| Wetlands are among the highest productive ecosystems,which providing tremendous ecosystem services to human being.Wetland use is the major direct driver of change to wetland ecosystem services.Climate change and anthropogenic activities are the key factors affecting wetland use.Human activities can change the wetland use directly and indirectly through altering hydrological regime of wetlands by construction of water projects.Landscape pattern will change following change of wetland use and then influence on wetland ecosystem,particularly plant diversity and nutrient cycle.Research on wetland landscape pattern changes and drivers,as well as the impacts on ecosystem services are hotspots in wetland ecology.Dongting Lake,locating at the middle and lower reaches of Yangtze River,was once the largest freshwater lake in China.Its surface water area lost nearly 50%due to the interaction of sedimentation and reclamation within a century.Because of artificial reed and poplar plantations,area of natural wetlands in the lake has been declining obviously.After the operation of Three Gorges Dam(TGD)in 2003,sediments transportation and hydrological regime changed significantly in the middle and lower reach of the Yangtze River,which cause a sharp decline in the amount of sediments deposited and altered the status of siltation within the Dongting Lake;meanwhile,water flow into the Dongting Lake from mainstream and water level during flooding season are dropping with earlier-started and longer-dated dry season To adapt to the relation change between the Yangtze River and the Dongting Lake and restore downgraded wetlands in Dongting Lake in reasonable way,key scientific problems are facing urgent need:how is landscape pattern being affected by these changes,how to distinguish the contribution of different drivers,and what are influences of wetland landscape change on the plant diversity,as well as soil carbon and nitrogen.In this study,spatiotemporal wetland landscape pattern change(WLPC)before and after the operation of the TGD in the Dongting Lake was analyzed basing on interpretation of LANDS AT images from 1987 to 2014.A generalized addictive model(GAM)was selected to qualitatively analyze the contribution of different drivers,hydrological regime and climatic factors,to WLPC.Basing the results came out;we then analyzed the impact of main direct anthropological disturbance driving WLPC,artificial poplar plantation,on plant species diversity and contents of soil carbon and nitrogen.The results showed that:(1)Wetland landscape pattern in Dongting Lake changed significantly:wet meadow and marsh land vegetation extended with open water and mudflat shrunk during the dry season.Wet meadows and reed land in East Dongting Lake(EDL)increased 4 765 hm2 and 3 840 hm2,respectively.In West Dongting Lake(WDL),poplar plantation increased to 10153.1 hm2,meanwhile,the wet meadows reduced 3 631.5 hm2.Open water and mudflats in the EDL and WDL shrunk 8 605.0 hm2 and 4 379.0hm2,respectively.The expansion of poplar plantation distributed along all the elevation gradient in WDL,happening at reed marshland,wet meadow,mudflat and shallow open water area.However,in EDL,poplar plantation only distributed in higher elevation areas,where were dominated by reed marsh.(2)The results of GAM showed that on annual time scale,the timing when water level dropped to 25m at Chenglingji hydro station,and winter rainfall have significant correlation with the expansion of wet meadows in EDL(p<0.001,D2=76.2%),which indicated that the hydrological regime has a significant impact on the WLPC in EDL.Meanwhile,annual time have only been found significant correlated to the expansion of the poplar plantation(p=0.234,n=29),which indicated that anthropological activities has significant impact on the WLPC in WDL,the impact of hydrological regime alternation on WLPC probably masked by strong anthropological activities.(3)WLPC in Dongting Lake have influenced significantly on plant diversity.Although plant diversity has increased in poplar plantations,it has changed the composition of wetland plant communities and is not conducive to plant community stability.The growth of xerophytic vegetation was found with increasing species richness and diversity.Plant species richness in 6-year,9-year and 13 year ditch digging poplar plantation were totally numbered from 16 to 28,including 6 species of xerophytic plants,and plant species richness in 12-year normal afforestation poplar plantation were18,including 3 species of xerophytic plants.While there were total 14 and 9 plants species number in reed marsh and wet meadow,respectively,and no xerophytic plants.Plant Shannon-wiener indexes were between 2.91 to 3.68,2.84 and 2.39 in poplar plantation woods,reed marsh and wet meadow,respectively.Ditch digging-ridge raising format of poplar plantation also caused higher species richness and diversity on ridge than in furrow and normal afforestation poplar plantation.(4)WLPC in Dongting Lake have also affected soil carbon and nitrogen content significantly.SOC and TN in 0-100cm deep soil in 12-year and 15-year normal afforestation poplar plantation was significantly lower than that of reed marsh and wet meadows(p<0.05),which showed that poplar plantations decreased the SOC and TN content of the native wetland soil,and resulted in the loss of soil carbon and nitrogen.At the same time,SOC and TN in 0-60cm deep soil in 12-year poplar plantations was significantly lower than those in 15-year poplar plantations,which showed that the pattern of normal afforestation can promote the SOC and TN with the increase of stand age.However,in ditch digging poplar plantation,SOC and TN in 0-100cm soil layers in 13-year poplar plantation were lower than 9-year and 6 year,regardless of ridge or gully,among them,the differences were significantly in the 0-60cm soil layers(p<0.05).The regularity of TN in ditch digging poplar plantation was weak than SOC,but still showed that TN in 0-60cm soil layers in 13-year poplar plantation were significantly lower than that of 9-year and 6-year(p<0.05),which showed that the pattern of ditch digging afforestation can reduce the SOC and TN with the increase of stand age.Poplar planting caused the loss of soil carbon and nitrogen.SOC and TN in 0-60cm deep soil in ridges of 13-year poplar plantation significantly tower than those of 6-year and 9-year(P<0.05),and SOC and TN in 0-100cm in 12-year normal afforestation poplar plantation also were significantly lower than those of reed marsh and wet meadows(P<0.05).Meanwhile,elder poplar plantation have promoted the accumulation of SOC and TN in 60-100cm deep soil On ridge of 13-year poplar plantation,the SOC and TN in 60-100cm were 9.57±0.09g/kg and 1.03±0.02g/kg,higher than 20-30cm deep soil(4.61±0.12 and 0.66±0.03g/kg)and 30-60cm(5.17±0.37 and 0.68±0.06g/kg).SOC and TN in 60-100cm deep soil in 12-year normal afforestation were 5.09±0.36g/kg and 0.63±0.05g/kg,higher than 20-30cm(3.76±0.31 and 0.47±0.04g/kg),and30-60cm(4.67±0.59 and 0.48±0.03g/kg).This study used GAM as quantitative tool to analyze the potential drivers of WLPC in Dongting Lake,and distinguished the different impacts of anthropogenic activities and hydrologic regime.In addition,this study revealed the impacts of special popular plantation,ditch digging-ridge rising,on plant diversity and soil carbon and nitrogen,and provides scientific data and case study support for wetland restoration in Dongting Lake. |
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