Quantitative analysis of the root system of Avicennia alba based on the pipe model theory

Abstract

Reliable estimation of the root biomass is essential to quantify the total amount and proportion of the carbon sink of a forest, especially for mangrove forests that exhibit various extensive root systems. One way to estimate root biomass is to use a quantitative root-system measurement based on the pipe model theory (PMT), which assumes that the total cross-sectional area of roots is identical among the different root-size classes for an individual plant. The PMT states that the cross-sectional area of the tree stem at the ground level supports a fixed total weight of roots. Therefore, estimation of the root weight of a tree is possible from measuring the stem diameter if that species conforms to the model. Using a water pump, the whole root system of three Avicennia alba trees, a pioneer species commonly dominating the mud flats and river edges of secondary mangrove forests in Thailand, were excavated. The living roots were sorted into diameter classes and their individual dry weights obtained. A significant inverse linear relationship was obtained between the root diameter and the number of roots of each sampled tree, with the regression slopes not statistically different from -2.0 (range -2.19 to -2.83 for the three trees) conforming to the PMT. The ratio between the total root weight and the basal area of the stem at the ground level ranged from 0.0340-0.0389, which is smaller and larger than that for Sonneratia caseolaris and Xylocarpus granatum, respectively, two other mangrove tree species previously reported to conform to the PMT.