A Guide to the Biology and Use of Forest Tree Seeds
L A N D M A N A G E M E N T H A N D B O O K
30 A Guide to the Biology and Use of Forest Tree Seeds
1996
Province of British Columbia
SPLENDOR OCCASU Ministry of Forests Research Program SINE A Guide to the Biology and Use of Forest Tree Seeds
Carole Leadem
Province of British Columbia
SPLENDOR OCCASU Ministry of Forests Research Program
SINE Canadian Cataloguing in Publication Data Leadem, Carole Louise Scheuplein, – A guide to the biology of forest tree seeds
(Land management handbook ; )
Includes bibliographical references: p. ---
. Seeds. . Trees - British Columbia - Seeds. . Gymnosperms - British Columbia - Seeds. . Angiosperms - British Columbia - Seeds. . Reforestation . I. British Columbia. Ministry of Forests. Research Branch. II. Title. III. Series.
.. .’’ --
Prepared by Carole Leadem B.C. Ministry of Forests Glyn Road Research Station Glyn Road Victoria, BC
for BC Ministry of Forests Research Branch Bastion Square Victoria, BC
Published by B.C. Ministry of Forests Forestry Division Services Branch Production Resources 1205 Broad Street, 2 Floor Victoria, BC
©␣ Province of British Columbia
Copies of this and other Ministry of Forests titles are available from: Crown Publications Inc. Fort Street Victoria, BC ACKNOWLEDGEMENTS
I am grateful to Dr. D. George Edwards, Canadian Izard, Paul Nystedt, and Heather Strongitharm—and Forest Service, for his patience, guidance, and con- Anna Gamble for the publication’s design. structive criticism, and for giving so generously of his The efforts of the editorial team are most greatly wealth of knowledge about tree seeds. appreciated: Dr. Annette Walker, Fran Aitkens, and Thanks are extended to the many B.C. Ministry of Susan Bannerman. Andrew MacKinnon verified the Forests reviewers who shared their expertise and pro- tree species and scientific authorities mentioned in vided useful comments: Rob Bowden-Green, Heather this handbook. Rooke, and Dave Kolotelo of the Tree Seed Centre in I thank the following suppliers of the seed samples Surrey; Karen Yearsley of the Research Branch; Tony used for this publication’s photographs: Don Pigott, Willingdon of the Surrey Nursery; and Clare Hewson Yellow Point Propagation, Ladysmith; Peter Hellenius, of the Interior Seed Orchards in Vernon. Thanks also Silva Enterprises, Prince George; and the Ministry of to Joe Wong of Woodmere Nursey in Telkwa, and Forests Tree Seed Centre, Surrey. Tom Gore of the Candace Laird of the Silviculture Institute of University of Victoria’s Biology Department kindly British Columbia. made available his extensive photographic expertise Joanne Clark provided valuable technical support and facilities. Peggy Frank drew the illustrations for in producing the text and figures, and helped in figures and . Donald Gunn drew the illustrations countless other ways with the final manuscript. for figures and , and the cover topic indicator. I appreciated the work, suggestions, and enthusiasm D. George Edwards supplied the x-ray photos for of the Production Resources staff — especially David Figure .
iii CONTENTS
Acknowledgements ...... iii
Introduction ......
The Basic Principles of Tree Seed Biology ...... . Seed Structure ...... . Development and Maturation ...... . Dormancy ...... . Germination ...... .. Hydration ...... .. Activation of growth processes ...... .. Emergence ...... .. Environmental factors ......
Applying the Principles of Tree Seed Biology ...... . Tree Seed Biology and Reforestation ...... . Seed Quality and Vigour ...... . Seed Collection and Storage ...... . Dormancy ...... . Germination ...... .. Hydration ...... .. Oxygen ...... .. Temperature ...... .. Light ...... .. Other factors ...... . Natural Regeneration ......
Conclusion ...... Appendix . Forest tree species occurring in British Columbia ...... Glossary ...... References ......
v
Dormancy-release treatments for tree seeds ......
Stratification regimes commonly used for conifers grown in British Columbia ......
Moisture content guidelines for tree seeds ......
Forest tree seed anatomy (longitudinal sections) ...... Wings aid in the dispersal of seeds ...... Some trees contain resin vesicles in their seed coats ......
Seeds of the same genus can vary in size and shape ......
Typical development and maturation cycles of British Columbia conifer seeds ...... Comparison of the major steps in the natural and artificial regeneration sequences of forest tree seedlings ...... Stages of germinant development ...... Absorption of far-red light converts the pigment phytochromefar-red back to phytochromered ...... Vigorous seeds complete germination first ......
Mature and immature embryos of Douglas-fir ...... The longevity of seeds increases as seed moisture content and storage temperature decreases ...... Effects of stratification regime on the germination of western hemlock seeds ...... Effects of stratification regime on the germination rates of Pacific silver fir seeds ......
Respiration of subalpine fir seeds during stratification ...... Germination of a) lodgepole pine, b) Sitka spruce, and c) Douglas-fir seeds at different temperatures after stratification for , , , and weeks ...... X-rays are used to determine whether seeds are fully developed, damaged, or have been attacked by insects ......
A young whitebark pine seedling struggles to establish in a high alpine meadow ......
