Biology Department, University of Central Florida, Orlando, FL, USA.
Biological Sciences, Nicholls State University, Thibodaux, LA, USA.
Glob Chang Biol. 2021 Mar;27(6):1226-1238. doi: 10.1111/gcb.15494. Epub 2021 Jan 5.
Increases in minimum air temperatures have facilitated transitions of salt marshes to mangroves along coastlines in the southeastern United States. Numerous studies have documented mangrove expansion into salt marshes; however, a present-day conversion of oyster reefs to mangrove islands has not been documented. Using aerial photographs and high-resolution satellite imagery, we determined percent cover and number of mangrove patches on oyster reefs in Mosquito Lagoon, FL, USA over 74 years (1943-2017) by digitizing oyster reef and "mangrove on oyster reef" areas. Live oyster reefs present in 1943 were tracked through time and the mangrove area on every reef calculated for seven time periods. There was a 103% increase in mangrove cover on live oyster reefs from 1943 (6.6%) to 2017 (13.4%). Between 1943 and 1984, the cover remained consistent (~7%), while between 1984 and 2017, mangrove cover increased rapidly with a 6% year increase in mangrove area on oyster reefs (198% increase). In 1943, 8.7% of individual reefs had at least one mangrove patch on them; by 2017, 21.8% of reefs did. Site visits found at least one mature Avicennia germinans on each tracked mangrove reef, with large numbers of smaller Rhizophora mangle, suggesting the post-1984 mangrove increases were the result of increased R. mangle recruitment and survival. Escalation in the coverage and number of mangrove stands on oyster reefs coincided with a period that lacked extreme freeze events. The time since a temperature of ≤-6.6°C (A. germinans mortality threshold) and ≤-4°C (R. mangle mortality threshold) were significantly correlated with the increased ratio of mangrove area:oyster area, total mangrove area, and number of mangrove patches, with greater variation explained by time since ≤ -4°C. The lack of freezes could lead globally to an ecosystem shift of intertidal oyster reefs to mangrove islands near poleward mangrove range limits.
气温的升高促进了美国东南部沿海盐沼向红树林的转变。许多研究都记录了红树林向盐沼的扩张；然而，目前还没有记录到牡蛎礁向红树林岛屿的转变。本研究利用航空照片和高分辨率卫星图像，通过对佛罗里达州 Mosquito Lagoon 的牡蛎礁和“牡蛎礁上的红树林”区域进行数字化，确定了 1943 年至 2017 年 74 年间（1943-2017 年）牡蛎礁上红树林斑块的百分比和数量。通过时间跟踪 1943 年存在的活牡蛎礁，并计算每个礁的七个时间段的红树林面积，确定了活牡蛎礁上的红树林面积。从 1943 年（6.6%）到 2017 年（13.4%），活牡蛎礁上的红树林覆盖率增加了 103%。1943 年至 1984 年期间，覆盖率保持稳定（~7%），而 1984 年至 2017 年期间，红树林覆盖率迅速增加，牡蛎礁上的红树林面积每年增加 6%（增加 198%）。1943 年，有 8.7%的单个礁上至少有一个红树林斑块；到 2017 年，这一比例上升到 21.8%。实地考察发现，在每个跟踪的红树林礁上都至少有一棵成熟的海桑，还有大量较小的红树，这表明 1984 年后红树林的增加是由于红树幼苗的增加和存活率的提高。牡蛎礁上红树林斑块的覆盖范围和数量的增加与一段没有极端冻结事件的时期相吻合。≤-6.6°C（海桑死亡率阈值）和≤-4°C（红树死亡率阈值）的温度的时间间隔与红树林面积：牡蛎面积、总红树林面积和红树林斑块数量的增加比例显著相关，时间间隔对>-4°C 的解释差异更大。缺乏冻结可能会导致全球范围内潮间带牡蛎礁向红树林岛屿的生态系统转变，而红树林岛屿位于向极红树林范围的限制附近。
