NDVI time series - early warning signals

Many freshwater forested wetlands along the southeastern U.S. coastline are rapidly transitioning from forest to marsh or open water, due to climate change related disturbances, such as saltwater intrusion and increasing flooding frequency. These changes in wetland state are considered a regime shift, and the timing and trajectory of change are not well understood. Recent studies have found early warning signals (EWS) of regime shifts in other ecosystems, but it is unclear if these can be detected for coastal wetlands. In this study, we examined the ability to detect EWS of regime shifts in coastal wetlands within the Albemarle Pamlico peninsula, North Carolina, U.S.A. We used 35 years of the Landsat record to examine trends and variance of normalized difference vegetation index (NDVI) time series for selected areas known to have undergone regime shifts. We found that NDVI time series trends combined with changes in standard deviation of NDVI allowed us to identify four scenarios ...美国东南部沿海沿岸的众多淡水森林湿地，正因气候变化相关扰动（如盐水入侵与洪涝频次增加）快速从森林生态系统向沼泽或开阔水域转变。这类湿地状态的变化被视为生态系统状态转换（regime shift），目前学界对其发生时机与变化轨迹尚未形成充分认知。已有研究在其他生态系统中发现了生态系统状态转换的早期预警信号（early warning signals, EWS），但尚不清楚该类信号能否在滨海湿地中被检测到。 本研究以美国北卡罗来纳州阿尔伯马尔-帕姆利科半岛的滨海湿地为研究对象，探究了生态系统状态转换早期预警信号的可检测性。我们利用35年的陆地卫星（Landsat）观测数据，对已知发生过状态转换的选定区域的归一化植被指数（normalized difference vegetation index, NDVI）时间序列的趋势与方差进行了分析。 研究结果表明，结合NDVI时间序列趋势与NDVI标准差的变化特征，我们可识别出四类情景……