Tree Hollows Research Articles

Making hollow trees: Inoculating living trees with wood-decay fungi for the conservation of threatened taxa - A guide for conservationists

Decaying wood and cavities in living trees are fundamental determinants of forest biodiversity.However, a long history of forestry and land-use change has created a fragmented network ofwoodland with a depleted stock of veteran trees that support these microhabitats. Decomposition isa slow process and it may take heart-rot fungi hundreds of years to establish before hollowing evenbegins. A major challenge to forest restoration, therefore, is how these habitats can be restored orreplicated. One approach is to inoculate trees with heart-rot fungi as a direct intervention toaccelerate tree hollowing. We identify two types of conservation inoculation that could be beneficialin forest conservation: (1) Veteranising inoculations designed to benefit cavity and decay dependantfauna; and (2) Translocation inoculations, to reintroduce locally extinct, dispersal-limited heart-rotfungi.

Does the need to drink influence nest site selection in a wide-ranging threatened cockatoo?

Conserving wide-ranging species is challenging and requires an understanding of resource selection at multiple spatial scales, including how functional resources interact. Understanding this interaction is likely to be particularly important if resources are rare, such as tree hollows, or likely to change into the future under climate change, such as water availability and distribution. We examined nest selection by Karak (Calyptorhynchus banksii naso), a threatened obligate tree hollow nester endemic to south-western Australia, globally one of the regions most affected by climate change. We aimed to identify factors influencing nest hollow selection at multiple spatial scales including interactions between functional resources. We found that nest selection occurred primarily at the hollow scale, with deeper, higher hollows selected, and at the landscape scale, with hollows closer to ephemeral and permanent drink sites selected. The preference for specific types of hollows indicated that suitable hollows are likely to be scarce in the landscape and that management prescriptions need to be developed to maintain the supply of suitable hollows. Maintenance of drink sites in an area experiencing significant declines in rainfall will require more novel management prescriptions, which could potentially include the provision of artificial drink sites. Overall, our study demonstrated the importance of understanding interactions between functional resources at large spatial scales for the effective conservation of wide-ranging species.

Influence of tree hollow characteristics and forest structure on saproxylic beetle diversity in tree hollows in managed forests in a regional comparison.

Tree hollows are among the rarest habitats in today's Central European managed forests but are considered key structures for high biodiversity in forests. To analyze and compare the effects of tree hollow characteristics and forest structure on diversity of saproxylic beetles in tree hollows in differently structured managed forests, we examined between 41 and 50 tree hollows in beech trees in each of three state forest management districts in Germany. During the two‐year study, we collected 283 saproxylic beetle species (5880 individuals; 22% threatened species), using emergence traps. At small spatial scales, the size of hollow entrance and the number of surrounding microhabitat structures positively influenced beetle diversity, while the stage of wood mould decomposition had a negative influence, across all three forest districts. We utilized forest inventory data to analyze the effects of forest structure in radii of 50–500 m around tree hollows on saproxylic beetle diversity in the hollows. At these larger spatial scales, the three forest management districts differed remarkably regarding the parameters that influenced saproxylic beetle diversity in tree hollows. In Ebrach, characterized by mostly deciduous trees, the amount of dead wood positively influenced beetle diversity. In the mostly coniferous Fichtelberg forest district, with highly isolated tree hollows, in contrast, only the proportion of beech trees around the focal tree hollows showed a positive influence on beetle diversity. In Kelheim, characterized by mixed forest stands, there were no significant relationships between forest structure and beetle diversity in tree hollows. In this study, the same local tree hollow parameters influenced saproxylic beetle diversity in all three study regions, while parameters of forest structure at larger spatial scales differed in their importance, depending on tree‐species composition.

Ecological network for species dependent on ancient broadleaf trees using Osmoderma barnabita as a model species: a new approach

Abstract Organisms that depend on rare and endangered microhabitats, such as old, hollow trees, have a high conservation value. To conserve these species, it is necessary to apply methods that identify habitats suitable for their survival. Until now, a comprehensive method has not been developed for predicting the occurrence of species associated with old, hollow trees. Here, we developed criteria to identify habitats for organisms associated with old, hollow trees, using Osmoderma barnabita as a model species. We used hexagonal tessellation to identify suitable habitats for hermit beetle. Criteria for predicting the occurrence of a hermit beetle were applied in practise, and as result 47.56% of the known hermit beetle localities coincided with the hexagons where the probability of occurrence of the species was the highest. Based on these results, an ecological network was created for species dependency on old, hollow trees, using the hermit beetle as a model species. The proposed approach makes it possible to plan and implement environmental protection measures for species living in territories spanning several countries. In this work, we propose a method to identify species occurrence in core areas and to determine ecological corridors and gaps. This method could tailor conservation planning efforts.

