In eastern North America, less than 1% of our temperate forests are considered old-growth, meaning that they have no history of being cleared since the arrival of European colonists. Old-growth forests serve as important reference points as to what forests are like in an uninterrupted state, and accordingly have unique attributes compared to second-growth (regrowing) forests. Huge old trees, mixed-age tree communities, and large amounts of deadwood all distinguish old-growth from second-growth forests. Moreover, unique and rare species of plants and animals inhabit old-growth, some of which are dependent on the stable supply of habitat through time found only in these undisturbed settings. For this reason, old-growth forests are often places of refuge for species while the surrounding landscape undergoes deforestation or other change. 

As part of my PhD at UGA, I am studying saproxylic insects in the southern Appalachian Mountains, where the majority of old-growth resides in the eastern US. Saproxylic insects are dependent on deadwood or its associated resources (e.g., fungi) for their life cycle. For this reason, they are sensitive to human influence in forests, which often reduces the amount of deadwood present. The southern Appalachians make a unique case study, as there are large, interrupted tracts of old-growth remaining, but they are surrounded by a landscape of much younger forests that were cleared during the early 1900s. With help from my advisors (Drs. Joseph McHugh and Michael Ulyshen) and collaborators (Drs. Michael Caterino and Michael Ferro), I assessed the dependence of two saproxylic beetle species on old-growth forests in this region: a pleasing fungus beetle, Megalodacneheros (Erotylidae), and an ironclad beetle, Phellopsis obcordata (Zopheridae). Although little is known about the natural history of these species, both require wood-decaying fungi to reproduce. Both are associated with old-growth, but little was known about their degree of dependence on it. The two species differ in their ability to fly, which probably influences their ecology. Some individuals of M. heros can and probably do fly for parts of their life, but all individuals of P. obcordata are entirely flightless and have tiny shriveled wings under their elytra.

Off the bat, we faced the challenge of actually finding these beetles in the remote and disorienting mountains of North Carolina, Tennessee, Virginia and Georgia. As a rough guide, we used label data from museum specimens (including those from the UGCA) to revisit some areas of the mountains where the species had been found previously. Some of these records were over a hundred years old (!) and it’s safe to say that we were unsuccessful more often than not. Additionally, we used historic disturbance maps of Great Smoky Mountains National Park (GSMNP) to hike trail sections in the park that would take us through old-growth forests. On these hikes we would look for shelf fungi growing on logs and dead trees, and we’d search for the beetles on or around the fungus. We additionally measured tree diameters and stand basal area both when the species were and weren’t present. After a few weeks, we started finding the beetles more regularly by making associations with forest types and fungi, which allowed us to observe their behavior and natural history. We also looked for the beetles and their fungi in second-growth forests to see if either were restricted to old-growth. Finally, we asked members of the public to help find new locations for these species in GSMNP and post those occurrence records on iNaturalist. The Discover Life in America organization featured the beetles on their “Smokies Most Wanted” page. This community science element was an important addition to our field surveys because we could only cover so much ground ourselves in the expansive landscape.

After a few weeks of field work, it was clear that our two species were responding quite differently to historic forest disturbance, and final results including the iNaturalist records verified this. Megalodacneheros and its host fungi could be found nearly anywhere below an elevation threshold, in old-and second-growth alike, and large trees didn’t seem to be necessary for their presence. Dead hemlock logs resembled a Cincinnati Bengals tailgate party with the shiny orange and black beetles aggregating in huge numbers on the red sporocarps of Ganoderma tsugae. This species and their host fungi were so prevalent that we had to stop counting to make the best use of our time, but we did document them using a new host, Ganoderma megaloma. Conversely, finding P. obcordata required patience, hard work, and (oftentimes) luck. These rugged, brown beetles were sometimes locally abundant, but populations seemed few and far between. We found them mostly on a newly documented host fungi, Fomitopsisochracea, growing on softwood trees at mid to high elevations, and Fomitopsisbetulinagrowing on birches. Phellopsis obcordata usually was found inside or very near old-growth sections of GSMNP, although there are a few scattered museum records more than 3 km from old-growth. Our surveys and iNaturalist records showed that their host fungi were found in second-growth, but not the beetles! Interestingly, however, P. obcordata didn’t seem to require any physical characteristics of old-growth forests, such as extremely large trees or logs.

Naturally, we were curious why our two study species differed in their dependence on old-growth. The host fungi of both species were present in both old- and second-growth forests, so why weren’t both beetles as well? The answer may lie in their ecology, specifically their ability to disperse to new habitat. Some Megalodacneheros individuals can fly and so individuals on average can disperse more easily to new host fungi that is far away. Accordingly, populations may be better at recolonizing or persisting in second-growth forests after disturbances. Conversely, Phellopsis obcordatabeetles are entirely flightless, and consequently are limited to finding new host fungi within a much shorter walking distance. Since no physical characteristics of old-growth were important for P. obcordata, this makes us believe that this species is dependent on the steady supply of host fungi through time and space found in old-growth. In second-growth forests, historic clearing events likely interrupted the supply for some time, and at present day the fungus hosts are distributed out of reach or too sparsely to support P. obcordata populations. This makes P. obcordata a great indicator of old-growth in the southern Appalachian Mountains, as it is sensitive to human disturbances in forests. Overall, we were able to show that old-growth forests can act as a refuge for sensitive species while the surrounding landscape undergoes human disturbance. For this reason, old-growth is home to rare and unique species like P. obcordata