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Highlights • Decay rates of above-ground CWD are given for nine New Zealand tree species. • Decay rates were also determined for below-ground CWD for two species. • The half-life of above-ground CWD averaged 25.7 years across species. • The decay rate varied between species and was influenced by diameter. • Below-ground CWD decay rates were similar or faster above-ground CWD decay rates. Abstract Decomposition rates of above-ground coarse woody debris (CWD) (>10 cm diameter) were determined for nine common New Zealand indigenous tree species, and of below-ground CWD for two species. CWD samples were obtained from fallen trees originating from known windfall events which had been decaying for 10–29 years on the forest floor. Species-specific decay rates were estimated from the density (dry mass divided by original volume) of CWD samples relative to the density of samples from live and freshly fallen trees. The estimated half-life (t 1/2) or time to lose 50% mass for above-ground CWD across all species was 25.7 years but varied between species and increased with increasing stem diameter. Adjusted to a common diameter at breast height of 60 cm and mean annual temperature of 10 °C, Prumnopitys taxifolia and Fuscospora cliffortioides were the slowest to decay with a t 1/2 of about 40 years, Fuscospora fusca , Weinmannia racemosa , Beilschmiedia tawa , Lophozonia menziesii and Prumnopitys ferruginea the next slowest with a t 1/2 of 27–34 years, while the remaining species (Dacrydium cupressinum and Dacrycarpus dacrydioides) decaying more rapidly with a t 1/2 of less than 20 years. A model was derived for predicting the exponential decay constant for the above-ground CWD of individual stems as a function of diameter at breast height and mean annual temperature adjusted for species. This model can be used to predict above-ground CWD carbon stock changes in New Zealand's natural forests following tree mortality. Below-ground CWD decay rates did not differ significantly from those of above-ground CWD for D. cupressinum , but were significantly faster for L. menziesii. [ABSTRACT FROM AUTHOR]

Highlights • Field measurements indicate New Zealand's natural forest CWD carbon stocks are in steady state. • However, measured inputs from mortality exceed modelled loses from decay. • It appears this discrepancy is due to stocks being underestimated using current measurement protocols. Abstract Natural forest comprises approximately 78% of New Zealand's total forest area and their dead wood carbon pools contribute to the national carbon balance reported under the United Nations Framework Convention on Climate Change. We investigate the accuracy of coarse woody debris (CWD) carbon stock estimates obtained from periodic inventories of New Zealand's natural forest. Dimensions and decay classes of standing-dead spars, fallen logs and stumps over 10 cm diameter were assessed in permanent plots during two inventory cycles (2002–07 and 2009–14). Carbon in CWD was estimated by multiplying CWD volume with species-specific mean wood and bark density, a density modifier based on decay class, and the carbon fraction. The measured change in CWD between inventories was obtained by subtracting 2002–07 from 2009–14 mean CWD carbon stocks. This measured change in CWD carbon stocks was compared with the modelled change derived by applying a decay model to 2002–07 stocks to determine the losses from decay and adding inputs from the tree mortality recorded between the two measurement cycles. Estimates of CWD carbon stocks from field measurements were 19.9 and 19.5 tC ha−1 in 2002–07 and 2009–14 respectively, with a statistically non-significant change of −0.4 ± 0.6 tC ha−1 (95% C.I.) over the period between measurements implying that CWD carbon is in a steady state. However, the modelling approach gave a significant increase of 2.5 ± 0.7 tC ha−1, implying that CWD stocks are increasing. It appears likely that the primary reason for the discrepancy between the direct measurement and modelling approaches is that CWD carbon stocks are substantially underestimated using current measurement protocols used in the inventory of New Zealand's natural forest. If CWD carbon stocks in New Zealand's natural forests are approximately in balance as implied by the direct measurement of CWD stocks, CWD inputs from mortality would have to be balanced by losses from decay. For this to be true, the CWD pool would have to approximate 35.4 tC ha−1 which is 78% higher than its measured value. [ABSTRACT FROM AUTHOR]

Abstract: 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 4years. 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 10–152mm and branches 10–130mm. 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. [Copyright &y& Elsevier]

