Occurrence Of Hollows Research Articles

How an unprecedented wildfire shaped tree hollow occurrence and abundance—implications for arboreal fauna

BackgroundTree hollows are an important habitat resource used by arboreal fauna for nesting and denning. Hollows form when trees mature and are exposed to decay and physical damage. In the absence of excavating fauna, hollow formation can take up to 200 years in Australian temperate Eucalyptus forests, making tree hollows a critical but slow forming habitat feature. The increasing frequency and severity of wildfires due to climate change has led to increased concern about the landscape-scale loss of nesting space for arboreal fauna, including endangered species such as the folivorous southern greater glider (Petauroides volans). To understand patterns of nesting resource availability, we assessed drivers of hollow occurrence in southeastern Australian mixed-species Eucalyptus forests and quantified the effects of an unprecedented large-scale wildfire, the 2019/2020 Black Summer bushfires, on hollow occurrence and abundance.ResultsTree size and shape, as well as site productivity and topography, were important predictors for hollow occurrence both before and after the fires. The occurrence of the southern greater glider was strongly dependent on high proportions of hollow-bearing trees. While high fire severities had a negative impact on southern greater glider occurrence, the number of hollow-dependent arboreal species was not affected. While the wildfires significantly reduced hollow abundance, we did not find significant effects on hollow occurrence. Fires altered the relationship between tree size and hollow occurrence expressed as a change in the probability of hollow occurrence, with a higher likelihood at smaller tree sizes after the fires.ConclusionsOur findings suggest that post-fire nesting space may be reduced at the tree-scale, while at the stand-scale, hollow-bearing trees persist as biological legacies. These persisting trees can support the recovery of hollow-dependent arboreal fauna, such as the endangered southern greater glider by providing denning and nesting space. Hollow-bearing trees that survived the fires have the potential to form new hollows faster compared to undisturbed mature trees.

Evaluation of the Occurrence of Hollows in Trees in Managed Forest in the Eastern Brazilian Amazon

Abstract This study proposed to generate information related to the abundance and the structural characteristics of hollows at the tree and species levels in an area of managed forest in the Amazon. A database was used that was composed of 29,899 trees from thirty-eight species that were inventoried, harvested, and measured in ten annual production units that were managed between 2008 and 2017 in the Tapajós National Forest, state of Pará, Brazil. The data was tabulated and reported in absolute and percentage values. Contingency tables were created to identify patterns of occurrence of hollows in commercial stems. A total of 24% of the harvested trees had hollows in at least one section of the commercial stem, and species had different propensities for hollow occurrence. It was found that the larger the tree diameter the greater the possibility for occurrence of hollows, and these hollows were proportionally larger. Most of the hollows in harvested trees were small. Patterns of occurrence in commercial stem hollows were observed, and these were dependent on tree species and size. Most trees had hollows distributed along the entire stem or only at the base of the stem. Study Implications: The presence of hollows in managed native forest trees in the Amazon depends on the species, and trees with larger diameters have a greater probability of having hollows. Furthermore, trees with larger diameters have hollows that have larger diameters, lengths, and volumes. Commercial trees have different patterns of hollows along the stem, and these patterns are dependent on tree species and size. The results of this study will contribute to management of trees with hollows in the Amazon Forest as well as to serve as a base for future studies related to this theme.

Hollow occurrence and tree spacing in Eucalyptus tereticornis

ABSTRACT A ground-based survey of 400 Eucalyptus tereticornis trees examined the incidence of visible hollows suitable for hollow-dependent fauna. As well as the expected relationship with stem diameter, data analysis revealed a relationship between the incidence of hollows (with entrance diameter ≥10 cm) and spacing between neighbouring trees. When the distance between trees is equivalent to 0.3 times total tree height, a tree is more likely to suffer branch breakage and subsequent hollow formation. This influence of spacing, likely due to branches clashing during storms, appears to contribute a twofold difference in the incidence of hollows. This offers opportunities for the management of regrowth forests and restoration plantings, especially where one of the objectives is to encourage the formation of hollows.

