Grade recoveries from sawing 22-year-old unpruned cypress clones
By Rosie Sargent, Toby Stovold, February 2021.
Download SWP-T116 (pdf)
A Scion clonal cypress trial established in 1997 provided the opportunity to determine the grade recoveries of sawn timber from two cypress clones; GH5 (Cupressus lusitanica) and Ovensii (Cupressus lusitanica x Chamaecyparis nootkatensis). As well as giving information on the wood properties from these new clones, this study demonstrates the viability of young trees for sawn timber production. Cypresses are typically pruned and thinned to maximise clearwood recovery while minimising the incidence of bark encased knots, which substantially downgrade the timber. Harvesting unpruned trees at a young age reduces the incidence of bark encased knots, and provides high appearance grade recoveries for a low-intensity silvicultural regime.
The two clones were grown as part of a cypress hybrid trial in Rotoehu forest, in New Zealand’s North Island. The GH 5 clone was harvested from a clonal block planted at 800 stems per hectare and were unpruned and unthinned. The GH 5 clone was notable for is wavy stem form but fine branching. The Ovensii clone was harvested from row plots and were mostly of straight form. Fourteen trees were harvested, and the 36 logs were sawn at Ruapehu sawmill in Raetihi with a sawn timber recovery of 58% from an average SED of 233mm and with approximately 50% nominal grade recoveries. The dried boards were visually graded for appearance and structural grades, and acoustic stiffness measured on a subset of the boards.
Overall volume recoveries were good, with 51% of the volume of logs harvested converted into graded timber. Grade recoveries were very high with over 90% of boards reaching the top appearance grade (modified Dressing grade, which allows a maximum intergrown knot size of 50mm), and almost all boards reaching No. 1 framing grade. Grade recoveries were slightly higher in the C. x ovensii boards, due to a lower incidence of bark encased knots compared with the C. lusitanica clones (GH5). Bark encased knots are an issue for older unpruned trees, and as the trees grow older the number of bark encased knots is expected to increase. The number of bark encased knots in the C. x ovensii boards were very low, and it is not clear how quickly these would increase over time, and whether the trees could be harvested at a later age without a significant increase in the incidence of bark encased knots.
A brief economic analysis was performed to gauge the viability of sawing young unpruned trees. Three scenarios, each with an emphasis on different products, were compared: Thermally modified cladding; Interior panelling; and exterior products from 100% heartwood boards. For both clones the thermally modified cladding scenario had the highest margins (product price minus processing cost). This suggests that producing thermally modified cladding from young unpruned trees could be viable, and that the continued development and testing of thermally modified cypress is a priority. For the C. x ovensii, the higher proportion of heartwood means that sawing into conventional outdoor products (which require 100% heartwood) could also be viable; the heartwood-only scenario had the same average margin as the thermal modification scenario (31%). For the GH5, the heartwood-only scenario gave the lowest average margin (16%). Considerable variations in processing cost and product prices were found between different sources, and it is not known how sensitive the cypress timber market would be to increased supply of timber. The processing of cypress (including thermal modification) will be investigated in more detail in upcoming WoodScape modelling work being undertaken at Scion.
Acoustic stiffness was similar in each species (average 8.7 GPa) with a lot of variation between boards. The basic (oven dry) density of C. x ovensii was significantly higher than the GH5 (average 450kg/m3 and 360 kg/m3 respectively).
Overall the silviculture regime used for growing (plant and leave 800 stems per hectare) gave good grade recoveries, and the C. x ovensii clone had a high proportion of heartwood (average 61%, compared to 30% for the GH5). It may be possible to grow this clone for longer, to increase the volume of heartwood in the logs without the formation of excessive numbers of bark encased knots, but further work would be needed to confirm this.