3 0

L A N D M A N A G E M E N T H A N D B O O K

SPLENDOR OCCASU

SINE

Province of British ColumbiaMinistry of Forests Research Program

1 9 9 6

A Guide to the Biology andUse of Forest Tree Seeds

SPLENDOR OCCASU

SINE

Province of British ColumbiaMinistry of Forests Research Program

Carole Leadem

A Guide to the Biology andUse of Forest Tree Seeds

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 Foreststitles are available from:Crown Publications Inc. Fort StreetVictoria, BC

Canadian Cataloguing in Publication DataLeadem, 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 ofForests. Research Branch. II. Title. III. Series.

.. .’’ --

iii

ACKNOWLEDGEMENTS

I am grateful to Dr. D. George Edwards, CanadianForest Service, for his patience, guidance, and con-structive criticism, and for giving so generously of hiswealth of knowledge about tree seeds.

Thanks are extended to the many B.C. Ministry ofForests reviewers who shared their expertise and pro-vided useful comments: Rob Bowden-Green, HeatherRooke, and Dave Kolotelo of the Tree Seed Centre inSurrey; Karen Yearsley of the Research Branch; TonyWillingdon of the Surrey Nursery; and Clare Hewsonof the Interior Seed Orchards in Vernon. Thanks alsoto Joe Wong of Woodmere Nursey in Telkwa, andCandace Laird of the Silviculture Institute ofBritish Columbia.

Joanne Clark provided valuable technical supportin producing the text and figures, and helped incountless other ways with the final manuscript.I appreciated the work, suggestions, and enthusiasmof the Production Resources staff — especially David

Izard, Paul Nystedt, and Heather Strongitharm—andAnna Gamble for the publication’s design.

The efforts of the editorial team are most greatlyappreciated: Dr. Annette Walker, Fran Aitkens, andSusan Bannerman. Andrew MacKinnon verified thetree species and scientific authorities mentioned inthis handbook.

I thank the following suppliers of the seed samplesused for this publication’s photographs: Don Pigott,Yellow Point Propagation, Ladysmith; Peter Hellenius,Silva Enterprises, Prince George; and the Ministry ofForests Tree Seed Centre, Surrey. Tom Gore of theUniversity of Victoria’s Biology Department kindlymade available his extensive photographic expertiseand facilities. Peggy Frank drew the illustrations forfigures and . Donald Gunn drew the illustrationsfor figures and , and the cover topic indicator.D. George Edwards supplied the x-ray photos forFigure .

v

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vi

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 INTRODUCTION

The reasons for an interest in forest tree seedsvary widely. Nursery workers, silviculturists, seedorchard managers, cone collectors, and seed dealershave a very practical need for knowledge. But manyothers have developed a general interest in seedbiology because they want to achieve a better under-standing of the natural world around them.

Seed maturation, dormancy, and germinationare still not completely understood. It remainssomewhat of a mystery how a seed can remainviable for many years in the forest duff, then,responding to some cue, break through its woodyseed coat and establish itself as an independentseedling. However, we know some of the factorscritical to those processes, and we know that theeffects of these factors may vary, depending on thephysiological state of the seed.

At the moment of natural seedfall, the potentialquality of seeds is as high as it will ever be. To main-tain that quality and to produce the best seedlingsfor reforestation, knowledge of tree seed biology isessential.

This handbook describes the basic principles thatgovern the biology of forest tree seeds and examineshow these principles might apply to reforestation.Its intent is to give an overall picture of how andwhy seeds may germinate and to provide someunderstanding of a remarkable process.

2 THE BASIC PRINCIPLES OF TREE SEED BIOLOGY

. Seed Structure

A seed is a unique package containing the essentialstructures of a new seedling and the nutrients tosupport early growth. This package is constructedduring a maturation period, after which the seedundergoes a period of dormancy, followed by areactivation process referred to as germination.Each step in the sequence is critical to optimumseed performance.

A fully developed seed consists of an embryo sur-rounded by nutritive tissue, all of which is enclosedin a protective seed coat. The anatomy of severaltree seeds is shown in Figure .

is a plant in miniature, containingrudimentary versions of the basic structures neededby the new seedling for growth and development:primary leaves (cotyledons), primary root (radicle),the stem below the cotyledons (hypocotyl), and thestem above the cotyledons (epicotyl).

Growth regions (meristems) located at the baseof the cotyledons and behind the root tip are thesource of new cells for the shoot and root growthof the seedling.

The embryo contains the genetic makeup of thenew seedling. The embryo is the product of fertiliz-ation, or the union of the egg from the female par-ent with the sperm contained in the male pollen.Both the female parent and male parent contributea single chromosome complement to the egg andthe sperm. A mature seed, therefore, has a , ordouble chromosome complement.

Words in bold are defined in the glossary.

Forest tree seed anatomy (longitudinal sections): red alder, an angiosperm (left); and Douglas-fir,a gymnosperm (right).

provides the energy supplies andraw materials needed by the germinating embryo.This tissue maintains the developing seedling untilits photosynthetic and water uptake systems areable to support it. It contains vitamins, plant growthregulators, minerals, and many organic compounds,all essential for normal embryo growth.

