Seed production of western larch in seed-tree systems in the southern interior of British Columbia

Seed production of western larch in seed-tree systems in the southern interior of British Columbia

Forest Ecology and Management 130 (2000) 7±15 Seed production of western larch in seed-tree systems in the southern interior of British Columbia Mich...

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Forest Ecology and Management 130 (2000) 7±15

Seed production of western larch in seed-tree systems in the southern interior of British Columbia Michael U. Stoehr* Glyn Road Research Station, Research Branch, Ministry of Forest, P.O. Box 9536, Stn. Prov. Govt., Victoria, BC, V8W 9C4 Canada Received 25 February 1999

Abstract A series of surveys and experiments were conducted on four sites to identify constraints to seed production and natural regeneration in western larch seed-tree systems in the southwestern interior of British Columbia, Canada. These surveys included pollen monitoring, a cone analysis to evaluate seed production potential, seed trapping to estimate seed rain and the installation of ®eld germination trials to assess the effects of germination substrate and seed losses due to bird and rodent predators. Pollen shedding was found to be adequate for moderate seed production with ®lled seed counts ranging from 9 to 30 per cone (10±34% of all seeds/cone). No signi®cant differences in seed yields per cone and cone characteristics were observed between uncut control stands and seed-tree stands. Seed rain was generally good in 1995, ranging from 70,000 to 4.6 million seed/ha. For the four seed-tree stands, the average ®lled seed percentage of the trapped seeds ranged from 9±30%. Seed rain and seed quality were much reduced in 1996. Field germination trials showed western larch seeds germinated and survived best (15±70%) on mineral soil but rodent and/or bird predation reduced germination success signi®cantly. Germination/ germinant survival on undisturbed forest ¯oor, covered with predator exclusion screens, was intermediate. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Larix occidentalis; Pollen monitoring; Alternate silvicultural systems; Cone analysis; Seed trapping

1. Introduction Western larch (Larix occidentalis Nutt.) occupies a limited natural range in the northwestern USA (Washington, Oregon, Idaho and Montana) and western Canada (British Columbia, Alberta) (Schmidt and Shearer, 1995; Farrar, 1995). It is a shade-intolerant *

Corresponding author. Tel.: ‡1-250-952-4120; fax: ‡1-250-952-4119. E-mail address: [email protected] (M.U. Stoehr)

pioneer species reproducing after ®res, but is also found in late-successional stages, reaching ages of 300 years or more (Fiedler and Lloyd, 1995). Western larch, the largest growing larch species, is valued for its rapid juvenile growth, good wood properties and resistance to insects and diseases (Carlson et al., 1995). In British Columbia, partial cutting systems, such as the seed-tree method (Smith, 1962) have been used with western larch as the residual species to achieve natural regeneration and/or as visual remedy for clear-

0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 9 ) 0 0 1 7 3 - 5

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cuts. However, regeneration success according to stand management prescriptions has been limited, especially on moister sites with rapidly establishing grass and broadleaf competition. Usually, these sites had to be planted with conifer species other than western larch because supplies of larch seeds are limited by sporadic natural seed production and because the seed orchard program is still at an early stage (Jaquish et al., 1995). The pollination mechanism and reproductive biology of western larch have been described in detail (Owens and Molder, 1979; Owens et al., 1994; Owens, 1995). The number of ®lled seeds produced is generally low, presumably due to lack of pollination (Owens and Molder, 1979). Also, late frost often leading to cone damage or early cone drop has been postulated as a major reason for seed crop failures (Schmidt and Shearer, 1995). These constraints on seed production and the generally long periods between good seed crops (Roe, 1966), make western larch a poor seed producer (Owens, 1995). The objectives of this study were to con®rm some of the constraints in seed production imposed on western larch by the seed-tree system and to identify some of the reasons for poor seedling establishment in seedtree cuts. 2. Material and methods 2.1. Description of study areas The study was conducted on four sites in southern British Columbia (Fig. 1), representing three different biogeoclimatic types (Meidinger and Pojar, 1991). Stand information and seed tree characteristics are outlined in Table 1. In each seed-tree stand, a 100 m  100 m area (1 ha) was chosen near the centre of the stand. A 10 m  10 m grid system was superimposed onto the study area, with the grids numbered from 1 to 100. This allowed the random placement of location for seed trapping and ®eld germination trials. 2.2. Pollen monitoring On each study site, estimates of western larch pollen production and daily pollen ¯ow were obtained from three pollen monitors installed at 4 m height in the early spring of 1995. The monitors utilized a seven-