A Hollow Argument: Understory Vegetation and Disturbance Determine Abundance of Hollow-Dependent Mammals in an Australian Tropical Savanna

Native mammals are suffering widespread and ongoing population declines across northern Australia. These declines are likely driven by multiple, interacting factors including altered fire regimes, predation by feral cats, and grazing by feral herbivores. In addition, the loss of tree hollows due to frequent, intense fires may also be contributing to the decline of hollow-dependent mammals. We currently have little understanding of how the availability of tree hollows influences populations of hollow-dependent mammals in northern Australian savannas. Here, we test the hypothesis that the abundance of hollow-dependent mammals is higher in areas with a greater availability of tree hollows. We used camera-trap data from 82 sites across the savannas of Melville Island, the largest island in monsoonal northern Australia. Royle–Nichols abundance-induced heterogeneity models were used to investigate the biophysical correlates of the abundance of three threatened mammals: northern brushtail possum (Trichosurus vulpecula arnhemensis), black-footed tree-rat (Mesembriomys gouldii), and brush-tailed rabbit-rat (Conilurus penicillatus). Our analyses included two variables that reflect the availability of tree hollows: the density of tree hollows, estimated from the ground, and the density of large eucalypt trees (Eucalyptus and Corymbia spp.). We found no evidence that the abundance of the three hollow-dependent mammals is positively associated with the availability of tree hollows on Melville Island. Despite their reliance on hollow-bearing trees for denning, the abundance of these mammals appears to be more strongly associated with other factors, such as the characteristics of the understory (i.e., shrub density), which affords protection from predators (including feral cats) and access to food resources. Future conservation management should aim to maintain a dense, diverse understory by managing fire and feral herbivores to facilitate the persistence of hollow-dependent mammals across northern Australia.

Logging and wildfire limit the distribution of a vulnerable arboreal mammal

Habitat loss and degradation are two of the greatest threats to biodiversity conservation globally. In Australia, the incidence of wildfire and native forest logging contribute substantially to these processes and have been linked to reduced species diversity and abundance. Arboreal species such as the Yellow-bellied Glider (Petaurus australis), are thought to be susceptible to these disturbances due to their reliance on large patches of forest and hollow-bearing trees. We aimed to assess the impact of logging and wildfire on site occupancy by the Yellow-bellied Glider in Mountain Ash forests of the Central Highlands of Victoria. We deployed autonomous recording units for 14 nights at 70 sites and searched recordings for Yellow-bellied Glider vocalisations. Yellow-bellied Gliders were detected at 30% (N = 21) of sites. Site occupancy increased with an increasing proportion of the landscape (within 400 m of the recorder) that had not been logged within the last 100 years or burnt by wildfire within the last 10 years. Habitat disturbance caused by logging and fire therefore limit the site occupancy of Yellow-bellied Gliders and likely impact its conservation status. It is therefore critical that appropriate management of timber resources protects large patches of old-growth forest providing food resources and hollows, and the connectivity between large forest patches.

Effects of the Australian 2019–2020 megafires on a population of endangered broad‐headed snakes Hoplocephalus bungaroides

