Chapter 5 - Morphological plasticity and survival thresholds of mangrove plants growing in active sedimentary environments

Abstract

Intertidal sedimentary environments are highly dynamic due to frequent sediment redistribution, tidal flooding, and wind exposure, yet mangrove plants are able to colonize and shape (sub) tropical coastlines. Morphological plasticity, the ability of organisms to modify their anatomical traits independent of their genotype in response to environment variations, allows mangrove plants to adjust to such dynamic environments. First, mangrove establishment on bare tidal flats is largely dependent on pioneer traits and disturbance-free periods (Windows of Opportunity). As soon as the first seedlings root, morphological plasticity, as a response to abiotic forcing, determines plant survival (e.g., through differential allocation of biomass). This unidirectional control on vegetation survival shifts to a bidirectional feedback system between abiotic processes and vegetation growth, once mangrove cover surpasses a critical density threshold to attenuate hydrodynamic energy. Fully grown mangrove trees, however, still show morphological plasticity, resulting from both tree-to-tree interactions and the abiotic environment. While crown displacement toward light available spaces is a fundamental adaptive plastic trait that reduces competition, it is also influenced by wind forcing, and as growth is limited by higher salinities, with greater salt stress, tree crowns also become more susceptible to wind dynamic loading. The mechanical imbalance caused by crown displacement might increase the risk of felling but might also be compensated by plastic responses of the root systems, a topic largely unexplored within mangroves. In this chapter, we aim to provide an overview of the current understanding of plastic traits in mangrove trees, with a focus on biophysical interactions in dynamic sedimentary environments, discussing ecological implications and current gaps of knowledge that are detrimental to achieve successful adaptive management and conservation strategies.