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Regeneration dynamics of Sitka spruce in artificially created forest gaps
Forest Ecology and Management 221 (2006) 260–266 www.elsevier.com/locate/foreco
Regeneration dynamics of Sitka spruce in artificially created forest gaps L.M. Page, A.D. Cameron * School of Biological Sciences, University of Aberdeen, Cruickshank Building, St. Machar Drive, Aberdeen AB24 3UU, UK Received 31 March 2005; received in revised form 14 July 2005; accepted 3 October 2005
Abstract This study examined the variation in the development of naturally regenerated and planted seedlings of Sitka spruce (Picea sitchensis (Bong.) Carr.) within gaps cut in a 32-year-old stand of the same species. The circular gaps were 20 m in diameter and designed to allow sunlight into only half of the gap floor at midsummer given the latitude of 568450 N. Eight plots (8 m 3 m) were laid out along a north–south transect through each gap (four within the gap and two each under the closed canopy north and south of the gap). Each plot was sub-divided and seedlings were planted into one part and the other part was left to naturally regenerate. In subsequent seasons, plots were further subdivided into ‘weed free’ and ‘vegetation left untouched’. Results showed that while the two central plots within the gaps had the highest value of canopy openness, the highest accumulated temperature and lowest soil moisture were recorded in plots that received direct sunlight. However, level of germination was significantly higher in the shaded area of the gap than in the part that received direct sunshine suggesting that higher moisture levels in shaded areas are important to successful germination. Minimal germination was recorded in the plots beneath the canopy. Seedling survival was significantly influenced by the influx of competing vegetation, but only in the part of the gaps that received direct sunlight. The success of Sitka spruce regeneration within gaps appears to depend on sufficient moisture and light to support regeneration and early growth, but not too much light to encourage the development of competing vegetation. The permanently shaded areas of the gaps appeared to offer ground conditions with sufficient moisture and light to ensure successful germination and early growth of seedlings, but without excessive competition from other vegetation. # 2005 Elsevier B.V. All rights reserved. Keywords: Sitka spruce; Forest gaps; Natural regeneration
1. Introduction The process of transforming largely even-aged plantation forests into more diverse structures has become an important feature of forest management in many temperate regions of the world. One of the simplest approaches to transformation involves creating discrete openings in regular stands and the establishment of small centres or groups of regeneration continuously over a long period until a ‘balanced’ distribution of size classes is achieved (Smith et al., 1997). Previous research on transformation using groups has concentrated on germination and early growth of the target species, particularly in relation to optimising gap size (e.g. Smith, 1986; Marquis, 1989; Malcolm et al., 2001) and gap light conditions (e.g. McLaughlin, 1978; Poulson and Platt, 1989; Canham, 1989; Canham et al., 1990; Lieffers et al., 1999). However, there is
* Corresponding author. Tel.: +44 1224 272673; fax: +44 1224 272685. E-mail address: [email protected] (A.D. Cameron). 0378-1127/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2005.10.006
insufficient understanding of how variation in micro-environmental conditions, such as temperature and soil moisture, within gaps influences the process of regeneration. The forest floor is a heterogeneous environment where a wide range of conditions can be experienced (Lieberman et al., 1989). Growth of young trees is closely associated with the amount of light received (e.g. Minckler, 1961); with the highest light level found in the centre of a gap, and the lowest at the edge. The closer young trees are to the edge of gaps, the greater the influence that the surrounding trees will have on their growth and development (Malcolm et al., 2001). However, the light environment within gaps is not uniform from the centre to the edge because of the angle of sunlight experienced in cool temperate regions at higher latitudes. In the northern hemisphere, some direct sunlight may reach the forest floor on the north side of even relatively small gaps while the south side of the gap remains in permanent shade. This creates an imbalance in the microenvironment across gaps in a north to south direction. Understanding the effect of this imbalance in environmental conditions on germination and establishment
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of natural regeneration and on the survival and growth of planted seedlings is essential if the group system is to be used successfully for transforming largely even-aged stands into irregular structures. The aim of this study was to determine whether the variation in exposure by sunlight within artificially created gaps influences the development of natural regeneration and planted seedlings of Sitka spruce (Picea sitchensis (Bong.) Carr.). Sitka spruce is classed as being moderately shade tolerant (Minore, 1979) and is considered a suitable species for group regeneration (Malcolm et al., 2001).
2. Materials and methods 2.1. Study site and creation of gaps The experimental site is located in Kindrogan Forest in Tay Forest District, Mid-Scotland (568450 N, 38340 W, National Grid Reference no. 041629). The mean annual rainfall is 1164 mm. Soils are mainly well-drained iron pans with an underlying geology of Upper Dalradian quartz–mica schist. The study area (compartment 5711a) is located on a north facing slope (68) at a mean altitude of 370 m. It was planted in 1965 with Sitka spruce and has been thinned once in 1993. Average top height of the stand at the time of the experiment was 17 m. Six locations for cutting gaps were randomly identified in spring 1997. Taking into account the angle of the sun at this latitude and the slope of the ground, a gap diameter of 20 m (measured from the stem) was calculated to allow direct sunlight to reach the floor of up to half the gap area for a few months of the year (Fig. 1). The trees were felled and the harvest residues removed. No machines were permitted to enter the gaps to avoid soil compaction. No other site preparation was undertaken. Initial ground vegetation was very sparse. The surface mainly comprised of litter (depth 3 cm) with a few patches of bryophytes (mainly Polytrichum spp.).
Fig. 1. Diagram of the cross-section of a gap and the location of the plots A–H. Plots E–H receive direct sunlight for part of the year whereas plots A–D are in permanent shade. The figure is approximately to scale so the sun paths are accurate.
