OUR WA­TER

4 good rea­sons to use plants for pro­tec­tion

NZ Lifestyle Block - - Contents - WORDS DR RICHARD STOREY

One of the most ef­fec­tive and cost-ef­fec­tive ways to im­prove the health of wa­ter­ways in ru­ral ar­eas is to fence them from stock ac­cess and plant the stream banks (ri­par­ian ar­eas) with na­tive trees. This cre­ates a buf­fer be­tween the stream and farm­ing ac­tiv­i­ties on land.

Landown­ers, com­mu­nity groups, in­dus­tries and coun­cils around NZ have poured enor­mous in­vest­ment into do­ing just that and, as a re­sult, thou­sands of kilo­me­tres of stream banks have been planted out in the last few years.

But what im­prove­ments in stream eco­log­i­cal health can be ex­pected from cre­at­ing ri­par­ian buf­fers, and how long does it take for im­prove­ments to oc­cur?

It is im­por­tant to know what you can ex­pect from your restora­tion ef­forts and how long you will need to wait. Un­for­tu­nately, de­spite the large num­ber of ri­par­ian buf­fers planted, very few of the streams have been mon­i­tored to see whether in­di­ca­tors of stream health im­prove. This is start­ing to change, and NIWA now has some data on how streams re­spond to ri­par­ian fenc­ing and plant­ing.

Sci­en­tists now know that some health in­di­ca­tors re­spond more strongly and more quickly than others.

1 Bac­te­ria and wa­ter clar­ity

These are the first in­di­ca­tors to show im­prove­ment. E. coli in­di­ca­tor bac­te­ria counts re­duce sig­nif­i­cantly, and wa­ter clar­ity in­creases within 2-4 years as live­stock stop defe­cat­ing in streams and tram­pling stream banks, as ri­par­ian ground­cover plants be­come thicker and bet­ter at fil­ter­ing runoff from pas­tures, and as tree roots grow to hold the stream banks to­gether.

2 Silt

Fine sed­i­ment (silt) de­posited on the stream bed may be washed out over the first few years, but it may ap­pear to get worse for a few years af­ter that as the stream banks ad­just to the new veg­e­ta­tion type.

3 Tem­per­a­ture

Wa­ter tem­per­a­ture de­creases over 3-20 years (in small to medium-sized streams) as ri­par­ian plants grow to shade

the stream. The amount of al­gae and aquatic weed grow­ing on the stream bed also re­duces over this time as shade in­creases.

4 Nu­tri­ents

Dis­solved nu­tri­ents (that cause growth of nui­sance al­gae) are more com­plex. Phos­phate, which mostly binds to soil par­ti­cles, usu­ally de­creases as ri­par­ian ar­eas be­come bet­ter fil­ters, and can be ex­pected to de­crease within 10 years of ri­par­ian fenc­ing or plant­ing.

But ni­tro­gen, which mostly dis­solves and reaches streams in the form of ni­trate via leach­ing and un­der­ground flow paths, may not be re­duced no­tice­ably by ri­par­ian plant­ing, un­less the ground­wa­ter flows through the roots of ri­par­ian plants, par­tic­u­larly wet­land species.

The amount and rate of im­prove­ment in wa­ter qual­ity de­pends on a num­ber of fac­tors. The longer and wider a ri­par­ian buf­fer strip is, the more ef­fec­tive it will be. The smaller the stream, the more (and faster) it will re­spond to ri­par­ian man­age­ment. The steeper the catch­ment, the harder it is to pre­vent con­tam­i­nants en­ter­ing the stream.

Al­though wa­ter qual­ity im­prove­ments de­pend on sev­eral fac­tors, they are gen­er­ally pre­dictable. How­ever, im­prove­ments in stream fauna (in­ver­te­brates and fish) are much less pre­dictable and ap­pear to be de­pen­dent on the abil­ity of sen­si­tive species to re­colonise the im­proved habi­tat and com­pete with tol­er­ant res­i­dents. In some places, well-con­nected to sources of sen­si­tive species, the in­ver­te­brate and fish species ex­pected in healthy streams have quickly re­turned, within 5-10 years.

In other places they have not re­turned even af­ter the stream habi­tat and wa­ter qual­ity have im­proved. Sci­en­tists at NIWA are try­ing to un­der­stand the con­di­tions that al­low aquatic an­i­mals to re­colonise re­stored streams.

Photo: Brian Smith

Plant­ing a va­ri­ety of na­tive plants pro­vides ad­di­tional ben­e­fits: shade, bank sta­bil­i­sa­tion, and leaf lit­ter for food and habi­tat.

Photo: Brian Smith

Fenc­ing keeps live­stock out of streams, and en­cour­ages thick grasses which fil­ter run-off from pas­tures.

Photo: John Steven­son

Tak­ing sam­ples to check on wa­ter qual­ity.

Photo: John Steven­son

A com­mu­nity group check­ing wa­ter sam­ples.

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