Pruning promotes the formation of an insufficient number of cavities for hollow-dependent birds in Iberian Holm-oak dehesas

Holm oak Quercus ilex dehesas cover large areas in the Mediterranean region, and constitute a paradigm of well preserved low-intensity agro-ecosystem supporting a high number of bird species. The canopy of holm oak trees is periodically pruned, but the role of pruning in the process of tree-hollow formation remains unstudied. Here I investigate the occurrence of tree-hollows in relation to tree characteristics (vitality, trunk diameter and fork height), infection by drilling Coleoptera and pruning intensity, and assess likely origin, use and suitability of hollows for non-excavator birds in dehesas of Spain. 26.3% of all sampled oaks had at least one hollow, and the number of pruning cuts predicted the occurrence of hollows. 97.0% of hollows were situated on pruning cuts suggesting that pruning may have favored decay cavity formation in Iberian dehesas. 43.6% of hollows were estimated suitable to hold the nest of a small-sized cavity-nesting bird, but only 11.9% might have potentially held the nest of a medium-sized cavity-nesting bird, rendering estimated 3.9 and 1.1 potential cavities/ha, respectively. The low densities of suitable hollows would point toward a lack of nest-sites which might limit cavity-nesting birds in holm oak dehesas. Future conservation actions targeting in protection of old and dead holm oak trees instead of selective removal, together with land-scale measurements of enhancement of riverine forests stands may enrich and ensure the persistence of cavity-using species in Iberian dehesas.

Analysis of MESSENGER high‐resolution images of Mercury's hollows and implications for hollow formation

AbstractHigh‐resolution images from MESSENGER provide morphological information on the nature and origin of Mercury's hollows, small depressions that likely formed when a volatile constituent was lost from the surface. Because graphite may be a component of the low‐reflectance material that hosts hollows, we suggest that loss of carbon by ion sputtering or conversion to methane by proton irradiation could contribute to hollows formation. Measurements of widespread hollows in 565 images with pixel scales <20 m indicate that the average depth of hollows is 24 ± 16 m. We propose that hollows cease to increase in depth when a volatile‐depleted lag deposit becomes sufficiently thick to protect the underlying surface. The difficulty of developing a lag on steep topography may account for the common occurrence of hollows on crater central peaks and walls. Disruption of the lag, e.g., by secondary cratering, could restart growth of hollows in a location that had been dormant. Images at extremely high resolution (~3 m/pixel) show that the edges of hollows are straight, as expected if the margins formed by scarp retreat. These highest‐resolution images reveal no superposed impact craters, implying that hollows are very young. The width of hollows within rayed crater Balanchine suggests that the maximum time for lateral growth by 1 cm is ~10,000 yr. A process other than entrainment of dust by gases evolved in a steady‐state sublimation‐like process is likely required to explain the high‐reflectance haloes that surround many hollows.

Remote sensing can locate and assess the changing abundance of hollow-bearing trees for wildlife in Australian native forests

Context Hollow-bearing trees are an important breeding and shelter resource for wildlife in Australian native forests and hollow availability can influence species abundance and diversity in forest ecosystems. A persistent problem for forest managers is the ability to locate and survey hollow-bearing trees with a high level of accuracy at low cost over large areas of forest. Aims The aim of this study was to determine whether remote-sensing techniques could identify key variables useful in classifying the likelihood of a tree to contain hollows suitable for wildlife. Methods The data were high-resolution, multispectral aerial imagery and light detection and ranging (Lidar). A ground-based survey of 194 trees, 96 Eucalyptus crebra and 98 E. chloroclada and E. blakelyi, were used to train and validate tree-senescence classification models. Key results We found that trees in the youngest stage of tree senescence, which had a very low probability of hollow occurrence, could be distinguished using multispectral aerial imagery from trees in the later stages of tree senescence, which had a high probability of hollow occurrence. Independently, the canopy-height model used to estimate crown foliage density demonstrated the potential of Lidar-derived structural parameters as predictors of senescence and the hollow-bearing status of individual trees. Conclusions This study demonstrated a ‘proof of concept’ that remotely sensed tree parameters are suitable predictor variables for the hollow-bearing status of an individual tree. Implications Distinguishing early stage senescence trees from later-stage senescence trees using remote sensing offers potential as an efficient, repeatable and cost-effective way to map the distribution and abundance of hollow-bearing trees across the landscape. Further development is required to automate this process across the landscape, particularly the delineation of tree crowns. Further improvements may be obtained using a combination of these remote-sensing techniques. This information has important applications in commercial forest inventory and in biodiversity monitoring programs.

