Root biomass under stem bases and at different distances from trees

• Root biomass under and out of stem bases was measured in a tropical dry forest.
• Biomass concentration under bases was decuple than out in an area nine times smaller.
• Total root biomass was 25 Mg ha−1, a root:shoot ratio of 0.56, 41% under stem bases.
• Root biomass in caatinga and maybe other dry forests has been underestimated.
• In caatinga biomass estimate should be updated from 1.88 to 2.50 Tg.

Aboveground and root biomass and the root:shoot ratio were determined in six areas of caatinga. The aboveground biomass was estimated using allometric equation. The root biomass was collected from 0.5 × 0.5 m2 trenches, opened under stem bases and in areas away from the stem bases, separated in 20 cm deep layers down to 1 m. Roots were separated into five diameter classes: < 2 mm; 2.1–5 mm; 5.1 mm–10 mm; 10.1–20.0 mm; and >20.1 mm. The total aboveground biomass was 46.5 Mg ha−1. The root biomass in trenches under the stem base was ten times higher than in the other trenches but the area of the stem bases was 11 times smaller, so root biomass under the bases was 41% of the total root biomass (10.6 and 25.6 Mg ha−1, respectively). The root:shoot ratio was 0.56. The highest proportion of roots was in the upper soil layer and under the stem base and most of them were coarse roots (>20.1 mm). It is essential to consider the root biomass below the stem base to obtain a proper estimation of the root biomass.