Photographic tableau of forest tree seeds ......
vi 1 INTRODUCTION
The reasons for an interest in forest tree seeds vary widely. Nursery workers, silviculturists, seed orchard managers, cone collectors, and seed dealers have a very practical need for knowledge. But many others have developed a general interest in seed biology because they want to achieve a better under- standing of the natural world around them. Seed maturation, dormancy, and germination are still not completely understood. It remains somewhat of a mystery how a seed can remain viable for many years in the forest duff, then, responding to some cue, break through its woody seed coat and establish itself as an independent seedling. However, we know some of the factors critical to those processes, and we know that the effects of these factors may vary, depending on the physiological state of the seed. At the moment of natural seedfall, the potential quality of seeds is as high as it will ever be. To main- tain that quality and to produce the best seedlings for reforestation, knowledge of tree seed biology is essential. This handbook describes the basic principles that govern the biology of forest tree seeds and examines how these principles might apply to reforestation. Its intent is to give an overall picture of how and why seeds may germinate and to provide some understanding of a remarkable process.
2 THE BASIC PRINCIPLES OF TREE SEED BIOLOGY
. Seed Structure
A seed is a unique package containing the essential provides the energy supplies and structures of a new seedling and the nutrients to raw materials needed by the germinating embryo. support early growth. This package is constructed This tissue maintains the developing seedling until during a maturation period, after which the seed its photosynthetic and water uptake systems are undergoes a period of dormancy, followed by a able to support it. It contains vitamins, plant growth reactivation process referred to as germination. regulators, minerals, and many organic compounds, Each step in the sequence is critical to optimum all essential for normal embryo growth. seed performance. Nutritive tissues of conifer (gymnosperm) tree A fully developed seed consists of an embryo sur- seeds, and broad-leaved (angiosperm) tree seeds rounded by nutritive tissue, all of which is enclosed differ in several important respects, although both in a protective seed coat. The anatomy of several types of tissue perform the same function. tree seeds is shown in Figure . In conifers, the nutritive tissue is referred to as is a plant in miniature, containing the megagametophyte, and contains a large pro- rudimentary versions of the basic structures needed portion of fats and proteins. The tissue derives by the new seedling for growth and development: entirely from the female parent and has a single primary leaves (cotyledons), primary root (radicle), chromosome complement (). The nutritive the stem below the cotyledons (hypocotyl), and the tissue of conifer seeds is physically separate, and stem above the cotyledons (epicotyl). food must move from the megagametophyte to the Growth regions (meristems) located at the base embryo by diffusion. of the cotyledons and behind the root tip are the Nutritive tissue of broad-leaved tree seeds is source of new cells for the shoot and root growth called the endosperm and carries a chromosome of the seedling. complement of . It is produced from the union of The embryo contains the genetic makeup of the one set of chromosomes derived from the male par- new seedling. The embryo is the product of fertiliz- ent, and of two sets derived from the female parent. ation, or the union of the egg from the female par- Many broad-leaved tree seeds store the major part ent with the sperm contained in the male pollen. of their food supply in the cotyledons. The structure Both the female parent and male parent contribute of angiosperm seeds allows for the direct transfer of a single chromosome complement to the egg and food supplies from the endosperm or cotyledons be- the sperm. A mature seed, therefore, has a , or cause they are physically attached to the embryo. double chromosome complement. 2.7 mm 9 mm Seed wing
Pericarp Seed coat
Seed coat Cotyledons
Cotyledon Hypocotyl Embryo Embryo Hypocotyl Megagametophyte
Radicle Radicle
Micropyle Micropyle
Forest tree seed anatomy (longitudinal sections): red alder, an angiosperm (left); and Douglas-fir, a gymnosperm (right).
Words in bold are defined in the glossary.
a) Seed length: 2.6 cm b) Seed length: 2.1 cm c) Seed length: 1.0 cm d) Seed width: 0.5 cm e) Total seed length (seed = 1 cm; wing = 1.6 cm) (seed = 0.7 cm; wing = 1.3 cm) (seed = 0.3 cm; wing = 0.7 cm) (seed = 5 mm) (with wings): 5.2 cm Wings aid in the dispersal of seeds: a)␣ Pacific silver fir; b)␣ ponderosa pine; c) white spruce; d) yellow-cedar; e) bigleaf maple. Average dimensions of seeds provided.
of the seed (testa) provides area in the coat. During germination, embryo elon- physical protection for the embryo and nutritive gation and degradation of nutritive tissue assist the tissue. As well, it regulates the movement of water, radicle to emerge through the micropyle. Seeds may oxygen, and carbon dioxide in and out of the seed. vary greatly in size, colour, and shape. Even seeds In many species, the seed coat has membranous of closely related species can appear very different protrusions called “wings” that enhance the wind (Figure 4). dispersal of mature seeds (Figure ). In species such as pine,2 hemlock, and spruce, the wings may be . Development and Maturation easily detached, but in species such as western redcedar and yellow-cedar, the wings cannot be During maturation, a fertilized ovule is transformed removed without damaging the seed. Conifers such into a fully developed seed, containing all the ele- as western redcedar, the true firs, western hemlock, ments to produce a new tree. In the early stages of and mountain hemlock contain resin vesicles in the reproductive cycle, ovules develop in female their seed coats (Figure ). These can be damaged by cones and pollen develops in male cones. Pollen is improper handling, resulting in reduced viability of released in the spring and carried by the wind from the seed. the male to the female cones. In most British The seed coat has a chromosome complement of Columbia conifers, fertilization takes place during 2. It develops from the tissues (integuments) of the spring or early summer, shortly after pollination. the female parent tree that surround the ovule be- Following fertilization, the embryo grows until, at fore fertilization. The opening (micropyle) through maturity, it occupies almost the full length of the which pollen enters the ovule remains as a weak seed.
v v
v
v v
v a) b) c)
Some trees contain resin vesicles in their seed coats: a)␣ western redcedar, seed length = 0.5 cm; b)␣ Pacific silver fir, seed length = 1.1 cm; c)␣ western hemlock, seed length = 0.4 cm.
