Increasing Gap Size Research Articles

SU-E-T-121: Determination of MLC QA Criteria for Non-Split IMRT Fields Based On Clinical Quantification

Purpose: To identify clinically-driven MLC performance criteria for non-split step-and-shoot IMRT fields by quantifying the dosimetric impact of Varian MLC leaf abutment miscalibration on the delivered dose. The maximum tolerable leaf gap that can be used as a criterion for MLC leaf gap QA will be determined. Methods: Ten IMRT plans with large (X > 15 cm), non-split fields were selected, 5 whole pelvis and 5 head and neck. All were planned using Pinnacle v9.0. The delivered dose for each plan was calculated using Monte Carlo simulations assuming leaf abutment gaps from 0 to 3 mm, in 0.5 mm increments. All calculations were done with 1 mm resolution. Three dimensional dose difference maps (ΔD) were calculated by subtracting the 0 mm dose matrix from each non-zero one. Cumulative ΔD histograms were constructed to determine the maximum tolerable gap where 5% volume received ΔD >= 3%. Results: The gap where % volume received ΔD >= 3% ranged from 0.7 to 2.2 mm (average 1.0 mm). For 8/10 plans this gap was >= 1.0 mm. The head and neck plans showed more sensitivity to gaps than the whole pelvis plans (p = 0.08). Increasing gap size did not adversely affect target coverage. Maximum OAR dose increased with increasing gap; a 3 mm gap led to a 4% dose increase. Conclusion: Non-split fields should be used for IMRT with caution. This method of treatment is acceptable as long as MLC leaves abut within 0.5 mm. Standard IMRT QA using a 4 mm calculation grid and gamma of 3%/3mm is not sensitive enough to detect dose differences from leaf abutment miscalibration. If treating this way, leaf abutment should be checked as part of weekly QA with a 0.5 mm criterion in addition to the TG-142 recommended MLC QA.

Disturbance size and severity covary in small and mid-size wind disturbances in Pennsylvania northern hardwoods forests

Do large disturbances differ from small ones in characteristics other than size? The importance of disturbances in forest dynamics is unquestioned, and the size of the disturbed area (size of gap) is the most common way of differentiating disturbances. But few studies have examined other disturbance characteristics to see if small and large disturbances are different. In northern hardwoods forests of northwestern Pennsylvania, a 2003 windstorm event created windthrow gaps ranging in size from 0.1ha to >100ha, allowing comparisons across a broad size spectrum, yet with similar species composition, soils, climate, and topography. We selected 17 gaps, ranging in size from 0.05 to 4.0ha, to compare damage characteristics across a size spectrum. Disturbance severity (both proportion of trees fallen, and proportion of basal area fallen) increased significantly with gap size. Similarity in floristic composition between pre- and post-disturbance stands, decreased with increasing gap size. Larger gaps also caused greater reduction in mean size (diameter) of remaining trees. As expected, larger gaps resulted in greater canopy openness, but canopy openness was significantly influenced by both gap size and severity of canopy disturbance. These findings demonstrate that disturbance size and severity covary in northern hardwood windthrow gaps. Moreover, because of nonrandom impacts across species and size classes, immediate changes in size structure and composition of affected stands were greater in larger gaps. Managers seeking to implement disturbance-based management can use these findings to more closely mimic natural damage effects during harvest operations.

Magnetic vortex stability in Ni80Fe20 split rings

Magnetoresistance measurements at 300 K and 1.7 K are used to quantify the vortex state magnetic field duration in a series of Ni80Fe20 ring structures with two nm-sized gaps. The measured gap sizes are from 40 nm to 510 nm. The vortex magnetic field range of stability reduces on increasing gap size but then stabilizes and is predominantly determined by extrinsic domain wall pinning defects. Virtual domain wall mediated switching with the applied magnetic field direction parallel to the gap, reduces a stochastic switching behavior in these structures, compared to a continuous ring. A comparison of experiment with the predicted switching behavior from a micro-magnetic simulation is made and the stray fields are calculated in the vortex remanent state. The reduction in the vortex field duration with increasing gap size is more gradual than that predicted by the micro-magnetic simulation up to a gap size of ∼100 nm.

