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Reproductive phenology, parental balance, and supplemental mass pollination in a sitka-spruce seed-orchard
Forest Ecology and Management, 31 ( 1990 ) 45-54
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Elsevier Science Publishers B.•., Amsterdam - - Printed in The Netherlands
Reproductive Phenology, Parental Balance, and Supplemental Mass Pollination in a Sitka-Spruce Seed-Orchard YOUSRY A. EL-KASSABY 1'~and SHEILA REYNOLDS 1
1Canadian Pacific Forest Products Limited, Tahsis Pacific Region, Saanich Forestry Centre, 8067 East Saanich Road, RR # 1, Saanichton, B.C. VOS 1MO (Canada) 2Faculty o[ Forestry, University of British Columbia, Vancouver, B.C. V6T 1 W5 (Canada) (Accepted 25 January 1989 )
ABSTRACT EI-Kassaby, Y.A. and Reynolds, S., 1990. Reproductive phenology, parental balance, and supplemental mass pollination in a Sitka-spruce seed-orchard. For. Ecol. Manage., 31: 45-54. Reproductive bud phenology and seed-cone crop of a Sitka-spruce [Picea sitchensis (Bong.) Carr. ] seed-orchard were studied to assess the validity of two seed-orchard assumptions, namely panmictic equilibrium, and parental balance. Also, the impact of supplemental mass pollination as a crop-management practice was evaluated. The study revealed significant variation in reproductive bud-development timing as well as proportion of reproductive strobili. A lack of mature male cones was observed while the majority of seed cones were in their receptive stage. It was also observed that approximately 80% of the seed-cone crop was produced by only 20% of the orchard clones. Supplemental mass pollination produced a 14-fold difference in the number of filled seeds per cone compared to that produced under wind pollination. In addition to the significant increase in seed-yield, supplemental mass pollination is proposed as a management option in Sitka-spruce seed-orchards to alleviate the effect of parental imbalance and panmictic disequilibruim.
INTRODUCTION
The Canadian Pacific Forest Products Ltd. Sitka spruce [Picea sitchensis (Bong.) Cart. ] tree-improvement program began in 1969 with the selection of parent trees and the establishment of the Nootka seed-orchard on the Saanich Peninsula of Vancouver Island, B.C. Testing was limited to open-pollinated families and provenance trials (Illingworth, 1976; Yeh and Rasmussen, 1985 ). Early cone production for the orchard started in 1976, and the first commercial crop was harvested in 1980. In order to attain the potential genetic improvement of a seed-orchard, seed quantity and quality should be maximized. Seed-yield in seed-orchards can be 0378-1127/90/$03.50
© 1990 Elsevier Science Publishers B.V.
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Y.A. EL-KASSABY AND S. REYNOLDS
increased by cultural and/or hormonal treatments (Philipson, 1983, 1985a,b ) and by supplemental mass pollination (Bridgwater and Bramlett, 1982; Hadders, 1984; Yazdani et al., 1986). Seed quality, on the other hand, is dependent on the fulfilment of certain assumptions regarding seed-orchards (Eriksson et al., 1973; Woessner and Franklin, 1973 ) such as: a) isolation from surrounding stands; b) synchronization of female and male reproductive strobili ripening; c) random mating; d) parental balance with equal compatibility for all possible crosses; and e) minimal rates of natural self-fertilization. In this paper, the validity of of two these assumptions, namely synchronization of female and male reproductive strobili ripening and parental balance, are assessed, and the effect of supplemental mass pollination as a crop-management practice is evaluated in the same 14-year-old Sitka-spruce seedorchard. MATERIALSAND METHODS The study was conducted in the Canadian Pacific Forest Products Ltd., Tahsis Pacific Region, 1.48-ha clonal Sitka-spruce seed-orchard in Saanichton, British Columbia (latitude 48°35'N, longitude 123°24'W). The orchard comprises 146 clones (averaging 9.3 ramets clone -1) selected from elevations between 0 and 415 m on western Vancouver Island, Washington and Oregon. The orchard, planted in a random single-tree mix over three unequal blocks, was established in 1971; however, newly grafted trees are being planted throughout the orchard due to mortality. Trees are spaced 3 m apart and kept at approximately 4 m in height by top-pruning. Reproductive-bud phenology data for the entire orchard (every sexually active tree) was collected by monitoring the orchard trees every second day throughout the 1985 pollination season. The phenological classification used followed Owens and Molder (1980) and Owens and Blake (1984). The critical stage of seed-cone (female) receptivity started when the seed-cone had just emerged fully from the bud scales [B + 2; (Owens and Blake, 1984) ]. The time between the beginning and end of pollen-shedding was used to identify the critical stage for the male reproductive cones (Owens and Molder, 1980). The orchard was divided into two parts; block 1 was left to be wind-pollinated (control), while blocks 2 and 3 received supplemental-mass-pollination treatment (SMP). The prevailing wind-direction and the size and relative position of the three blocks were the factors considered in dividing the orchard between the two treatments (control versus SMP). Supplemental pollen used in blocks 2 and 3 was a mixture of 1-year-old pollen collected and stored in airtight jars at - 2 0 oC, and fresh pollen collected from blocks 2 and 3 during the pollination period. The first pollen application consisted entirely of 1-year-old pollen, the second from a mixture of 1: 1 one-
REPRODUCTIVE PHENOIX)GY, P A R E N T A L BALANCE, A N D S M P IN A SITKA-SPRUCE SEED-ORCHARD
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year-old: fresh pollen, and third consisted entirely of fresh pollen. The number of clones represented in any pollen mix was between 25 and 40, with an equal quantity from each clone. To account for within-tree variation, receptive trees were pollinated three to six times and at two levels (the upper crown, reached by ladders and the lower crown, reached by ground-crews). Pollen was applied using a hand-operated insecticide sprayer with a tubular wand. Pollen was pumped through the device until it was observed coming from the end of the wand. The amounts of pollen applied were not strictly controlled. The number of seed cones and the number of aborted cones on each tree in the two treatments were recorded during harvest. Samples of cones were collected from 15 trees for each treatment (67 and 81 cones for SMP and control, respectively). The individuality of cone lots and subsequent seedlots had been retained. Cones were air-dried at room temperature, seeds were extracted, dewinged and cleaned by hand. The number of filled seeds per cone for each sample tree for each treatment was obtained by radiograph on Kodak Industrex Instant 600 paper, using a Faxitron 804 X-ray unit and an exposure factor of 360 mA at 12.5 kV. Differences in number of filled seeds cone- 1 and number of aborted cones tree-1 between treatments (SMP and control) were checked for significance ( P < 0.05) by comparing bounds of confidence intervals. RESULTS AND DISCUSSION
The cone-receptivity period extended from 14 April to 17 May (Fig. 1 ). The maximal number of receptive seed cones and pollen-shedding cones occurred from 30 April to 1 May and 6-7 May, respectively. Pollen shedding began 10 days after the commencement of seed-cone receptivity. Maximal seed-cone receptivity was earlier than the peak of pollen shedding by 6 days. Receptive cone-bearers outnumbered pollen-producers for all but one of the inspections (Fig. 1 ). On 22, 24 and 28 April, respective totals of 38, 54 and 86 receptive trees were available, while respective totals of 0, 1 and 11 pollenshedding trees were available at these dates. The peak of receptivity ( 102 trees) occurred between 30 April and 1 May, while only 18 trees were available as a pollen source for that period. For ease of discussion, the receptive period (33 days) was divided into three equal classes: early (14-25 April), intermediate (25 April-6 May), and late (6-18 May). The early receptive class was characterized by a steady increase in the number of receptive trees {from 1 to 57) and the absence of pollen donors (only one tree on 24-25 April). The majority of receptive trees (57-102) was present in the intermediate class, while the number of pollen donors increased from 1 to 57, providing a steady increase in pollen production. The late period, on the other hand, was characterized by a sharp decline in the number of pollen donors (from 57 to 3 ) which accompanied a constant number of receptive trees for most of that period. This long receptive period is not unique to this species nor to the orchard. In fact, long receptive periods have been
48
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observed for several coniferous seed-orchards (Polk, 1966; Eriksson et al., 1973; Jonsson et al., 1976; Griffin, 1982; O'Reilly et al., 1983; E1-Kassaby et al., 1984, 1986; Askew, 1986; Fashler and E1-Kassaby, 1987 ).
