Transitional Mangrove Scrub North of La Paz and Magdalena Bay in Baja California, Mexico: A Historical Review

Abstract

The location and topography of the 1,300 km Baja California peninsula west of the Sonoran Desert provide the most extreme conditions for mangrove forest growth in the Northern Hemisphere. Despite their small size, however, the dwarf shrub mangroves (<2 to 4 m) are important for global CO2 sequestration, and monitoring of their expansion or decline is needed as part of assessing global carbon fluxes and biodiversity. Mangrove distributions at 42 coastal sites in 1972/73 are compared to historical accounts and modern data. These data show that there has been almost no northward migration of Rhizophora mangle during the past century although Avicennia germinans and Laguncularia racemosa have spread north on the Gulf Coast. The dwarf mangrove plant biodiversity in 1972/73 is related to geoform, temperature (T) and seasonal precipitation (P), but mangrove species cluster by annual P and T. The present northernmost Baja California mangrove limits are at 26.79° N on the Pacific Coast, and at 29.04° N on the Gulf Coast mainland, close to limits recorded in 1921. However, there are notable (46 to 4,649 ha) increases in areal cover and canopy height at these northern sites, and in other areas remote from urbanization. Case histories for seven locations point to increasing frequency and severity of tropical storms as main drivers of change on both coasts. However, anthropogenic changes from tourism promotion are major hazards to survival of the Gulf Coast forests, despite international, governmental and public efforts to legislate nature preserves. On the Pacific Coast, rising seawater temperatures may create coastal current conditions that potentially allow the extension of red and white mangrove ranges into northwestern Baja California. However, hazards associated with ocean warming that promotes tropical storm frequency and intensity can have predominantly negative impacts on mangrove production. These extreme hazards counteract the benefits of local and regional protection and management efforts