Defining sustainability of plantation forests through identification of site quality indicators influencing productivity—A national view for New Zealand

Abstract New Zealand is committed to developing sustainable forest management practices as evidenced through Government involvement in international forestry agreements such as the Montreal Process, and the forestry sector's adoption of forest certification mechanisms. In support of this commitment, it has been identified that there is little quantitative evidence of the interactions of plantation forestry on site quality and long-term site productivity. To address this issue, a nationwide study of site quality was initiated at 35 key sites covering the range of edaphic and environmental conditions representing the productivity envelope for New Zealand plantation forests. At each location, within the productivity envelope, eight short-term site quality plots were planted at a very high stand density (40,000 stems ha−1) to rapidly identify key soil indicators of growth which may be useful for determining site sustainability. In addition, a permanent sample plot was established by planting seedlings at conventional stem densities (500–1100 stems ha−1). At each site, a factorial design was applied with the following three factors: species (Pinus radiata and Cupressus lusitanica), fertiliser (no fertiliser and nutrients supplied in excess of crop demands) and disturbance (low and high disturbances). After two years of increment, initial analyses are presented which partition treatment and site effects on increment and identify key soil properties that influence increment of the two species. Volume increment over the two-year period was most strongly influenced by site, ranging 50-fold and 15-fold across sites for C. lusitanica and P. radiata, respectively. For the treatments, species accounted for most of the variance in increment, with mean volume increment across all sites of P. radiata significantly exceeding that of C. lusitanica by 56%. Fertilisation also significantly influenced volume increment inducing mean gains of 30%. Disturbance had a significant, but comparatively weak effect, reducing mean volume increment by 9%. After correction had been made for environment (temperature and rainfall), soil properties that were most strongly related to volume increment for both species included CN ratio, total soil nitrogen, total soil phosphorus, organic phosphorus and depth of the A horizon. When soil properties were included in combination, the best predictive models of volume increment formulated for both species included rainfall, temperature, the product of total soil nitrogen and total soil phosphorus and depth of the A horizon.