The effect of forestry management activities on stream water quality within a headwater plantation Pinus radiata forest

Abstract

Many long-term forestry water quality studies focus on the impact of the harvesting phase of forestry operations. As a result, the water quality impacts of other activities such as forest (re)planting, thinning and fertilising are less well understood. Here we report the results from 23 years of monthly water quality monitoring from a steep headwater catchment within the Waikato region, New Zealand. Three experimental sub-catchments were planted in different proportions of Pinus radiata (PW2 = 100%, K-Pine 57%; PW3 = 36%). PW2 and PW3 were historical pastoral sites in the first rotation of plantation forestry and K-Pine was harvested and subsequently planted in a second rotation of forestry. All sites underwent two phases of stand thinning six and eight years after planting. The most significant effect of forestry was the increase in nitrate-N (NO3-N) and total nitrogen (TN) concentrations in response to both stand planting and stand thinning operations. Planting resulted in significant increases in NO3-N, TN and DRP over a four-year planting period at PW2. Significant increases in NO3-N and TN were detected over a four-year thinning period, at all three experimental sites. We propose that stream NO3-N and TN concentrations increased in response to stand planting at PW2 because of the change in physical conditions (e.g. increased stream shading, proliferation of nitrogen fixing weeds and decreased spatial extent of seepage wetlands) caused by the change in land use. Increasing nitrogen concentrations due to decreasing stream runoff could also be contributing a factor. Any decrease in NO3-N and TN concentrations in response to increased uptake by the rapidly growing P. radiata appears to have been masked by these other factors. However, the increase in NO3-N and TN concentrations in response to stand thinning at all three experimental sites suggests that NO3-N uptake by growing trees is an important process. Over the entire forestry period, median NO3-N concentrations at PW2 (first rotation site) increased by a factor four (400–1590 µg l−1). The median NO3-N during the post-thinning period was also one order of magnitude higher than those measured at K-Pine (second rotation site) during the post-thinning period. We attribute these differences to the NO3-N legacy within soil and/or groundwater of the previous sheep and beef cattle grazing land use. Long-term use of the PW2 catchment for forestry may eventually reduce NO3-N concentrations to the lower levels measured at K-Pine. Therefore, while NO3-N concentrations and loads may increase in response to the establishment of plantation forestry we envisage that these will decline if plantation forestry continues beyond the first rotation. This research highlights: (i) the complex nature of the response of stream water quality to catchment land use changes, and (ii) the value of studies that monitor the impact of land use/cover change over the long-term.