Fig. 1. Location of study areas within Kamloops Forest Region, British Columbia.

day clock, described by Webber and Painter (1996). Because the precise time of seed-cone receptivity could not be determined, the total available pollen load was approximated as the amount captured during the central 80% portion of the pollen shedding period (Fig. 2, between arrows). Daily pollen captures during these time periods were averaged and summed to determine the total available pollen load (Sorensen and Webber, 1997) for each of the four sites separately. 2.3. Cone analysis Cones from 10 trees in each of the seed-tree cuts and control stands (uncut) were collected in August 1995.

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Table 1 Stand description of western larch seed tree stands in southern British Columbia Location

Ashton Creek Becker Lake Coldstream Derenzy Lake

Elevation (m)

Ecological classificationa

Stand density (trees/ha)

Age (years)

Seed tree size DBH (cm)

Height (m)

1260 1200 1100 1450

ICH IDF IDF MS

23 102 148 60

94 120 140 90

29 40 40 23

25 30 30 23

Site index (50)

Soil type

Year logged

19 24 26 16

sandy loam sandy loam sandy loam silty loam

1993 1992 1993 1992

a Ecological classification is based on Meidinger and Pojar (1991). ICH is `Interior Cedar Hemlock', IDF is `Interior Douglas fir' and MS is `Montane Spruce'.

Ten cones per selected tree were randomly chosen and cone length and diameter (at the widest point) measured. All measured cones were completely dissected (cone analysis) to determine seed yields per cone. Filled seeds per cone were determined using X-ray analysis of all extracted seeds per cone. Cones from the control stands were analyzed for differences in

cone and seed yield characteristics between control stands and seed-tree stands. To be able to analyze a single variable that combines the assessed seed and cone traits (total number of seeds per cone; number of ®lled seeds per cone), and cone size measurements, a principal component analysis (Proc. PRINCOMP of SAS) (SAS Institute, 1988) using the correlation

Fig. 2. Average daily western larch pollen captures in April and May, 1995 at four study sites in south central British Columbia. Pollen counts between arrows were used to determine total pollen load.

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matrix was run to generate the ®rst component scores for univariate analysis of variance (ANOVA) (Pimentel, 1979). The ANOVA was based on the following linear model: Yijkl ˆ m ‡ Si ‡ Tj ‡ S  Tij ‡ e…ij†k ‡ s…ijk†l where Yijkl is the principal component score for a single cone l collected from tree k in stand type j on site i; m the overall grand mean; Si the effect due to sites, a random block main effect with i ˆ 1,2,3,4. Tj the stand type effect (seed-tree stand versus uncut control stand), a random main effect with j ˆ 1,2. S  Tij the two-way interaction between the main effects; e(ij)k and s(ijk)l the experimental error (i.e., trees within stand types within sites) and sampling error (i.e., the individual component scores for each cone within a tree), respectively. ANOVA and variance component calculations were carried out using SAS Proc. GLM and Proc. VARCOMP (SAS Inst., 1988), respectively. 2.4. Seed trapping Twenty seed traps (0.5 m2 each) per site were placed in randomly designated grids in late August of 1995 and in 1996. The seed traps were made of a window-screen base and a wire mesh cover to prevent bird and/or rodent predation. Each year immediately following snow melt, seed traps were emptied, the seeds cleaned and the number of ®lled seeds determined by X-ray analysis. 2.5. Field germination In the fall of 1996, on each of the four sites, 10 randomly designated grids were used for the ®eld germinations. In each selected grid, three germination plots (0.5 m2 each) were established side by side by (1) placing a predator exclusion shelter (a wooden frame covered with ®ne mesh wire placed on the litter layer, designated as an undisturbed, covered treatment); (2) by placing a predator exclusion screen on the mineral soil exposed by scraping off the litter layer to expose the soil (i.e., a disturbed and covered treatment) and (3) by preparing a germination plot as in (2), but without the wire screen (i.e., a disturbed, uncovered treatment). In all plots, 50 ®lled seeds each were sprinkled on to the germination bed. Germina-

