Predicting cone crop potential in conifers by assessment of developing cone buds and cones

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1 Predicting cone crop potential in conifers by assessment of developing cone buds and cones J. J. PHILIPSON Forestry Commission Research Division, Northern Research Station, Roslin, Midlothian EH25 9SY, Scotland Summary The reproductive cycles of the main conifers in Britain are described, from cone bud initiation to the shedding of mature seed, and buds and cones are described at characteristic developmental stages. Potential cone crops could be predicted by estimating the number of developing cone buds, newly flushed cones or mature seed cones. These predictions will be valuable for estimating potential natural regeneration in the forest and will allow forest managers to plan silvicultural operations to encourage natural regeneration if this is desired. The most suitable time of assessment will depend on the species; earlier predictions will be less accurate and more difficult to carry out, while later predictions will give less advance warning of potential seed crops and potential natural regeneration. Introduction Research has been carried out in North America to predict cone crop potential in a number of species in the Pinaceae in order to assist with the planning of seed cone collections (Dobbs et al., 1976). Predictions have been made on the basis of identifying and counting reproductive buds during the months before flushing, by estimating the number of cones when they emerge, and by assessing and evaluating the crop of mature seed cones before seed is shed. Early prediction of coning is valuable for assessing the potential for natural regeneration and can help forest managers to plan silvicultural operations (Clark, 1992). It must be stressed that only the potential coning of the crop is being predicted as many factors may subsequently reduce the O Institute of Chartered Forester*, 1997 final crop of mature seed which is shed (Dobbs etal., 1976). Frosts in spring can damage emerging cones, poor weather, particularly rain at the time of pollen shedding, can reduce pollination and insect damage and premature conelet drop may occur. Some of the latter points were discussed by Harmer (1994) with reference to broadleaves and he considered that the only sure way of determining the seed crop is after seed fall. However, research, particularly in Fennoscandia, has used climatic data to calculate the probability that seed cones produce a mature crop of seed (Henttonen etal., 1986) and there are techniques using X-ray analysis and conductivity methods to give an early indication of whether developing seeds have the potential to fully mature (Sahlen and Gjelsvik, 1993; Forestry, Vol. 70, No. 1, 1997

2 88 FORESTRY Sahlen and Bergsten, 1994). The current paper outlines reproductive cycles, describes developing cone buds and cones, and considers methods for predicting cone crop potential in the major conifers in Britain, and the findings will be relevant to these species in other parts of their geographical range. The species studied are: Sitka spruce {Picea sitchensis (Bong.) Carr.), Norway spruce {Picea abies (L.) Karst), Douglas fir (Pseudotsuga menziesii (Mirb.) Franco), European larch {Larix decidua Miller), Japanese larch {Larix kaempferi (Lambert) Carr.), lodgepole pine {Pinus contorta Douglas), Corsican pine {Pinus nigra var. maritima (Aiton) Melville) and Scots pine {Pinus sylvestris L.). Types of reproductive cycle Two types of reproductive cycle are exhibited in the conifers considered here. The spruces, larches and Douglas fir have a 2-year cycle. In these species cone buds are initiated and differentiate in the early summer (year one). In the following year (year two) the cone buds flush, pollination and fertilization occur and embryo and seed development are completed. Seed fall begins in the autumn and may continue into the spring of a third year. In the pines, in contrast, the reproductive cycle occurs over 3 years. Cone buds differentiate in year one, and in year two they flush and pollination occurs; ovule development begins but is not completed. Fertilization occurs in the spring of year three and seeds are mature by the autumn. Seed shed begins in the autumn of year three but cones containing full seed can be retained on the trees for several years. Spruces Cone bud development has been studied in several of the spruces (see Owens and Blake, 1985) and is broadly similar in Sitka spruce and Norway spruce. The description given here is based on research for Sitka spruce, and where development of Norway spruce differs this is highlighted. Position of cones In spruces seed cones occur towards the top of the crown and pollen cones are concentrated in the mid to upper crown, though in Norway spruce the region bearing pollen cones can extend to the lower crown. Both types of cone are borne on 1-year-old shoots. In Sitka spruce seed-cone buds develop from lateral buds on vigorous shoots, or from smaller terminal apices on