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2.2. Sample plots and treatments Eight plots (8 m 3 m) were laid out along a N–S transect through each gap (Fig. 1). Four plots were located within the gap area (C–F) and two each under the closed canopy south (A and B) and north (G and H) of the gap. The plots were partially protected from deer damage by 1.2 m high fences. Three 2 m 2 m subplots were laid out within each main plot, 0.5 m apart and 0.5 m from the fence. Each sub-plot was randomly assigned to one of three treatments: (1) planted with Sitka spruce seedlings; (2) sown with Sitka spruce seed; (3) left untouched for natural regeneration to establish from the seed rain. The purpose of sowing seed was to provide a fallback should natural regeneration fail. This was a necessary precaution since good seed years only occur with Sitka spruce every 3–5 years on average (Harris, 1990). The 2 m 2 m subplots were designed to be subdivided in the second year of the experiment into four 1 m2 square plots. Two of these smaller plots were randomly selected for weeding while the remaining two were left untouched. The weeded areas were hand weeded throughout the spring and summer of the second growing season to remove all herbaceous vegetation, namely Deschampsia caespitosa (L.) Beauv. and Juncus effuses (L.). Therefore, the experimental design consisted of six gaps (replicates), eight plots per gap and two weeding treatments per plot. Forty-nine, 2-year-old Sitka spruce seedlings (22–25 cm tall) of Queen Charlotte Island (QCI) provenance were planted within the 2 m 2 m subplots at 0.32 m spacing in April 1998. Seed, also of QCI provenance, were sown in the 2 m 2 m subplots in parallel rows at a density of 30 m2 in April 1998. No weeding treatment was applied. The experimental design consisted of six gaps (replicates) and eight plots per gap. 2.3. Measurements Weekly measurements were made on the natural regeneration plots during the first two growing seasons (May– September). Each new germinant was marked with a numbered tag indicating the day that it was first observed. Subsequently, weekly monitoring noted stage of development (i.e. 1- or 2year-old seedlings) and mortality. Valid measurements beyond this were not possible due to the erratic nature of seedling losses. Weekly measurements were also made in the planted seedling plots. Due to the relatively large size of the planted seedlings in relation to the developing ground vegetation, weeding was not necessary. Level of mortality and height growth were assessed over five growing seasons. Browsing damage by deer, observed as loss of apical or side shoots, was recorded within the plots in the second year of measurement (1999). 2.4. Environmental monitoring Temperature was monitored within each plot using Orion Tinytalk II Data Loggers (Gemini Data Loggers (UK) Ltd.)
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located between the seedling and seeded plots. Due to the high cost of equipment, instruments were not placed in the two extreme plots under the canopy. The data loggers were programmed to record temperature every 48 min so that maximum and minimum daily temperatures could be determined in addition to accumulated temperature. Accumulated temperature was measured in day-degrees above 5 8C. An attempt was made to monitor relative humidity in the same way as temperature; however, the humidity loggers could not function properly in the condensing atmosphere of the gaps. Soil moisture probes (Theta Probe type ML1, Delta T Devices Ltd.) were connected to Delta 2LE Data Loggers and placed close to the temperature loggers. These probes monitor moisture at a soil depth of approximately 5 cm. The light environment was determined at the beginning of each growing season for 5 years by taking hemispherical photographs (hemiphots) at each plot, following the methods of Whitmore (1993) and Easter and Spies (1994). Hemiphots were taken with a Nikon 8 mm super wide-angle lens attached to a Nikon FM2 camera. The camera was attached to a tripod at 1.5 m above ground level and the top of the image orientated to magnetic north. The hemiphots were analysed using WINPHOT image analysis software (Steege, 1994), which provided a value for canopy openness. 2.5. Statistical analysis Analysis of variance was carried out on environmental data (canopy openness, accumulated temperature and soil moisture); germination, seedlings growth and browsing damage, and planted seedling growth. All data were analysed per plot (i.e. mean plot values were used). Correlation coefficients and level of significance of relationships between variables were determined. These were calculated using the appropriate environmental data for each year of the study to take account of gradually changing conditions within the gaps. 3. Results 3.1. Environmental conditions Given the similarity of environmental data recorded within the gaps over 5 years, only those for year 1 only are described here. Percentage canopy openness varied from 5% beneath the canopy, to 54% around the centre of the gap (Fig. 2). Accumulated temperature varied considerably across the gap. Not surprisingly, higher values were found in plots receiving direct sun (E and F) in comparison with the shaded plots (C and D) (P < 0.05) and those beneath the canopy (B and G) (P < 0.01) (Fig. 3). Soil moisture was significantly higher in the gap than under the canopy (P < 0.01) (Fig. 4). The plots in the northern part of the gap that received direct sunlight (E and F) had a lower soil moisture level in comparison with plots in the shade (C and D) (P < 0.05). Plots under the canopy (A, B, G and H) had the lowest soil moisture level in comparison with plots within the gaps (P < 0.05).
Fig. 2. Canopy openness (%) (measured in year 1) measured at 1.5 m above ground level in plots beneath the canopy (plots A, B, G and H) and within 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. Bars = 1 S.E.
3.2. Seedling germination Natural regeneration from the seed rain was evident in the plots. However, it was obvious that the sown seed had not survived since it had been sown in straight lines at a fixed distance between rows, and there were no significant differences between the number of seedlings between nonsown and sown plots. Direct sowing was a fallback treatment only, should natural regeneration from the seed rain have failed. Therefore, the sown treatment was ignored. The number of germinating seedlings varied across the gaps. The highest level of germination occurred in the permanently shaded plots (C and D) in comparison with the plots receiving direct sun (P < 0.05) during the two years of observation (1998 and 1999) (Fig. 5). The weeded plots had a higher level of germination than the non-weeded plots within the gap (plots C–F), but this was only significantly different in plot E (P < 0.01) (within the ‘direct sun’ plot nearest to the plot centre) (Fig. 6). 3.3. Survival and growth of planted seedlings The level of mortality was greatest under the canopy, where very few seedlings in the plots furthest from the gaps survived the first growing season and all had died by the end of the second growing season. Level of mortality increased in all plots over time with the highest levels under the canopy (Fig. 7).
Fig. 3. Accumulated temperature (8C) (measured in year 1) measured at ground level in plots beneath the canopy (plots B and G) and within 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. Plots A and H were not recorded due to high cost of data loggers. Bars = 1 S.E.
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Fig. 4. Soil moisture content (m3/m3) (measured in year 1) measured at approximately 5 cm soil depth in plots beneath the canopy (plots A, B, G and H) and within 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. Bars = 1 S.E.
Annual increment of the planted seedlings was greatest in the gaps in comparison with those under the canopy (Fig. 8). At the end of the first growing season, seedling height was not significantly different between plots, but from the second season onwards, seedling height was greater in plot E (i.e. plot closest to the gap centre receiving direct sunlight) in comparison with the other plots (P < 0.05). Shoot increment increased over time in plots within the gaps, but decreased under the canopy. 3.4. Browsing damage on planted seedlings There was no browsing in the first growing season (1998); however, by the summer of 1999 it was observed that deer had browsed some of the planted seedlings. While some variation in percentage browsing was observed between plots, these differences were not significantly different (Fig. 9). 3.5. Correlations between seedling germination and growth, and environmental parameters Number of germinating seedlings was strongly and positively correlated with openness (1999, r = 0.87) and soil
Fig. 5. Number of living and dead germinants after 2 years (measured October 1999) within plots beneath the canopy (plots A, B, G and H) and within 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. Bars = 1 S.E.
