Prop root production response of planted Rhizophora to abrupt subsidence-induced sea level rise

Abstract

The 7.2 Mw Bohol earthquake in 2013 caused significant ground subsidence in some coastal zones northwest of the province, shifting the tidal inundation regime of intertidal habitats. These habitats included more than 500 ha of mangrove plantations particularly in island barangays which substrates are now 85% inundated annually from the 40% pre-quake conditions. Consequently, in less than three years after the quake, the effort of the subsided individual trees to produce new prop roots above the pre-quake height limits was found to be immediate, substantial, and tidally-linked. This prop root response resulted in an increase in the total length and volume by 74% and 39%, respectively, at 2.5-year post-quake relative to the 20-year pre-quake cumulative. With a mean elongation rate of ca. 1.1 m y−1, some 1.5–2.2 year period was necessary for these new prop roots to reach the substrate, and effectively establish a new equilibrium. The results highlight the role of prop roots on the aeration and stability requirements of Rhizophora, and provide quantitative evidence against the common practice of harvesting prop roots for firewood, charcoal, and other various purposes. The study presents a short-term glimpse of the prop root response of Rhizophora to the otherwise long-term gradual process of sea level rise.