Adjacent Crowns Research Articles

Numerical Analysis of Dual Droplet Simultaneous Oblique Impact on a Water Film

The simultaneous oblique impact of multiple droplets on a liquid film is an intricate phenomenon prevalent in diverse natural and industrial processes. However, previous studies have primarily focused on single droplet impact, while an in-depth understanding of the more complex multi-droplet scenario remains lacking. The current study aims to numerically investigate the simultaneous oblique impact of two droplets on a water film using a three-dimensional Volume of Fluid (VOF) model. The effects of the Weber number and the impact angle on the crown behavior are carefully analyzed. The results demonstrate that increasing the Weber number enhances the central uprising sheet height but has minor influences on the upstream crown radius and central sheet radius. In contrast, the increase in the impact angle leads to a decreased upstream crown radius and an increased central sheet radius, while the central sheet height remains relatively unaffected. In addition, the splashing threshold for the dual droplet impact cases is significantly lower than that of the single droplet impact cases due to the interactions between the adjacent crowns. The present results provide novel insights into the underlying physics and useful supports in developing predictive models for the intricate multi-droplet impact phenomenon.

The significance of vertical platform discrepancies and splinting on marginal bone levels for adjacent dental implants.

The aim of this retrospective study was to investigate the influence of vertical platform discrepancies for splinted and non-splinted adjacent implants on radiographic marginal bone loss (RMBL). Data from January 2000 to February 2021 were collected from the electronic charts of 156 patients with 337 implants at the UCSF School of Dentistry. Five different implant restoration categories were evaluated for radiographic evidence of proximal RMBL. Patients with (1) two adjacent single crowns, (2) two adjacent splinted crowns, (3) three-unit bridges supported by two implants, (4) three adjacent single crowns, and (5) three adjacent splinted crowns. Inclusion required baseline radiograph taken at the time of prosthesis delivery or final impression, and follow-up radiographs at least 12 months after restorations have been in function. Measurements assessed included vertical distance between adjacent implant platforms and proximal RMBL around implants. Odds ratios (ORs) and 95% confidence interval (95% CI) of implants with ≥1 mm RMBL between different type of restorations were calculated. In general, prostheses supported by splinted adjacent implants demonstrated a significant association with the presence of ≥1 mm RMBL (OR=2.55, 95% CI=1.17-5.17, p=0.018) when compared to prostheses supported by non-splinted adjacent implants. In addition, prostheses with a vertical platform discrepancy ≥0.5 mm demonstrated a significant association with the presence of ≥1 mm RMBL (OR=4.30, 95% CI=1.85 to 10.01, p=0.007) when compared to prostheses with a vertical platform discrepancy <0.5 mm. When adjacent implants had ≥0.5 mm vertical platform discrepancy, the majority (66.67%) of three splinted adjacent crowns had at least one implant with ≥1 mm RMBL. This was followed by two splinted adjacent crowns (58.97%), three-unit bridge (25.93%), two single adjacent crowns (24.24%), and three single adjacent crowns (18.18%). When adjacent implants had ≥1 mm vertical platform discrepancy, there was an increased percentage of implants with ≥1 mm RMBL. The restorative design associated with the highest percent of implants with bone loss was three splinted adjacent crowns (70%), two splinted adjacent crowns (61.11%), three single adjacent crowns (40%), and three-unit bridge and two single adjacent implants (21.05%). Three splinted adjacent crowns were significantly associated with ≥1 mm RMBL when compared to three-unit bridge (OR 6.56, 95% CI 1.59-27.07). Similarly, two splinted crowns were significantly associated with ≥1 mm RMBL when compared to two single crowns (OR=2.50, 95% CI=1.08-5.79). Two or three adjacent implants placed with a vertical platform discrepancy, when splinted together, are associated with higherincidence of ≥1 mm RMBL than non-splinted restorations.

