Decomposition of coarse woody roots and branches in managed Pinusradiata plantations in New Zealand – A time series approach

A time series decay trial was used to determine the decay rate of Pinus radiata (D. Don) woody root and branch material in recently harvested or newly planted forests at eight locations covering a range of climate and soil types in New Zealand. Root and branch initial density was measured in live material and then annually over a period of 4 years. Changes in the density over time were used to estimate the decay rate constants using a single exponential model. Coarse roots decayed faster than branches at most sites. There was no diameter effect on decay rate with a diameter range for roots of 10–152 mm and branches 10–130 mm. Carbon concentration did not change with mass loss but there was a significant increase in nitrogen concentration. The mass of nitrogen in roots and branches was 1.2 times greater than the initial live nitrogen mass when 66–77% of the original live mass remained. Nitrogen mass decreased once mass loss exceeded this value. Exponential models based on mean annual temperature and decay time provided the best estimates of the decay rate of woody roots and branches arising from harvest activities.

► No diameter effect on decay rate.
► Woody roots decay faster than branches.
► Models including MAT provide the best estimate of residue decay in New Zealand.
► No increase in carbon concentration with decay.
► Increase in nitrogen concentration with decay.