Hollows on Mercury: Materials and mechanisms involved in their formation

Recent images of the surface of Mercury have revealed an unusual and intriguing landform: sub-kilometre scale, shallow, flat-floored, steep-sided rimless depressions typically surrounded by bright deposits and generally occurring in impact craters. These ‘hollows’ appear to form by the loss of a moderately-volatile substance from the planet’s surface and their fresh morphology and lack of superposed craters suggest that this process has continued until relatively recently (and may be on-going). Hypotheses to explain the volatile-loss have included sublimation and space weathering, and it has been suggested that hollow-forming volatiles are endogenic and are exposed at the surface during impact cratering. However, detailed verification of these hypotheses has hitherto been lacking.In this study, we have conducted a comprehensive survey of all MESSENGER images obtained up to the end of its fourth solar day in orbit in order to identify hollowed areas. We have studied how their location relates to both exogenic processes (insolation, impact cratering, and solar wind) and endogenic processes (explosive volcanism and flood lavas) on local and regional scales. We find that there is a weak correlation between hollow formation and insolation intensity, suggesting formation may occur by an insolation-related process such as sublimation. The vast majority of hollow formation is in localised or regional low-reflectance material within impact craters, suggesting that this low-reflectance material is a volatile-bearing unit present below the surface that becomes exposed as a result of impacts. In many cases hollow occurrence is consistent with formation in volatile-bearing material exhumed and exposed during crater formation, while in other cases volatiles may have accessed the surface later through re-exposure and possibly in association with explosive volcanism. Hollows occur at the surface of thick flood lavas only where a lower-reflectance substrate has been exhumed from beneath them, indicating that this form of flood volcanism on Mercury lacks significant concentrations of hollow-forming volatiles.

Hollow occurrence and abundance varies with tree characteristics and among species in temperate woodlandEucalyptus

Tree hollows are a critical but diminishing resource for a wide range of fauna around the world. Conservation of these fauna depends on sustainable management of tree species that produce the hollows on which they depend. This study addressed the need for empirical data about intraspecific and interspecific variation in hollow occurrence and abundance in woodland trees in Australia. We measured and performed hollow surveys on 1817 trees of seven species of woodland Eucalyptus in central-western New South Wales, Australia. Trees were surveyed at 51 one-hectare sites and about 30% of trees surveyed had multiple stems. Generalized linear mixed models that accounted for nestedness of stems within trees and trees within sites detected a significant amount of variation in hollow occurrence and abundance. Models for individual tree stems of live trees showed hollow probability and abundance increased with diameter at breast height (DBH) and with increasing senescence (form). Stems of Eucalyptus microcarpa Maiden had a higher probability of having hollows than similar DBH stems of Eucalyptus camaldulensis Dehnh., Eucalyptus melliodora A.Cunn. ex Schauer or Eucalyptus populnea ssp. bimbil L.A.S.Johnson & K.D.Hill. Dead stems in live trees were more likely to have hollows than live stems of similar DBH. Each stem in a multi-stemmed tree had a lower probability of hollow occurrence and lower abundance of hollows than single-stemmed trees of similar DBH. For stems of dead trees, hollow occurrence and abundance increased with DBH and differed depending on stage of senescence. A comparison of our data with other studies indicates regional variation of hollow abundances within tree species.

Is Active Management the Key to the Conservation of Saproxylic Biodiversity? Pollarding Promotes the Formation of Tree Hollows

Trees with hollows are key features sustaining biodiversity in wooded landscapes. They host rich assemblages of often highly specialised organisms. Hollow trees, however, have become rare and localised in Europe. Many of the associated biota is thus declining or endangered. The challenge of its conservation, therefore, is to safeguard the presence of hollow trees in sufficient numbers. Populations of numerous species associated with tree hollows and dead wood are often found in habitats that were formed by formerly common traditional silvicultural practices such as coppicing, pollarding or pasture. Although it has been occasionally mentioned that such practices increase the formation of hollows and the availability of often sun-exposed dead wood, their effect has never been quantified. Our study examined the hollow incidence in pollard and non-pollard (unmanaged) willows and the effect of pollarding on incremental growth rate by tree ring analysis. The probability of hollow occurrence was substantially higher in pollard than in non-pollard trees. Young pollards, especially, form hollows much more often than non-pollards; for instance, in trees of 50 cm DBH, the probability of hollow ocurrence was ∼0.75 in pollards, but only ∼0.3 in non-pollards. No difference in growth rate was found. Pollarding thus leads to the rapid formation of tree hollows, a habitat usually associated with old trees. It is therefore potentially a very important tool in the restoration of saproxylic habitats and conservation of hollow-dependent fauna. If applied along e.g. roads and watercourses, pollarding could also be used to increase landscape connectivity for saproxylic organisms. In reserves where pollarding was formerly practiced, its restoration would be necessary to prevent loss of saproxylic biodiversity. Our results point to the importance of active management measures for maintaining availability, and spatial and temporal continuity of deadwood microhabitats.