Scientific and common names of forest tree species occurring in British Columbia are listed in Appendix 1.
For Douglas-fir, redcedar, spruces, true firs, and hemlocks, the development and maturation cycle takes about months (Figure ). In pines, complete development takes months because fertilization is delayed for one year after pollination. In yellow- cedar, pollination and fertilization take place in the same growing season, but the total cycle usually lasts about months. Dehydration is an essential part of the maturation process. Water is lost from the seed, cell membranes assume a more condensed form, and physiological processes such as respiration diminish to very low levels. Simple compounds are changed to starches, fats, and proteins. These complex compounds can remain stable over many years, enabling seeds to lie dormant or to be stored for long periods. During germination the process is reversed as Seeds of the same genus can vary in size and storage compounds are broken down into simpler shape (counterclockwise from top): Korean pine, Coulter pine, limber pine, whitebark pine, forms (such as sugars and amino acids) that can be ponderosa pine, western white pine, Caribbean easily used by the embryo. Mature seeds are released pine, jack pine (2 seeds), lodgepole pine. naturally from cones in late summer and fall. Depending on the species, dispersal sometimes would leave vulnerable seedlings exposed to harsh continues into the following spring. winter conditions. In nature, dormant seeds remain inactive until favourable growing conditions occur . Dormancy the following spring. Some may remain dormant for two growing seasons or more. Seeds can maintain In the seeds of many tree species, maturation is ac- viability for many years in a dormant state. companied by the induction of a state of dormancy. Seeds are released from dormancy through This is an advantage for seeds that mature in late changes that occur during their exposure to cold, wet summer to early fall, since immediate germination conditions over winter, and they (usually) germinate
Reproductive cycles Year 1 Year 2 Year 3
Bud initiation Mature seeds
Douglas-fir, redcedar spruces, true firs, and hemlocks
Pines
Yellow-cedar
Legend:
Reproductive bud development Pollination Dormancy Fertilization Embyro development
Typical development and maturation cycles of British Columbia conifer seeds (adapted from Eremko et al. 1989).
as temperatures rise in the following spring. Some- Seed membranes are not fully operational during times seeds do not germinate because water and the early phase of hydration, and substances are eas- gases cannot permeate the seed coats. In nature, such ily leached from the seeds. Within a few minutes to seed coat dormancy may be removed by chemical ac- several hours, however, membranes resume full func- tion in the soil solution, which breaks down resistant tion. At this time, water is taken actively into coats or leaches chemical inhibitors from the seeds. the seed, and respiration and other physiological Dormancy may also be broken by seeds passing processes increase to characteristic metabolic levels. through the guts of birds or other animals. .. Activation of growth processes Once seeds are fully hydrated, moisture content and . Germination respiration remain relatively constant as the essential growth processes of germination take place. During Germination is the reactivation of physiological this phase, physiological activity is high as stored processes in the seed that result in the development reserves are mobilized, (the genetic coding) is of an embryo into an independent seedling. The activated, and cell repair and cell division begin. The three phases of germination are hydration, activa- storage compounds broken down during respiration tion of growth processes, and emergence of the release their energy to drive the processes of germin- embryo (Figure ). ation. Other raw materials will be used to form .. Hydration proteins, membranes, and other cellular structures When the moisture content of a mature seed falls of the developing seedling. below %, it can survive extended storage periods. .. Emergence With this degree of dehydration, however, metabolic The embryo grows primarily through cell division activity is virtually non-existent. Seeds must be rehy- and elongation of existing cells. Cell elongation is drated before germination can proceed. promoted by the transport of sugars, which increases Dry seeds take up water rapidly, but early hydra- the tendency of the embryo cells to take up water. tion is essentially a passive process. Thus dead and live The increased water pressure assists in the growth of seeds cannot be distinguished from one another on the radicle, enabling it to break through the seed coat the basis of their initial water uptake; physical prop- (Figure ). Once the radicle emerges, water uptake re- erties of the seed coat, such as waxiness, hairiness, sumes. Oxygen is now more readily available to the and thickness, appear to more important factors embryo, and respiration rises sharply to supply the governing the entry of water into the seed. energy needs of the new seedling.
Germination Biological Stage Dormancy Hydration Activation Emergence
Mature seeds Germinants
Natural regeneration Seed banks Seeds soaked Overwinter in soil Warm conditions by fall rains in spring
Artificial regeneration Storage Soak in water Stratification Sow in nursery (-18°C, <10% mc) (2–5°C, >25% mc)
Comparison of the major steps in the natural and artificial regeneration sequences of forest tree seedlings.
Epicotyl Hypocotyl
Root Root
a) b)
Stages of germinant development: a) Garry oak, an angiosperm, illustrates hypogeal germination, in which cotyledons remain below the ground; b) white spruce, a gymnopserm, exhibits epigeal germination in which cotyledons are raised above the ground.
