Forest disturbance and arthropods: Small-scale canopy gaps drive invertebrate community structure and composition

Combinative effects of thinning and prescribed burning on fuel reduction and soil arthropods: A case study in a Mediterranean pine forest

Wildfire pressure involves today to implement silvicultural practices that provide a good compromise between reducing fire risk and maintaining ecological functioning. Thinning reduces tree density and low branches, but results in the deposition of a considerable biomass of woody debris on the ground (up to 4800 g m2 in this study). They can be eliminated by prescribed burning, but this raises questions about the fire intensity that can be generated and the impact on soil fauna. We undertook the monitoring of a thinning and prescribed burning operation, separated and combined, in November 2020, in a Pinus laricio stand prone to fire risk, located in Bavella, Corsica. Fuel load was determined, and temperature measurements in the soil were performed using K-type thermocouples. Soil arthropod populations were monitored using pitfall traps, in particular Collembola, Acari, Aranae, and Coleoptera. The combination of thinning and burning resulted in a fire intensity of 75.8 versus 8.4 kW m-1 for burning alone. Maximum temperature rise measured at -2 cm below the surface was less than 5°C for both treatments. The combination of thinning and burning did not result in higher fire intensity at ground level than burning alone, and the soil showed high insulation capacity. Most of the woody debris that burned was small-diameter, and large-diameter debris remained unconsumed. This burning, performed during a period of low biological activity, had no effect on soil arthropods, and the presence of large debris may have provided refuge areas. Collembola group was the faster to recover, and were followed by cohorts of predators in summer, especially Acari. Our results suggest that a combination of burning and thinning in autumn may be beneficial for fire prevention. However, the decomposition of woody debris in relation to fire risk, and the occurrence of pests after these treatments need to be monitored.

Adaptive monitoring in action-what drives arthropod diversity and composition in central European beech forests?

Recent studies suggest that arthropod diversity in German forests is declining. Currently, different national programs are being developed to monitor arthropod trends and to unravel the effects of forest management on biodiversity in forests. To establish effective long-term monitoring programs, a set of drivers of arthropod diversity and composition as well as suitable species groups have to be identified. To aid in answering these questions, we investigated arthropod data collected in four Hessian forest reserves (FR) in the 1990s. To fully utilize this data set, we combined it with results from a retrospective structural sampling design applied at the original trap locations in central European beech (Fagus sylvatica) forests. As expected, the importance of the different forest structural, vegetation, and site attributes differed largely between the investigated arthropod groups: beetles, spiders, Aculeata, and true bugs. Measures related to light availability and temperature such as canopy cover or potential radiation were important to all groups affecting either richness, composition, or both. Spiders and true bugs were affected by the broadest range of explanatory variables, which makes them a good choice for monitoring general trends. For targeted monitoring focused on forestry-related effects on biodiversity, rove and ground beetles seem more suitable. Both groups were driven by a narrower, more management-related set of variables. Most importantly, our study approach shows that it is possible to utilize older biodiversity survey data. Although, in our case, there are strong restrictions due to the long time between species and structural attribute sampling.

Impact of forest disturbance on microarthropod communities depends on underlying ecological gradients and species traits

Windstorms and salvage logging lead to huge soil disturbance in alpine spruce forests, potentially affecting soil-living arthropods. However, the impacts of forest loss and possible interactions with underlying ecological gradients on soil microarthropod communities remain little known, especially across different environmental conditions. Here we used DNA metabarcoding approach to study wind-induced disturbances on forest communities of springtails and soil mites. In particular, we aimed to test the effect of forest soil disturbance on the abundance, richness, species composition, and functional guilds of microarthropods. We sampled 29 pairs of windfall-forest sites across gradients of elevation, precipitation, aspect and slope, 2 years after a massive windstorm, named Vaia, which hit North-Eastern Italy in October 2018. Our results showed that wind-induced disturbances led to detrimental impacts on soil-living communities. Abundance of microarthropods decreased in windfalls, but with interacting effects with precipitation gradients. Operative Taxonomic Units (OTU) richness strongly decreased in post-disturbance sites, particularly affecting plant-feeder trophic guilds. Furthermore, species composition analyses revealed that communities occurring in post-disturbance sites were different to those in undisturbed forests (i.e., stands without wind damage). However, variables at different spatial scales played different roles depending on the considered taxon. Our study contributes to shed light on the impacts on important, but often neglected arthropod communities after windstorm in spruce forests. Effects of forest disturbance are often mediated by underlying large scale ecological gradients, such as precipitation and topography. Massive impacts of stronger and more frequent windstorms are expected to hit forests in the future; given the response we recorded, mediated by environmental features, forest managers need to take site-specific conservation measures.