The Role of Homogeneous Chemistry during Ignition of Propane Combustion in Pt-Catalyzed Microburners

The aim of this work is to numerically investigate the ignition behavior of homogeneous-heterogeneous (HH) combustion of propane in a Pt-catalyzed microburner to delineate the role of homogeneous chemistry during cold-start ignition. A two dimensional model with one-step homogeneous and catalytic mechanisms in a parallel-plate microburner is considered. The ignition characteristics (ignition temperature and ignition time) are explored for catalytic microreactor with and without homogeneous chemistry. We show that the catalytic reaction lights-off first, followed by the homogeneous reaction. Consequently, the homogeneous chemistry does not affect the ignition behavior of the catalytic microburner. The effect of inlet velocity, wall thermal conductivity and gap size on ignition characteristics is explored. The ignition characteristics are not affected by homogeneous chemistry even at larger gap sizes, despite the fact that the homogeneous contribution at steady state increases with increasing gap size of the microburner.

The effects of gap size and age on natural regeneration of Picea mongolica in the semi-arid region of Northern China

The aim of this work was to study the effects of gap size and age on natural regeneration of endangered tree species Picea mongolicaW. D. Xu, which is only found in the semi-arid region of Northern China. For this purpose, six random transect belts of varying size were established in Baiyinaobao Natural Reserve. The number, height and age of P. mongolica saplings were inventoried. Results show that the total number of saplings per hectare and corresponding number and relative share of saplings in height class I (0.1–0.99 m) decreased along with increasing gap size, opposite to height class III (6–9 m). In height class II (1–5.99 m) the number of saplings per hectare and their relative share increased first slightly and then decreased with the increase of gap size. The number of saplings and their relative share in each height class changed similarly with the increase in the distance to the edge of the mature stand (p < 0.05). Gap age also significantly affect the number of saplings per hectare or their height distribution except in height class II. The distance to the mature stand edge played a very important role in the success of natural regeneration for species P. mongolica.

Do deer and shrubs override canopy gap size effects on growth and survival of yellow birch, northern red oak, eastern white pine, and eastern hemlock seedlings?

Innovative forestry practices that use natural disturbance and stand developmental processes as models to increase forest complexity are now being considered as a way to conserve biodiversity while managing for a range of objectives. We evaluated the influence of harvest-created gap size (6, 10, 20, 30, and 46m diameter gaps and uncut references) over 12 growing seasons on planted tree seedling growth and survival for four tree species that tend to experience poor recruitment in both managed and unmanaged northern hardwood forests in eastern North America. We expected, based on silvics, that the three mid-tolerant species (yellow birch [Betula alleghaniensis], red oak [Quercus rubra], and white pine [Pinus strobus]) would perform best in intermediate-sized gaps, and the one shade tolerant species (hemlock [Tsuga canadensis]) would perform best in small gaps. However, all four of the species grew taller with increasing gap size, while survival was highest in intermediate gap sizes. Although gap size had statistically significant effects on growth and survival, the magnitude of the effects were modest. With the exception of a small portion of white pine individuals (35% of survivors were >150cm tall), trees were short (<1m) and few survived (<30%) 12years after planting. Evidence from deer exclosures and individual gaps with high shrub (Rubus idaeus) densities suggest that browsing and shrub competition resulted in poor tree growth and survival, and may have constrained the magnitude of many potential tree seedling responses to gap size. Our study highlights the management challenges of using gap size as a tool to influence future forest composition in forests with overly abundant deer and pervasive shrub layers and underscores the importance of silvicultural prescriptions that include measures for reducing these impacts.

Regeneration responses to gap size and coarse woody debris within natural disturbance-based silvicultural systems in northeastern Minnesota, USA

