DNA and Isotope Methods for Timber

In animals and fresh plant tissue, DNA extraction is relatively straightforward because living cells with intact nuclei are abundant. In mature wood, the xylem cells (vessels, tracheids, fibers) are dead and have undergone programmed autolysis, breaking down most of their nuclear content. What DNA remains is fragmented, often to pieces shorter than 100 base pairs, and is embedded in a cell wall matrix of cellulose, hemicellulose, and lignin, along with species-specific secondary metabolites such as tannins in Quercus and anthraquinones in some Dalbergia species.

• Grinding: Samples are cryogenically ground to a fine powder (80 mesh) in liquid nitrogen to maximize surface area. Coarser grinding leaves intact cell walls from which extraction is inefficient.
• Inhibitor removal: Polyvinylpyrrolidone (PVP) or CTAB (cetyltrimethylammonium bromide) buffers at high salt concentrations precipitate polyphenols before DNA purification. Without this step, tannins co-purify with DNA and block Taq polymerase in subsequent PCR.
• Short-fragment PCR: Because wood DNA is degraded, primer pairs are designed to amplify fragments of 80-150 bp rather than the 500-1000 bp typical for fresh tissue. Mini-microsatellite primers are the standard in timber forensics. Longer amplicons simply fail.
• Replication: Low DNA yield and PCR inhibitors make allelic dropout (failure to amplify one allele at a heterozygous locus) a real risk. Best practice replicates each PCR at least twice and does not report a homozygous result that could be due to dropout without replication confirming it.

In most angiosperms and conifers, chloroplasts are inherited maternally, passed from the maternal parent to seeds without mixing with paternal chloroplasts during fertilization. This means the chloroplast genome does not recombine. Mutations and microsatellite repeat-length changes in cpDNA accumulate over generations and are carried by seed dispersal, but not mixed by pollen flow. As a result, cpSSR haplotype distributions across a species' range are geographically structured: populations separated by barriers (oceans, mountain ranges, unfavorable habitat) carry distinct haplotype combinations that can be mapped.

The population assignment workflow requires a reference database of cpSSR haplotypes sampled from known-location trees across the species' range. For Swietenia macrophylla (big-leaf mahogany), the best-developed reference database was published by Lemes et al. (2010) with samples from across the Amazon basin. An unknown mahogany specimen is genotyped at 5-7 cpSSR loci, the resulting haplotype is compared to the reference database using likelihood or Bayesian assignment tests, and the population-of-origin is reported with a probability estimate.

Stable isotopes are incorporated into wood cellulose and lignin as a tree grows, recording the chemical composition of its environment. The key ratios for timber forensics are:

• Delta-D and delta-18O (hydrogen and oxygen): reflect local precipitation composition, which varies with latitude, altitude, distance from the coast, and climate. These are the most used ratios for broad geographic discrimination.
• Delta-13C (carbon): reflects atmospheric CO2 isotope composition and plant water-use efficiency. In tropical species it can discriminate between lowland rainforest and montane forest environments.
• 87Sr/86Sr (strontium): reflects the geology of the soil parent material. Strontium ratios are geologically controlled and can distinguish timber from limestone-derived soils from timber from granitic soils, or from volcanic versus ancient metamorphic parent material.

The analytical workflow involves combustion or acid dissolution of the wood sample, then measurement of isotope ratios by isotope ratio mass spectrometry (IRMS) for light elements and by multi-collector ICP-MS for strontium. Cellulose is sometimes extracted before analysis to remove non-structural compounds that can introduce noise, particularly for hydrogen and oxygen.

A published study on Swietenia macrophylla by Boner and Forstel (2004) demonstrated that hydrogen and oxygen isotopes alone could correctly assign timber to country-of-origin at accuracy rates above 80 percent when compared to a calibrated isoscape. Combining strontium ratios raised assignment accuracy further. For high-enforcement species with good reference data, isotope assignment is now considered court-admissible evidence in several jurisdictions.

Inductively coupled plasma mass spectrometry simultaneously measures concentrations of 20-50 trace elements in a dissolved sample. The relative pattern of elements, barium, strontium, manganese, zinc, iron, rare earth elements, and others, reflects the soil chemistry at the tree's growth site. Because soil chemistry varies regionally with geology, weathering history, and contamination history, the multi-element profile constitutes a geographic fingerprint that complements isotope ratios.

Wood powder is dissolved in nitric acid (microwave digestion), diluted, and run through the ICP-MS. Data reduction applies multivariate statistics, typically linear discriminant analysis or random forest classifiers, to assign an unknown sample to a reference population. The method is rapid (30-60 minutes per sample after digestion) and can process large batches, making it practical for port-of-entry screening of consignments. It also works on highly processed products where DNA has been degraded, because inorganic elements survive processing steps that destroy nucleic acids.

Each processing step degrades the analytical signal. Kiln drying at 100-120 degrees C fragments DNA further. Steaming for bending fragments it still more. Chemical kraft pulping for paper manufacturing uses sodium hydroxide and high temperature and effectively destroys all DNA: paper manufactured from Dalbergia pulp cannot be identified as Dalbergia by any PCR-based method. For these end products, isotope and element methods are the only remaining tools, and even these lose accuracy as the processing chemistry alters elemental and isotope ratios.

• Veneer (0.5-2 mm): DNA still recoverable by short-fragment cpSSR methods. Anatomy also works. Isotopes reliable if cellulose is extracted.
• Kiln-dried lumber: DNA recoverable with optimized protocols from sapwood. Anatomy fully intact. Isotopes reliable.
• Charcoal: DNA destroyed by pyrolysis. Anatomy sometimes preserved as ghost structure in charcoal (anthracology), readable by SEM. Stable isotopes alter during charring; strontium and some elements still informative.
• Wood flour / composite board: DNA degraded below amplifiable size. Element fingerprinting may still work if adhesive matrix does not dilute the signal. Highly challenging for species ID; anatomy is destroyed.

In practice, timber forensics casework is multi-method. Anatomy identifies the genus at minimum cost and time. If genus identification is sufficient for the seizure (because the genus as a whole is CITES-listed), the case may stop there. If species or geographic origin is required, DNA and isotope work follow. ICP-MS element fingerprinting is added when high-value shipments justify the cost or when the number of samples is large enough that throughput matters more than per-sample cost.

The US Forest Service's WoodID program and its associated molecular laboratory use exactly this tiered approach. Anatomy and macroscopic examination screen the shipment. Suspect samples go to DNA extraction. When cpSSR assignment is inconclusive, isotope analysis is commissioned from the stable isotope laboratory. The method chain mirrors the cost and complexity hierarchy: anatomy is cheap and fast, DNA is moderate, isotopes are slower and more expensive, and the combination is reserved for the highest-value or most-contested cases.