Forestry Advance Access published online on July 7, 2009
Forestry, doi:10.1093/forestry/cpp020
Relationships between canopy transmittance and stand parameters in Sitka spruce and Scots pine stands in Britain
1 Forest Research, Northern Research Station, Roslin, Midlothian EH25 9SY, Scotland
2 School of the Environment and Natural Resources, College of Natural Sciences, Bangor University, Gwynedd LL57 2UW, Wales
3 Present address: Forest Research, Linmere, Delamere, Northwich CW8 2JD, England
* Corresponding author. E-mail: sophie.hale{at}forestry.gsi.gov.uk
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Summary |
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The changing emphasis within British forestry from a clearfell/replant system focussed on timber production, to multipurpose forestry encompassing biodiversity and recreation, has resulted in a need for changes to forest management. Manipulation of the forest canopy through thinnings is a powerful tool for forest managers to modify the canopy transmittance, and therefore the below-canopy light levels. This helps to achieve specific objectives such as habitat management or seedling growth as part of transformation of an even-aged stand to a continuous cover forestry regime. In this study, hemispherical photography was used to assess canopy transmittance in a range of Sitka spruce (Picea sitchensis (Bong.) Carr) and Scots pine (Pinus sylvestris L.) stands in Britain. Species-specific relationships were developed between canopy transmittance and easily-measured stand parameters. The models that provided the best fit to the data were based on basal area and stocking for Sitka spruce and basal area alone for Scots pine. The models indicate that a Sitka spruce stand with a basal area of 30 m2 ha–1 should have a stocking density <450 stems ha–1 to favour growth of Sitka spruce seedlings. Similarly, a Scots pine stand should have a basal area <27 m2 ha–1 to achieve transmittance suitable for growth of Scots pine seedlings. In conjunction with a knowledge of the light requirements of different vegetation types, these models can provide a valuable contribution to guidance on current and changing forest management practice.
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Introduction |
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TopSummaryIntroductionMethodsResultsDiscussionConclusionsConflict of Interest StatementAcknowledgementsReferences |
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The objectives of forest management in Britain are changing from predominantly timber production to include a wide range of other functions, such as biodiversity and recreation. Accompanying these developments is a change in emphasis from clearfelling/replanting to continuous cover forestry (CCF) (Anon, 2004
). CCF is an approach to forest management whereby the overstorey is retained, at one or more levels, during the establishment phase (Mason et al., 1999). It incorporates a wide range of silvicultural systems, such as shelterwoods, and group or single-tree selection. There is a presumption towards natural regeneration, although underplanting may sometimes be preferable.
The understorey light environment is one of the primary factors influencing the growth of vegetation beneath a forest canopy (Lieffers et al., 1999). The proportion of incident radiation that is transmitted through the canopy (canopy transmittance) depends on the size, distribution and density of the tree crowns (Brunner, 1998; Jennings et al., 1999). Manipulation of the stand is therefore a powerful tool for forest managers to modify the light environment to help to achieve management objectives (Messier, 1996), by controlling the growth of ground vegetation and seedlings. However, practical guidance is needed, specific to the species and conditions relevant to British forestry, to enable managers to estimate the light regime beneath a stand. This will enable them to make informed decisions on how to manipulate a stand to obtain the desired light levels, in support of a number of issues, such as transformation of even-aged plantations to continuous cover management and habitat management within native woodlands.
Many complex, distance-dependent single-tree models have been developed for predicting canopy transmittance and understorey light (e.g. Wang and Jarvis, 1990; Brunner, 1998), but the intensive data requirements of these models (e.g. individual tree positions and crown dimensions) mean that they are of more use as research tools rather than management tools for forestry. For practical purposes, there is therefore a requirement for models, for the key species in Britain, that link canopy transmittance to stand parameters that are routinely measured by foresters.
