Deadwood Beetles
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Increase diversity, abundance and biomass
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Cairngorms Connect
Scotland
Problem: Deadwood is a key component of natural forests, providing habitat for a vast array of species. One highly diverse group of deadwood-dependent species is deadwood beetles, which performs a crucial role in nutrient cycling within forests. In forests managed for commercial production, the amount and/or diversity of deadwood is typically lower than that of natural forests.
Action: Cairngorms Connect is steadily increasing and diversifying the deadwood amount in formerly commercial plantations. This is achieved by active deadwood creation and passive management strategies such as allowing stands to grow on to maturity and natural senescence.
Indicator: The abundance and diversity of deadwood, or saproxylic, beetles correlates with the amount and diversity of deadwood in these forested areas. Saproxylic beetles indicate the presence of the ecological function deadwood beetles provide, namely breaking down plant matter and promoting the cycling of nutrients and soil enrichment, thus increasing overall forest productivity and biodiversity.
Methods
Insect samples were collected at six control and six treatment plots in plantation-origin stands of Scot’s Pine in 2020 (before deadwood creation), 2021 and 2022 (1 year and 2 years after deadwood creation, respectively), in a before-after control-impact (BACI) design. Samples were also collected in a semi-natural area of Scot’s Pine forest, to provide a comparison of deadwood invertebrate communities in this more natural habitat.
At each monitoring point, one flight interception trap was used to catch flying beetles. Traps were checked monthly between May and September and all beetles caught were manually identified to family level. Note that because of practical problems, after the 2020 field season the type of trap used across all sites was changed, from a Steiniger trap to a vane trap design, baited with 50 ml industrial methylated spirits (see Lessons Learned section below). Steiniger traps were also used at the semi-natural plots only in 2021.
Deadwood surveys were carried out at all sampling plots in 2020 and 2022. Active deadwood creation was carried out in winter 2020-21 at the six treatment plots only. This involved creating standing and lying deadwood by three different methods: ring-barking to create standing deadwood, chainsaw felling to create lying deadwood plus short stumps, and winching trees to create lying deadwood with exposed root plates (simulating windfall).
Standing deadwood created by the ring-barking method. Photo: Pip Gullet
Old standing deadwood; deadwood creation method was by cutting. Photo: Pip Gullet.
Lying deadwood created by pulling the tree over using a winch. Photo: Pip Gullet
A vane trap, hanging among newly created standing deadwood. Photo: Pip Gullet
Results
Surveys of dead trees showed that deadwood creation activities successfully increased the number of dead trees in treatment plots, from 100 (±43) in 2020 to 168 (±90) in 2022. In contrast, the number of dead trees in control plots decreased slightly from 138 (±74) to 119 (±72) and in semi-natural plots showed a small increase from 12 (±11) in the 2020 to 16 (±15) in 2022.
Figure 1. Saproxylic beetle abundance across six monitoring plots in each of three forest types across three monitoring years. Abundance was significantly lower in semi-natural plots compared to both treatment and control plantation-origin areas (one-way ANOVA: F(2,15) = 6.98, P<0.01). This is likely due to the different trap types used in the semi-natural plots (see below for more details).
Insect sampling across three years and 18 plots resulted in a total of 8990 invertebrates, of which 4721 were beetles. A total of 42 beetle families were recorded, of which 25 contained predominantly saproxylic species. In 2020, the baseline year, neither saproxylic beetle abundance or saproxylic beetle family diversity showed any significant differences between treatment, control or semi-natural plots (one-way ANOVAs: P>0.05). In 2021, the year following deadwood creation, the abundance of saproxylic beetles was significantly lower in semi-natural plots compared to both treatment and control plantation-origin areas (Figure 1), although these differences may have been affected by differences in trap types between treatments and were not maintained in 2022. There was no significant difference in the Simpson’s diversity of saproxylic beetle families between the three plot types in any year (Figure 2).
Figure 2. Saproxylic family Simpson’s diversity across six monitoring plots in each of three forest types across three monitoring years. There was no significant difference in the diversity of saproxylic beetle families between the three plot types in any year (one-way ANOVAs; P>0.05 for all comparisons).
Interpretation
The lack of difference in deadwood communities between treatment and control plots is likely due to the fact that, particularly since 2006, active forest management has been widespread across this landscape, including in the control plots used in this study. This meant that there was a lot of deadwood resource available across all the old plantation plots. Indeed, surveys found that on average more dead trees were present in control plots compared to treatment plots at the start of the study. The lack of clear effects is also likely to reflect spatial heterogeneity in management history across the site. The abundance and richness of saproxylic beetles is influenced by numerous factors, such as microclimate and deadwood diversity (Seibold et al. 2016), which is likely to increase levels of variation within each treatment type.
It is also important to note that the results suggest that the type of trap used strongly affected the number of invertebrates and beetles caught. Specifically, the far lower beetle numbers recorded in all plot types in the baseline year and in semi-natural plots in year 1 appear to be a result of using Steininger traps, which have a far smaller trapping surface area compared to vane traps. Therefore, the project switched to using more effective vane traps from 2021 onwards (see Lessons Learned). This change in trapping methods meant that it was not possible to directly compare beetle indices between the baseline and post-treatment years.
Pytho depressus, or the log bark beetle. Photo: Ewan Craig
There was a tendency for the number of deadwood-specialist beetles to be higher in areas of plantation-origin forest compared to areas of semi-natural forest. This difference is probably driven by the lower abundance of dead trees in semi-natural forest patches, which reflects the fact that ‘semi-natural’ woodland in this landscape is still far from being natural or ‘old growth’ forest, due to a long history of human intervention. The high abundance of saproxylic beetles in plantation-origin plots suggests that active deadwood creation, carried out both before and during this study, is successfully increasing the abundance and diversity of both deadwood and saproxylic beetles in this habitat.
Three different methods have been used to create deadwood in old plantations – cutting, winching and ring-barking. Trees killed by each of these methods will take different lengths of time to die, meaning that many of the changes in invertebrate communities are expected to take longer than two years. Continuing to monitor both the invertebrates and the deadwood in these plots over the next 5-10 years will be important to assess the medium- and longer-term effects of restructuring.
News Articles
Cairngorms Connect website: Beetle Treasure in the Caledonian Pine Forest
Cairngorms Connect website: Deadwood Beetle Monitoring with Cairngorms Connect