less vigorous shoots, whereas pollen-cone buds usually develop from small axillary or small terminal apices on less vigorous shoots (Owens and Molder, 1976; Fletcher, 1992). In Norway spruce the position of the buds on the shoots is more distinct, and this may aid early identification of buds. The female buds appear only in terminal positions and the male buds occur in lateral positions along the length of the shoot, though they are predominantly on the lower half (basal end) of the shoot (Eis and Craigdallie, 1981). Reproductive cycle In year one bud differentiation begins at about the cessation of lateral shoot elongation, towards the end of July (Owens and Molder, 1976; Fletcher, 1992), when there is an increase in the cell division within the bud and buds undergo the transition to reproductive or vegetative structures. At this stage the anatomical changes can only be detected with difficulty after sectioning, staining and examination at the cellular level with the microscope (for details of cytological changes see Owens and Molder, 1976). In September the buds are still small and the external shape and size of female, male and vegetative buds are similar: broadly conical, greenish-brown and covered in a bloom of light grey resin (Eis and Craigdallie, 1981). Buds become dormant during November, at which time they contain all their needles, microsporophylls or bracts and ovuliferous scales for the next season (Owens and Molder, 1976). The timing of cone bud differentiation in Norway spruce in Britain is likely to be similar to that described for Sitka spruce, though male-cone buds may be initiated slightly earlier than female-cone buds. During winter dormancy it is extremely diffi-

3 PREDICTING CONE CROP POTENTIAL IN CONIFERS 89 cult to identify buds on external appearance in January (Moir and Fox, 1975) or before March of year two (Fletcher, 1992). Eis and Craigdallie (1981) report that dissection of the buds does facilitate identification; however their photographs indicate that some early spring bud growth may have occurred. The dissection procedure would be either to slice the bud longitudinally or carefully to cut away the bud scales to reveal the apex. At this stage of development (i.e. February of year two) the pattern of sporophylls in male and female buds can be distinguished in the dissected buds from the pattern of needle primordia in the vegetative buds (Moir and Fox, 1975). (For photographs see Dobbs et al., 1976; Eis and Craigdallie, 1981; and Owens and Molder, 1984a for buds of interior spruce, i.e. Picea glauca (Moench) Voss and Picea engelmannii Parry.) By late April of year two the buds have begun to swell and bud types can be identified on external appearance. The vegetative buds are smallest, they have a conical shape intermediate between that of the two types of cone bud, and a pointed apex. The male buds are rounded and the female buds have an elongated shape. In order to identify buds on mature trees, shoots would have to be sampled from the tree, either by climbing the trees, felling, or by using pruning shears. By mid May the cones have flushed and are readily visible on the trees. The pollen cones appear red and as they elongate and pollen is shed they turn yellow. Once seed cones have opened they turn to the vertical position and are green, reddish-green or red. Seed cone numbers could be estimated on the trees using binoculars once female cone elongation is completed; the best time for assessment would probably be before vegetative shoots have begun to flush in late May or early June. Pollen shedding and pollination occur in late May and fertilization occurs in late June. By late August the seed cones are green and pendant on the trees. During September the pendant seed cones are reddish brown in the tops of the trees and this is another opportunity for coning to be estimated, possibly using binoculars. The mature cones of Norway spruce are larger than those of Sitka spruce, being about cm in length. Seed shedding In Sitka spruce shedding usually begins in mid to late October (Fletcher, 1992) and is recorded to occur in two phases, firstly in early autumn of year two and again in January or February of year three (Harris, 1969; Mair, 1973), but the time of shedding is variable and may occur throughout this period when conditions are dry. In Norway spruce development in year two lags behind that of Sitka spruce by about 2-3 weeks (Eis and Craigdallie, 1981; Gordon and Faulkner, 1992). Thus cone buds first flush in late May in Britain, and Gordon and Faulkner (1992) suggest crop assessments in June, but seed shedding begins at about the same time as in Sitka spruce, in late October. The timing of developmental stages of the reproductive cycle of several species is given in Table 1. Douglas fir Position of cones There is considerable overlap of the seed cone and pollen cone bearing regions of the crown in Douglas fir, and both types of cone can occur on the same shoot (Eis and Craigdallie, 1981; Owens and Blake, 1985). On each shoot the terminal