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Fig. 6. Number of living and dead germinants after 2 years (measured October 1999) within weeded (w) and not weeded (nw) plots within the 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. No vegetation colonised the plots beneath the canopy (A, B, G and H). Bars = 1 S.E.
moisture (1998, r = 0.77; 1999, r = 0.84), but not accumulated temperature (Table 1). Growth of the planted seedlings, i.e. shoot extension, was positively related with openness, soil moisture and accumulated temperature. Level of mortality of the planted seedlings, however, was strongly and negatively associated with the environmental parameters measured. The correlation between mortality and canopy openness increased each year for five growing seasons. These results suggest that seedling mortality increased with lower light levels, decreasing temperature and soil moisture. The relationship between the various environmental measures and shoot extension of the planted seedlings was significant in most years (Table 1). 4. Discussion Variations in microclimate conditions recorded within the gaps in this study were similar to those found by Wright et al. (1998) and Gray and Spies (1997). While the two central plots within the gaps had the highest value of canopy openness, the highest accumulated temperature and lowest soil moisture were recorded in plots that received direct sunlight (Figs. 2–4). The relatively small gap size used in the current study, with a diameter/height ratio of approximately 1, shows that the
Fig. 7. Level of mortality (%) of planted seedlings from 1998 to 2002 of planted seedlings within plots beneath the canopy (plots A, B, G and H) and within 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. Bars = 1 S.E.
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Fig. 8. Annual increment (mm) from 1999 to 2002 of planted seedlings within plots beneath the canopy (plots A, B, G and H) and within 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. None of the seedlings survived in plots A and H. Bars = 1 S.E.
Fig. 9. Percentage browsing after two growing season of planted seedlings within plots beneath the canopy (plots A, B, G and H) and within 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. None of the seedlings survived in plots A and H. Bars = 1 S.E.
amount of light reaching the forest floor was relatively low. The openness value within the gap centre was only 52–54%. Openness levels approaching 100% are only found in gaps with diameter/height ratios greater than 2 (Berry, 1964; Minckler, 1961). Even with this relatively small gap size, a steep openness/indirect site factor gradient was found, both across the gap itself and when passing from gap to below canopy positions. At the gap edge, openness fell from between 27 and 32% to between 10 and 13%, 5 m under the canopy. A further decrease to only 6–8% with another 5 m distance under the canopy reduced openness to levels typically found under closed canopy (Hale, 2001). The influence of the gap did not extend as far under the canopy as might have been expected based on other studies (Canham, 1989; Canham et al., 1990), although
lower dead branches tend to remain on the stems of Sitka spruce for many years and contribute to the shading of the forest floor. Level of germination was significantly higher in the shaded area of the gap than in the part that received direct sunshine. Surface conditions are important for germination (Harmon, 1987) and that litter surfaces will not be favourable if they become too dry (Nixon and Worrell, 1999). A significant positive relationship was found between soil moisture and level of germination in the current study (Table 1). Surface moisture content is crucial to germination and a significant positive association is known to exist between moisture content and germination rates of Sitka spruce (Howells, 1966). Excessive drying of the seedbed can lead to poor survival of Sitka spruce seedlings (Harmon, 1987). Sitka spruce seeds are very small
Table 1 Correlation coefficients (r values) of environmental factors and seed germination and growth of planted seedlings in 20 m diameter gaps within 32-year-old planted Sitka spruce forest Openness (%) Openness (%) Accumulated temperature (8C), 1998 Accumulated temperature (8C), 1999 Soil moisture (m3/m3)
1 0.694* 0.673* 0.814**
Natural regeneration Number of germinants/m2, 1998 Number of germinants/m2, 1999
0.551 0.872**
Planted seedlings Cumulative mortality Cumulative mortality Cumulative mortality Cumulative mortality Cumulative mortality Shoot Shoot Shoot Shoot
extension extension extension extension
(%), (%), (%), (%), (%),
(mm), (mm), (mm), (mm),
1998 1999 2000 2001 2002
1999 2000 2001 2002
Browsed/damaged (%), 1999 * **
P < 0.05. P < 0.01.
0.786* 0.834** 0.852** 0.919** 0.961** 0.905** 0.787 0.901** 0.966** 0.002
1998 1 0.980** 0.536
1999
1 0.604
0.216 0.170
0.119 0.206
0.711* 0.685* 0.654 0.755* 0.794*
0.774* 0.754* 0.737* 0.800** 0.787*
0.898** 0.929** 0.781* 0.812* 0.664
0.909** 0.909** 0.805* 0.794* 0.671
Soil moisture (m3/m3)
1 0.772* 0.840** 0.947** 0.960** 0.954** 0.923** 0.866** 0.838* 0.724 0.907* 0.825* 0.089
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(400,000/kg), and consequently have small carbohydrate reserves; therefore seedling growth is supported for a very short period during establishment. If the emerging radicle is desiccated during this period, the seedling will rapidly die. Successful germination and early growth of the similarly small-seeded Engelmann spruce (Picea engelmannii Parry) has been found to require microsites where surface drying occurs gradually (Knapp and Smith, 1982). Although average soil moisture values were used in the current study, the ground surface in the plots receiving direct sun was observed to dry out rapidly in comparison with the plots in the permanent shade. The low level of germination in the plots located under the canopy was more likely to have been an effect of low light levels (less than 13% openness) rather than low soil moisture levels. The mortality of both natural regeneration and planted seedlings was high, and similar to levels of mortality found for Norway spruce regeneration in gaps (Diaci, 2002) and under shelterwoods (Nilson and Lunqvist, 2001). Perhaps the most significant contributor to the loss of naturally regenerated seedlings was competing ground vegetation. Strong ingrowth of mainly D. caespitosa and J. effuses within the gaps had a smothering effect on the seedlings. Ground vegetation is known to be a serious competitor for newly germinated and older seedlings (Diaci, 2002), but in the current study, this competition was only significant in the plots receiving direct sun (plots E and F, Fig. 6). The development of ground vegetation in the permanently shaded plots appears to have been restricted by the lower light levels. Mortality among the planted seedlings was strongly related to canopy openness; the more