis desert. Heihe River originates in Qilian Mountains in Qinghai Province and flows through Hexi Corridor of Gansu Province; it finally reaches the Inner Mongolia Autonomous Region Ejina, with the total length being of 821km. The lower reaches of Heihe River are a typical arid area. Although the plant species is limited here and the growing status, morphology as well as physiological characteristics of the plant extremely depend on the distribution and depth of groundwater, there still grows a globally rare desert plant-Populus euphratica Oliv. As the second largest desert riparian Populus euphratica Oliv forest, its area has withered extremely due to shortage of water resource and the influence of human activity. Now its distribution area is of 2.94 x104 hm2, which has decreased 2.06×10 4 hm2 comparing with 30 years ago. Now the Populus euphratica Oliv forest is heavily damaged by pests and diseases, the growing status of this tree is bad and its natural regeneration ability is very weak. Thus Populus euphratica Oliv faces the danger of disappearing from the region. Because Populus euphratica Oliv is a key species in maintaining the ecological balance of desert riparian forests and it is determined by FAO as the forest genetic resource needing to be protected prior to others, the protection and reconstruction of Populus euphratica Oliv is imperative. At present, our country has already taken the Ejina Populus euphratica Oliv forest as a national protection forest, which is mainly for the protection of Populus euphratica Oliv, plant communities and biodiversity. Then the status of Populus euphratica Oliv has improved. However, according to the restrictive environmental factor (groundwater), we still know little about how to effectively distinguish the growing status of Populus euphratica Oliv, how to make reasonable rehabilitation and reconstruction as well as how to scientifically predict the changes of Populus euphratica Oliv in the future. Therefore, this article took natural Populus euphratica Oliv of Ejina as its study object and analyzed the growing status, the morphology of leaves as well as the physiological characteristics of Populus euphratica Oliv in different groundwater levels. We hope that our study can provide a scientific basis for the protection and reconstruction of Populus euphratica Oliv. The main conclusions of this study are as follows:1. As to different groundwater levels with different degrees of water stress, the growing status of natural Populus euphratica Oliv are quite different. The population density (PD), ratio of died branches to full branches (RBF), the crown and height of Populus euphratica Oliv all vary as the groundwater changes. Generally speaking, the growing status of Populus euphratica Oliv can be divided into 3 levels:(1) population density (-700-～200tree/hm2) is rather high where the groundwater table is less than～3 m, and ratio of died branches to full branches is very low (<～10%). This groundwater level forms the optimum wet conditions for for Populus euphratica Oliv to grow and develop. (2) the density(～200-～50 tree/hm2) is much reduced where the groundwater table is between～3 and～5 m, and ratio of died branches to full branches is promoted (～10%-～60%). At this time, water stress is happened and this groundwater table is appropriate to growth and development of Populus euphratica Oliv. (2) the density(<～50 tree/hm2) is very low when the groundwater table is beyond～5 m, and ratio of died branches to full branches is very high (-60% -～100%). This groundwater table is not suitable for the growth and development of Populus euphratica Oliv.2. The morphological and physiological parameters of Populus euphratica Oliv were significantly changed with different groundwater tables. In this study, three leaf-related morphological and physiological parameters of Populus euphratica Oliv are stomatal density, specific leaf area and stable carbon isotopic composition. Both of them can indicate different different groundwater tables.3. Variation of stomatal density (SD) of Populus euphratica Oliv can indicate different groundwater tables. salt stress dictates leaf stomatal density (SD) of shallow groundwater conditions (less than～2.7 m) and that water stress dictates leaf stomatal density (SD) of deep groundwater conditions (larger than～2.7 m), the narrow zone around～2.7 m probably being a stress-free zone (neither salt stress nor water stress). Within the water-stressed portion (from～2.7 to～8.5 m) or the bell-shaped portion, SD climbing segment (～2.7 -～3.7 m) is leaf-area controlled and the declining segment (～5.2 -～8.5 m) is stomatal-number controlled with the plateau segment (～3.7 -～5.2 m) being the transition.4. Variation of specific leaf area (SLA) of Populus euphratica Oliv can indicate different groundwater tables (water stress). Specific leaf area (SLA) decreases exponentially with increasing groundwater tables. Our research show that specific leaf area (SLA) is more sensitive to groundwater tables when the table is shallow and～3 m of groundwater table seems to be a threshold where SLA becomes less sensitive to groundwater table. 5. Variation of stable carbon isotopic composition (d13C) of Populus euphratica Oliv can also indicate different groundwater tables (water stress). The d13 is also strongly dependent on groundwater table. Our research show that stable carbon isotopic composition (d13C) becomes more sensitive when the groundwater table is deep and-7 m of DG seems to be a threshold where the d13C signature becomes more sensitive to DG.6. In the preparation and calculation of stomatal density, this study has inherited the traditional ways of adhesive cellophane tape emulated and microscopic photographed method, it also made the creative use of remote sensing image processing technology (object-oriented classification) to calculate the Populus euphratica Oliv leaves stomatal density. We used the professional object-oriented classification software (eCognition) to give a multiscale segmentation for the stomatal images. Then we imported the generated classification images into the ArcGIS software to calculate stomatal density. Finally, we made a batch program using R language to deal with a lot of photos'data. Using macro-image processing technology to solve micro-problems, and achieve the effect of fast processing speed, high precision calculation.
|
PDF Full Text Request