Wider crown shyness between broad‐leaved tree species than between coniferous tree species in a mixed forest of Castanopsis cuspidata and Chamaecyparis obtusa

AbstractPersistent openings (gaps) between adjacent crowns at similar heights can be found in tall forests. Such openings are called “crown shyness” and are formed through the collision of tree crowns during high winds. While crown shyness has been studied in plantations, much less studied in natural forests despite its importance for light acquisition, competition, and coexistence. In this study, we focused on a mixed forest in Japan, and examined a hypothesis that broad‐leaved tree species, which often have top‐wide crowns, have wider crown shyness than coniferous tree species, which often have narrow cone‐shape crowns, because top‐wide crowns may be more exposed to wind and more likely to collide with their neighbors. Crown shyness, which was measured by drone imagery, was clearly found between individuals of the broad‐leaved tree species Castanopsis cuspidata but not between the coniferous tree species Chamaecyparis obtusa. This interspecific difference may be associated with differences in crown shape; Castanopsis had wider (1.57‐fold) and shallower (0.64‐fold) crowns than Chamaecyparis. Within Castanopsis, the size of crown shyness tended to negatively correlated with stem diameter and positively correlated with stem slenderness. Our study showed that the difference in crown shape among species is one of the key factors in determining the size of crown shyness. Top‐wide crowns of broad‐leaved trees compared to conifers may have a competitive advantage in light interception, but it may come at the cost of crown collisions with adjacent crowns and resulting in persistent openings at the canopy layer.

10-Year Clinical Comparative Study of Ceramic and Composite Veneered Metal Crowns.

The rejection of composite veneerings in fixed partial dentures is primarily caused by the inadequacy of the bonds between the metal framework and the composite veneering. The development of improved veneering composites necessitates an investigation of their clinical suitability compared with ceramic materials. Nineteen patients with at least two suitable, adjacent natural teeth for crowning were treated with 64 high noble alloy crowns. The adjacent crowns were veneered with ceramic (IPS inline) and composite materials (SR Adoro). Seven follow-up examinations were carried out over a period of 10 years. The crowns were investigated for mechanical defects, periodontal parameters, and discolorations. The survival rates of the ceramic veneered metal crowns (CeMCs) and composite veneered metal crowns (CoMCs) at the 10-year follow-up were 87.1 and 87.9%, respectively. The success rates of the crowns after 10 years were 83.9% for CeMCs and 51.5% for CoMCs (log-rank test, p = 0.009). No significant differences between the groups were found in the periodontal parameters (Kruskal-Wallis one-way analysis of variance, ANOVA p ⟩ 0.05). After 10 years, discoloration patterns of the two materials differed significantly (Mann-Whitney-U-test, p = 0.017). Thus, despite the improvements associated with CoMCs, CeMCs remain the gold standard for veneered metal crowns.

FCRN-BASED MULTI-TASK LEARNING FOR AUTOMATIC CITRUS TREE DETECTION FROM UAV IMAGES

Abstract. Citrus producers need to monitor orchards frequently, and would benefit greatly from having automated tools to analyze aerial images acquired by drones over the plantations. However, analysing large aerial data sets to enable producers to take management decisions that would optimize productivity and sustainability over time and space remains challenging. Motivated by the success of deep learning in computer vision, this work proposes a novel approach based on Fully Convolutional Regression Networks and Multi-Task Learning to detect individual full-grown trees, tree seedlings, and tree gaps in citrus orchards for inventory tracking. We show that the proposal can identify eight-year-old orange trees with accuracy between 95–99% in high-density commercial plantations where adjacent crowns overlap. This quality of detection was achieved on RGB orthomosaics with a pixel size of about 9.5 cm and requires the nominal spacing between adjacent trees as a priori information. Our results also highlight that detecting tree seedlings and tree gaps remains a challenge. For these two categories, classification sensitivity (recall) was between 59–100% and 63–94%, respectively.

Scelidosaurus harrisonii from the Early Jurassic of Dorset, England: cranial anatomy