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.

Long-term changes in avian biomass and functional diversity within disturbed and undisturbed Amazonian rainforest

Recent long-term studies in protected areas have revealed the loss of biodiversity, yet the ramifications for ecosystem health and resilience remain unknown. Here, we investigate how the loss of understory birds, in the lowest stratum of the forest, affects avian biomass and functional diversity in the Amazon rainforest. Across approximately 30 years in the Biological Dynamics of Forest Fragments Project, we used a historical baseline of avian communities to contrast the avian communities in today's primary forest with those in modern disturbed habitat. We found that in primary rainforest, the reduced abundance of insectivorous species led to reduced functional diversity, but no reduction of biomass, indicating that species with similar functional traits are less likely to coexist in modern primary forests. Because today's forests contain fewer functionally redundant species-those with similar traits-we argue that avian communities in modern primary Amazonian rainforests are less resilient, which may ultimately disrupt the ecosystem in dynamic and unforeseen ways.

Intensive site preparation for reforestation wastes multi-trophic biodiversity potential in commercial oak woodlands

Lowland oak forests host high biodiversity throughout Europe, and to maintain their high biodiversity potential we need to understand how current silviculture practices influence these ecosystems. The most common harvest approach in oak woodlands remains clear-cutting followed by site preparation for reforestation. The site preparation can vary in intensity and possibly affect forest biodiversity in various ways. We studied the impact of site preparation intensity on ground-dwelling arthropod predators and detritivores in commercial oak woodlands. The intensive management included mechanical site preparation with soil milling, stump removal, and herbicide application. The extensive management consisted of natural succession after reforestation with only such slight disturbances as regular hand-provided mowing of vegetation. We recorded 120 spider species, 92 species of ground beetles, 10 species of centipedes, 17 species of millipedes, and 7 species of woodlice, including a relatively large number of threatened species. We found that intensive post-logging management strongly homogenized the habitat structure, and this led to low multi-trophic taxonomic and functional diversity in comparison to that under extensive management. At sites with extensive management, there was not only high functional diversity but also high functional redundancy. Species of conservation concern almost vanished from clearings under intensive management. The high multi-trophic diversity and functional redundancy indicate that extensive site preparation may enhance ecosystem multi-functionality, including primary productivity and ecosystem resilience. Extensive post-harvest management is therefore strongly preferable, and it is also economically more feasible. Intensive post-harvest management should be prohibited in lowland forests within transition zones to localities under nature protection or in natural oak forests.

Three-dimensional stratification pattern in an old-growth lowland forest: How does height in canopy and season influence temperate bat activity?

The study of animal-habitat interactions is of primary importance for the formulation of conservation recommendations. Flying, gliding, and climbing animals have the ability to exploit their habitat in a three-dimensional way, and the vertical canopy structure in forests plays an essential role for habitat suitability. Forest bats as flying mammals may seasonally shift their microhabitat use due to differing energy demands or changing prey availability, but the patterns are not well understood. We investigated three-dimensional and seasonal habitat use by insectivorous bats in a temperate lowland old-growth forest, the Belovezhskaya Pushcha in Belarus. We acoustically sampled broadleaved and mixed coniferous plots in the forest interior and in gaps in three heights during two reproductive periods (pregnancy/lactation vs. postlactation). In canopy gaps, vertical stratification in bat activity was less pronounced than in the forest interior. Vertical activity patterns differed among species. The upper canopy levels were important foraging habitats for the open-space forager guild and for some edge-space foragers like the Barbastelle bat Barbastella barbastellus and the soprano pipistrelle Pipistrellus pygmaeus. Myotis species had highest activity levels near the ground in forest gaps. Moreover, we found species-dependent seasonal microhabitat shifts. Generally, all species and species groups considered except Myotis species showed higher activity levels during postlactation. Myotis species tended toward higher activity in the forest interior during postlactation. P. pygmaeus switched from high activity levels in the upper canopy during pregnancy and lactation to high activity levels near the ground during postlactation. We conclude that a full comprehension of forest bat habitat use is only possible when height in canopy and seasonal patterns are considered.