tion was evaluated in the early summer of the following year by counting surviving germinants within each germination plot. Germination percentages (germinant survivals) were arcsine transformed and subjected to ANOVA according to the model outlined for the cone analysis with three treatments, but without the term for sampling error. 3. Results and discussion In the four study areas in 1995, pollen ¯ight for western larch began around 18 April and ended in early May (Fig. 2). The mean available pollen load over the sampling period was 16.2 grains/mm2 for Ashton Creek, 52.0 grains/mm2 for Becker Lake, 11.5 grains/mm2 for Coldstream and 9.3 grains/mm2 for Derenzy Lake. It is possible that full initial pollen shed was not captured at Derenzy Lake because the access to the study site was delayed by large amounts of snow. However, the available pollen supply on all sites does not appear to be limiting to good seed set. For example, in Douglas ®r with a similar pollination mechanism as western larch (Owens et al., 1994), Sorensen and Webber (1997) found that a pollen load of 9 grains/mm2 would yield between 18 and 30 ®lled seeds per cone. Whereas a tested relationship between pollen load and ®lled seed yield per cone for western larch does not exist, with the exception of Derenzy Lake, ®lled seed yields derived from this cone analysis are in the range of these estimates (Table 2). In this study, total seeds per cone include those produced by infertile scales at the tip and base of the cone and ranged from 83 to 119 seeds per cone (Table 2). These estimates are consistent with the number of seeds per cone found by Shearer (1990) over a range of four years in a study in nine stands in Idaho and Montana (ranging from 104 to 118 seeds per cone). Overall, ®lled seed percentage per cone was higher in this study, ranging from 10 to 34% with an average of 22% (Table 2), while Shearer (1990) found an average of 14%. ANOVA of the ®rst principal component of seed and cone traits revealed that there are no signi®cant differences between control trees and trees in the seed-tree stand (Table 3). Thus, stand density (i.e., seed trees vs. uncut controls) did not affect the cone and seed traits studied and accounted for only 1% of the total variation. Site effects could not

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Table 2 Means (standard errors) for data collected from a cone analysis of western larch cones from seed trees and adjacent uncut stands at four sites in southern central British Columbia Site

Stand typea

Nb

Cone characters # of seed/cone

# of filled seed/cone

% filled seed/cone

Cone length (mm)

Cone diameter (mm)

Ashton Creek

S C

87 100

83.4 (4.2) 92.6 (4.1)

19.1 (1.8) 24.1 (2.4)

23.5 (1.4) 27.1 (1.8)

28.3 (0.5) 29.0 (0.6)

20.1 (0.3) 20.0 (0.4)

Becker Lake

S C

97 92

85.8 (4.0) 119.2 (2.9)

12.6 (1.3) 19.3 (1.4)

17.5 (1.3) 18.2 (1.0)

25.9 (0.4) 27.4 (0.5)

19.6 (0.3) 20.0 (0.3)

Coldstream

S C

98 80

93.4 (3.6) 103.0 (4.2)

30.1 (2.4) 28.4 (2.0)

33.9 (1.8) 30.9 (1.7)

29.7 (0.4) 29.1 (0.6)

21.4 (0.2) 20.5 (0.3)

Derenzy Lake

S C

99 100

88.9 (2.5) 96.9 (2.6)

12.1 (1.2) 9.2 (0.8)

14.8 (1.3) 10.2 (0.8)

23.3 (0.4) 25.5 (0.4)

