Trees decay at very slow rates over the space of decades and constitute as much as 10-25% of the above ground biomass in forests (Ulyshen, 2014). Dead wood is a significant factor in nutrient cycling and is a host to many organisms, from unicellular microorganisms to complex invertebrates (Franklin et al., 1997).
Organisms that depend on dead or decaying wood in their life cycle are called ‘saproxylic’ (Speight, 1989). Wood-inhabiting species are composed of fungi and insects. Insects that depend or live in decaying wood contain the orders of: Coleoptera (beetles), Diptera (flies), Hymenoptera (wasps, ants, bees), Isoptera (termites), Lepidoptera (moths), and Hemiptera (true bugs). Other than insects, members of the Order Nematoda (nematodes) can also be found living in decaying wood (Stokland et al., 2012).
The diversity of saproxylic fauna is determined by a variety of factors including; the species of tree, level of decay, stand age, and the cause of tree death (Langor et al., 2008). In a study by Dahlberg and Stokland (2004), it was found that in a large sample of saproxylic insects, a small percentage of them exhibited a preference to a specific tree species. This was due in part to tree composition. The structural or chemical factors in trees that affect host-tree relationships include lignin, hemicellulose content, and secondary compounds produced by the tree as a defense mechanism (Stokland et al., 2012). The latter constitute an important challenge for initial bark colonizers, such as bark beetles, which need to face the chemical defense system of the tree, highly present in the cells under the bark of recently deceased trees. Nevertheless, patterns of host-tree preferences are also linked to decay preference.
Image 1: Some invertebrates found in decaying logs. The inverts found in this petri dish include: Fire-coloured beetle larvae (pyrochroid Dendroides canadensis), centipedes, and other saproxylic feeders. (Identification retrieved from bugguide.net).
Saproxylic species which associate strictly with one host-tree occur at early stages of decay. Thus, saproxylic insects with more specific host-tree requirements are more often found in recently dead trees. Heavily decayed trees tend to support the diversity of saproxylic species with less specific host requirements (Stokland et al., 2012). As the physical and chemical properties change with the decomposition of the tree, invertebrate communities will change as well, as they have specific nutritional needs that are provided by different stages of decay.
The Morgan Arboretum is a reconstitution of many types of forests, the largest forest type being Sugar Maple-American Beech. A study by Williamson (1975), suggested that early colonizers (pioneers) of this type of forest are Ash trees, and since they are eventually outcompeted by Sugar Maple and American Beeches, we thought we would be most likely to find many fallen Ash trees at different stages of decay. Additionally, these trees may colonize a recently disturbed habitat eventually make a stand of Ash trees as a means of secondary succession.
Image 2: On the left, a primarily a Maple and Ash stand. This is the first forest we sampled from on our first data collection day. Ash in red and Maple in pink. On the right, a close up of Ash bark. Notice the signature ‘diamond’ or ‘X’ patterns in the bark.
Ash trees are also currently threatened by the Emerald Ash borer, a pest insect introduced from Asia. This pest has rapidly spread to a good proportion of the United States and Canada and has already killed millions of ash trees since its discovery in 2002 (BenDor et al. 2006).
Considering the increase in Ash tree death due to the Emerald Ash Borer, the sudden surge in fallen Ash could influence the saproxylic organisms that rely on these trees. A sudden increase in fallen ashes may be beneficial in the short term for saproxylic organisms that require them for their nutritional needs (Littlewood et al., 2014). Some orders such as: Hemiptera, Diptera, and Lepidoptera are obligate to Ashes and they may suffer devastating population crashes as they rely on a continual supply of these trees (Littlewood et al., 2014). Since the Emerald ash borer threatens Ashes in North America, it is important to learn more about the relationship between the saproxylic invertebrates and this type of tree.
Based on what we discovered above, we asked the following research question:
How does the level of decay in Ash trees affect the diversity of the saproxylic invertebrates that inhabit them?
To answer our research question, we established a scale to measure the different levels of decay of dead trees.