Soil Biodiversity as Affected by Different Thinning Intensities in a Pinus laricio Stand of Calabrian Apennine, South Italy

Forest soil biodiversity, which drives natural ecosystem multifunctionality, can be altered by incorrect forestry management practices. Pinus laricio is the most representative and widespread conifer species in Calabria, South Italy, and appropriate management is needed to maintain Pinus laricio forest for its great economic and natural value. In Europe, thinning is considered the most effective silvicultural treatment to maintain/increase the ecological value of coniferous stands. In this study, moderate thinning (MT), intense thinning (HT), and clear cut (CC) treatments were used to manage Pinus laricio stands with the aim of identifying the thinning intensity that is less detrimental to soil biodiversity. The effects of the different thinning intensities were evaluated, in two contrasting seasons (summer and winter), on the abundance, and diversity of arthropods, fungi, and bacteria colonies as well as on selected soil properties (organic matter, humification index, bulk density, pH) related to soil habitability. Results evidenced that the abundance, species richness, and diversity of arthropods, as well as fungi, bacteria colonies, and soil properties, changed with the treatments and seasons. Under HT, the greatest biodiversity and the highest amounts of arthropods, fungi, and bacteria were found in both seasons. This study finds evidence for Connell’s intermediate disturbance hypothesis, highlighting that the greatest organic carbon content and humification index, as well as the lowest bulk density, found in HT reduced the likelihood of competitive exclusion between occurring species, thereby promoting high species richness and diversity. This study gives insights into ecological relationships between understory composition related to tree species abundance and soil community.

Ground-dwelling arthropods of pinyon-juniper woodlands: Arthropod community patterns are driven by climate and overall plant productivity, not host tree species

Pinyon-juniper (PJ) woodlands have drastically changed over the last century with juniper encroaching into adjacent habitats and pinyon experiencing large-scale mortality events from drought. Changes in climate and forest composition may pose challenges for animal communities found in PJ woodlands, especially if animals specialize on tree species sensitive to drought. Here we test habitat specialization of ground-dwelling arthropod (GDA) communities underneath pinyon and juniper trees. We also investigate the role of climate and productivity gradients in structuring GDAs within PJ woodlands using two elevational gradients. We sampled 12,365 individuals comprising 115 taxa over two years. We found no evidence that GDAs differ under pinyon or juniper trees, save for a single species of beetle which preferred junipers. Climate and productivity, however, were strongly associated with GDA communities and appeared to drive differences between sites. Precipitation was strongly associated with arthropod richness, while differences in GDA composition were associated with environmental variables (precipitation, temperature, vapor pressure, and normalized difference vegetation index). These relationships varied among different arthropod taxa (e.g. ants and beetles) and community metrics (e.g. richness, abundance, and composition), with individual taxa also responding differently. Overall, our results suggest that GDAs are not dependent on tree type, but are strongly linked to primary productivity and climate, especially precipitation in PJ woodlands. This implies GDAs in PJ woodlands are more susceptible to changes in climate, especially at lower elevations where it is hot and dry, than changes in dominant vegetation. We discuss management implications and compare our findings to GDA relationships with vegetation in other systems.

Dynamic Responses of Ground-Dwelling Invertebrate Communities to Disturbance in Forest Ecosystems

In forest ecosystems, natural and anthropogenic disturbances alter canopy structure, understory vegetation, amount of woody debris, and the properties of litter and soil layers. The magnitude of these environmental changes is context-dependent and determined by the properties of the disturbance, such as the frequency, intensity, duration, and extent. Therefore, disturbances can dynamically impact forest communities over time, including populations of ground-dwelling invertebrates that regulate key ecosystem processes. We propose conceptual models that describe the dynamic temporal effects of canopy gap formation and coarse woody debris accumulation following disturbances caused by invasive insects, wind, and salvage logging, and their impacts on ground-dwelling invertebrate communities. Within this framework, predictions are generated, literature on ground-dwelling invertebrate communities is synthesized, and pertinent knowledge gaps identified.