Roots of a tree serve three functions:
- Water and Nutrient Capture from the soil,
- Structural integrity – holding the tree up,
- Energy storage.
Unfortunately these three factors are mutually exclusive and not tied together in any way, that manifests which one of these is a problem.
For example: A very large old Grey Box of about 300 – 500 years old (Say 400), some 50m tall and 1.5m DBH. The tree had a major internal decay that had been burnt out by fire. The first 200 years of wood, was now a hollow. The next some 150 years of wood ranged in the sate of decay from just identifiable as wood, under charcoal remnants, to strong and hard at the barrier zone. Then there is the outside 75 mm, some 30 – 50 years of wood produced since the fire event.
The location of the tree, which had about a 10 degree lean towards the road, some 13 metres away, was in a drainage swale beside a rural highway. The site had some 600mm of top soil over an anaerobic sticky grey clay sub soil. The root zone was subject to regular inundation during rain events, this was compounded by the impermeable road surface and camber that shed water into the swale. The site was substantially flat river plain so draining of the swale was due more to evapotranspiration and evaporation than runoff.
The base of the tree was strongly asymmetric, see diagram above, with three large buttresses that had formed over recent years. Clearly, the tree had had a substantially round even bole at the time of the fire event. However, following the fire that had burnt the wound altered and decayed wood within the trunk and down into the roots. Only the three main structural root surfaces, had not been killed by the fire.
These roots sprouted new adventitious roots and these had locally colonised the site. They continued to grow forming the buttresses at the base of the tree up to some 6m above ground line. This was due to the root trunk connection where only the top of the old root being connected to the underside and the topside of the new root. Normally the roots, a cylinder, connect with the trunk, another cylinder, the roots effectively unzipping the underside of the root, to form a smooth continuum. In this normal situation, there is little or no buttressing and a substantially circular bole, which is the trunk root interface.
In the case of this tree, the new roots that formed in the moist silt top soil, supplied all the water and nutrient needs of the tree’s crown. The crown was healthy in appearance but had shed many large limbs at 15 and 30 metres. The crown was rather sparsely foliated in comparison to what would be expected in the crown of a fully healthy tree, sprout shoots within the crown were also clearly evident.
The tree above 6 metres had wound altered trunk wood but it was still solid wood all the way to the top of the tree apart from some additional small pockets of advanced decay where branches had failed.
Thus, we had solid heavy wood above a bole, lower trunk and root system, that was substantially hollow, decayed, wound altered and brittle, with no structural roots forming due to the local moist nutrient rich to soil. As the wound altered trunk wood and the centres of the old structural roots continued to decay the structural integrity of the root system continued to diminish. The small feeding roots continued to sustain the relatively small crown some 50m above the ground.
The 10 degrees of lean, towards the road, exaggerating the load on the root system. From the bole diagram above you can also see the location of the root buttresses, the root load and the fall direction. The three buttress roots are so located that one is towards the road, and is the most exaggerated due the reaction growth induced by the load. The other two roots are at the rear sides of the tree, and are substantially both parallel to the road. These roots are under a tensional load which wood is very poor at resisting and up lift load generated by the lean of the tree over the front root. The inevitable failure occurred at ground line. Inspection, after the failure, confirmed the visual evidence that was available but had been ignored.
These factors combined and the tree failed, late one winter night, falling across the highway. The result, a 50 m tree with a 1 metre diameter trunk, with its centre line located about 1 metre above the road surface just around a bend in the road. The driver of the truck that hit the tree had reported the hazard tree on a number of occasions but no satisfactory action was taken.
The important lesson from this is that very few roots, non-structural in nature, can sustain the crown of a tree when those roots are in ideal conditions for the supply of nutrients and water. The nutrients were not fully abundant as manifested by the thinner than expected crown. However, the swale directed any available water to the roots thus sustaining the tree. This inevitably also plasticising the soil, thereby reducing its load carrying capacity, so returning the load to the root plate that should have been widespread and thus able to cope with the localised soil weakness. In this case there were only small feeding roots and small structural roots that were in the wrong location, from a structural viewpoint, and could not resist gravity.
In – Shigometry Decay & Vitality Assessment we saw that the symplast, living tissue, of a tree varies with health and condition. This tree had a thin crown and thus a very thin symplast spread out thinly over a large internal 1.5m BHD, 50m tall tree. This tree had, in effect, the symplast and crown equal to that of a 10 year old healthy sapling. The total trunk volume was 37.5 m3, the Symplast volume at best was 7.2 m3. The trunk weighed some 30 tonnes of which less than 5 tonnes was symplast.