AbstractThe 2019–2020 Australian megafires were unprecedented in their intensity and extent. These wildfires may have caused high mortality of adult broad‐headed snakes Hoplocephalus bungaroides which shelter inside tree hollows during summer. We evaluated the impacts of two high‐intensity wildfires (2002 Touga Fire and 2020 Morton Fire) on a broad‐headed snake population in Morton National Park, south‐eastern Australia. We analysed a 29‐year mark–recapture data set to estimate survival rates of adults in years with and without wildfires, and with and without human disturbance to rock outcrops. To examine the short‐term effects of fire on occupancy, we analysed presence–absence data collected from 25 sites during 2019 and 2020. Estimates of occupancy were higher for 2020 (0.93 ± 0.09) than for 2019 (0.66 ± 0.14), while detection rates were constant (0.40 ± 0.06). Over the period 1992–2020, the best supported Cormack–Jolly–Seber model was one in which adult survival rates were high and stable (0.81 ± 0.04), but were 23% lower in years when humans disturbed rocks (0.63 ± 0.08). A model in which adult mortality was 20% higher in years with human disturbance and 14% higher in years with wildfires was also supported. Estimates of abundance revealed that the population declined by 34% after the Touga Fire, and 26% after the Morton Fire. Over the 29‐year study, the population has declined by 60%. Our results highlight how mortality events from wildfires need to be evaluated in the context of other threatening processes. For this population, the removal of snakes and associated habitat disturbance poses a more serious threat to population viability than infrequent wildfires.

Occupancy of chainsaw-carved hollows by an Australian arboreal mammal is influenced by cavity attributes and surrounding habitat

The creation of mechanically carved tree cavities to provide supplementary shelter for hollow-dependent wildlife is increasingly popular in conservation management programs. However, there is limited empirical evidence quantifying how the features of their design and broader placement within the landscape influence use by target fauna. In this study, we took a multi-scale approach to investigating the use of chainsaw hollows (CHs) by a target native arboreal marsupial, Krefft’s Glider Petaurus notatus. We hypothesized that Krefft’s Gliders would respond to attributes of: (1) the CH itself; (2) the tree into which it was installed; and (3) the habitat within the reserve in which it was located. We monitored occupancy (over 2.7 years, using a pole camera) of 48 CHs installed in live Eucalyptus trees across multiple reserves in Melbourne, Australia, plus visitation (over ∼3 months, using camera traps) to 40 of the CHs. Mixed models were used to relate the occupancy of CHs by Krefft’s Gliders to field-collected data on various attributes of CHs at all three scales. From a total of 35,219 visitations across the 40 CHs, we recorded 13 native hollow-dependent species (five mammals, eight birds) visiting CHs. During 14 pole-camera inspections, Krefft’s Glider occupied 60% of the 48 CHs at least once, with 4% used as maternal dens during breeding. Four exotic species were also recorded visiting and/or occupying CHs. Orientation and volume both significantly affected occupancy: Krefft’s Gliders were more likely to occupy larger CHs, and those facing north and west (possibly related to microclimate). Tree-scale attributes did not affect occupancy, but Krefft’s Gliders were more likely to occupy CHs further from reserve edges, and where local density of Black Wattles (Acacia mearnsii: an important food resource) and low vegetation cover (100–200 cm above ground) was increased. Our findings show that CHs can provide supplementary shelters for Krefft’s Gliders when natural hollows are limited. Practitioners should consider both the physical characteristics of the cavity (e.g. volume) and the surrounding habitat (e.g. availability of foraging resources) to enhance the use of CHs by target species. The ongoing availability of CHs to target species will also be influenced by their use by exotic species, and potential for closure as woundwood grows. In an Australian context, a lack of knowledge about the specific features of natural hollows to which hollow-dependent fauna respond, means it is critical for policy and management to prioritize the retention of mature hollow-bearing trees.

Remembrances of a Honey Bee Biologist.

Thomas Seeley's research has focused on analyzing the collective intelligence and natural lives of honey bees. This account describes how the author encountered honey bees as a boy and became a beekeeper; how he switched his career path from medicine to biology to study the behavior and social life of honey bees; and how he focuses on understanding how a honey bee colony functions when it lives in the wild, rather than in a beekeeper's hive. He has shown how a honey bee colony works as a single decision-making unit to adaptively allocate its foragers among flower patches and to choose its nesting site in a hollow tree. These findings buttress the view that, in some social insect species, the colony is a group-level vehicle of gene survival. Beyond his research, he has written three books to synthesize these findings for biologists and share these discoveries with beekeepers.

Influence of European Beech (Fagales: Fagaceae) Rot Hole Habitat Characteristics on Invertebrate Community Structure and Diversity.