AbstractScelidosaurus harrisonii is an early (Late Sinemurian) armoured ornithischian dinosaur whose remains have, to date, only been recovered from a restricted location on the south coast of Dorset (Charmouth), England. This dinosaur has been known since 1859, but only on the basis of a partial description found in two articles published in the early 1860s by Richard Owen. The original material, discovered in 1858, comprised the majority of the skull and its associated postcranial skeleton, and represents the first ever, more or less complete dinosaur discovered. In addition to the original material, a number of further discoveries have been made at Charmouth; these latter supplement the information that can be gleaned from the original specimen. This article describes the skull of Scelidosaurus.The external surface of individual skull bones in ontogenetically relatively mature individuals displays exostoses, a patina of fibrous or granular-textured bone that anchored an external shielding of keratinous scales. There is a small, edentulous rostral beak, behind which is found a row of five heterodont premaxillary teeth. There is a minimum of 22 maxillary teeth and 27 dentary teeth in jaws of the largest well-preserved individuals known to date. Both dentitions (upper and lower) are bowed medially and are sinuous longitudinally. Maxillary and dentary crowns are tilted lingually on their roots, trapezoidal in outline and have crenellate (coarsely denticulate) margins. Adjacent crowns of teeth have mesiodistally (anteroposteriorly) expanded bases that overlap slightly and are consequently arranged en echelon. The dentitions are flanked by deep cheek pouches. Tooth abrasion is usually discontinuous along the dentition. In one individual nearly all teeth seem to be fully emerged and there is little evidence of abrasion. There is no physical evidence of a predentary, but the presence of this (typically ornithischian) element may be inferred from the structure of the symphyseal region of the dentary. The external narial and antorbital fenestrae are comparatively small, whereas the orbit and temporal fenestrae are large and open. A sclerotic ring was undoubtedly present and supported the eyeball, but it is too poorly preserved to allow it to be reconstructed with accuracy. A prominent supraorbital brow ridge overhangs the orbit. There are three osteoderms: palpebral, middle supraorbital and posterior supraorbital, sutured to the dorsal margin of the orbit. The occiput provides an area for attachment of a pair of curved, keratin-sheathed, osteodermal horns.Epistyloid bones project from the ventrolateral region of the braincase; their distal ends flank the anterolateral region of the neck. Rugose facets on either side of the basioccipital are suggested to have provided attachment sites for the epistyloid bones. Internally, the skull has a deeply vaulted snout and the nasal chambers are roofed by what are here named epivomer bones that appear to have been sutured to the dorsolateral edges of the vomers. Unusually, among dinosaurs generally, an epipterygoid is preserved attached to the dorsolateral surface of the pterygoid; there is no obvious point of articulation for the epipterygoid against the lateral wall of the braincase. A deep pit on the posterior surface of the quadrate of an immature specimen is suggestive of the existence of a remnant of cranial pneumatism. This pit becomes occluded in larger, more mature specimens.

GrabCut algorithm for dental X‐ray images based on full threshold segmentation

Teeth are difficult to be destroyed due to their corrosion resistance, high melting point and hardness. Dental biometrics can therefore provide assistance in human forensic identification, especially to the unknown corpses. One of the key issue in dental based human identification is the segmentation of Dental X-ray images. In this paper, a novel segmentation algorithm has been proposed for this purpose. The proposed algorithm is based on full threshold segmentation. We first obtain the outline image set Iwholen and crown image set Icrownm of the complete target tooth. Morphological open operation is then applied to the difference images of Iwholen and Icrownm. Subsequently, the most complete target tooth image and its corresponding crown image are selected. Getting independent target tooth image Icontour and its crown image Icrown from these two images. Median filtering is applied to the synthetic image of Icontour and Icrown, and the resulted image will be used as the Mask for GrabCut to obtain the target tooth image. Experimental results show our proposed algorithm can effectively overcome the problems of uneven grayscale distribution and adhesion of adjacent crowns in dental X-ray images. It can also achieve a high segmentation accuracy and outperform related methods to be compared.

Influence of the exclusion distance among trees on gap fraction and foliage clumping index of forest plantations

