Landscapes
globally have been greatly altered by agricultural land use. Restoring native
vegetation has necessarily occurred at small spatial scales, with limited ability
to address landscape scale effects. However, extensive replanting
is an economically feasible option for climate change mitigation in emerging carbon-driven economies. Much replanting
occurs in riparian zones, offering concomitant ecological health benefits for aquatic
systems because riparian vegetation provides important subsidies of terrestrial
organic carbon to streams. We measured organic carbon fluxes in riparian replantings
on 13 agricultural streams over two years to assess if replanting affected
in-stream organic carbon dynamics. We then up scaled to estimate fluxes from
similar sized streams in an entire catchment (second and third order streams;
approximately 30% of the network). We sought to assess the regional implications
of incorporating additional trees in future landscapes. At reach scales we
found that replanting vegetation increased the provision of leaf litter and doubled
the standing stock of coarse particulate organic carbon in-stream within three
decades. Combined with greater shading, this increased organic carbon
processing and reduced net ecosystem productivity by 50%. At catchment scales
changes in organic carbon fluxes were high per unit of stream, but small
compared to the potential for terrestrial carbon storage. We predict that extensive
replanting of riparian vegetation will cause agricultural streams to become
more retentive of organic carbon. This reflects a restoration of terrestrial
and aquatic processes towards that of past, forested catchments. Replanting
riparian vegetation in agricultural landscapes can effectively return multiple
benefits within decadal timescales.