Hollows of veteran trees (i.e., rot holes) provide habitat for many rare and threatened saproxylic invertebrates. Rot holes are highly heterogeneous, particularly in terms of substrate and microclimate conditions. There is, however, a dearth of information regarding the differences in biological communities inhabiting rot holes with different environmental conditions. Invertebrates were sampled from European beech (Fagus sylvatica) rot holes in Windsor, Savernake, and Epping Forests (United Kingdom). For each rot hole, physical and environmental conditions were measured, including tree diameter, rot hole dimensions, rot hole height, substrate density, water content, and water potential. These parameters were used to assess the influence of environmental conditions and habitat characteristics on invertebrate communities. Rot hole invertebrate communities were extremely diverse, containing both woodland generalist and saproxylic specialist taxa. Large variation in community structure was observed between rot holes and across woodlands; all sites supported threatened and endangered taxa. Environmental conditions in rot holes were highly variable within and between woodland sites, and communities were predominantly structured by these environmental conditions. In particular, turnover between invertebrate communities was linked to high β-diversity. The linked heterogeneity of environmental conditions and invertebrate communities in rot holes suggests that management of deadwood habitats in woodlands should strive to generate environmental heterogeneity to promote invertebrate diversity. Additional research is required to define how management and conservation activities can further promote enhanced biodiversity across rot holes.

Unexpected overlapping use of tree hollows by birds, reptiles and declining mammals in an Australian tropical savanna

Unexpected overlapping use of tree hollows by birds, reptiles and declining mammals in an Australian tropical savanna

Landscape‐scale distribution of nest predators and its relationship with regent honeyeater nest success

AbstractOvercoming low breeding success is a major challenge for reversing population decline in threatened species. High nest predation rates are a common cause of low productivity in birds, but implementing evidence‐based management actions to reduce nest predation is challenging. Targeted breeding management requires a good understanding of where threatened species breed, the identity and distribution of nest predator species and the impact of these nest predators on breeding success. For rare and nomadic species, this information is hard to come by. The breeding success of regent honeyeater Anthochaera phrygia has declined over recent decades, and nest predation is the primary cause of breeding failure. We conducted point‐count surveys of nine avian and three mammalian nest predator species across 80% of the regent honeyeater’s contemporary breeding area in south eastern Australia. We used occupancy models to determine the presence of predator species at survey sites and spatial models to predict predator abundance and species richness across the breeding area. We incorporated predator predictions for regent honeyeater nest locations into nest fate and daily nest survival models. Predator abundance was correlated positively with tree hollow abundance and proximity to water and negatively with shrub cover. Regent honeyeater nest success showed no significant relationship with predator abundance or predator species richness. Given how abundant and widespread avian and mammalian nest predators were throughout the breeding area, improving regent honeyeater nest success is likely to require implementation of simultaneous nest protection measures to avoid potential compensatory nest predation. Further research is required to better understand the relationship between nest predator abundance and regent honeyeater nest success.

Target Species and Other Residents—An Experiment with Nest Boxes for Red Squirrels in Central Poland

The red squirrel typically nests in dreys and tree hollows, but also (when given an opportunity) in large nest boxes. We assessed the occupancy rate of nest boxes by red squirrel and non-target species (120 boxes in the continuous forest, habitat mosaic and urban park, checked annually for eight years). Habitat type explained the variability in the occupancy of nest boxes by different species/taxa. Red squirrels used nest boxes in all habitats but occupancy rates were highest in the urban park (>50% of the boxes at maximum) and lowest in the forest. This could be explained by high population density, competition for shelters and willingness to explore alternative sheltering opportunities by urban squirrels. The yellow-necked mouse inhabited nest boxes infrequently and mostly in habitat mosaic. Tits mostly occurred in the forest and least often in the park, which suggests limited availability of natural cavities in managed forest. Nest box occupancy by starlings increased with an anthropopression level, which reflects high densities of urban and rural populations of the species. Hymenoptera (mainly wasps) were present only in rural areas, which may be due to their persecution by humans or use of anti-mosquito pesticides in urban parks. Additionally, 24 insect species were found to inhabit squirrel dreys.

Ornithologists' Help to Spiders: Factors Influencing Spiders Overwintering in Bird Nesting Boxes.

