New Forests (2011) 41:221–233 DOI 10.1007/s11056-010-9223-y

Effects of artificial shade on early performance of white spruce seedlings planted on clearcuts Rongzhou Man • Ken J. Greenway

Received: 12 March 2010 / Accepted: 20 September 2010 / Published online: 22 October 2010 Ó Her Majesty the Queen in Right of Canada 2010

Abstract Chlorophyll fluorescence, chlorophyll content, growth, and mortality of white spruce (Picea glauca [Moench] Voss) seedlings were monitored for 2 years after planting under three scenarios of artificial shade: no-shade (control), shade in summer only, and shade all year. The shade frames allowed 50–60% light transmission, with limited effects on air temperature, relative humidity, soil temperature, and soil moisture around seedlings. Based on fluorescence yield and chlorophyll content measurements, summer-only shade reduced photoinhibition and photooxidation, especially in summer and fall; extending to all year shading did not further reduce either photoinhibition or photooxidation. Shade tended to reduce seedling diameter and mortality, but after 2 years the cumulative effect on mortality was not statistically significant. Study results support the establishment of white spruce seedlings under partial forest canopy, especially on sites with harsh environmental conditions. Keywords

Photoinhibition  Fluorescence  Chlorophyll content  Mortality

Introduction White spruce (Picea glauca [Moench] Voss) is sensitive to planting stress and during the first few years after planting growth is often restricted and needles can become chlorotic (Vyse 1981; Burdett et al. 1984; Nienstaedt and Zasada 1990). Planting stress can lead to seedling mortality (Mullin 1963; Vyse 1981; Burdett et al. 1984), which according to a

R. Man (&) OMNR, Ontario Forest Research Institute, 1235 Queen St. E, Sault Ste. Marie, ON P6A 2E5, Canada e-mail: [email protected] K. J. Greenway Alberta Sustainable Resource Development, 9920–108th Street, Edmonton, AB T5K 2M4, Canada e-mail: [email protected]

123

222

New Forests (2011) 41:221–233

survey of Alberta silvicultural foresters by the Alberta Research Council (R. Man, personal communication 2002), can reach 20–50% 4–5 years after planting depending on stock, handling, planting, weather, and microsite conditions (Man and Lieffers 1999; Archibold et al. 2000; MacDonald and Thompson 2003; Tan et al. 2008). This slow initial growth and high seedling mortality extends the time required for plantations to meet reforestation standards, lengthens harvesting cycle, and increases costs as additional seedlings are planted to offset expected losses to mortality. Among the environmental factors likely contributing to the poor performance of planted seedlings, water stress has been suggested as the most important in newly planted seedlings. Water stress results from the lack of connection between the roots and surrounding soil (Margolis and Brand 1990; Grossnickle 2005). Recovery of normal seedling water relations and growth depends largely on new root growth (Margolis and Brand 1990; Grossnickle 2005; Tan et al. 2008), supported by carbohydrates from stored reserves, current photosynthesis, or both (Van Den Driessche 1987; Philipson 1988). Because carbohydrate reserves available for root growth last only a short time after planting (Philipson 1988), protecting seedlings from planting stress during transplanting by ensuring their photosynthetic systems remain functional may increase survival and growth of planted seedlings. This may be especially true for hot-lifted seedlings that are physiologically active requiring immediate moisture for continued growth, but that are subject to extreme conditions such as water stress and direct sunlight when planted on clearcuts. Photoinhibition is the impairment of the photosynthetic system by strong light (Powles 1984) and is manifested as both decreased fluorescence yield and photosynthetic capacity ¨ quist 1985). It is common under natural conditions, especially for shade(Strand and O tolerant plants with low light-saturated photosynthetic capacities, and can be exacerbated by environmental extremes such as water stress (Powles 1984) and freezing temperatures (Lundmark and Ha¨llgren 1987; Man and Lieffers 1997a), or internal factors that reduce a plant’s photosynthetic capacity. In extreme cases of excess solar energy, the resulting damage may lead to photooxidation, the destruction of chlorophyll molecules, and longterm sub-optimal functioning of the photosynthetic system (Powles 1984). Seedlings transplanted to clearcuts can face extreme microclimatic conditions (Man and Lieffers 1999) and therefore may be under significant physiological stress and susceptible to photoinhibition and/or photooxidation. An effective way to limit photoinhibition is to limit the quantity of incident light. White spruce is a shade-tolerant species and its photosynthetic systems saturate at about 600–800 lmol m-2 s-1 (Greenway 1995; Man and Lieffers 1997a), far below the solarnoon light levels in mid-summer which, for example in Alberta, Canada, typically reaches 1500–1800 lmol m-2 s-1 (Man and Lieffers 1997b). An overstory canopy can reduce environmental extremes (Childs and Flint 1987; Groot and Carlson 1996; Man and Lieffers 1999) and benefit establishment of white spruce seedlings (Eis 1967; Marsden et al. 1996; Man and Lieffers 1997a, 1999). Because the overstory canopy affects light, temperature, and moisture, the role of reduced light alone in aiding seedling establishment is not clear. This study was designed to assess the effects of light reduction on the survival and growth of planted white spruce seedlings 2 years after planting in harvested cutblocks, separate from the effects of shade on other microclimates. The ultimate goal is to recommend silvicultural practices that enhance the establishment and early growth of white spruce seedlings on harvested sites.