open the canopy, the lower the losses (Table 1). Nearly all the seedlings planted under the canopy in the plots furthest from the gap edge (A and H) died within one season. Even those planted in plots under the canopy nearest to the gap edge (B and G) had virtually all died by the fifth growing season (Fig. 7). Planted seedlings also died within the gap, but at a significantly lower level than under the canopy. While browsing by both red (Cervus elaphus L.) and roe (Capreolus capreolus L.) deer damaged some of the planted seedlings, much of this was relatively minor in nature and it is difficult to determine whether this was a major cause of seedling death. Overall, this study shows that the success of Sitka spruce regeneration within gaps depends on sufficient moisture and light to support regeneration and early growth, but not too much light to encourage the development of competing vegetation. Natural regeneration of Sitka spruce will not survive where there is too little light. The permanently shaded areas of the gaps appeared to offer ground conditions with sufficient moisture and light to ensure successful germination and early growth of seedlings, but without excessive competition from other vegetation. The area of the gaps that received direct sunlight was also capable of supporting regeneration, but losses occurred through physical smothering of germinating seedlings by competing vegetation. However, seedlings that did survive in this part of the gaps appeared to benefit from the higher light levels and grew more quickly than seedlings located in the permanent shade. Clearly, a balance is required to, on the one hand, restrict light to limit growth of competing vegetation
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during the phase when seedlings germinate and become establish, and on the other hand provide sufficient light to support adequate growth. Creating smaller gaps to restrict direct sunlight within the gap may not be the answer as the reduction in light may be insufficient to support growth. Page et al. (2001) showed that if Sitka spruce seedlings do not receive sufficient light, they move into a phase of retarded growth known as ‘check’. This may not be a major problem with Sitka spruce in the long term as it is a moderately shade tolerant species (Minore, 1979) and can respond to increases in light levels when up to 15 years old (Deal and Farr, 1994). Eventually, higher light levels than those required for regeneration and early growth will be required to ensure adequate photosynthesis and growth as trees grow in size. These results indicate that transforming regular Sitka spruce stands into irregular structures using the group system may not be the best approach given the large variation in gap environment and its effect on regeneration and weed growth. Alternatively, Schu¨tz (2001) suggested a gradual reduction of the whole canopy to, firstly, stabilise the stand through the development of longlived, ‘cover-building’ trees and, secondly, to create suitable conditions for natural regeneration. This approach is supported by Page et al. (2001) who found that maintaining an over storey basal area of less than 30 m2/ha is necessary for the successful regeneration of Sitka spruce. Once the stand has stabilised and regeneration is occurring, Schu¨tz (2001) highlighted the importance of creating ‘light gaps’ to ensure that regeneration develops in the middle storey, to be used for recruitment to the upper storey, but avoiding the formation of the uniform condition. In time, an irregular structure will form. Acknowledgements We gratefully acknowledge the financial support from Shotton Paper Company Plc. We would like to thank Mr. C. Taylor of Tay Forest District, Forest Enterprise for permitting access to the study site at Kindrogan. References Berry, A.B., 1964. Effect of strip width on proportion of daily light reaching the ground. Forest. Chron. 40, 130–131. Canham, C.D., 1989. Different responses to gaps among shade-tolerant tree species. Ecology 70, 548–550. Canham, C.D., Denslow, J.S., Platt, W.T., Runkle, J.R., Spies, T.A., White, P.S., 1990. Light regimes beneath closed canopies and tree-fall gaps in temperate and tropical forests. Can. J. For. Res. 20, 620–631. Deal, R.L., Farr, W.A., 1994. Composition and development of conifer regeneration in thinned and unthinned natural stands of western hemlock and Sitka spruce in southeast Alaska. Can. J. For. Res. 24, 976–984. Diaci, J., 2002. Regeneration dynamics in a Norway spruce plantation on a silver fir-beech forest site in the Slovenian Alps. For. Ecol. Manage. 162, 27–38. Easter, M.J., Spies, T.A., 1994. Using hemispherical photography for estimating photosynthetic photon flux density under canopies and in gaps in Douglasfir forests of the Pacific Northwest. Can. J. For. Res. 24, 2050–2058. Gray, A.N., Spies, T.A., 1997. Microsite controls on tree seedling establishment in conifer forest canopy gaps. Ecology 78, 2458–2473. Hale, S., 2001. Light regimes beneath Sitka spruce plantations in northern Brita: preliminary results. For. Ecol. Manage. 151, 61–66.
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Harmon, M.E., 1987. The influence of litter and humus accumulations and canopy openness on Picea sitchensis (Bong.) Carr. and Tsuga heterophylla (Raf.) Sarg. seedlings growing on logs. Can. J. For. Res. 17, 1475–1479. Harris, A.S., 1990. Picea sitchensis (Bong.) Carr. Sitka spruce. In: Burns, R.M., Honkala, B.H. (Eds.), Silvics of North America, vol. 1: Conifers. US Department of Agriculture Forest Service Agriculture Handbook No. 654, pp. 260–267 Howells, G., 1966. Some factors affecting development of natural regeneration of Sitka spruce. Ph.D. Thesis. UCNW, Bangor. Knapp, A.K., Smith, W.K., 1982. Factors influencing understorey seedling establishment of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) in southeast Wyoming. Can. J. Bot. 60, 2753–2761. Lieberman, M., Lieberman, D., Peralta, R., 1989. Forests are not just Swiss cheese: canopy stereogeometry of non-gaps in tropical forests. Ecology 70, 550–552. Lieffers, V.J., Messier, C., Stadt, K.J., Gendron, F., Comeau, P.G., 1999. Predicting and managing light in the understorey of boreal forests. Can. J. For. Res. 29, 796–811. Malcolm, D.C., Mason, W.L., Clarke, G.C., 2001. The transformation of conifer forests in Britain—regeneration, gap size and silvicultural systems. For. Ecol. Manage. 151, 7–23. Marquis, D.M., 1989. Alternative silvicultural systems-east. In: Proceedings of the National Silviculture Workshop: Silvicultural Challenges and Opportunities in the 1990s, USDA Forest Service Timber Management, pp. 36–45. McLaughlin, S.P., 1978. Upper storey attributes, light, throughfall and the interpretation of upper storey-under storey relationships. For. Sci. 24, 550– 553. Minckler, L.S., 1961. Measuring light in uneven-aged hardwood stands. USDA Forest Service, Central States Forest Experiment Station. Technical Paper No. 184, 9 pp.
