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

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, 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. 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.Oceanographic and sea-level effects on mangroves X. López-Medellín et al.

Poleward expansion of mangroves is a threshold response to decreased frequency of extreme cold events

“…It is unlikely that regionalscale increases in mangrove area would arise from local scale changes in nutrient inputs, sedimentation, or hydrology, as these local drivers typically have local scale impacts (37,38). Another alternative is that sea level rise has contributed to inland expansion of mangroves across our study area (30 (39). In addition, there was no latitudinal trend in the amount of low-elevation habitat vulnerable to inland expansion by mangroves (Methods and Fig.…”

“…Local observations of mangroves encroaching into salt marshes are accumulating (25)(26)(27)(28)(29)(30), but regional assessments of mangrove expansion are lacking. Hypothesized mechanisms for these local mangrove encroachments include increased temperature, increased precipitation, revegetation of areas cleared for agriculture, altered tidal regimes, and/or increases in nutrient levels and sedimentation (29,30). However, to date, we are aware of no study that has linked landscape-scale changes in mangrove abundance or spatial extent to changes in climate.…”

The state of the world’s mangroves in the 21st century under climate change

“…Rather, mangroves moved inland and adjusted vertically along the fringes, resulting in a 35% increase in total mangrove coverage in some portions of the Ten Thousand Islands region of Florida (Krauss et al, 2011). Such sea-level-rise induced expansion has also been documented along the coasts of the Gulf of Mexico, southeast Australia, and the Pacific coast of Mexico (Rogers et al, 2006;Saintilan et al, 2009;López-Medellín et al, 2011). While it is true that mangroves have the ability in some cases to migrate landward and invade adjacent wetlands in response to sea-level rise, net loss or gain of mangrove area has been shown to vary by region as a function of the local rates of sealevel rise and coastal subsidence (Ellison & Strickland, 2015), landform slope and tidal forcing (Doyle et al, 2010), vertical accretion (Lovelock et al, 2015), sedimentation rates , and the absence or presence of actual migration corridors (Enwright et al, 2016).…”

Mangroves Response to Climate Change: A Review of Recent Findings on Mangrove Extension and Distribution

“…López-Medellín et al (2011) reported a 20% increase in mangrove extent through landward migration into halophytic shrub land, associated with sea level rise, and particularly pronounced during El Nino seasons. The effects of strengthening of the El Niño, suggested by some global climate models (IPCC 2013) could accelerate mangrove expansion, as suggested by studies in this and other areas.…”