Impacts of forest harvest removal and fertiliser additions on end of rotation biomass, carbon and nutrient stocks of Pinus radiata

Abstract

Forest harvesting practices remove tree biomass and the nutrients that they contain. There is uncertainty around the impact of this nutrient removal on the long-term sustainable production of forests over multiple rotations. To explore the impact of harvest management practices on planted forest sustainability, three long-term trials in Pinus radiata D. Don forests were assessed at the end of their second rotation of 26 to 27 years to determine the impact of harvest residue removal and fertiliser additions on sustainable forest productivity. The harvest residue removal treatments were stem only (SO), whole-tree (WT), and whole-tree plus forest floor (FF) removal with and without urea-nitrogen fertiliser additions (cumulative range across three sites 950 – 3200 kg N ha−1) from early rotation out to mid-rotation (last fertiliser application age range across sites 8 – 22 years). The FF removal treatment reduced soil carbon and nitrogen stocks throughout the subsequent rotation, whereas forest floor carbon and nutrient stocks had recovered from the FF removal treatment by the end of the rotation. Forest productivity was reduced at one site with the FF removal treatment, taking 3 years longer for stems to reach 35 cm diameter at breast height than other residue removal treatments. Between WT and SO harvest residue treatments there were very limited impacts on soil carbon and nutrient stocks, no impact on forest floor mass, carbon and nutrient stocks and the sum of the above ground live biomass, carbon and nutrient stocks or tree productivity. Across all harvest removal treatments fertiliser addition increased soil carbon and nitrogen stocks for sites with low initial soil stocks. The increased soil carbon and nitrogen stocks from fertiliser addition were associated with forest productivity gains with the time to reach 35 cm (diameter at breast height) for one site reduced by 3 – 5 years. Fertiliser addition to sites with FF removal was able to mitigate soil carbon and nitrogen stocks only when very large amounts of fertiliser were added (cumulative amounts of 1150 and 3200 kg N ha−1). The end of rotation results from this long-term study demonstrate the importance of harvest residues and forest floor material at low fertility sites for long-term nutrient sustainability in planted forests.