Martin Toop, of Hamilton, asks :-
All I have read say forests are needed to store carbon. Surely we can plant trees for many other important reasons, eg employment, moisture and soil retention, cleaner rivers, and high value tree crops to replace dairy. Shouldn't these other aspects be more widely propagated?
Les Basher, a soil scientist at Landcare Research (Nelson) responded.
Planted forests, or trees, provide a wide range of benefits beyond storing carbon. These include erosion control, improved water quality, habitat for native species, and flood mitigation as well as being a recreational resource, a significant employer and earner of export earnings.
Hillslopes with established forests can reduce landslide erosion (compared to pasture) by up to 90% in large storms, and they have also been shown to be effective in reducing rates of earthflow movement and in controlling gully erosion. Space-planted trees (willows and poplars) are widely used in pastoral farmland to control erosion and have been shown to reduce shallow landslides by 95% compared to paired pasture control sites.
Afforestation of pasture can improve water quality by directly shading streams and lake margins and by reducing nutrient and bacterial inputs. Forests can reduce flood peak flows, with the effect being greatest (30-90% reduction) for smaller floods and in small catchments where forest cover is extensive. The effect is smaller (20-50% reduction) for large catchments with partial forest cover, and for larger floods when the whole catchment may become saturated.
While planted forest typically supports fewer native species than a native forest at the same site, they provide habitat for at least 118 threatened native species in New Zealand (e.g. brown kiwi). They can also provide connectivity between native forest remnants.
Currently the New Zealand forest products industry employs 17,695 people directly in growing and processing the trees, and another 37,000 in forest-related activities. The industry is the 3rd largest export earner making up 3.2% of GDP and contributing $4.8 billion in export earnings (figures sourced from New Zealand Forest Owners Association).
Many planted forests provide important recreational opportunities. A 2011 study of Whakarewarewa Forest at Rotorua found there were 94,000 users per year and estimated the economic value from walking and mountain biking was $15.4 million per year, exceeding the value of timber production ($4.6 million).
However there are also some down sides to planting forests. Afforestation of pasture catchments typically reduces the total water yield, typically by about 30%, which can be very important especially in drier areas of New Zealand. Harvesting of forests from steep, erodible slopes also temporarily increases erosion and sediment yield and disrupts habitat for resident fauna.