.. Environmental factors to respond. The light stimulus is received through Moisture, oxygen, and favourable temperatures are the phytochrome system, which operates as an all essential for germination. Water is needed to ac- on/off switch for many physiological processes in tivate physiological membranes; oxygen is required plants (Figure ). Germination is usually stimulated for respiration that fuels the germination process. by exposure to red light ( nm) and inhibited by The The environmental cues affecting germination exposure to far-red light ( nm). The intensity of operate in various and interrelated ways. Satisfaction light required to activate the phytochrome system is of a single requirement is generally not sufficient to low, and – lx (comparable to bright moonlight) is trigger germination. generally sufficient. Temperature is one of the most important factors affecting seeds. Water uptake, gas diffusion, respira- Red light tion, and other metabolic processes all proceed faster at higher temperatures. Germination is dependent on all these processes, and thus is strongly affected by temperature. Dormancy Pred All seeds have an optimum temperature or tem- perature range for germination. Some species have a fairly narrow optimum temperature range, while others germinate over a wide range. For most British Columbia conifers, optimal temperatures Pfar-red Germination for germination are between and °C. Generally, the rate of germination is inhibited when tempera- Far-red light tures fall below °C. Some species are reportedly ca- pable of germinating in the snowpack, but do so Absorption of far-red light converts the pigment slowly over several months. Prolonged exposure to phytochromefar-red (ususally the active form) back temperatures of °C or higher is usually lethal to to phytochromered (the inactive form). This reaction germinating seeds. is reversible, depending on the relative amounts of red and far-red light. In sunlight, red light is pre- Some conifer seeds require light to stimulate ger- dominant, whereas far-red light is predominant in mination, but seeds must be fully hydrated in order canopy-filtered light.
3 APPLYING THE PRINCIPLES OF TREE SEED BIOLOGY
. Tree Seed Biology and Reforestation under a variety of conditions. They are potentially a more sensitive indicator of seed performance be- A successful reforestation program depends on a cause vigour declines more rapidly than viability. continuous supply of healthy seedlings. This process As yet, no single test has been found to adequately begins with successful seed collection, storage, pro- quantify seed vigour, but most attempts have been cessing, and sowing operations, then continues with based on characteristics that distinguish vigorous the careful growing, lifting, and storage of seedlings. from non-vigorous seeds. These characteristics Various treatments, based on the physiological include: requirements of seeds, are used to stimulate and Rapid germination Seeds that germinate rapidly enhance germination. It is vital to understand the are better able to compete for available water, biological and environmental conditions associated light, and nutrients (Figure ). with germination and to know when and why special treatments may be needed to enhance the process. With Germination under various temperatures this knowledge, it is possible to realize the maximum All seeds germinate well under optimum potential of seeds and produce high-quality seedlings. temperatures, but vigorous seeds germinate well under a wide range of temperatures. . Seed Quality and Vigour Respiration rate Respiration may vary, depending on the seed’s moisture content or stage of germi- Viable seeds may vary widely in their ability to nation, but higher than normal rates signal produce vigorous, healthy seedlings. The potential impaired physiological activity. of a seed to develop into an independent seedling is referred to as seed quality, and is usually assessed us- Stress tests Seeds are incubated under very low ing germination and vigour tests. Germination tests temperatures, or under very high temperature are the most often used; they are standardized and and humidity conditions. Vigorous seeds germi- relatively easy to perform, but the results will depend nates better, and are more resistant to attack by on seed preconditioning and the test environment. fungi or moulds. Vigour tests attempt to predict performance
Vigorous seeds complete germination first.
. Seed Collection and Storage The key concern is that physiological activity, especially respiration, is kept to a minimum so that The health of the female parent directly affects seed seed resources are not depleted during storage. Seeds quality because the nutritive tissue of conifers is damaged during handling, or those stored under derived entirely from the female parent. A healthy, conditions that permit elevated respiration, will vigorously growing female parent tree will be able to consume valuable resources, leaving fewer resources contribute more resources to the megagametophytic available for germination and subsequent seedling tissue. The heavy investment of resources by the growth. female parent during seed production is one of the reasons why good seed crops are generally followed by poorer ones. In seed orchards, cultural treatments such as watering or fertilization of parent trees favour the development of seed storage tissues. The ability to evaluate seed maturity is essential to the correct timing of cone collections, since seed quality is at its peak in fully mature seeds. Immature seeds are more difficult to process and more suscep- tible to damage during extraction, and they do not store well. Cones must be handled carefully after collection and during field storage. Freshly collected cones must be kept well ventilated and dry. This helps prevent the growth of moulds and other pathogens and minimizes heat accumulation from physiological activity. a) As cones and seeds mature, they usually take on a characteristic colour. However, colour alone is not always sufficient to establish seed maturity. Seeds should be examined for embryo development, but since embryos often reach their full length before seeds are fully mature, the condition of nutritive tissue should also be noted (Figure ). Nutritive tis- sues contain simple compounds that are transformed to more complex products as seeds mature, and these changes in the chemical nature of the nutritive tissues are reflected in their appearance. Nutritive tissues appear watery or translucent in immature seeds, changing to a more opaque appearance as the seed matures. In the fully mature seed, nutritive tissues look similar to the meat of a coconut. b) The conditions under which seeds are stored are critical to maintaining seed quality. The amount of Mature and immature embryos of Douglas-fir: seed reserves is fixed when the connection to the a)␣ The storage tissue of a mature seed completely fills the interior of the seed coat, and the embryo parent tree is broken (i.e., when the cones open and extends at least 90% of the length of the embryo release the seeds, or the cones are picked for seed cavity. collection). These reserves will have to support all b)␣ In an immature seed, the embryo does not extend the full length of the embryo cavity, and metabolic activities of the seed until the embryo after cutting, the storage tissues tend to pull away develops into an independent seedling. from the inside of the seed coat.
