The two arborists were supposed to investigate the eight mature olive trees that the building manager was concerned with. There was a scaffold over the restaurant that supported sun shades that could retract in segments, and duct system for climate control. The building manager shared that 10 years ago, the olive trees were lifted up to the roof with a helicopter in 1.5 m boxes. Since then, the trees have been creating a garden-like environment. However over the last few years, the branches have been dying back and they were pruned off. The arborists noticed that about half the remaining branches had leaves with necrotic tips. Furthermore, each tree had one drip irrgation dripper and below the mulch the soil appeared to have enough moisture. There was a possibility that the duct system, which was used to transport hot air could have dried the leaves but the patten of dieback does not correspond with the location of the duct system. In addition, the problem only started to appear in the last four years. If the hot air was the issue, the symptoms would have appeared almost immediately 10 years ago.
The arborists that suspected that the leaves could have been infected by bacteria, which has been found recently in urban olive trees. The particular bacteria species, Xylella, can cause a systemic infection that restricts the flow of water and result in drought stress symptoms in the leaves. They took leaf samples and then took photos of the drainage system (not obvious in the photo below) and tree containers in the space underneath the restaurant floor. Much later after the site visit, the lab test did not detect Xylella in the leaves or stem tissue. The tree containers were shown to be not much larger than the 1.5m boxes that they were transplanted in, and the arborists deduced that the trees were likely to be root-bound.
One of the arborists dug around a tree container with a trowel and found some adventitious roots, which are produced by trees with issued below ground like girdling roots or poor soil conditions. Girdling roots are a sign that the trees were root-bound, which means that they could have out-grown their containers.
The other arborist calculated the amount of soil volume recommended based on the crown projection (three cubic feet of soil per square foot of crown projection) and found that the soil volume in the container was far less than the recommended amount. He proffered his view that the soil volume for each tree should be larger and there was no way to correct the problem without "demolishing and rebuilding the entire restaurant and steel casings [that suspends the entire floor above the roof]". The arborist counselled the building manager that he could try to keep the trees alive for as long as he could but plan for their eventual replacement, as the tree containers would limit their lifespan.
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The (fictional?) consultants were observant and knowledgeable enough to consider the duct system across the scaffolding and the possibility of a recent bacteria infection occurence, which turned out to be negative from those olive trees. However, they may not have considered that olive trees does not tolerate too much water and are unhappy in soggy soils. The top few inches of soil should be allowed to dry between waterings, and the irrigation system should be adjusted towards this result. In addition, the drainage system of the tree containers should be checked that it is free from blockage, which would allow excess water to drain freely. The necrotic tips of the leaves and adventitious roots could be symptoms that the trees are receiving too much water from the irrigation system.
While it is likely that the trees have girdling roots and do not have the recommended amount of soil volume in the slightly larger than 1.5m tree containers, the arborists could have offered favourable solutions other than to rebuild the entire restaurant from ground up (or in this case, roof up) or to wait for the trees to die. Perhaps, in order to do improvement work within that crawlspace meant breaking a dozen occupational safety and health rules, and hence the other solutions were not suggested.
Tree roots generally spread three to four times wider than the crown spread, and when trees are grown in containers, the roots tend to spiral around within the container creating a root-bound condition. Roots dessicate when exposed to air, and this principle is how air-pruning pots/containers work - by pruning roots that try to extend outward using air - and solving the root-bound issue.
Image Air-Pot.com
The tree containers could be enlarged by first removing the floor of the restaurant. But that does not mean giving up valuable real estate for the trees' well-being. Next, structural cells (trade name: Silva Cells) could be installed in the enlarged soil space, which would hold up decking or slabs and prevent compaction of the soil. The soil added should be sandy and suitable for the mediterranean climate plants. The opportunity should be taken to upgrade the drainage system. And the sides of the tree containers should be air-pruning panels instead of steel panels.
Image Air-Pot.com