123

New Forests (2011) 41:221–233

223

Materials and methods Study area The study area is located about 100 km northwest of Lac La Biche (55°110 N, 111°570 W) in the Central Mixedwood Natural Subregion of the Alberta Boreal Forest Natural Region. Two adjacent cutblocks harvested in September 1997 were selected: 29851 (SE 29-71-13 W4) and 28361 (SW 29-71-13 W4) in Alberta Pacific Forest Industries Ltd. forest management area. Before harvesting, the sites supported 120-year-old mature trembling aspen (Populus tremuloides Michx.) stands, with few, mainly understory, conifers including white spruce (Picea glauca [Moench] Voss) and balsam fir (Abies balsamea [L.] Mill.). Dominant woody shrubs were beaked hazelnut (Corylus cornuta Marsh.), prickly rose (Rosa acicularis Lindl.), lowbush cranberry (Viburnum edule [Michx.] Raf.), and red-osier dogwood (Cornus stolonifera Michx.). Soil texture is fine sand on north- and northeast-facing slopes of 2–4% grade. Average elevation is about 650 m. Overall, the sites are dry, except for some depressions with standing water. Annual precipitation is about 400 mm. Seedlings and treatments The sites were straight-bladed in early June 2008 and operationally planted with 1200 oneyear-old, container-grown (415 B), hot-lifted white spruce seedlings per hectare July 10–11, 1998. At the time of the operational planting, four seedlings of the same planting stock were planted at 0.5 m from each operationally planted seedling in the same pattern as a five on a die. Each of the two cutblocks had 60 experimental seedling plots in an area of about 1 ha. Plots were randomly assigned to one of three levels of shading: none, summer only, or all year shade. Shade consisted of an overhead, 1 m2, black mosquito screen tilted approximately 22° (from horizontal) to the south. The shade frames over all 5 seedlings were designed to allow about 50% transmission of summer light, similar to that under partially harvested canopies of mixedwood stands (Man and Lieffers 1999) or mature aspen stands (Constabel and Lieffers 1996), while minimizing the effect of shading on temperature and humidity under the frames. Side shade (east and west only) was provided by a 50 cm piece of screen hanging loosely from the frames that did not restrict airflow. Alloy conduit tubing held the shade frames 35 cm above the seedlings at the time of planting. The frame position was raised as the seedlings grew. Both all year and summer shade started immediately after planting. In the summer shade treatment, shade frames were removed between October 7, 1998 and April 26, 1999, and between October 14, 1999 and May 3, 2000. In the all year shade treatment, shade frames were placed nearly upright from November to March to reduce snow accumulation. Competing vegetation was removed by site preparation in the first year before planting and hand-cleared in the second and third years. The latter involved removing vegetation taller than seedlings manually and mechanically with brushsaw in early June and early August, respectively, within a perimeter of 1 m from the edge of each plot. Measurements The effects of shade on seedling microclimates were monitored during the growing seasons of 1999 and 2000. Light transmission through shade frames was determined with an

