Scaling relations and substrate conditions controlling the complexity of Rhizophora prop root system

Abstract

The complex structures of the prop root system of the mangrove genus Rhizophora attenuate storm surges and tsunamis and promote sedimentation. While quantification of the mangrove's drag force is essential for assessing the mangroves' ability to reduce the risk of disaster and vulnerability to rising sea-level, the projected area of prop root system having multiple orders of prop roots (e.g., first-, second-, and third-order prop roots, where the root order indicates the level of branching from the stem) are still unexplored. To contribute to the quantitative evaluation of the mangrove's drag force, we investigated the vertical profile of whole-tree prop root projected area and the number of prop roots of 156 trees sampled from Indonesia, the Philippines, and Japan. Our results showed that prop roots above the first-order contribute up to 80% of the whole-tree prop root projected area, highlighting the importance of the presence of second-, third-, and even higher-order prop roots on the drag force exerted by mangroves. Based on field data, an empirical model for the prop root system, described by a scaling factor (S), and maximum and minimum root heights (HRmax, HRmin), was developed by assuming that the size distribution of prop roots follows a constant scaling factor S. S and HRmax showed significant correlations with stem diameter at breast height suggesting the scaling relations in the prop root system. The model, which employs the site- and species-specific regression models for S and HRmax, reproduced the vertical distribution of the number of prop roots – a good predictor of the prop root projected area – with reasonable accuracy. However, the site- and species-specific scaling relations are highly variable depending on the environment, suggesting morphological plasticity in the prop root system. Rhizophora trees that grow on a shallow sediment layer (around 0.1 m thickness) tend to produce more prop roots compared with those on a thicker sediment layer suggesting a morphological response of the prop root system to increase below-ground root biomass where below-ground root development is physically constrained. Multivariate analysis indicated that hard substrate also possibly contributes to higher complexity of prop root system. However, prediction of the site- and species-specific scaling relations from these environmental variables still needs to be improved. Further research is needed to explain the mechanisms of morphological response of prop root systems to environmental gradients and to establish a generalized model for predicting the prop root system in various environments.