Basal area is a routinely easily-assessed stand parameter derived from measurements of tree diameters, and several studies have investigated relationships between basal area and transmittance in both conifer and broadleaf forests (e.g. Kuusipalo, 1985; Mitchell and Popovich, 1997; Comeau, 2001; Hale, 2001; Hale, 2003; Sonohat et al., 2004). The theoretical basis of using basal area as a surrogate for canopy transmittance is simplistic, as it essentially ignores site-to-site and age- or size-related variation in the relationship between diameter, sapwood area and leaf area (Gower et al., 1987; Mencuccini and Bonosi, 2001; McDowell et al., 2002; Mencuccini et al., 2005). It also ignores the influence of the spatial distribution of trees crowns and the gaps between them; for example, two stands with the same basal area will have very different canopy transmittance if one is closed canopy and the other has just been thinned (García, 1990). Despite these limitations, relationships between decreasing transmittance and increasing basal area have been found in relatively open stands (Palik et al., 1997; Comeau, 2001; Mitchell, 2001; Balandier et al., 2002; Comeau and Heinemann, 2003; Hale, 2003). In closed canopies, however, the relationship does not hold, with low transmittance irrespective of the basal area (Mitchell and Popovich, 1997; Comeau, 2001; Hale, 2001; Parker et al., 2002). Recent stand management is likely to have a critical influence on the relationship between stand parameters and canopy transmittance. Some authors have considered that it is not the actual basal area that is important, but the proportion removed from a closed stand (Jenkins and Chambers, 1989; Hale, 2003) and the time since this intervention. Stand age and stocking density (stems ha–1) are other parameters that have been considered in conjunction with the basal area (Comeau and Heinemann, 2003; Sonohat et al., 2004) as predictors of understorey light regime.
This study assesses the canopy transmittance of stands of Sitka spruce (Picea sitchensis (Bong.) Carr) and Scots pine (Pinus sylvestris L.), which are the dominant conifers in British forestry (Smith and Gilbert, 2003). It builds on work presented in Hale (2001) and Hale (2003). In Sitka spruce stands, the primary management objective requiring information on canopy transmittance is the transformation of uniform stands to uneven-structured stands, by ensuring seedling growth beneath the canopy (Mason et al., 2004). In many cases, there will be a trade-off between light requirements for seedling success and the growth of competing vegetation (Messier, 1996). Transformation will also be a management objective in some Scots pine plantations. In keeping with the objectives of the Deer Commission Scotland, Forestry Commission Scotland and Scottish Natural Heritage (Anon, 1996; Edwards, 2006; Anon, 2007; Edwards and Duncan, 2007), guidance is required on favourable conditions for obtaining natural regeneration in native pinewoods in either the stand reinitiation or old growth phases (Oliver and Larson, 1990; Edwards and Mason, 2006). Additionally, in many areas the objective of pinewood management is to create or maintain a specific habitat for rare or threatened species of fauna, such as capercaillie (Tetrao urogallus L.) (Moss and Picozzi, 1994; Kortland, 2006) or flora (Anon, 1994; Broome et al., 2004).
The aim of this study is to determine simple relationships between stand parameters and canopy transmittance for Sitka spruce and Scots pine stands that can be incorporated into guidance for forest managers.
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Methods |
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Site descriptions
Measurements of canopy transmittance were obtained from a series of existing experimental plots in both Sitka spruce and Scots pine stands and from additional plots established specifically for this study. In total, 36 Sitka spruce measurement plots were used located across six forests in Scotland, England and Wales and 43 Scots pine plots located across five forests in Scotland (Table 1). The existing plots were 50 x 50 m (one was 40 x 45 m; see Hale, 2003); the additional plots were 30 x 30 m (Sitka spruce) and 40 x 40 m (Scots pine). All plots were surrounded by a buffer of at least 25 m (in most cases equivalent to at least one tree height) from the nearest edge. Summary details of the plots in each forest are given in Table 1. Data from the Sitka spruce stands at Kielder and Aberfoyle were previously presented in Hale (2001) and Hale (2003). Seven of the Sitka spruce measurement plots and one of the Scots pine plots had never been thinned. For the majority of the thinned plots, details of the thinning history were not available. The Sitka spruce plots at Aberfoyle included one previously unthinned plot which was re-assessed after five successive thinnings over a 5-month period (Hale, 2003). The plots at Glasfynydd comprised three plots assessed three times each, after successive thinnings over a number of years. All assessments from these plots are included in the analysis. All other plots were assessed only once. The Scots pine plots at Curr Wood were in a plantation established in 1880, progressing from understorey reinitiation into old growth stage. Two plots at Glenmore were in native pinewoods with a wide range of tree ages on site (
20 to >225 years). All other plots were in first rotation plantation forests.
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Measurement of transmittance
Hemispherical photographs were taken at nine measurement points (seven points in the 30 x 30 m plots) distributed systematically across the plot (Figure 1). The diameter at 1.3 m of all trees in each plot was measured, giving basal area and stocking. A combination of film and digital cameras were used; Hale and Edwards (2002) showed that results from the two systems are comparable over a wide range of canopy openness. A self-levelling mount for the camera was used for all photographs, with the camera levelled at a height of 1.3 m. All photographs were taken either in uniform overcast sky conditions or beneath a clear sky after sunset (Rich, 1990; Fournier et al., 1996).
