Early rotation biomass and nutrient accumulation of Pinus radiata forests after harvest residue management and fertiliser treatment on contrasting types of soil

Abstract

The productivity of planted forests depends upon the maintenance of soil fertility and an understanding of the relationships between site fertility and stand productivity. The effects of harvest residue and forest floor management, and fertiliser additions on ecosystem biomass, carbon and nutrient accumulation were evaluated 5 years after establishment of Pinus radiata stands growing on four contrasting soil types at Woodhill, Tarawera, Kinleith, and Golden Downs forest in New Zealand. The harvest residue removal treatments were stem only harvest, whole tree harvest, and whole tree harvest combined with forest floor removal, all with and without mostly urea-nitrogen fertiliser additions (cumulative range across the four sites 400–1150 kg N ha−1). Woodhill forest biomass and nutrient stock results at age 5 years have been previously reported and were added to with new carbon and nitrogen analysis. The results showed for all sites except on recent sands at Woodhill, where soil nutrients were very low, that harvest residue and forest floor removal had limited impact on 5 year old P. radiata productivity, above ground live biomass and nutrient accumulation, and mineral soil nutrient concentrations. Across all sites the whole tree harvest combined with forest floor removal negatively impacted the 5 year old forest floor biomass and nutrient stocks, with variable impact due to whole tree only removal. The addition of large experimental amounts of mineral nitrogen fertiliser increased tree productivity across sites, which was reflected in tree biomass for all sites except Tarawera. Based on these relatively short-term findings, when P. radiata forests in New Zealand are experiencing a peak nutrient demand (highest nutrient demand amount per year), there were limited negative impacts of harvest residue and forest floor removal on forest productivity, tree biomass and nutrient accumulation, and mineral soil nutrient concentrations. The exception was for a sand dune derived soil that had very low soil nutrients resulting in a greater proportion of the forest ecosystem nutrient pool to be held within the tree and forest floor biomass, emphasising the importance of harvest residue and forest floor retention at low nutrient sites.