DESICCATION OF WHITE SPRUCE SEEDLINGS PLANTED IN THE SOUTHERN BOREAL FOREST OF BRITISH COLUMBIA

M. J. KRASOWSKI ~, T. LETCHFORD ~, A. CAPUTA ~, and W. A. BERGERUD z tBritish Columbia Ministry of Forests (BCMoF) Research Branch, Red Rock Research Station, Rural Route #7, Rural Mail Delivery #6, Prince George, British Columbia V2N 2J5, Canada ZBCMoF Research Branch, 31 Bastion Square, Victoria British Columbia V8W 3E7, Canada.

Abstract. Three consecutive years of seedling injury evaluation data from two research sites established in the southern part of British Columbia boreal forest are presented. In 2 out of 3 years the injury was substantial. Planted white spruce is injured during its first overwintering while naturally established seedlings are not. A brief summary of 1992/93 post-winter assessments of 1-year old operationally established plantations is also presented. Most of the injury is sustained in late March and April. Microclimatic records indicate that desiccation is the main cause of observed injury. Prompt dehydration and severe injury resulted from exposing seedlings to controlled environment conditions similar but less harsh than those recorded in late March and April on monitored forest sites. Positioning of young roots of planted seedlings deep in the soil is viewed as a factor contributing to the injury. Keywords. FREEZE-DESSICATION, WHITE SPRUCE, OVERWINTER INJURY, ARTIFICIAL REGENERATION, BOREAL FOREST.

1. Introduction

Injuries to white spruce seedlings occur frequently in the southeastern part of the British Columbia boreal forest, approximately between 55~ and 57~ (Krasowski et al., 1993). In this area, winters are long, with extreme temperatures down to -50 ~ and frequent high winds. Warm Chinooks (called Foehn winds in Europe) occur several times in winter, reducing or eliminating snowpack (Krasowski et al., 1993). Possible reasons for the observed injuries are: direct freezing damage due to extremely logo and rapidly changing temperature, seedling dehardening by warm temperatures occurring during winter (caused by Chinook winds), photoinjury due to high solar radiation during freezing air temperatures, and seedling desiccation due to exposure of shoots and foliage to high vapour pressure deficit while soil and roots are frozen. Correct identification of the causes of low-temperature related injuries is difficult~ Symptoms often appear long after the damage was done and the suspected cause might be a secondary effect of another injury (Tranquillini, 1982). We are investigating overwinter injury to newly planted white spruce seedlings to determine precisely what stress causes it and when it takes place. In this publication, we provide an overall assessment of the injury problem for the area and determine its cause and timing of occurrence.

Water, Air and Soil Pollution 82:133-146, 1995. 9 1995 KluwerAcademic Publishers. Printed in the Netherlands.

134

M.J. KRASOWSKI ETAL.