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The film photography system used a Sigma 8-mm 180º lens mounted on a Nikon camera (FM2, Nikon Corporation, Tokyo, Japan), with Ilford HP5 400ASA (Ilford Imaging, Cheshire, UK) black and white film. To obtain the best contrast between sky and canopy, photographs were taken at the correct automatic exposure and
and one f-stop below this (Chen et al., 1991; Frazer et al., 1997; Hale, 2001). The negatives were processed commercially. The digital system used a Coolpix camera (Coolpix 950, Nikon Corporation) with a fisheye lens (FC-E8, Nikon Corporation), using the settings detailed in Hale and Edwards (2002) to give an image size of 1600 x 1200 pixels. The images were downloaded directly from the camera to a PC hard drive.
For each photopoint, the image with the best contrast between canopy and sky was selected for analysis, by visual assessment on a PC screen. The selected negatives were scanned using a 35-mm negative scanner (LS1000, Nikon Corporation) to give a resolution of 1000 pixels across each image. The digital images from all photopoints were analysed using the Hemiview software (Delta-T Devices, Cambridge, UK), which allows the digital grey-scale image to be classified into ‘sky’ and ‘canopy’, by a user-defined threshold. The threshold was set separately for each image. Photographs from all Scots pine plots and most Sitka spruce plots were analysed by a single operator. Photographs from seven Sitka spruce plots were analysed by another operator with cross-checks to ensure comparability of results.
Hemiview was used to calculate indirect site factor (ISF) and direct site factor (DSF) which represent, respectively, the proportion of diffuse and direct radiation transmitted through the canopy. Assuming that in Britain 65 per cent of incident radiation is diffuse (UK Meteorological Office, 2002), total transmittance (or global site factor (GSF)) was calculated as a weighted average of ISF and DSF as follows:
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GSF represents the proportion of all radiation that is transmitted through the canopy over the course of the year. Throughout the remainder of this paper, ‘transmittance’ refers to GSF. It ranges from 0 (no light transmitted, i.e. completely closed) to 1 (all light transmitted, i.e. completely open). The value of transmittance presented for each plot is the average of the seven or nine photopoints in that plot.
Due to the time constraints associated with collecting these datasets, and in particular the limitation imposed by the stringent requirements for weather conditions (overcast, but not raining), it was not possible to collect sufficient data to have separate datasets for model development and validation.
Statistical analysis
A series of generalized linear mixed models (GLMMs) were fitted to the canopy transmittance data. Following exploratory data analysis of the stand-level data variables, basal area and stocking density were selected as candidate fixed effects and, as a number of measurement plots were located both within the same forest and some reassessed over time, random effects were added for forests and plots within forests. Other features of the GLMM included a log-link function to shape the expected relationship between GSF, basal area and stocking density and a weighting function applied to each GSF measurement, calculated as the reciprocal of the replicate GSF variability per plot.
Standard model fitting procedures were applied. Non-significant factors, variables and interaction terms were removed from the model and residuals were investigated to assess how well the model described the data. Scots pine and Sitka spruce data were analysed separately. All statistical analyses used SAS (SAS Institute Inc., 2003) software.
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Results |
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Canopy transmittance (GSF from Hemiview), averaged over the seven or nine points in each plot, ranged from 0.04 to 0.40 for the Sitka spruce stands and from 0.12 to 0.89 for the Scots pine stands. Standard deviations were typically in the range 0.01–0.03 for Sitka spruce and in the range 0.01–0.05 for Scots pine. Both species showed a general decrease in transmittance with increasing basal area (Figure 2). Scots pine stands had a higher transmittance for a given basal area than Sitka spruce stands (
0.35 at a basal area of 30 m2 ha–1 compared with 0.25 for Sitka spruce at the same basal area).