17.5 (0.3) 18.6 (0.3)

a

S denotes seed tree stand, C denotes adjacent control (uncut) stand. N denotes the number of cones analyzed. When available, 10 cones were collected from 10 trees for a total of 100 trees, except at the Coldstream site, where only nine control trees were sampled. In the analysis, cones with less than 10 seeds were considered underdeveloped and excluded from the study. b

be statistically tested here as sites were considered blocks (Anderson and McLean, 1974). Nevertheless, the magnitude of the means squares indicates that sites are an important source of variation, accounting for 13% of the total variation. The bulk of the variation was accounted for by trees within stands and cones within trees (Table 3). Any possible genetic effects of stand density were not evaluated in this study. A possible increase in sel®ng as a result of fewer parent trees in the seed-tree stands, manifested as a higher proportion of empty seeds, could not be con®rmed as the percentage of ®lled seeds was higher on two sites in seed-tree stands, but lower on the two other sites (Table 2). In contrast,

it is possible that inbreeding is decreased as pollen can move more freely over longer distances in a more open stand, promoting outcrossing. Furthermore, removal of neighboring, often related trees during seed-tree stand establishment further may have reduced the chance of inbreeding. However, El-Kassaby and Jaquish's (1997) mating system study in seed-tree stands using isozyme markers was similarly inconclusive. The amount of seeds trapped varied from year to year for all four sites. While more seeds were trapped at the Becker Lake and Derenzy Lake sites in 1995 than in 1996, the reverse was true for Ashton Creek and Coldstream (Table 4). The percentage of ®lled

Table 3 Analysis of variance using first principal component scores of various seed and cone characters of larch trees growing on four sites in two stand types in southern British Columbia Source of variation

df

Mean square

F-valuea

Pr > F

Site (S) Stand type (T) ST Trees/S/T (exp. error)b Sampling errorc Total

3 1 3 71 674 752

74.3 26.6 7.3 12.9 1.2

N/A 3.6 0.6

0.15 0.6

a

Stand type MS was tested against S  T MS. Only nine control trees were sampled at Coldstream causing the the loss of 1 df. c The total number of cones analysed was 753 (see Table 2). b

Variance component (%) 13.3 1.0 0.0 43.7 42.0 100.0

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Table 4 Average number (standard errors) and percent filled seeds per trap dispersed from western larch seed trees in 1995 and 1996 at four study sites in south central British Columbia 1995 Seed raina

Site

Ashton Creek Becker Lake Coldstream Derenzy Lake a b

1996 Seed raina

Nb

Total # of seed (trap)

# filled seed (trap)

% filled seed (trap)

Nb

Total # of seed (trap)

# filled seed (trap)

% filled seed (trap)

20 20 20 20

3.5 227.7 177.9 91.1

0.3 68.3 48.7 19.7

8.6 30.0 27.4 21.6

18 17 20 20

7.4 131.8 446.5 28.5

0.2 1.6 7.6 1.3

2.7 1.2 1.7 4.6

(0.5) (10.5) (11.8) (3.0)

(0.1) (3.9) (4.0) (3.0)

(1.4) (29.9) (53.0) (4.3)

(0.1) (0.3) (1.0) (0.5)

Multiply by 20,000 to extrapolate the number of seeds produced per hectare. Number of seed traps used.

seeds per trap was higher at all four sites in 1995 (8.6± 30.0%) than in 1996 (1.2±4.6%). The 1995 range of ®lled seed percentages were similar to those (14.4± 33.9%) obtained from the analysis of seed-tree cones (Table 2). The seed quality in 1996 was extremely poor for unknown reasons. Unfortunately, pollen production was not monitored in 1996. As already noted, the effects of sites appeared large. However, when seed rain estimates were adjusted for the number of residual trees in each stand, differences became much smaller. For example, in 1995, on an

average, 0.15 seeds were contributed by each tree to a trap at the Ashton Creek site (3.5 seed/trap divided by 23 trees). For Becker Lake, Coldstream and Derenzy Lake, these average are 2.2, 1.2 and 1.5, respectively. Field germinations/germinant survival were evaluated in early summer of 1997. On all four sites, germination/germinants survival was best on exposed mineral soil when seeds were protected by screens to exclude rodents and/or birds (Fig. 3). The next most successful environment for germinant survival was the undisturbed forest ¯oor with screens. The importance

Fig. 3. Average percent germinant survival of western larch sown in fall 1996 on plots installed on exposed mineral soil subject to seed predation and sheltered from seed predation and on undisturbed forest floor plots that were sheltered from seed predation on four study sites in south-central British Columbia.