2. Materials and Methods 2.1. EXTENTAND TIMINGOF INJURY Three-year data are presented for two unsheltered research sites established near Dawson Creek. The sites were manually brushed and planting spots screefed. Each year, container-grown seedlings of local white spruce seedlots were planted in spring (late May) and in summer (early August). Seedlings were numbered and tagged. They were assessed before their first winter to account for earlier damage, then again in May or June. Seedling condition was classed as: (1) good (_< 30% dead foliage); (2) moderate (> 30% to < 60%); (3) poor (> 60% to < 100%); and (4) dead. Each year, remnants of the previous year's planting were removed, then new microsites were chosen for planting. Unlike in the first two years, there was no destructive sampling from the sites in 1992/93, so many more seedlings could be assessed. One of the affected forest districts (Fort St. John) contracted out an assessment of all their sites planted in the spring of 1992 (little summer planting took place in this district in 1992). The contractor performed the survey in the summer of 1993, recording the incidence of deaths and injury attributed to winter-related causes. A brief summary is presented. To determine the timing of the injury, seedlings were randomly sampled throughout two winters from the research sites (two per row of planting time treatment). In 1990/91, 10 spring- and 10 summer-planted seedlings were sampled on each collection date. In 1991/92, 24 spring-planted and 48 summer-planted seedlings were sampled each time. Sampling intervals depended on weather changes. Snow, if present, was carefully removed. Seedlings were cut at the ground level, packed into plastic bags, and brought to the research station in a cooler. After overnight thawing at +4 ~ in the dark, the severed ends were freshly cut under water. Seedlings in water-filled jars were placed under growth-promoting conditions. Visual assessment was done 2 weeks later. 2.2. CLIMATEMONITORING Observations were similar on both sites, therefore, data from one site only" are presented. Microsite and weather variables were monitored with Campbell Scientific Inc. (CSI) model 21X data loggers. Air temperature at 2, 10, and 40 cm heights, and soil temperature at 5, 10, and 50 cm depths, were monitored with thermocouples at three microsites. Averaged data are presented. Relative humidity and air temperature at 1.5 m were measured with a CSI model HMP 35CF probe. Photosynthetically active radiation (PAR) was measured at 1.5 m height with a Li-Cor model 190 SB quantum sensor. Soil thermocouples were scanned every 5 minutes, while all other sensors were scanned every 30 seconds. Shown data are condensed to daily maximum and minimum for temperature and to daily total average for PAR. Vapour pressure deficit (VPD) was calculated from air temperature and relative humidity. An average VPD between 12:00 and 14:00 hours is presented. Snow depth was averaged from several measurements per site on each site visit. Between the visits, snow depth was inferred from air temperatures at different heights and soil temperature data.

WHITE SPRUCE SEEDLING DESSICATION IN SOUTHERN BOREAL FOREST

135

2.3. GROWTH-CHAMBEREXPERIMENTS These were done to determine if conditions similar to those recorded in March/April on the research sites result in seedling injury. In 1992, two seedlots (SL) planted on the research sites were also planted at Red Rock Research Station (about 300 km to the south). The seedlots were from similar longitudes (about 121~ and latitudes (close to 56~ but SL 8779 was from about 1000 m and SL 8782 from about 800 m elevation. Spring-planted stock was 2+0 while summer-planted stock was 1+0. Seedlot 8782 was a larger stock than SL 8779. The former was nursery-grown in PSB 415B containers (Beaver Plastic Ltd., Edmonton, Alberta), with root cavity 18 cm deep and 105 ml volume. The 8779 was grown in PSB 313B containers, 15 cm deep and 65 ml volume. Therefore, possible stock type effect could be concealed by SL effects. Individually numbered seedlings were planted in spring and in summer into 25 cm long segments of 10 cm di~imeter PVC pipe filled with field soil. The tubes were then buried in soil outdoors in 96 groups of four. Each group contained only one seedling of each seedlot / planting time combination. In January, February, and April 1993, 24 random groups were removed from the ground and thawed overnight at +5 ~ in the dark. Next, water was added to each tube until soil saturation and bottom ends of pipe segments were closed and sealed. Each tube was placed into a plastic bag sealed around the seedling stem. Two growth chambers (G) were set at +10/+4 ~ (day/night), 300 ~tmol m -2 s -1 of incandescent plus fluorescent light, and VPD of 0.55/0.20 kPa (day/night). The lights were on for 8, 10, and 12 h in January, February, and April trials, respectively. In each growth chamber, six random tubes per SL/planting time (P) were placed into a controlled temperature bath; another six tubes were outside the bath. Bath temperature was decreased from +4 ~ to -2 ~ over 3-4 h and kept at the latter temperature constantly. Consequently, two treatments (T) were created: (1) frozen roots, and (2) control. Two seedlings per SL/P were randomly sampled from each treatment in each growth chamber after 7, 14, and 21 day treatment durations (D) from the start. They were placed at recrvery conditions of 25/20 ~ 400 ~tmol m 2 sl, with ample watering. Visual evaluation of injury (%) was done after two weeks. Seedling mean water content (fresh weight / dry weight) was calculated from weights of four pairs of leaves sampled from the upper 2 cm of each seedling being taken into recovery conditions, except at the beginning when two random seedlings per T/SL/P/G were sampled to match the factorial design. SAS (SAS Institute Inc., Cary, North Carolina) software was utilised for data analyses. The experimental design was a factorial, with a seedling nested in all other factors. Seedlot and growth chamber were considered random factors.