REPRODUCTIVE PHENOLOGY, PARENTAL BALANCE, AND SMP IN A SITKA-SPRUCE SEED-ORCHARD
49
It must be emphasized that the length of receptive period in this orchard is also affected by the weather conditions during the pollination season. The receptive period extended to 34, 28 and 32 days for 1986, 1987 and 1988, respectively. This observation was similar to that reported on a Douglas-fir [Pseudotsuga menziessii (Mirb.) Franco] orchard in this area under natural conditions (E1-Kassaby et al., 1984) and/or cooling treatment (Fashler and EI-Kassaby, 1987). In addition, the first pollen-sheddingbegan 4 and 7 days after the commencementof seed-cone receptivity for 1987, and 1986 and 1988, respectively. The proportion of pollen-sheddingtrees for these 3 years (198688) during the early and intermediate classes was similar to that observed in 1985, and supplemental mass pollination was the only reliable source of pollen available. The length of the receptive period that accompanied either a lack of, or a disproportionate number of, pollen donors suggests that the orchard population is in a panmictic disequilibrium, and that there was a substantial shortage of pollen. The duration of receptivity of individual Sitka-spruce cones ranges from 6 to 8 days (Owens and Blake, 1984) and the duration of pollen-shedding averages 7 days (Owens and Molder, 1980). Therefore, clones with receptive seed-cones in the early class cannot be pollinated by pollen produced by any of the three (early, intermediate, and late) pollination classes described here. In addition, intermediate and late-receptive seed cones will also produce a continuum of small effective breeding populations throughout the rest of the pollination period, which is characterized by an apparent lack of pollen. This situation was observed in Douglas-fir and loblolly-pine (Pinus taeda L.) seedorchards (E1-Kassaby et al., 1984,1988; Askew, 1986; Fashler and E1-Kassaby, 1987). A significant (P<0.05) difference in the average number of filled seeds cone -~ was observed in the 13.9-fold difference between SMP (75.1 +32.6) and control (5.4 + 5.7). This observed increase in seed-yieldappears to be higher than the average seed-set expected under wind pollination in natural stands (J.N. Owens, Biology Department, University of Victoria, B.C. personal communication, 1988). This corresponds with the observed lack of pollen donors throughout the receptive period. Although pollen viability of the 1-year-old pollen used in the SMP treatment was not determined, the high seed-set obtained indicated adequate pollen viability. The number of aborted cones tree-1 showed no significant difference between SMP (1.6 + 0.9) and control (1.3 + 0.4), indicating that Sitka-spruce cones develop to full size in the absence of pollen (i.e., are parthenocarpous). It has been reported that Sitka-spruce cones reached harvest size with even less than 1% seed set (Owens and Blake, 1984, pp. 1141 and 1148). In Picea, it was reported that unfertilized ovules develop partially, then abort, and that unpollinated cones are reduced in size (Dogra, 1967; Zasada et al., 1978). Differences in seed-set between SMP and control treatments have demon-
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Y.A. EL-KASSABY AND S. REYNOLDS
.strated that supplemental mass pollination is a viable crop-management practice in Sitka-spruce seed-orchards to achieve crop potential. Supplemental mass pollination has been reported to increase seed-yield in young Douglas fir (Daniels, 1978) and loblolly pine (Bridgwater and Bramlett, 1982) seed orchards; however, no increase was observed in older orchards (Bridgwater and Bramlett, 1982; and own unpublished data, 1988). In addition, the very low filledseed count in the control treatment has suggested that Sitka-spruce background pollen was negligible in the orchard area. In fact, this orchard is 10 km from the nearest Sitka-spruce orchard, and 40 km from the nearest natural trees (Sooke, B.C., 48°21'N, 123°50'W). Parental balance in the 1985 cone crop was assessed using the cumulative cone-yield curve {Griffin, 1984). The cone crop was ranked by clone from high to low cone-yield, and the cumulative yield in percentage was plotted against the percentage of clones censused (Fig. 2). Assuming that reproductive energy is equal to reproductive success (i.e., the number of filled seeds cone- 1is equal across clones), it appears that 20% of the clones produced 80% of the total seed crop (i.e., 29 of 146 clones produced the majority of that year's cone crop). This '20: 80' ratio is identical to that reported by the North Carolina Tree Improvement Co-operative for several seed orchards {Anonymous, 1976), and similar to that reported for Picea abies (Eriksson et al., 1973), Picea mariana (O'Reilly et al., 1983), Pinus radiata {Griffin, 1982), Pinus taeda (Schmidtling, 1983), and Pseudotsuga menziesii (EI-Kassaby et al., 1989). The degree of distortion in the parental balance in seed-orchards is influ-
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REPRODUCTIVE PHENOLOGY, PARENTAL BALANCE, A N D S M P IN A SITKA-SPRUCE SEED-ORCHARD