bud is usually vegetative, seed-cone buds are produced towards the apical end of the shoots often in sub-terminal positions and pollen-cone buds occur along the length of the shoot but are most frequent towards the middle or base of the shoot (Eis and Craigdallie, 1981) and often occur in clusters (Dobbs et al., 1976; Fletcher, 1992). Reproductive cycle As with the spruces, Douglas fir buds first become anatomically determined when lateral shoot elongation is nearly complete, about the end of July (Allen and Owens, 1972; Owens, 1973; Owens and Blake, 1985). From mid August bud types can be identified from their morphological appearance, according to a study from North America (Eis and Craigdallie, 1981) but at this stage it may be necessary to dissect the bud to aid identification. Bud development continues until winter dormancy in late October

4 Table I: Timing of certain stages in the reproduaive cycle of selected conifers in Britain Species Beginning of reproductive bud differentiation Macroscopic identification of reproductive buds (dissection required) Reproductive bud identification on external morphology Flowers emerge Seed shed begins Sitka spruce Norway spruce Douglas fir Larches Pines End July Year 1 End July Year 1 End July Year 1 End July Year 1 Males: July September Year 1 Females: August-October Year 1 February-March Year 2 February March Year 2 Late August Year 1 September November Year 1 (male and female buds distinguishable) Males: October Year 1 Late April Year 2 Early May Year 2 October Year 1 September-October Year 1 (male and female buds not distinguishable) Males: October Year 1 Females: April Year 2 Mid May Year 2 Late May Year 2 Late April Year 2 March Year 2 May-Year 2 (Seed conelets present September Year 2) October Year 2 October Year 2 End September Year 2 Japanese larch: October Year 2 European larch: January Year 3 Lodgepole pine (var. contorta): October Year 3 Scots and Corsican pine: February March Year 4) \j 7) m on H <a <

5 PREDICTING CONE CROP POTENTIAL IN CONIFERS 91 to early November, and during this time identification becomes progressively easier (Eis and Craigdallie, 1981). In Britain buds become dormant in October and can be identified externally at this time (Fletcher, 1992). Dormant vegetative buds are slender, without any swelling at the base, and dark coloured with conspicuous shaggy edged bud scales. When the buds are sliced open the vegetative apex is about 2 mm long and the rest of the bud cavity is empty and the green needle primordia can be identified. Dormant male buds are ovate and widest at the middle, they have smooth edged glossy bud scales and are lighter in colour than the vegetative buds. The bud cavity is filled with the spirally arranged microsporophylls, which have a characteristic raspberry-like form. The female-cone buds are of similar size to, or slightly larger than, the male buds and are ovate, widest at the base, and more pointed than the male buds. The bud cavity is filled by the developing apex and the three pronged bracts are identifiable (Dobbs et al., 1976; Eis and Craigdallie, 1981). Pollen-cone buds flush in mid April of year two, and seed-cone buds have emerged and are becoming erect by mid April, and thus the cones can be assessed on the trees in late April. The pollen cones are initially pinkish-red, as they elongate they turn downwards and become pinkish-yellow and shed pollen at about the end of April. The erect seed cones are about 3 cm long and normally reddish and are characterized by long extended bracts which are reflexed back at the time of pollination (Fletcher, 1992). Pollination occurs towards the end of April, seed cones then become pendant and fertilization takes place in early June and the cones elongate and reach their final length in early July. As the seed cones mature in late August the bracts turn a light brown colour. Seed shedding Seed fall begins in late September and the majority of the seed has been shed by early November (Fletcher, 1992). As with other species the time of seed fall depends on climatic conditions and is favoured by drying winds (Mair, 1973). European and Japanese larch Position of cones Cones occur throughout the crown in larches and there may be considerable overlap in the male and female flowering regions. The seed cones occur most commonly in the upper crown on more vigorous branches which are often upswept or slightly pendant. Pollen cones occur in the lower crown on less vigorous branches which are typically pendant (Owens and Blake, 1985; Fletcher, 1992). The cone buds differentiate on short shoot buds and in L. decidua and L. leptolepis differentiate on shoots which are at least one year old. Thus emerging cones occur on shoots which are at least 2 years old. Reproductive cycle Studies of western larch (L. occidentalis Nutt.) (Owens and Molder, 1979a, b) and European and Japanese larch (Fletcher, 1992) indicate that cone bud differentiation begins at about the end of vegetative lateral long shoot elongation, which is in late July for European and