Balancing ecology and tree safety in urban parks

Trees provide a huge number of ecosystem services, ranging from the amenity value they provide to the ecology they support through their mere existence. In a setting where there is no target zone, a tree can be allowed to simply persist in any sort of state without there being the risk of injury or damage to property, though where the tree resides within a busy environment this is not the case. Therefore, a particular conflict arises between retaining viable and niche decaying (or hollow) habitat in an old tree and reducing the risk associated with the retention of such a tree. In this study, the authors assess how balancing such conflicting demands impacts upon saproxylic beetles including the endangered Osmoderma eremita.

The study area for this project was a large area (159ha) of parkland in the city of Rome, Italy. The parkland is characterised by areas of woodlands and scattered mature trees, with a species composition that features exotic cedars, pines, and cypress, though is dominated by old Quercus ilex (holm oak). These holm oaks were the focus of this research, and 1,247 individuals were surveyed in the areas of the park that were designated for Visual Tree Assessment surveys by the park’s managers (presumably because these oaks were seen to be of greatest potential risk to the public).

The gardens of Villa Borghese. Source: Wikimedia Commons.

Alongside the VTA, (initially) all holm oaks were surveyed for saproxylic beetles (every fourth day) between 14:00 and 19:00 from 19th July to 28th August. This involved exploring all hollows up to 5m above ground level. After a period of time, only those oaks with more significant decay were surveyed. Further, from 1st September to 30th September, between 09:00 and 18:00, the holm oaks were surveyed three times for beetle larvae and frass.

In all, ten species of saproxylic beetle were identified from 66 of the 1,247 holm oaks. However, 41% of the beetle population was found within the holm oaks considered to be at highest risk of falling (category D3 – see table below), with some species being found exclusively in such trees. For Osmoderma eremita, four D3 trees were host to the species, and the remaining seven host trees were deemed less likely to fail.

The number of beetles found, of different species, in the holm oaks surveyed. D3 relates to the holm oaks at highest risk of falling, and the data shows how much of the population of beetles exists within these trees.

In relation to the properties of the trees that contained the beetles, it was found that host holm oaks had an average DBH of 71.5cm, height of 16m, and spacing (relative to other trees) of 13m (these are big trees!). 63% of the hollows within such oaks, which were home to saproxylic beetles, had a fair amount of mould, and many of the cavities were moderate in size (an average of 18cm in width – only a few cavities of 50cm width were found to have beetles), and the decaying wood within was situated some way from the opening of the hollow. Of these hollows, many had a north-eastern or south-eastern orientation, were no higher than 2m from ground level, and were free from bird nests. Holm oaks with damage to the root collar were also found to be most likely to be host to more species of beetle (higher species richness), most probably because of the greater amount of internal hollowing and decay close to the base of the tree (where the largest hollows can form).

The authors suggest that, in order to preserve viable habitat for Osmoderma eremita, there is a need to retain living and large holm oaks that are close together (no more than 100m apart, but ideally far less), that have a lot of internal wood mould, and have nest-free cavities situated on NE and SE sides of the tree – this ensures there is high humidity but also a relatively cool internal temperature, and a lack of predators in the form of nesting birds. Therefore, the removal of such hazardous (category D3) oaks found to be host to the species is damaging in the sense that the beetle’s habitat is fragmented and also largely removed, meaning remaining beetle populations may become isolated (even when found only a few hundred metres from other suitable holm oaks) and the carrying capacity of a landscape is reduced.

An overhead shot of some of the park. We can see how dense the tree cover is. Source: Personal Drones.

Concerningly, because 41% of holm oaks host to saproxylic beetles were considered to be of high risk to members of the public (because of their poor condition), there is an evident conflict of interest associated with management of many trees that are ecologically critical for rare and threatened insects. In fact, large trees home to beetles including Osmoderma eremita were already on a felling schedule, which is catastrophic for the saproxylic species’ longevity on the site. In place of felling, it is suggested that old trees are retained via the installation of supporting structures that reduce the risk of large limbs falling, or such hazardous limbs have some of the more unnecessary side-branches removed to reduce mechanical loading. However, if limbs must be removed, they should be left on the ground beneath the tree, and the cuts made should mimic a natural fracture created under over-loading conditions.

This begs the question – are park managers, who are responsible for many mature and veteran trees, aware of their ecological benefits, and are they prepared to extra spend money on retaining such trees for their habitat whilst also satisfying the need to reduce the risk to visitors (and do they even have the expertise to request this)? Because many of the holm oaks found to be host to saproxylic beetles were already on a felling list, it suggests that park managers (at least in this instance) were more than prepared to accept the loss of habitat in the pursuit of public safety (did they even know about the beetles being there?). How willing would park managers be to change their stance, and strike more of a balance between the two conflicting aspects of tree management?

Source: Carpaneto, G.M., Mazziotta, A., Coletti, G., Luiselli, L. and Audisio, P., 2010. Conflict between insect conservation and public safety: the case study of a saproxylic beetle (Osmoderma eremita) in urban parks. Journal of Insect Conservation. 14 (5). p555-565.

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