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The influence of native replanting on stream ecosystem metabolism in a degraded landscape: can a little vegetation go a long way?

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Summary

  1. The effectiveness of revegetation is usually gauged by responses in biodiversity, which may differ between isolated replanted patches. The ecological value of revegetation may be detected more effectively by monitoring ecosystem processes. In-stream metabolism has been much modified by the degradation of riparian vegetation in agricultural landscapes around the world. We sought to determine whether the spatial scale typical of most riparian replanting projects (i.e. <1 km long) influences stream metabolism.
  2. Metabolism is a functional measure that incorporates gross primary production (GPP), ecosystem respiration (ER) and the difference between them [net ecosystem productivity (NEP)]. We hypothesised that replanted reaches would have lower GPP (due to greater canopy shading) and greater ER (governed by larger terrestrial energy inputs) compared with pasture reaches.
  3. We measured metabolism in paired reaches within four agricultural streams. Two streams had an untreated pasture reach contrasted with a reach replanted with native woody vegetation >17 years ago. The other two streams had similar riparian vegetation condition adjacent to both reaches, to act as reference sites.
  4. Mean daily GPP (0.27–4.9 g O2 m−2 day−1) and ER (1.1–22 g O2 m−2 day−1) were within the range of values recorded previously in agricultural streams elsewhere. Replanted reaches had rates of NEP lower than upstream untreated reaches at treatment sites, but NEP was similar between reaches at reference sites.
  5. The effects of replanting on stream ecosystem processes are detectable even when the spatial scale of restoration is relatively small in a whole-of-catchment context. Land managers can protect and restore vegetation at feasible spatial scales and benefit ecosystem processes. Ecosystem metabolism should be included in the range of responses that need to be monitored to provide a complete picture of the effectiveness of stream restoration.
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