Increment coring and heartwood decay in Norway spruce

The use of an increment borer to obtain a core sample from a tree may be used for a variety of reasons, including tree ageing, determining past climatic conditions in which the tree has existed through, and ascertaining wood properties (generally associated with establishing internal decay). However, taking a core sample is not without its issues. For instance, around the wound site, the tree will form reaction zones in an attempt to compartmentalise the damage, and such damage may also breach existing reaction and barrier zones established from prior instances of wounding and internal spread of decay. Therefore, there is a theoretical risk of such cored trees suffering from the injury, and potentially having a reduced life expectancy / increased mortality rate because of an increased spread of internal decay.

In order to test this theoretical assumption, the authors of this study commissioned a survey of unmanaged Picea abies (Norway spruce) stands in the Scatlè Forest, which lies within the Swiss Alps. In two small segments of this forested area, when the entire forest became protected under Swiss law in 1965, all of the trees were inventoried and many of the spruce (approximately 25%, amounting to 619 spruce) of over 8cm in diameter at breast height were cored. In 2011, the authors re-visited 22 of these trees, and paired them with 22 ‘control’ (un-cored) trees in similar micro-sites within the very near locale (basically, one could say they were paired with a close neighbour). At these study trees, they used sonic tomography (PiCUS Sonic) and electrical resistivity tomography (PiCUS Treetronic) to ascertain internal wood properties, and considered any trees to be decayed only if it had an evidently and extensively decayed heartwood region in both of the PiCUS tests. The tomograms below show, as an example, how the two PiCUS tests outlined trees with decay and trees without decay.

The tomograms marked A and B are sonic tomograms (of the same spruce), whilst C and D are electrical resistance tomograms (of another spruce). The top two images are from a spruce considered largely free of decay, whilst the bottom two are considered to be of a spruce that is significantly decayed within the heartwood zone.

Following these tests, the authors took samples from ten (five neighbouring pairs) of the trees (six with observable decay and four without observable decay, from the tomograms – the six decayed trees were not necessarily considered to be extensively decayed, however), to determine whether the two fungal heart rot pathogens Heterobasidion annosum and Armillaria sp. were present. These two fungal pathogens are viewed as significant agents of damage, within coniferous stands across Europe. These samples were taken at heights of 1m, the height of the tomograph measurements (around 45-90cm), and at the butt.

In light of the data collected, it was found that only four of the 44 Norway spruce had heartwood decay, and just one of these four had been cored in 1965 (just over 9% of trees surveyed were thus deemed to be decayed). Curiously, this is lower than the accepted ‘background rate’ of decay within coniferous stands across Europe, which is considered to be between 15-80% (depending upon the study). The reason for this, it is suggested, is that because the stand is unmanaged, the damage to remaining trees associated with timber extraction has not occurred. This has therefore led to the trees not being wounded to such an extent as conifers in harvested stands would be wounded. Furthermore, because the cores were taken in 1965, it can be said that they are unlikely to have enabled for decay to extend into the region, as if they had increased the risk of decay, then more of the cored trees would have shown up as being decayed with the tomographic surveying. When recognising that Heterobasidion annosum can potentially extend in the stem by 30-40cm a year, and the cores were taken at 40-90cm up the stem in 1965, if there cores would have facilitated in heartwood decay then they would have done so already.

From the ten trees cored for living samples of Heterobasidion annosum and Armillaria sp., it was found that 75% were colonised, discoloured, and in six cases observably decayed by either of the fungi. Generally, these decayed trees occupied a space in close proximity to one another, and of these trees, Heterobasidion annosum was observed to be a much more comment (seven trees) agent of decay than Armillaria sp. (one tree). The close proximity of decayed spruce to one another is perhaps not surprising, when one recognises that both fungi can colonise via active pathogenesis (root-to-root contact only, in the case of Heterobasidion annosum). Therefore, it is perhaps more likely that decay is not facilitated by increment coring, but instead root grafts between spruce trees. In addition, because Heterobasidion annosum has a life expectancy of around 40 years, after which it has a high mortality rate (per tree), it seems yet further unlikely that the cores taken in 1965 caused the decay and discolouration in the ten tested trees. It is far more likely, instead, that infection came (long) after, given that 75% of the ten tested trees had living mycelium found in the core samples taken. Not only this, but artificial core inoculation of trees with Heterobasidion annosum has a very low success rate, and therefore in this sense taking core samples is also very rarely going to induce decay by the fungus.

To conclude, it can be asserted that increment coring spruce trees is not at all routinely going to induce decay by Heterobasidion annosum or Armillaria sp. Of course, other decay fungi may potentially utilise cores that are not studied here (Phaeolus schweinitzii, for example), and because the PiCUS is not as effective at discerning only slightly decayed wood, the 9% of trees deemed to have extensive heartwood decay (of which only one had been cored) may be a slightly conservative estimate. Further to this, because the sample size was only small, larger-scale studies may be needed to add weighting to the findings contained within this report. Nonetheless, it can hardly be stated that increment coring has massively adverse impacts to tree health, if the results here are anything to go by.

Source: Wunder, J., Manusch, C., Queloz, V., Brang, P., Ringwald, V., & Bugmann, H. (2013) Does increment coring enhance tree decay? New insights from tomography assessments. Canadian Journal of Forest Research. 43 (8). p711-718.

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