Japanese larch in Britain. Cone bud development continues into October and all the primordia have differentiated by winter dormancy in early November. In September to October of year one reproductive buds are larger than the vegetative structures and sometimes slightly pointed and identification on external appearance may be possible, but it is not possible to distinguish between male and female buds based on external appearance. Male flowering in larch can be much more prolific than female flowering and an assessment at this stage could give an incorrect prediction of seed cone potential. Roe (1966), however, reported that in western larch the male and female winter buds are distinguishable based on external characteristics. By dissecting the buds it may be possible to distinguish between pollen- and seed-cone buds from about October (see photographs of sectioned buds of L. occidentalis in Dobbs et al., 1976). The vegetative buds are small and round and in section the needles are described as having an artichoke like appearance. The female bud is the

6 92 FORESTRY largest, it is oval and in section has a conical core and overlapping bracts. The male bud is intermediate in size and has a raspberry like appearance in section. In the spring of year two cone bud development resumes in February to March and meiosis is completed in the pollen cones. At this time larch is susceptible to low temperatures and frosts which can damage the cone buds and may contribute to the production of infertile or poor quality pollen (Eriksson, 1970; Owens and Blake, 1985). Bud break is dependent on temperature and in Japanese larch can be a few weeks in advance of that for European larch (Fletcher, 1992). Pollen-cone buds enlarge rapidly during the completion of meiosis and flush in early March, up to 3 weeks in advance of pollination. At flushing they remain oval and a reddish tinge of the microsporophylls is apparent; the cones then elongate and they become yellow at pollen shedding. The seed-cone buds become larger and pointed before bud burst; after flushing elongation of the cone continues for 2-3 weeks and it becomes erect. European larch cones are normally red, and Japanese larch cones are green. Meiosis in the seed cone occurs at the time of pollination and the cone continues to elongate at this time. A good period to assess cones on the trees is just before vegetative buds are flushing, in early April. Fertilization occurs 6 8 weeks after pollination and the seeds are mature in late September to early October; the mature seed cones could be assessed at this time. Seed shedding Seed shed in Japanese larch can begin in October and continues into the spring of year three, as winter frosts encourage the release of seed. Seed shedding in European larch begins in January to February of year three and is usually completed during the summer. Drying winds encourage the cones to open and release seed but the cones of European larch do not always open readily and occasionally cones may remain closed for several years before releasing seed (A.M. Fletcher, personal communication). Seed cones of both species are retained on the trees for several years and if mature cones were assessed in the autumn care would be required to distinguish between current year cones and older cones. Pines Cone development is similar in Scots pine, lodgepole pine and Corsican pine. The description given here is based on a description given for these three species by Fletcher (1992) and on descriptions for lodgepole pine by Eis and Craigdallie (1981) and Owens and Molder (1984b). Position of cones Both male and female cones may occur over the entire crown (Eis and Craigdallie, 1981), though seed cones are more abundant on the more vigorous shoots in the upper crown and pollen cones occur more frequently on less vigorous shoots in the lower crown (Fletcher, 1992). The two types of cone are usually borne on separate shoots, though in lodgepole pine both types commonly occur on the same shoot. The male cones are spirally arranged in clusters at the base of the current year shoots. The female cones occur nearer the apex of the shoot, either singly in lateral positions, or in groups in subterminal positions adjacent to next year's vegetative buds. When male and female cones occur on the same shoot they may be separated by about 15 cm of vegetative shoot at the time of pollination (Eis and Craigdallie, 1981). Reproductive cycle Pines produce two types of shoot, long shoots or branches and short shoots or needles. Reproductive buds differentiate within the long-shoot buds, seed cones replacing future long shoots, and pollen cones replacing future short shoots. Pollen cone initiation occurs at the base of the long shoots in late June to early July (Scots pine, Fletcher, 1992) or in August to September (lodgepole pine, Owens and Blake, 1985). Seed cones are initiated in more distal positions in late summer (August, for Scots pine, Fletcher 1992; October, for lodgepole pine, Owens and Blake, 1985). Thus in pines the time of differen-