3. Results 3.1. EXTENTOF INJURY Table I shows the percentage of seedlings per condition class in 1991, 1992, and 1993 post-winter assessments. Data are pooled across planting dates because there were no consistent planting-time related trends. Although the immediate mortality was low,

136

M. I. KRASOWSKIET AL.

injury was massive after the 1991/92 and 1992/93 winters. Few seedlings were free from injury and many had dead foliage on parts of the stem. There was almost no direct spring frost-related injury in any year. Spring frosts kill swelling buds and young elongating shoots, and these symptoms are easy to identify.

TABLE I Summary of post-winter seedling condition assessments made on seedlings planted in 1990, 1991 and 1992 on two research sites. Data are pooled across planting times on each site. SLK = Stewart Lake site; BM = Bear Mountain site.

Planting/ assessment year

Site

Number of seedlings assessed

Seedling condition* - (% of seedlings assessed) Good Moder Poor Dead

90/91

SLK

265

76

13

8

3

90/91

BM

295

68

16

8

8

91/92

SLK

454

36

28

27

9

91/92

BM

347

25

38

25

12

92/93

SLK

949

24

46

26

4

92/93

BM

954

21

49

28

2

* Condition codes relate to % foliage damage as follows: Good: < 30%; Moderate: > 30% to < 60%; Poor: > 60% to < 100%; and Dead: 100%.

The poor state of 1-year old white spruce plantations following the 1992/93 winter was widely noticed throughout the Peace River area. The contracted survey of springplanted sites in the Fort St. John Forest District showed an average mortality of 24% on sites planted without mechanical site preparation. Average mortality was 12% on plowed sites but only 6% on mounds. The latter had 66% uninjured seedlings, while plowed sites had 38%. Sites on south-facing slopes had the greatest average mortality (21%), while those with northern aspects had the least (12%). Site size was apparently of little relevance: average mortality on sites larger than 10 ha was 19%, and was 15% on smaller ones, while 46% and 49% of seedlings, respectively, were uninjured. Worst mortality (72%) was on a large, plowed site. One-year old (1+0) planting stock had an average mortality of 16%, compared to 21% for 2+0 seedlings. Worst mortality (72%) occurred in 2+0 stock, while the worst for 1+0 was 48%. Best condition was 98% uninjured seedlings for 1+0, and 87% for 2+0 planting stock. 3.2. TIMINGOF INJURY Fall assessment of the research sites made in 1990 showed minimal injury occurring before November. Little injury was found in samples collected between November and the spring of 1991, but the spring injury assessment also showed low injury levels. Few 1991 planted seedlings showed any injury in the fall of 1991 except for the pale-green appearance of some summer-planted seedlings resulting from post-planting drought. Most of these later died, contributing to higher mean injuries in November and December collections in which a few of these seedlings happened to be sampled