123

224

New Forests (2011) 41:221–233

Accupar linear Ceptometer (Decagon Instrument Ltd.). Measurements were made on all shaded seedling plots on either clear, sunny days or uniform, cloudy days. At each seedling plot, measurements were taken above and under the shade frames and light transmission was calculated as a ratio of light transmitting through the frames. Soil temperature was measured using thermocouples (24-gauge copper/constantan) inserted horizontally at 10 cm depth. Volumetric soil moisture was measured with a single diode time domain reflectometer (TDR) probe inserted vertically at a depth of 18.5 cm in the centre of each seedling plot. Instantaneous soil temperature and moisture readings were obtained with a thermocouple reader (Transcat, 5102 RTC, Mississauga, ON) and the TDR unit (Moisture Point TK-917, Environmental Sensors Inc., Victoria, BC), respectively, three times a year (May, August, and October) on 10 randomly selected seedling plots of each shading treatment. Air temperatures and humidity were measured with Hobo sensors (Onset Computer Corp., Pocasset, MA). Six sensors were installed at seedling height (30 cm above ground) beneath shade frames and on control plots (without shade) in each cutblock during the periods of soil temperature and moisture measurements in spring, summer, and fall. The sensors, shielded from direct radiation and rain with aluminum plates, were programmed to log data every minute. Photoinhibition was assessed with chlorophyll fluorescence technique, a non-destructive and rapid assessment of in vivo photosynthetic activities (Maxwell and Johnson 2000). The assessments were done with a portable chlorophyll fluorescence meter (PAM 2000, Heinz—Walz GmbH, Germany) in spring (early to mid-May), summer (early to midAugust), and fall (mid- to late October), concurrently with the measurements of seedling microclimates. Total chlorophyll content for assessment of photooxidation using the DMSO extraction and colourimetric assay technique (Hiscox and Israelstam 1979) was also measured. Fluorescence readings were taken under low light conditions shortly before dark on two to three seedlings randomly selected from each plot. Readings were averaged to generate plot-level values. The effective fluorescence yield (F0 m-Ft)/F0 m (where F0 m is the maximum fluorescence and Ft is steady-state fluorescence) of a pre-illuminated sample, measures the efficiency of Photosystem II photochemistry and therefore overall photosynthesis (Maxwell and Johnson 2000). Under low light conditions, the effective fluorescence yield approximates optimal fluorescence yield, Fv/Fm (where Fv = Fm-Fo, Fm = maximum fluorescence and Fo = minimal fluorescence) of a dark-adapted sample (Binder et al. 1997), a fluorescence variable widely used as a reliable diagnostic indicator of photoinhibition (Maxwell and Johnson 2000). Current-year needles were collected from the same seedlings immediately after fluorescence measurements and combined for each plot to determine total chlorophyll content. Needles were kept in a cooler filled with ice during transport from the field to the lab where analysis was conducted shortly after samples arrived. To evaluate seedling performance over time after planting, a visual assessment of seedling health, percent needle chlorosis of live seedlings, and cumulative mortality of total planted seedlings, was carried out twice a year, in early May before budflush and midOctober when the shade frames (for summer-only shade treatment) were removed. Growth response of seedlings to shade was assessed in mid-October each year after current year growth had finished. Root collar diameter and annual height increment were assessed for all seedlings with normal growth (i.e., without browsed or damaged terminal shoots; after 2 years, about 4.6% of planted seedlings were damaged across all treatments). Measurements were averaged by seedling plot.

123

New Forests (2011) 41:221–233

225

Data analysis The SAS Mixed Model procedure (SAS Institute Inc. 2008) was applied to determine shade effects on soil temperature, soil moisture, fluorescence yield, chlorophyll content, root collar diameter, annual height increment, needle chlorosis, and seedling mortality. The experimental design was a randomized complete block with two blocks (sites) (random effect) and three levels of shade (fixed effect). There were 20 subsamples (seedling plots) of each shade treatment at each site. Measurements repeated over time after planting on the same seedling plots were analyzed with the repeated option within the Mixed Model. Selection of the best covariance structure was made according to the values of AIC (Akaike’s Information Criterion) and SBC (Schwarz’s Bayesian Criterion) (Littell et al. 1996). For needle chlorosis and mortality, seedlings within each combination of block by shade were combined to calculate percentages. Effects were considered statistically significant when the probability of a Type I error was 0.05 or less. Whenever a significant difference was detected, orthogonal comparisons between shade and control and between summer only and all year shade were used to determine treatment mean differences.