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Previous work has shown hemispherical photography to be a poor predictor of canopy transmittance in dense canopies (Machado and Reich, 1999
; Comeau, 2000; Hale, 2003). For this reason, plots where the average GSF was <0.1, or where more than two photopoints had an GSF <0.1, were excluded from the statistical analysis. This resulted in the exclusion of 10 Sitka spruce plots, which occurred across the basal area range from 35 to 59 m2 ha–1. The parameters of the log-link model fitted to the data are shown in Table 2. For Sitka spruce, a model using basal area and stocking provided the best fit to the data. Adding an interaction term for basal area and stocking produced no significant improvement. Residuals were approximately normally distributed, with over 80 per cent of them lying within ±0.02 (same units as transmittance) of the measured value. The fitted model indicates a decrease in transmittance as basal area increases and as stocking increases.
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= exp(a + b x G + c x S), where The best model fit for Scots pine used basal area alone. Addition of terms for stocking and the interaction between basal area and stocking did not significantly increase the fit. Residuals were approximately normally distributed, with over 80 per cent of them lying within ±0.05 of the measured value.
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Discussion |
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The model fitted to the Sitka spruce data predicts that transmittance is higher for a given basal area in a stand with fewer (and by inference, larger) trees. This is because tree-scale clumping will result in the foliage being more irregularly distributed across the stand (Chen and Cihlar, 1995a
, b). Scots pine canopies have a higher transmittance than Sitka spruce because Scots pine stems support a lower leaf area than Sitka spruce (Landsberg et al., 1973; Beadle et al., 1982; Ford, 1982; Bormann, 1990; Mencuccini and Bonosi, 2001). In contrast to the model for Sitka spruce, transmittance in the Scots pine stands was found to be best described by basal area alone; stocking did not significantly improve the fit. This may be attributed in part to the lower leaf area density of Scots pine canopies compared with Sitka spruce (Whitehead, 1978; Whitehead et al., 1984; Mencuccini and Grace, 1995), which allows higher transmittance through the tree crowns themselves as well as through gaps between the trees. This will reduce the importance of clumping at the tree scale. The absence of stocking in the model may also be due to a high correlation between basal area and stocking for the Scots pine stands, which was not found for the Sitka spruce stands. This is not simply an artefact of the wider spread of data in the Scots pine stands, because the correlation remained strong even when assessed over the same basal area range as found in the Sitka spruce dataset. This may be a function of the different management strategies that are normally adopted for these two species, but further assessments would need to be done in a wider range of stands to confirm this.
The combination of basal area and stocking density to estimate canopy transmittance has parallels with the use of stand density index (SDI; Reineke, 1933) as a measure of site occupancy. Reineke proposed the relationship SDI = N·(Dq/25.4)b for fully stocked, non-managed stands, invariant across locations and species, where N is stems ha–1, Dq is the quadratic mean diameter (cm) and b = 1.605. The ratio of SDI to a maximum SDI for a given species is used as a measure of stand occupancy in the development of stand density management diagrams (Jack and Long, 1996). However, the value of the exponent, b, has since been shown to vary with species, age, tree size and location (Zeide, 2005). SDI could provide a useful tool for stand management in British forestry when it has been parameterized for the relevant species growing in British conditions, and work is currently ongoing for Sitka spruce.
We know of no other directly comparable published measurements comparing transmittance of Sitka spruce forest canopies with stand parameters, either in Britain or abroad, for comparison with our results. Sonohat et al. (2004) studied the relationship between transmittance of light and stand parameters in a range of coniferous canopies in France and Belgium, including Scots pine and Norway spruce (Picea abies (L.) Karst), although stocking was not considered in their analysis. The values of transmittance that they recorded in Scots pine stands were slightly lower than those presented here for a given basal area. This may be due to differences in measurement technique or site differences between the two studies could lead to different relationships between stem diameter and leaf area (Mencuccini and Bonosi, 2001). As in the present study, they found that transmittance in Scots pine stands was best described using basal area alone, whereas other species required the addition of other parameters (time since last thinning, intensity of thinning and stand age) to improve the relationship. In line with the results presented here, their results showed that transmittance in spruce stands (in their case, Norway spruce) was lower than in Scots pine stands of equivalent basal area.
Implications for stand management
Previous work has suggested that the basal area of a Sitka spruce stand must be <30 m2 ha–1 to transmit sufficient light for seedling growth (Page et al., 2001). However, standard management of Sitka spruce tends to maintain a basal area of over 30 m2 ha–1. Other studies have also shown that the minimum transmittance for Sitka spruce and Scots pine seedling growth is 0.2 and 0.35, respectively, to achieve acceptable growth (defined as 50 per cent of the growth that would be achieved under full light (Hale et al., 2004; Mason et al., 2004)). Results from the model derived in this study suggest that a Sitka spruce stand with a basal area of 30 m2 ha–1 should have a stocking of <460 stems ha–1 to achieve the light levels required for Sitka spruce seedling growth. This is supported by observations from the plots used for this study. Seven plots with basal area in the range 28–32 m2 ha–1, and a stocking of <350 stems ha–1, all had transmittance >0.25 and good natural regeneration. In contrast, one plot with a basal area of 25 m2 ha–1, and 590 stems ha–1, had low transmittance (0.13) and no regeneration.
