Squeeze of Brazilian subtropical mangroves from a Holocene and Anthropocene perspective

Abstract

Globally, mangroves are expected to expand into subtropical latitudes and higher elevations due to rising temperatures and sea-level over the coming decades. However, local geomorphology and coastal urbanization may strongly modulate these responses. Insights into mangrove responses to environmental changes can be gained from the last millennium, a key period marking the transition from natural to anthropogenic influence. Remote sensing and multi-proxy analyses from a subtropical Brazilian coast revealed the presence of mangroves on elevated flats (>1.5 m above mean sea level, msl) around ∼1700 cal yr BP. These mangroves migrated to lower flats (∼70 cm below current msl) by ∼530 cal yr BP (1420 CE), in response to a relative sea-level (RSL) fall. This trend contrasts with the RSL rise since the end of the Little Ice Age (LIA, ∼1300–1850 CE), which has since driven landward mangrove migration in the area. Additionally, recent warming in South America has facilitated the poleward expansion of Rhizophora species, contributing to an increase in mangrove canopy height (from 1 to 3.5 m) over recent decades. While global warming favors latitudinal expansion and canopy growth, the availability of suitable land is critical. Elevated terraces, which served as refuges for coastal wetlands during the mid-Holocene high sea-level stand, are now urbanized, limiting future establishment. Thus, mangrove expansion into temperate latitudes and landward migration hinge on horizontal space availability and the degree of urban development. Gentle terrain (slopes of 1.3–0.8 %) supports mangrove persistence, while steep slopes (>14 %) and urban encroachment threaten their survival. Without adequate space for migration, the region may face a net mangrove loss—termed “mangrove squeeze”—between 2100 and 2160 CE.