A hypergeometric model is proposed explicitly instead of two previous stochastic models (the Poisson model and Neyman-A model) to describe the topological relationship of trees and the influence of the exclusion distance on gap fraction and clumping index of forest plantation canopies. Gap fraction (GF) and clumping index (CI) play key roles in plant light interception, and therefore they have strong impacts on plant growth and canopy radiative transfer processes. Trees are usually assumed to be randomly distributed in natural forests in many previous studies. However, few studies have shown how trees are distributed in forest plantations and how these distribution patterns affect GF and CI in these forests. In this paper, a simple and general distance factor defined as relative allowable shortest distance between centers of two adjacent crowns divided by the mean diameter of the crowns (RASD) is proposed to describe quantitatively the degree of mutual exclusion among trees in forest plantations of various tree distribution patterns. A hypergeometric model is proposed instead of two previous stochastic tree distribution models (the Poisson model and Neyman-A model) to describe the topological relationship of trees and the influences of the exclusion distance on the GF and CI of the forest plantation canopies. The results show that: (1) the hypergeometric model is more suitable than the Poisson model and Neyman-A model for describing the topological relationship of trees in forest plantations; (2) the exclusion distance has strong impacts on GF and CI: there are significant differences between the results of the hypergeometric model and the Poisson model. Larger RASD causes lower GF and larger CI. The simulations are verified by field measurements in four forest plantation stands. Similarly, impacts of RASD on GF and CI are also found for other two crown shapes (prolate and oblate ellipsoids).

Twenty-five year response of non-crop trees to partial release during precommercial crop tree management

An underappreciated component of precommerical crop tree release (PCTR) is the inevitable partial release of non-crop trees. While the response of fully released crop trees is increasingly understood, few studies have examined the response of non-crop trees. The effects of precommercial crop tree release at canopy closure on upper canopy persistence, mortality, and diameter growth over 25-years were examined on seven study areas established in Connecticut in 1988. Each area had nine 8m×8m plots for each of two treatments: PCTR and unmanaged controls. The equivalent of 156 crop trees per hectare were completely released by cutting all stems with adjacent crowns. This resulted in the inadvertent partial release on two or more sides of 480 upper canopy, non-crop trees per hectare. Diameters and crown classes of all stems (DBH>2cm) were measured annually. For those stems in the upper canopy at when treated, partial release increased the proportion of oaks, but not maples or birches, which persisted in the upper canopy. Partial release increased the proportion of intermediate oaks that ascended into the upper canopy and reduced mortality. Partial release increased 25-year diameter growth of oaks. However, releasing upper canopy, sapling oaks on only one side did not increase upper canopy persistence or diameter growth. PCTR increased the proportion of oaks among the largest 300 trees per hectare twenty-five years after treatment. Where predicted oak densities are below management goals, precommercial crop tree release should be considered as a tool to increase survival and growth of quality oak saplings.

Individual Tree Delineation in Deciduous Forest Areas with LiDAR Point Clouds

Detection and delineation of individual trees is important for the estimation of forest information using Light Detection and Ranging (LiDAR) point clouds. However, over-segmentation of canopy height models (CHMs) is frequently caused by subcrowns and branches, whereas under-segmentation is caused by adjacent crowns, especially in deciduous forests. To eliminate such problems, an approach for delineating individual trees based on curvature attributes derived from LiDAR data is proposed. To detect accurate treetops for reducing over-segmentation, the dominant sizes of tree crowns are first determined and used to fit the dome-shaped tree crowns in CHMs, and then the minimum curvature values of fitted CHMs at different scales are calculated for treetop detection. For delineation of individual trees, tree features such as height variation and curvature are chosen to form a feature vector as a pixel-level logistic regression classifier. Then an ensemble classifier combining multiple logistic regression classifiers is trained to analyze the similarity between neighboring points to reduce under-segmentation. Finally, segment maps at different scales are integrated to generate the final segment map. Our experimental results on 2 test areas demonstrate that the proposed method has promising applications for eliminating over-segmentation and under-segmentation in deciduous forests.

Papilla preservation between two implants: a modified socket-shield technique to maintain the scalloped anatomy? A case report.

A scalloped anatomy maintained between two adjacent crowns supported by immediate implants is presented in this clinical case. This is a modified concept of the socket-shield technique, which allows preservation of the papilla due in part to preservation of interproximal root fragments. Clinical results at 11 months after implant placement are presented.