As Sitka spruce in Britain is commonly grown in upland areas, the risk of wind damage to the stand must be considered. It is essential that any stand manipulation aiming to increase seedling growth is done after assessing the vulnerability of the stand to wind damage, preferably with use of a wind risk model such as ForestGALES (Gardiner et al., 2004, 2008). In addition to leaving the stand prone to wind damage, over-thinning could allow the growth of competing vegetation, such as grasses (e.g. Deschampsia flexuosa L. (Trin.) and Molinia caerulea L. (Moench)) and rushes (e.g. Juncus inflexus L.), which might prevent seedlings from thriving.
Obtaining Scots pine seedling growth beneath a Scots pine canopy is complicated by the fact that if there is sufficient light for seedling growth, this will also favour the growth of competing vegetation such as heather (Calluna vulgaris (L.) Hull) and bilberry (Vaccinium myrtillus L.). The percentage cover of heather has been found to increase steadily from 10 per cent at relative light levels of 0.2, up to 90 per cent in full light (Parlane et al., 2006). Bilberry has a wide shade tolerance, with an optimal light level for bilberry cover of 0.35; above 0.5, it tends to be outcompeted by heather (Parlane et al., 2006). For Scots pine, the model predicts that the minimum transmittance for seedling growth of 0.35 will be achieved by a basal area of 24 m2 ha–1 or less. If light is thought to be limiting Scots pine regeneration, then stand manipulation can be used to alter the light regime by reducing the basal area. Ideally, timing of intervention should be coincident with a good seed fall, to allow seedlings to establish before the site is colonized by competing vegetation. If a stand has a sufficiently low basal area to suggest that light is not limiting, then other measures, such as ground disturbance, must be considered if seedling growth is desired.
Limitations of the models
The Sitka spruce stands used in this study covered a narrower range of basal areas than the Scots pine stands, with very few plots <20 m2 ha–1. This is because traditional forest management retains relatively high basal areas throughout a rotation (Edwards and Christie, 1981), so stands with low basal area are uncommon. As stands become more widely managed using CCF, however, low basal area Sitka spruce stands may become more widespread. The youngest stands in this study were 35 years old (Sitka spruce) and 39 years old (Scots pine), whereas in many cases, stand manipulation to increase canopy transmittance should start at an earlier age to encourage growth of wind-firm stands (Mason, 2002; Hale et al., 2004). The relationships developed here should be applied cautiously outwith the data range used for this study. Ideally, further measurements should be made to extend the data range available and to allow for model validation.
For most of the stands in this study, the thinning history was not known in detail. Other studies have shown that the proportion of basal area removed, and the time since intervention, have an important influence on the understorey light regime (Jenkins and Chambers, 1989; Beaudeat and Messier, 2002; Hale, 2003; Prévost and Pothier, 2003; Sonohat et al., 2004). Many of the Sitka spruce stands in this dataset had been thinned recently (immediately before or within one growing season of measurements being made). The relationship derived for Sitka spruce may therefore be biased towards recently thinned stands and might overestimate the transmittance of stands that had not been thinned within the last 3–4 years.
With the exception of the native pinewood stands, all the data were collected in even-aged plantation forests. The relationships derived between stand parameters and canopy transmittance may therefore have limited applicability to irregular-structured stands. Again, the paucity of such stands in Britain makes this a challenging area to investigate, complicated by the fact that irregular stands are commonly also composed of a mixture of species.
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Conclusions |
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This study has provided the most comprehensive quantification of the canopy transmittance beneath Sitka spruce and Scots pine stands in Britain to date. The models developed relating transmittance to stand parameters can form a valuable contribution to guidelines on forest management techniques that will be required to meet changing objectives, specifically the early stages of transformation from even-aged to irregular structure, and they should be incorporated into overall forest guidance rather than used in isolation. In particular, individual stand assessments are important to gauge the status of other factors that influence stand dynamics, e.g. seed supply, site fertility and browsing pressure. This can then ensure that interventions are timed to optimize the likelihood of management objectives being met. However, further understanding is needed on the light requirements of species of vegetation that commonly compete against tree seedlings for resources in these woodland types, particularly the brackens and grasses found beneath Sitka spruce stands. More work is also required on the light requirements of saplings, in order to allow forest managers to ensure that successful regeneration is subsequently recruited into the overstorey.