7 PREDICTING CONE CROP POTENTIAL IN CONIFERS 93 tiation of pollen cones differs from that of seed cones. Pines have a reproductive cycle that spans 3 years. After cone bud initiation, development continues into the autumn of year one. By October all the microsporophylls in the pollen-cone buds have been initiated. In the developing seedcone bud bract scale initiation begins in September but only those bracts at the base of the developing cone are initiated before winter dormancy in November (see photographs of developing buds in Eis and Craigdallie, 1981; and Owens and Molder, 1984b). During winter dormancy vegetative buds are long and slim, without any swelling at the base. If pollen-cone buds have formed, a swelling at the base of the vegetative bud indicates their presence. If the vegetative bud is sliced open at the time of winter dormancy the needle primordia may be detected at the tip of the bud and clusters of pollen cones at the base. If more numerous pollen cones have been initiated the sheath of bud scales which normally covers them is ruptured to reveal a cluster of buds, and identification is thus easier. The bud containing the female cones is similar to the vegetative bud, and partly because the female cones are not fully developed at this time it is usually impossible to identify a potential seed cone crop by external examination (Eis and Craigdallie, 1981; Fletcher, 1992). Cone buds flush in May of year two and pollen shedding and pollination occurs in late May or early June. The pollen cones are visible as yellow clusters at the base of elongating shoots. The female cones can be detected by their reddish bracts, and are visible in the lateral or sub-terminal positions of the shoot. The female cones are reported to be conspicuous because elongation of needles has not been completed, but cones in the upper crown may not be readily visible from the ground. Following pollination, ovule development in the seed cones begins but is halted by mid summer, and the cones close to form purple conelets. At the end of the growing season of year two the conelets are greenish-brown and are about 15 mm long. These conelets could be assessed but sometimes they are shed at this immature stage and conelet drop may be as much as per cent in Scots pine (A.M. Fletcher, personal communication). Some shedding of full-size cones in year three, before seed is shed, has also been reported (Brown, 1971; Bramlet, 1974). Fertilization occurs in early July of year three and embryo development is complete in late August for lodgepole pine or November for Scots and Corsican pine (Fletcher, 1992) and these mature cones could be assessed in the late summer. Seed shedding The timing of seed shed varies between species. In coastal provenances of lodgepole pine (P. contorta var. contorta) shedding begins in autumn of year three and is complete by spring of year four. In interior provenances (P. controta var. latifolia), although cones of some origins will open in October of year three, others will not open until they have received very high temperatures in a fire, and thus they do not open readily in Britain, and cones may be retained on the trees for many years. In Scots pine and Corsican pine cones will begin to open during February and March of year four and shedding is usually complete by June (A.M. Fletcher, personal communication). Summary of the stages at which coning could be assessed and predicted Reproductive buds of spruces cannot readily be recognized on external morphology during the year of bud initiation. Assessments of coning potential could be attempted in April of year two if developing buds were sampled from the trees and identified by external morphology, but this method may not give an accurate estimate. Male and female cones could be assessed on the trees using binoculars in late May of year two after the cone buds have flushed, about 5 months before seed shedding begins, and this may be the most suitable time to estimate cone crop potential in spruces. There may be some further losses of cone and seed potential after this, due to frost or poor pollination, and the number of semi-mature, green, seed cones present could be estimated during June or July. Douglas fir cone buds can be identified on the basis of external morphology in October of year one, 12 months before seed shedding, and this is

