1. Channel complexity affects the physical structure, biotic communities and functioning of stream ecosystems. Large wood (LW) is a key element in the creation and maintenance of physically complex stream channels in forested areas.
2. In an attempt to enhance stream habitat quality and ecosystem functioning and to reduce inputs of organic matter to a downstream reservoir, LW was experimentally introduced into four mountain streams in the Basque Country (northern Spain), ranging in channel width from 3 to 13 m. Following a before–after/control–impact (BACI) design, streams were monitored during 1 year prior to wood addition and during 2 years after addition in one control and one experimental reach per stream.
3. Areal cover of benthic organic matter in the entire channel was measured from regular transects and the mass of stored organic matter from random Surber samples. Breakdown of organic matter was assessed in litter bag experiments performed with black alder leaves. When 50% of the initial mass in the bags remained, invertebrates associated with leaf bags were collected.
4. Wood placement produced a 2- to 70-fold increase in the storage of organic matter, especially in thick deposits upstream from wood jams, with values in excess of 2 kg AFDM per m2 in the small streams. The accumulation of organic matter produced by wood introduction decreased with increasing stream size.
5. Despite the large increase in the availability of organic matter, litter breakdown rates were unaffected by the experimental reaches, suggesting large increases in the total amount of organic matter consumed at the reach scale.
6. Numbers of invertebrates and shredders per gram of leaf litter did not respond to wood addition. Average body mass of invertebrates associated with leaf litter showed a non-significant decreasing trend, which might reflect increased recruitment.
7. Although the effects of wood addition can depend on wood stability and stream size, adding LW to restore channel complexity can improve environmental conditions for invertebrate communities and affect stream ecosystem functioning, enhancing the efficiency to use organic matter inputs on a reach scale.