Importance of a distal proximal contact on load transfer by implant-supported single adjacent crowns in posterior region of the mandible: a photoelastic study

ObjectiveThis study aimed to evaluate the importance of a distal proximal contact on the load transfer to the posterior region of the mandible by non-splinted adjacent implant-supported crowns using photoelastic stress analysis. Material and MethodsA rectangular model (68x30x15 mm) was made of polymethylmethacrylate resin to simulate half of the mandibular arch. One model was completed with resin replicas representing the first premolar and second molar and with two 3.75 mm dia.x11 mm internal hexagon threaded implants replacing the second premolar and first molar. The other model was manufactured in the same way but without the second molar. Both models were duplicated using photoelastic resin. The roots of the teeth replicas were covered with a layer of polyether impression material to simulate the periodontal ligament. Two different vertical loads were applied to the crowns as follows: 1 - single static point load alternately applied to the crowns replacing the second premolar and first molar (50 N); 2 - simultaneous static point loads applied to both of the crowns replacing the second premolar and first molar (100 N). The resulting isochromatic fringe pattern in the photoelastic model was monitored and photographed.ResultsAll loading conditions studied showed that the presence of the second molar has changed the load transmission and the pattern of stresses.ConclusionResults showed that the presence of a second molar proximal contact can help minimize the stresses around the implants.

The Integrity® bare-metal stent made by continuous sinusoid technology

The Integrity® Coronary Stent System (Medtronic Vascular, CA, USA) is a low-profile, open-cell, cobalt–chromium-alloy advanced bare-metal iteration of the well-known Driver®/Micro-Driver™ Coronary Stent System (Medtronic Vascular). The Integrity stent is made with a process called continuous sinusoid technology. This process allows stent construction via wrapping a single thin strand of wire around a mandrel in a sinusoid configuration, with laser fusion of adjacent crowns. The wire-forming process and fusion pattern provide the stent with a continuous preferential bending plane, intended to allow easier access to, and smoother tracking within, distal and tortuous vessels while radial strength is maintained. Continuous sinusoid technology represents innovation in the design of stent platforms and will provide a future stent platform for newer technology, including drug-eluting stent platforms, drug-filled stents and core wire stents.

The first record of Helicoprion Karpinsky (Helicoprionidae) from China

A new species, Helicoprion jingmenense sp. nov., from the Lower Permian Qixia Formation of Hubei Province, China, is described based on a nearly complete symphyseal tooth spiral preserved as part and counterpart. The specimen is a large elliptical symphyseal tooth spiral with \( 4{\raise0.5ex\hbox{$\scriptstyle 1$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle 3$}} \) volutions, with cutting blade being relatively wide, and its width being greater than the height after 3½ volution, middle portion being higher than the cutting blade, narrowed base being short. There is a distinct space not covered by enameloid in two adjacent tooth crowns in the outermost two volutions; the height of the compound root is moderate, and the ventral groove is about 1/10 of the same tooth crown. In comparison with other known Helicoprion species of the world, the new species resembles H. ferrieri and H. bessonowi. However, it can be easily distinguished from H. ferrieri by the wider cutting blade, the shorter narrowed base and lower compound root, and also from H. bessonowi by its more narrowed compound root and less than 39 tooth crowns per volution. Helicoprion jingmenense sp. nov. is obviously different from helicoprionid species previously found in China in general shape, size and histology. However, it is very difficult to identify their relationships because of the fragmentary nature of holotypes of the latter. Because the new specimen is the first record of Helicoprion and the most complete fossil dentition of helicoprionids from China, the new species is of significance for biostratigraphy and paleobiogeography.

Effects of splinted prosthesis supported a wide implant or two implants: a three‐dimensional finite element analysis

Three-dimensional finite element (FE) models of splinted prosthetic crowns were studied and stress analyses were evaluated with different types of implant support, including standard, wide or two implant(s) for partial, posterior edentulous restorations. The FE models were constructed based on a cadaver mandible containing the 2nd premolar and the 1st molar. The crowns of these two teeth were modeled as connected and disconnected to mimic the splinted and non-splinted designs, respectively. One standard implant was placed at the premolar region, while three types of implant support, one at a time (the standard implant, wide implant and two implants), were used to support the molar crown. A 100 N oblique load was applied to the buccal cusp on each crown. The FE simulation was validated experimentally via strain gauge measurement. The experimental data were well correlated with the FE predictions (r(2)=0.97). When compared with the standard implant used in the molar area, the wide implant and two implants reduced the peak stress in crestal bone by 29-37% for both splinted and non-splinted cases. Inserting the standard implant into both the premolar and molar area, the bone stresses were identical for splinted and non-splinted designs. However, splinting the adjacent crowns has shown to decrease the bone stresses at the premolar region by 25%, while the wide implant or two implants were placed at the molar region. The biomechanical advantages of using the wide implant or two implants are almost identical. The benefit of load sharing by the splinted crowns is notable only when the implants on the premolar and molar regions have different supporting ability.