Physiological activity is minimal in dehydrated Dormancy-release treatments for tree seeds seeds kept at low temperatures. Conifer seeds may be successfully stored for many years at –°C and –% Treatment Description moisture content (Figure ). Under these storage conditions, germination in some collections of Stratification Moist chilling at –°C; removes Douglas-fir, white spruce, lodgepole pine, and yellow metabolic blocks, weakens seed pine seeds has remained high (more than %), even coats, increases germination after years. If either seed moisture or storage tem- promoter levels perature is increased, seed quality may be adversely Light Exposure to specific wavelengths; affected. stimulates the phytochrome system Leaching Soaking in water; removes inhibitors . Dormancy from seed coats
When seeds are used for growing seedlings in a Scarification Chemical (sulphuric acid) or forest nursery, dormancy presents a major problem. mechanical (abrasion) treatment: breaks down seed coats Unless dormancy is removed, the seeds may germ- inate haphazardly, or not at all. Seed performance in Plant growth Enhance natural levels in favour the nursery can be maximized only if the appropri- regulators of germination ate treatments are used to promote germination. High oxygen Supply respiration; remove For nursery use, dormant seeds can be artificially concentrations metabolic blocks stimulated to germinate using treatments that emu- late natural conditions (Table ). The choice of a suitable dormancy-release treatment can increase germination rates, and broaden the range of envi- Stratification is the most consistently effective ronmental conditions under which germination can dormancy-release treatment for British Columbia occur. conifer seeds. The treatment simulates winter condi- tions by exposing hydrated seeds to cold temperatures (–°C). Seeds are soaked in water (hydrated) usually for hours, drained, then placed in a plastic bag and refrigerated for several weeks. With the exception of western redcedar seeds (not considered to be dor- + mant), all British Columbia conifer seeds require stratification for best germination (Table ). Stratification enables seeds to germinate more quickly and completely (Figure ), and can some- times eliminate the need for other special conditions, such as light or closely controlled temperatures. Damaged seeds, or those of low vigour, may deterio- rate during stratification; in such cases, the seeds
Germination (%) after storage should be sown without chilling. + The true firs (Pacific silver, grand, and subalpine) respond best to a two-part stratification called strati- fication-redry. Seeds are hydrated for hours and then stratified for weeks; seed moisture content is Low High high, usually above %. The seeds are then dried to Storage temperature / seed moisture content –% moisture content and chilled for an addi- tional weeks.
The longevity of seeds increases as seed moisture content and storage temperature decreases.
Stratification regimes commonly used for conifers grown in British Columbia
a b ______Common practice ______Alternative practice Species Soak Stratification Soak Stratification (°C) (–°C) (°C) (–°C)
western redcedar h wk h wk yellow-cedar h wk d wk
Douglas-fir h wk h wk
lodgepole pine h wk h wk ponderosa pine h wk h wk whitebark pine h wk h wk western white pine d wk h + c
mountain hemlock h wk h wk western hemlock h wk h wk
Engelmann spruce h wk h wk Sitka spruce h wk h wk white spruce h wk h wk
amabilis fir h + wkd h + wke grand fir h wk h + wke noble fir h wk h + wke subalpine fir h + wkd h + wke western larch h wk h wk a Currently used by B.C. Ministry of Forests Tree Seed Centre. b These methods have been shown to improve germination, but require additional time or handling. c Warm/cold stratification: 4W + 8C hydrated seeds are kept at 20°C for 4 weeks, then transferred to 2–5°C for 8 weeks. d Modified stratification-redry treatment (see footnote e): hydrated seeds are stratified for 4 weeks, dried to 30% moisture content, and stratified for an additional 8 weeks. e Stratification-redry treatment: hydrated seeds are stratified for 4 weeks, dried to 30% moisture content, and stratified for an additional 12 weeks.
When sown in the nursery, true fir seeds given the 100 stratification-redry treatment begin and complete germination quickly (Figure ). This is a significant 80 advantage in the nursery because crops that progress rapidly and uniformily from the germinant to the seedling stage are easier to cultivate and less costly 60 to handle. 12 6 3 0 Redrying may be effective because seeds exhibit 40 Stratification (wk) lower respiration rates when they are chilled at a Germination (%) lower moisture content (Figure ). Just as athletes 20 require less oxygen during strenuous activity, seeds that have received the redry treatment consume less 0 oxygen and respire less of their stored reserves. This 5 10 15 20 leaves more resources available for germination and Time (days) the critical seedling establishment period. The strati- Effects of stratification regime on the germination of western hemlock seeds (data from Edwards fication-redry treatment generally has not been 1973). shown to improve the germination of other tree species, but it has been found to be effective for some seeds sources of Douglas-fir. 10
. Germination 8 Stratification-redry
.. Hydration 6 Mature seeds must be fully hydrated to germinate. This involves soaking the seeds in water, or placing Stratification (8 wk) 4 them in a highly humid environment. Movement of water within the seed progresses No stratification 2 slowly, and complete hydration of the inner tissues Germination rate (% per day) may take several days. The amount of time that seeds 0 must be soaked to completely hydrate the mega- 0 7 14 21 28 gametophyte and embryo differs among species. Time (days) The ability of biological membranes to retain Effects of stratification regime on the germination water within cells has inspired a technique that rates of Pacific silver fir seeds (data from Leadem separates high- from low-vigour seeds. This separa- 1986). tion is possible because, when dried in air, high- and low-vigour seeds have different water retention capabilities. Seeds are first soaked in water, kept at 16 Stratification o (no moisture control) – C for several days to ensure that tissues are 14 physiologically functional, and then dried for a few Stratification-redry 12 (moisture control) hours. During this drying, low-vigour or damaged -1 -1 seeds lose their moisture more quickly than high- 2 10 LO g min ) LO vigour seeds. When the partially dried seeds are µ 8 placed in water, low-vigour seeds tend to float, 6 whereas high-vigour seeds tend to sink. Seed moisture content is an indicator of the 4 Respiration ( physiological state of the seed. Moisture content can 2 be used as a guide to determine whether seeds are in 0 a condition suitable for stratification or sowing, or 0 4 8 12 16 20 24 28 Weeks at 2°C whether they can be stored or shipped without dam- age (Table ). Respiration of subalpine fir seeds during stratification (data from Leadem 1989).
