| Coastal wetland is an ecosystem which is affected first by global climate change. In this paper, chenier wetlands in the Yellow River Delta was taken to be a case, the characteristics and mechanism of water niche spatial-temporal differentiation of three dominant shrub species(Tamarix chinensis L., Ziziphus jujuba var. spinosa Hu and Periploca sepium Bge.) were studied in this study. The stable isotopic technology was mainly used combined with traditional method, such as field locating observation and laboratory analysis, and so on. Soil particle distribution and soil salinity spatial difference were analyzed in different shrubs, and the effects of different rain pulse on the stable oxygen isotope composition in three dominant shrub species xylem water, soil water and groundwater in chenier wetlands in the Yellow River Delta were studied. The spatial- temporal changes of stable oxygen isotope in potential water sources and plant xylem water were discussed, and the water use strategies of three dominant shrub species were revealed by calculating the contributions of potential water sources to plant xylem water through Iso-Source software model. The water niche breadths of different shrub species were obtained by Levins and Shannon-Wiener formula,and the Levins overlap degree model was also applied to study the spatial–temporal variability of water niche breadth of different shrub species. The water niche differentiation characteristics of different species in the same habitat was also discussed. The main results as follows.1. Effect of precipitation on stable isotopic composition of soil water and plant xylem water in chenier wetlandThe response of ?18O values of soil water to different rainfall pulse showed difference. The ?18O values of soil water from 0-40 cm soil layers in all shrubs were sensitive to moderate rain. The response time of ?18O values of soil water from 40-100 cm soil layers in T. chinensis and Z. jujuba mixed shrubs to the precipitation was relatively later. The moderate rain had little effect on the ?18O values of soil water from 60-100 cm depth in T. chinensis shrubs, lowland and upland Z. jujuba shrubs. The ?18O values of soil water from all soil layers in P. sepium shrubs could respond to the precipitation immediately. The two light rain only had a temporary effect on the ?18O values of soil water from 0-20 cm depth. In T. chinensis and Z. jujuba mixed shrubs, T. chinensis shrubs, lowland Z. jujuba shrubs and P. sepium shrubs, the ?18O values of plant xylem water responded to the moderate rain immediately, and rapidly declined following the precipitation. However, in upland Z. jujuba shrubs, the ?18O values of plant xylem water had little response to the precipitation. The light rain had non-significant effect on the ?18O values of plant xylem water.The water use patterns of different shrubs species were different before and after precipitation. In chenier wetland, three domidant shrub species all used the soil water before the precipitation, but the main depths of their water sources were different. Fllowing the moderate rain, T. chinensis and Z. jujuba mainly use the rain water except the one lived in upland Z. jujuba shrubs, and P. sepium use a little part of rain water and its mainly water source was still soil water, while upland Z. jujuba mainly used the soil water from 60-100 cm depth. With the sampling time developing after precipitation, the main depths of water sources were significantly different between T. chinensis and Z. jujuba in mixed shrubs, the water source of T. chinensis was always from 0-40 cm soil layers in T. chinensis shrubs, and upland Z. jujuba mainly used the soil water from 60-100 cm depth during the whole process. The light rain lessly affected the water use strategies of shrub species.2. The water use strategies of dominant shrubs species in chenier wetlandThere were significant differences in the ?18O values of soil water and groundwater among growing seasons. The influence of seasonal change on the ?18O values of soil water was weakened along the soil depth. The ?18O values of soil water from 0-40 cm depth had large variation ranges and were very sensitive to the seaconal change, and the variation ranges of of the ?18O values of soil water from 0-40 cm depth were relatively smaller. The stable oxygen isotope was most enriched in the upper soil layers. Generally speaking, the oxygen isotopic values of soil water were more enriched in July and Octomber and most depleted in September. The variation trendency of ?18O values of soil water in T. chinensis shrubs was different compared to other shrubs, the oxygen isotopic values of soil water from 40-100 cm depth were more enriched compared to that in upper soil layers. The oxygen isotopic values of soil water in T. chinensis shrubs were moer enriched compared to other shrubs. The fluctuation range of ?18O values of groundwater was smaller than that of soil water in chenier wetlands.The main water sources of three dominant shrubs species showed clearly spatial-temporal difference. In T. chinensis and Z. jujuba mixed shrubs, the main water source of Z. jujuba was 0-60 cm soil water from June to August and 20-40 cm soil water in September, and 83.8% xylem water was from 0-20 cm soil water. In the same shrubs, the depth of main water source of T. chinensis was deeper than Z. jujuba. The main water source of T. chinensis was 60-100 cm soil water in June, shifted to shallow groundwater in July and August and 20-40 cm soil water in September, and 70.4% xylem water was from 60-100 cm soil water in Octomber. In T. chinensis shrubs, T. chinensis mainly used 20-40 cm