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enced by the crop size and tree type (i.e.,grafts vs. seedlings; EI-Kassaby et a].,1989). Parental-balance ratios of'35: 80' and '42:80' were obtained for this orchard for 1986 and 1988 (good cone years) and 1987 (moderate cone year). The observed improvement in the parental-ba]ance ratio across the four years (1985-88) could be caused by differences in cone-crop size or age of trees. The study has revealed three problems in this Sitka-spruce seed-orchard: 1 ) inadequate pollen supply; 2) panmictic disequilibrium; and 3) parental imbalance. Effective seed-orchard management can alleviate all these problems. An annual reproductive-phenology evaluation, accompanied by SMP of receptive seed-cones, can solve allthe aforementioned problems. The difference in filledseed-set that was observed between SMP and control treatments (75.1 versus 5.4 seeds cone -I ) indicate seed-set is dependent on SMP when phenology of receptivity and pollen availabilitydiffer strongly. Figure I indicates that pollen production started in the intermediate- and ended in the late-pollination period. The collection,extraction, and over-winter storage of this pollen will provide the seed-orchard manager with the option of pollinating early-, intermediate- and late-receptive seed cones with pollen collected from any period. In other words, differences in reproductive phenology can be overcome by the SMP treatment, producing one close, panmictic, breeding population. Parental imbalance due to the differentialcone production among the orchard clones also can be reduced by the use of pollen from less-fecund clones in the SMP mixes. This method may over-emphasize the presence of low-seed-cone-producing clones in the pollen pool. The use of SMP for adjusting panmictic disequilibrium and parental imbalance in seed orchards has been proposed and used for Douglas-fir seed orchards (EI-Kassaby et a]. 1984, 1986, 1989; Fashler and EI-Kassaby, 1987; Reynolds and EI-Kassaby, 1989). The SMP treatment has been empirically proven to be successful in incorporating desired genotypes into orchard seed crops (Yazdani et a].,1986). Clones of low fecundity could be stimulated to produce cones through the use of cultural (top- and root-pruning, girdling,heat and drought) and/or hotmona] (exogenous gibberellin) treatments (Tompsett and Fletcher, 1977, 1979; Philipson, 1983,1985a,b; Marquard and Hanover, 1984,1985; Ross, 1985, 1988a). However, it must be emphasized that all cone-induction treatments (cultural and hormonal) are stress treatments (Ross and Pharis, 1985a,b; Ross, 1988b), and, consequently, their application might generate permanent genetic changes in the resultant seed crop (Durrant, 1962; Durrant and Timmis, 1973; Wills, 1984; AI-Saheal and Larik, 1987; Cullis, 1987). Therefore, genetic evaluation of stress treatments is advised. Nitrogenous fertilizationand hormonal application to forest trees have changed germinative speed (Allen, 1961; Puritch et al.,1979 }. In addition it is commonly known, though not reported, that the gibbereUin applications produce smaller cone size. Changes in cone size have been shown to affect seed size and subsequent seedling size (Sorensen and Campbell, 1985). Therefore, a combination of changes in germinative speed
52
Y.A. EL-KASSABY AND S. REYNOLDS
and final seedling size in container nurseries could affect the genetic diversity of seedling crops through nursery practices, specifically thinning and culling (Campbell and Sorensen, 1984). ACKNOWLEDGEMENTS
The authors thank M.D. Meagher and the two referees for their constructive comments, M. Crown for valuable discussions, C. Cook, J. Devitt, B. Goodwin, C. Haberlin, D. MacLeod and B. Newbery for their patience during seed extraction, M. Robertson for typing, and A. Philip for drafting. Special thanks to D.G.W. Edwards and D. Taylor of the Canadian Forestry Service for use of the seed X-ray facilities. This study was supported by the British Columbia Ministry of Forests, Silviculture Branch, Section 88 (1). REFERENCES Allen, G.S., 1961. Testing Douglas-fir seed for provenance. Int. Seed Test. Assoc. Proc., 26: 388402. A1-Saheal, Y.A. and Larik, A.S., 1987. Genetic control of environmentally induced DNA variation in flax genotrophs. Genome, 29: 643-646. Anonymous, 1976. Twentieth annual report in cooperative tree improvement and hardwood research program. North Carolina State Univ., Raleigh. Askew, G.R., 1986. Implications of non-synchronous flowering in clonal conifer seed orchards. In: A.V. Hatcher and R.J. Weir (Editors), Proc. IUFRO Joint Meeting of Working Parties on Breeding Theory, Progeny Testing and Seed Orchards, Williamsburg, Virginia, October 1986. Tree Improvement Co-op., North Carolina State Univ., Raleigh, pp, 182-191. Bridgwater, F.E. and Bramlett, D.L., 1982. Supplemental mass pollination to increase seed yields in loblolly pine seed orchards. South. J. Appl. For., 6" 100-104. Campbell, R.K. and Sorensen, F.C., 1984. Genetic implications of nursery practices. In: M.L. Duryea and T.D. Lands (Editors), Forest Nursery Manual for Production of Bareroot Seedlings. Dr. W. Junk, Boston, pp. 183-191. Cullis, C.A., 1987. The generation of somatic and heritable variation in response to stress. Am. Nat., 130(s): 62-73. Daniels, J.D., 1978. Efficacy of supplemental mass-pollination in Douglas-fir seed orchards. Silvae Genet., 27: 52-58. Dogra, P.D., 1967. Seed sterility and disturbances in embryogeny in conifers with particular reference to seed testing and tree breeding in Pinaceae. Stud. For. Suec., 45: 1-97. Durrant, A., 1962. The environmental induction of heritable changes in Linum. Heredity, 17: 2761. Durrant, A. and J.N. Timmis, 1973. Genetic control of environmentally induced changes in Linum. Heredity, 30: 369-379. E1-Kassaby, Y.A., Fashler, A.M.K. and Sziklai, 0., 1984. Reproductive phenology and its impact on genetically improved seed production in a Douglas-fir seed orchard. Silvae Genet., 33: 120125. E1-Kassaby, Y.A., Davidson, R. and Weber, J.W., 1986. Genetics of seed orchards: a Douglas-fir case study. In: A.V. Hatcher and R.J. Weir {Editors), Proc. IUFRO Joint Meeting of Working Parties on Breeding Theory, Progeny Testing and Seed Orchards, Williamsburg, Virginia, October 1986. Tree Improvement Co-op., North Carolina State Univ., Raleigh, pp. 410-421.