137

WHITE SPRUCE SEEDLING DESSICATION IN SOUTHERN BOREAL FOREST

70 60" o~

50-

o) 40E -(3 oE3) 30._~

u0-O 2010I

f

1

14NOV91

11DFC91

30JAN92

I

I

02MAR92 24MAR92

Fig. 1. Mean foliage injury (black points) and standard errors (vertical bars) in samples collected from two research sites during the 1991/92 winter. Dashed line indicates the averagefoliage injury across these sites calculated from seedling assessmentmade in June 1992. (Figure 1). Apart from these deaths, foliage injury in all collections was very low compared to that found in June 1992 (Figure 1). This indicates that the bulk of injuries found in June took place after mid-March. 3.3. CLIMATEDATA Figure 2 shows monthly mean air temperatures at 1.5 m over three winters on the research sites compared with the 1951-1980 averages for the Dawson Creek airport. The winters of 1990/91 and 1991/92 were warmer than average. In 1990/91, snow cover was deep and long lasting. In 1991/92 and 1992/93, snow accumulation was high in mid-winter but the snow was gone by late winter (Figure 2). This probably explains the difference in seedling injury between 1990/91 and the 2 later years. In presentation of detailed data, we will concentrate on 1992/93. The winter started cold in late October (Figures 3a-c, 4a) and the soil froze at 5 cm in November (Figure 5a), then progressively deeper (Figure 5b), freezing at 50 cm by late December (Figure 5c). Air temperature (Figures 3a, 4a) decreased to about -40 ~ in early January but it was milder near the ground (Figures 3b,c) due to snow cover (Figure 2). Two Chinook events with temperatures well above 0 ~ occurred in late January and in February (Figures 3a-c, 4a) removing the snow, and soil temperature at 5 and 10 cm temporarily rose to about 0 ~ C (Figures 5a, 5b). Then the soil depleted of snow cover froze again (Figures 5a, 5b). Diurnal air temperature fluctuation from early March until late April

138

M.J. KRASOWSKIETAL.

NOV

DEC

JAN

FEB

MAR

I

I

I

I

40

APR

1992-93

.--... 20

E O

o

I

I

...c 40 CL o 20 '(3 0

~

1991-92

rO 40 (,/-) 20

._

990-91

5

O

9 ..,;y

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1990--91. ,

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1991-92

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.',,, ~"

%*~ ~ 1992-93 ',,, '~

~

/1951-80

~ ~ s

I

I

I

I

DEC

JAN

FEB

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APR

Fig. 2. Snow depth averages and monthly mean temperatures at the Bear Mountain site for the three study periods compared with 1951-1980 climate normais at the Dawson Creek airport, (Atmospheric Environment Service, 1983).

(Figures 3a-c, 4a) delayed soil thawing. The soil remained frozen at 5 cm until about April t0 (Figure 5a) and until even later at 10 and 50 cm depths (Figures 5b, 5c). Photosynthetically active radiation (Figure 4b), and VPD (Figure 4c), increased with increasing day length and air temperature towards spring, although the January Chinook caused a peak in VPD that was not matched until early April. Springtime VPD values were in and above the range in which desiccation injury was induced to seedlings with frozen roots in growth-chamber experiments. 3.4. GROWTH-CHAMBEREXPERIMENTS Data collection in the April experiment was terminated after 1 week due to a power failure. Since results of all three experiments were similar, the February experiment details are shown as an example. Mean seedling injury after 1, 2, and 3 weeks of experiment duration are shown in Figure 6a. Analysis of variance (Table II) identifies treatment as the most significant source of variance for the injury; planting time is the next major one. Figure 6a well illustrates the results of the analysis shown in Table II.

139

WHITE SPRUCE SEEDLING DESSICATION IN SOUTHERN BOREAL FOREST

1---. 20 (D s ~

o

(1) -10 Q. E -2o -go

O

2O

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10 ~-~

0

d.) (b -10 -20 -ao

c) 2 cm

..--- 20

2

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E -2o -go

21OCT

11NQV 30NOV 20DEC

09JAN

29JAN

18FEB

10MAR

30MAR

19APR

09MAY

1992-93

Fig, 3. Daily m a x i m u m and minimum microsite air temperature at (a) 40 cm, (b) 10 cm, and (c) 2 cm. Daily values were averaged from three microsites at Bear Mountain.

140

M . J . KRASOWSKI ET AL.