The use of silvicultural systems based on the natural disturbance patterns for a given region has been suggested as an approach for promoting late-successional forest characteristics and maintaining native biodiversity in managed forests. To this end, we examined the effectiveness of natural disturbance-based harvest gaps in maintaining and restoring native tree species diversity within second-growth northern hardwood systems in northeastern Minnesota, USA. In particular, tree regeneration and downed coarse woody debris (CWD) were measured 6- and 7-years post-treatment in 46 gaps that emulated the historic range of natural canopy opening sizes for northern hardwood forests in the upper Great Lakes region. Measurements were compared across gap size-classes (>0–0.02, >0.02–0.04, >0.04 ha) and with closed canopy portions of these systems. In addition, the factors affecting the recruitment of Betula alleghaniensis across gaps and within closed canopy were investigated due to the historical importance of this species in these forests. Seedling and sapling densities increased in harvest gaps; however, results indicated that these gaps did little to increase tree diversity, including the recruitment of shade mid-tolerant species, such as B. alleghaniensis. Gaps were dominated primarily by Acer saccharum and age distributions of dominant saplings within gaps indicated that most individuals established 10–40 years prior to gap formation. Establishment of B. alleghaniensis was strongly related to highly decayed, large coniferous pieces of CWD with little recruitment occurring on the undisturbed forest floor. Age distributions of B. alleghaniensis established on CWD suggest this species can persist on this substrate for at least a decade. Levels of CWD increased with increasing gap size; however, all gaps had lower levels of CWD compared to the surrounding intact forest. Our results suggest that management regimes based on natural canopy gap sizes within northern hardwood systems will do little to restore native tree diversity if provisions for creating the suitable seedling microsites historically generated by natural disturbance (e.g., exposed mineral soil, highly decayed wood) are not included within management prescriptions.

Gap-, stand-, and landscape-scale factors contribute to poor sugar maple regeneration after timber harvest

Natural regeneration in canopy gaps is a key process affecting long-term dynamics of many forests, including northern hardwood forests. The density and composition of regenerating trees are often highly variable, reflecting sensitivity to a suite of driving factors operating at different scales (e.g., harvest gap to regional landscape), including production of seeds, physical characteristics of gaps and stands, competition with non-tree vegetation, and browsing by animals. Multivariate analyses over broad geographic areas provide insights into the relative effects of these factors and permit exploration of spatial patterns in regeneration. We examined the effects of gap-, stand-, and landscape-scale factors on densities of tree seedlings (<1 m tall) and saplings (1–2 m tall) in 59 selection-harvested northern hardwood stands located across a 4500 km 2 region of Michigan's Upper Peninsula. We used Bayesian multilevel modeling to account for the hierarchical structure of the data and assess uncertainty in parameter estimates. Sugar maple ( Acer saccharum) saplings were absent from 61% of 154 m 2 plots centered in harvest gaps ( n = 347) despite its high shade tolerance and overstory dominance, but densities were high in other gaps. Densities of sugar maple seedlings and/or saplings were negatively associated with a combination of greater stand-scale densities of white-tailed deer ( Odocoileus virginianus), greater gap-scale cover of non-tree vegetation, and lower gap-scale light availability, with deer density having the greatest effect. Densities of unpalatable and commercially less valuable ironwood ( Ostrya virginiana), the second most common regeneration species, were positively related to gap-scale seed-production potential but were unrelated to factors affecting sugar maple. Ironwood tended to replace sugar maple saplings in areas with high deer density. At the landscape scale, densities of sugar maple seedlings and saplings decreased with decreasing latitude and snow depth and increasing winter deer densities. These inverse spatial patterns suggest that deer herbivory can lead to landscape-scale variation in regeneration success. However, the spatial distribution of habitat types (a proxy for soil moisture and nutrient conditions) confound this observation, with higher densities of sugar maple generally located on stands with less nutrient-rich habitat types. Results demonstrate that combinations of factors operating at different scales, and with different relative magnitudes of impact, contribute to high variation in regeneration composition and density following timber harvest. Selection silvicultural practices, as currently applied, do not ensure regeneration of desirable species; practices might require modifications in general (e.g., increasing gap size) and to match them to regionally varying factors like deer density.