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E1-Kassaby, Y.A., Ritland, K., Fashler, A.M.K. and Devitt, W.J.B., 1988. The role of reproductive phenology upon the mating structure of a Douglas-fir seed orchard. Silvae Genet., 37: 76-82. EI-Kassaby, Y.A., Fashler, A.M.K. and Crown, M., 1989. Variation in fruitfulness in a Douglasfir seed orchard and its effect on crop management decisions. Silvae Genet., 38: 113-121. Eriksson, G., Jonsson, A. and Lindgren, D., 1973. Flowering in a clonal trial ofPicea abies Karst. Stud. For. Suec., 110: 5-45. Fashler, A.M.K. and El-Kassaby, Y.A., 1987. The effect of water spray cooling treatment on reproductive phenology in a Douglas-fir seed orchard. Silvae Genet., 36: 245-259. Griffin, A.R., 1982. Clonal variation in radiata pine seed orchards. I. Some flowering, cone and seed production traits. Aust. J. For. Res., 12: 295-302. Griffin, A.R., 1984. Clonal variation in radiata pine seed orchards. II. Flowering phenology. Aust. J. For. Res., 14: 271-281. Hadders, G., 1984. Supplemental and controlled mass pollination in seed orchards of Scots pine (Pinus sylvestris L.). Foreningen Skogstradsforadling, Institutet for Skogsforbattring, ~,rsbok 1984, pp. 47-64. Illingworth, K., 1976. Sitka spruce international ten provenance experiment in British Columbia Phase I and II (nursery) results. In: J. O'Driscoll (Editor), IUFRO Sitka Spruce International Ten Provenance Experiment. Dept. Lands, Forests and Wildlife Serv., Dublin, Ireland, pp. 278-310. Jonsson, A., Ekberg, I. and Eriksson, G., 1976. Flowering in a seed orchard of Pinus sylvestris L. Stud. For. Suec., 135: 1-38. Marquard, R.D. and Hanover, J.W., 1984. Relationship between GA4/7 concentration, time of treatment, and crown position on strobilus production of Picea glauca. Can. J. For. Res., 14: 547-553. Marquard, R.D. and Hanover, J.W., 1985. Floral response of Picea glauca to gibberellin A4/7, naphthaleneacetic acid, root-pruning, and biennial treatment. Can. J. For. ires., 15: 743-746. O'Reilly, C., Parker, W.H. and Barker, J.E., 1983. Effect of pollination period and strobili number on random mating in a clonal seed orchard of Picea mariana. Silvae Genet., 31: 90-94. Owens, J.N. and Blake, M.D., 1984. The pollination mechanism of Sitka spruce (Picea sitchensis). Can. J. Bot., 62: 1136-1148. Owens, J.N. and Molder, M., 1980. Sexual reproduction of Sitka spruce (Picea sitchensis). Can. J. Bot., 58: 886-901. Philipson, J.J., 1983. The role of gibberellin A4/7,heat and drought in the induction of flowering in Sitka spruce. J. Exp. Bot., 34: 291-302. Philipson, J.J., 1985a. The promotoin of flowering in large field grown Sitka spruce by girdling and stem injections of gibberellin A4/7.Can. J. For. Res., 15: 166-170. Philipson, J.J., 1985b. The effect of top pruning, girdling, and gibberellin A4/7 application on the production and distribution of pollen and seed cones in Sitka spruce. Can. J. For. Res., 15: 1125-1128. Polk, R.B., 1966. Reproductive phenology and precocity as factors in seed orchard development. In: H.B. Kriebel (Editor), 5th Central States Forest Tree Improvement Conf., October 1966. Ohio Agricultural Research and Development Center, Wooster, OH, pp. 977-1007. Puritch, G.S., McMullan, E.E. Meagher M.D. and Simmons, C.S., 1979. Hormonal enhancement of cone production in Douglas-fir grafts and seedlings. Can. J. For. Res., 193-200. Reynolds, S. and E1-Kassay, Y.A., 1989. Parental balance in Douglas-fir seed orchards - - cone crop vs. seed crop. Silvae Genet. (in press). Ross, S.D., 1985. Promotion of flowering in potted Picea engelmannii (Perry) grafts: effects of heat, drought, gibberellin A4/7and their timing. Can. J. For. Res., 15: 618-624. Ross, S.D., 1988a. Effects of temperature, drought, and gibberellin A4/7and timing of treatment,
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on flowering in potted Picea engelmanni and Picea glauca grafts. Can. J. For. Res., 18: 163171. Ross, S.D., 1988b. Pre- and post-pollination polyhouse environment effects on pollen and seed development in potted Picea engelrnannii grafts. Can. J. For. Res., 18: 623-627. Ross, S.D. and Pharis, R.P., 1985a. Promotion of flowering in crop trees: different mechanisms and techniques, with special reference to conifer. In: M.G.R. Cannell and J.E. Jackson (Editors), Attributes of Trees as Crop Plants. Institute of Terrestrial Ecology, Monks Wood Exp. Stn., Abbots Ripton, Huntingdon, Great Britain, pp. 383-397. Ross, S.D. and Pharis, R.P., 1985b. Status of flowering in conifers: a constraint to tree improvement: In: F. Caron, A.G. Corriveau and T.J.B. Boyle (Editors), Proc. 20th Meeting, Canadian Tree Improvement Assoc., Part 2, pp. 11-28. Schmidtling, R.C., 1983. Genetic variation in fruitfulness in a loblolly pine (Pinus taeda L.) seed orchard. Silvae Genet., 32: 76-80. Sorensen, F.C. and Campbell, R.K., 1985. Effect of seed weight on height growth of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco vat menziesii) seedlings in a nursery. Can. J. For. Res., 15: 1109-11115. Tompsett, P.B. and Flescher, A.M., 1977. Increased flowering of Sitka spruce [Picea sitchensis (Bong.) Carr. ] in a polythene house. Silvae Genet., 26: 84-86. Tompsett, P.B. and Fletcher, A.M., 1979. Promotion of flowering on mature Picea sitchensis by gibberellin and environmental treatments. The influence of timing and hormonal concentration. Physiol Plant., 45: 112-116. Wills, C., 1984. The possibility of stress-triggered evaluation. In: G.S. Mani (Editor), Lecture Notes in Biomathematics (53) Evolutionary Dynamics of Genetic Diversity. Springer, Berlin, pp. 299-312. Woessner, R.A. and Franklin, E.C., 1973. Continued reliance on wind-pollinated southern pine seed orchards - is it reasonable? In: R.J. Dinus, B.A. Thielges and O.O. Wells (Editors), Proc. 12th Southern Tree Improvement Conf., June 1973. Louisiana State Univ., Baton Rouge, pp. 64-73. Yazdani, R., Hadders G. and Szmidt, A.E., 1986. Supplemental mass pollination in a seed orchard of Pinus sylvestris L. investigated by isozyme analyses. Scand. J. For. Res., 1: 309-315. Yeh, F.C. and Rasmussen, S., 1985. Heritability of height growth in 10-year-old Sitka spruce. Can. J. Genet. Cytol., 27: 729-734. Zasada, J.C., Foote, M.J., Deneke F.J. and Parkerson, R.H., 1978. Case history of an excellent white spruce cone and seed crop in interior Alaska: cone and seed production, germination, and seedling survival. USDA For. Serv. Gen. Tech. Rep. PNW-65, 53 pp.
