Assessing drivers of plantation forest productivity on eroded versus non-eroded soils on hilly and steep land in eastern North Island, New Zealand: From catchment to regional scale

Abstract

The impacts of soil erosion on forest productivity in a Pinus radiata plantation forest were assessed to determine the effects of key soil properties on tree volume. Fifteen plots on soils markedly eroded (truncated) by landsliding were established in a plantation forested atchment (Pakuratahi) comprising mainly hilly and steepland in a dominantly mudstone/sandstone terrain overlain in places with a mantle of pumiceous tephras in Hawke’s Bay, eastern North Island, New Zealand. Fifteen control plots were established on noneroded soils alongside the eroded soils on landslide scars for comparison. Soils within these eroded and non-eroded sites were analysed and associated trees were measured for height and volume. The analyses of 0.1 m of the soil profile within eroded sites, compared with those of non-eroded sites, revealed that soil properties in eroded sites had significantly lower values: total nitrogen (N) 52%, total carbon (C) 47%, total phosphorus (P) 43% and soil organic matter 36% (SOM) (at 99% confidence interval). The C:N ratio, soil pH, and mean soil depth in eroded plots were also significantly reduced (at 95% confidence interval). Trees on the eroded sites had 14% less volume compared with those growing in soils at the non-eroded sites. Regression analysis revealed a significant relationship between tree volume and total C, total N, and SOM at 99% confidence interval and total P at 95% confidence interval. Total recoverable volume was less in trees from eroded plots. Log quality and value was assessed using Atlas Forecaster. High-quality pruned logs and large unpruned structural grade sawlogs returned smaller volumes from the eroded plots than from the non-eroded plots. Lower-grade sawlogs returned a higher volume in the eroded plots. Total recoverable volume for a 25-year rotation growing on eroded sites was valued at $68,494 (2013 value) - 9% less than the estimated value from the control plots ($75,989). Eroded plots yielded 16% less volume from high quality pruned logs which accounted for a reduction in revenue of around $4,000 per hectare at eroded sites. A reduction in total recoverable revenue was estimated at $7,500 per hectare on eroded sites. Erosion scars were measured and the average soil loss was estimated and reported at 0.1 m depth. On average, an eroded plot lost around 277 m³ of soil. In total, around 415 tonnes of soil had been displaced from all measured eroded sites. Using digital layers orthorectified from aerial photographs, it was estimated that Pakuratahi catchment has erosion scars covering around 12 hectares of a total area of 774 ha. Extrapolating the soil data from the plots to the catchment, it was estimated that 2385 t of the soil profile, 271 t of C, 20 t of N, and 3 t of P have been displaced from the slopes of Pakuratahi catchment in the last 100 years. Using a non-linear regression model the potential soil loss following an 80-year and a 100-year extreme rainfall event was predicted. Total soil loss in Pakuratahi catchment was estimated to be 2630 t and 7890 t for an 80-year and 100-year event, respectively. To gain perspective of the value of avoided erosion or erosion mitigation, at a regional scale in Hawke’s Bay, areas with terrain attributes comparable to those of Pakuratahi catchment were identified. The area currently under plantation forestry in such landscapes is around 65 km². The conservative estimate of soil loss on unforested land following an extreme rainfall event for this area is up to 1.69 x 10⁵ t. Based on the estimated soil loss calculated from Pakuratahi eroded sites, the value of afforestation of these potentially erodible soils is around NZ$288 per tonne of soil. Afforestation of erodible land provides a valuable ecosystem service through land stabilisation but this service is currently not recognised financially in New Zealand.