Oceanographic anomalies and sea-level rise drive mangroves inland in the Pacific coast of Mexico

Study Number:

60

Author:

X. López-Medellín, E. Ezcurra, C. González-Abraham, J. Hak, L. S. S. & J. O. Sickman

Abstract:

Question: Although mangrove forests are generally regarded as highly threatened, some studies have shown that mangrove canopies in the Pacific coast of Mexico have been increasing in recent decades. We investigated the possible causes driving this reported mangrove expansion.
Location: The mangrove lagoons of Magdalena Bay in Baja California, Mexico.
Methods: We used 50-year-old aerial photographs and 24-year-old satellite images to compare long-term vegetation change, surveyed a coastal vegetation transect to analyse flooding levels, compiled six decades of tidal and oceanographic information, as well as hurricane data to analyse changes in storm frequency or sea-level conditions (climate change), and used isotopic analysis to date the age of trees along the gradient.
Results: A significant increase in mangrove cover has occurred in backwaters of the lagoons during the last 40 years, and especially during the El Niño anomalies of the 1980s and 1990s, while at the same time the mangrove fringe has been receding.
Conclusions: The observed change can be attributed to the combined action of the warm surface waters of El Niño events and sea-level rise (climate change). Jointly, these two effects are sufficient to flood large areas of previously non-flooded salt flats, dispersing mangrove seedlings inland. The inland expansion of mangroves, however, does not ease conservation concerns, as it is the seaward fringes, and not the inland margins, that provide the most valuable environmental services for fisheries and coastal protection.

Main Results and Conclusions:

• Results of studying mangrove reactions to rising sea levels:
  • “…the spectral signature of the mangrove canopy in LANDSAT images increased significantly between 1986 and 2001 (Fig. 1 and Table 1). The net increase concentrates mostly in the landward fringe, where the mangrove forest meets the desert. During the two decades of the study period, a significant area of salt flats became colonized by new mangrove growth”(146).
  • “During the last two decades, the area covered by mangrove canopies has increased in Magdalena Bay by more than 20%”(148).
• Concerns:
  • “An area occupied by new-growth mangrove saplings may have a canopy spectral signature similar to that of a mature forest, but ecologically it does not have the complexity of an old-growth stand. At the same time, many areas of fringe mangrove have been suffering considerable loss (Whitmore et al. 2005) as a result of forest clearing (deforestation), dredging, sedimentation, increased wave action from motorboats (coastal development) and, as this paper now shows, also as a result of increased sea-levels (climate change)”(149).
  • “The on-going colonization by small black mangrove saplings in the landward part of the tidal flats does not necessarily compensate for the loss of old-growth mangrove forest in the seaward fringe” (150).

Works Cited:

Whitmore, R.C., Brusca, R.C., Le ´on de la Luz, J.L., Gonz´ alez-Zamorano, P., Mendoza-Salgado, R., Amador-Silva, E.S., Holgu´ın, G., Galva´n-Magan˜ a, F., Hastings, P.A., Cartron, J.- L.E., Felger, R.S., Seminoff, J.A. & McIvor, C.C. 2005. The ecological importance of mangroves in Baja California Sur: conservation implications for an endangered ecosystem. In: Cartron, J.E., Ceballos, G. & Felger, R.S. (eds.) Biodiversity, ecosystems and conservation in northern Mexico. pp. 298–332. Oxford University Press, New York, NY, US