Predicting long-term sapling dynamics and canopy recruitment in northern hardwood forests

Prediction of forest composition and structure over multiple generations of trees is often hampered by limited data on understory tree dynamics and the highly variable process of canopy recruitment in forest openings. In this paper, we describe a model of sapling dynamics and overstory recruitment for CANOPY, a spatially explicit, crown-based, individual-tree model of gap dynamics. The model incorporates gap size as a predictor of sapling recruitment and height growth, and it mimics the processes of sapling release, gap capture, and lateral gap closure. Calibration data were derived from 12 data sets with a wide range of stand ages and disturbance history in northern hardwood stands in the Great Lakes region, USA. The model accounted for 30%–62% of the variation in sapling density, composition, and growth rates. Predicted effects of increasing gap size on growth rate were similar to observed trends. Growth equations that included gap size as an independent variable generally gave better predictions of sapling density, species composition, and growth rates than equations based on conventional plot-level competition metrics. Long-term, 1000-year simulations produced estimates of stand basal areas and tree density in each size class that are close to the mean observed values for old-growth stands in the region.

The effects of small-scale disturbance on forest birds: a meta-analysis

Small-scale disturbance is a significant process in all major forest biomes. Some silvicultural practices, particularly group selection harvesting, intend to emulate natural small-scale disturbance by harvesting small clearcuts in the continuous forest. We conducted a meta-analysis on the effects of small-scale harvesting on North American breeding forest birds. We extracted species richness and relative abundance of several functional bird groups and guilds from published studies and compared them between gap-dominated and unlogged forest as a function of forest type and the size and age of the gap. The abundance of many bird groups was higher in the gap-dominated than in the continuous forest. Species preferring interior parts of the forest had the most negative association with the presence of gaps but this relationship was not statistically significant. Abundances of many bird groups increased with increasing gap size, while its effect on abundance of some bird groups disappeared quickly. Our review suggests that silvicultural practices that bring about small gaps do not negatively affect the abundances of most forest birds and often even enhance it. However, more studies are needed to examine optimal size and abundance of gaps in a forest and whether emulated small-scale disturbance effectively mimics natural processes.

Surfactant Properties Differentially Influence Intravascular Gas Embolism Mechanics

Gas bubble motion in a blood vessel causes temporal and spatial gradients of shear stress at the cell surface lining the vessel wall as the bubble approaches the cell, moves over it and passes it by. Rapid reversals occur in the sign of the shear stress imparted to the cell surface during this motion. These may result in injury to the cell. The presence of a soluble surfactant in the bulk medium reduces the level of the shear stress gradients imparted to the cell surface as compared to an equivalent surfactant-free system and is an important therapeutic aid. This is particularly true for a very small vessel. In this study, we analyze various physical and chemical properties of any given soluble surfactant to ascertain the relative significance of the property of the surfactant on the reduction in the level of the shear stress gradients imparted to the cell surface in such a vessel. While adsorption, desorption, and maximum possible monolayer interface surfactant concentration significantly impact the shear stress levels, physical properties such as the bulk or surface diffusivity do not appear to have large effects. At a given diameter, surfactants with k(a)/(k(d)d>O(10)⁻⁵ and Γ(∞)/C(0)d>9.5 x 10⁻⁴ are noted to be preferable from the point of view of an increased gap size between the bubble and vessel wall, and a corresponding reduction in the shear stress level imparted to an endothelial cell. The shear stress characteristics of nearly occluding bubbles, in contrast with smaller sized bubbles under identical conditions, are most affected by the introduction of a surfactant in regard to shear stress levels. These observations could form a basis for choosing surfactants in treating gas embolism related illnesses.

Herbivory in canopy gaps created by a typhoon varies by understory plant leaf phenology

1. Availabilities of light and soil nitrogen for understory plants vary by extent of canopy gap formation through typhoon disturbance. We predicted that variation in resource availability and herbivore abundance in canopy gaps would affect herbivory through variation in leaf traits among plant species. We studied six understory species that expand their leaves before or after canopy closure in deciduous forests. We measured the availabilities of light, soil nitrogen, soil water content, and herbivore abundance in 20 canopy gaps (28.3–607.6 m2) formed by a typhoon and in four undisturbed stands. We also measured leaf traits and herbivory on understory plants.2. The availabilities of light and soil nitrogen increased with increasing gap size. However, soil water content did not. The abundance of herbivorous insects (such as Lepidoptera and Orthoptera) increased with increasing gap size.3. Concentrations of condensed tannins, total phenolics, and nitrogen in leaves and the leaf mass per area increased in late leaf expansion species with increasing gap size, whereas none of the leaf traits varied by gap size in early leaf expansion species.4. Herbivory increased on early leaf expansion species with increasing gap size, but decreased on late leaf expansion species. In these late leaf expansion species, total phenolics and C : N ratio had negative relationships with herbivory.5. These results suggested that after typhoon disturbance, increased herbivory on early leaf expansion species can be explained by increased herbivore abundance, whereas decreased herbivory on late leaf expansion species can be explained by variation in leaf traits.