8 94 FORESTRY a possible time to assess cone crop potential. The emerging cones can be recognized and estimated on the trees in late April of year two, 5-6 months before seed shedding, and this would provide a more accurate estimate of the crop potential, or mature seed cones could be estimated in the early autumn. Larch cone buds can be recognized on external morphology in October of year one, but dissection would be required to distinguish between male and female buds. However, larch cones are particularly prone to damage by spring frosts and these early assessments may be unreliable. Cone crop potential could be obtained by estimating the female cones on the trees after pollination in early April of year two, when some of the danger of frost damage has receded. Mature seed cones could be estimated in September of year two, but it may be difficult to distinguish between current year cones and older cones. In the pines the male and female cones emerge in late May of year two, 18 months to 2 years before seed shed, and this represents a possible time at which cone crop potential could be estimated. In September of year two the woody female conelets could be estimated, but these do not all develop into mature seed cones. Semimature cones could be estimated in July of year three. Crop rating Assessments of cones on individual trees must be related to the total size of the crop. Although this is difficult, relative values between different areas or years will be of value. A system has been developed for assessing Douglas fir crops by sampling developing buds in the autumn of year one (Allen, 1941; Finnis, 1953; both quoted in Dobbs et al., 1976). Six trees are selected within the stand, one branch in each of the upper, middle and lower portions of the crown is collected, e.g. by climbing the tree to remove the branch, and the number of vegetative and female buds counted. The average ratio of female to vegetative buds gives the 'flower intensity'; a ratio of 0.1 is interpreted as a crop rating of 'very light', 0.15 is 'light', 0.2 is 'medium' and 0.25 is 'heavy'. Systems are also described for assessing visible cones on the crop. Gordon and Faulkner (1992) for example, recommend the crop is viewed with binoculars from a suitable vantage point, observing as many trees as possible and assessing the crop as follows: absent (no cones on any trees), light (a few cones on about one tree in every 50), moderate (a significant number of cones on about per cent of the trees) and heavy (very many cones on 5 10 per cent of the trees, a significant number on many other trees, and at least a few on nearly every other tree). The criteria used to classify coning, e.g. as 'light' or 'medium', are very difficult to define and will depend on the species and on local experience. Gordon and Faulkner (1992) suggest that for Sitka spruce 'heavy' coning might mean 1000 or exceptionally 2000 visible cones on a tree. The assessment methods and criteria will have to be developed for the species studied, bearing in mind that seed cone production in spruces is confined to the upper whorls, whereas in species such as larch coning may be widespread throughout the crown. In developing these assessments it would be useful actually to count the number of cones on some trees in each class, to help in the standardization of the criteria. Evaluation of individual seed cones and seeds The seed cones in the developing crop can also be evaluated to estimate the number of full seeds they contain. The cones of most species can be evaluated during late August before seed shed. Dobbs et al. (1976) suggest sampling a minimum of six trees that are well distributed throughout the stand, and selecting nine cones from each tree. The cones should be obtained from various positions and levels throughout the conebearing region of the trees. These cones will have to be sampled by shooting off branches, climbing the trees, or from felled trees. Each cone should be sliced in half longitudinally using a cutting tool or sharp knife. The number of filled seed exposed in one half section is then counted. For seed evaluation purposes 'filled' means those seeds in which white storage tissue, the endosperm, is visible; it is not necessary to determine the presence of the embryo during

9 PREDICTING CONE CROP POTENTIAL IN CONIFERS 95 this assessment (Eremko et al., 1989). This half section count provides an estimate of the number of filled seed per cone. For example, Dobbs et al. (1976) state that Douglas fir cones contain about three full seeds for each full seed revealed in a half section. These authors list average, acceptable seed counts in half sections for several species for medium and heavy crop ratings, e.g. in Douglas fir, larches and spruces, respectively, five, six and seven seeds are regarded as acceptable in a medium crop. In Britain four seeds per half cone would be regarded as acceptable for larch (A.M. Fletcher, personal communication). Semi-mature lodgepole pine cones can be cut longitudinally in July, but once they have hardened this is difficult and Eremko et al. (1989) recommend they are cut transversely. If cones are collected just before seed shedding, their seed may be extracted and assessed by cutting the seeds longitudinally to reveal the white endosperm and embryo. Even if quantitative readings are not taken, slicing the cones open for examination will reveal potential problems of insect or fungal attack, or indicate problems of empty seed production. Conclusions Potential cone crops can thus be predicted for several species, but, depending on the stage when the assessments are made, the accuracy and cost of the predictions will vary. The earliest predictions, which necessitate the sampling