Department of Ecosystem Science and Management, Texas A&M University, College Station, Texas, United States of America.
Department of Marine Biology, Texas A&M University at Galveston, Galveston, Texas, United States of America.
PLoS One. 2018 Mar 1;13(3):e0193617. doi: 10.1371/journal.pone.0193617. eCollection 2018.
Global changes, such as increased temperatures and elevated CO2, are driving shifts in plant species distribution and dominance, like woody plant encroachment into grasslands. Local factors within these ecotones can influence the rate of regime shifts. Woody encroachment is occurring worldwide, though there has been limited research within coastal systems, where mangrove (woody shrub/tree) stands are expanding into salt marsh areas. Because coastal systems are exposed to various degrees of nutrient input, we investigated how nutrient enrichment may locally impact mangrove stand expansion and salt marsh displacement over time. We fertilized naturally co-occurring Avicennia germinans (black mangrove) and Spartina alterniflora (smooth cordgrass) stands in Port Aransas, TX, an area experiencing mangrove encroachment within the Northern Gulf of Mexico mangrove-marsh ecotone. After four growing seasons (2010-2013) of continuous fertilization, Avicennia was more positively influenced by nutrient enrichment than Spartina. Most notably, fertilized plots had a higher density of taller (> 0.5 m) mangroves and mangrove maximum height was 46% taller than in control plots. Fertilization may promote an increase in mangrove stand expansion within the mangrove-marsh ecotone by shifting Avicennia height distribution. Avicennia individuals, which reach certain species-specific height thresholds, have reduced negative neighbor effects and have higher resilience to freezing temperatures, which may increase mangrove competitive advantage over marsh grass. Therefore, we propose that nutrient enrichment, which augments mangrove height, could act locally as a positive feedback to mangrove encroachment, by reducing mangrove growth suppression factors, thereby accelerating the rates of increased mangrove coverage and subsequent marsh displacement. Areas within the mangrove-marsh ecotone with high anthropogenic nutrient input may be at increased risk of a regime shift from grass to woody dominated ecosystems.
全球变化，如气温升高和二氧化碳升高，正在推动植物物种分布和优势的变化，如木本植物侵入草原。这些生态交错带内的局部因素会影响生态系统的转变速度。木本植物的入侵正在全球范围内发生，但在沿海系统中的研究有限，那里的红树林（木本灌木/树）正在向盐沼地区扩展。由于沿海系统受到各种程度的养分输入的影响，我们研究了养分富集如何在局部影响红树林扩展和盐沼位移随时间的变化。我们在德克萨斯州的帕拉尼斯港（Port Aransas）对自然共存的黑木（Avicennia germinans）和互花米草（Spartina alterniflora）进行了施肥，该地区是墨西哥湾北部红树林-沼泽生态交错带内发生红树林入侵的地区。经过四个生长季节（2010-2013 年）的连续施肥，营养丰富对黑木的影响比互花米草更为积极。最值得注意的是，施肥的样地中，更高（>0.5 米）的红树林密度更高，红树林的最大高度比对照样地高出 46%。施肥可能会通过改变黑木的高度分布，促进红树林在红树林-沼泽生态交错带中的扩展。达到特定物种特定高度阈值的黑木个体，减少了对邻近植物的负面影响，并且对冷冻温度的抵抗力更高，这可能会增加红树林相对于沼泽草的竞争优势。因此，我们提出，养分富集可以通过增加红树林的高度，在局部地区作为红树林入侵的正反馈，减少抑制红树林生长的因素，从而加速红树林覆盖面积增加和随后的沼泽地取代的速度。在红树林-沼泽生态交错带内，受到人类活动养分输入影响较高的地区可能面临从草本为主的生态系统向木本为主的生态系统转变的风险增加。