In Vitro Evaluation of the Effect of Fracture Configuration on the Mechanical Properties of Standard and Novel Interlocking Nail Systems in Bending

To investigate the effect of fracture configuration on the mechanical properties of standard interlocking nails (ILNs) and a novel angle-stable ILN (ILNn) in bending. In vitro experimental study. Synthetic tibial gap fracture bone models. Bone models, featuring a 5 or 120 mm central defect, respectively, mimicking a simple diaphyseal and a comminuted fracture involving both metaphyses, were implanted with 6 or 8 mm screwed or bolted standard ILNs (ILN6s, ILN6b, ILN8s, ILN8b, respectively) or an ILNn. Specimens were tested in 4-point bending. Construct angular deformation (AD) and slack were statistically compared (P<.05). With increasing gap size, standard ILN construct AD increased significantly by approximately 27% in ILN8b and by up to 105% in ILN6s. Similarly, standard ILN construct slack significantly increased by approximately 33% in ILN8b (from approximately 4.2 degrees to approximately 5.6 degrees) and by up to approximately 130% in ILN6s (from approximately 7 degrees to approximately 16 degrees). Conversely, there was no difference in the ILNn construct AD (approximately 4 degrees) regardless of gap size. ILNn AD was the lowest of all groups and occurred without slack. This study demonstrated that the angle-stable ILNn provided construct stability regardless of fracture configuration, whereas the intrinsic slack of standard ILNs could jeopardize construct stability in a fracture configuration involving the metaphyses. Use of standard ILNs may be optimal in diaphyseal fractures where circumferential nail/cortical contact could augment repair stability. Conversely, the angle-stable ILNn may represent a reliable fracture stabilization method for diaphyseal fractures as well as fractures involving the metaphyseal regions.

Spatial distribution pattern of soil seed bank in canopy gaps of various sizes in temperate secondary forests, Northeast China

To promote the natural regeneration of secondary forests under gap disturbance is one of important approaches for realizing their restoration and sustainable management. However, soil seed bank, one of regeneration strategies, is poorly understood in gaps of secondary forests. Objectives of the study were to evaluate the effects of sizes of experimentally created gaps on seed distribution in soil, and of seed banks on vegetation recovery at early formation stage of gaps in temperate secondary forests, Northeast China. It was found that with increasing gap size, species richness in the seed bank significantly increased (r = 0.691, P < 0.05, n = 10), but correspondence in species-composition of seed reservoir between gaps and canopies adjacent to gaps decreased (r = −0.83, P < 0.01, n = 10). Gap size and soil depth and their two-way interactions had significant differences on the distribution of seed bank density (F = 4.101, P < 0.05; F = 34.483, P < 0.01; F = 5.232, P < 0.01, respectively). The Sokal and Sneath similarity indices in species-composition between extant and potential (seed bank) vegetation were generally low, ranging from 0.06 (interior of large gaps) to 0.24 (canopies adjacent to large gaps). From the view of seed bank to restore temperate secondary forests, establishment of medium-sized artificial gaps (i.e., gap size with 500–150 m2) could be more feasible for seed invasion. To conclude, the regeneration potential out of soil seed banks is limited, and the restoration of gaps at early formation stage may be mostly dependent on other reproductive strategies (e.g., vegetative propagation of species).

