Land cover for the barrier islands of the Delmarva Peninsula in Virginia, 1984-2016

We quantified state change from sand, grassland, and woody vegetation in seven undeveloped barrier islands over three time intervals because the ecological and economic value of upland barrier systems is significant, yet often overlooked. We focused on seven islands with woody land cover: Cedar, Parramore, Hog, Cobb, Wreck, Smith, and Fishermans. Landsat TM5 satellite images were obtained from the USGS Global Visualization Viewer for the following dates: September 21, 1984, September 12, 1998, August 15, 2011 and September 12, 2016. Images were chosen from available dates within the growing season and were cloud-free in order to minimize uncertainties due to heterogeneous atmospheric conditions. Barrier islands provide the first line of defense against storms for millions of people living in coastal areas. Upland vegetation (that is, grassland, shrubland, and maritime forest) has received little attention, even though this land surface is most strongly affected by development pressures. We use remote sensing analysis to assess state change on seven undeveloped Virginia barrier islands over 32 years (1984-2016) that are free from direct human influence. Our analysis highlights the spatial-temporally dynamic nature of barrier island upland land area and vegetation, with rapidly changing ecosystem states. Between 1984 and 2011, upland vegetation was dramatically reduced by 29% whereas woody vegetation cover increased 40% across all islands. Although conversions between sand, grassland, and woody vegetation were variable within each island, three major patterns of vegetative land cover change were apparent: overall loss of vegetative cover,frequent transitions between grass and woody cover, and gain in woody cover. These patterns are valuable for understanding natural evolution of barrier islands in response to sea-level rise. Evaluation of temporal dynamics in barrier upland is needed to characterize underlying processes including island resilience or chronic stress, and is a prerequisite to sustainable coastal management- and resilience-based planning, especially when implementing ecosystem-based solutions.

本研究量化了7座未开发障壁岛在三个时间区间内沙地、草地与木本植被的状态变化。障壁岛高地生态系统具有重要的生态与经济价值，却常被忽视。本次研究聚焦于7处拥有木本土地覆被的障壁岛，分别为锡达岛（Cedar）、帕拉莫尔岛（Parramore）、霍格岛（Hog）、科布岛（Cobb）、雷克岛（Wreck）、史密斯岛（Smith）与费希尔曼斯岛（Fishermans）。本研究从美国地质调查局（USGS）全球可视化查看器（Global Visualization Viewer）获取了Landsat TM5卫星影像，影像拍摄日期分别为1984年9月21日、1998年9月12日、2011年8月15日与2016年9月12日。研究选取了生长季内可用日期且无云的影像，以最大限度降低异质性大气条件带来的不确定性。障壁岛是沿海地区数百万居民抵御风暴的第一道防线。高地植被（即草地、灌丛与滨海森林）虽受开发压力的影响最为显著，却鲜有研究关注。本研究采用遥感分析方法，对1984-2016年共32年间7座未受人类直接干扰的美国弗吉尼亚州障壁岛的状态变化进行评估。本次分析揭示了障壁岛高地土地面积与植被的时空动态特征，其生态系统状态变化迅速。1984年至2011年间，所有岛屿的高地植被总量大幅减少29%，而木本植被覆被率则提升了40%。尽管各岛屿内部沙地、草地与木本植被之间的转换情况存在差异，但仍显现出三类主要的植被土地覆被变化模式：植被覆被整体减少、草地与木本覆被间频繁转换，以及木本覆被增加。这些模式有助于理解障壁岛应对海平面上升的自然演化过程。开展障壁岛高地的时间动态评估，有助于阐明岛屿恢复力或长期胁迫等内在过程，同时也是开展可持续海岸管理与基于恢复力的规划的前提条件，在实施基于生态系统的解决方案时尤为如此。