Time-since-fire and inter-fire interval influence hollow availability for fauna in a fire-prone system

Tree hollows are a critical, yet potentially limiting habitat resource for many animal species. Fire influences hollow availability, and the associated indirect effects on fauna can threaten the persistence of hollow-dependent species in fire-prone systems. We investigated the influence of two temporal aspects of fire regimes (time-since-fire, inter-fire interval) on hollow occurrence in a semi-arid, fire-prone region in south-eastern Australia. Empirical data on the characteristics of hollow-bearing eucalypt stems and fire-history attributes were compiled for 581 study sites. Mixed models were used to examine the relative influence of time-since-fire and inter-fire interval on hollow occurrence. Time-since-fire and inter-fire interval both affected the probability of hollow occurrence, but in different ways. Time-since-fire influenced the occurrence of hollows in live and dead stems. As time-since-fire increased, so too did the probability of live and dead stems containing hollows. Live stems did not provide hollows before 40years post-fire, while the probability of dead stems containing hollows peaked at 50–60years. Inter-fire interval influenced the availability of hollows in dead stems. Longer inter-fire intervals resulted in an increased density of dead hollow-bearing stems. In this region, hollow-dependent fauna will benefit from increased fire-free periods, both in terms of individual fire events and the intervals between repeated fires. These results highlight the complex way in which fire affects the availability of faunal habitat resources, and the extended time periods over which such influences operate. Understanding the effects of fire regimes on slow-developing habitat resources over long time-frames is imperative for sound ecological fire management.

Effects of forest management on structural features important for biodiversity in mixed-age hardwood forests in Australia's subtropics

The contiguous forest landscapes of the Brigalow Belt Bioregion in Australia's subtropics, known as ‘the western hardwood forests’ are thought to be important for biodiversity conservation through the provision of continuous habitat features in an otherwise extensively cleared region. However, these forests have a long management history, as they are also important for timber production and stock grazing. Knowledge of the effect of these and other disturbances on the distribution and abundance of key structural or habitat features are limited and forest management activities thus continue uninformed. To address this knowledge gap, we investigated the effects of prescribed and non-prescribed disturbances on the distribution and abundance of six key habitat features (live and dead trees with hollows, large mature live trees, shrub abundance, downed coarse woody debris and litter cover) at 120 sites located in spotted gum Corymbia citriodora dominant forests within the western hardwoods.Logged stands had a lower mean abundance of live trees with hollows (2.5±0.6ha−1) than unlogged stands (6.2±0.8ha−1). This is below current timber harvesting prescriptions which outline the retention of 6 live trees with hollows ha−1. Since only 13.5% of western hardwood forest under state forest tenure remains unlogged, live trees with hollows are currently limited throughout the majority of the western hardwood forests. Recent wildfire in the region also meant reduced numbers of live trees with hollows. Logging affected the abundance of large, live trees, with highest densities occurring in the less intensively logged stands. Given the close relationship between increased tree diameter and hollow occurrence across a number of tree species, the small area of unlogged forest remaining in the region suggests that the pool of trees that may eventually form hollows may also be limited. The density of dead trees with hollows was strongly reduced by fire; by both low-frequency but high-intensity fires (wildfire) and by high-frequency but low-intensity burns associated with stock grazing management. Increased grazing was also associated with decreased abundance of shrubs and less downed coarse woody debris. Other than decreasing with increased logging intensity, litter cover did not appear to be influenced by other disturbance regimes, at least not by the long term correlational approach of our investigation. Overall, our results suggest that although forest management in the western hardwood forests could currently be described as low-intensity, it is extensive and impacts are discernible and cumulative. We highlight the need for a landscape approach to be taken in managing the various land uses for both production and biodiversity within the contiguous western hardwood forests.

