The performance of American Hardwood
The American Hardwood Export Council (AHEC) is engaged in the largest ever Life Cycle Assessment (LCA) project to cover hardwood products.
An important starting point for LCA in the wood sector is to assess the sustainability of harvesting at source. Forests act as carbon sinks, since trees absorb carbon dioxide from the atmosphere through photosynthesis and store it as carbon. When the trees are harvested, much of the carbon remains stored in all the resulting products, thus mitigating climate change. In order for wood products to be credited with this carbon storage effect, it’s important that they derive from a renewable source where growth exceeds harvest. For this reason, the AHEC project incorporates a detailed analysis of US-government forest inventory data gathered at regular intervals over the last sixty years. For walnut, this data shows that the US harvest averages around 1 million m3 each year, well below the annual growth rate of 3.6 million m3. Even after harvesting, an additional 2.6 million m3 of walnut accumulates in US forests every year.
Raw material and finished product are moved all around the world by sea freight. The AHEC LCA study confirms that the environmental impact is far lower than that of other forms of transport such as rail and road. The initial Life Cycle Inventory (LCI) data on US hardwood lumber exported to Europe confirms that forest extraction and the sawmilling process make up a small part of the overall impact and that it is the kiln drying of the lumber, not the transport that is the biggest factor in determining impacts such as Global Warming Potential (GWP). Heavy dependence on biomass rather than fossil-fuel energy during kilning is a particularly important factor in keeping the overall carbon footprint of American hardwoods low. In fact, transporting American hardwoods by ship six thousand kilometres across the Atlantic requires little more energy than an overland journey of five hundred kilometres.
Case study: Playhouse Theatre
The initial brief for architects Studio 101 for the Playhouse Theatre at the Geelong Performing Arts Centre (GPAC) was to brighten the space, make it more welcoming and to review its acoustics performance. The Geelong-based practice specializes in timber-based design solutions and is drawn to wood. Australian timber such as blackbutt or cypress might have been considered, but the architects say the choice was much more about “honesty of expression in materiality … Other Australian hardwoods can become quite lightish, and we certainly didn’t want bounced or reflected light. Whereas walnut, with its mid-colour toning and warmth has none of those issues.” Although finishing and maintaining timber elements can require a little extra work, Studio 101 says variability was a key factor in the choice of the walnut: “It is not too inconsistent and has a nice overall texture, plus each board has its own texture and colour toning.”
It identified the following key benefits American walnut brought to the GPAC project:
• it is select grade for visual consistency
• it has the right durability and hardness for vertical linings and
ceiling soffits (Janka rating 4.5)
• it has rich, mid-range colour toning and tight, delicate grain
• it has a wonderful tactile quality
• it is harvested from sustainably managed hardwood forests
• it is readily available at the time of specification in veneer and
shiplap cladding
• it has excellent acoustic properties
• it is “deemed to satisfy” fire-hazard indices.
But what about the environmental performance of American hardwood? Such is the size and productivity of the US walnut resource that it takes no more than five minutes for the walnut used at the Geelong Performing Arts Centre to be replaced by new growth in the forest. The walnut used at Geelong will act as a carbon store for as long as it remains in the building. This storage benefit may be extended considerably if the walnut is then reused or recycled. At the end of the building’s life, if the walnut is not recycled, it could be burnt as a renewable fuel generating up to 37,000 KWh of energy.
As for the carbon footprint, the AHEC LCA study shows that the 15 m3 of walnut kiln-dried lumber used in the project stores the equivalent of 31 tonnes of carbon dioxide (see chart). This is well in excess of the carbon emissions – totalling a little under 18 tonnes – required to extract, process, and transport the walnut lumber all the way from the US to Australia. In LCA terminology, the walnut used in Geelong has a “Global Warming Potential” of minus 13 tonnes. The assumptions made here about transport distances are very generous. It is assumed that the walnut travels 220 km by road before kilning, then another 1325 km by road before reaching the US port of export. It is then shipped the long way round the globe, from the US east coast via Suez to Singapore for transhipment to Melbourne (a grand total of 26,000 km). The wood came to Geelong via a local Melbourne importer, so the 200 km allowed for road transport once inside Australia also seems generous.
Despite these huge distances, transport does not emerge as an overwhelmingly significant factor in the overall carbon footprint.
Carbon emissions during transport are no greater, in fact, than those that occur during sawing and kilning. Efficiency of wood processing is at least as important as transport in determining the overall impact.