Workshop Report: Poplar and willow planting for Land Overlay 3A, Gisborne, East Coast region
Comparative effectiveness of wide-spaced pole planting and radiata pine
The comparative effectiveness of poplars versus radiata pine for erosion control was raised at the workshop. The results of empirical studies of poplar height, diameter at breast height (dbh), root biomass and root strength were presented to benchmark the effectiveness of poplar pole plantings at various densities compared to radiata pine planting at 1250 spha. Such an approach enables a theoretical threshold root biomass to be calculated to provide effective soil reinforcement. East Coast poplar tables for dbh and stems per hectare have recently been correlated with root biomass per hectare (Knowles 2006). Allowing for variation in growth at different sites, this correlation indicates that mature poplar stands need to be about 70–100 spha to achieve the same root protection as closed-canopy pine forest and that about 160–200 spha are needed to achieve quick stabilisation by younger trees on severely eroding sites. These indications are supported by a similar correlation for a poplar stand in the Manawatu (McIvor 2007), with the proviso that McIvor inclines towards the lower end of the spha range, in view of poplars’ superior fine-root mass.
The 2004 storm damage data for a random sample of 79 poplar and willow plantings in the Manawatu–Wanganui region have been converted to spha, enabling comparison with Knowles’ and McIvor’s tables (Hicks 2007). The converted data show little fresh erosion where spha on unstable ground exceeds 50 spha, but substantial erosion where spha are less than 50. Fresh erosion was measured at just one site with greater than 100 spha. The sites had been planted in response to earlier severe erosion, during wet winters from the 1950s through the 1970s. Most stands are more than 20 years old and corresponds with Knowles’ and McIvor’s mature category. Hicks’ data supports their conclusions, that final stand density needs to be at least in the 50 to 100 spha range for adequate protection of soil; and that initial planting density in excess of 100 spha helps achieve stability at severely eroding sites (particularly if it can be maintained long-term).
However, it seems clear that more information on poplar and willow root biomass development is required to enable this approach to be used with confidence.
Contact for Enquiries
East Coast Forestry Project
Ministry of Agriculture and Forestry
Cnr Grey St & Childers Rd
1st Floor ZGFM Building
PO Box 2122
Gisborne 4040
Tel: +64 6 986 8691
Fax: +64 6 986 8697
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