Moisture content guidelines for tree seeds a) Lodgepole pine
100 Moisture Physiological content (%) status 80
60 <5 All water is chemically bound; removal may be detrimental 40 5–10 Seeds may be stored for prolonged periods at low temperatures (–°C)
Germination (%) at 42 days 20 <20 Seeds may revert to dormant state 0 010 152025303540 25–30 Reduced risk for premature germination Temperature (°C) during stratification (–°C) b) Sitka spruce 30–45 + Moisture level of fully imbibed seeds in preparation for stratification or sowing 100
80 .. Oxygen Oxygen requirements differ by species, but since 60 most tree seeds are able to germinate at concentra- tions well below atmospheric levels ( % by vol- 40 ume), oxygen is not generally considered to be a limiting factor. In flooded soils, however, available Germination (%) at 42 days 20 oxygen can be limited because the air in soil pore spaces is displaced by water. Seed coats also act as 0 barriers to oxygen during germination, as indicated 015202530354010 ° by the marked increase in respiration after radicles Temperature ( C) protrude through the coat. c) Douglas-fir .. Temperature In nature, germination usually occurs over a range 100 of temperatures that are higher during the day and 80 lower at night. Thus, a diurnal temperature is often used during germination tests. A temperature regime of °C day (-h light) and °C night (-h dark) is 60 commonly used for most British Columbia conifers. Tree seeds usually germinate more quickly under 40 high temperatures, but more is not necessarily better. 20 For example, although Pacific silver fir seeds usually Germination (%) at 42 days germinate quickly under warm conditions (°C ° 0 days and C nights), total germination is often 015202530354010 greater when seeds are incubated under cool condi- Temperature (°C) ° ° tions ( C days and C nights). Such a positive Legend: 0 wk 6 wk response at low temperatures may reflect an 3 wk 18 wk adaption of true fir seeds to the cool environments they would ordinarily encounter in nature. Germination of a) lodgepole pine, b) Sitka spruce, and c) Douglas-fir seeds at different temperatures Stratification broadens the temperature range for after stratification for 0, 3, 6, and 18 weeks (data germination (Figure ). Unstratified seeds tend to from Jones and Gosling 1994).
germinate well within a fairly narrow temperature range, but beyond that range, germination may decline notably. It is impossible to predict what envi- ronmental conditions will exist when seeds are sown in forest tree nurseries. Stratification allows earlier sowing and more reliable germination under unfa- vourable early season temperatures. .. Light Seeds lying on or near the soil surface receive enough light to trigger germination if all other conditions have been satisfied. However, seeds buried too deeply in the soil would not receive enough light for (a) (b) germination. In the nursery, the light requirement is generally met during routine handling of hydrated seeds. The light requirement for germination may be affected by treatments such as stratification. For example, unstratified seeds of species that require light for germination can be made to germinate in darkness once they have been stratified. .. Other factors Failure to germinate is not always linked to dormancy. Poor germination may be caused by immaturity— seeds may have been picked too early or collected from high elevation or high latitude areas that experi- (c) (d) ence shortened growing seasons. Such seeds may benefit from artificial ripening in the cones after they X-rays are used to determine whether seeds are have been collected and before they are extracted. fully developed, damaged, or have been attacked by insects. a) mature: c = cotyledon, e = mega- Pests, either fungal or insect, may severely diminish metophyte, r = radicle, s = seed coat; b) immature the quality of seeds (Figure ). seed; c) insect larva; d) damaged seed.
. Natural Regeneration Logging may affect seed germination by altering critical environmental variables. Total canopy The success of natural regeneration programs removal may result in higher soil temperatures and depends on how well seedbed environments meet extreme temperature fluctuations. In northern areas the requirements for seed germination and seedling where moisture is not limiting, complete canopy establishment. The suitability of different seedbeds removal is sometimes beneficial because soil tem- varies with their light, moisture, and temperature peratures at high elevations or northern exposures characteristics. are often too cold for germination (Figure ). The Many conifer seeds germinate better on mineral increased exposure of logged areas may stimulate soils because they provide more available moisture the germination of species such as pines, but may and higher average temperatures than organic soils. inhibit the germination of species adapted to shad- However, in bright sunlight the surface temperatures ed habitats. Partial canopy removal may create of mineral soils can become so high as to be lethal more favourable conditions for germination be- to germinating seeds. Rotting wood can be a good cause of the moderating influence of forest cover substrate because it retains moisture well, whereas on light, moisture, and temperature conditions of surfaces that have been burned tend to develop high the seedbed. temperatures that reduce the moisture-holding The regeneration potential of a site cannot be capability of upper soil layers. evaluated on the basis of a single factor isolated Seedbed environments can be enhanced by from other environmental or site variables. Careful selecting appropriate silvicultural systems and site consideration must be given to species, aspect, com- preparation methods. Site preparation can improve petition, soil, contour, and the other factors com- natural regeneration by increasing the amount of prising the physical and biological matrix upon exposed mineral soil and by bringing buried seeds which successful natural regeneration depends. to the surface. Burning is probably detrimental to most conifer species since seeds are generally on or close to the surface of the forest floor, and are therefore vulnerable to mortality from even a low- severity fire.