Minore, D., 1979. Comparative autecological characteristics of northwestern tree species: a literature review. US Department of Agriculture Forest Service. General Technical Report No. PNW-87. Nilson, K., Lunqvist, L., 2001. Effect of stand structure and density on development of natural regeneration in two Picea abies stands in Sweden. Scand. J. For. Res. 16, 253–259. Nixon, C.J., Worrell, R., 1999. The potential for the natural regeneration of conifers in Britain. Forestry Commission Bulletin No. 120. HMSO, Edinburgh. Page, L.M., Cameron, A.D., Clarke, G.C., 2001. Influence of overstorey basal area on density and growth of advanced regeneration of Sitka spruce in variably thinned stands. For. Ecol. Manage. 151, 25–35. Poulson, T.L., Platt, W.L., 1989. Gap light regimes influence canopy tree diversity. Ecology 70, 553–555. Schu¨tz, J.-Ph., 2001. Opportunities and strategies of transforming regular forests to irregular forests. For. Ecol. Manage. 151, 87–94. Smith, D.M., 1986. The Practice of Silviculture, 7th ed. John Wiley and Sons Inc., New York. Smith, D.M., Larson, B.C., Kelty, M.J., Ashton, P.M.S., 1997. The Practice of Silviculture: Applied Forest Ecology. Wiley, New York, p. 537. ter Steege, H., 1994. Hemiphot: A Programme to Analyze Vegetation Indices, Light and Light Quality from Hemispherical Photographs. Tropenbos Documents No. 3. Tropenbos, Wageningen, The Netherlands. Whitmore, T.C., 1993. Use of hemispherical photographs in forest ecology: measurement of gap size and radiation totals in a Bornean tropical rain forest. J. Trop. Ecol. 9, 131–151. Wright, E.F., Coates, K.D., Bartenucci, P., 1998. Regeneration from seed of six tree species in the interior cedar–hemlock forests of British Columbia as affected by substrate and canopy gap position. Can. J. For. Res. 28, 1352– 1364.
Regeneration dynamics of Sitka spruce in artificially created forest gaps L.M. Page, A.D. Cameron * School of Biological Sciences, University of Aberdeen, Cruickshank Building, St. Machar Drive, Aberdeen AB24 3UU, UK Received 31 March 2005; received in revised form 14 July 2005; accepted 3 October 2005
Abstract This study examined the variation in the development of naturally regenerated and planted seedlings of Sitka spruce (Picea sitchensis (Bong.) Carr.) within gaps cut in a 32-year-old stand of the same species. The circular gaps were 20 m in diameter and designed to allow sunlight into only half of the gap floor at midsummer given the latitude of 568450 N. Eight plots (8 m 3 m) were laid out along a north–south transect through each gap (four within the gap and two each under the closed canopy north and south of the gap). Each plot was sub-divided and seedlings were planted into one part and the other part was left to naturally regenerate. In subsequent seasons, plots were further subdivided into ‘weed free’ and ‘vegetation left untouched’. Results showed that while the two central plots within the gaps had the highest value of canopy openness, the highest accumulated temperature and lowest soil moisture were recorded in plots that received direct sunlight. However, level of germination was significantly higher in the shaded area of the gap than in the part that received direct sunshine suggesting that higher moisture levels in shaded areas are important to successful germination. Minimal germination was recorded in the plots beneath the canopy. Seedling survival was significantly influenced by the influx of competing vegetation, but only in the part of the gaps that received direct sunlight. The success of Sitka spruce regeneration within gaps appears to depend on sufficient moisture and light to support regeneration and early growth, but not too much light to encourage the development of competing vegetation. The permanently shaded areas of the gaps appeared to offer ground conditions with sufficient moisture and light to ensure successful germination and early growth of seedlings, but without excessive competition from other vegetation. # 2005 Elsevier B.V. All rights reserved. Keywords: Sitka spruce; Forest gaps; Natural regeneration
1. Introduction The process of transforming largely even-aged plantation forests into more diverse structures has become an important feature of forest management in many temperate regions of the world. One of the simplest approaches to transformation involves creating discrete openings in regular stands and the establishment of small centres or groups of regeneration continuously over a long period until a ‘balanced’ distribution of size classes is achieved (Smith et al., 1997). Previous research on transformation using groups has concentrated on germination and early growth of the target species, particularly in relation to optimising gap size (e.g. Smith, 1986; Marquis, 1989; Malcolm et al., 2001) and gap light conditions (e.g. McLaughlin, 1978; Poulson and Platt, 1989; Canham, 1989; Canham et al., 1990; Lieffers et al., 1999). However, there is
* Corresponding author. Tel.: +44 1224 272673; fax: +44 1224 272685. E-mail address: [email protected] (A.D. Cameron). 0378-1127/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2005.10.006
insufficient understanding of how variation in micro-environmental conditions, such as temperature and soil moisture, within gaps influences the process of regeneration. The forest floor is a heterogeneous environment where a wide range of conditions can be experienced (Lieberman et al., 1989). Growth of young trees is closely associated with the amount of light received (e.g. Minckler, 1961); with the highest light level found in the centre of a gap, and the lowest at the edge. The closer young trees are to the edge of gaps, the greater the influence that the surrounding trees will have on their growth and development (Malcolm et al., 2001). However, the light environment within gaps is not uniform from the centre to the edge because of the angle of sunlight experienced in cool temperate regions at higher latitudes. In the northern hemisphere, some direct sunlight may reach the forest floor on the north side of even relatively small gaps while the south side of the gap remains in permanent shade. This creates an imbalance in the microenvironment across gaps in a north to south direction. Understanding the effect of this imbalance in environmental conditions on germination and establishment
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of natural regeneration and on the survival and growth of planted seedlings is essential if the group system is to be used successfully for transforming largely even-aged stands into irregular structures. The aim of this study was to determine whether the variation in exposure by sunlight within artificially created gaps influences the development of natural regeneration and planted seedlings of Sitka spruce (Picea sitchensis (Bong.) Carr.). Sitka spruce is classed as being moderately shade tolerant (Minore, 1979) and is considered a suitable species for group regeneration (Malcolm et al., 2001).
