Community Characteristics Of Willow Bay And The Dynamic Mechanisms Of Succession In The Mu Us Sandy Land

This study used quantitative ecological,statistical and geostatistical methods through field investigations and indoor experiments,and revealed the floristic characteristics,community characteristics,and dynamic mechanisms of succession of Willow Bay communities in the Mu Us Sandy Land.This will provide a basis for the protection and utilization of Mu Us interdunes.The main results are as follows.1.Floristic characteristics of Willow Bay in the Mu Us Sandy Land(1)A total of 126 vascular plant taxa(including subspecies),belonging to 34 families and 84 genera,were recorded.Gramineae contained the highest number of species,followed by Compositae and Leguminosae,which accounted for 42.4% of all species in the research area.At the genus level,Artemisia L.had the most species,followed by Carex L.and Juncus L.(2)The plant life-form spectrum was dominated by perennial herbs(57.2%),followed by annual and biennial herbs(33.9%).Shrubs and subshrubs accounted for 7.3% and 1.6%,respectively.The plant water ecotypes spectrum were dominated by mesophytes(58.9%),followed by xerophytes(22.6%)and hygrophytes(16.9%),with hydrophytes constituting the smallest proportion(1.6%).(3)The floristic geographical elements were dominated by temperate components(64.1%),followed by East Asian(13.6%),world(13.6%),and Paleo-Mediterranean components(8.7%).2.Community types of Willow Bay in the Mu Us Sandy LandAccording to the principles of physiognomic classification,the Willow Bay communities could be divided into 10 formations,27 association groups,and 94 associations,of which there were 38 associations distributed in the interdunes and 56 distributed in the dunes.This study reflected the complexity and diversity in Willow Bay communities,as well as the differences between dunes and interdunes.3.Dynamic mechanisms of the succession of the Willow Bay interdune community(1)The succession of the Willow Bay interdune community went through stages of settlement,development,maturation,and decline.The main features of the four stages are:(1)Selection of seed settlement conditions and pioneer community characteristicsSalix psammophila is the first leaf flowering in early spring and has small light seeds.Compared to Salix cheilophila,S.psammophila has an earlier period of seed maturation and seed dispersal.S.psammophila seeds spread more easily via wind and water than Hippophae rhamnoides subsp.sinensis.Moreover,S.psammophila seeds have a high yield,no dormant period,and a high germination rate and short germination time under suitable conditions.These biological and ecological characteristics have caused S.psammophila to become the pioneer species in Willow Bay communities.In the Mu Us Sandy Land,the settlement of S.psammophila seeds mainly occurs on the wet bare sandy zones in the aquatic-terrestrial areas of freshwater or lower alkaline lakes.The soil texture was rough,the ratio of coarse to fine sand was 8:5,soil saturation conductivity(EC)varied between 29 and 40 μs/cm,pH varied between 8.7 and 9.0,and ratio of total carbon to nitrogen(C/N)varied between 6.2 and 8.6.In addition,the subdominant species and associated species were characterized by mesic or hygric forbs,sedges,and rushes,while there were no xerophytes.(2)Community development stageIn comparison with the community settlement period,the height and canopy values of S.psammophila obviously increased,while the density and cover values decreased.During this period,the herbaceous layer was still dominated by mesic or hygric forbs and sedges and rushes,but no xerophytes.The soil texture became finer,ratio of coarse to fine sand was 3:2,and soil moisture content,EC,and C/N values gradually increased.(3)Community maturation stageDuring this period,the community cover and height significantly increased.In addition,S.cheilophila and H.rhamnoides subsp.sinensis increased and were locally established.The herbaceous layer was dominated by xeric or mesic axial-rooted forbs and rhizome grasses.The soil texture became finer,ratio of coarse to fine sand was 1:2,and soil moisture content,EC,pH,and C/N values continued to increase gradually.(4)Community decline stageDuring this period,the community cover and height significantly decreased.The shrub layer was dominated by S.cheilophila,and the number of S.psammophila and H.rhamnoides subsp.sinensis decreased.The herbaceous layer was dominated by xeric or mesic axial-rooted forbs and rhizome grasses,xerophytes significantly increased and salt-tolerant plants appeared,such as Cleistogenes squarrosa,Artemisia ordosica,Puccinellia hauptiana,and Triglochin maritima.The community habitat developed into two extremes: deserts and salt marshes.(2)Maintenance mechanism of the Willow Bay community in comparison with the characteristic of adjacent communities in the dune compound ecosystem(1)The maintenance mechanism of the Willow Bay communityAs the community develops,Willow Bay is engulfed by mobile dunes and cannot survive due to the lower groundwater levels.In the lower part of the old windward slope,new blowout pits and moist environments are established,which form the new Willow Bay settlement area and reopen a cycle again.With the movement of dunes,the old Willow Bay community gradually becomes the dominant species on the leeward slope.In relatively dry sandy environments,hygrophyte and perennial mesic herbs are gradually replaced by perennial xeric herbs or annual herbs,which become the main herbs in the leeward slope forest.Meanwhile,the original community on the leeward slope becomes a windward slope plant community and its cover becomes sparser because of the harder soil and drier environment of the windward slope.Due to lower dune mobility and accumulation of precipitation,the interdune community of the Willow Bay decline stage is replaced by a saline meadow or swamp.However,these environment will eventually be engulfed by dunes,forming new conditions for Willow Bay settlement.(2)Willow Bay communities at different developmental stages compared with adjacent communitiesWillow Bay grows in areas of good permeability that are relatively humid.In dune and interdune compound ecosystems,the soil mainly consist of coarse and fine sand,and the proportion of silt is relatively low throughout the different developmental stages.Therefore,the environmental and species compositional characteristics at the earlier stages are similar to those communities of Agrostis divaricatissima and Calamagrostis pseudophragmite,which prefer moderately humid and saline soil conditions in wetland habitat types.With the increase in fine sand,clay and salinity,the environmental and species compositional characteristics at the later stages are similar to some communities that are tolerant to salt and drought,such as P.hauptiana,Achnatherum splendens,Iris lactea var.chinensis,A.ordosica.Concerning the spatial distribution,these species can replace each other in response to developments in the community,changes in the environment,and external disturbances.4.Protection of Willow BayWillow Bay is a unique ecosystem in the Mu Us Sandy Land,and its development is an important indicator for the health of the area.In the Mu Us Sandy Land,the development of Willow Bay is an important process in the evolution from mobile to fixed dunes.Willow Bay has a higher biodiversity,productivity and fertility than the adjacent communities.Therefore,it plays an irreplaceable role in preventing wind,fixing sand,improving the soil,and maintaining high biodiversity in the region.