30 r0

a) Air temperature

20 ~o o

o~

E

-10

i ~ -20

-30

40

b)

PAR

,7

-(D 30 co O 20

i~1 ~ ~.,~I 10

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0.5

21OCT

11NOV 30NOV 20DEC

09JAN

29JAN

18FEB 10MAR 30MAR

19APR

09MAY

1992-93

Fig. 4. Daily weather variables recorded at the Bear Mountain site at 1.5 m. (a) Maximum and minimum air temperature. (b) Total photosynthetically active radiation (PAR). (c) Average vapour pressure deficit (VPD) between 12:00 and 14:00 hours.

WHITE SPRUCE SEEDLING DESSICATION IN SOUTHERN BOREAL FOREST

141

6

9

4

2 O_

~-2 -4

6

v

2 0 0_

~-2 -4

~ (D

6

~

4

bO cm

2 (I) C)_ 0

E -4

21 OCT

11NOV

30NOV

20DEC

09JAN

29JAN

18FEB

10MAR 30MAR

1 9 A P R 09MAY

1992-93

Fig. 5. Daily maximum and minimum microsite soil temperature at (a) 5 cm, (b) 10 cm and (c) 50 cm. Daily values were averaged from three microsites.

142

M . J, K R A S O W S i O

ET AL.

a) 100

8O a,

6O o

40

C

20

treated 3ntrol

0 i

-

7 days

otat

~UI I I

14 days

spr sum 21 days

b) 120

/ ,,d

100

V

80

(,3 60 40 control

20 0

eated

,z__.. sar

0 days

7 days

14 days

21 days

Fig. 6. (a) Mean foliage injury, and (b) mean water content of treated and control seedlings by planting time, after 7, 14, and 21 days of treatment in a growth chamber.

WHITE SPRUCE SEEDLINGDESSICATIONIN SOUTHERNBOREALFOREST

143

T A B L E II

Analysis o f v a r i a n c e on p e r c e n t a g e foliage injury sustained in F e b r u a r y o f the g r o w t h - c h a m b e r desiccation experiments, d f is degrees o f freedom. SS is sums of squares (type Ill). Source a

P-value

T e r m used for error

G

df 1

SS 376

F-value 1.7

0.20

b

SL

1

1 473

6.6

0.013

b

T

1

55 200

244.4

0.0016

c

P

1

15 708

69.5

0,0072

c

D

2

5 821

12.9

0.049

c

TxP

1

5 704

25.3

0.024

c

TxD

2

2 698

6.0

0.11

c

PxD

2

2 277

5.0

0.13

c

TxPxD

2

1 691

3.7

0.18

c

SLxTxPxD

11

2 723

0,8

0.63

SLxGxTxPxD

GxTxPxD

11

3 119

0.9

0.54

SLxGxTxPxD

1.4

0.22

b

SLxGxTxP•

12

3 662

b

48

10 7 3 0

a T = treatment; G = growth chamber; SL = seedlot; P = planting time; and D = duration. b Sampling error. e Pseudo F-tests output by SAS were used here. The Error term used is: MS(SLxTxPxD) + MS(GxTxPxD) - MS(SL• = 225.88 with degrees of freedom (df) = 2.47.

Foliage water content declined much faster in the frozen root treatment than in the controls (Figure 6b). The same sources of variance as in the injury analysis were significant in water content analysis and in the same ranking order. However, in the latter analysis, TxPxD effects were significant (Table III). Figure 6b clearly explains the results of the analysis shown in Table III except the significant SL effect. This SL effect was caused by greater and faster decline of water content in SL 8782 than in SL 8779.