2. Materials and methods 2.1. Study site and creation of gaps The experimental site is located in Kindrogan Forest in Tay Forest District, Mid-Scotland (568450 N, 38340 W, National Grid Reference no. 041629). The mean annual rainfall is 1164 mm. Soils are mainly well-drained iron pans with an underlying geology of Upper Dalradian quartz–mica schist. The study area (compartment 5711a) is located on a north facing slope (68) at a mean altitude of 370 m. It was planted in 1965 with Sitka spruce and has been thinned once in 1993. Average top height of the stand at the time of the experiment was 17 m. Six locations for cutting gaps were randomly identified in spring 1997. Taking into account the angle of the sun at this latitude and the slope of the ground, a gap diameter of 20 m (measured from the stem) was calculated to allow direct sunlight to reach the floor of up to half the gap area for a few months of the year (Fig. 1). The trees were felled and the harvest residues removed. No machines were permitted to enter the gaps to avoid soil compaction. No other site preparation was undertaken. Initial ground vegetation was very sparse. The surface mainly comprised of litter (depth 3 cm) with a few patches of bryophytes (mainly Polytrichum spp.).
Fig. 1. Diagram of the cross-section of a gap and the location of the plots A–H. Plots E–H receive direct sunlight for part of the year whereas plots A–D are in permanent shade. The figure is approximately to scale so the sun paths are accurate.
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2.2. Sample plots and treatments Eight plots (8 m 3 m) were laid out along a N–S transect through each gap (Fig. 1). Four plots were located within the gap area (C–F) and two each under the closed canopy south (A and B) and north (G and H) of the gap. The plots were partially protected from deer damage by 1.2 m high fences. Three 2 m 2 m subplots were laid out within each main plot, 0.5 m apart and 0.5 m from the fence. Each sub-plot was randomly assigned to one of three treatments: (1) planted with Sitka spruce seedlings; (2) sown with Sitka spruce seed; (3) left untouched for natural regeneration to establish from the seed rain. The purpose of sowing seed was to provide a fallback should natural regeneration fail. This was a necessary precaution since good seed years only occur with Sitka spruce every 3–5 years on average (Harris, 1990). The 2 m 2 m subplots were designed to be subdivided in the second year of the experiment into four 1 m2 square plots. Two of these smaller plots were randomly selected for weeding while the remaining two were left untouched. The weeded areas were hand weeded throughout the spring and summer of the second growing season to remove all herbaceous vegetation, namely Deschampsia caespitosa (L.) Beauv. and Juncus effuses (L.). Therefore, the experimental design consisted of six gaps (replicates), eight plots per gap and two weeding treatments per plot. Forty-nine, 2-year-old Sitka spruce seedlings (22–25 cm tall) of Queen Charlotte Island (QCI) provenance were planted within the 2 m 2 m subplots at 0.32 m spacing in April 1998. Seed, also of QCI provenance, were sown in the 2 m 2 m subplots in parallel rows at a density of 30 m2 in April 1998. No weeding treatment was applied. The experimental design consisted of six gaps (replicates) and eight plots per gap. 2.3. Measurements Weekly measurements were made on the natural regeneration plots during the first two growing seasons (May– September). Each new germinant was marked with a numbered tag indicating the day that it was first observed. Subsequently, weekly monitoring noted stage of development (i.e. 1- or 2year-old seedlings) and mortality. Valid measurements beyond this were not possible due to the erratic nature of seedling losses. Weekly measurements were also made in the planted seedling plots. Due to the relatively large size of the planted seedlings in relation to the developing ground vegetation, weeding was not necessary. Level of mortality and height growth were assessed over five growing seasons. Browsing damage by deer, observed as loss of apical or side shoots, was recorded within the plots in the second year of measurement (1999). 2.4. Environmental monitoring Temperature was monitored within each plot using Orion Tinytalk II Data Loggers (Gemini Data Loggers (UK) Ltd.)
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located between the seedling and seeded plots. Due to the high cost of equipment, instruments were not placed in the two extreme plots under the canopy. The data loggers were programmed to record temperature every 48 min so that maximum and minimum daily temperatures could be determined in addition to accumulated temperature. Accumulated temperature was measured in day-degrees above 5 8C. An attempt was made to monitor relative humidity in the same way as temperature; however, the humidity loggers could not function properly in the condensing atmosphere of the gaps. Soil moisture probes (Theta Probe type ML1, Delta T Devices Ltd.) were connected to Delta 2LE Data Loggers and placed close to the temperature loggers. These probes monitor moisture at a soil depth of approximately 5 cm. The light environment was determined at the beginning of each growing season for 5 years by taking hemispherical photographs (hemiphots) at each plot, following the methods of Whitmore (1993) and Easter and Spies (1994). Hemiphots were taken with a Nikon 8 mm super wide-angle lens attached to a Nikon FM2 camera. The camera was attached to a tripod at 1.5 m above ground level and the top of the image orientated to magnetic north. The hemiphots were analysed using WINPHOT image analysis software (Steege, 1994), which provided a value for canopy openness. 2.5. Statistical analysis Analysis of variance was carried out on environmental data (canopy openness, accumulated temperature and soil moisture); germination, seedlings growth and browsing damage, and planted seedling growth. All data were analysed per plot (i.e. mean plot values were used). Correlation coefficients and level of significance of relationships between variables were determined. These were calculated using the appropriate environmental data for each year of the study to take account of gradually changing conditions within the gaps. 3. Results 3.1. Environmental conditions Given the similarity of environmental data recorded within the gaps over 5 years, only those for year 1 only are described here. Percentage canopy openness varied from 5% beneath the canopy, to 54% around the centre of the gap (Fig. 2). Accumulated temperature varied considerably across the gap. Not surprisingly, higher values were found in plots receiving direct sun (E and F) in comparison with the shaded plots (C and D) (P < 0.05) and those beneath the canopy (B and G) (P < 0.01) (Fig. 3). Soil moisture was significantly higher in the gap than under the canopy (P < 0.01) (Fig. 4). The plots in the northern part of the gap that received direct sunlight (E and F) had a lower soil moisture level in comparison with plots in the shade (C and D) (P < 0.05). Plots under the canopy (A, B, G and H) had the lowest soil moisture level in comparison with plots within the gaps (P < 0.05).
Fig. 2. Canopy openness (%) (measured in year 1) measured at 1.5 m above ground level in plots beneath the canopy (plots A, B, G and H) and within 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. Bars = 1 S.E.
