To maximize utilization of our forest resources, detailed knowledge of wood property variation and the impacts this has on end-product performance is required at multiple scales (within and among trees, regionally). As many wood properties are difficult and time-consuming to measure our knowledge regarding their variation is often inadequate as is our understanding of their responses to genetic and silvicultural manipulation. The emergence of many non-destructive evaluation (NDE) methodologies offers the potential to greatly enhance our understanding of the forest resource; however, it is critical to recognize that any technique has its limitations and it is important to select the appropriate technique for a given application. In this review, we will discuss the following technologies for assessing wood properties both in the field: acoustics, Pilodyn, Resistograph and Rigidimeter and the lab: computer tomography (CT) scanning, DiscBot, near infrared (NIR) spectroscopy, radial sample acoustics and SilviScan. We will discuss these techniques, explore their utilization, and list applications that best suit each methodology. As an end goal, NDE technologies will help researchers worldwide characterize wood properties, develop accurate models for prediction, and utilize field equipment that can validate the predictions. The continued advancement of NDE technologies will also allow researchers to better understand the impact on wood properties on product performance.

Average bark-to-bark resistance of the IML Resistograph PD400 (hereafter referred to as ‘Resi’) was found to provide strong linear correlations with the basic density of 12-mm-diameter increment cores taken from standing plantation eucalypt trees. Relationships between Resi values and approximately 2 000 cores (predominantly Eucalyptus globulus but some E. nitens) were examined across seven studies (representing samples from nine distinct sites) in Tasmania, Victoria and Western Australia. Custom-written software was developed to process the Resi traces to automatically perform a linear baseline correction of the trace, and extract: Baseline correction was needed to counter the variable effects among trees of needle drag across the diameter, largely a function of tree diameter and wood density. Individual traces were collected in less than 20 s tree⁻¹. The sampling conditions of 150 cm min⁻¹ speed of forward movement (feed speed) and 3 500 revolutions per minute (rpm) were identified as optimal for the plantation eucalypts studied. The relationship between different feed speeds and rpm were linear and coefficients determined to allow average Resi resistance to be converted to a common set of sampling conditions. A simple linear regression was identified in each study to define a slope and intercept to convert the Resi values to basic density and determine the variance between them. Resi traces from different studies were not always collected using the same instrument and this is believed to explain most of the between-study variance in slope and intercept. Trace processing software was built into a web-based package that is available to make trace processing easy, with defined variables downloadable for use in routine plantation assessment. Users can change the default slope and intercept values to suit their individual instruments or species as required. Further work is required to fully define the effects of instrument, site and species on these relationships. The IML Resistograph PD400 was found to be an accurate and quick (40–70 trees hour⁻¹ person⁻¹) infield tool for estimating diameter and wood density in standing trees. When combined with automated, web-based processing software the methodology is among the lowest cost options conceivable for wood density assessment of plantation eucalypts.

The ring width–wood density relationship (RW–WDR) is somewhat controversial. Most of the literature reports negative, but weak relationships, while others found no significant relationships at all. This study analyses the RW–WDR using a dataset of twenty even-aged Norway spruce trees grown at two different levels of air pollution in the Eastern Ore Mountains, Germany. Ring width, latewood proportion and wood density were measured in each tree ring. The year-to-year (temporal) variability of the RW–WDR was calculated by sequentially taking the rings grown in the same calendar year in trees within a site. In addition, RW–WDR was also calculated across tree rings (pith to bark) of given trees. During the observed period, the temporal RW–WDR fluctuated between negative and positive correlations with certain climatic patterns, as well as forest operations, playing a prominent role. It was shown that increased late-season rainfall favoured a more positive relationship. The temporal RW–WDR illustrated, for given trees of a site, that smaller rings formed in certain years might be related to higher wood density while other years may result to lower wood density. This finding is useful for an improved understanding regarding the effect of ring width on wood density.

Mature age Pinus radiata D. Don trees were sampled across nine sites in northern New South Wales, Australia, that were expected, based on site quality and inventory metrics, to exhibit significant variation in productivity and wood quality. Twenty trees per site were harvested and 13 mm diameter, pith-to-bark increment cores were extracted from three trees per site from eight of the nine sites for SilviScan analysis. Outerwood increment cores were collected from all trees for basic density measurement. The same trees were also sampled using an IML PD400 (Resi) instrument. Radial mean properties of wood basic density derived from Resi traces were found to correlate strongly with the mean density data derived from SilviScan analyses and from increment cores. The Resi-derived basic density of 10 mm radial segments was strongly correlated with SilviScan measures of basic density averaged at similar intervals.

The IML PD series Resi is a device used to assess the drilling resistance of wood. The IML PD series Resi instrument is being widely adopted for commercial wood quality assessment due to its speed, cost-effectiveness, and precision when combined with web-based trace processing. Collecting Resi data with fixed feed speed and RPM settings is challenging due to inherent basic density variations within and between tree species. Altering these settings affects the drilling resistance amplitude of the Resi data, impacting basic density predictions. This study introduces the concept of chip thickness to combine feed speed and RPM into a single parameter to minimise the effects of different sampling conditions on the basic density predictions. Regression models, with chip thickness as the regressor variable, account for 97% to 99% of variance in mean Resi outerwood amplitude across six species. The demonstrated adaptability of chip thickness for adjusting feed speed and RPM settings, along with species-specific functions correlating it with Resi amplitude, holds promise for standardizing amplitude values across diverse feed speeds and RPM settings. Optimal sampling conditions needed to predict basic density lie within the 30%–40% amplitude range. To drill a ~30 cm diameter tree, the recommended fastest settings were 200 cm/min and 3500 RPM for Southern Pine (Pinus elliottii var. elliottii (Engelm) × Pinus caribaea var. hondurensis (Sénéclauze)) and Radiata Pine (Pinus radiata (D. Don.)), 200 cm/min and 2500 RPM for Hoop Pine (Araucaria cunninghamii (Mudie)), 50 cm/min and 5000 RPM for Spotted Gum (Corymbia citriodora subsp. variegata (F. Muell.)), 200 cm/min and 4500 RPM for White Cypress (Callitris glaucophylla (Thompson & Johnson)), and 150 cm/min and 3500 RPM for Shining Gum (Eucalyptus nitens (H. Deane & Maiden) Maiden) based on the billets sampled.