Invertebrate community response to coarse woody debris removal for bioenergy production from intensively managed forests

Variation in Larval Thermal Tolerance of Three Saproxylic Beetle Species

Temperature is a key abiotic condition that limits the distributions of organisms, and forest insects are particularly sensitive to thermal extremes. Whereas winged adult insects generally are able to escape unfavorable temperatures, other less-vagile insects (e.g., larvae) must withstand local microclimatic conditions to survive. Here, we measured the thermal tolerance of the larvae of three saproxylic beetle species that are common inhabitants of coarse woody debris (CWD) in temperate forests of eastern North America: Lucanus elaphus Fabricius (Lucanidae), Dendroides canadensis Latreille (Pyrochroidae), and Odontotaenius disjunctus Illiger (Passalidae). We determined how their critical thermal maxima (CTmax) vary with body size (mass), and measured the thermal profiles of CWD representing the range of microhabitats occupied by these species. Average CTmax differed among the three species and increased with mass intraspecifically. However, mass was not a good predictor of thermal tolerance among species. Temperature ramp rate and time in captivity also influenced larval CTmax, but only for D. canadensis and L. elaphus respectively. Heating profiles within relatively dry CWD sometimes exceeded the CTmax of the beetle larvae, and deeper portions of CWD were generally cooler. Interspecific differences in CTmax were not fully explained by microhabitat association, but the results suggest that the distribution of some species within a forest can be affected by local thermal extremes. Understanding the responses of saproxylic beetle larvae to warming habitats will help predict shifts in community structure and ecosystem functioning in light of climate change and increasing habitat fragmentation.

Longhorned Beetle Functional Diversity in Response to Biomass Harvesting

Woody debris left after timber harvest may be used to produce bioenergy. This will help reduce reliance on nonrenewable fossil fuels. There are ecological impacts to the use of this woody material as many species use coarse woody debris for food and shelter. An understanding of how the removal of woody biomass after forest harvest changes the functional diversity of forest communities is needed to make sustainable management decisions. Many longhorned beetles (Coleoptera: Cerambycidae) provide ecosystem services by reducing woody debris in forests and pollinating flowering plants including trees. I used a taxonomically and functionally diverse community of longhorned beetles to examine changes to functional diversity following different levels of biomass removal in a secondary forest in south-central Indiana, USA. The functional diversity was measured prior to forest harvest and biomass removal and for 2 yr after treatment. I predicted that the abundance and functional group richness of the longhorned beetles would correlate positively with the volume of woody biomass left on the site, and that the community would show resilience by returning to the functional portfolio that was in place in the pre-harvest communities and occurred in the control. The abundance and functional group richness increased with more woody debris left on site. There was much variance in the trajectory of the community following harvest, with no clear trend toward pre-harvest functional portfolio that would indicate resilience. A 3-yr study is likely not long enough to capture this resilience, especially on sensitive sites.

Ground Beetle (Coleoptera: Carabidae) Response to Harvest Residue Retention: Implications for Sustainable Forest Bioenergy Production

Research Highlights: Our study adds to the scant literature on the effects of forest bioenergy on ground beetles (Coleoptera: Carabidae) and contributes new insights into the responses of ground beetle species and functional groups to operational harvest residue retention. We discovered that count of Harpalus pensylvanicus (DeGeer)—a habitat generalist—increased owing to clear-cut harvests but decreased due to harvest residue reductions; these observations uniquely allowed us to separate effects of additive forest disturbances to demonstrate that, contrarily to predictions, a generalist species considered to be adapted to disturbance may be negatively affected by altered habitat elements associated with disturbances from renewable energy development. Background and Objectives: Despite the potential environmental benefits of forest bioenergy, woody biomass harvests raise forest sustainability concerns for some stakeholders. Ground beetles are well established ecological indicators of forest ecosystem health and their life history characteristics are connected to habitat elements that are altered by forest harvesting. Thus, we evaluated the effects of harvest residue retention following woody biomass harvest for forest bioenergy on ground beetles in an operational field experiment. Materials and Methods: We sampled ground beetles using pitfall traps in harvest residue removal treatments representing variable woody biomass retention prescriptions, ranging from no retention to complete retention of all merchantable woody biomass. We replicated treatments in eight clear-cut stands in intensively managed loblolly pine (Pinus taeda L.) forests in North Carolina and Georgia. Results: Harvest residue retention had no effect on ground beetle richness and diversity. However, counts of H. pensylvanicus, Anisodactylus spp., and “burrower” and “fast runner” functional groups, among others, were greater in treatments with no woody biomass harvest than those with no harvest residue retention; all of these ground beetles may confer ecosystem services in forests. We suggest that H. pensylvanicus is a useful indicator species for burrowing and granivorous ground beetle response to harvest residue reductions in recently harvested stands. Lastly, we propose that retaining 15% retention of total harvest residues or more, depending on regional and operational variables, may support beneficial ground beetle populations.

Saproxylic Diptera assemblages in a temperate deciduous forest: implications for community assembly

Saproxylic insects, those associated directly or indirectly with decaying wood for all or part of their life cycle, compose a large proportion of forest organisms. Flies (Diptera) are often the most abundant and species-rich group of insects in forest microhabitats, yet most work to date on saproxylic insect diversity and ecology has focused on beetles (Coleoptera). We compared saproxylic Diptera assemblages reared from two tree species (sugar maple and American beech) at two stages of decay (early/young and advanced/old) for a total of 20 logs in an eastern Canadian Nearctic old-growth forest. We found that communities are distinct within both species type and decay stage of wood. Early decay stage wood is more variable in community composition than later decay stage; however, as the age of the decaying wood increases, the abundance of Diptera increases significantly. Most indicator species are discernible in later decay stage and wood type. We venture to suggest that stochastic and deterministic processes may play a role in driving Diptera communities in temperate deciduous forests. To retain the highest saproxylic Diptera diversity in a forest, a variety of decaying wood types at different stages of decomposition is necessary.