3.2. Seedling germination Natural regeneration from the seed rain was evident in the plots. However, it was obvious that the sown seed had not survived since it had been sown in straight lines at a fixed distance between rows, and there were no significant differences between the number of seedlings between nonsown and sown plots. Direct sowing was a fallback treatment only, should natural regeneration from the seed rain have failed. Therefore, the sown treatment was ignored. The number of germinating seedlings varied across the gaps. The highest level of germination occurred in the permanently shaded plots (C and D) in comparison with the plots receiving direct sun (P < 0.05) during the two years of observation (1998 and 1999) (Fig. 5). The weeded plots had a higher level of germination than the non-weeded plots within the gap (plots C–F), but this was only significantly different in plot E (P < 0.01) (within the ‘direct sun’ plot nearest to the plot centre) (Fig. 6). 3.3. Survival and growth of planted seedlings The level of mortality was greatest under the canopy, where very few seedlings in the plots furthest from the gaps survived the first growing season and all had died by the end of the second growing season. Level of mortality increased in all plots over time with the highest levels under the canopy (Fig. 7).
Fig. 3. Accumulated temperature (8C) (measured in year 1) measured at ground level in plots beneath the canopy (plots B and G) and within 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. Plots A and H were not recorded due to high cost of data loggers. Bars = 1 S.E.
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Fig. 4. Soil moisture content (m3/m3) (measured in year 1) measured at approximately 5 cm soil depth in plots beneath the canopy (plots A, B, G and H) and within 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. Bars = 1 S.E.
Annual increment of the planted seedlings was greatest in the gaps in comparison with those under the canopy (Fig. 8). At the end of the first growing season, seedling height was not significantly different between plots, but from the second season onwards, seedling height was greater in plot E (i.e. plot closest to the gap centre receiving direct sunlight) in comparison with the other plots (P < 0.05). Shoot increment increased over time in plots within the gaps, but decreased under the canopy. 3.4. Browsing damage on planted seedlings There was no browsing in the first growing season (1998); however, by the summer of 1999 it was observed that deer had browsed some of the planted seedlings. While some variation in percentage browsing was observed between plots, these differences were not significantly different (Fig. 9). 3.5. Correlations between seedling germination and growth, and environmental parameters Number of germinating seedlings was strongly and positively correlated with openness (1999, r = 0.87) and soil
Fig. 5. Number of living and dead germinants after 2 years (measured October 1999) within plots beneath the canopy (plots A, B, G and H) and within 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. Bars = 1 S.E.
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Fig. 6. Number of living and dead germinants after 2 years (measured October 1999) within weeded (w) and not weeded (nw) plots within the 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. No vegetation colonised the plots beneath the canopy (A, B, G and H). Bars = 1 S.E.
moisture (1998, r = 0.77; 1999, r = 0.84), but not accumulated temperature (Table 1). Growth of the planted seedlings, i.e. shoot extension, was positively related with openness, soil moisture and accumulated temperature. Level of mortality of the planted seedlings, however, was strongly and negatively associated with the environmental parameters measured. The correlation between mortality and canopy openness increased each year for five growing seasons. These results suggest that seedling mortality increased with lower light levels, decreasing temperature and soil moisture. The relationship between the various environmental measures and shoot extension of the planted seedlings was significant in most years (Table 1). 4. Discussion Variations in microclimate conditions recorded within the gaps in this study were similar to those found by Wright et al. (1998) and Gray and Spies (1997). While the two central plots within the gaps had the highest value of canopy openness, the highest accumulated temperature and lowest soil moisture were recorded in plots that received direct sunlight (Figs. 2–4). The relatively small gap size used in the current study, with a diameter/height ratio of approximately 1, shows that the
Fig. 7. Level of mortality (%) of planted seedlings from 1998 to 2002 of planted seedlings within plots beneath the canopy (plots A, B, G and H) and within 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. Bars = 1 S.E.
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Fig. 8. Annual increment (mm) from 1999 to 2002 of planted seedlings within plots beneath the canopy (plots A, B, G and H) and within 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. None of the seedlings survived in plots A and H. Bars = 1 S.E.
Fig. 9. Percentage browsing after two growing season of planted seedlings within plots beneath the canopy (plots A, B, G and H) and within 20 m diameter gaps (plots C–F) within 32-year-old planted Sitka spruce forest. None of the seedlings survived in plots A and H. Bars = 1 S.E.
amount of light reaching the forest floor was relatively low. The openness value within the gap centre was only 52–54%. Openness levels approaching 100% are only found in gaps with diameter/height ratios greater than 2 (Berry, 1964; Minckler, 1961). Even with this relatively small gap size, a steep openness/indirect site factor gradient was found, both across the gap itself and when passing from gap to below canopy positions. At the gap edge, openness fell from between 27 and 32% to between 10 and 13%, 5 m under the canopy. A further decrease to only 6–8% with another 5 m distance under the canopy reduced openness to levels typically found under closed canopy (Hale, 2001). The influence of the gap did not extend as far under the canopy as might have been expected based on other studies (Canham, 1989; Canham et al., 1990), although
lower dead branches tend to remain on the stems of Sitka spruce for many years and contribute to the shading of the forest floor. Level of germination was significantly higher in the shaded area of the gap than in the part that received direct sunshine. Surface conditions are important for germination (Harmon, 1987) and that litter surfaces will not be favourable if they become too dry (Nixon and Worrell, 1999). A significant positive relationship was found between soil moisture and level of germination in the current study (Table 1). Surface moisture content is crucial to germination and a significant positive association is known to exist between moisture content and germination rates of Sitka spruce (Howells, 1966). Excessive drying of the seedbed can lead to poor survival of Sitka spruce seedlings (Harmon, 1987). Sitka spruce seeds are very small
Table 1 Correlation coefficients (r values) of environmental factors and seed germination and growth of planted seedlings in 20 m diameter gaps within 32-year-old planted Sitka spruce forest Openness (%) Openness (%) Accumulated temperature (8C), 1998 Accumulated temperature (8C), 1999 Soil moisture (m3/m3)
1 0.694* 0.673* 0.814**
Natural regeneration Number of germinants/m2, 1998 Number of germinants/m2, 1999
0.551 0.872**
Planted seedlings Cumulative mortality Cumulative mortality Cumulative mortality Cumulative mortality Cumulative mortality Shoot Shoot Shoot Shoot
extension extension extension extension
(%), (%), (%), (%), (%),
(mm), (mm), (mm), (mm),
1998 1999 2000 2001 2002
1999 2000 2001 2002
Browsed/damaged (%), 1999 * **
P < 0.05. P < 0.01.