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of predator exclusion to germination/germinant survival was shown in comparisons between mineral soil with screens and mineral soil without screens (Fig. 3). Germination/germinant survival was reduced by half without screen protection due to rodent/bird predation. Deer mice (Peromyscus maniculatus) populations have been observed to increase sharply following good seed years in a larch/Douglas ®r stand (Halvorson, 1982), indicating a close relationship between a food source and a population peak. Elsewhere, most Douglas ®r and western hemlock seeds were eaten by rodents and birds in an Oregon clearcut (Gashwiler, 1970). The losses observed in this study may include other types of predation such as insect damage and/or fungal decay, although these were not monitored. Fungal attack was an important cause of seed loss in high elevation Engelmann spruce (Picea engelmannii Parry) and subalpine ®r (Abies lasiocarpa (Hook.) Nutt.) (Zhong and van der Kamp, 1999). In this germination study, ANOVA showed that treatment effects were highly signi®cant (p < 0.0001), accounting for 12% of the total variation. Almost 50% of the total variation (Table 5) was due to site effects caused by local microclimatic conditions near the forest ¯oor. For example, at Ashton Creek, broad-cast burning had been used as method of site preparation. The resulting black forest ¯oor together with a southwest aspect may have resulted in high temperatures during the early part of summer, even on germination plots with the black (burnt) litter layer removed. Several sun-scorched seedlings were observed on this site. Another study has found lowest larch seedling survival on south and west facing slopes in a spot seeding study with western larch (Shearer and Halvorson, 1967). In contrast, on the north-facing, high elevation Derenzy Lake site, temperatures may

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have been too low for germination or too low for germinant survival. However, recent evidence suggests that the seed bed does not affect germinant survival of Engelmann spruce and subalpine ®r in similar forest types (Feller, 1998). Conifers generally rely on seed fall and not a persistent seed bank for successful natural regeneration (Johnson and Fryer, 1996; Strickler and Edgerton, 1976; Farmer, 1997 and references therein). Western larch produces good seed crops every ®ve or so years (Schmidt and Shearer, 1995), at these times it has the potential for producing large numbers of seeds up to 1.2 million seed/ha in Montana (Shearer, 1959). Over 8 million seed/ha were produced in this study. Like in most conifers, the number of ®lled or sound (viable) seeds produced here is low, ranging from 10±30% (this study and Owens, pers. comm.). The causes of this reduction in seed set are not known, but it is believed to be a combination of inadequate pollination, embryo abortions due to sel®ng and abortion due to unknown causes (Owens, 1995). However, western larch is not unique for having low ®lled seed yields. The proportion of ®lled seeds per cone is on average 50% in Douglas ®r (based on the number of potentially fertile seed) (Owens, pers. comm.; Webber, pers. comm.), around 15% in western red cedar (Thuja plicata Donn ex D. Don) (Owens, pers. comm.) and between 41 and 45% in short leaf pine (Pinus echinata Mill.) (Wittwer et al., 1997). Natural regeneration failure in western larch is partly due to poor seed quality, high seed predation, long periods between good seed years, an unsuitable seed bed and partially due to perceived failure as a result of `impatience' built into regeneration legislation (i.e., a requirement for rapid and fully stocked regeneration after harvesting). There are other factors

Table 5 Analysis of variance following arc sine transformation of germinant survival percentages for western larch seed sown in fall of 1996 on plots installed on exposed mineral soil or on undisturbed forest floor and/or protected from seed predation at four study sites in south central British Columbiaa Source of variation

df

Type III MS

F-ratio

Pr > F

Site (S) Seed bed (T) ST Exp. error Total

3 2 6 108 119

8397.2 2711.0 122.1 217.6

N/A 12.46 0.56

0.0001 0.761

a

Note: Site was not tested as they are considered blocks.