Results Effects of shade on seedling environments Mean PAR transmission through the shade frames was 50–60% for both sunny and overcast conditions, based on two periods of measurements with both uniform clear and cloudy conditions, September 3–4, 1998 and August 10–11, 2000. In the 2 years (1999, 2000), soil in the control was about 1°C warmer and 2% drier than the shaded plots (Fig. 1); but neither of the differences were statistically significant (Table 1). A typical diurnal pattern of air temperatures recorded during August 11–13, 1999 is shown in Fig. 2a. Seedling plots without shade frames were warmer during the day and cooler at night than plots with shade (generally less than 1°C difference, see Fig. 2b). The corresponding differences in relative humidity were generally less than 4% (Fig. 2c). Effects of shade on seedling performance Regardless of shade treatment, both fluorescence yield and total chlorophyll content increased from spring to summer and decreased in the fall, except for chlorophyll content of control seedlings in the first year after planting (1999) (Fig. 3). On average, shaded seedlings tended to have greater fluorescence yield and chlorophyll content than that found in unshaded seedlings, while the differences between summer and all-year shade were not significant (Tables 1 and 2). The effects of shade on fluorescence yield and chlorophyll content varied with season (weakest in spring) and years after planting (generally decreased over time) (significant shade by time interaction, Table 1). Needle chlorosis was highest in the spring of 1999 (10–30% of live seedlings), before the peak of seedling mortality in the fall of 1999 (Fig. 4). Shade did not significantly affect chlorosis or mortality, despite a general trend of less needle chlorosis and seedling mortality in shaded seedlings (Table 1; Fig. 4). Shade did not affect height growth. However, control seedlings (without shade) had greater diameter growth than seedlings in either of the shaded treatments and the differences increased over time (significant shade by time interaction, Table 1 and Fig. 5). By

123

226

New Forests (2011) 41:221–233

Soil temperature ( °C)

25

(a)

20

15

10

5

0

40

(b)

Shade treatment:

Soil moisture (v%)

No shade Summer only All year

30

20

10

0 May 18

Aug 12 1999

Oct 13

May 4

Aug 10 Oct 17 2000

Time after planting Fig. 1 Soil temperature at 10 cm (a) and moisture from 0 to 18.5 cm (b) (least square means ± SE, n = 20) by shade treatment (none, summer, all year) and time after planting (spring-May, summer-August, and fall-October in 1999 and 2000)

the end of the second growing season (2000), root collar diameter of the control seedlings was 5% larger than that of seedlings in the summer-only shade treatment and 9% larger than those in the all-year shade treatment.

Discussion Seedling environment Shade frames provided 50–60% light transmission, allowing 800–1000 lmol m-2 s-1 on clear days under the shade frames at noon in mid-summer. This light level is adequate to saturate the photosynthetic systems of white spruce (Greenway 1995; Man and Lieffers 1997a) and is sufficient to support maximum height growth (Logan 1969; Lieffers and

123

New Forests (2011) 41:221–233

227

Table 1 F and P (in parentheses) values from ANOVA for measured parameters by shade treatment (none, summer, all year) and time after planting Measurements

Shade

Time

Shade 9 time

Soil temperature

1.94 (0.15)

569.92 (\0.01)

0.59 (0.82)

Soil moisture

0.82 (0.44)

27.30 (\0.01)

0.22 (0.94)

Fluorescence yield

51.73 (\0.01)

400.74 (\0.01)

Chlorophyll content

42.01 (\0.01)

20.84 (\0.01)

2.28 (0.01)

Needle chlorosis

3.67 (0.35)

6.64 (\0.01)

0.80 (0.61)

Mortality

9.96 (0.22)

9.04 (\0.01)

3.62 (0.02)

Total root collar diameter

4.38 (0.02)

538.53 (\0.01)

1.98 (0.14)

Annual height increment

0.24 (0.78)

2.41 (0.12)

0.75 (0.47)

5.23 (\0.01)

Degrees of freedom were 2 for shade and 5 (soil temperature, soil moisture, fluorescence yield and chlorophyll content), 4 (needle chlorosis and seedling mortality), and 1 (total root collar diameter and annual height increment) for time after planting. Random factors included blocks and seedling plots. P values below 0.05 are considered statistically significant

Differences between shade & control

Air temperature ( °C)

