Ecosystem recovery in restored headwater streams: the role of enhanced leaf retention.
There is controversy over how the success of ecological restoration should be measured. Traditionally, emphasis has been placed on species diversity and other community attributes, whereas the restoration of ecosystem processes has received less attention. Here, we combine replicated field experiments and a field survey to provide an ecosystem-level measure of stream restoration success. Numerous headwater streams in Finland, and in many other parts of the world, have been channelized for timber transport, resulting in channels with simplified structure and flow. Recently, programmes have been launched to restore these streams to their pre-channelization condition. While the efficacy of restoration in improving fish habitat has been tested, little is known about effects on other stream biota or on the retention of leaf litter, despite its importance in trophic dynamics of forested headwater streams. Using a before-after-control-intervention (BACI) designed experiment with multiple reference and experimental streams, we examined restoration-induced changes in retention efficiency by conducting leaf-release experiments before (1993) and after (1996) restoration. Substrate heterogeneity increased, but moss cover decreased dramatically following restoration. Retention efficiency in restored streams was higher than in channelized, but lower than in natural, streams. Algae-feeding scrapers were the only macroinvertebrate group whose density increased significantly after restoration. 4. Aquatic mosses were a key retentive feature in both channelized and natural streams, but their importance to retention was strikingly reduced by restoration. During restoration work, mosses are detached from large areas of the stream bed, exposing bare stone surfaces for colonization by periphytic algae. A more effective restoration technique would involve the use of moss transplants, or the addition of large woody debris, to increase retentiveness and, thus enhance the availability of organic material to benthic consumers. This case study on rivers illustrates how restoration projects benefit from an ecosystem perspective and from measures of ecosystem processes in assessing restoration success.