Climate Change: Defining Reference Conditions and Restoring Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 223

The high natural variability of large wood standing stock and ongoing climate
change makes it difficult to define reference and target conditions for streams.
Therefore, the use of 'passive restoration' methods (restoring the process
of wood recruitment on larger scales) instead of 'active restoration' (placement of
wood structures on a reach scale) is desirable for a number of reasons. First,
local, active restoration measures are at risk of neglecting the more general
processes at the catchment scale causing degradation, and often treat symptoms
rather than causes of stream degradation, thus being prone to failure in the long
run (e.g. Kauffman et al . 1997). Along these lines, it has been proposed that
restoration projects are more likely to be successful if they are undertaken in the
context of entire catchments (Wohl et al . 2005). The findings of Kail & Hering
(2005) lend further support to the conjecture that restoration activities using
large wood must consider other factors such as catchment land use. Secondly,
active restoration measures may create conditions that do not correspond to the
potential natural state. It is doubtful that the target condition for stream
restoration (the potential natural state) can be accurately described for stream
reaches if controls like discharge and sediment supply have been altered by
human activity and, therefore, historic conditions cannot be used as reference or
target conditions.
Moreover, climate change will probably have a strong impact on stream
hydraulics, morphology, biota and hence the potential natural state. In contrast,
the establishment of natural riparian vegetation and natural wood input will
initiate natural channel dynamics, ultimately resulting in natural channel
dimensions which correspond to the potential natural state. Thirdly, even in a
densely populated region like Central Europe the process of natural wood input
can be restored. A conservative estimate of Kail and Hering (2005) showed that
about 7% of the streams in Central Europe can potentially be restored by large
wood recruitment from native or non-native riparian forests. In many parts of
Central Europe, land-use pressure may decrease in the future, due to decreasing
population size, alternative agricultural techniques and increasing flood risks
following climate change. In the long term, this may increase chances for the
establishment of riparian forests and passive restoration, particularly in remote
parts of mountainous areas.
Ecosystem connectivity and species' dispersal ability
For the restoration of freshwater ecosystems to be considered successful, the
reference conditions need to be attained or at least closely approximated as
discussed above. According to the European Water Framework Directive (European
Commission 2000) this involves more than just the restoration of abiotic reference
conditions such as water nutrient status and pH. It also involves the restoration of
reference communities, i.e. the assemblages of the fauna and flora typical of
unperturbed conditions. Whereas, as a first step, abiotic reference conditions can
be restored by applying external measures (such as reduced nutrient loading or
addition of large woody debris), the restoration of reference communities is in
most cases expected to follow a natural recovery. This often requires that species
that have been absent from the restored site re-colonize from outside source