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Elsevier Science Publishers B.•., Amsterdam - - Printed in The Netherlands
Reproductive Phenology, Parental Balance, and Supplemental Mass Pollination in a Sitka-Spruce Seed-Orchard YOUSRY A. EL-KASSABY 1'~and SHEILA REYNOLDS 1
1Canadian Pacific Forest Products Limited, Tahsis Pacific Region, Saanich Forestry Centre, 8067 East Saanich Road, RR # 1, Saanichton, B.C. VOS 1MO (Canada) 2Faculty o[ Forestry, University of British Columbia, Vancouver, B.C. V6T 1 W5 (Canada) (Accepted 25 January 1989 )
ABSTRACT EI-Kassaby, Y.A. and Reynolds, S., 1990. Reproductive phenology, parental balance, and supplemental mass pollination in a Sitka-spruce seed-orchard. For. Ecol. Manage., 31: 45-54. Reproductive bud phenology and seed-cone crop of a Sitka-spruce [Picea sitchensis (Bong.) Carr. ] seed-orchard were studied to assess the validity of two seed-orchard assumptions, namely panmictic equilibrium, and parental balance. Also, the impact of supplemental mass pollination as a crop-management practice was evaluated. The study revealed significant variation in reproductive bud-development timing as well as proportion of reproductive strobili. A lack of mature male cones was observed while the majority of seed cones were in their receptive stage. It was also observed that approximately 80% of the seed-cone crop was produced by only 20% of the orchard clones. Supplemental mass pollination produced a 14-fold difference in the number of filled seeds per cone compared to that produced under wind pollination. In addition to the significant increase in seed-yield, supplemental mass pollination is proposed as a management option in Sitka-spruce seed-orchards to alleviate the effect of parental imbalance and panmictic disequilibruim.
INTRODUCTION
The Canadian Pacific Forest Products Ltd. Sitka spruce [Picea sitchensis (Bong.) Cart. ] tree-improvement program began in 1969 with the selection of parent trees and the establishment of the Nootka seed-orchard on the Saanich Peninsula of Vancouver Island, B.C. Testing was limited to open-pollinated families and provenance trials (Illingworth, 1976; Yeh and Rasmussen, 1985 ). Early cone production for the orchard started in 1976, and the first commercial crop was harvested in 1980. In order to attain the potential genetic improvement of a seed-orchard, seed quantity and quality should be maximized. Seed-yield in seed-orchards can be 0378-1127/90/$03.50
© 1990 Elsevier Science Publishers B.V.
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Y.A. EL-KASSABY AND S. REYNOLDS
increased by cultural and/or hormonal treatments (Philipson, 1983, 1985a,b ) and by supplemental mass pollination (Bridgwater and Bramlett, 1982; Hadders, 1984; Yazdani et al., 1986). Seed quality, on the other hand, is dependent on the fulfilment of certain assumptions regarding seed-orchards (Eriksson et al., 1973; Woessner and Franklin, 1973 ) such as: a) isolation from surrounding stands; b) synchronization of female and male reproductive strobili ripening; c) random mating; d) parental balance with equal compatibility for all possible crosses; and e) minimal rates of natural self-fertilization. In this paper, the validity of of two these assumptions, namely synchronization of female and male reproductive strobili ripening and parental balance, are assessed, and the effect of supplemental mass pollination as a crop-management practice is evaluated in the same 14-year-old Sitka-spruce seedorchard. MATERIALSAND METHODS The study was conducted in the Canadian Pacific Forest Products Ltd., Tahsis Pacific Region, 1.48-ha clonal Sitka-spruce seed-orchard in Saanichton, British Columbia (latitude 48°35'N, longitude 123°24'W). The orchard comprises 146 clones (averaging 9.3 ramets clone -1) selected from elevations between 0 and 415 m on western Vancouver Island, Washington and Oregon. The orchard, planted in a random single-tree mix over three unequal blocks, was established in 1971; however, newly grafted trees are being planted throughout the orchard due to mortality. Trees are spaced 3 m apart and kept at approximately 4 m in height by top-pruning. Reproductive-bud phenology data for the entire orchard (every sexually active tree) was collected by monitoring the orchard trees every second day throughout the 1985 pollination season. The phenological classification used followed Owens and Molder (1980) and Owens and Blake (1984). The critical stage of seed-cone (female) receptivity started when the seed-cone had just emerged fully from the bud scales [B + 2; (Owens and Blake, 1984) ]. The time between the beginning and end of pollen-shedding was used to identify the critical stage for the male reproductive cones (Owens and Molder, 1980). The orchard was divided into two parts; block 1 was left to be wind-pollinated (control), while blocks 2 and 3 received supplemental-mass-pollination treatment (SMP). The prevailing wind-direction and the size and relative position of the three blocks were the factors considered in dividing the orchard between the two treatments (control versus SMP). Supplemental pollen used in blocks 2 and 3 was a mixture of 1-year-old pollen collected and stored in airtight jars at - 2 0 oC, and fresh pollen collected from blocks 2 and 3 during the pollination period. The first pollen application consisted entirely of 1-year-old pollen, the second from a mixture of 1: 1 one-
REPRODUCTIVE PHENOIX)GY, P A R E N T A L BALANCE, A N D S M P IN A SITKA-SPRUCE SEED-ORCHARD
47
year-old: fresh pollen, and third consisted entirely of fresh pollen. The number of clones represented in any pollen mix was between 25 and 40, with an equal quantity from each clone. To account for within-tree variation, receptive trees were pollinated three to six times and at two levels (the upper crown, reached by ladders and the lower crown, reached by ground-crews). Pollen was applied using a hand-operated insecticide sprayer with a tubular wand. Pollen was pumped through the device until it was observed coming from the end of the wand. The amounts of pollen applied were not strictly controlled. The number of seed cones and the number of aborted cones on each tree in the two treatments were recorded during harvest. Samples of cones were collected from 15 trees for each treatment (67 and 81 cones for SMP and control, respectively). The individuality of cone lots and subsequent seedlots had been retained. Cones were air-dried at room temperature, seeds were extracted, dewinged and cleaned by hand. The number of filled seeds per cone for each sample tree for each treatment was obtained by radiograph on Kodak Industrex Instant 600 paper, using a Faxitron 804 X-ray unit and an exposure factor of 360 mA at 12.5 kV. Differences in number of filled seeds cone- 1 and number of aborted cones tree-1 between treatments (SMP and control) were checked for significance ( P < 0.05) by comparing bounds of confidence intervals. RESULTS AND DISCUSSION
The cone-receptivity period extended from 14 April to 17 May (Fig. 1 ). The maximal number of receptive seed cones and pollen-shedding cones occurred from 30 April to 1 May and 6-7 May, respectively. Pollen shedding began 10 days after the commencement of seed-cone receptivity. Maximal seed-cone receptivity was earlier than the peak of pollen shedding by 6 days. Receptive cone-bearers outnumbered pollen-producers for all but one of the inspections (Fig. 1 ). On 22, 24 and 28 April, respective totals of 38, 54 and 86 receptive trees were available, while respective totals of 0, 1 and 11 pollenshedding trees were available at these dates. The peak of receptivity ( 102 trees) occurred between 30 April and 1 May, while only 18 trees were available as a pollen source for that period. For ease of discussion, the receptive period (33 days) was divided into three equal classes: early (14-25 April), intermediate (25 April-6 May), and late (6-18 May). The early receptive class was characterized by a steady increase in the number of receptive trees {from 1 to 57) and the absence of pollen donors (only one tree on 24-25 April). The majority of receptive trees (57-102) was present in the intermediate class, while the number of pollen donors increased from 1 to 57, providing a steady increase in pollen production. The late period, on the other hand, was characterized by a sharp decline in the number of pollen donors (from 57 to 3 ) which accompanied a constant number of receptive trees for most of that period. This long receptive period is not unique to this species nor to the orchard. In fact, long receptive periods have been
48
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observed for several coniferous seed-orchards (Polk, 1966; Eriksson et al., 1973; Jonsson et al., 1976; Griffin, 1982; O'Reilly et al., 1983; E1-Kassaby et al., 1984, 1986; Askew, 1986; Fashler and E1-Kassaby, 1987 ).
