Ligia E. Urrego

Mangrove dynamics in the southwestern Caribbean since the ‘Little Ice Age’: A history of human and natural disturbances

Relatively little is known about the long-term response of Caribbean mangroves to human and natural disturbances during the ‘Little Ice Age’ (LIA). We present new palynological information on the dynamics of the Bahia Honda mangrove from the eastern coast of San Andres Island in the southwestern Caribbean for the late Holocene. Major changes in the Bahia Honda pollen record show the combined effects of natural events (strong storms and sea-level rise), and human disturbances. These changes are supported by 14C dates, sedimentological and palynological information. A storm (most probably a hurricane) was recorded around AD 1600, caused sediment reworking and the subsequent loss of about 2000 years of the vegetation record. The devastation of tree vegetation by this event allowed the expansion of heliophytic vegetation (e.g. grasses and vines). Mangroves and coastal vegetation started to recover at AD 1700, reaching their maximum extent within a few decades, when microforaminifera shells became abundant at the coring site, thus suggesting a relative sea-level rise because of the geomorphic reconfiguration of the coastal plain after the storm. Furthermore, the pollen evidence indicates more humid regional climates during the late LIA (AD 1700—1850). Mangrove and coastal vegetation declined sharply as a consequence of the establishment of coconut plantations around AD 1850. The recovery of the mangroves after AD 1960 is a result of the combined effect of relative sea-level rise and drastic changes in the local economy from coconut plantations to commerce.

Colombian Caribbean mangrove dynamics: anthropogenic and environmental drivers

espanolSe realizo una revision de los efectos del incremento en el nivel del mar, los cambios en la salinidad y las perturbaciones humanas sobre la dinamica de los manglares. Los efectos de estos factores se analizaron a escalas seculares en seis registros palinologicos tomados en el Caribe Colombiano. A pesar del incremento acelerado en el nivel del mar, los incrementos en las temperaturas del mar y del aire y las disminuciones en la precipitacion en los dos ultimos siglos, los manglares han aumentado su representacion en los registros de palinologicos en varios sitios de la region, ubicados tanto en el continente como en las islas oceanicas. Tal expansion de los manglares se asocia con su capacidad de mantener el substrato contrarrestando el incremento en el nivel mar con el aumento en las cargas de sedimentos aluviales, desde 1850 AD en los sitios continentales, y con el aumento en la acumulacion de turba autoctona en los suelos de la isla de San Andres. Las perdidas de manglares en el pasado, se asociaron a la erosion costera, y/o al efecto combinado del incremento regional en las sequias y en la salinidad. Estos procesos han ocasionado la muerte de los manglares o cambios en la composicion de especies. Cuando se incrementan la salinidad del substrato y la deposicion de arenas, pasan de ser manglares dominados por Rhizophora mangle a los dominados por Avicennia germinans. La capacidad de rebrote de los tallos danados de Avicennia germinans parece ser el rasgo que le ha permitido a esta especie expandirse despues de la ocurrencia de sequias intensas y prolongadas y de vientos fuertes, tormentas tropicales y huracanes. Las perturbaciones humanas se representaron por la expansion de Laguncularia racemosa y Avicennia germinans a expensas de Rhizophora mangle, o por el reemplazo de la vegetacion arborea por vegetacion herbacea (pastizales o cultivos) y especialmente dominada por el helecho Acrostichum aureum. Alrededor de 1850 AD, los manglares y al vegetacion de playa fueron reemplazados por plantaciones de coco en la isla de San Andres y en la bahia de Cispata las areas pantanosas fueron cubierta con cultivos de arroz. Aunque despues de 1900 AD se abandonaron estos cultivos debido a una incursion marina, ha prevalecido la dominancia de Laguncularia racemosa en los registros de polen indicando la intervencion humana permanente desde entonces. EnglishWe review the main effects of sea level rise, salinity changes, and human disturbances on mangrove forest dynamics. The effects of these drivers on mangrove communities are evaluated at centennial time scales in the light of six palynological records from the Colombian Caribbean. Despite the accelerated rates of sea level rise, increases in sea surface and air temperatures, and decreases in precipitation of the last two centuries, mangroves have shown an increasing representation in pollen records at continental and marine locations of the region. In continental settings, such expansions have been related to the offsetting of sea levels by the increasing loads of fluvial sediments since 1850 AD, and by increases in autochthonous peat accumulation in San Andres Island. Losses of mangrove cover in the past have been related to coastal erosion, regional droughts, and salinity increases. Such processes have commonly caused mangrove die-off or changes in forest species composition. When the substrate has become more saline, or sand sedimentation has increased significantly, the composition of mangrove communities has switched towards the dominance of Avicennia germinans at expenses of Rhizophora mangle. Resprout capacity of damaged stems of Avicennia germinans seems to have been the trait that has allowed this species to increase after strong and prolonged droughts, and occurrence of strong winds, tropical storms, and hurricanes. Human disturbances are represented either by the expansion of Laguncularia racemosa and Avicennia germinans and decreases of Rhizophora mangle, or by mangrove replacement by herbaceous vegetation (grasses or crops), especially the fern Acrostichum aureum. Around 1850 AD, mangroves and beach vegetation were replaced by coconut plantations in San Andres Island, and in the Cispata bay swampy areas were covered with rice crops. Although after 1900 AD these crops were abandoned because of a marine incursion, Laguncularia racemosa has prevailed so far indicating pervasive anthropogenic disturbances.