Measuring and modelling the crown and light transmission characteristics of juvenile aspen

Crown dimensions, crown leaf areas, and leaf area densities were determined on individual trees in 96 juvenile Populus tremuloides Michx. stands in central Alberta, Canada. Crown radius, height, leaf area, and leaf area density were well explained by stem diameter at 30 cm (D30). Leaf area index, estimated using the LAI-2000, reached a maximum of 4 m2·m–2 in some stands by age 9 and decreased after age 25. Leaf area indices for the same stands, estimated by allometric relationships, were greater than 7 m2·m–2. Horizontal overlap between adjacent tree crowns declined from 68 to 38% of crown width with increasing average D30 from 1.5 to 15.0 cm. These data were used to calibrate MIXLIGHT, a spatially explicit light transmission model, to predict the range of light conditions in the understory of juvenile stands. Light predictions were validated in 18 additional plots in which light transmission was measured. Measured light transmission ranged from 4 to 68% of above canopy light among sites and from 16 to 53% within a mapped site. MIXLIGHT predicted transmission well for average stand light levels (R2 = 0.80). For individual positions within the mapped stand there was a strong relationship between predicted and observed light (R2 = 0.92), but there was underprediction at high light and over at low light.

The Effect of Inter‐Implant Distance on the Height of Inter‐Implant Bone Crest

The biologic width around implants has been well documented in the literature. Once an implant is uncovered, vertical bone loss of 1.5 to 2 mm is evidenced apical to the newly established implant-abutment interface. The purpose of this study was to evaluate the lateral dimension of the bone loss at the implant-abutment interface and to determine if this lateral dimension has an effect on the height of the crest of bone between adjacent implants separated by different distances. Radiographic measurements were taken in 36 patients who had 2 adjacent implants present. Lateral bone loss was measured from the crest of bone to the implant surface. In addition, the crestal bone loss was also measured from a line drawn between the tops of the adjacent implants. The data were divided into 2 groups, based on the inter-implant distance at the implant shoulder. The results demonstrated that the lateral bone loss was 1.34 mm from the mesial implant shoulder and 1.40 mm from the distal implant shoulder between the adjacent implants. In addition, the crestal bone loss for implants with a greater than 3 mm distance between them was 0.45 mm, while the implants that had a distance of 3 mm or less between them had a crestal bone loss of 1.04 mm. This study demonstrates that there is a lateral component to the bone loss around implants in addition to the more commonly discussed vertical component. The clinical significance of this phenomenon is that the increased crestal bone loss would result in an increase in the distance between the base of the contact point of the adjacent crowns and the crest of bone. This could determine whether the papilla was present or absent between 2 implants as has previously been reported between 2 teeth. Selective utilization of implants with a smaller diameter at the implant-abutment interface may be beneficial when multiple implants are to be placed in the esthetic zone so that a minimum of 3 mm of bone can be retained between them at the implant-abutment level.

Geometric-optical bidirectional reflectance modeling of the discrete crown vegetation canopy: effect of crown shape and mutual shadowing

In the case where a vegetation cover can be regarded as a collection of individual, discrete plant crowns, the geometric-optical effects of the shadows that the crowns cast on the background and on one another strongly condition the brightness of the vegetation cover as seen from a given viewpoint in the hemisphere. An asymmetric hotspot, in which the shape of the hotspot is related to the shape of the plant crowns in the scene, is created. At large zenith angles illumination shadows will preferentially shadow the lower portions of adjacent crowns. Further, these shadows will be preferentially obscured since adjacent crowns will also tend to obscure the lower portions of other crowns. This effect produces a 'bowl-shaped' bidirectional reflectance distribution function (BRDF) in which the scene brightness increases at the function's edges. Formulas describing the hotspot and mutual-shadowing effects are derived, and examples that show how the shape of the BRDF is dependent on the shape of the crowns, their density, their brightness relative to the background, and the thickness of the layer throughout which the crown centers are distributed are presented. >