REPRODUCTIVE PHENOLOGY, PARENTAL BALANCE, AND SMP IN A SITKA-SPRUCE SEED-ORCHARD
49
It must be emphasized that the length of receptive period in this orchard is also affected by the weather conditions during the pollination season. The receptive period extended to 34, 28 and 32 days for 1986, 1987 and 1988, respectively. This observation was similar to that reported on a Douglas-fir [Pseudotsuga menziessii (Mirb.) Franco] orchard in this area under natural conditions (E1-Kassaby et al., 1984) and/or cooling treatment (Fashler and EI-Kassaby, 1987). In addition, the first pollen-sheddingbegan 4 and 7 days after the commencementof seed-cone receptivity for 1987, and 1986 and 1988, respectively. The proportion of pollen-sheddingtrees for these 3 years (198688) during the early and intermediate classes was similar to that observed in 1985, and supplemental mass pollination was the only reliable source of pollen available. The length of the receptive period that accompanied either a lack of, or a disproportionate number of, pollen donors suggests that the orchard population is in a panmictic disequilibrium, and that there was a substantial shortage of pollen. The duration of receptivity of individual Sitka-spruce cones ranges from 6 to 8 days (Owens and Blake, 1984) and the duration of pollen-shedding averages 7 days (Owens and Molder, 1980). Therefore, clones with receptive seed-cones in the early class cannot be pollinated by pollen produced by any of the three (early, intermediate, and late) pollination classes described here. In addition, intermediate and late-receptive seed cones will also produce a continuum of small effective breeding populations throughout the rest of the pollination period, which is characterized by an apparent lack of pollen. This situation was observed in Douglas-fir and loblolly-pine (Pinus taeda L.) seedorchards (E1-Kassaby et al., 1984,1988; Askew, 1986; Fashler and E1-Kassaby, 1987). A significant (P<0.05) difference in the average number of filled seeds cone -~ was observed in the 13.9-fold difference between SMP (75.1 +32.6) and control (5.4 + 5.7). This observed increase in seed-yieldappears to be higher than the average seed-set expected under wind pollination in natural stands (J.N. Owens, Biology Department, University of Victoria, B.C. personal communication, 1988). This corresponds with the observed lack of pollen donors throughout the receptive period. Although pollen viability of the 1-year-old pollen used in the SMP treatment was not determined, the high seed-set obtained indicated adequate pollen viability. The number of aborted cones tree-1 showed no significant difference between SMP (1.6 + 0.9) and control (1.3 + 0.4), indicating that Sitka-spruce cones develop to full size in the absence of pollen (i.e., are parthenocarpous). It has been reported that Sitka-spruce cones reached harvest size with even less than 1% seed set (Owens and Blake, 1984, pp. 1141 and 1148). In Picea, it was reported that unfertilized ovules develop partially, then abort, and that unpollinated cones are reduced in size (Dogra, 1967; Zasada et al., 1978). Differences in seed-set between SMP and control treatments have demon-
50
Y.A. EL-KASSABY AND S. REYNOLDS
.strated that supplemental mass pollination is a viable crop-management practice in Sitka-spruce seed-orchards to achieve crop potential. Supplemental mass pollination has been reported to increase seed-yield in young Douglas fir (Daniels, 1978) and loblolly pine (Bridgwater and Bramlett, 1982) seed orchards; however, no increase was observed in older orchards (Bridgwater and Bramlett, 1982; and own unpublished data, 1988). In addition, the very low filledseed count in the control treatment has suggested that Sitka-spruce background pollen was negligible in the orchard area. In fact, this orchard is 10 km from the nearest Sitka-spruce orchard, and 40 km from the nearest natural trees (Sooke, B.C., 48°21'N, 123°50'W). Parental balance in the 1985 cone crop was assessed using the cumulative cone-yield curve {Griffin, 1984). The cone crop was ranked by clone from high to low cone-yield, and the cumulative yield in percentage was plotted against the percentage of clones censused (Fig. 2). Assuming that reproductive energy is equal to reproductive success (i.e., the number of filled seeds cone- 1is equal across clones), it appears that 20% of the clones produced 80% of the total seed crop (i.e., 29 of 146 clones produced the majority of that year's cone crop). This '20: 80' ratio is identical to that reported by the North Carolina Tree Improvement Co-operative for several seed orchards {Anonymous, 1976), and similar to that reported for Picea abies (Eriksson et al., 1973), Picea mariana (O'Reilly et al., 1983), Pinus radiata {Griffin, 1982), Pinus taeda (Schmidtling, 1983), and Pseudotsuga menziesii (EI-Kassaby et al., 1989). The degree of distortion in the parental balance in seed-orchards is influ-
CONE PRODUCTION I00 o
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Y
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REPRODUCTIVE PHENOLOGY, PARENTAL BALANCE, A N D S M P IN A SITKA-SPRUCE SEED-ORCHARD
51
