Underbrush is the constituent part of the wood biomass, which is one of a few renewable resources with an yearly increasing demand and importance. It is urgent to know what amounts of biomass suitable for fuel are available in Lithuanian forests.

The aim of this study is to estimate underbrush biomass and to determine its preconditioning factors.

To estimate the amount and distribution of underbrush biomass, data on amount of the stems and height of underbrush trees collected by the Lithuanian National Forest Inventory (NFI) and stored in the data base of the State Forest Service were used along with a developed equation describing variation in underbrush biomass over height.

The results showed that the mean aboveground underbrush dry biomass in Lithuanian forests was 860 kg/ha, totalling to about 1.6 mil. tons. The largest portion of underbrush biomass was found for common hazel (about 41 %). Mean dry underbrush biomass on very poor („a“) trophotope sites fails to reach even 0.1 t/ha, while fertile „d“ and „f“ trophotope sites comprises 1.4 and 2.2 t/ha, respectively. The highest amount of dry underbrush biomass is found in aspen stands, about 2.1 t/ha, in birch stands, about 1.9 t/ha, and in ash stands, about 1.6 t/ha, while the lowest one was found in pine stands, about 0.4 t/ha, and in spruce stands, about 0.6 t/ha.

It was found that underbrush biomass is mostly predetermined by site trophotope, dominant tree species and stand stocking level.

Key words: underbrush, dry biomass, tree height, site trophotope, dominant tree species, stand stocking level.