Characteristics Of Plant Colonization And Population Recruitment On Loess Shallow Landslides

Landslides are a severe form of disturbance to slope ecosystems.The objectives of our study were to characterize plant colonization and population recruitment on loess shallow landslides under the semi-arid climate of the hilly-gullied region of the Loess Plateau,which can provide theoretical support for vegetation restoration on landslides and other degraded land in this region.Therefore,30 landslides triggered by the continuous rainstorms in Ansai County in July 2013 were selected,and the surrounding undisturbed vegetated slopes were considered as the main propagule source areas for landslides and thus used as control areas.Through investigation,monitoring and sampling in the field as well as observation and measurements in the laboratory,we characterized soil and plant communities of the landslides and their main propagule source areas;through species-and trait-based approach,we quantified changes in soil properties and plant community properties after landslides,analyzed soil-plant interactions in the process of plant colonization on the landslides,and also estimated the restoration potential of landslides and the resistance of slope ecosystems to landslide disturbance in this region;through field monitoring,we characterized seedling and ramet recruitment on both south-facing and north-facing landslide scars in terms of their temporal dynamics,their relative contributions to population recruitment and their levels,estimated the coupling in species composition between the adult and the young layers of plant communities,and also predicted future restoration progress and succession of plant communities on landslide scars.The main conclusions are as follows:1.Soils of scars,deposits and control areas were almost loamy soils,being composed of more clay particles;soils of scars and deposits were more fine-grained relative to soils of control areas.Soils of control areas tended to be loose,while soils of scars and deposits tended to be compact.Soil available water contents in the three geomorphologic classes were similar and quite low.Soil moisture contents in both surface and deeper layers were always higher in scars and deposits than in control areas.With the growing season of the dry year(2015),soil water in 0–5 cm layers in the three geomorphologic classes were all unavailable for plants from May to July,but the duration of being unavailable are shorter in scars and deposits than in control areas;with the growing season of the normal year(2016),soil water in 0–5 cm and 0–80 cm layers in the three geomorphologic classes were almost available for plants.The period from April to June is the blooming period of plants on scars and deposits,but soil moisture contents in the two geomorphologic classes were lowest during this period;the period from September to October is the period of seed production of plants in control areas,but soil moisture content in control areas was lowest during this period.Soil organic matter,inorganic nitrogen and available phosphorus contents in the three geomorphologic classes were low to extremely low,while soil available potassium contents in the three geomorphologic classes were moderate to very high.Soil clay to sand ratio and soil moisture content were significantly higher but soil available phosphorus content was significantly lower in north-facing scars than in south-facing scars.2.The cover and species richness of plant communities and the numbers of families,genuses and species on control areas were quite small.Therefore,the main propagule sources of landslides were disadvantaged in terms of cover and species quantity.The cover and species richness of plant communities and the numbers of families,genuses and species on deposits have approached those in control areas.For plant communities on scars,their species richness and the numbers of families,genuses and species have approached those in control areas,while their average cover was less than 20 %.Within control areas,the variation in species composition among plant communities was quite large;the main dominant species are also common dominant species of plant communities in the hilly-gullied region of the Loess Plateau.Therefore,the main propagule sources of landslides were advantaged in terms of species quality.Within deposits,the variation in species composition among plant communities was quite large as well;except for Cleistogenes caespitosa,the main dominant species of plant communities in control areas were also the main dominant species of plant communities on deposits.Within scars,the variation in species composition among most plant communities was quite small;except for Dendranthema chanetii,the main dominant species of plant communities in control areas were also the main dominant species of plant communities on scars.Moreover,most dominant species of plant communities on scars and deposits are also common dominant species of plant communities in the hilly-gullied region of the Loess Plateau.Most individuals in plant communities on scars,deposits and control areas were all long-lived and clonal,but they were all herbaceous,short and unable to resprout after disturbance;all of their seeds were of low production,large weight,long-distance dispersal,anchorage mechanisms and high germination speeds and rates.For plant communities on north-facing scars,relative to plant communities on south-facing scars,species richness was significantly larger;life span,clonality and resprouting capacity were superior,while woodiness,height and seed production were inferior.In addition,the requirement of sunlight was obviously different between the main dominant species of plant communities on south-facing scars and those on north-facing scars.3.Under the semi-arid climate of this region,landslides do not have profound effects on soil properties as well as species richness,species composition and functional identities of plant communities;the cover of plant communities on scars was significantly lower than those on deposits and control areas,but the vegetation recovery of scars should be only a matter of time.The surrounding plant communities play an important role in providing available propagules for landslides,especially for scars.Neither scars nor deposits have to experience a long-term succession process to approach the surrounding species composition.Moreover,deposits only took several years to approach the surrounding vegetation cover.These findings imply great potential for vegetation restoration of landslides and the resistance of slope ecosystems to landslide disturbance in this region.4.The period from early winter until late summer should be the most critical period for recruitment efficiency on scars.Seedling recruitment made a relatively important contribution to population recruitment on north-facing scars,and seedling recruitment and ramet recruitment made equal contributions to population recruitment on south-facing scars.There was recruitment limitation on scars,especially on south-facing scars,which can constitute a barrier to vegetation restoration of scars.There were also high degrees of uncoupling between the adult and the young layers on scars,especially on south-facing scars,indicating future potential changes in species composition driven by Artemisia gmelinii,Stipa bungeana and Poa sphondylodes on north-facing scars and those driven by A.scoparia and Ixeridium sonchifolium on south-facing scars.