SUMMARY 1. Breakdown of wood was compared at three sites of the Agüera catchment (Iberian Peninsula): two oligotrophic first‐order reaches (one under deciduous forest, the other under Eucalyptus globulus plantations) and one third‐order reach under mixed forest, where concentration of dissolved nutrients was higher.
2. Branches (diameter = 3 cm, length = 10 cm) of oak (Quercus robur), alder (Alnus glutinosa), pine (Pinus radiata) and eucalyptus, plus prisms (2.5 × 2.5 × 10 cm) of alder heartwood were enclosed in mesh bags (1 cm mesh size) and placed in the streams. Mass loss was determined over 4.5 years, whereas nutrient, lignin and ergosterol were determined over 3 years. In order to describe fungal dynamics, ergosterol was also determined separately on the outer and inner parts of some branches.
3. Breakdown rates ranged from 0.0159 to 0.2706 year−1 with the third‐order reach having the highest values whatever the species considered. The most rapid breakdown occurred in alder heartwood and the slowest in pine branches; breakdown rates of oak, eucalyptus and alder branches did not differ significantly.
4. The highest nitrogen and phosphorus contents were found in alder, followed by oak, while pine and eucalyptus had low values. During breakdown, all materials rapidly lost phosphorus, but nitrogen content remained constant or slightly increased. Lignin content remained similar.
5. Peaks of ergosterol ranged from 0.023 to 0.139 mg g−1 and were higher in alder than in other species in two of the three sites. The third‐order reach generally had the greatest increase in ergosterol, especially in alder branches, eucalyptus and alder heartwood. The overall species/site pattern of fungal biomass was thus consistent with the observed differences in breakdown.
6. When compared with leaves of the same species decomposing at these sites, wood breakdown appeared to be less sensitive to the tree species but more sensitive to stream water chemistry. Although wood breakdown is slower and its inputs are lower than those of leaf litter, its higher resistance to downstream transport results in a relatively high standing stock and a significant contribution to the energy flux.
The capacity of stream channels to retain leaf litter (retentiveness) was measured in 21 reaches of the Agüera basin (northern Spain) at different discharges, using plastic strips as leaf analogs. Strips were calibrated against seven local leaf species occurring in the area. Retention was highest for alder, followed by plastic strips, oak, beech, chestnut, eucalyptus, hazel, and sycamore. Inter-specific differences in retention were great, and not clearly related to leaf form or size. This result shows that a great deal of caution is necessary to compare results obtained by authors using different leaf species. The Agüera stream channels were highly retentive, especially Aquat. Sci. 65 (2003) 158 -166 Aquatic Sciences in the headwaters. At baseflows, the average travel distance of strips was 3.6 m in 1st-order reaches, increasing to 16.6 m in 3rd-order streams. Travel distances of strips increased twofold in 3rd-and 2nd-order reaches and 5-fold in 1st-order streams during periods of high discharge. Leaf litter retentiveness was related to channel gradient, width, and substrate. Cobbles and wood showed high retention efficiencies, and the role of wood as a retention factor increased at high discharges. Retentiveness enhances storage and subsequent utilization of organic materials in forested streams, and thus should be taken into account when managing streams.
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