The role of urban trees – is it all too anthropocentric?

This post is essentially a thought-piece, so in place of a referenced post it’s more of a ‘brain dump’ (of which I cannot promise absolute coherency and flow). I suppose the purpose of this post is therefore to stimulate some form of discussion, at least internally (though ideally also externally) – this discussion will, ideally, be laden with academic references and personal experiences.

Over the last year in particular, as I have begun to really begin researching the role urban trees have, and what they are perceived to provide in the planning stages of developments, I have become increasingly concerned with the seemingly over-riding desire to use trees for the benfits of humans. In this sense, their role is one of a sort of dynamic ‘furniture’, be it a street that is furnished wth trees, a garden, an urban park, or even an urban woodland or forest. Attention is paid to what trees give humans, and this may range from the already-mentioned aesthetic aspects, to the social ecology of humans (promoting integration between individuals, and outdoor recreation), and to the economic benefits (reduced recovery times in hospitals, a better sense of self-esteem and associated lower levels of stress, the encouragement of consumer spending, raising the value of a property, and so on). The list, with regards to the anthropocentric provisionings of trees, is quite literally vast. However, at least anecdotally, there seems to be little desire to utilise trees in the urban environment for the benefit of biodiversity and, even if there is a desire, it seems to rank beneath the value trees directly provide to humans. We can even see this with (peri-)urban woodlands, which are utilised generally for recreational purposes, and these (usually intensive) recreational activities are very much damaging to lower tier plants and soil flora and fauna. In turn, these adverse impacts cause the tree populations to suffer (be it from seedling recruitment, reduced nutrient uptake, water uptake, and so on).

Whilst I fully understand why trees are viewed in such an anthropocentric manner, we must not forget that our urban and peri-urban environments reside within a mosaic of habitats at the landscape-scale, and biodiversity does not stop at the doorstep of our ever-expanding towns and cities. By-and-large, native faunal biodiversity has evolved alongside the presence of native floral biodiversity, and this relationship over many millennia has produced thousands of obligate associations, and has formed the basis of functioning terrestrial ecosystems. In our urban areas, this relationship is sometimes aggressively severed by the abundant presence of exotic tree species that lack any obligate associations with native fauna (and have the potential to be invasive), a lack of tree species diversity, and fragmentation that creates many ‘urban tree islands’ (oft found in tree-less streets and estates), which are somewhat akin to their ‘forest island’ counterparts. At the same time, ivy stems are removed from trees, and usually under the guise of the ivy reducing amenity value in an urban system that thrives off of order and uniformity, or the concern that the ivy will kill the tree (which is usually not going to be the case). Even where the means of servance is risk-based, is there justification in severing the stem at the base? Probably not.

Granted, with the fact that many invasive pests and pathogens are wiping-out native tree and plant species with relative ease, the ideal reliance upon native tree species is perhaps going to become an ever-growing problem. For example, ash dieback (an oriental plant pathogen) is expected to cause massive mortality of the European ash (Fraxinus excelsior), of which there is currently no means of remedy. It is suspected that oriental ash species will possess resistance to the pathogen, as they have evolved alongside the pathogen and thus have developed means of effectively limiting its impact on the population level (the same applies to ash trees and the insect pest emerald ash borer), though the introduction of such exotic ash species will not reverse, by any stretch, the intricate associations European ash have sustained with, for instance, obligate lichen species (of which many are endangered and will go extinct if the European ash die). One means of retaining the presence of native urban tree species is to create cultivars that are resistant, at least partially, and in the short-term this is ideal. However, once the pathogen has been exposed to the specific cultivar(s) over a period of time, it will itself adapt to the change in host type and likely once again ravage the (genetically undiverse, as many cultivars share some, or even all, genetic qualities) tree population(s). Even in recognising this, there still appears to be little consideration given to selecting resistant exotic tree species that can provide marked ecological benefits for urban biodiversity, with more focus instead being simply on sustaining a form of urban forest that gives benefits to humans (who generally don’t discriminate between tree species, but simply demand tree presence).

An avenue of ash being killed by the insect pest Agrilus planipennis (emerald ash borer). Once these ash are removed, the entire area will be significantly lacking in large trees, which will have significantly adverse impacts upon biodiversity. Source: ACTrees.

Beyond tree breeding for resistance, other cultivars and clones exist that have ‘urban form’, and these cultivars may also be at huge risk if exposed to an associated pest or pathogen. As urban environments suffer from in-fill and denser developments, restricted space leads to specific cultivars being selected that have fastigiate form, can tolerate root compaction, and so on. Where these cultivars exist as monocultured avenues, there may be an even more marked problem. Of course, standard tree avenues of a signle species are also very susceptible (notably if they are very similar, on a genetic level), and could quickly cease to be. This would then mean any biodiversity associated with the avenues is lost, too – notably, as dead trees are usually removed entirely from an urban site, even in spite of deadwood being of huge ecological importance.

