Allowing deadwood to accumulate for the benefit of saproxylic beetles

Within managed woodlands, deadwood is usually significantly lacking. This may be because deadwood (both standing and fallen) is removed during extraction processes, or because stands are felled before an age where the woodland would naturally begin to accumulate deadwood. Cause aside however, the lack of deadwood (particularly larger fragments of over 10cm in diameter) is bad news for saproxylic insects. Conversely, where deadwood is allowed to accumulate (usually via lapsed management; at times perhaps “benign neglect”), the suitability of a woodland for such insects is enhanced. Additionally, as saproxylic insects may vary in their host preference, a site with a variety of tree species is likely to yield a larger and more diverse population.

In the feature study of this article, an investigation was commissioned into how such a “benign neglect” strategy impacts upon saproxylic beetle populations. The location of the study was the Bavarian Forest National Park (Germany) – which employed such a tactic in managing areas of the park (for Ips typographus, at least partially) – and focussed on assessing the relationship between beetle populations and host specifity, and also the minimum threshold of deadwood to support “comprehensive” beetle populations.

The Bavarian Forest National Park covers an area that is around 24,500ha, and stand compositions varies across altidunial ranges. At higher altitudes of 1,150-1,430m, Picea abies (Norway spruce) is the dominant species, whilst at lower altitudes above 650m stands consist predominantly of silver fir (Abies alba), beech (Fagus sylvatica), and spruce (Picea abies). Until the 19th century, these stands were largely unmanaged, and were therefore old-growth forests with a likely abundance of deadwood (and associated beetle species). However, once forestry practices were introduced, old-growth stands became highly fragmented and exist today in patches normally no greater than 100ha. This change in stand structure lead to some beetle species becoming extinct. At this point in time therefore, the national park can be segmented into three categories: (1) old-growth forest, where there are many trees of over 400 years in age and there has been no logging for over 50 years; (2) unmanaged forest, where bark beetle-infected (Ips typographus) trees are left, and; (3) managed forest, where bark beetle-infected trees are actively salvaged for timber through the logging process (“salvage logging”). The site is (evidently) home to the bark beetle, which is causing mortality of many spruce trees.

Dead spruce stands, caused by Ips typographus colonisation. Beneath, re-growth can be observed. Canopy openness is very high, as is deadwood abundance. Source: Environment and Society Portal.

In this study, from the three above-mentioned stand categories, the authors identified a total of 293 plots at 0.1ha each. These plots also spanned across the entire altitudinal range of the park. From these, a randomly-selected 126 were surveyed for saproxylic beetles (21 in old-growth, 52 in unmanaged forests, and 53 in managed forests). Surveying for insects involved pitfall traps, flight traps, and direct searching. In these locations, deadwood accumulation was also calculated. All deadwood was categorised by species, and the minimum threshold for a piece of deadwood being calculated was 12cm in diameter.

From the deadwood recorded across each stand type, there was a discernible difference between deadwood presence in unmanaged stands and managed stands (see the below figure). Interestingly, old-growth forests had less deadwood than unmanaged sites, though this may be because the bark beetles in the unmanaged sites were causing marked deadwood accumulations (and such deadwood was not removed, unlike in salvage-logged sites). In addition, old-growth relicts were previously logged (as little as 51 years prior to the study). Furthermore, perhaps the stands had peaked and were now declining in deadwood abundance, because the dominance of large and mature trees would have ‘seen off’ competition many years previously. Any deadwood associated with a loss in competition would have since been degraded by fungi and insects.

Other variables recorded were the ‘penetration rate’ of bark beetles (Ips typographus) into spruce hosts (higher rates were associated with higher canopy ‘openness’), ‘senility’ (deadwood from individual fir and beech trees in a state of serious decline), and ‘control’ (altitude and habitat continuity).

The difference in deadwood abundance across the range of stand types, separated by species.

In relation to the species diversity and abundance of saproxylic beetles (excluding Ips typographus), 12,253 individuals were found from 280 species. 244 beetle species were found through the use of flight traps (103 exclusively), 164 by direct searching (22 exclusively), and 33 by pitfall trap (1 exclusively). 78 of the 280 species (27.8%) were red-listed.

