Examining The Growth Dynamics Of Invasive Spartina Alterniflora Using Unmanned Aerial Vehicle Remote Sensing And In-situ Observations

The coastal wetland ecosystems provide many important ecosystem services with extremely high production.The area changing and the response to rising sea level of coastal wetlands are attracting more and more attention from coastal ecologists.The spatio-temporal dynamics of wetland vegetation growth is closely related to the geomorphological changing of coastal wetlands.The growth characteristics of wetland vegetation could be used to describe the wetland geomorphology changing.To Improve the understanding of the spatio-temporal dynamics and potential environmental control mechanisms of wetland vegetation growth is important for understanding the changing and survival of coastal wetlands under the influence of wetland vegetation.Spartina alterniflora(S.alterniflora),as a common and aggressive wetland vegetation,has many positive and negative effects on coastal wetland ecosystems.Rapid monitoring of the growth dynamics,exploring the invasion mechanism and assessing carbon sink effects of S.alterniflora are very important for scientific evaluation of the ecological effects of S.alterniflora invasion on coastal wetland ecosystems.This research took the subtropical coastal wetland in Zhangjiang Estuary on the southeastern coast of China as a research area.Using multi-temporal unmanned aerial vehicle(UAV)images and LiDAR technology to characterize the spatio-temporal dynamics of growth dynamics of S.alterniflora on the patch and landscape scale,by combined with field experiment,in situ meteorological monitoring and tidal water level dynamic observation.The greenness index which calculated from UAV images were used to construct S.alterniflora aboveground biomass model.And then,quantifying the spatio-temporal variation pattern of S.alterniflora biomass and making preliminary assessment of spatial distribution difference of carbon sink capacity of S.lterniflora in this site,by combined with the ratio of above-belowground biomass from previous research and spatial data of relative elevation and inundation duration.The main results are as follows:(1)The isolated S.alterniflora patches in the intertidal zone in Zhangjiang Estuary wetland had expanded rapidly in recent years(2014?2017)with a mean lateral expansion rate of 4.28 m/a.(2)Based UAV images,the greenness index could well track the phenology and biomass changing of S.alterniflora.The dual control of temperature and precipitation affected the seasonal growth of S.alterniflora.The growth potential of S.alterniflora patches reached peak in the early/mid-summer with high temperature and sufficient precipitation.The S.alterniflora patches grew at annual biomass increasing rate of 37.5?1145.5 kg/a in the research site.(3)S.alterniflora could adapt to hypoxic environment of long inundation duration by adjusting canopy height and stem density.The canopy height had a significant positive correlation with inundation duration in the range of 3?17 h/d.Inundation duration of 17 h/d may was the threshold of inundation-height changing pattern.(4)In the range of inundation duration of 3?18 h/d,mean lateral expansion rate and annual biomass increasing rate of S.alterniflora patches,are negatively(positively)related to the inundation duration(relative elevation).We suggest that the optimal growth environment of S.alterniflora patches is the inundation duration of 3?8 h/d(relative elevation of 0.15?0.58 m).The growth potential of S.alterniflora in wetlands was affected by tidal and meteorological factors.The spatial heterogeneity was controlled by tidal inundation,while the temporal variation was controlled by meteorological factors.This study confirms the potential of multi-temporal UAV images to accurately characterize the spatio-temporal dynamics of S.alterniflora invasion.Greenness index based on RGB cameras can also well quantified the phenology and biomass changing of S.alterniflora.