4. Discussion We have shown that injury to planted white spruce seedlings in the southern boreal forest of British Columbia can be a significant problem in some years. Most of the injury is apparently sustained in late March and in April when local conditions are typical freeze-desiccation conditions (Sakai, 1970; Tranquillini, 1982; Christersson and Von Fircks, 1988). These are certainly not conditions promoting photoinjury, which requires subfreezing temperatures coupled with intense light (121quist, 1983; Powles, 1984). The timing of the injury does not support the notion that it is caused directly by low temperature. Our unpublished data suggest that frost-hardiness of these northern seedlots is adequate until March. Although warm Chinook periods may decrease it, the decrease is not sufficient to explain the observed injuries. Finally, in the controlledenvironment experiments, we showed that it took 1-3 weeks to cause heavy seedling damage or even death by imposing desiccating conditions similar (although not as

144

M.J. KRASOWSKIET AL. T A B L E III

Analysis of variance on foliage water content (percent of dry weight) in February mn of growth-chamber desiccation experiments, df is degrees of freedom. SS is sums of squares (type III). Source a

df

SS

F-value 39.0

0.4

P-value

Error

0.53

Tree

G

1

SL

1

2 665.6

27.7

0.0001

Tree

T

1

65 9 9 7 . 2

547.0

0.0001

c

P

1

3 685.9

30.5

0.0005

c

D

3

28 2 1 7 . 0

228.7

0.0001

b

TxP

1

1 247.1

10.3

0.012

b

TxD

3

I 1 985.2

97.1

0.0001

b

PxD

3

279.9

2.3

0.17

b

3

561.2

4.5

0.046

b

15

137.6

1.23

NS

TxPxD SLxTxPxD GxTxP'xD

15

99.7

0.90

NS

SLxGxTxPxD

16

111.5

1.16

0.31

128

12 3 1 7 . 6

Tree(SLxGxTxPxD)

a "r = treatment; G = growth chamber; SL = seedlot; P = planting time; and D = duration. b Pseudo F-test using the denominator of 0.9091 MS(SLxTxPxD) + 0.9091 MS (GxT• MS(SL•215215 + 0.0707MS(error) = 123.3689 with 6.90 df. c Pseudo F-test using the denominator of 0.8182 + MS(SLxTxPxD) + 0.8182 MS(GxTxPxD) - 0 . 8 MS(G• + 0.1636 MS(error) = 120.655 with 8.14 df (degrees of freedom).

harsh) to those recorded on the research sites. Sakai (1970) used similar experiments to demonstrate that desiccation was the cause of injury and mortality to white spruce planted in Japan. He concluded that desiccation was the most important factor limiting successful reforestation in Japan. We have not found any specific recommendations for dealing with freezedesiccation problems in reforestation. The situation we described is not a typical example of post-planting stress that can be resolved by better handling and preconditioning of planting stock (Rietveld, 1989). It appears that some types of mechanical site preparation (such as mounding) could lower the risk of desiccation; however, this type of treatment has limited applicability. The lower incidence of injury on mounds indicates that the solution to the problem is to be found below the ground. Appropriate root growth after planting has been postulated as one of the cardinal prerequisites of successful seedling establishment (Sands, 1984; Rietveld, 1989). Revel et al. (1990) recommended that the summer planting program in the Prince George Forest Region (including the area of interest to this study) be completed in early August at the latest, so that there is enough time for substantial root growth before winter. In the course of this study, we observed numerous new roots in summer-planted and, in particular, in spring-planted seedlings, but most of these roots grew from the bottom of the root plug. The number of new roots that a seedling puts out after planting has been often implicated in determining its survival and performance (see Rietveld, 1989 for review), but positioning of these roots in the soil has rarely been considered. Container-grown seedlings used in British Columbia during the last decade tend to

