Subsidiary Information on Roots

The Iodine or Starch Test can give an indication as to whether a root is dead or alive. It relies on the fact that all trees and shrubs in Britain store starch in some of their cells, and this substance reacts with iodine to instantly turn a violet colour; see the two photomicrographs, showing  a cross section of an oak root before and immediately after application of iodine:

       Oak: before

        Oak: after

When a cell dies any starch it contained is digested by nearby micro-organisms, and eventually disappears. Therefore, if no cell contents in the root turn violet, we can usually state that the particular root is DEAD with confidence. But remember that this may not reflect the state of the tree itself; a tree can have many dead roots in a healthy root system. To say categorically that a root is alive is not possible with this test; note the significance of this as regards evidence in court.  If there is starch present, the violet colour appears; but what if the root died a few months ago and soil bugs have not yet got at the starch (as can readily be the case in a heavy anaerobic clay)? Such a root would test as ‘alive’, whereas in reality all we can say is that it was RECENTLY ALIVE when removed from the soil. The rate of loss of the starch varies greatly, depending on soil factors (oxygenation, water and organic content), type and size of root (very thin roots can lose any starch in a few days), and indeed it can remain detectable for years under certain conditions (our archived roots, kept clean and dry, retain their starch content for many years!). We have found starch in Oak roots deep in an anaerobic clay from trees known to have been felled 10 or more years previously! Therefore a felled tree may retain some apparently live roots long after its removal, and thus we feel that the term ‘recently alive’, although admittedly unsatisfactory, best reflects the situation.  At least take the results of this test with due reservations; and for such reasons we only carry it out on request, £5 per root. Click on our publication Dead or Alive? for further information.

DNA testing is available in specialist laboratories, and can sometimes be useful in matching roots to twigs, but after commissioning some pilot research in 1997, we decided to rely solely on our anatomical and morphological methods. The question must be asked "Why do I need to identify this root?" and usually the answer is solely the requirement to distinguish between trees under different ownerships (e.g. on the pavement or at a neighbour's, or your own), and this rarely warrants sophisticated, environmentally unfriendly laboratory techniques; our microscopic examination is adequate.  Furthermore, while comprehensive DNA testing can lead to very specific identification, and can therefore be justified in a small proportion of cases, there are pitfalls. What of natural grafting of roots, when a group of two or more trees of the same species can 'share' one root network? Or of trees grafted onto different rootstocks (a very common horticultural practice)? What of the many cloned varieties, all with identical profiles? And what of a bundle of say 20 roots found together - when all 20 must be DNA-tested in order to obtain any comprehensive indication of provenance - compared with our quick examination with a hand lens (see page on Root Identification [What sort of sample?]), when we reach an acceptable conclusion in most cases - and usually with 19 of the 20 for free! Thus it should be appreciated that in root investigations, in many instances DNA matching alone does not provide the additional incontrovertible evidence expected of this technique... not to mention the question of value-for-money...  Are clients appraised of such limitations?

An assessment of the age of a root is sometimes requested. This is often little more than guesswork; some types of root do not have annual growth rings (unlike their timber). Furthermore, a root can remain in its ‘1-year-old’ state for several years before possibly increasing, so assessing thin or even medium-sized roots is particularly unreliable.

Some clients request that the diameter of roots identified is included in reports. This is of little relevance, except as a crude means of labelling. Again, the information can be very misleading; some types of root have thick bark which shrinks on drying, so that a thin or medium-sized fresh root can as much as halve in diameter within days.

We are sometimes asked to comment on the root activity of a system sampled for our identification.  As co-author of Tree Roots and Buildings, we are in theory best suited to give such information.  However, we do not offer it as a matter of routine, as every situation is individual, and it is dangerous or even irresponsible to predict the water demand of a particular tree without a site inspection.  Such factors as site configuration, underground structures (both artificial and natural), as well as variety, size, vigour and history of the tree, influence its root activity, and all make for generalized statements based only on theory to be of limited, superficial value.  Categories such as 'High', 'Medium' and 'Low' water-demand should be considered only as starting points in the assessment of individual trees.

Our overall comment: Beware of reports filled with ‘impressive’ information in the form of lists, tables, figures, scientific jargon… Most of it is at best of marginal value, and at worst misleading! We do not pursue pseudo-science; let’s be open, keep things simple! And remember, whatever we say, we are prepared to support in court.

 Top of Page