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Scientists can make use of different techniques to identify the species to which a piece of wood belongs, and its geographical origin. Depending on the technique used, the test piece can be tiny (e.g. veneer a few millimetres thick or a sliver cut from a piece of art), small (e.g. a few cubic centimetres cut from a piece of furniture) or its actual size (non-destructive methods). In addition to the sample size needed for the analysis, each technique has its advantages and limitations. Consequently, one of GTTN’s goals is to facilitate the integration of these different methods.

Aside from the well-established wood anatomical method, timber-tracking techniques are innovative and still evolving. The most pressing need is to extend the collection of reference data, which form the basis for the verification of species and origin of a traded wood-based product.

 

Wood anatomy

Method

The wood’s macroscopic and microscopic structure is used to identify its genus (and species). The macroscopic structure is the combination of characters visible to the naked eye, such as colour, porosity, and colour patterns. The microscopic structure is the combination of characters that are only visible when putting a thin (µm) slice of wood under a light microscope, allowing a look at the different cell characteristics of which the wood is composed.

Recent developments in the field are automated recognition of species (machine vision) or logs (biometric log traceability) making use of image reference collections.

Advantages

The identification of wood via its anatomy is the oldest timber tracking method and can therefore build on decennia of experience. Extensive databases with reference samples for around 14,000 species are available from several wood collections worldwide.

Limitations

Some wood species resemble each other very closely in their wood anatomy, hindering species identification but still allowing distinction of the genus. While wood anatomy changes with local environmental conditions, it cannot give information about the wood’s geographic origin (as environmental conditions can be the same in different locations).

Wood molecular biology

Method

Genetic DNA is extracted from the wood cells by first pulverising the wood and then using several chemicals to isolate the DNA from other cell content. Specific parts of the DNA are then read and compared with reference data to identify species or geographic origin of the wood sample.

Advantages

Even the most anatomically similar wood samples can be identified up to species level. As well as information about taxonomic origin, DNA also contains information about the wood’s geographic origin as, over the years, species evolved and adapted to their location (as seeds and pollen have a limited dispersal distance). Depending on the species and available reference data, the spatial resolution reaches region, country or even concession level.

Limitations

Although used on the ground today, it is a relatively new method that is still improving and expanding its reference database. Developing reference data is a titanic work, where the DNA of a species has to be analysed in the search for pieces (referred to as genetic markers) that are distinct between species or geographical locations. The level of distinctness attainable, and hence the diagnostic power, varies. Except for the outermost wood of a tree, wood cells are dead, meaning that the DNA is degraded, which makes it more challenging to extract DNA that is useful for genetic analysis. Furthermore, the processing of wood (heat, glues, coatings, …) hinders extraction of useable DNA. This, in combination with the screening of the DNA for its species or origin specific positions, makes DNA analyses relatively time intensive.

Wood chemistry

In addition to structural and genetic profiling, wood can also be characterised by its chemical composition, which is determined by tree species, as well as by climate and geology as these determine local growing conditions. Three wood chemistry methods, further explained below, can aid the determination of tree species and/or geographical origin.

Stable isotopes
Method

Stable isotopes are stable variants of a same atomic element, with the same amount of protons, but with different numbers of neutrons. Water, air and soil are characterised by stable isotope ratios (for elements such as C, H, O, S, N) that are influenced by climate and geology and are thus location specific. When a tree grows and takes up water, nutrients and carbon dioxide, these stable isotope ratios are passed on to the wood, imprinting it with a geographical marker that can be used to identify the wood’s origin.

Advantages

The method is well developed and has been used already for years for determining the provenance of food products.

Limitations

Availability of reference data is crucial, as for all methods. As the chemical elements are taken up out of the environment, the isotope composition varies between tree rings. The position where the test samples are taken is thus of high significance. The spatial resolution varies but current developments may lead to finer scale origin identifications by combining with Strontium isotopes, rare earth and trace elements. Similar to DNA, the attainable spatial resolution, and hence the diagnostic power, varies. Isotope ratio analysis cannot be used for species identification.

Mass spectrometry
Method

The chemical composition of wood is determined by geographical and environmental but also by genetic factors. This can therefore be used to identify wood species and, to some extent, also geographical origin. Chemicals in and on the surface of wood can be volatised by applying a stream of helium ions heated to 350°C. These are then ionised in a mass spectrometer to generate a chemical profile, which can then be compared to spectrometric reference profiles.

Advantages

This method requires nearly no sample preparation, is non-destructive (a sliver of wood is sufficient) and fast. Closely related species can be differentiated and, for some taxa, the geographic origin can also be distinguished.

Limitations

A limited reference collection is available to date but the reference collection is expanding continuously. Furthermore, although most phytochemical profiles are species-specific, not all of them are.

Near-infrared spectroscopy
Method

As for mass spectrometry, phytochemical properties can also be assessed using near infrared spectroscopy (NIRS) signatures. The absorption spectra of timber are measured when exposed to near-infrared electromagnetic energy. They provide information on both the chemical and physical structure of wood.

Advantages

This method requires almost no sample preparation, is non-destructive and fast. Closely related species can be differentiated and the technique has the capacity to discriminate between geographic provenances. An integrated approach with analysis of isotopes and trace elements can yield high levels of accuracy.

Limitations

NIRS reference data are still limited. As a stand-alone method, the identification accuracy is variable.

 

Further reading:

► Dormontt, E. E., et al. (2015). “Forensic timber identification: It’s time to integrate disciplines to combat illegal logging.” Biological Conservation 191: 790-798.

► Lowe, A. J., et al. (2016). “Opportunities for Improved Transparency in the Timber Trade through Scientific Verification.” BioScience 66(11): 990-998.

► NEPCon (2017). “Timber Testing Techniques. A guide to laboratory techniques to determine species and origin of timber products.” Thematic article series no. 1.

► Schmitz, N., et al. (2019). “The Timber Tracking Tool Infogram. Overview of wood identification methods’ capacity.”

More detailed information can also be found in the Document library