Of all the species, 113 were generalist, 90 were found exclusively in coniferous deadwood, and 77 were found exclusively in broadleaved deadwood. Greater total populations were found when there were a greater number of species present (as shown by the graph below). However, the presence of Ips typographus was found to be significant in terms of its impact upon populations of other beetle species found only in coniferous deadwood (of which some were red-listed). Single-tree senility of beech and fir was found to be the biggest driver of broadleaved specialist presence, whilst the increasing penetration rate of the bark beetle was found to be the sole driver of increasing coniferous specialists (though not so for red-listed species, where numbers declined at higher penetration rates). Generalist species simply benefited from a greater abundance of deadwood.

How total beetle population correlated with total beetle species, for each site surveyed.

Across all sites and for all beetle species, it was found that an increase in canopy openness and deadwood abundance (‘resources’) lead to an increase in beetle populations. In this sense, it can be stated that more resources in a more preferable setting will provide habitat for more individuals of a greater number of species. As a matter of fact, population increases were rapid in response to even only a moderate increase in such resources, across generalist and specialist beetle species. Even where bark beetles did colonise spruce stands, the resultant increase in deadwood was beneficial for many beetle species, and therefore the bark beetle could be described as beneficial for other saproxylic beetles that specialise in conifers. This may, in part, be because of the bark beetle’s effect of opening up canopies, folowing death of the host spruce. However, stand openess had a negative effect upon broadleaved specialists, and this may be because broadleaved specialists are more likely to be fungivores (and fungi specialise in cool, humid, and thus low-light conditions). For red-listed species exclusively, deadwood presence in ‘naturally disturbed’ (as in, not logged) stands is most important.

Of course, certain beetle species will fare more preferably in open stands with a high amount of deadwood, whilst others will frequent closed canopies with less deadwood. Despite this, where there is little deadwood, beetle communities may become ‘impoverished’ (suffer from ‘famine’), and there are typically more beetle species in open stands with high amounts of deadwood than in closed stands. In this sense therefore, there is a minimum viable deadwood presence, for all species of beetle – it simply varies depending upon the species, which is not surprising. Unfortunately, the authors did not specify conclusive thresholds, and state that it is hard to calculate them because species may respond differently to differing amounts of deadwood, and there may not necessarily be a ‘cut-off’ point, but simply a continued (and perhaps steady) decline in population until there is not one at all. Highly intricate research would be required to calculate such thresholds, that would need to also be species-specific.

Fallen and standing deadwood within a dense mixed forest stand within the Bavarian Forest (there are more incredible photos to see via the source link). Source: Philip Klinger.

In spite of this, the authors do recommend that for dense stands of mixed woodland, canopy openness should be around 20%, and the abundance of deadwood from beech and fir should be increased to 30-60 cubic metres per hectare (at least). For more open stands (20-50% canopy openness, or greater), the focus should be only to increase deadwood abundance (to similar amounts as in denser stands). Where stands are logged therefore, retaining deadwood to such amounts would be beneficial in a conservation sense.

This is all certainly highly complex therefore, and from reading the paper somewhat taxing to digest (at times!). Granted, because beetle species are highly variable in their preferences, a broad study such as this is always to be welcomed as it ‘sets up the playing field’. Despite this, the authors note that this is research that shoud be defined only as a case study – other sites may vary in their characteristics, constituent populations, and thus ‘ideal’ states. Nevertheless, for montane stands in Central Europe, this research may be considered very important. Perhaps the most critical thing we can draw from this is that managed stands need more deadwood – period. Because beetle diversity and populations incresed alongside deadwood presence, the openness created by logging could perhaps be a good thing when compiled with the selected retention of deadwood – at least, for species that are not red-listed (as these require disturbance only of natural origin, of which logging is not).

Source: Müller, J., Noss, R., Bussler, H., & Brandl, R. (2010) Learning from a “benign neglect strategy” in a national park: Response of saproxylic beetles to dead wood accumulation. Biological Conservation. 143 (11). p2559-2569.

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