Naturally formed forest patches known as tree islands are found within lower-statured wetland matrices throughout the world, where they contrast sharply with the surrounding vegetation. In some coastal wetlands they are embedded in former freshwater marshes that are currently exposed to saltwater intrusion and mangrove encroachment associated with accelerating sea-level rise. In this study we resurveyed tree composition and determined environmental conditions in tree islands of the coastal Florida Everglades that had been examined two decades earlier. We asked whether tree islands in this coastal transition zone were differentiated geomorphologically as well as compositionally, and whether favorable geomorphology enabled coastal forest type(s) to maintain their compositional integrity against rising seas. Patterns of variation in geomorphology and soils among forest types were evident, but were dwarfed by differences between forest and adjacent wetlands. Tree island surfaces were elevated by 12–44 cm, and ²¹⁰Pb analyses indicated that their current rates of vertical accretion were more rapid than those of surrounding ecosystems. Tree island soils were deeper and more phosphorus-rich than in the adjoining matrix. Salinity decreased interiorward in both tree island and marsh, but porewater was fresher in forest than marsh in Mixed Swamp Forest, midway along the coastal gradient where tropical hardwoods were most abundant. Little decrease in the abundance of tropical hardwood species nor increase in halophytes was observed during the study period. Our data suggest that geomorphological differences between organic tree island and marl marsh, perhaps driven by groundwater upwelling through more transmissive tree island soils, contributed to the forests’ compositional stability, though this stasis may be short-lived despite management efforts.

自然形成的森林斑块被称为树岛，分布在世界各地的低矮湿地基质中，与周围的植被形成鲜明对比。在一些沿海湿地，它们被嵌入以前的淡水沼泽中，这些沼泽目前暴露于与加速海平面上升相关的咸水入侵和红树林侵蚀。在这项研究中，我们重新调查了佛罗里达大沼泽地沿海树木岛屿的树木组成并确定了其环境条件，这些岛屿在 20 年前就已被检查过。我们询问了这个沿海过渡带中的树岛是否在地貌和组成上有所不同，以及有利的地貌是否使沿海森林类型能够在海平面上升的情况下保持其组成完整性。森林类型之间地貌和土壤的变化模式很明显，但由于森林和邻近湿地之间的差异而显得相形见绌。树岛表面升高了 12-44 厘米，并且²¹⁰铅分析表明，它们目前的垂直吸积速度比周围生态系统的速度更快。树岛土壤比相邻的基质更深，磷含量更高。树岛和沼泽的内部盐度降低，但森林中的孔隙水比混合沼泽森林中的沼泽更新鲜，位于热带硬木最丰富的沿海梯度的中间。在研究期间，热带硬木物种的丰度几乎没有减少，盐生植物也没有增加。我们的数据表明，有机树岛和泥灰沼泽之间的地貌差异，可能是由地下水通过更具渗透性的树岛土壤上涌驱动的，有助于森林的组成稳定性，尽管尽管做出了管理努力，但这种停滞可能是短暂的。