24 20

(a) Control plots (a) Open site

16 12

2

(b) Air temperature (°C)

Shaded plots

0 -2 2 0 -2 -4 -6

(c) Relative humidity (%) Shaded plots

0:00

04:00

8:00

12:00

16:00

20:00

24:00

Time of day

Fig. 2 Air temperatures at 30 cm in control plots (a) and relative differences of air temperature (b) and relative humidity (c) from the measurements under shade frames, August 11–13, 1999

Stadt 1994). In spring and fall, however, solar radiation can be well below 1000 lmol m-2 s-1 with decreasing sun angle, and for a large part of the day the light transmitted through the shade frames may not be sufficient for maximum height growth (Man and Lieffers 1997b).

123

228

New Forests (2011) 41:221–233

(a) Fluorescence yield

0.8

0.6

0.4

0.2

0.0

Needle chlorophyll content (mg/g dry wt)

4

(b)

Shade treatment: No shade Summer only

3

All year

2

1

0 May 17

Aug 10 Oct 14 1999

May 3

Aug 10 2000

Oct 17

Fig. 3 Fluorescence yield (a) and needle chlorophyll content (b) (least square means ± SE, n = 40) by shade treatment (none, summer, all year) and time after planting (spring-May, summer-August, and fallOctober in 1999 and 2000)

Table 2 Orthogonal contrasts of treatment means of physiological responses of white spruce seedlings to three levels of shade in a harvested area in Alberta, Canada Response variables

No shade vs. shade

Summer-only vs. all year shade

Difference

Difference

P value

P value

Fluorescence yield

-0.0987

\0.0001

0.0015

0.7884

Chlorophyll content

-0.9479

\0.0001

0.0718

0.2365

0.0609

0.0082

0.0158

0.2370

Total root collar diameter

The shade frames met the objective of reducing light transmission while minimizing effects on other microclimatic conditions. Some precipitation may be intercepted by shade frames, but the reduced light level and lower air temperatures would also reduce seedling transpiration and soil evaporation (Spittlehouse and Childs 1990) as suggested by the soil moisture data (Fig. 1b). The warmer and drier air during the day and cooler air at night

123

New Forests (2011) 41:221–233

229

Needle chlorosis (% of live seedlings)

(a) 30

20

10

Cumulative seedling mortality (% of total planted)

0

(b) Shade treatment: No shade Summer only All year

10

0 Oct 8 1998

Apr 26 Oct 13 1999

May 4 Oct 17 2000

Time after planting Fig. 4 Percent needle chlorosis of live seedlings (a) and cumulative mortality of total planted seedlings (b) (least square means ± SE) by shade treatment (none, summer, all year) and time after planting (springApril/May and fall-October 1998, 1999, and 2000)

observed in control plots (without shade) are similar to observations made under shelterwood canopies (Childs et al. 1985; Childs and Flint 1987; Valigura and Messina 1994; Man and Lieffers 1999), but to a lesser degree. Seedling physiology Shaded seedlings generally had higher fluorescence yields and chlorophyll contents than unshaded seedlings, indicating a reduced level of photoinhibition and photooxidation and hence more efficient energy transfer from chlorophyll to the photosynthetic system (Groninger et al. 1996). These traits are common in plants adapted to growing in low light conditions (Boardman 1977). Man and Lieffers (1997a) showed that transplanted white spruce seedlings grown on an open site had lower photosynthetic capacity than seedlings under the shade of a forest canopy, particularly in spring and fall. Similarly, Groninger et al. (1996) found that Fv/Fm increased with shade for four Virginia Piedmont tree species. On the other hand, Khan et al. (2000) reported that Fv/Fm ratio decreased with increasing shade from 0 to 75% on ponderosa pine (Pinus ponderosa Dougl. ex Laws.), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), western red cedar (Thuja plicata Donn ex D. Donn), and western hemlock (Tsuga heterophylla (Raf.) Sarg.) in the Pacific Northwest.

