AbstractIn terrestrial ecosystems, a large portion (20–80%) of the dissolved Si (DSi) in soil solution has passed through vegetation. While the importance of this “terrestrial Si filter” is generally accepted, few data exist on the pools and fluxes of Si in forest vegetation and the rate of release of Si from decomposing plant tissues. We quantified the pools and fluxes of Si through vegetation and coarse woody debris (CWD) in a northern hardwood forest ecosystem (Watershed 6, W6) at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire,USA. Previous work suggested that the decomposition ofCWDmay have significantly contributed to an excess ofDSi reported in stream‐waters following experimental deforestation of Watershed 2 (W2) at theHBEF. We found that woody biomass (wood + bark) and foliage account for approximately 65% and 31%, respectively, of the total Si in biomass at theHBEF. During the decay of American beech (Fagus grandifolia) boles, Si loss tracked the whole‐bole mass loss, while yellow birch (Betula alleghaniensis) and sugar maple (Acer saccharum) decomposition resulted in a preferential Si retention of up to 30% after 16 yr. A power‐law model for the changes in wood and bark Si concentrations during decomposition, in combination with an exponential model for whole‐bole mass loss, successfully reproduced Si dynamics in decaying boles. Our data suggest that a minimum of 50% of theDSi annually produced in the soil of a biogeochemical reference watershed (W6) derives from biogenic Si (BSi) dissolution. The major source is fresh litter, whereas only ~2% comes from the decay ofCWD. Decay of tree boles could only account for 9% of the excessDSi release observed following the experimental deforestation of W2. Therefore, elevatedDSi concentrations after forest disturbance are largely derived from other sources (e.g., dissolution ofBSi from forest floor soils and/or mineral weathering).