% Variance component 49.4 11.6 0.0 39.0 100.0

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that contribute to slow natural regeneration. For example, when larch seed-tree stands are established at the time of initial harvesting, larch trees are often part of the dominant and/or codominant strata of the forest. After partial cutting, these residual larch trees often have underdeveloped crowns that are not conducive to seed production as there is a close relationship between crown size and seed cone production (Shearer, 1986). Ironically, site preparation to expose mineral soil is carried out immediately after the seedtree cut, at a time when the seed trees may not be fully developed for seed production. By the time seed production is high (through a combination of fully developed crowns and a naturally occurring good seed year), created mineral soil patches may be already over grown and competing vegetation prevents larch establishment. Thus, on productive sites, larch natural regeneration will not be adequate. As a consequence, other conifers are planted. In conclusion, western larch may be suitable for management through a seed-tree system despite biological constraints in seed production and current management practices requiring immediate reforestation, if the conditions for natural seed production are right. This includes the coincidence of a good seed year with fresh site preparation in a stand with an adequate number of residual seed trees. Broad-cast burning, especially if ¯ammable material is removed from the base of the seed trees (Shearer, pers. comm.) has been found to be very effective, especially on north and east facing aspects (Shearer, 1976). This study has shown that more than 60 seed-trees/ha enhance the chance of good seed rain. Furthermore, recent results showed that seed production in mature western larch can be improved by using hormonal cone induction (Shearer et al., 1999). Considering factors such as natural regeneration on higher elevation (>1500 m), less productive and/or drier sites where competition is not severe and planting may not be economically feasible, western larch may be satisfactorily regenerated naturally. Acknowledgements The technical assistance during the establishment of the ®eld trials of R. Painter, now Tree Improvement Branch, Ministry of Forests, Victoria, is greatly appre-

ciated. Assistance from C. Hollefreund, D. Hedlin and M. Grif®n, all from the Research Branch, Ministry of Forests, Victoria, BC, throughout this study is deeply appreciated, as is the assistance, interest shown and co-operation of Regional and/or District staff of Ministry of Forests including R. Hudson and M. Faliszewski, Kamloops Region, J. Wright, Salmon Arm District, D. Purdy and G. Waneck, Vernon District and M. Jobke, Penticton District. Helpful reviews and valuable suggestions for improvement of the manuscript were made by A. Vyse, V. Sit, R. Parish and J. Webber, BC Ministry of Forests and R. Shearer, USDA Forest Service in Montana. Two anonymous reviewers provided additional suggestions to improve the manuscript. References Anderson, V.L., McLean, R.A., 1974. Design of Experiments. A Realistic Approach. Marcel Dekker, New York, pp. 418. Carlson, C.E., Byler, J.W., Dewey, J.E., 1995.Western larch: pest tolerant conifer of the Rocky Mountains. In: Schmidt, W.C., McDonald, K.J. (Comps.), Ecology and Management of Larix Forest: A Look Ahead. USDA, Forest Service Intermountain Research Station General Technical Report, GTR-INT-319, pp. 123±129. El-Kassaby, Y.A., Jaquish, B., 1997. Population density and mating pattern in western larch. J. Hered. 87, 438±443. Farmer, R.E., Jr., 1997. Seed Ecophysiology of Temperate and Boreal Zone Forest Trees. St. Lucie Press, Delray Beach, FL, p. 253. Farrar, J.L., 1995. Trees in Canada. Fitzhenry and Whiteside and the Canadian Forest Service, p. 502. Feller, M.C., 1998. Influence of ecological conditions on Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasciocarpa) germinant survival and initial seedling growth in south-central British Columbia. For. Ecol. Manage. 107, 55±69. Fiedler, C.E., Lloyd, D.A., 1995. Autecology and synecology of western larch. In: Schmidt, W.C, McDonald, K.J. (Comps.), Ecology and Management of Larix Forest: A Look Ahead. USDA, Forest Service Intermountain Research Station General Technical Report, GTR-INT-319, pp. 118±122. Gashwiler, J.S., 1970. Further study of conifer seed survival in a western Oregon clearcut. Ecol. 51, 849±854. Halvorson, C.H., 1982. Rodent occurrence, habitat disturbance, and seed fall in a larch fir forest. Ecol. 63 (2), 423±433. Jaquish B.C., Howe, G., Fins, L., Rust, M. 1985. Western larch tree improvement programs in the Inland Empire and British Columbia. In: Schmidt, W.C, McDonald, K.J. (Comps.), Ecology and Management of Larix Forest: A Look Ahead. USDA, Forest Service Intermountain Research Station General Technical Report, GTR-INT-319, pp. 452±460.

