Abundance, characteristics, and movement of woody debris in four Basque streams

We studied the amount, size and dynamics of woody debris in 4 reaches of th e Aguera stream catchment (northern Iberian Peninsula): three sites (1st-, 2nd- and 3rd-order) surrounded by deciduous forests, and one 1st-order site under a plantation of Eucalyptus globulus. From July 1995 to February 1997 , on six occasions, all wood pieces larger than 1 cm in diameter were measu red; logs (diameter > 5 cm) were tagged and their positions recorded. The v olume of wood was relatively low and decreased downstream: 13,700 cm(3)/m(2 ) at the headwater deciduous site, 490 cm(3)/m(2) in the 2nd-order reach, a nd 100 cm(3)/m(2) in the 3rd-order reach. The woody standing crop at the he adwaters within eucalyptus plantations was only 960 cm(3)/m(2). The average size of individual pieces decreased downstream. The mobility of logs was l owest in the headwaters: 47 % of the logs tagged in the 1st-order reaches m oved during the year, 86 % in the 2nd-order reach, and 76 % in the 3rd-orde r reach. Temporal variations were small and associated with sporadic events such as floods or human activities. Mid and low reaches of Basque streams have few old-growth riparian forests, and most fallen logs are removed; thi s results in low amounts of wood, thus limiting the role of woody debris in organic matter retention or habitat diversity. Similarly, plantations of e ucalyptus seem to result in low volume and small size of woody debris in th e headwaters.