Tom, our most recent 4-week Worms Scholarship recipient, has a particular interest in trees, and the recent weather in Jerusalem and the effect on the trees of the city prompted him to note down some of his thoughts for you. Hope you enjoy it.
Due to the unusually heavy snow experienced in Jerusalem this Winter, many of the city’s trees have been damaged. One only has to walk through the Sacher Park, opposite the scholars’ flat, to begin to understand the scale at which canopy coverage has been reduced in the city. Having probably never been loaded with snow in this manner before, and not being adapted for this as are many northern temperate trees, the limbs have simply failed by cracking, splitting or falling off entirely.
The remedial work, removal and eventual replacement will come at a huge cost to the city. There will be many trees with structural defects that are inconspicuous at present, which may necessitate their removal in future. All the trees that have remedial work undertaken have lost their form, and will never regain their natural shape. This renders them more susceptible to extreme weather in future, as trees are constantly trying to optimise their structure to distribute physical stress in a uniform manner. The primary framework that is laid down by the tree as it grows upwards and outwards for the first time are strongly attached to one another. The secondary growth that re-sprouts following damage, arises from dormant buds in the outer rings of wood. It will take a large increase in girth before these branch origins are enveloped, and they can never truly be seen as strongly attached.
The financial burden imposed by stochastic events on a city tree stock is not limited to Jerusalem; many of you will remember Dutch Elm’s Disease which nearly bankrupted municipal governments through the cost of treatment, removal and replacement. Less serious but more prevalent are cases of nuisance, in which a tree on city land incurs a cost to a private landowner. Where I live in leafy Richmond, West London many pavements are disrupted by the ever increasing girth of the numerous mature street trees. This necessitates repeated pavement repairs, issues with accessibility and creates trip hazards. Also in Richmond is the delightful Oak Processionary Moth (http://www.forestry.gov.uk/fr/oakprocessionarymoth). I have experienced the skin irritation myself, and it is a truly unpleasant ordeal.
Then there is the constant and serious risk of injury from a failed branch or tree. A good example of one afflicting European city trees is a pathogen known as Massaria Disease (http://www.forestry.gov.uk/fr/massaria), which causes a lesion on the upper side of branches on the very widely-used and often mature London Plane (Platanus acerifolia (Aiton) Willd. syn. Platanus x hispanica Mill. ex. Munchh – Yes they have changed its name, it is now recognised as a species and not a hybrid, don’t ask, another blog in itself!).
Mature London Plane branches are big and heavy, and advanced Massaria can cause them to fail, so this cannot be allowed to happen. The smaller branches are also a problem. The increased deadwood production necessitates very regular (up to 3 month cycles in exceptional circumstances i.e. tourist hotspots with intensive pedestrian use) climbing inspections and inflates the cost of responsible maintenance by several orders of magnitude.
Trees are expensive. Trees cause damage to property. Trees can cause harm and tragedy even when rigorously and responsibly maintained to above and beyond the standard required by law. So why have them in a city?
I am a big fan of trees both aesthetically and spiritually so am understandably biased towards the maintenance and upkeep of trees in cities, but part of my job also involves the use of trees in the built environment. Many have viewed trees as an unnecessary nuisance and a drain on public funds, but this could not be further from the truth. Aside from the more intangible benefits such as feelings of well-being and lower levels of stress, there are now a suite of ways to measure the benefits that trees provide to a city. Put simply, they are engineering marvels. They are able to modify the environment in ways that would be prohibitively expensive for us to simulate.
Trees actively cool their environment, providing shade for comfort and reducing the demand on air conditioning systems. They effectively reduce the phenomena known as the urban heat island effect, and lower mortality rates during heat waves. Trees intercept rain water, absorb it and transpire it into the atmosphere as vapour. Often their planting pits are the only source of infiltration in a heavily paved environment. Urban drainage in the UK is a huge problem, and as we have seen earlier this year, our drainage infrastructure can’t deal with a large amount of water. Too much is paved and not enough is allowed to percolate into the soil and be taken up by trees. They filter and remove volatile compounds and pollutants from the air. They sequester carbon, removing it from the air and locking it up in their trunks. The bigger they are the more carbon they are retaining. House prices are higher in neighbourhoods with good urban forest amenities. All these points are irrefutably supported by a wealth of scientific literature.
Anyway, I digress, the Jerusalem Botanical Garden forms a part of the urban forest of Jerusalem with a very high density of trees. The snow unfortunately showed no mercy to the collections, and many are in a bad way. All of the work on the city trees has been done from lifts, but this isn’t feasible within a botanical collection for reasons such as access, plant health and a need for a higher level of prescriptive care. Head Gardener Eli has used the opportunity to equip the team with climbing arboricultural kit and training. He and Piers are now qualified to climb and are busy planning and prioritising work to ensure the safety of the visitors, vitality of the trees and to retain as much of the aesthetic character of the damaged trees as possible.
Arboriculture is a total specialisation, requiring a high degree of strength and stamina, a steely resolve, an intimate knowledge of available equipment (and there is A LOT), the creativity to integrate new innovations into existing climbing systems, an acute awareness of health and safety, more knots and rope techniques than the best boy scout and importantly, a deep and applied knowledge of tree biology. I was lucky enough to train with the Arboricultural Unit at Kew, and was able to assist Piers as he learned to use his new kit. I supported him from the ground and directed his work. It always looks easy from the ground, but from within the canopy things are much less clear. A methodical approach is essential.
Luckily Piers is already a very capable rock climber, so is very fit and has absolutely no fear. He has been able to use his knowledge of rock climbing rope systems and apply it to his tree climbing systems. The major difference between the two is your life-support system (i.e. main rope attachment) in rock climbing is there to do just that. If you fall your rope will immediately tighten and arrest your fall. It is not used during climbing however, as you are meant to climb unassisted. In a tree system your life-support is also there as a point of attachment to the tree. You rely on it to enable you to get into positions and work. It becomes like a third leg. Piers used his new system to very ably dead wood some large damaged Eucalyptus grandis over hanging paths and a seating area, and he removed weight from the ends of branches under tension. He did all of this in 30 degree heat with a handsaw, and didn’t complain once! Once he is qualified to use a chainsaw within the tree things will become a lot easier. Hopefully I have been able to help the guys start a long term strategy for the health, safety and beauty of JBG’s trees.