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Conflict of Interest Statement |
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None declared.
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Acknowledgements |
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Many thanks to staff at the Forest Research field stations, Talybont and Newton, and also a succession of students, for taking hemispherical photographs and stand assessments; to Arne Pommerening and the Tyfiant Coed team (Bangor University) and Alice Broome (Forest Research) for providing stand data from their field experiments; to Forestry Commission and private managers for providing access to their forests and to Miriam Baldwin (Forest Research) for preliminary data analysis.
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References |
|---|
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|---|
Anon. UK Biodiversity Action Plan (1994) London: HMSO. 189.
Anon. Deer (Scotland) Act 1996 (1996) London: HMSO.
Anon. The UK Forestry Standard – The Government's Approach to Sustainable Forestry (2004) Edinburgh: Forestry Commission. 74.
Anon. Joint working on wild deer: present rules for interpreting measures of habitat condition and impacts. (2007) Draft Staff Guidance, Version 3 (unpublished internal document) Scottish Natural Heritage.
Balandier P, Ruchard F, Pauwels D, Jouvie R. Predicting Light Transmission Through Canopies of Larch Stands (Larix sp.) in France and Belgium. Improvement of Larch (Larix sp.) for Better Growth, Stem Form and Wood Quality, Gap (Hautes-Alpes) – Auvergne and Limousin. Paques LE, ed. (2002) F-45166 Olivet Cedex, France: INRA, Unite d’Amelioration, de Genetique et de Physiologie des Arbres forestiers. 340–349.
Beadle CL, Talbot H, Jarvis PG. Canopy structure and leaf area index in a mature Scots pine forest. Forestry (1982) 55:2–19.
Beaudeat M, Messier C. Variation in canopy openness and light transmission following selection cutting in northern hardwood stands: an assessment based on hemispherical photographs. Agric. For. Meteorol. (2002) 110:217–228.[CrossRef]
Bormann BT. Diameter-based biomass regression models ignore large sapwood-related variation in Sitka spruce. Can. J. For. Res. (1990) 20:1098–1104.[CrossRef]
Broome A, Quine C, Trout R, Poulsom E, Mayle B. Research in Support of the UK Biodiversity Action Plan: Forest Management and Priority Species. (2004) In Forest Research Annual Report and Accounts 2003-2004. TSO, Edinburgh. pp. 112–125.
Brunner A. A light model for spatially explicit forest stand models. For. Ecol. Manage. (1998) 107:19–46.[CrossRef]
Chen JM, Cihlar J. Plant canopy gap-size analysis theory for improving optical measurements of leaf-area index. Appl. Opt. (1995a) 34:6211–6222.[CrossRef][Web of Science]
Chen JM, Cihlar J. Quantifying the effect of canopy architecture on optical measurements of leaf area index using two gap size analysis methods. IEEE Trans Geosci Remote Sens (1995b) 33:777–787.[CrossRef]
Chen JM, Black TA, Adams RS. Evaluation of hemispherical photography for determining plant area index and geometry of a forest stand. Agric. For. Meteorol. (1991) 56:129–143.[CrossRef]
Comeau P. Measuring Light in the Forest. Extension Note 42 (2000) British Columbia: Ministry of Forests Research Program. 7.
Comeau P. Relationships between stand parameters and understorey light in boreal aspen stands. B C J. Ecosyst. Manage. (2001) 1:1–8.
Comeau PG, Heinemann JL. Predicting understorey light microclimate from stand parameters in young paper birch (Betula papyrifera Marsh) stands. For. Ecol. Manage. (2003) 180:303–315.[CrossRef]
Edwards C. A baseline woodland monitoring system in native pinewoods: agreed methodology. In: Internal Protocol (unpublished internal document) (2006) Forest Research.
Edwards PN, Christie JM. Yield Models For Forest Management. Forestry Commission Booklet 48 (1981) London: HMSO.
Edwards C, Duncan C. Monitoring the Woodland Structure in Ballochbuie Pinewood. Unpublished report (2007) Edinburgh: Forest Research, Forestry Commission.
