Saproxylic organisms, old oaks, and canopy openness

In light (or the lack of, as may become evident!) of reading the first five chapters of Vera’s work in Grazing Ecology and Forest History (not yet done with the book), the recognising of oaks not regenerating under a closed canopy, because of a lack of light (and now you get the joke), is something I’d explore further. An article I found wasn’t quite aligned with this above statement directly, but sounded interesting and thus I’ll share it below. But first, a little wider context.

Often, mature oaks found in regenerating woodlands (because of the abandonment of grazing activities) will have an open-grown crown structure, with a wide-spreading crown from a low stem break, either through pollarding, or natural crown formation. Such oaks are usually very old, and of impressive size. These large trees, by virtue of their size and age, will also be host to saproxylic organisms (insects, fungi, and so on), and such a habitat may very well have also been present prior to the woodland regenerating around the old oaks. Before regeneration, these oaks may have been wholly exposed, or existed in a small scrub area, from which grazing livestock were barred from entering (to protect the tree, in order to safeguard the mast each year). Therefore, one can explore how the change in surrounds to the host trees impacts upon these saproxylic organisms, and what this may mean for future woodland management practices if conservation is a key consideration.

Generally, saproxylic organisms prefer lighter conditions. This is, of course, not true across the board, though particularly for insects, the more open conditions provide for better habitat quality. With regards to the diversity of the landscape, saproxylic organisms may also vary as diversity changes, and therefore the authors of the study being looked at in this post assess: (1) the species composition of saproxylic organisms in old oaks in regenerating thermophilic (temperate) oak woodlands in Krivoklatsko, Prague; (2) species richness in these oaks, and; (3) whether these organisms are affected by tree site conditions (solitary tree, woodland edge, open forest, dense forest). The study looked specifically at saproxylic fungi, lichens,  beetles, ants, bees, and wasps, within the Krivoklatsko area.

From the surveying of fungi and lichens and trapping of arthropods, a total of 78 species of fungi, 36 species of lichen, 153 species of beetle, and 32 species of ant, bee, and wasp, were found. Generally-speaking, open habitats supported a wider and more homogenous fungal species range than closed-canopy locations, where populations were less diverse and less homogenous (as in, the species found were not very similar across all locations). However, fungi did seem to opt more readily for denser stands and woodland edges, in place of generally more open habitats, and in woodland edges species richness was greatest. This is probably because of the moister wood and cooler temperatures, which likely suit fungi more, though woodland edges may also be host to the greatest amount of deadwood as a result of windthrown trees being more plentiful. For lichens, diversity increased alongside the level of openness. Beetles perhaps took this to the greatest extreme, showing preference for open-grown trees. The greatest richness was also found in these solitary trees, followed by woodland edges – open and closed stands were generally poor, for beetle species. The ants, bees, and wasps were shown to prefer transitonal areas where the woodland edge met scrub (an ecotone, or the mantle and fringe), and this may be due to the higher abundance of potential nectar sources associated with shrubby species and herbaceous vegetation not found in the forest. Open forest stands were also supportive of these species, to a similar degree, and probably for the same reasons. The richness of species was also positively correlated with increasing levels of openness (up to a point – not solitary trees), by-and-large. The graphs below help illustrate the above.

saproxylic organisms tree light
The size of the circles relates to the species richness at the studied locations, and the graphs in the upper right corner refer to the weighted species mean in each of the four area categories. Fungi (a), lichen (b), beetles (c), and ants, bees, and wasps (d).

In response to the above data, what can certainly be suggested is whether a non-intervenionist approach to stand management is good, with regards to preserving saproxylic organisms; of which many are endangered. Ultimately, the goals of the site will dictate management practices, though the idea of leaving stands to become high and possess a dense canopy is not necessarily going to be optimal for lichens, beetles, and ants, bees, and wasps that rely upon deadwood principally (or near exclusively). In this sense, re-introducing management practices, namely coppicing around older trees, or other good saproxylic habitats, may be of marked benefit. Such practices also ensure that actual stand diversity does not shift in favour of, in time, near exclusively (or wholly) shade-tolerant species, such as beech (Fagus sylvatica), Norway spruce (Picea abies), and European silver fir (Abies alba), and also provides scope for re-intoducing lost practices (perhaps even extensive grazing) and revive potentially dwindling economies. If a stand is large enough, this management doesn’t even need to necessarily be widespread. A good mosaic of different habitats, ranging from open ones to higher canopies, may support the greatest number of species (assuming the patches are large enough, and abundant enough), and as mentioned, also support traditional rural practices that have unfortunately been so very lost in many parts of Europe.

At the same time, attention should be given to paving the way for future old oaks, in thermophilic oak woodlands, and as oak cannot regenerate under a closed canopy, the only way to provide for future generations is to re-open the canopy in large enough areas to permit oak regeneration. Given that our large wild herbivores are largely gone, we must assume the role of those herbivores, be it through the introduction of ungulates, or woodland management practices.

This begs the question – why is a non-interventionist approach to woodland management so popular?

Source: Horak, J., Vodka, S., Kout, J., Halda, J., Bogusch, P., & Pech, P. (2014) Biodiversity of most dead wood-dependent organisms in thermophilic temperate oak woodlands thrives on diversity of open landscape structures. Forest Ecology and Management. 315 (1). p80-85.

If you found this post of use, please do share it (and / or the source article), and post below (or on Arbtalk) with your thoughts.

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Saproxylic organisms, old oaks, and canopy openness

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