The crown‐gap ratio (C) and crown cover: Derivation and simulation study

The crown-gap ratio C is defined as the mean distance between adjacent crowns divided by the mean crown diameter. Previous field studies have established that for a limited range of vegetation stands crown cover is related to C by a general function, viz. crown cover = k/(1 + C)2, where k is a constant. In the present study we investigated the general derivation of the function which relates crown cover to C. The function was then used to determine values for k, a range of point distributions being simulated from semi-regular to clumped, with mixtures of crown sizes. The relationship between crown cover and 1/(1 + C)2 was linear in all cases tested (R2= 1.0). The value of k was shown to depend on the sampling technique used, the degree of clumping, and the range in crown sizes permitted. The variation in k was reduced by using a specific sampling method and limiting the range of spatial distributions and crown sizes considered. The constraints imposed were: 1 Sampling followed a zig-zag transect (Delaunay two-sided model). 2 Point distributions were not overly clumped. 3 The range of crown diameters approximated a Gaussian distribution with a relative range of 1–4. Given these constraints the value of k was determined to be 0.806 with a variance of less than 2%. A table is given to convert values for C into crown cover percentages and, if required, to calculate foliage cover per cent. The constraints imposed are considered to be within conditions normally encountered in the field, making the use of C an accurate and simple method by which to estimate crown or foliage cover per cent.

Seed Dispersal, Seed Predation, and Juvenile Mortality of Aglaia sp. (Meliaceae) in Lowland Dipterocarp Rainforest

Black hornbills (Anthracoceros malayanus) appear to be the principal long-distance seed dispersers of Aglaia sp. (Meliaceae) at Pasoh Forest Reserve in Peninsular Malaysia. The squirrel Callosciurus prevostii removed some of the large seeds at least as far as adjacent crowns and sometimes dropped them after consuming only the orange, oily sarcotesta. It chased other squirrel species, which are probably seed predators, and hornbills out of the fruiting crown. Seeds on the ground beneath the parent crown were removed more rapidly than those farther away by rodent and (possibly) phasianid seed predators. Sitophilus sp. (Curculionidae) was also an Aglaia seed predator or parasite. Aglaia juveniles grew slowly under closed canopy, and small ones (<50 cm in height) had only 9.7 percent mortality per year during a 4-year period. Seedling survival was positively size dependent. For unknown reasons, small seedlings near the parent had a higher death rate than those at a greater distance; this was apparently a long-term pattern, because large seedlings and saplings did not occur within 10 and 35 m, respectively, of the parent tree's base. For this species an advantage of seed dispersal is avoidance of disproportionate seed and seedling mortality near the parent. Although Aglaia grows much faster in gaps than under closed canopy, it may require several episodes of growth in successive gaps before becoming reproductively mature. IN THEIR REVIEW OF SEED DISPERSAL, Howe and Smallwood (1982) remarked that little is known about its ecological and evolutionary advantages to the plant. Here we combine observations on seed dispersal and predation and juvenile mortality and spatial distribution to show how dispersal helps the tropical tree Aglaia sp. (Meliaceae) avoid seed and seedling mortality that is greater near the parent than farther away. Webb et al. (1967), Janzen (1970), and Connell (1971) suggested that such a mortality pattern, generated by seed predators, herbivores, or allelopathy, would promote the maintenance of high tree species richness in tropical forests. More recently, Connell (1978, 1979) has withdrawn his support for this hypothesis because field studies have shown that seed and seedling mortality is not invariably higher near conspecific adults. However, Clark and Clark's (1984) review of 24 data sets on mostly neotropical, woody plants showed that most evidence indicates either densityor distance-dependence in progeny mortality, as originally predicted by Janzen and Connell. Hubbell (1979, 1980) challenged the Janzen-Connell hypothesis on empirical and theoretical grounds, daiming the latter to demonstrate that spacing between conspecific adults could contribute little to the maintenance of high species richness. His theoretical analysis has been questioned by Becker et al. (1985), who consider the issue unresolved for nonequilibrium