A young whitebark pine seedling struggles to establish in a high alpine meadow. The seeds must first escape predation by the Clark’s nutcracker, then overcome mechanical and physiological dormancy.
4 CONCLUSION
A handbook of this sort cannot provide answers to all of the questions that may arise with seed- related activities, but hopefully it explains some of the factors that govern tree seed biology. Understanding the biological principles should enable a prediction of the types of responses that might be expected under a particular set of circum- stances. It is our responsibility, as the present-day stewards of British Columbia’s natural resources, to use this information to best sustain the long-term capabilities of our forests.
The seed is the start the seed is the end and what takes place between the two is biochemical mystery
APPENDIX 1 Forest Tree Species Occurring in British Columbia
Scientific name/authority Common name
Angiosperms Acer macrophyllum Pursh bigleaf maple Alnus rubra Bong. red alder Arbutus menziesii Pursh arbutus Betula neoalaskana Sarg. Alaska paper birch Betula papyrifera Marsh. paper birch Cornus nuttallii Aud. ex T. & G. Pacific dogwood Fraxinus latifolia Benth. Oregon ash Malus fusca (Raf.) Schneid. Pacific crabapple Populus balsamifera L. ssp. balsamifera balsam poplar Populus balsamifera L. ssp. trichocarpa (T. & G.) Brayshaw black cottonwood Populus tremuloides Michx. trembling aspen Prunus emarginata (Dougl.) Walp. bitter cherry Quercus garryana Dougl. Garry oak Rhamnus purshiana DC. cascara Salix amygdaloides Anderss. peach-leaf willow Salix bebbiana Sarg. Bebb’s willow Salix discolor Muhlenb. pussy willow Salix exigua Nutt. sandbar willow Salix lucida Muhl. ssp. lasiandra (Benth.) E. Murray Pacific willow Salix scouleriana Barratt ex Hook. Scouler’s willow
Gymnosperms Abies amabilis (Dougl. ex Loud.) Forbes Pacific silver fir Abies grandis (Dougl. ex D. Don in Lamb.) Lindl. grand fir Abies lasiocarpa (Hook.) Nutt. subalpine fir Chamaecyparis nootkatensis (D. Don in Lamb.) Spach yellow-cedar Juniperus scopulorum Sarg. Rocky Mountain juniper Larix laricina (Du Roi) K. Koch tamarack Larix lyallii Parl. in DC. subalpine larch Larix occidentalis Nutt. western larch Picea engelmannii (Parry ex Engelm.) Engelmann spruce Picea glauca (Moench) Voss white spruce Picea mariana (P. Mill.) B.S.P. black spruce Picea sitchensis (Bong.) Carr. Sitka spruce Pinus albicaulis Engelm. whitebark pine Pinus banksiana Lamb. jack pine Pinus contorta Dougl. ex Loud. var. contorta shore pine Pinus contorta Dougl. ex Loud. var. latifolia Engelm. lodgepole pine Pinus flexilis James limber pine Pinus monticola Dougl. ex D. Don in Lamb. western white pine Pinus ponderosa Dougl. ex P. & C. Lawson ponderosa pine Pseudotsuga menziesii (Mirb.) Franco var. glauca (Beissn.) Franco Rocky Mountain Douglas-fir Pseudotsuga menziesii (Mirb.) Franco var. menziesii coastal Douglas-fir Taxus brevifolia Nutt. Pacific yew Thuja plicata Donn ex D. Don in Lamb. western redcedar Tsuga heterophylla (Raf.) Sarg. western hemlock Tsuga mertensiana (Bong.) Carr. mountain hemlock
Appendix 1 Continued
5 Alnus rubra red alder Betula glandulosa 3 4 scrub birch Acer macrophyllum bigleaf maple
9 6 Alnus crispa ssp. sinuata Sitka alder
2 Prunus emarginata bitter cherry
7 Betula papyrifera paper birch Arbutus menziesii 1 8 arbutus Cornus nuttallii Pacific dogwood Quercus garryana Garry oak
Taxus brevifolia 1 2 3 4 Pacific yew Thuja plicata western redcedar 8 9 57 Pinus contorta var. 6 latifolia 12 lodgepole pine 11 Abies grandis 10 grand fir Picea sitchensis 13 Sitka spruce 14 15 Larix occidentalis western larch Abies amabilis Juniperus scopulorum Pinus albicaulis Pacific silver fir Rocky Mountain juniper whitebark pine Tsuga mertensiana Pinus ponderosa Abies lasiocarpa mountain hemlock ponderosa pine subalpine fir Chamaecyparis nootkatensis Pseudotsuga menziesii var. Pinus flexilis yellow-cedar menziesii coastal Douglas-fir limber pine
Photographic tableau of forest tree seeds: angiosperms (top) and gymnosperms (bottom).
GLOSSARY
Activation Phase following hydration and ending with Hydration Uptake of water into the seed and incorpo- actual emergence of the radicle; includes mobilization ration of water into seed tissues. of stored reserves, repair of , and reactivation of enzymes needed for the metabolic processes essential to Hypocotyl Part of the axis of an embryo or stem of a germination. seedling between the cotyledons and the radicle; usu- ally identifiable between the root collar and the base of Angiosperms Flowering plants; distinguished from the cotyledons. gymnosperms by having the ovules borne within the closed cavity of the ovary; after fertilization the ovary Integument Outer cell layer or layers that surround becomes a fruit, enclosing one or more seeds. the ovule and give rise to the seed coat.