Does hollow occurrence vary with forest type? A case study in wet and dry Eucalyptus obliqua forest

The distribution of hollow-using fauna is frequently related to forest type. If hollow occurrence varies with forest type, the use of generic prescriptions for managing the hollow resource may be inappropriate. This study examined the relationship between the occurrence and abundance of tree hollows and site and tree attributes in wet and dry Eucalyptus obliqua forest in Tasmania, Australia. A total of 388 trees at 39 sites were examined before and after being felled. Evidence of invertebrate damage contributing to hollow formation was more common in wet forest than either dry or damp forest. Evidence of hollow formation by fire and fungi was more prevalent in drier than wetter forest. There was no difference among forest types in the proportion of hollows showing evidence of limb breakage. Examination of the site and tree factors related to hollow presence and abundance was done using Classification Trees, Random Forests and generalised linear models. The variables found to be most practical for predicting hollow occurrence were the number of hollows observed before the tree was felled, tree diameter and the amount of dead wood in the canopy. Tree age, an important variable to consider when planning harvest rotations, was strongly associated with hollow presence but had less bearing on hollow abundance. The size of hollows that were found in a tree was related to greater senescence. Although significantly more hollows of all sizes were found in wet forest than either dry or damp forest, the age at which trees began to produce hollows was similar among the different forest types. The models predicted that trees needed to be at least 100 years old before they are likely to contain a hollow and that medium-sized hollows rarely occur in trees younger than 140 years old. Trees grow slightly more slowly in dry forest than wet which means that smaller diameter trees were found to contain hollows in dry forest than wet and that even the largest trees may not contain large hollows in wet forest. Despite this, no differentiation between the forest types was found in the predictive models. It is recommended that trees retained for faunal habitat be at least 100 cm in diameter.

Errors associated with two methods of assessing tree hollow occurrence and abundance in Eucalyptus obliqua forest, Tasmania

Tree hollows provide important habitat for fauna, but difficulties associated with detecting tree hollows can impede the formulation of appropriate management action. This paper examines the accuracy and errors associated with two methods commonly used to assess the presence/absence or abundance of tree hollows; ‘ground-based’ surveys and ‘tree-felling’ surveys. Three hundred and forty-six trees in Tasmania's State Forest were surveyed for hollows both before and after being felled. In order to assess the type and frequency of errors associated with each method, the fate of every potential hollow identified during the ground-based survey was determined after the tree had been felled. Three main types of error were identified: hollows misidentified during the ground-based survey; hollows not detected during ground-based surveys; and hollows not found during tree-felling surveys. Bayesian models were used to examine the association between site, tree and hollow variables and the occurrence of these errors. The likelihood that a tree would have a misidentified hollow increased with the number of potential hollows that were seen during the ground-based survey. Smaller hollows were more likely to be misidentified during ground-based surveys than larger hollows, particularly in trees that have little dead wood. For both of the survey methods the rate at which hollows were not detected was found to increase with tree size and hollow abundance. Hollows located in the branches were more likely to remain undetected during both survey methods, while small hollows, hollows high off the ground and those facing upwards were more likely to be missed during ground-based surveys. Although ground-based surveys provide inaccurate hollow counts, they are useful for assessing relative rather than true hollow abundance, which is valuable in the selection of habitat trees for retention in production forests. Tree-felling surveys provide a more accurate measure of actual hollow counts, particularly when a correction is made for the proportion of tree that cannot be surveyed. Tree-felling surveys are therefore useful for doing research such as developing models predicting hollow abundance.

The occurrence of hollows in eucalypts and Ironwood Erythrophleum chlorostachys in the Gulf region of the Northern Territory and its implications for timber harvesting

The occurrence of hollows in eucalypts and Ironwood Erythrophleum chlorostachys was examined in different habitats in Limmen National Park in the Gulf Falls and Uplands bioregion of the Northern Territory. For each tree sampled, the diameter of the trunk at breast height (DBH) and numbers of hollows in three size categories were recorded. It was found that the riparian zone had the highest density of hollows per unit land area, followed by the rocky hill zone, and then the sandy flat zone. The relationship between hollows per tree and DSH was compared for four species: Eucalyptus camaldulensis (the dominant riparian tree), Eucalyptus leucophloia (the dominant tree on rocky hills), Eucalyptus miniata (the dominant tree on sandy flats), and Ironwood. E. miniata and Ironwood were found to have fewer hollows for a given DBH. It is suggested that the low incidence of hollows in Ironwood is a result of the species' know resistance to termites. The low number of hollows per Ironwood tree, the scarcity of Ironwood in the study area, and the small size of Ironwood trees relative to most of the eucalypts resulted in very few Ironwood hollows overall. This considered, harvesting of Ironwood (a preferred timber species) in this area is unlikely to have an impact on the availability of hollows to fauna. Further research is required to clarify discrepancies with other studies, and assess whether these results apply more broadly, for instance to wetter areas with larger Ironwood trees.