soil water in June and August, 20-60 cm soil water in July, 0-40 cm soil water in September, and 0-20 cm soil water in Octomber. In lowland and upland Z. jujuba shrubs, in June, July, August and Octomber, Z. jujuba mainly used the soil water from 60-100 cm layer. In September, 70.1% xylem water of lowland Z. jujuba was from 0-60 cm soil water, and upland Z. jujuba still used the 60-100 cm soil water. In P. sepium shrubs, 83.3% xylem water of P. sepium was from 0-60 cm soil water in June, it mainly used 40-100 cm soil water in July, 0-20 cm soil water in August, 20-100 cm soil water in September, and 20-40 cm soil water in Octomber.3. Water use patterns of shrubs species in chenier wetland during wet and dry seasonsThe ?18O values of soil water in dry season were siginificantly higher than that in wet season. The vertical variation ranges of soil water ?18O values in dry season were larger than those in wet season. The difference on soil water ?18O values between wet and dry seasons decreased along the soil depth, especially in lowland Z. jujuba shrubs and P. sepium shrubs, there were non-significant difference on the ?18O values of 60-100 cm soil water between wet and dry season. In T. chinensis shrubs, the ?18O values of 60-100 cm soil water were higher than those in all shrubs on dune crest whether in wet season or dry season. The ?18O values of shallow groundwater in dry season were higher than those in wet season, and those in T. chinensis shrubs were remarkably higher than those on dune crest. The ?18O values of plant xylem water also presented significantly variations at temporal, spatial scale and among different species.In wet and dry seasons, the water use strategies of plant were difference among shrub species. In T. chinensis and Z. jujuba mixed shrubs, in wet season, T. chinensis mainly used the mixture water of 60-100 cm soil water and shallow groundwater, and Z. jujuba mainly used 60-100 cm soil water, in dry season, the main water source of T. chinensis shifted to shallow groundwater, and Z. jujuba mainly used 20-100 cm soil water. In T. chinensis shrubs, the main water source of T. chinensis in wet season was 20-40 cm soil water and shifted to 20-100 cm soil water in dry season. Lowland Z. jujuba mainly used the soil water from 40-60 cm depth in wet season, and 60-100 cm soil water in dry season. The main water sources of upland Z. jujuba were all 60-100 cm soil water in wet and dry season. The main water sources of P. sepium was 0-20 cm soil water in wet season and 40-100 cm soil water in dry season. In general, the order of the main depths of water source of three species was T. chinensis > Z. jujuba > P. sepium.4. The spatial-temporal heterogeneity in water niche of dominant shrub speciesThe water niche breadth of shrubs species showed a evidently spatial-temporal heterogeneity. In mixed shrubs, the water niche breadth of Z. jujuba was broader in June, July and August, it started to declined in September and reduced to minimum in October. In lowland Z. jujuba shrubs, the water niche breadth of Z. jujuba was lower in June, July, August and October, and it was relatively higher in September. In upland Z. jujuba shrubs, the water niche breadth of Z. jujuba was significantly lower than that of Z. jujuba in other shrubs, and there was little difference on water niche breadth of Z. jujuba among growing seasons. The water niche breadth of T. chinensis in mixed shrubs was the highest in June, which declined gradually in July and August and lightly increased in September, and declined again in October. In T. chinensis shrubs, the water niche breadth of T. chinensis was lower in June, August and October, which was the highest in July. In P. sepium shrubs, the water niche breadth of kept stable in the whole growing season, which was higher in June, July, September and October and there was no significant different among these seasons, and it decreased to the minimum in August.In wet and dry seasons, the variations of palnt water niche breadth in different shrubs were different. In mixed shrubs, the water niche breadth of Z. jujuba was lower in wet season than that in dry season, and the variation trend of water niche breadth of T. chinensis was opposite to Z. jujuba. In lowland and upland Z. jujuba shrubs, the water niche breadth of Z. jujuba had no significant difference between wet and dry season. Especially, in upland Z. jujuba shrubs, the water niche breadth of Z. jujuba almost had no difference between wet and dry season, at the same time, which was significantly lower than that of Z. jujuba in other two shrubs. In T. chinensis shrubs, the water niche breadth of T. chinensis was lower in wet season than that in dry season. The water niche breadth of was also lower in wet season than that in dry season.The water niche overlap of T. chinensis and Z. jujuba in mixed shrubs were different among growing seasons or between wet and dry seasons. Their water niche overlap was the lowest in October and the highest in September, the order of the water niche overlapping-degree from large to small was sorted as September > June > August > July >October, which indicated that two species had similar water sources as soil water was sufficient and low plant water consumption, while their water niche differentiated obviously during the season with deficient soil water. In wet season, the water niche overlap of T. chinensis and Z. jujuba was great, on the contrary, two co-existing species reduced their water niche overlap in dry season to avoid their competition for water when the soil moisture was deficient. |
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