and dissection of developing cone buds, will carry the greatest risk of being inaccurate and will be the most expensive to carry out. Assessment of emerging cones on the trees will give a better estimate of the number of cones in the crop, but frosts, poor pollination and conelet drop may reduce the accuracy of these predictions. Assessments of mature cones will give the most accurate prediction of the potential seed crop but will provide less advance warning of the seed crop and potential natural regeneration. These predictions of potential coning will allow forest managers to anticipate the potential for natural regeneration and give them time to implement management techniques to encourage natural regeneration if desired. Research is required to establish whether the sampling methods described, often based on experience from North America, will provide reliable predictions of cone crops. Once the earliest predictions have been made these must be correlated with predictions based on later stages of cone development and with the final crop of seed cones and the density of natural regeneration that subsequently becomes established. Acknowledgements I thank Dr A.M. Fletcher and C.J. Nixon for their helpful comments during the preparation of this manuscript. References Allen, G.S A basis for forecasting seed crops of some coniferous species. ]. For. 39, Allen, G.S. and Owens, J.N The Life History of Douglas-fir. Environment Canada, Canadian Forestry Service, Ottawa, 139pp. Bramlet, D.L.C Seed potential and seed efficiency. In Seed Yield from Southern Pine Seed Orchards. J. Kraus (ed.). Georgia Forest Research Council, 1-7. Brown, I.R Flowering and seed production in grafted clones of Scots pine. Silvae Genet., 20, Clark, G.C The natural regeneration of spruce. Scott. For. 46, Dobbs, R.C., Edwards, D.G.W., Konishi, J. and Wallinger, D Guideline to collecting cones of BC conifers. A BC Forest Service/Canadian Forestry Service Joint Report No. 3, 98pp. Eis, S. and Craigdallie, D Reproduction of conifers. A Handbook for Cone Crops Assessment. Canadian Forestry Service, Pacific Forest Research Centre, BC, Canada. Report BC-X-219, 46pp. Eremko, R.D., Edwards, D.G.W. and Wallinger, D A guideline to collecting cones of British Columbia Conifers. FRDA Report 55, 114pp; Forestry Canada, Victoria, BC, (revised version of Dobbs etal., 1976). Eriksson, G Low-temperature induced irregularities in pollen mother cells of Larix leptolepis. Stud. For. Suec. 85, 14pp. Finnis, J.M A note on the bud count method of forecasting cone crops of Douglas-fir. For. Chron. 29, Fletcher, A.M Flower, fruit and seed development and morphology. In Seed Manual for Forest Trees. A.G. Gordon (ed.). Forestry Commission Bulletin No. 83. HMSO, London,

10 96 FORESTRY Gordon, A.G. and Faulkner, R Identification and assessment of cone and seed crops. In Seed Manual for Forest Trees. A.G. Gordon (ed.). Forestry Commission Bulletin No. 83. HMSO, London, Harmer, R Natural regeneration of broadleavcd trees in Britain. II Seed Production and Predation. Forestry 67, Harris, A.S Ripening and dispersal of a bumper western hemlock Sitka spruce seed crop in Southeast Alaska, 11pp. US Forest Service Research Note PNW-105. Henttonen, H., Kanninen, M., Nygren, M. and Ojansun, R The maturation of Pinus sylvestris seeds in relation to temperature climate in northern Finland. Scand. J. For. Res. 1, Mair, A.R Dissemination of tree seed. Sitka spruce, western hemlock and Douglas-fir. Scott. For. 27, Moir, R.B. and Fox, D.P Bud differentiation in Sitka spruce, Picea sitchensis (Bong.) Carr. Silvae Genet. 24, Owens, J.N The Reproductive cycle of Douglasfir. Pacific Forestry Research Centre, Victoria, BC, Canada, 23pp. Owens, J.N. and Blake, M.D Forest tree seed production. Petawawa National Forestry Institute, Canadian Forestry Service. Information Report PI- X-53, 161pp. Owens, J.N. and Molder, M Bud development in Sitka spruce II. Cone differentiation and early development. Can. J. Bot. 54, Owens, J.N. and Molder, M. 1979a Bud development in Larix occidentalis. I. Growth and development of vegetative long shoot and vegetative short shoot buds. Can. J. Bot. 57, Owens, J.N. and Molder, M. 1979b Bud development in Larx occidentalis. II. Core differentiation and early development. Can. J. Bot. 57, Owens, J.N. and Molder, M. 1984a The Reproductive Cycle of Interior Spruce. British Columbia, Ministry of Forests, Information Services Branch, Victoria, BC, Canada, 30pp. Owens, J.N. and Molder, M. 1984b The Reproductive Cycle of Lodgepole Pine. British Columbia, Ministry of Forests, Information Services Branch, Victoria, BC, Canada, 29pp. Roe, A.L A procedure for forecasting western larch seed crops. USDA Forest Service, Research Note 1NT-49, 7pp. Sahlen, K. and Bergsten, U Predicting anatomical maturity of Pinus sylvestris L. seeds in northern Fennoscandia. Scand. J. For. Res. 9, Sahlen, K. and Gjelsvik, S Determination of Pinus sylvestris seed maturity using leachate conductivity measurements. Can. J. For. Res. 23, Received 1 November 1995