123

New Forests (2011) 41:221–233

Total root collar diameter (cm)

230

1.0

(a) Shade treatment:

0.8

No shade Summer only All year

0.6

0.4

0.2

Annual height increment (cm)

0.0

(b) 15

10

5

0 1999

2000

Time after planting Fig. 5 Total root collar diameter (a) and annual height increment (b) (least square means ± SE) by shade treatment (none, summer, all year) and time after planting (1998, 1999, and 2000)

The smaller differences of fluorescence yield and chlorophyll content between shaded and unshaded seedlings in the spring than those in the summer and fall (Fig. 3) may be due to the winter depression of photosynthesis from photoinhibition and/or photooxidation in all seedlings. In winter, temperature is the controlling factor for photosynthesis (Man and Lieffers 1997a, b), which was not greatly modified by the shade frames. The higher fluorescence yield and chlorophyll content with shade treatments likely resulted from reduced radiation and its interactions with freezing temperatures during recovery of photosynthesis from winter depression (Lundmark and Ha¨llgren 1987, Lundmark et al. 1988; Dang et al. 1992). In summer and early fall, photoinhibition is likely light-dependent, even though the effects of extreme temperatures (Man and Lieffers 1997a) and water stress (Grossnickle 2005; Marsden et al. 1996) can be important. Seedling morphology Shaded seedlings appeared to be healthy in terms of needle chlorosis and cumulative mortality, though the treatment effects were not significant after 2 years. Because of the

123

New Forests (2011) 41:221–233

231

limited effects of shade on temperature and moisture, changes in seedling growth under shade frames were smaller than those under forest canopies (Childs and Flint 1987; Man and Lieffers 1999). Much of the needle chlorosis and seedling mortality occurred during the first winter when planted white spruce seedlings are vulnerable to winter desiccation (Krasowski et al. 1996) and other stresses (Vyse 1981, Burdett et al. 1984, Nienstaedt and Zasada 1990). This suggests that the most difficult period for white spruce seedlings is during the first year after which seedlings adapt to site conditions. The root collar diameter and height growth of white spruce seedlings in the present study agree with other observations on seedling growth along a gradient of light conditions (Walters et al. 1993; Wang et al. 1994). The light level under the shade frames exceeded the threshold for maximum height growth of white spruce (Logan 1969; Lieffers and Stadt 1994), but was not adequate for maximum diameter growth (Lieffers and Stadt 1994; Man and Lieffers 1999). It is expected that the difference in seedling diameter will increase with the continuation of shade. Implications for white spruce regeneration The results of this study support the practices of partial cut or other silvicultural systems in which white spruce are regenerated under a forest canopy. In a shelterwood system, the mature stands are removed in several cuts, providing protection to the regenerating young trees in the understory (Smith 1986). The residual density of shelterwood can be manipulated to reduce the negative effects of shade on growth, especially after seedling establishment. In seed-tree and modified clearcut systems, the live or dead residuals reduce direct sunlight and hence temperature and moisture extremes; thereby protecting seedlings from photoinhibition and photooxidation. In boreal mixedwood regions, post-planting tending is commonly conducted in the first 2 years after planting. The fast-growing hardwood regeneration and woody shrubs compete with planted seedlings (Flint and Childs 1987; Munson et al. 1993; Jobidon 2000; Man et al. 2008) and reduce growth, survival, and hence regeneration success over the long term (Posner and Jordan 2002; Jobidon et al. 2003; Man et al. 2009). On less competitive or harsh sites, however, tending may be postponed or reduced in intensity to take advantage of protection by woody and herbaceous vegetation during establishment of planted seedlings. Other than photoinhibition and photooxidation, vegetation may help reduce temperature and moisture extremes, resulting in improved seedling water relations (Marsden et al. 1996) and reduced overwinter desiccation (Krasowski et al. 1996) and incidence of summer frosts (Posner and Jordan 2002).

Conclusions The artificial shade that allowed 50–60% light transmission positively affected fluorescence yield, chlorophyll content, needle chlorosis and cumulative mortality of newlyplanted white spruce seedlings on clearcuts, but the effect on root collar diameter growth was negative. The results suggest that lower light levels may help maintain the integrity of photosynthetic systems during establishment of planted white spruce seedlings. Photoinhibition and photooxidation alone may not cause seedling death, but can influence seedling health on sites with harsh environmental conditions.

123

232

New Forests (2011) 41:221–233

Acknowledgments This study was supported financially by Alberta-Pacific Forest Industries Inc. and Alberta Research Council. The authors thank Marie Gorda, Amar Varma, and Dave Kelsberg for their help with treatment set-up and data collection, and L. Buse and two anonymous reviewers for their constructive comments on an earlier draft of the manuscript.

