Reduction in Suspended Sediment Loads Following Canopy Closure: The Impact of Pasture‐To‐Pine Plantation ( <i>P.radiata</i> ) Conversion in a Headwater Catchment

Plantation forestry is an important land use in New Zealand (currently 7% of the area), and recent central government incentives for tree planting will likely result in further increases in the future. Plantation forestry in New Zealand often occurs on steep, high‐erodible land, often as a form of catchment restoration and/or erosion control. However, the potential for episodically high erosion and impacts on waters during tree harvest is a concern. We studied suspended sediment loads (SSLs) from the originally grazed pasture Mangaotama headwater catchment, which was partially converted to a pine plantation, over 31 years (encompassing almost a complete rotation) within a BACI (before‐after‐control‐impact) design. Suspended sediment sampling in both the impact catchment and a nearby control (the native‐forested Whakakai Catchment) was used to prepare sediment ratings for flow. We show that the SSLs within the Mangaotama catchment decreased by a factor of three in response to increasing forest cover (mostly P. radiata plantation) from ~2% to ~62% of the catchment. The establishment of plantation forest may have resulted in some reduction in SSLs shortly after livestock exclusion and planting, but the most significant reduction occurred after canopy closure (~8 years after planting). This post‐canopy closure SSL reduction was a response to reduced frequency and magnitude of flow events, particularly large events which transport most suspended particular material (SPM). We estimate that a full 28‐year rotation of P. radiata ( including harvest) would export ~30% less SPM than the same catchment under grazed pasture over the same time period. Given the scarcity of such empirical evidence from New Zealand catchments, these findings should usefully inform the debate around the environmental ramifications of converting pastoral land to plantation forestry.