Veteranisation of exotic tree species for cavity-nesting birds

Courtesy of forestry practices, and also the general clearance of ageing trees either to facilitate development or to remove a high level of supposed risk from their retained presence in urban and woodland environments, there is a growing lack of a future veteran / ancient tree population. This is concerning, as such trees have huge ecological value, and typically a value that is far greater to biodiversity than younger trees (notably for saproxylic insects and cavity-nesting birds). At this moment in time, the majority of existing veteran and ancient trees are likely native species, by sheer virtue of the fact that introduced species likely do not predate the 1600s-1700s (and those that are of marked age are generally few and far between, having been planted only in high-profile areas). However, given many environments now feature tree species that have been introduced artificially, future veteran and ancient tree populations may have to exist beyond just those species which are native to an environment. Similarly, if veteranisation techniques are to be practiced in order to artificially bridge the gap between two generations of ‘true’ veteran trees, such practices may have to also be undertaken on introduced tree species. This will in fact be particularly important, as many native tree species suffer from high mortality rates associated with exotic pests and diseases (Dutch elm disease, ash dieback, acute oak decline, and so on, in the realm that is the UK), which may increase our reliance upon exotic tree species.

In order to assess whether an introduced tree species can support native biodiversity, the authors of this study sought to ascertain whether veteranisation techniques (the creation of basal water-filled cavities and trunk cavities for nesting birds) undertaken to the London plane (Platanus x acerifolia) would yield positive results for select bird species, and for how long. This species was chosen as, in the study site of northern Italy (Bosco Fontana), the plane is considered a pest that is damaging the native ancient and semi-natural Quercus robur stands (alongside an invasive tree species: Quecus rubra). Ecologically, the implications of such an un-natural stand formation and succession could be considered highly significant, as there are almost certainly going to be a huge number of obligate associates to the native oak species, which in turn attract predators and parasitoids, and so on and so forth. Conversely, no species will have co-evolved with the London plane or red oak, and thus associations may very well be few and far between.

map italy plane tree
A map of the site, with the green dots being individual plane trees used in the study.

The 112 plane trees featured in the study were all of a DBH between 40-75cm, and spaced at least 20m apart from one another, in many segments of the study location (see above map). On each tree, between two and three basal pockets and one larger nesting cavity upon the trunk (1-5m up) were created. The basal pockets were designed to accumulate water and slowly rot (for the benefit of water-reliant fauna), and the largest slits were at the bottom of the trunk and decreased in size as they progressed upwards (unfortunately, no data is provided as to their benefits for biodiversity in the study). Upper nesting cavities were designed to house secondary cavity-nesting birds, and resemble naturally-occurring cavities frequent in veteran trees. Their sizes varied, in accordance with the intended bird species that would use the cavity as a nesting site, and these cavity types (and abundance) are listed in the table below.

nesting bird tree cavity
Tree cavities and the intended nesting birds – an overview of the study cavity types.

For each tree, the artificial stem cavities noted in the above table were routinely monitored over three years for signs of habitation, and in a select few trunk cavities the internal cavity temperature was recorded for a period of six moths during 2001. Internal cavity temperatures were compared to nest boxes also installed on some trees. Two additional (partial) investigations were also conducted on the plane trees, in 2008 and 2010. The DBH of all trees was measured, to determine whether the veteranisation techniques had harmed the tree in any marked manner.

After a period of 8 years following on from the creation of the habitats, 77% of the plane trees remained alive, and the treatment type (trunk cavity sizes) and the size of the tree had an impact upon the mortality rate. Those that did die most routinely failed along the main stem, suggesting that the decay initiated by the habitat creation practices was a possible cause (though, not a significant attributing factor). Of the cavities within the main stem, the most constant temperatures were recorded for the large nesting cavity created for Strix aluco, suggesting that conditions within the host stem were optimal for habitation. Temperatures were also far more constant than those within nest boxes, which means that, wherever feasible, nest boxes should be avoided and instead nesting cavities constructed within the tree (even for exotic tree species).

Looking at bird species that colonised the cavities, it was found that five of the nine bird species targeted utilised the 87 (80%) of the artificial habitats (M. striata, P. caeruleus, P. major, S. aluca, S. europaea). For P. caeruleus and P. major, their presence significantly increased with time, and they were even found to use cavities created for other bird species. This trend (increased abundance as time progressed) was also observed for many other of the bird species found, as can be seen in the graphs below. However, by the eighth year, the cavities were largely unsuitable for nesting birds, perhaps because of the decay around the cavities.

cavity nesting bird tree
The different graphs relate to the cavity sizes listed in the table earlier in this post.

Evidently, the use of the tree cavities by nesting birds means that exotic tree species can be relied upon to provide for some habitat, assuming they are continually created as the previous ‘batch’ become unsuitable (due to decay, insect infestation, and so on). Of course, some bird species didn’t use the cavities even though they were locally present, though given over half of the bird species did use the cavities it shows a good level of willingness by the bird species to associate themselves with the planes. From this, we can recognise that there is value in exotic tree species, and that veteranisation techniques should not necessarily be isolated solely to native tree species (if native tree species are even available, that is!). The authors do however note that this practice should be limited to broadleaved species, as the traumatic resin canals created by conifers flood the cavities with resin and this makes the cavities unsuitable for nesting birds. By a similar token, such practices should take place away from well-used locations, meaning that if the practices are undertaken in a woodland then they should be done away from pathways. This benefits both the need for risk management and nesting birds, which would generally be more suited to locations that are less disturbed.

Source: Zapponi, L., Minari, E., Longo, L., Toni, I., Mason, F., & Campanaro, A. (2014) The Habitat-Trees experiment: using exotic tree species as new microhabitats for the native fauna. iForest-Biogeosciences and Forestry. 8 (1). p464-470.

Veteranisation of exotic tree species for cavity-nesting birds

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