|其他摘要||In desert environments, water is the principal factor limiting the ecological processes, and precipitation and groundwater are two primary water sources for plants. Precipitation is partitioned into throughfall and stemflow by canopy during reaching the ground, which alter water infiltrition and redistribution and play a crucial role on survial and growth of plants. However, precipitaion can hardly satisfy the growth needs of plants, and groundwater is another important water source for plants. Within the current background of global climate change, significant increase in precipitation has been recorded in the arid region of central Asia over the past 50 years. In addition, in the transition region between the Gurbantonggut desert and oases, the groundwater table has fallen significantly as a result of the overexploitation of groundwater. All these changes would definitely lead to the changes of plant water usage in this region. Haloxylon ammodendron and Haloxylon persicum are the dominant species in the Gurbantonggut desert. This study aims to investigate precipitation partitioning by canopy of H. ammodendron and H. persicum; and the responses of H. ammodendron and H. persicum when throughfall and stemflow were eliminated, from the perspective of ecophysiological activities and morphological adjustment of individuals. The dynamics of water use of these two species were also studied.
The results showed that, during the growth season of 2012 and 2013, throughfall, stemflow and interception increased linearly with the increasing precipitation amount (p<0.001). Throughfall accounted for the largest percentage of total precipitation. Based on the observation from the natural precipitation, the average throughfall in H. ammodendron and H. persicum was 61.6% and 73.5% of total precipitation, respectively. The average stemflow in H. ammodendron and H. persicum was 7.3% and 5.7% of total precipitation, respectively. Interception in H. ammodendron and H. persicum was 31.0% and 20.8% of total precipitation, respectively. Stemflow occurred when total amount of precipitation was more than 0.8 mm for both shrubs. Funneling ratios of H. ammodendron were in the range of 1.72–128.91, and in the range of 0.12–122.71 for H. persicum. H. ammodendron showed greater potential than H. persicum in collecting stemflow, which means that H. ammodendron has the capacity to store more precipitation water in the deep soil layer to cope with the drought.
In order to investigate response of H. ammodendron and H. persicum to variation in precipitation partitioning (throughfall and stemflow), the experiments were carried out under natural precipitation and manipulated precipitation treatments (no throughfall, no stemflow, and no throughfall and no stemflow, respectively). During the growing season of 2011 to 2013, the branch water potential, chlorophyll fluorescence parameters and photosynthetic parameters of H. ammodendron appeared to be insensitive to precipitation changes, but the branch growth rate differed significantly among four precipitation treatments. Assimilating branches under the treatment of no throughfall, no stemflow, and no throughfall and no stemflow had slower growth or earlier defoliation than natural precipitation treatments in the middle or late growing season, especially after the large precipitation pulses. The result showed that efficient morphological adjustment of H. ammodendron individuals contributes to its maintenance of photosynthesis and acclimation to variation in water condition. During the growing season of 2012 and 2013, the branch water potential, chlorophyll fluorescence parameters, photosynthetic parameters of H. persicum showed no significant difference among four treatments, and its individual morphological adjustment appeared to be insensitive to precipitation changes. However, treatments of no throughfall and no stemflow resulted in lower branch water potential, photosynthesis capacity and the more defoliation of assimilating branches, implying that it may have an impact on growth of H. persicum. H. persicum was less sensitive to the removal of throughfall and stemflow than H. ammodendron, implying that H. ammodendron has a quicker response to the precipitation change than H. persicum.
During the growth season of 2013, we used stable oxygen isotopes to study the dynamics of water usage of two Haloxylon species and their responses to the soil water fluctuations resulted from summer precipitation. H. ammodendron growing on inter-dune and H. persicum growing on the adjacent dune crest had distinct water use patterns and were able to shift water utilization according to the upper soil water content: when the upper soil water content was abundant in early spring, H. ammodendron mainly used shallow soil water(0–40 cm) while H. persicum mainly used middle soil water(40–100 cm); when the upper soil water content was depleted in summer, H. ammodendron mainly used groundwater while H. persicum mainly used deep soil water(100–300 cm). H. ammodendron and H. persicum increased utilization of upper water sources following the relatively large summer precipitations. However, H. ammodendron mainly took up groundwater and H. persicum mainly deep soil water.
In conclusion, H. ammodendron and H. persicum have the ability to collect, store and utilize precipitation water. Both species enhanced their resistance to water deficiency, and improved precipitation use efficiency by “autonomous water collection”. Therefore, H. ammodendron and H. persicum could tolerate a certain degree of drought and climate change, and maintain the stability of communities under natural conditions. Namely, the redistribution of precipitation water by these two species is ecological important.|