The impact of earlier flood recession on metacommunity diversity of wintering waterbirds at shallow lakes in the middle and lower Yangtze River floodplain

The hydrological regime in wetlands plays an important role in the process of wintering waterbird metacommunity assemblage.However,increasing frequency of extreme climate and the intensification of human activities,such as the construction of sluices and dams,have resulted in frequently abnormal hydrological regime in the middle and lower Yangtze River floodplain.In recent years,earlier flood recession has become one of the main hydrological problems faced in the shallow lakes,having a great impact on wetland biodiversity.It is necessary to understand the impact of earlier flood recession on waterbirds,an indicator of wetland biodiversity,and the metacommunity concept is helpful to elucidate the underlying mechanism involved in the processes of assemblage by waterbird communities.In this study,we surveyed the wintering waterbirds at three sub-lakes of Caizi Lakes during 2019-2020 and 2020-2021 and compared the richness,abundance,alpha and beta diversity of waterbirds in and among local metacommunities under earlier flood recession and normal hydrological regime.The results showed that the earlier flood recession reduced the species richness in the early stage and abundance in the late stage,it also reduced the Shannon-Wiener index in the early stage and increased the dissimilarity between and within waterbird metacommunities in the late stage.The partition of beta diversity showed that the turnover component played a major role in the process of waterbird metacommunity assemblage.It was found that the earlier flood recession reduced the richness,abundance in different stages of flood recession,which also increased the turnover of waterbirds.Metacommunities with high habitat heterogeneity had better resistance to abnormal hydrological regime,which resulted in high dissimilarity between and within metacommunities.The results of this study provide important information for waterbird conservation and water level management at shallow lakes in the middle and lower Yangtze River floodplain.

Effects of Rainfall and Underlying Surface on Flood Recession——The Upper Huaihe River Basin Case

The effects of rainfall and underlying surface conditions on flood recession processes are a critical issue for flood risk reduction and water use in a region.In this article,we examined and clarified the issue in the upper Huaihe River Basin where flood disasters frequently occur.Data on 58 rainstorms and flooding events at eight watersheds during 2006–2015 were collected.An exponential equation(with a key flood recession coefficient)was used to fit the flood recession processes,and their correlations with six potential causal factors—decrease rate of rainfall intensity,distance from the storm center to the outlet of the basin,basin area,basin shape coefficient,basin average slope,and basin relief amplitude—were analyzed by the Spearman correlation test and the Kendall tau test.Our results show that 95%of the total flood recession events could be well fitted with the coefficient of determination(R2)values higher than 0.75.When the decrease rate of rainfall intensity(Vi)is smaller than 0.2 mm/h2,rainfall conditions more significantly control the flood recession process;when Vi is greater than 0.2 mm/h2,underlying surface conditions dominate.The result of backward elimination shows that when Vi takes the values of0.2–0.5 mm/h2 and is greater than 0.5 mm/h2,the flood recession process is primarily influenced by the basin’s average slope and basin area,respectively.The other three factors,however,indicate weak effects in the study area.

The influence of cultivation frequency on weed species composition and diversity in flood recession farming in the Okavango Delta, Botswana

Introduction:Flood recession farming,locally known as molapo farming,is an important livelihood activity for the subsistence farming community around the Okavango Delta.The study was aimed at investigating the influence of cultivation frequency on non-crop species composition.It was carried out in the peripheral settlements of Shorobe,Makalamabedi and Lake Ngami.Methods:Vegetation sampling was conducted from March to April 2016 in 36 fields of varying historical cultivation frequencies.The cultivation frequencies were 5,10 and 15 years with an uncropped control site.Sampling was done in 1-m^(2) quadrats placed 10 m apart along a line transect.Shannon’s diversity and equitability indices were used to compare diversity for each cultivation frequency within and across the study sites.The Kruskal–Wallis test was used to investigate significant differences in diversity between cultivation frequencies.Rank of abundance was used to determine the association of weeds with the cultivation frequencies.Results:Cultivation frequency did not influence species diversity(p>0.05)at either Lake Ngami or Shorobe.Species evenness was significantly different(p<0.05)between the cultivation frequencies at Shorobe,but not at either Makalamabedi or Lake Ngami.There was a significant difference(p<0.05)in diversity indexes across the four cultivation frequencies.The 5-year frequency recorded the highest median score(Md=1.352)while the 15-year frequency recorded the lowest median score(Md=1.035).Corchorus tridens was the most abundant noncrop species at the 5-and 15-year frequencies.Cynodon dactylon was most abundant in the 10-year frequency while Cyperus esculentus was most abundant in uncropped fields.Conclusions:Cultivation frequency influenced weed species diversity and composition in flood recession farming.Flood recession farming fields were dominated by common weed species which are also troublesome in dryland arable farming.Uncropped sites were composed of wetland species that tolerate some dry periods or seasonal flood plains.