Edwards C, Mason WL. Stand structure and dynamics of four Scots pine (Pinus sylvestris L.) woodlands in northern Scotland. Forestry (2006) 79:261–277.
Ford ED. High productivity in a polestage Sitka spruce stand and its relation to canopy structure. Forestry (1982) 55:1–17.
Fournier RA, Landry R, August NM, Fedosejevs G, Gauthier RP. Modelling light obstruction in three conifer forests using hemispherical photography and fine tree architecture. Agric. For. Meteorol. (1996) 82:47–72.[CrossRef]
Frazer GW, Trofymow JA, Lertzman KP. A method for estimating canopy openness, effective leaf area index, and photosynthetically active photon flux density using hemispherical photography and computerized image analysis techniques. In: Information Report BC-X-373 (1997) Victoria, British Columbia: Pacific Forestry Centre. 73.
García O. Growth of thinned and pruned stands. In: New Approached to Spacing and Thinning in Plantation Forestry—James RN, Tarlton GL, eds. (1990) N Z Ministry of Forestry, Rotorua, New Zealand. 1–10.
Gardiner B, Suarez J, Achim A, Hale S, Nicoll B. ForestGALES 2 – A PC-Based Wind Risk Model for British Forests. User Guide (2004) Forestry Commission, Edinburgh.
Gardiner BA, Byrne K, Hale SE, Kamimura K, Mitchell S, Peltola H, et al. A review of mechanistic modelling of wind damage risk to forests. Forestry (2008) 81:447–463.
Gower ST, Grier CC, Vogt DJ, Vogt K. Allometric relations of deciduous (Larix occidentalis) and evergreen conifers (Pinus contorta and Pseudotsuga menziesii) of the Cascade Mountains in central Washington. Can. J. For. Res. (1987) 17:630–634.[CrossRef]
Hale SE. Light regime beneath Sitka spruce plantations in northern Britain: preliminary results. For. Ecol. Manage. (2001) 151:61–66.[CrossRef]
Hale SE. The effects of thinning intensity on the below-canopy light environment in a Sitka spruce plantation. For. Ecol. Manage. (2003) 179:341–349.[CrossRef]
Hale SE, Edwards CE. Comparison of film and digital hemispherical photography across a wide range of canopy densities. Agric. For. Meteorol. (2002) 112:51–56.[CrossRef]
Hale SE, Levy PE, Gardiner B. Trade-offs between seedling growth and stand stability in alternative silvicultural systems: a modelling analysis. For. Ecol. Manage. (2004) 187:105–115.[CrossRef]
Jack SB, Long JL. Linkages between silviculture and ecology: an analysis of density management diagrams. For. Ecol. Manage. (1996) 86:205–220.[CrossRef]
Jenkins MW, Chambers JL. Understorey light levels in mature hardwood stands after partial overstory removal. For. Ecol. Manage. (1989) 26:247–256.[CrossRef]
Jennings SB, Brown ND, Sheil D. Assessing forest canopies and understorey illumination: canopy closure, canopy cover and other measures. Forestry (1999) 72:59–73.
Kortland K. Forest Management for Capercaillie (2006) Capercaillie BAP Group. 43.
Kuusipalo J. On the use of tree stand parameters in estimating light conditions below the canopy. Silva Fenn. (1985) 19:185–196.
Landsberg JJ, Jarvis PG, Slater MB. The radiation regime of a spruce forest. In: Plant Response to Climatic Factors Proceedings of the Uppsala Symposium, 1970—Slayter RO, ed. (1973) Paris: UNESCO. 411–417.
Lieffers VJ, Messier C, Stadt KJ, Gendron F, Comeau PG. Predicting and managing light in the understorey of boreal forests. Can. J. For. Res. (1999) 29:796–811.[CrossRef]
Machado J-L, Reich PB. Evaluation of several measures of canopy openness as predictors of photosynthetic photon flux density in deeply shaded conifer-dominated forest understorey. Can. J. For. Res. (1999) 29:1438–1444.[CrossRef]
Mason WL. Are irregular stands more windfirm? Forestry (2002) 75:347–355.
Mason WL, Kerr G, Simpson J. What is Continuous Cover Forestry? Information Note 29 (1999) Edinburgh: Forestry Commission. 8.
Mason WL, Edwards C, Hale SE. Survival and early seedling growth of conifers with different shade tolerance in a Sitka spruce spacing trial and relationship to understorey light climate. Silva Fenn. (2004) 38:357–370.