enced by the crop size and tree type (i.e.,grafts vs. seedlings; EI-Kassaby et a].,1989). Parental-balance ratios of'35: 80' and '42:80' were obtained for this orchard for 1986 and 1988 (good cone years) and 1987 (moderate cone year). The observed improvement in the parental-ba]ance ratio across the four years (1985-88) could be caused by differences in cone-crop size or age of trees. The study has revealed three problems in this Sitka-spruce seed-orchard: 1 ) inadequate pollen supply; 2) panmictic disequilibrium; and 3) parental imbalance. Effective seed-orchard management can alleviate all these problems. An annual reproductive-phenology evaluation, accompanied by SMP of receptive seed-cones, can solve allthe aforementioned problems. The difference in filledseed-set that was observed between SMP and control treatments (75.1 versus 5.4 seeds cone -I ) indicate seed-set is dependent on SMP when phenology of receptivity and pollen availabilitydiffer strongly. Figure I indicates that pollen production started in the intermediate- and ended in the late-pollination period. The collection,extraction, and over-winter storage of this pollen will provide the seed-orchard manager with the option of pollinating early-, intermediate- and late-receptive seed cones with pollen collected from any period. In other words, differences in reproductive phenology can be overcome by the SMP treatment, producing one close, panmictic, breeding population. Parental imbalance due to the differentialcone production among the orchard clones also can be reduced by the use of pollen from less-fecund clones in the SMP mixes. This method may over-emphasize the presence of low-seed-cone-producing clones in the pollen pool. The use of SMP for adjusting panmictic disequilibrium and parental imbalance in seed orchards has been proposed and used for Douglas-fir seed orchards (EI-Kassaby et a]. 1984, 1986, 1989; Fashler and EI-Kassaby, 1987; Reynolds and EI-Kassaby, 1989). The SMP treatment has been empirically proven to be successful in incorporating desired genotypes into orchard seed crops (Yazdani et a].,1986). Clones of low fecundity could be stimulated to produce cones through the use of cultural (top- and root-pruning, girdling,heat and drought) and/or hotmona] (exogenous gibberellin) treatments (Tompsett and Fletcher, 1977, 1979; Philipson, 1983,1985a,b; Marquard and Hanover, 1984,1985; Ross, 1985, 1988a). However, it must be emphasized that all cone-induction treatments (cultural and hormonal) are stress treatments (Ross and Pharis, 1985a,b; Ross, 1988b), and, consequently, their application might generate permanent genetic changes in the resultant seed crop (Durrant, 1962; Durrant and Timmis, 1973; Wills, 1984; AI-Saheal and Larik, 1987; Cullis, 1987). Therefore, genetic evaluation of stress treatments is advised. Nitrogenous fertilizationand hormonal application to forest trees have changed germinative speed (Allen, 1961; Puritch et al.,1979 }. In addition it is commonly known, though not reported, that the gibbereUin applications produce smaller cone size. Changes in cone size have been shown to affect seed size and subsequent seedling size (Sorensen and Campbell, 1985). Therefore, a combination of changes in germinative speed
52
Y.A. EL-KASSABY AND S. REYNOLDS
and final seedling size in container nurseries could affect the genetic diversity of seedling crops through nursery practices, specifically thinning and culling (Campbell and Sorensen, 1984). ACKNOWLEDGEMENTS
The authors thank M.D. Meagher and the two referees for their constructive comments, M. Crown for valuable discussions, C. Cook, J. Devitt, B. Goodwin, C. Haberlin, D. MacLeod and B. Newbery for their patience during seed extraction, M. Robertson for typing, and A. Philip for drafting. Special thanks to D.G.W. Edwards and D. Taylor of the Canadian Forestry Service for use of the seed X-ray facilities. This study was supported by the British Columbia Ministry of Forests, Silviculture Branch, Section 88 (1). REFERENCES Allen, G.S., 1961. Testing Douglas-fir seed for provenance. Int. Seed Test. Assoc. Proc., 26: 388402. A1-Saheal, Y.A. and Larik, A.S., 1987. Genetic control of environmentally induced DNA variation in flax genotrophs. Genome, 29: 643-646. Anonymous, 1976. Twentieth annual report in cooperative tree improvement and hardwood research program. North Carolina State Univ., Raleigh. Askew, G.R., 1986. Implications of non-synchronous flowering in clonal conifer seed orchards. In: A.V. Hatcher and R.J. Weir (Editors), Proc. IUFRO Joint Meeting of Working Parties on Breeding Theory, Progeny Testing and Seed Orchards, Williamsburg, Virginia, October 1986. Tree Improvement Co-op., North Carolina State Univ., Raleigh, pp, 182-191. Bridgwater, F.E. and Bramlett, D.L., 1982. Supplemental mass pollination to increase seed yields in loblolly pine seed orchards. South. J. Appl. For., 6" 100-104. Campbell, R.K. and Sorensen, F.C., 1984. Genetic implications of nursery practices. In: M.L. Duryea and T.D. Lands (Editors), Forest Nursery Manual for Production of Bareroot Seedlings. Dr. W. Junk, Boston, pp. 183-191. Cullis, C.A., 1987. The generation of somatic and heritable variation in response to stress. Am. Nat., 130(s): 62-73. Daniels, J.D., 1978. Efficacy of supplemental mass-pollination in Douglas-fir seed orchards. Silvae Genet., 27: 52-58. Dogra, P.D., 1967. Seed sterility and disturbances in embryogeny in conifers with particular reference to seed testing and tree breeding in Pinaceae. Stud. For. Suec., 45: 1-97. Durrant, A., 1962. The environmental induction of heritable changes in Linum. Heredity, 17: 2761. Durrant, A. and J.N. Timmis, 1973. Genetic control of environmentally induced changes in Linum. Heredity, 30: 369-379. E1-Kassaby, Y.A., Fashler, A.M.K. and Sziklai, 0., 1984. Reproductive phenology and its impact on genetically improved seed production in a Douglas-fir seed orchard. Silvae Genet., 33: 120125. E1-Kassaby, Y.A., Davidson, R. and Weber, J.W., 1986. Genetics of seed orchards: a Douglas-fir case study. In: A.V. Hatcher and R.J. Weir {Editors), Proc. IUFRO Joint Meeting of Working Parties on Breeding Theory, Progeny Testing and Seed Orchards, Williamsburg, Virginia, October 1986. Tree Improvement Co-op., North Carolina State Univ., Raleigh, pp. 410-421.