Chromosome A rod-shaped carrier of hereditary ma- Maturation Final stage of seed development terial (genes) inside the nucleus of cells. characterized by dehydration of seed tissues and the induction (in most British Columbia conifers) of Cotyledon First leaf produced by the embryo of a seed dormancy. plant. Megagametophyte The nutritive tissue () of gym- Dormancy Physical or physiological condition of a nosperm seeds, which surrounds and nourishes the viable seed that prevents germination even in the pres- embryo. Often incorrectly referred to as endosperm. ence of otherwise favourable germination conditions. (See also endosperm.)
Embryo Rudimentary plant within the seed; that part Meristem Undifferentiated tissue that is capable of of a seed that develops from the union of the egg cell undergoing cell division; located at the tips or growing and sperm cell, which after germination becomes the points of vegetative or reproductive organs. young plant. Micropyle Opening in the integument of an ovule Emergence Protrusion of the hypocotyl and cotyle- through which the pollen grain or pollen tube passes to don above the soil surface. reach the embryo sac, and through which the embryo radicle emerges during germination. Endosperm Nutritive tissue () of an angiosperm seed, which surrounds and nourishes the embryo. Ovule Structure in seed plants containing nutritive (See also megagametophyte.) tissue and an egg cell, which is surrounded by one or two integuments; when the egg is fertilized, the ovule Epicotyl That portion of the seedling stem above the develops into a seed. cotyledons. Pericarp In angiosperms, a fruit wall which develops Fertilization Penetration of a pollen tube into the from the ovary wall; it may be dry, hard, or fleshy. ovule, in which the male sperm nucleus is discharged into the ovule to unite with the egg nucleus. Photosynthesis The combination of carbon dioxide and water by chlorophyll-containing plants, using sun- Germination Resumption of active growth in the light as an energy source. embryo, which results in the protrusion of the embryo from the seed and development of the embryo into an Phytochrome Protein pigment of plants that exists in independent plant. either of two forms; it changes from one form to the other by absorption of red or far-red light. Gymnosperms Conifers and their allies; distinguished from angiosperms by having unprotected ovules (not Pollen Spore body () of vascular plants that enclosed in a fruit). contains the male sex cells.
Pollination Process by which pollen is transferred in Testa Seed coat; protective covering of the embryo angiosperms from the anther, where it is produced, to of seed plants formed from the integument; usually the stigma. In gymnosperms, pollen is dispersed by hard and dry. wind from male to female cones. Viability The state of being capable of germination Radicle Portion of the axis of an embryo from which and subsequent growth and development of the the root develops. seedling.
Respiration Metabolic reactions from which a plant or Vigour Seed properties that determine the potential animal derives energy. for rapid emergence and development of normal seedlings under a wide range of field conditions. Stratification Dormancy-breaking treatment in which seeds are exposed to moist, cold ₍– °) conditions for several weeks (or months, depending on the species).
REFERENCES
Edwards, D.G.W. . Effects of stratification Leadem, C.L., R.D. Eremko, and I. Davis. . on western hemlock germination. Can. J. For. Seed biology, collection and post-harvest Res. ():–. handling. In Regenerating British Columbia’s forests. D. Lavender, R. Parish, C.M. Johnson, ______. . Maturity and quality of tree seeds: G. Montgomery, A. Vyse, R.A. Willis, and a state-of-the-art review. Seed Sci. Technol. D. Winston (editors). Univ. B.C. Press, :–. Vancouver, B.C. pp. –.
______. . Collection, processing, testing, and Osborne, Daphne J. . Physiological and biochemi- storage of true fir seeds: a review. In Biology and cal events in seed development. Adv. Res. Technol. management of true fir in the Pacific Northwest. Seeds :–. C.D. Oliver and R.M. Kenady (editors). U.S. Dep. Agric. For. Serv., Pac. N.W. Range Exp. Sta., and Owens, J.N. and M. Molder. . The reproductive Univ. Wash., Coll. For. Resour., Seattle, Wash. cycle of interior spruce. B.C. Min. For., Victoria, pp. –. B.C. pp.
Eremko, R.D., D.G.W. Edwards, and D. Wallinger. . ______. . The reproductive cycle of lodgepole A guide to collecting cones of British Columbia pine. B.C. Min. For., Victoria, B.C. pp. conifers. B.C. Min. For., Res. Br., Victoria, B.C. FRDA Rep. No. . pp. ______. . The reproductive cycles of western and mountain hemlock. B.C. Min. For., Victoria, Jones, S.F. and P.G. Gosling. . “Target moisture B.C. pp. content” prechill overcomes the dormancy of temperate conifer seeds. New For. :–. ______. . The reproductive cycles of western redcedar and yellow-cedar. B.C. Min. For., Leadem, C.L. . Stratification of Abies amabilis Victoria, B.C. pp. seeds. Can. J. For. Res. ():–. ______. . The reproductive cycles of true firs. ______. . Stratification and quality assessment of B.C. Min. For., Victoria, B.C. pp. Abies lasiocarpa seeds. B.C. Min. For., Res. Br., Victoria, B.C. FRDA Rep. No. . pp.
______. . Respiration of tree seeds. In Dormancy and barriers to germination. D.G.W. Edwards (compiler and editor). For. Can., Pac. For. Cent., Victoria, B.C. pp. –.