The Saproxylic Activity Index: A New Tool for the Rapid Assessment of Deadwood Species during Forest Restoration

Moya L. Burns, Mike Smith, Eleanor M. Slade, Richard A. Ennos

Department of Zoology, University of Oxford, Oxford, UK.
Department of Zoology, University of Oxford, Oxford, UK; Department of Agricultural Sciences, University of Helsinki, Helsinki, Finland.
Forest Research, Northern Research Station, Roslin, UK.
School of Biological Sciences, University of Edinburgh, Edinburgh, UK.

DOI: 10.4236/ojf.2014.42020                

Abstract

Restoring deadwood habitat is vital in order to recreate fully functioning forest ecosystems. Letting this process occur naturally can take in excess of one hundred years, thus management practises typically try to accelerate this via the artificial addition of deadwood. Since the species which rely on deadwood often have poor dispersal abilities, restoring deadwood habitat rarely results in the full restoration of the saproxylic fauna. Furthermore, standard deadwood monitoring protocol only records the amount and type of substrate available and is not capable of determining whether saproxylic insects have been restored. Full species inventories are time-consuming, costly and require great expertise. We present a rapid biodiversity assessment tool which we believe is the first protocol for measuring saproxylic activity which is accessible to non-specialists. Utilising the exit bore holes which saproxylics create on deadwood can provide an indication of the density, richness and diversity of species present; we call this the Saproxylic Activity Index. We show that this index can detect differences in the activity of insects between substrates. As saproxylic insects provide important ecosystem functions, such as aiding in the decay of deadwood and recycling nutrients, a measure of their activity levels may indicate the rate of restoration of these ecosystem processes. We believe that further exploration of this method provides an exciting opportunity for the functional restoration of saproxylic fauna to become incorporated into mainstream forest management.

 

Keywords

Citizen Science; Temperate Forest; Beetles; Bioindicators; Volunteers; Coleoptera; Biodiversity; Surveys

Received December 1^st, 2013; revised January 7^th, 2014; accepted January 21^st, 2014

Figure 2.

Bore hole richness accumulation curves for (a) standing deadwood and (b) fallen deadwood.

Figure 3.

Bore hole (a) density and (b) diversity in relation to area of substrate (fallen and standing deadwood combined). Bore hole (c) den- sity and (d) diversity in relation to direction of rot-site (fallen and standing deadwood combined). Error bars show standard error of the mean. Only one rot site occurred facing towards the north-east (NE).

Rot type was found to have a significant effect on all measures of the Saproxylic Activity Index. Bore hole density varied significantly between rot types (df = 7, F = 3.27, p = 0.004). Rot type G (Red/ Brown Fibrous Moist) was found to have the greatest mean density of insect bore holes and rot type F (Red/ Brown Cubical Dry) the least (Figure 4(a)). Bore hole richness (df = 7, F = 3.11, p = 0.009) and diversity (df = 7, F = 2.30, p = 0.042) also differed significantly among rot types. Bore hole richness was greatest on rot type B (White Fibrous Dry) and lowest on rot type P (Black Fibrous Dry) (Figure 4(b)). Rot type A (White Cubical Dry) supported the greatest diversity of bore holes (Figure 4(c)). Since rot type A was observed only once in the field, this data point was removed from the analysis and an a-posteriori ANOVA confirmed that even excluding this there was a significant difference in bore hole diversity between rot types (df = 6, F = 2.80, p = 0.027).

Discussion

These results open an exciting new avenue of research for the future rapid assessment of saproxylic activity during forest restoration. Rot type was found to affect the density, richness and diversity of insect bore holes, indicating that this method of rapid biodiversity assessment can reflect real differences in the activity of saproxylic insects. Deadwood is already used as an indicator of “old growth” conditions being reached during forest restoration. We argue that direct evidence of the activ- ity of saproxylic invertebrates, such as that demonstrated

Rot types are classified using the English Nature Veteran Trees Initiative. A = White Cubical Dry, B = White Fibrous Dry, C = White Fibrous Moist, F = Red/Brown Cubical Dry, G = Red/Brown Fibrous Dry, H = Red/Brown Fibrous Moist, P = Black Fibrous Dry, R = Black Fibrous Moist.