Simple SummaryNesting boxes are often used to support hole-nesting birds, but are also attractive as a shelter for many invertebrates, especially for overwintering. We studied assemblages of spiders overwintering in nesting boxes in a lowland forest and the factors influencing their abundance and activity during winter. The results show that the majority of arboreal spider species use nesting boxes to overwinter and that their abundance increases with the presence of nest material. Some spider species are also active at low temperatures in winter and can resettle emptied nesting boxes during the winter season. By hanging nesting boxes on trees, ornithologists support not only hollow nesting birds, but also overwintering spiders.Spiders are common inhabitants of tree hollows, as well as bird nesting boxes, especially in autumn and winter. Some species of spiders use bird nesting boxes for overwintering. We investigated spider assemblages in nesting boxes and how temperature influences the abundance of overwintering spiders in nesting boxes in lowland forest in the Czech Republic. The study was conducted in the European winters of 2015–2017. In total, 3511 spider specimens belonging to 16 identified species were collected from nesting boxes over three years in late autumn and winter. Almost all species were arboreal specialists. The dominant species were Clubiona pallidula, Anyphaena accentuata, Platnickina tincta, and Steatoda bipunctata. Although the tree species had no effect on the abundance of overwintering spiders, the presence of nest material affected the abundance of spiders in the nesting boxes (preferred by C. pallidula and P. tincta). In general, spiders resettled nesting boxes during winter only sporadically, however A. accentuata reoccupied boxes continuously, and its activity was positively correlated with the outside temperature. Nesting boxes support insect-eaters all year around—birds during spring and summer and spiders during autumn and winter.

Stigmaeus exilis, a new fusiform species of Stigmaeus Koch (Acariformes: Stigmaeidae) from Sansa, Turkey

A new species of the genus Stigmaeus (Stigmaeidae), S. exilis sp. nov., is described based on the adult females collected from soil in a dead walnut tree hollow from Sansa Gorge, Turkey. Illustrations and phase contrast microscopic images of the new species are also given here. This species can be distinguished by having thin and slender body, anterior margin of prodorsum microtuberculate, posterior margin of propodosomal shield indenting forward by striae, apodemal marking present, eyes and postocular bodies absent, median zonal shield entire, suranal shield entire and bearing three pairs of setae, and femur II with four setae.

Spout hollow nest boxes provide a drier and less stable microclimate than natural hollows

AbstractArtificial tree hollows (e.g., nest‐boxes) are commonly deployed to mitigate the loss of mature trees within human‐disturbed landscapes. Their effectiveness as a habitat resource, and thus conservation management tool, is strongly influenced by the suitability of internal microclimate conditions. In south‐eastern Australia, spout hollows are a nesting resource used by a diverse community of vertebrate species. We tested the suitability of a novel nest box design (spout boxes) that mimicked the physical characteristics of spout hollows. We monitored the occupancy (n = 193) and internal microclimate (n = 131) of natural hollows and spout boxes within a woodland where natural tree hollows were once abundant. Both natural hollows and spout boxes were occupied and used for breeding by birds and mammals. Natural hollows had consistently higher humidity, and thermal maxima and minima were buffered, when compared with spout boxes. These differences were largely explained by wall thickness. Spout boxes displayed even more extreme temperature variation and lower humidity when not shaded. While more extreme microclimate conditions did not prevent usage, tolerable thresholds for hollow‐dependent species may soon be exceeded under current climate change projections. Managers need to carefully consider nest box design and positioning to ensure the suitability of these supplementary resources for conservation purposes.

Relict of primeval forests in an intensively farmed landscape: what affects the survival of the hermit beetle (Osmoderma barnabita) (Coleoptera: Scarabaeidae) in pollard willows?

The hermit beetle (Osmoderma barnabita) is an internationally protected specialist of tree hollows considered a relic of primeval forests. The backbone of its distribution in eastern Czechia, however, is a system of pollard willows in intensively farmed lowlands. Pollarding, a traditional agroforestry practice inducing formation of tree hollows, was partly abandoned in the area during the twentieth century. To assess the state of the system of pollard willows and to investigate parameters of pollards and their stands which affect the beetle’s presence. We inventoried pollard trees and beetle distribution across ca 2000 km2 of lowlands along the Dyje, Morava and Odra rivers and their tributaries in eastern Czechia and westernmost Slovakia. We found 10 441 pollard willows in 324 stands: most stands contained trees of large diameters and poor health; young trees were rare. Probability of O. barnabita presence increased with number of trees in stand, decreased with distance to the nearest occupied stand, and was further affected by the health state of trees. Our results show that high hollow density in pollards allows for the existence of specialized, dispersal limited forest organisms in virtually deforested landscapes. They suggest stands should contain at least 200–300 pollard willows to sustain the beetle population. Although the beetle is still widespread over the study area, the existing populations are isolated and subjected to extinction debt. Large-scale restoration of pollarding practice, planting and pollarding of numerous young trees, and increasing connectivity is vital to facilitate the survival of this hermit beetle population but also support other hollow associated organisms.