Overcoming bias in ground-based surveys of hollow-bearing trees using double-sampling

Hollow-bearing trees are an important ecological resource for many forest-dwelling species throughout the world. Assessments of the abundance and characteristics of tree hollows usually involve ground-based surveys. These are likely to be biased because of the distance over which the assessments are made. We demonstrate the use of double-sampling theory to correct this bias in order to provide the most efficient estimate of hollow abundance. A sample of 40 Eucalyptus leucoxylon (yellow gum) trees located at Yarra Bend Park, Melbourne, were climbed and assessed for hollows through close-up inspection. The results were compared to ground-based surveys conducted by individuals with varying levels of experience in assessing tree hollows. Strong correlations ( r=0.57−0.83) were found between climbing and ground surveys and amongst ground surveys. Those ground surveyors who took more time with their surveys generally identified a higher proportion of hollows (Spearman’s r 2=0.72), although all ground surveys underestimated hollow frequency. The proportion of hollows identified by each observer (range 0.09–0.44) remained approximately constant across all tree diameter size classes, indicating systematic underestimation of hollow frequency. The relative efficiency of ground surveys in comparison to climbing surveys is dependent on the time taken for each survey method and the strength of the relationship between them. Although climbing surveys provided the standard for hollow detection, in all cases, the quicker ground surveys were found to increase the efficiency of hollow assessments. Therefore, it was determined that efficient surveys of hollow occurrence could be undertaken by inexperienced surveyors with periodic climbing surveys to measure and correct for bias.

Hollows in jarrah ( Eucalyptus marginata) and marri ( Corymbiacalophylla) trees : II. Selecting trees to retain for hollow dependent fauna

From a sample of 665 hollows found in 154 jarrah ( Eucalyptus marginata) and 85 marri ( Corymbia calophylla), we identified 204 hollows in 84 trees that were potentially suited to one or more of 10 species of hollow using birds and mammals. Occurrence of these hollows increased with tree age, tree size, and species (marri bore more usable hollows than jarrah) and increased amounts of dead wood in tree crowns. Hollow occurrence was most likely in trees with moderately senescent crowns with damage to intermediate sized branches, and the largest hollows were more likely to occur in more highly senescent crowns. Evidence of termite invasion at the tree butt was not related to occurrence of hollows. For all but one of the birds and mammals we considered, dead trees were no more likely than live trees to contain hollows. Our study indicates that for the purpose of forest management planning, 130 years can be taken as the typical minimum age for the formation of usable hollows in jarrah and marri. The current minimum prescribed diameter for “habitat trees” (trees retained in logged areas to supplement existing hollows), which corresponds to a mean age of 171 years, is thus a realistic minimum size for these retained trees. We recommend raising the prescribed range of crown senescence for retained habitat trees to increase the probability of providing large hollows suited to large species such as red-tailed black cockatoo, and common brushtail possum, and maternal hollows used by smaller species. Retaining the largest trees with appropriate crown attributes will substantially increase the probability that these trees will bear usable hollows.

Phenetic variability of stipe morphology in relation to season, exposure, and depth in the non-digitate complex of Laminaria Lamour. (Phaeophyta, Laminariales) in Nova Scotia

Abstract In the non-digitate complex of Laminaria in Nova Scotia two reported species, L. agardhii Kjell. and L. longicruris de la Pyl. are difficult to distinguish using the diagnostic criteria of stipe length and stipe hollow occurrence. A study was made of these traits in relation to season, exposure, and depth. A marked seasonal variability in stipe morphology was found. When only mature plants were examined there was found to be a definite tendency for stipe length and hollow occurrence to increase with decreasing exposure. The differentiation of populations in relation to exposure was clinal with overlap in the variation patterns. When stipe morphology was examined in relation to depth, significant differentiation was found only in a population sampled in a sheltered locality. In exposed localities, no such differentiation occurred. In the sheltered site, the variation with depth was attributed to differing developmental class structures at various depths. The major taxonomic finding of the analyses...