WHITESPRUCESEEDLINGDESSICATIONIN SOUTHERNBOREALFOREST

145

grow most new roots from the bottom of the root plug and very few from its sides (McMinn, 1978; von der G6nna, 1989). This is a very undesirable characteristic of planting stock destined for areas with freeze-desiccation problems. Krasowski et al. (1993) reported that naturally regenerated seedlings are not significantly affected by this injury. They tend to grow many roots in the shallow organic/mineral soil interface, which is one of the main features distinguishing naturals from planted seedlings (Eis, 1970; Halter and Chanway, 1993). Consequently, naturally regenerated seedlings may take advantage of superficial soil thawing during warm spring days and take up some water. Krasowski et al. (unpubl.) found that severely dehydrated seedlings can restore reasonable foliage water content after just 1 day with soil temperatures barely above zero. While seedling height and previous-year growth rate (in spring-planted seedlings) are related to the extent of desiccation injury (Krasowski et al., unpubl.), we believe that the key to the successful resolution of the problem is in altering the below-ground rather than the aboveground planting stock attributes. It is difficult to predict what impact global warming may have on the severity of desiccation injury in this geographic area. The impact would depend not only on the temperature change but also on changes in precipitation. The desiccation problem and heavy soils in the area may restrict potential influx of species from the south.

5. Conclusions Serious injury to planted white spruce in the southern boreal forest of British Columbia apparently results from seedling desiccation caused by local climate (deep freezing of fine texture soils) and by rooting characteristics of planted seedlings. Remedy to the problem should be sought by employing shallow-rooting planting stock, and by using mechanical site preparation that promotes early soil warming.

Acknowledgments This study was funded from the Forest Resource Development Agreement (FRDA II), project FR02 and FR05 funds granted to M.J. Krasowski. Thanks to Bonnie Hooge and Peter Ott for technical help and to Karen Dale for secretarial assistance.

References Atmospheric Environment Service: 1983, Canadian Climate Normals, 1951-1980, Temperature and Precipitation, British Columbia. Environment Canada, Downsview, Canada, 268 pp. Christersson, L. and Von Fircks, H.: 1988~ Silva Fennica 22, 195-201. Eis, S.: 1970, Dep. Fish., Can. For. Serv. Publ. #1276, 10 pp. Halter, M. R. and Chanway, C. P.: 1993~ Ann. For. Sci. 50, 71-77. Krasowski, M. J., Herring L. J. and Letchford, T.: 1993, Winter Freezing Injury and Frost Acclimation in Planted ConiJkrous Seedlings, FRDA Report #206, ISSN 0835-0752; 36 pp. Krasowski, M. J., Letchford, T., Caputa, A. and Bergerud, W.: 1995, In: New Forests, Spec. Issue of Proc. Intern. IUFRO Symp. on Seedling Quality "Making the Grade", Sault Ste. Marie, Ont., Canada, 11-15 September, 1994. (unpublished manuscript).

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M. J. KRASOWSKIETAL.

McMinn, R. G.: 1978, In: Van Erden, E. and Kinghorn, J. M. (eds.), Proc. Root Form Plant. Trees Symp., Victoria, British Columbia., Min. For. Rep.#8, pp. 379-412. Oquist, G.: 1983, Plant, Cell, Environ. 6, 281-300. Powles, S. B.: 1984, Ann. Rev. Plant Physiol. 35, 15-44. Revel, J., Lavender, D. P. and Charleson, L.: 1990, Summer Planting of White Spruce and Lodgepole Pine Seedlings, FRDA Report #145, ISSN 0835-0752, 14 pp. Rietveld, W. J.: 1989, N.J.A.F. 6, 99-107. Sakai, A.: 1970, Ecology 51, 657-664. Sands, R.: 1984,Aust. For. Res. 14, 67-72. Tranquillini, W.: 1982, In: Lange, O. O., Osmond, P. S., and Zeigler, H. (eds.), Encyclopedia of Plant Physiology, New Series, Vol. 12B. Springer-Verlag, Berlin-Heidelberg, Federal Republic of Germany, pp. 379-400. yon der G6nna, M.: 1989, First Year Performance and Root Egress of White Spruce and Lodgepole Pine Seedlings in Mechanically Prepared and Untreated Planting Spots in North Central British Columbia, M.Sc. thesis, Faculty of Forestry. University of British Columbia, Vancouver, Canada, 130 pp.