Smaller and more numerous harvesting gaps emulate natural forest disturbances: a biodiversity test case using rove beetles (Coleoptera, Staphylinidae)

ABSTRACTAim To evaluate changes in the abundance, species richness and community composition of rove beetles (Coleoptera, Staphylinidae) in response to three configurations of experimental gap cuts and to the effects of ground scarification in early succession yellow birch‐dominated boreal forest. In each experimental treatment, total forest removed was held constant (35% removal by partial cutting with a concomitant decrease in gap size) but the total number of gaps was increased (two, four and eight gaps, respectively), resulting in an experimental increase in the total amount of ‘edge’ within each stand.Location Early succession yellow birch‐dominated forests, Quebec, Canada.Methods Pitfall traps, ANOVA, MIXED procedure in sas®, post hoc Tukey's adjustment, rarefaction estimates, sum‐of‐squares and distance‐based multivariate regression trees (ssMRT, dbMRT).Results Estimates of species richness using rarefaction were highest in clearcut and two‐gap treatments, decreased in smaller and more numerous gaps and were significantly higher in scarified areas than in unscarified areas. ANOVA indicated a significant impact of harvesting on the overall standardized catch. Post hoc Tukey's tests indicated that the total catch of all rove beetles was significantly higher in uncut forests than in the treated areas. Both sum‐of‐squares and distance‐based multivariate regression trees indicated that community structure of rove beetles differed among treatments. Assemblages were grouped into (a) control plots, (b) four‐ and eight‐gap treatments and (c) two‐gap and clearcut treatments.Main conclusions Rove beetle composition responded significantly to increasing gap size. Composition among intermediate and small‐sized gap treatments (four‐ and eight‐gap treatments) was more similar to uncut control forests than were larger gap treatments (two‐gap) and clearcuts. Effects of scarification were nested within the harvested treatments. When the total area of forest removed is held constant, smaller, more numerous gaps are more similar to uncut control stands than to larger gaps and falls more closely within the natural forest heterogeneity.

Numerical study of optical properties of single silver nanobowtie with anisotropic topology

Optical properties of silver nanobowtie with anisotropic topology, which is separated with nanogap that is horizontal (Y direction) and vertical (Z direction) to the line between the two constituent triangles (i.e., the bowtie axis), are studied. A discrete dipole approximation (DDA) method is used to determine the extinction efficiency and the electric field (E-field) distribution, then the corresponding enhancement factor (EF) and the full width half maximum (FWHM) of the E-field are obtained and analyzed. Our results show that the wavelength of localized surface plasmon resonance (LSPR)-induced blue-shifts and the maximum EF decrease were observed with increasing gap size for the parallel polarized (Z-polarized) excitation. It is also found that the light spot at the "hot spot" in the gap becomes larger and deviates from the rectangular pattern with the increase of the gap in the Z direction (Z-gap), and the FWHM in the Z direction of the localized E-field in the gap broadens with either an increase in the gap in the Y direction (Y-gap) or the Z-gap. These behaviors are due to the decrease of the coupling efficiency of near-field plasmon coupling. In contrast, the wavelength of the LSPR and the maximum EF excited by the perpendicularly polarized (Y-polarized) light show less dependence on the gap size than that of the Y-polarized light.

Parametric Equation for Static Strength of Tubular Circular Hollow Section Joints with Complete Overlap of Braces

This paper presents the development of a parametric equation for predicting the ultimate capacity of tubular circular hollow section joint with complete overlap of braces based on the experimental and numerical investigation. In the experiment, a joint specimen with complete overlap of braces is tested to failure under lap brace axial compression for verification of the finite-element (FE) model. The comparison of results showed that the FE model simulates very well the current and the previous test specimens with differences in ultimate capacity and lap brace axial displacement of less than 1.52 and 3.5%, respectively. In the parametric study, the joint ultimate strength is found not significantly affected by the boundary conditions and the chord prestresses. The developed parametric equation based on 3,888 FE models has been verified against the current and the previous test and FE data. A simplified equation in characteristic format for limit state design is also proposed. A comparison with the existing T/Y-joint equations showed that the joint ultimate strength at small gap size is higher. However, the joint ultimate strength decreases with increasing gap size and is found equivalent to that of T/Y-joints at a gap size greater than through brace diameter.

