Below-ground root yield and distribution in natural and replanted mangrove forests at Gazi bay, Kenya

Abstract

Estimation of total biomass in woody ecosystems is important because of its relevance to nutrient turnover and the potential to store carbon. Most work on mangrove biomass, particularly in the Western Indian Ocean Region, has concentrated on the above-ground component; comparatively little is known on below-ground biomass. The current study was conducted at Gazi bay on the southern coast of Kenya. The objective was to determine the below-ground biomass of three species of mangrove, Rhizophora mucronata Lamarck, Avicennia marina (Forsk.) Vierh and Sonneratia alba J. Smith, in natural and replanted stands. The effects of distance from the tree base and of soil depth on root biomass and size distributions were also studied using coring. Live below-ground biomass (mean S.E.) ranged from 7.5 0.4 t/ha to 35.8 1.1 t/ha, 48.4 0.7 t/ha to 75.5 2.0 t/ha and 39.1 0.7 t/ha to 43.7 1.7 t/ha for R. mucronata, S. alba and A. marina, respectively, depending on the age of the stand. Including dead roots produced total biomass values of 34.9 1.8–111.5 5.6 t/ha, 78.9 3.3–121.5 7.3 t/ha and 49.4 1.1–84.7 5.4 t/ha for R. mucronata, S. alba and A. marina. These values imply carbon contents of live roots ranging between 3.8 0.2 C t/ha and 17.9 0.6 C t/ha, 24.2 0.4 C t/ha and 37.7 1.0 C t/ha and 19.5 0.4 C t/ha and 21.9 0.9 C t/ha for R. mucronata, S. alba and A. marina stands, respectively, and 17.4 0.9 C t/ha and 55.7 2.8 C t/ha, 39.4 1.7 C t/ha and 60.7 3.6 C t/ha and 24.7 0.6 C t/ha and 42.4 2.9 C t/ha for R. mucronata, S. alba and A. marina stands, respectively if dead roots are included. Stand densities were 4650 177 stems/ha, 3800 212 stems/ha and 3567 398 stems/ha for R. mucronata 6-year old, 12-yearold and natural stands respectively. Mean stem diameter, and basal area were highest in the 12-year-old plantation while below-ground root biomass increased with age. Stand density for S. alba, was highest in the 12-year-old plantation (7900 141 stems/ha) while the 9-year-old stand had trees with the largest diameter (7.7 0.9 cm). Below-ground biomass was highest in the 12-year old (75.5 2.0 t/ha) and lowest in the natural stand (48.4 0.7 t/ha). Stand density for A. marina was highest in the 12-year-old plantation (4300 919 stems/ha) while mean stem diameter (7.9 0.7 cm) and basal area (16.2 2.1 m2/ha) were highest in the natural stand. Below-ground biomass in the 12-year-old (43.7 1.7 t/ha) and natural stands (39.1 0.7 t/ha) was similar. Root densities decreased with soil depth and with distance from the base of trees for all species and stands. Fine roots (diameter <5 mm) constituted between 24% and 45% of the total stand live root biomass. The information generated is important in establishing the total biomass and thus the potential amount of carbon sequestered by mangroves in the study area.