Veteran trees have divergent effects on beetle diversity and wood decomposition.

Veteran hollow trees are keystone structures in ecosystems and provide important habitat for a diverse set of organisms, many of which are involved in the process of decomposition. Since veteran trees are 'islands' of high biodiversity, they provide a unique system in which to study the relationship between biodiversity and decomposition of wood. We tested this relationship with a balanced experiential design, where we quantified the taxonomic and functional diversity of beetles directly involved in the process of decomposing wood, and measured the decomposition of experimentally added bundles of small diameter wood around 20 veteran trees and 20 nearby young trees in southern Norway. We found that the diversity (both taxonomic and functional) of wood-decomposing beetles was significantly higher around the veteran trees, and beetle communities around veteran trees consisted of species with a greater preference for larger diameter wood. We extracted few beetles from the experimentally added wood bundles, regardless of the tree type that they were placed near, but decomposition rates were significantly lower around veteran trees. We speculate that slower decomposition rates around veteran trees could have been a result of a greater diversity of competing fungi, which has been found to decrease decay rates. Veteran trees provide an ecological legacy within anthropogenic landscapes, enhance biodiversity and influence wood decomposition. Actions to protect veteran trees are urgently needed in order to save these valuable organisms and their associated biodiversity.

Physical and biotic factors driving the diversity of spider assemblages in tree hollows of MediterraneanQuercusforests

AbstractTree hollows are keystone structures that promote forest biodiversity. This study analyses the spatio‐temporal diversity of spiders in these microhabitats.Forty‐eight emergence traps were installed in tree cavities ofQuercus pyrenaicaforests in the Iberian Peninsula. Traps were collected monthly during a complete year.Generalised linear models and canonical correspondence analysis were used to evaluate the effect of two physical (hollow volume and height above ground) and three biotic variables (beetle richness and abundance and isopod abundance) on the spider diversity and composition of the entire assemblage and of the foraging guilds. Moreover, we examined the temporal segregation of the species using beta diversity metrics.Tree hollows hosted a rich spider assemblage (87 species) including 10 endemic and one vulnerable species. We registered four tree‐bark obligate and one hollow‐dependent species.Hollow height and beetle richness were the variables that better explained richness, while all variables analysed influenced spider abundances. However, contrary to the observed effect for other taxa (i.e. beetles), bigger cavities did not host more spider species.Temporal beta diversity throughout the year was very high and primarily explained by species turnover suggesting a temporal species segregation.We conclude that tree hollows constitute a key habitat for forest spiders and that special attention must be paid to the preservation of this microhabitat in forest ecosystems.

Nest Architecture and Nesting Site Preference of Tetragonula iridipennis Smith in North-Western Plains of India

Abstract Stingless bees constitute an important group of bee diversity providing a vital ecological service i.e. pollination. From India, nine species of stingless bees have been reported out of which Tetragonula iridipennis Smith is widely distributed. Studies revealed that the majority (86.67%) of these bees’ nests were present in brick walls and the remaining (13.33%) were in cavities of hollow tree trunks. Most of the colonies nested at a height of 3.50 m from the ground. Most of the nests (56.67%) had entrances oriented to the east. No distinct entrance tube was observed in 26.92 per cent of the colonies. Overall average entrance tube dimensions (longer side diameter × shorter side diameter × tube length) were 11.62 × 11.73 × 14.04 mm. In all the nests, brood area was surrounded by pollen pots and a few honey pots. The mean dimensions (depth × width) of brood cells, honey pots and pollen pots were 2.79 × 2.62 mm, 5.33 × 4.52 mm and 6.93 × 5.19 mm, respectively. All of the sixty colonies whether in brick/stone walls or in hollow tree trunks overwintered successfully. Our results can be useful for developing conservation measures for T. iridipennis and its augmentation for crop pollination.