Regeneration of timber species following selective logging in a moist semi-deciduous forest in Ghana

The natural regeneration and growth of timber tree species were studied in four logging gaps; loading bays, tree-fall gaps, primary skid trails and secondary skid trails with forest shade as control in a moist semi-deciduous forest in Ghana. There was prolific tree seedling emergence in all the sites in the first few months following logging but declined after 8 months. Species diversity as well as seedling numbers increased with reducing gap size from the loading bays to secondary skid trails. Weed biomass, however, increased with increasing gap size. Pioneer species were relatively abundant in loading bays than all the ecological species guilds while the non-pioneers showed preference for smaller gaps. Ceiba pentandra (pioneer), for example, was the only species that survived in the loading bays for the 27-month period of the study but could not survive in the forest shade. The limited regeneration of commercial tree species and increased colonization by competing weeds in larger gaps indicate the need for better control over harvest operations through gap size reduction and post-harvest competition control treatments to ensure sustainable production of economic tree species. Keywords: Ghana, moist semi-deciduous forest, logging, regeneration Ghana Journal of Forestry Vol. 17 & 18 2005: pp. 20-35

Regeneration after 8 years in artificial canopy gaps in Mountain Ash ( Eucalyptus regnans F. Muell.) forest in south-eastern Australia

We report on a study of regeneration of Mountain Ash ( Eucalyptus regnans) forest in S.E. Australia in artificially created canopy gaps (0.01–2 ha) and clearfelled coupes (4–27 ha) with different seedbed treatments. Treatments were applied in 1988, 1989, and 1990. Our results are based on measurements 8 years after harvesting. Gap size had a significant influence on sapling height and diameter growth. The effect was substantial for the canopy openings larger than 2 ha, but inconsistent across the smaller gap sizes. Mean height of the largest diameter saplings was significantly greater in the clearfells (13.2 m) than in the 2 ha gaps (10.7 m), while mean heights in the smaller gaps ranged from 6 to 8.8 m. Mean sapling diameters (at 1.3 m) in clearfells and 2 ha gaps were 9.6 and 7.9 cm respectively compared with 3.8–5.8 cm for the smaller gaps. There was lower stocking in all treatments for the third regeneration year (1990), when regeneration operations were delayed by high summer and autumn rainfall. Stocking ranged from 12 to 33% for the smallest gaps, increasing with increasing gap size to 34–64% for 2 ha gaps and 48–76% for clearfells. Results were consistent with a previous study 3 years after treatment, indicating that stocking at 8 years is still dominated by initial stand establishment rather than by stand development processes. There was no significant influence of seedbed preparation on any of the variables tested at year 8; this was in contrast with earlier findings at year 3 when seedling height was significantly higher on burnt sites compared with mechanically disturbed sites. The growth of non-eucalypt competitors was also influenced by gap size and year of regeneration treatment. The main competitor species differed between larger and smaller clearings and between sites. Results indicate that using a silvicultural system based on smaller gaps (<2 ha) in this forest type is likely to result in reduced timber production potential compared with clearfelling.

The Influence of Gap Size on Sagebrush Cover Estimates With the Use of Line Intercept Technique

Sagebrush cover is often estimated with the use of the line intercept method. However, a lack of standardized protocols may lead to variable estimates of sagebrush canopy cover. Our objectives were to determine the influence of gap size on 1) sagebrush canopy cover estimates, 2) time needed to read a transect, and 3) among-observer variability in sagebrush canopy cover estimates. We utilized 5-, 10-, and 15-cm gaps, and defined a gap as a lack of continuous live or dead shrub canopy. In instances where a segment of dead cover was less than the gap size and adjoined live cover, the dead cover was measured as live. We evaluated canopy cover at 6 Wyoming big sagebrush (Artemisia tridentata Nutt. ssp. Wyomingensis Beetle & A. Young) sites in southeast Oregon. At each site, four 2-person teams measured sagebrush canopy intercept along 50-m transects. Each transect was read by multiple teams to allow for assessment of among-observer variability. Intercept values were converted to percent canopy cover and we used analysis of variance to determine the influence of site and gap size on measurement time and cover estimates. Observer variability was highest at the intermediate gap size (i.e., 10 cm). Transect measurement time was longest with the use of a 5-cm gap (P < 0.001). Total cover estimates were not related to gap size (P = 0.270). Live canopy cover estimates increased (P < 0.001) from 12.1% to 14.5% with increasing gap size, and cover of dead material decreased (P = 0.015) from 4.4% to 3.2%. These differences are small in magnitude and would not likely change a gross assessment of vegetation status. However, use of a standardized gap size will enhance comparability of canopy cover estimates among studies and will decrease between-year sampling error for repeat monitoring.