McDowell N, Barnard H, Bond BJ, Hinckley TM, Hubbard RM, Ishii H, et al. The relationship between tree height and leaf area: sapwood area ratio. Oecologia (2002) 132:12–20.[CrossRef][Web of Science]
Mencuccini M, Bonosi L. Leaf/sapwood area ratios in Scots pine show acclimation across Europe. Can. J. For. Res. (2001) 31:442–456.[CrossRef]
Mencuccini M, Grace J. Climate influences the leaf area/sapwood area ratio in Scots pine. Tree Physiol. (1995) 15:1–10.
Mencuccini M, Martínez-Vilalta J, Hamid HA, Korakaki E, Lee S, Michiels B. Size-mediated ageing reduces vigour in trees. Ecol. Lett. (2005) 8:1183–1190.[CrossRef][Web of Science]
Messier C. Managing light and understorey vegetation in boreal and temperate broadleaf-conifer forests. In: Silviculture of Temperate and Boreal Broadleaf-Conifer Mixtures—Comeau PG, Thomas KD, eds. (1996) Victoria, BC: BC Ministry of Forests. 59–81. Land Management Handbook No. 36.
Mitchell AK. Growth limitations for conifer regeneration under alternative silvicultural systems in a coastal montane forest in British Columbia, Canada. For. Ecol. Manage. (2001) 145:129–136.[CrossRef]
Mitchell JE, Popovich SJ. Effectiveness of basal area for estimating canopy cover of ponderosa pine. For. Ecol. Manage. (1997) 95:45–51.[CrossRef]
Moss R, Picozzi N. Management of forests for Capercaillie in Scotland. In: Bulletin No. 113 (1994) London: HMSO. 32.
Oliver CD, Larson BC. Forest Stand Dynamics (1990) New York: McGraw-Hill. 467.
Page LM, Cameron AD, Clarke GC. Influence of overstorey basal area on density and growth of advance regeneration of Sitka spruce in variably thinned stands. For. Ecol. Manage. (2001) 151:25–35.[CrossRef]
Palik BJ, Mitchell RJ, Houseal G, Pedersen N. Effects of canopy structure on resource availability and seedling responses in a longleaf pine ecosystem. Can. J. For. Res. (1997) 27:1458–1464.[CrossRef]
Parker GG, Davis MM, Chapotin SM. Canopy light transmittance in Douglas-fir – western hemlock stands. Tree Physiol. (2002) 22:147–157.
Parlane S, Summers RW, Cowie NR, van Gardingen PR. Management proposals for bilberry in Scots pine woodland. For. Ecol. Manage. (2006) 222:272–287.[CrossRef]
Prévost M, Pothier D. Partial cuts in a trembling aspen – conifer stand: effects on microenvironmental conditions and regeneration dynamics. Can. J. For. Res. (2003) 33:1–15.[CrossRef]
Reineke LH. Perfecting a stand-density index for even-aged forests. J. Agric. Res. (1933) 46:627–638.[CrossRef]
Rich P. Characterizing plant canopies with hemispherical photographs. Remote Sens. Rev. (1990) 5:13–29.
SAS Institute Inc. The GLIMMIX Procedure (Experimental), Version 9.1 (2003) Cary, NC: SAS Institute Inc.
Smith S, Gilbert J. National Inventory of Woodland and Trees – Great Britain (2003) Edinburgh: Forestry Commission. 68.
Sonohat G, Balandier P, Ruchard F. Predicting solar radiation transmittance in the understorey of even-aged coniferous stands in temperate forests. Ann. For. Sci. (2004) 61:629–641.[CrossRef]
UK Meteorological Office. Land Surface Stations data (1900-2000) (2002) British Atmospheric Data Centre. http://badc.nerc.ac.uk/data/surface/ (accessed on 1 October, 2002).
Wang YP, Jarvis PG. Description and validation of an array model – MAESTRO. Agric. For. Meteorol. (1990) 51:257–280.[CrossRef]
Whitehead D. The estimation of foliage area from sapwood basal area in Scots pine. Forestry (1978) 51:137–149.
Whitehead D, Edwards WRN, Jarvis PG. Conducting sapwood area, foliage area, and permeability in mature trees of Picea sitchensis and Pinus contorta. Can. J. For. Res. (1984) 14:940–947.[CrossRef]
Zeide B. How to measure stand density. Trees (2005) 19:1–14.[CrossRef]
Received 25 February 2009.
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