Excessive risk management in urban settings, primarily out of the fear of litigation, is another huge issue that does not reconcile with the need for urban forests to provide for biodiversity. Older trees are usually going to present the greatest risk, due to their stag-headedness and other retrenchment processes that see them become what is considered ‘strucurally unsafe’, though it is these trees that give the most benefits to biodiversity – namely, saproxylic insects and fungi, though also other species that have had dozens (or hundreds, or even thousands) of generations exposed to the specific old trees in situ. Such risk to humans generally sees the trees managed, perhaps via crown reduction (or even felling), and in the case of reduction (and felling) work deadwood (which is ecologically crucial) may likely be removed if a target area exists beneath. At times, this deadwood won’t even remain at the base of the tree (if only reduction work is set), even in large parks (where retention is particular feasible), and this essentially creates an ecosystem (from single tree scale up to city-wide scale) with a huge void of deadwood-associated species. Outside of parks, the grabbing of large pieces of deadwood by members of the public that are destined solely for the log burner is a similar ecological travesty (this is an education issue).

A line of five Salix alba within a well-used urban park that were monolithed, as a result of extensive fungal decay caused by Ganoderma australe. These willows could easily have been felled, though the retention of the very large decaying stems is of marked benefit to saproxylic species.

Leaving trees themselves temporarily, the observation of most (if not all) urban cities having a fascination with intensively-cut amenity grass is another ecological issue. Large tracts of grass are usually cut for no reason other than to enable for the scene to look formal, and whilst this may be understandable in areas in parks where sports recreation is undertaken (which means management isn’t solely for formality), huge roadside verges are also cut in the same manner. Such verges, or even areas of parks not used for sports recreation, really need not be cut, and instead could be left unmowed for most of the growing season and allowed to ‘wild’ for the benefit of biodiversity (arthropods and small mammals, namely). Scope exists to keep grass long beneath many trees, in fact. Such a practice would in part resolve the issue of ground compaction around trees, and if the tree harbours many insect species, the wildflowers and ‘weeds’ may provide a source of nectar for such insects, and providing the conditions for a small micro-ecosystem to manifest on the single-tree level.

During urban development processes, we also observe areas of woodland, pasture, parkland, or otherwise, removed for housing and other forms of development. At times, relicts of former land use (namely trees) are retained, though not always in good conditon and certainly in isolation from their once more diverse ecological setting. The general bias in governmental (worldwide) plans for economic development sees attention to the ecological aspects associated with such developments lacking, and the lack of desire, scope, and ability to ensure tree planting, protection, and retention measures are properly developed, understood, practiced, and enforced, means that many new urban developments lack mature trees and are graced by only an insufficient number of new trees that are usually improperly cared for, and are often unable to provide, even long-term (if they survive), for local biodiversity. Small gardens so highly common in newer developments, compiled with a lack of roadside verges (maximising space for housing and off-road parking) and an abundance of aerial and subterranean services, means that scope for planting large (or even small) trees may not even exist. What hope is there ever going to be for these settings, on an ecological level? Rarely is there scope for demolishing these areas and entirely re-building, given the costs involved. The fragmented nature of the development process also doesn’t serve to aid with the provisioning for biodiversity, with horticultural and arboricultural measures usually only being considered during the latter stages of the process. Even when they are considered, it is questionable as to what weight they have, unless the site is particularly contentious or the potential customers are very affluent and demanding of high-quality landscaping and an associated mature tree presence.

A quite newly-built housing development that is largely void of space that can accommodate trees. We can see a few large trees in the background, though a lack of a roadside verge (on the property side), and a front garden of any decent size, severely limits the potential for any sort of tree planting. Source: Architecture Photos.

Not wanting to continue and make this post into something akin to War and Peace, I’ll round up here. There is no way I can cover everything I want to without making the post much longer, and that would go against the point of a blog post. I suppose the real crux behind all of my points here is that there is a general desire for amenity and human practicality when it comes to creating and managing tree populations, in place of the desire being to principally provide on an ecological level. In my eyes, this is counter-productive, as whilst it satisfies the need for humans it neglects needs of the ecosystem, which we, as humans, can never escape. Our urban environments are ecosystems just as much as our forests are, and if the health of the ecosystem declines then our own health will decline alongside. Trees, across the world, form the basis of many terrestrial ecosystems, and therefore they must also form the basis of our urban ones, and it is important that this is recognised and held in the highest of regards by those in the realm of decision-making and influence (this extends to pretty much every individual). It is also important that the tree species chosen are chosen for ecological reasons, as if they are chosen primarily for other reasons then the area will fall foul to ecological decline (in comparison to an area developed with ecology in the highest mind).

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