M.U. Stoehr / Forest Ecology and Management 130 (2000) 7±15 Johnson, E.A., Fryer, G.I., 1996. Why Engelmann spruce does not have a persitent seed bank. Can. J. For. Res. 26, 872±878. Meidinger, D., Pojar, J. (Eds.), 1991. Ecosystems of British Columbia, BC, Ministry of Forests Special Rep. No. 6, Victoria, BC, p. 330. Owens, J.N., 1995. Reproductive biology of larch. In: Schmidt, W.C, McDonald, K.J. (Comps.) Ecology and Management of Larix Forest: A Look Ahead. USDA, Forest Service Intermountain Research Station General Technical Report, GTR INT-319, pp. 97±109. Owens, J.N., Molder, M., 1979. Bud development in Larix occidentalis II. Cone differentiation and early development. Can. J. Bot. 57, 1557±1572. Owens, J.N., Morris, S.J., Catalano, G.L., 1994. How the pollination mechanism and prezygotic and postzygotic events affect seed production in Larix occidentalis. Can. J. For. Res. 24, 917±927. Pimentel, R.A., 1979. Morphometrics, the multivariate analysis of biological data. Kemdall/Hunt Publishing, Dubuque, IO, p. 276. Roe, A.L., 1966. A procedure for forecasting western larch seed crops. Ecology and Management of Larix Forest: A Look Ahead. USDA, Forest Service Intermountain Research Station General Technical Report, Note INT-49, p. 7. SAS Institute, 1988. SAS/STAT User's Guide. Rel. 6.03. SAS Institute, Carey, NC, p. 1028. Schmidt, W.C., Shearer, R.C.,1995. Larix occidentalis: a pioneer of the North American West. In: Schmidt, W.C, McDonald, K.J. (Comps.), Ecology and Management of Larix Forest: A Look Ahead. USDA, Forest Service Intermountain Research Station General Technical Report, GTR-INT-319, pp. 33±37. Shearer, R.C., 1959. Western larch seed dispersal over clearcut blocks in northwestern Montana. Montana Acad. Sci. Proc. 19, 130±134.

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Shearer, R.C., 1976. Early establishment of conifers following prescribed broadcast burning in western larch/Douglas fir forests. In: Proc. Conf. Tall Timbers Fire Ecology and Symp. on Fire and Land Mgt., October 8±10, Tallahassee, FL, pp. 481± 500. Shearer, R.C., 1986. Cone production in Douglas fir and western larch in Montana. In: Shearer, R.C. (Comp.), Conifer tree seed in the inland mountain West Symposium. USDA, Forest Service General Technical Report, INT-203, pp. 63±67. Shearer, R.C., 1990. Seed and pollen cone production in Larix occidentalis. In: Turnbull, J.W. (Ed.), ACIAR Proc. on Tropical Tree Seed Research, No. 28, pp. 14±17. Shearer, R.C., Halvorson, C.H., 1967. Establishment of western larch by spring spot seeding. J. For. 65 (3), 188±193. Shearer, R.C., Stoehr, M.U., Webber, J.E., Ross, S.D., 1999. Seed cone production enhanced by injecting 38-year-old Larix occidentalis Nutt. with GA4/7, New For., in press. Smith, D.M., 1962. The Practice of Silviculture. Wiley, New York, pp. 578. Sorensen, F.C., Webber, J.E., 1997. On the relationship between pollen capture and seed set in conifers. Can. J. For. Res. 27, 63± 68. Strickler, G.S., Edgerton, P.J., 1976. Emergent seedlings from coniferous litter and soil in eastern Oregon. Ecol. 57, 801±807. Webber, J.E., Painter, R.A., 1996. Douglas Fir Pollen Management Manual, 2nd ed. Res. Br., Min. For., Victoria, BC, Work. Pap. 02/96, p. 91. Wittwer, R.F., Tauer, C.G., Huebschmann, M.M., Huang, Y., 1997. Estimating seed quantity and quality in shortleaf pine (Pinus echinata Mill.) cones from natural stands. New For. 14, 45±53. Zhong, J., van der Kamp, B.J., 1999. Pathology of conifer seed and timing of germination in high elevation subalpine fir and Engelmann spruce forests of the southern interior of British Columbia. Can. J. For. Res., in press.