Sampling was done once a week for three weeks. We went to the Morgan Arboretum and sampled 17 dead ash trees. Logs were randomly chosen and we sampled multiple 30 by 30 cm2 plots on the trunk, chosen via a random number generator. For each tree chosen, we determined their level of decay (using the table above) and we measured the length and circumference. Invertebrates were collected on ⅔ of each log. We used a chisel and mallet to peel off the bark and retrieve any invertebrates we found.
Each specimen found was accounted for and was released back where we found them afterwards. We took pictures of every different invert we found to identify them later in addition to the level of decay of each tree.
See our video explaining our sampling method!
Image 3: Collecting the invertebrates on the left and data inventory of the invertebrates on the right.
We noticed that the level of decay greatly impacted the diversity of inverts we found. We did not come across any decaying ash trees that scored a 4 or a 5 as per our table (above), but we came across many that ranged from 1-3. We expected there to be less inverts present in newly fallen trees (scoring a 1) but we were surprised to find that there were a decent amount present (even though the bark was extremely hard to remove). We mostly found potato bugs, millipedes and some spiders (which are not saproxylic). Under the bark of trees that scored a 2 or a 3 or between both, we found many creatures including; earthworms, beetles (very few though), a variety of beetle larvae, millipedes, centipedes, some fly larvae, and potato bugs.
Our research and findings so far have shown that Ash trees prove to be good host to a variety of saproxylic invertebrates, which worried us at first since there was not a lot of literature on the host-specificity of saproxylic inverts to Ash trees. The vast majority of the literature we found in terms of Ash trees was linked to the Emerald Ash Borer, so we were relieved to find that Ash trees served as an adequate feeding, hibernation, and cover site for many saproxylic organisms.
Image 4: A. Wireworm (click beetle larvae), B. Millipede, C. Xylophagus larvae, D. Click beetle larvae, E. Potato bug, F. European Nightcrawlers (earthworms). These were all found under the bark of our sample Ash trees. (Identification retrieved from bugguide.net).
BenDor, T. K., Metcalf, S. S., Fontenot, L. E., Sangunnett, B., & Hannon, B. (2006). Modeling the spread of the Emerald Ash Borer. Ecological Modelling, 197(1–2), 221-236. doi:http://dx.doi.org/10.1016/j.ecolmodel.2006.03.003
Bughunter. (2005, 23 January 2008). Wireworm (Click Beetle Larvae). Retrieved from http://bugguide.net/node/view/14440
Dahlberg, A., Stokland, J. N. (2004). Substrate Requirements of Wood inhabiting species: a compilation of 3600 species. (Vol. Report 7, pp. 75): Skogsstyrelsen, Jönköping.
Franklin, J. F., Shugart, H. H., Harmon, M. E. (1987). Tree death as an ecological process. Bioscience, 37(8), 550-556. doi:10.2307/1310665
Langor, D. W., Hammond, H. E. J., Spence, J. R., Jacobs, J., Cobb, T. P. (2008). Saproxylic insect assemblages in Canadian forests: diversity, ecology, and conservation. The Canadian Entomologist, 140(4), 453-474. doi:10.4039/n07-LS02
Littlewood, N. A., Nau, B. S., Pozsgai, G., Stockan, J. A., Stubbs, A., & Young, M. R. (2014, February 05). Invertebrate species at risk from Ash Dieback in the UK. Journal of Insect Conservation, 19(1), 75-85. doi:10.1007/s10841-014-9745-2
Speight, M. C. D. (1989). Saproxylic invertebrates and their conservation (Vol. 42). France: Council of Europe, Strasbourg.
Stokland, J. N. (2001). The Coarse Woody Debris Profile: An Archive of Recent Forest History and an Important Biodiversity Indicator. Ecological Bulletins(49), 71-83. http://www.jstor.org/stable/20113265
Stokland, J. N., Siitonen, J., & Jonsson, B. G. (2012). Biodiversity in dead wood. New York: Cambridge University Press.
Ulyshen, M. D. (2016). Wood decomposition as influenced by invertebrates. Biological Reviews, 91, 70-85. doi:10.1111/brv.12158
Williamson, G. B. (1975). Pattern and Seral Composition in an Old-growth Beech-Maple Forest. Ecology, 56(3), 727-731. doi:10.2307/1935509