Figure 4.

Bore hole (a) density (b) richness and (c) diversity in relation to rot type (fallen and standing deadwood combined). Error bars show standard error of the mean. Rot type A only occurred once in the survey area.

in this paper, is needed in order to truly establish the success of restoration of deadwood species to a site; the Saproxylic Activ- ity Index offers a viable method by which this can be achieved.

Indicators need to be simple to use and effective in order to show whether management targets are being met (Ferris & Humphrey, 1999). The Saproxylic Activity Index has the po- tential to meet these requirements. It is quick, easy to use, and readily accessible to non-specialists, such as habitat managers and citizen scientists. Moreover, it is a non-destructive method of sampling saproxylic invertebrate presence. This is of partic- ular importance due to the already threatened nature of this group (Harvey et al., 2011).

We found that rot type influenced bore hole density, richness and diversity. As wood decays, the host range for saproxylic insects broadens and the flora of decay becomes the most im- portant factor determining the saproxylic fauna (Larkin & Elbourn, 1964; Jonsell et al., 1998), this may explain the rela- tionship found between rot type and the Saproxylic Activity Index. Rot type A, which only occurred in one site had the highest bore-hole diversity, which may indicate that rare rot types provide habitats for rare species.

It has been hypothesised that saproxylic invertebrates may prefer trees with larger diameters because they have a more stable microclimate (Ranius & Jansson, 2000). Some studies which have surveyed saproxylic insect fauna using capture methods have provided support for this theory, although they point out that this pattern may be species specific (Ranius & Jansson, 2002). Our study found no relationship at Dalkeith Old Wood between diameter and the Saproxylic Activity Index, and we suggest that this may be because of the homogeneity of the diameter of standing and fallen deadwood at the study site, due to the uniformity of tree age (Johnston, 2002). Alternatively, it may be that if such diameter effects are not apparent across all saproxylic species then a coarse measure of diversity such as the Saproxylic Activity Index will be unable to detect individu- al species responses.

A wood pasture by its nature has a far more open canopy than a plantation or semi-natural forest (Hol & Smith 2002). Most published studies looking at saproxylic invertebrate di- versity in relationship to exposure have been carried out in closed canopy forests (Ranius & Nilsson 1997). This may ex- plain why no pattern between rot site direction and the Saprox- ylic Activity Index was found in Dalkeith Old Wood.

The relationship between invertebrate bore hole diversity and actual saproxylic invertebrate species diversity is not known, and little information exists on which species produce which size and shape of bore holes. Moreover, some beetles are di- morphic in size, with females larger than males (Hespenheide, 1976), adding further complication to the idea that bore holes of a certain type are representative of a certain species. While we do not suggest that this method provides a substitute for full saproxylic surveys, in times of limited resources and expertise, the Saproxylic Activity Index can act as a coarse measure of the overall saproxylic density, richness and diversity of a site. In particular we suggest that if surveys using the Saproxylic Ac- tivity Index are conducted yearly during a forest restoration treatment, then changes in levels of saproxylic activity can be tracked over time.

Conclusion

Extensive studies aimed at developing umbrella groups of saproxylics which can be used as indicators have been largely unsuccessful and results have not been transferable between regions (Jansson et al., 2009). In contrast, measuring activity levels of saproxylics as a measure of ecosystem functioning offers an exciting new area of research, especially if consistent values representing old growth forest can be determined against which survey results can be compared. We hope this paper will stimulate further research into the relationship between the Saproxylic Activity Index and measurable saproxylic insect diversity and ecosystem functioning, such as the rate of wood decay. We anticipate that in future direct evidence of saproxylic activity may become commonplace in rapid biodiversity as- sessments carried out by forest managers and citizen scientists.

Acknowledgements

Many thanks are owed to Scottish Natural Heritage for al- lowing this study to be carried out at Dalkeith Old Wood and for providing background information on the site.

References

Conflicts of Interest

The authors declare no conflicts of interest.

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