Fencing streams not enough

Fencing rural streams is only the start when improving waterways, a new study has found. Holistic and sustainable land management is required to restore waterways, freshwater scientists at the Cawthron Institute have found.

‘‘There is no simple solution to restoring a healthy river ecosystem,’’ said lead author Katharina Doehring.

‘‘Fencing is just one component. Stock can’t get into the river and can’t deposit faeces in fenced sections of the river, and that’s a tick.’’

Fencing reduces E coli and sediment inputs, other good results. Planting on the stream edge is also encouraged.

But short sections of New Zealand streams and rivers that have been fenced and planted don’t then repair themselves, the researchers found, even over 30 years.

The only indicator that improved in this study was shade, brought about by planting along stream edges.

As they mature, bushes and trees shade the water and can lower its temperature, a desirable result.

Doehring and colleagues Joanne Clapcott and Roger Young sampled ecosystem function at 11 pairs of fenced and unfenced stream sites in Waikato in 2011-2012.

The stream sections were short – a few hundred metres –

but some had been fenced for 34 years and some for five years.

The researchers expected to find that fencing improved ecosystem function – and the longer a site had been fenced, the better stream health would be.

‘‘After 30 years, we expected to see a change in stream health,’’ they wrote. They were ‘‘surprised’’ this wasn’t the case.

Despite all the planting that was done in the stream sections studied, most of the indicators showed no difference in ecosystem function, one of five river health components, Doehring said.

‘‘You’d expect after 30 years that we’d get some good news and we didn’t.’’

Dissolved oxygen, algal production, mean wood mass loss rate and a host of other technical measures largely showed no differences.

Only organic matter decomposition (a measure of microbial activity) responded consistently over time after fencing.

The bigger picture is that streams and rivers are connected ecosystems and restoration needs to happen across whole catchments.

A few hundred metres of fencing and planting won’t compensate for upstream impacts. ‘‘Our waterways need to be managed at a catchment or large scale [because] small-scale efforts have little effect on stream ecosystem health,’’ Doehring said.

‘‘Ideally the landowners in a catchment work together and develop a co-ordinated environmental management plan for that catchment, ideally starting in the headwaters and working downstream,’’ she said.

Sustainable land management practices such as stocking rates, targeted fertiliser application, precision irrigation and protection-creation of wetlands will need to be looked at.

‘‘There isn’t a single answer,’’ she said. It depends what landowners can afford and the values expected of waterways.

Meanwhile, Doehring and others have started work on a national database of sustainable land management actions to improve water quality as part of the Our Land and Water National Science Challenge.

At present, nobody knows what restoration has been done and where.

It is hoped that landowners, farmers and industry will report their sustainable land use actions to the database on the Land, Air, Water Aotearoa (Lawa) website.

Over time, knowledge would build up, best practices, gaps and other insights will be identified, and effective actions encouraged.

Doehring urged landowners to continue fencing and planting, as they are important first steps.

‘‘You’d expect after 30 years that we’d get some good news and we didn’t.’’ Katharina Doehring, above