Abstract: Growing interest in the use of planted forests for bioenergy production could lead to an increase in the quantities of harvest residues extracted. We analysed the change in C and N stocks in the forest floor (LFH horizon) and C and N concentrations in the mineral soil (to a depth of 0.3m) between pre-harvest and mid-rotation (stand age 15years) measurements at a trial site situated in a Pinus radiata plantation forest in the central North Island, New Zealand. The impacts of three harvest residue management treatments: residue plus forest floor removal (FF), residue removal (whole-tree harvesting; WT), and residue retention (stem-only harvesting; SO) were investigated with and without the mean annual application of 190kgNha−1 year−1 of urea-N fertiliser (plus minor additions of P, B and Mg). Stocks of C and N in the forest floor were significantly decreased under FF and WT treatments whereas C stocks and mass of the forest floor were significantly increased under the SO treatment over the 15-year period. Averaged across all harvesting treatments, fertilisation prevented the significant declines in mass and C and N stocks of the forest floor which occurred in unfertilised plots. The C:N ratio of the top 0.1m of mineral soil was significantly increased under the FF treatment corresponding to a significant reduction in N concentration over the period. However, averaged across all harvesting treatments, fertilisation prevented the significant increase in C:N ratio of the top 0.1m of mineral soil and significantly decreased the C:N ratio of the 0–0.3m depth range. Results indicate that residue extraction for bioenergy production is likely to reduce C and N stocks in the forest floor through to mid-rotation and possibly beyond unless fertiliser is applied. Forest floors should be retained to avoid adverse impacts on topsoil fertility (i.e., increased C:N ratio). Based on the rate of recovery of the forest floor under the FF treatment, stocks of C and N in the forest floor were projected to reach pre-harvest levels at stand age 18–20. While adverse effects of residue extraction may be mitigated by the application of urea-N fertiliser, it should be noted that, in this experiment, fertiliser was applied at a high rate. Assessment of the sustainability of harvest residue extraction over multiple rotations will require long-term monitoring. [Copyright &y& Elsevier]

• End of rotation results released from the New Zealand LTSP trials. • Forest floor retention at harvest is beneficial to long-term site nutrition. • Whole tree versus stem only harvest had no impact on soil or forest productivity. • Fertiliser addition increased soil carbon and nitrogen stocks where they were low. 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. [ABSTRACT FROM AUTHOR]

Global interest in addressing knowledge gaps relating to the effect of forest harvest intensity on soil fertility and long-term site productivity has resulted in the installation of numerous experiments, including Long-Term Site Productivity (LTSP) trials. To explore this issue in the context of the New Zealand planted forest estate, six LTSP sites were established from 1985 to 1994 across differing climate and soil conditions, then subjected to varying levels of organic matter removal during the harvest of the trees. Here we present data describing live above ground, forest floor and mineral soil carbon and nutrient pools immediately prior to, and following, harvesting at each site. Harvest residue management practices employed included the removal of stem only, whole tree, whole tree plus forest floor, whole tree plus forest floor and topsoil, and the addition of double harvest slash material. The data provides an understanding of biomass, carbon and nutrient pools at harvest and the impact of different harvest removal treatments on these pools. With the maturation of the trees at the LTSP sites, the data acquires even greater future value by enabling changes in soil properties to be quantified and correlated to variations in the biological properties at the site, including site productivity and critical microbial parameters. Overall, these data sets comprise a foundation for New Zealand to address the question - can the productivity of intensively managed planted Pinus radiata be maintained or enhanced through the judicious management of organic matter and nutrient pools over successive growing and harvesting cycles?, (© 2019 The Authors.)

A combination of paired site, time series, and survey approaches were used to estimate the effect of land use change on mineral soil carbon (C), and to identify factors associated with variation. Land-uses compared included podocarp/hardwood forest, improved pasture, and pine plantation. Soil C was significantly related to soil pH that ranged between 3.9-5.9 (0-0.05 m), 3.6-6.0 (0.05-0.10 m), and 4.5-6.1 (0.10-0.50 m) in indigenous forest. Time series data obtained by periodically re-sampling soil (0-0.10 m) in permanent plots in a pine forest previously under pasture showed that mineral soil C decrease by approximately 4 Mg ha(-1) by the end of the first rotation. The time series data compared closely with mean results obtained at paired-site throughout New Zealand. Soil C concentration was highly variable in all land-uses, and the evidence suggests that chemical stabilisation of C occurred under acid conditions in native forest, through complexation with Al, and that effects persisted long after conversion of the native forest to other land-uses. The implications of these findings for the design of sampling protocols for soil C are discussed.