References Archibold OW, Acton C, Ripley EA (2000) Effect of site preparation on soil properties and vegetation cover, and the growth and survival of white spruce (Picea glauca) seedlings, in Saskatchewan. For Ecol Manag 131:127–141 Binder WD, Fielder P, Mohammed GH, L’hirondelle SJ (1997) Applications of chlorophyll fluorescence for stock quality assessment with different types of fluorometers. New For 13:63–89 Boardman NK (1977) Comparative photosynthesis of sun and shade plants. Ann Rev Plant Physiol 28:355–377 Burdett AN, Herring LJ, Thompson CF (1984) Early growth of planted spruce. Can J For Res 14:644–651 Childs SW, Flint LE (1987) Effect of shadecards, shelterwoods, and clearcuts on temperature and moisture environments. For Ecol Manag 18:205–217 Childs SW, Holbo HR, Miller EL (1985) Shadecard and shelterwood modification of the soil temperature environment. Soil Sci Soc Am J 49:1018–1023 Constabel AJ, Lieffers VJ (1996) Seasonal patterns of light transmission through boreal mixedwood canopies. Can J For Res 26:1008–1014 Dang QL, Lieffers VJ, Rothwell RL (1992) Effects of summer frosts and subsequent shade on foliage gas exchange in peatland tamarack and black spruce. Can J For Res 22:973–979 Eis S (1967) Establishment and early development of white spruce in the interior of British Columbia. For Chron 43:174–177 Flint LE, Childs SW (1987) Effect of shading, mulching and vegetation control on Douglas-fir seedling growth and soil water supply. For Ecol Manag 18:189–203 Greenway KJ (1995) Plant adaptations to light variability in the boreal mixedwood. Ph.D. thesis, University of Alberta Groninger JW, Seiler JR, Peterson JA, Kreh RE (1996) Growth and photosynthetic responses of four Virginia Piedmont tree species to shade. Tree Physiol 16:773–778 Groot A, Carlson DW (1996) Influence of shelter on night temperatures, frost damage, and bud break of white spruce seedlings. Can J For Res 26:1531–1538 Grossnickle SC (2005) Importance of root growth in overcoming planting stress. New For 30:273–294 Hiscox JD, Israelstam GF (1979) A method for the extraction of chlorophyll from leaf tissue without maceration. Can J Bot 57:1332–1334 Jobidon R (2000) Density-dependent effects of northern hardwood competition on selected environmental resources and young white spruce (Picea glauca) plantation growth, mineral nutrition, and stand structural development–a 5-year study. For Ecol Manag 130:77–97 Jobidon R, Roy V, Cyr G (2003) Net effect of competing vegetation on selected environmental conditions and performance of four spruce seedling stock sizes after 8 years in Que´bec (Canada). Ann For Sci 60:691–699 Khan SR, Rose R, Haase DL, Sabin TE (2000) Effects of shade on morphology, chlorophyll concentration, and chlorophyll fluorescence of four Pacific Northwest conifer species. New For 19:171–186 Krasowski MJ, Letchford T, Caputa A, Bergerud WA, Ott PK (1996) The susceptibility of white spruce seedlings to overwinter injury and their post-injury field responses. New For 12:261–278 Lieffers VJ, Stadt KJ (1994) Growth of understory Picea glauca, Calamagrostis canadensis and Epilobium angustifolium in relation to overstory light transmission. Can J For Res 24:1193–1198 Littell RC, Milliken GA, Stroup WW, Wolfinger RD (1996) SAS system for mixed models. SAS Institute Inc. Cary, NC, USA Logan KT (1969) Growth of tree seedlings as affected by light intensity. IV. Black spruce, white spruce, balsam fir, and eastern white cedar. Can For Serv, Pub No 1256 Lundmark T, Ha¨llgren JE (1987) Effects of frost on shaded and exposed spruce and pine seedlings planted in the field. Can J For Res 17:1197–1201 Lundmark T, Ha¨llgren JE, Hede´n J (1988) Recovery from winter depression of photosynthesis in pine and spruce. Trees 2:110–114 MacDonald GB, Thompson DJ (2003) Responses of planted conifers and natural hardwood regeneration to harvesting, scalping, and weeding on a boreal mixedwood site. For Ecol Manag 182:213–230