0.786* 0.834** 0.852** 0.919** 0.961** 0.905** 0.787 0.901** 0.966** 0.002
1998 1 0.980** 0.536
1999
1 0.604
0.216 0.170
0.119 0.206
0.711* 0.685* 0.654 0.755* 0.794*
0.774* 0.754* 0.737* 0.800** 0.787*
0.898** 0.929** 0.781* 0.812* 0.664
0.909** 0.909** 0.805* 0.794* 0.671
Soil moisture (m3/m3)
1 0.772* 0.840** 0.947** 0.960** 0.954** 0.923** 0.866** 0.838* 0.724 0.907* 0.825* 0.089
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(400,000/kg), and consequently have small carbohydrate reserves; therefore seedling growth is supported for a very short period during establishment. If the emerging radicle is desiccated during this period, the seedling will rapidly die. Successful germination and early growth of the similarly small-seeded Engelmann spruce (Picea engelmannii Parry) has been found to require microsites where surface drying occurs gradually (Knapp and Smith, 1982). Although average soil moisture values were used in the current study, the ground surface in the plots receiving direct sun was observed to dry out rapidly in comparison with the plots in the permanent shade. The low level of germination in the plots located under the canopy was more likely to have been an effect of low light levels (less than 13% openness) rather than low soil moisture levels. The mortality of both natural regeneration and planted seedlings was high, and similar to levels of mortality found for Norway spruce regeneration in gaps (Diaci, 2002) and under shelterwoods (Nilson and Lunqvist, 2001). Perhaps the most significant contributor to the loss of naturally regenerated seedlings was competing ground vegetation. Strong ingrowth of mainly D. caespitosa and J. effuses within the gaps had a smothering effect on the seedlings. Ground vegetation is known to be a serious competitor for newly germinated and older seedlings (Diaci, 2002), but in the current study, this competition was only significant in the plots receiving direct sun (plots E and F, Fig. 6). The development of ground vegetation in the permanently shaded plots appears to have been restricted by the lower light levels. Mortality among the planted seedlings was strongly related to canopy openness; the more open the canopy, the lower the losses (Table 1). Nearly all the seedlings planted under the canopy in the plots furthest from the gap edge (A and H) died within one season. Even those planted in plots under the canopy nearest to the gap edge (B and G) had virtually all died by the fifth growing season (Fig. 7). Planted seedlings also died within the gap, but at a significantly lower level than under the canopy. While browsing by both red (Cervus elaphus L.) and roe (Capreolus capreolus L.) deer damaged some of the planted seedlings, much of this was relatively minor in nature and it is difficult to determine whether this was a major cause of seedling death. Overall, this study shows that the success of Sitka spruce regeneration within gaps depends on sufficient moisture and light to support regeneration and early growth, but not too much light to encourage the development of competing vegetation. Natural regeneration of Sitka spruce will not survive where there is too little light. The permanently shaded areas of the gaps appeared to offer ground conditions with sufficient moisture and light to ensure successful germination and early growth of seedlings, but without excessive competition from other vegetation. The area of the gaps that received direct sunlight was also capable of supporting regeneration, but losses occurred through physical smothering of germinating seedlings by competing vegetation. However, seedlings that did survive in this part of the gaps appeared to benefit from the higher light levels and grew more quickly than seedlings located in the permanent shade. Clearly, a balance is required to, on the one hand, restrict light to limit growth of competing vegetation
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during the phase when seedlings germinate and become establish, and on the other hand provide sufficient light to support adequate growth. Creating smaller gaps to restrict direct sunlight within the gap may not be the answer as the reduction in light may be insufficient to support growth. Page et al. (2001) showed that if Sitka spruce seedlings do not receive sufficient light, they move into a phase of retarded growth known as ‘check’. This may not be a major problem with Sitka spruce in the long term as it is a moderately shade tolerant species (Minore, 1979) and can respond to increases in light levels when up to 15 years old (Deal and Farr, 1994). Eventually, higher light levels than those required for regeneration and early growth will be required to ensure adequate photosynthesis and growth as trees grow in size. These results indicate that transforming regular Sitka spruce stands into irregular structures using the group system may not be the best approach given the large variation in gap environment and its effect on regeneration and weed growth. Alternatively, Schu¨tz (2001) suggested a gradual reduction of the whole canopy to, firstly, stabilise the stand through the development of longlived, ‘cover-building’ trees and, secondly, to create suitable conditions for natural regeneration. This approach is supported by Page et al. (2001) who found that maintaining an over storey basal area of less than 30 m2/ha is necessary for the successful regeneration of Sitka spruce. Once the stand has stabilised and regeneration is occurring, Schu¨tz (2001) highlighted the importance of creating ‘light gaps’ to ensure that regeneration develops in the middle storey, to be used for recruitment to the upper storey, but avoiding the formation of the uniform condition. In time, an irregular structure will form. Acknowledgements We gratefully acknowledge the financial support from Shotton Paper Company Plc. We would like to thank Mr. C. Taylor of Tay Forest District, Forest Enterprise for permitting access to the study site at Kindrogan. References Berry, A.B., 1964. Effect of strip width on proportion of daily light reaching the ground. Forest. Chron. 40, 130–131. Canham, C.D., 1989. Different responses to gaps among shade-tolerant tree species. Ecology 70, 548–550. Canham, C.D., Denslow, J.S., Platt, W.T., Runkle, J.R., Spies, T.A., White, P.S., 1990. Light regimes beneath closed canopies and tree-fall gaps in temperate and tropical forests. Can. J. For. Res. 20, 620–631. Deal, R.L., Farr, W.A., 1994. Composition and development of conifer regeneration in thinned and unthinned natural stands of western hemlock and Sitka spruce in southeast Alaska. Can. J. For. Res. 24, 976–984. Diaci, J., 2002. Regeneration dynamics in a Norway spruce plantation on a silver fir-beech forest site in the Slovenian Alps. For. Ecol. Manage. 162, 27–38. Easter, M.J., Spies, T.A., 1994. Using hemispherical photography for estimating photosynthetic photon flux density under canopies and in gaps in Douglasfir forests of the Pacific Northwest. Can. J. For. Res. 24, 2050–2058. Gray, A.N., Spies, T.A., 1997. Microsite controls on tree seedling establishment in conifer forest canopy gaps. Ecology 78, 2458–2473. Hale, S., 2001. Light regimes beneath Sitka spruce plantations in northern Brita: preliminary results. For. Ecol. Manage. 151, 61–66.
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