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E1-Kassaby, Y.A., Ritland, K., Fashler, A.M.K. and Devitt, W.J.B., 1988. The role of reproductive phenology upon the mating structure of a Douglas-fir seed orchard. Silvae Genet., 37: 76-82. EI-Kassaby, Y.A., Fashler, A.M.K. and Crown, M., 1989. Variation in fruitfulness in a Douglasfir seed orchard and its effect on crop management decisions. Silvae Genet., 38: 113-121. Eriksson, G., Jonsson, A. and Lindgren, D., 1973. Flowering in a clonal trial ofPicea abies Karst. Stud. For. Suec., 110: 5-45. Fashler, A.M.K. and El-Kassaby, Y.A., 1987. The effect of water spray cooling treatment on reproductive phenology in a Douglas-fir seed orchard. Silvae Genet., 36: 245-259. Griffin, A.R., 1982. Clonal variation in radiata pine seed orchards. I. Some flowering, cone and seed production traits. Aust. J. For. Res., 12: 295-302. Griffin, A.R., 1984. Clonal variation in radiata pine seed orchards. II. Flowering phenology. Aust. J. For. Res., 14: 271-281. Hadders, G., 1984. Supplemental and controlled mass pollination in seed orchards of Scots pine (Pinus sylvestris L.). Foreningen Skogstradsforadling, Institutet for Skogsforbattring, ~,rsbok 1984, pp. 47-64. Illingworth, K., 1976. Sitka spruce international ten provenance experiment in British Columbia Phase I and II (nursery) results. In: J. O'Driscoll (Editor), IUFRO Sitka Spruce International Ten Provenance Experiment. Dept. Lands, Forests and Wildlife Serv., Dublin, Ireland, pp. 278-310. Jonsson, A., Ekberg, I. and Eriksson, G., 1976. Flowering in a seed orchard of Pinus sylvestris L. Stud. For. Suec., 135: 1-38. Marquard, R.D. and Hanover, J.W., 1984. Relationship between GA4/7 concentration, time of treatment, and crown position on strobilus production of Picea glauca. Can. J. For. Res., 14: 547-553. Marquard, R.D. and Hanover, J.W., 1985. Floral response of Picea glauca to gibberellin A4/7, naphthaleneacetic acid, root-pruning, and biennial treatment. Can. J. For. ires., 15: 743-746. O'Reilly, C., Parker, W.H. and Barker, J.E., 1983. Effect of pollination period and strobili number on random mating in a clonal seed orchard of Picea mariana. Silvae Genet., 31: 90-94. Owens, J.N. and Blake, M.D., 1984. The pollination mechanism of Sitka spruce (Picea sitchensis). Can. J. Bot., 62: 1136-1148. Owens, J.N. and Molder, M., 1980. Sexual reproduction of Sitka spruce (Picea sitchensis). Can. J. Bot., 58: 886-901. Philipson, J.J., 1983. The role of gibberellin A4/7,heat and drought in the induction of flowering in Sitka spruce. J. Exp. Bot., 34: 291-302. Philipson, J.J., 1985a. The promotoin of flowering in large field grown Sitka spruce by girdling and stem injections of gibberellin A4/7.Can. J. For. Res., 15: 166-170. Philipson, J.J., 1985b. The effect of top pruning, girdling, and gibberellin A4/7 application on the production and distribution of pollen and seed cones in Sitka spruce. Can. J. For. Res., 15: 1125-1128. Polk, R.B., 1966. Reproductive phenology and precocity as factors in seed orchard development. In: H.B. Kriebel (Editor), 5th Central States Forest Tree Improvement Conf., October 1966. Ohio Agricultural Research and Development Center, Wooster, OH, pp. 977-1007. Puritch, G.S., McMullan, E.E. Meagher M.D. and Simmons, C.S., 1979. Hormonal enhancement of cone production in Douglas-fir grafts and seedlings. Can. J. For. Res., 193-200. Reynolds, S. and E1-Kassay, Y.A., 1989. Parental balance in Douglas-fir seed orchards - - cone crop vs. seed crop. Silvae Genet. (in press). Ross, S.D., 1985. Promotion of flowering in potted Picea engelmannii (Perry) grafts: effects of heat, drought, gibberellin A4/7and their timing. Can. J. For. Res., 15: 618-624. Ross, S.D., 1988a. Effects of temperature, drought, and gibberellin A4/7and timing of treatment,
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on flowering in potted Picea engelmanni and Picea glauca grafts. Can. J. For. Res., 18: 163171. Ross, S.D., 1988b. Pre- and post-pollination polyhouse environment effects on pollen and seed development in potted Picea engelrnannii grafts. Can. J. For. Res., 18: 623-627. Ross, S.D. and Pharis, R.P., 1985a. Promotion of flowering in crop trees: different mechanisms and techniques, with special reference to conifer. In: M.G.R. Cannell and J.E. Jackson (Editors), Attributes of Trees as Crop Plants. Institute of Terrestrial Ecology, Monks Wood Exp. Stn., Abbots Ripton, Huntingdon, Great Britain, pp. 383-397. Ross, S.D. and Pharis, R.P., 1985b. Status of flowering in conifers: a constraint to tree improvement: In: F. Caron, A.G. Corriveau and T.J.B. Boyle (Editors), Proc. 20th Meeting, Canadian Tree Improvement Assoc., Part 2, pp. 11-28. Schmidtling, R.C., 1983. Genetic variation in fruitfulness in a loblolly pine (Pinus taeda L.) seed orchard. Silvae Genet., 32: 76-80. Sorensen, F.C. and Campbell, R.K., 1985. Effect of seed weight on height growth of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco vat menziesii) seedlings in a nursery. Can. J. For. Res., 15: 1109-11115. Tompsett, P.B. and Flescher, A.M., 1977. Increased flowering of Sitka spruce [Picea sitchensis (Bong.) Carr. ] in a polythene house. Silvae Genet., 26: 84-86. Tompsett, P.B. and Fletcher, A.M., 1979. Promotion of flowering on mature Picea sitchensis by gibberellin and environmental treatments. The influence of timing and hormonal concentration. Physiol Plant., 45: 112-116. Wills, C., 1984. The possibility of stress-triggered evaluation. In: G.S. Mani (Editor), Lecture Notes in Biomathematics (53) Evolutionary Dynamics of Genetic Diversity. Springer, Berlin, pp. 299-312. Woessner, R.A. and Franklin, E.C., 1973. Continued reliance on wind-pollinated southern pine seed orchards - is it reasonable? In: R.J. Dinus, B.A. Thielges and O.O. Wells (Editors), Proc. 12th Southern Tree Improvement Conf., June 1973. Louisiana State Univ., Baton Rouge, pp. 64-73. Yazdani, R., Hadders G. and Szmidt, A.E., 1986. Supplemental mass pollination in a seed orchard of Pinus sylvestris L. investigated by isozyme analyses. Scand. J. For. Res., 1: 309-315. Yeh, F.C. and Rasmussen, S., 1985. Heritability of height growth in 10-year-old Sitka spruce. Can. J. Genet. Cytol., 27: 729-734. Zasada, J.C., Foote, M.J., Deneke F.J. and Parkerson, R.H., 1978. Case history of an excellent white spruce cone and seed crop in interior Alaska: cone and seed production, germination, and seedling survival. USDA For. Serv. Gen. Tech. Rep. PNW-65, 53 pp.