123

New Forests (2011) 41:221–233

233

Man CD, Comeau PG, Pitt DG (2008) Competitive effects of woody and herbaceous vegetation in a young boreal mixedwood stand. Can J For Res 38:1817–1828 Man R, Lieffers VJ (1997a) Seasonal photosynthetic responses to light and temperature in white spruce (Picea glauca) seedlings planted under an aspen (Populus tremuloides) canopy and in the open. Tree Physiol 15:437–444 Man R, Lieffers VJ (1997b) Seasonal variations of photosynthetic capacities of white spruce (Picea glauca) and jack pine (Pinus banksiana) saplings. Can J Bot 75:1766–1771 Man R, Lieffers VJ (1999) Effects of shelterwood and site preparation on microclimate and establishment of white spruce seedlings in a boreal mixedwood forests. For Chron 75:837–844 Man R, Rice JA, MacDonald GB (2009) Long-term response of planted conifers, natural regeneration, and vegetation to harvesting, scalping, and weeding on a boreal mixedwood site. For Ecol Manag 258: 1225–1234 Margolis HA, Brand DG (1990) An ecophysiological basis for understanding plantation establishment. Can J For Res 20:375–390 Marsden BJ, Lieffers VJ, Zwiazek JJ (1996) The effect of humidity on photosynthesis and water relations of white spruce seedlings during the early establishment phase. Can J For Res 26:1015–1021 Maxwell K, Johnson GN (2000) Chlorophyll fluorescence—a practical guide. J Exp Bot 51:659–668 Mullin RE (1963) Planting check in spruce. For Chron 39:252–259 Munson AD, Margolis HA, Brand DG (1993) Intensive silvicultural treatment: impacts on soil fertility and planted conifer response. Soil Sci Soc Am J 57:246–255 Nienstaedt H, Zasada JC (1990) Picea glauca (Moench) Voss. White spruce. In: Burns RM, Honkala BH (eds) Silvics of North America, vol 1 Conifers. USDA Forest Service, Agriculture Handbook 654, pp 204–226 Philipson JJ (1988) Root growth in Sitka spruce and Douglas-fir transplants: dependence on the shoot and stored carbohydrates. Tree Physiol 4:101–108 Posner SD, Jordan PA (2002) Competitive effects on plantation white spruce saplings from shrubs that are important browse for moose. For Sci 48:283–289 Powles SB (1984) Photoinhibition of photosynthesis induced by visible light. Ann RevPlant Physiol 35:15–44 SAS Institute Inc (2008) SAS/STAT user’s guide, Version 9.2 edition. SAS Institute Inc. Cary, NC, USA Smith DM (1986) The Practice of silviculture, 8th edn. Wiley, New York, NY Spittlehouse DL, Childs SW (1990) Evaluating the seedling moisture environment after site preparation. In: Gessel SP, Lacate DS, Weetman GE, Powers RF (eds) Sustained productivity of forest soils. 7th N. Am. For. Soils Conf, Vancouver. Univ British Columbia, pp 80–94 ¨ quist G (1985) Inhibition of photosynthesis by freezing temperatures and high light levels in Strand M, O cold-acclimated seedlings of Scots pine (Pinus sylvestris).–I. Effects on the light-limited and lightsaturated rates of CO2 assimilation. Physiol Plant 64:425–430 Tan W, Blanton S, Bielech JP (2008) Summer planting performance of white spruce 1 ? 0 container seedlings affected by nursery short-day treatment. New For 35:187–205 Valigura RA, Messina MG (1994) Modification of Texas clear-cut environments with loblolly pine shelterwoods. J Environ Manag 40:283–295 Van Den Driessche R (1987) Importance of current photosynthate to new root growth in planted conifer seedlings. Can J For Res 17:776–782 Vyse A (1981) Growth of young spruce plantations in interior British Colombia. For Chron 57:174–180 Walters MB, Kruger EL, Reich PB (1993) Growth, biomass distribution and CO2 exchange of northern hardwood seedlings in high and low light: relationship with successional status and shade tolerance. Oecologia 94:7–16 Wang GG, Qian H, Klinka K (1994) Growth of Thuja plicata seedlings along a light gradient. Can J Bot 72:1749–1757

123