Nutritive tissues of conifer (gymnosperm) treeseeds, and broad-leaved (angiosperm) tree seedsdiffer in several important respects, although bothtypes of tissue perform the same function.

In conifers, the nutritive tissue is referred to asthe megagametophyte, and contains a large pro-portion of fats and proteins. The tissue derivesentirely from the female parent and has a singlechromosome complement (). The nutritivetissue of conifer seeds is physically separate, andfood must move from the megagametophyte to theembryo by diffusion.

Nutritive tissue of broad-leaved tree seeds iscalled the endosperm and carries a chromosomecomplement of . It is produced from the union ofone set of chromosomes derived from the male par-ent, and of two sets derived from the female parent.Many broad-leaved tree seeds store the major partof their food supply in the cotyledons. The structureof angiosperm seeds allows for the direct transfer offood supplies from the endosperm or cotyledons be-cause they are physically attached to the embryo.

Pericarp

Seed coat

Cotyledon

Hypocotyl

Radicle

Micropyle

Seed wing

Seed coat

Cotyledons

Hypocotyl

Megagametophyte

Radicle

Micropyle

9 mm2.7 mm

Emb

ryo

Emb

ryo

d) Seed width: 0.5 cm(seed = 5 mm)

c) Seed length: 1.0 cm(seed = 0.3 cm; wing = 0.7 cm)

b) Seed length: 2.1 cm(seed = 0.7 cm; wing = 1.3 cm)

a) Seed length: 2.6 cm(seed = 1 cm; wing = 1.6 cm)

e) Total seed length(with wings): 5.2 cm

v

v

v

v v

v

Some trees contain resin vesicles in their seed coats: a)␣ western redcedar, seed length = 0.5 cm; b)␣ Pacific silver fir, seedlength = 1.1 cm; c)␣ western hemlock, seed length = 0.4 cm.

of the seed (testa) providesphysical protection for the embryo and nutritivetissue. As well, it regulates the movement of water,oxygen, and carbon dioxide in and out of the seed.In many species, the seed coat has membranousprotrusions called “wings” that enhance the winddispersal of mature seeds (Figure ). In species suchas pine,2 hemlock, and spruce, the wings may beeasily detached, but in species such as westernredcedar and yellow-cedar, the wings cannot beremoved without damaging the seed. Conifers suchas western redcedar, the true firs, western hemlock,and mountain hemlock contain resin vesicles intheir seed coats (Figure ). These can be damaged byimproper handling, resulting in reduced viability ofthe seed.

The seed coat has a chromosome complement of2. It develops from the tissues (integuments) ofthe female parent tree that surround the ovule be-fore fertilization. The opening (micropyle) throughwhich pollen enters the ovule remains as a weak

area in the coat. During germination, embryo elon-gation and degradation of nutritive tissue assist theradicle to emerge through the micropyle. Seeds mayvary greatly in size, colour, and shape. Even seedsof closely related species can appear very different(Figure 4).

. Development and Maturation

During maturation, a fertilized ovule is transformedinto a fully developed seed, containing all the ele-ments to produce a new tree. In the early stages ofthe reproductive cycle, ovules develop in femalecones and pollen develops in male cones. Pollen isreleased in the spring and carried by the wind fromthe male to the female cones. In most BritishColumbia conifers, fertilization takes place duringthe spring or early summer, shortly after pollination.Following fertilization, the embryo grows until, atmaturity, it occupies almost the full length of theseed.

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.

a) b) c)

Scientific and common names of forest tree species occurring in British Columbia are listed in Appendix 1.

Typical development and maturation cycles of British Columbia conifer seeds (adapted from Eremko et al. 1989).

Seeds of the same genus can vary in size andshape (counterclockwise from top): Korean pine,Coulter pine, limber pine, whitebark pine,ponderosa pine, western white pine, Caribbeanpine, jack pine (2 seeds), lodgepole pine.

Reproductive cycles Year 1 Year 2 Year 3

Douglas-fir, redcedarspruces, true firs, and hemlocks

Pines

Yellow-cedar

Pollination

Fertilization

Embyro development

Dormancy

Reproductive bud development

Legend:

���������

���� ��

��

Mature seedsBud initiation

For Douglas-fir, redcedar, spruces, true firs, andhemlocks, the development and maturation cycletakes about months (Figure ). In pines, completedevelopment takes months because fertilization isdelayed for one year after pollination. In yellow-cedar, pollination and fertilization take place in thesame growing season, but the total cycle usually lastsabout months.

Dehydration is an essential part of the maturationprocess. Water is lost from the seed, cell membranesassume a more condensed form, and physiologicalprocesses such as respiration diminish to very lowlevels. Simple compounds are changed to starches,fats, and proteins. These complex compounds canremain stable over many years, enabling seeds to liedormant or to be stored for long periods.

During germination the process is reversed asstorage compounds are broken down into simplerforms (such as sugars and amino acids) that can beeasily used by the embryo. Mature seeds are releasednaturally from cones in late summer and fall.Depending on the species, dispersal sometimescontinues into the following spring.

. Dormancy

In the seeds of many tree species, maturation is ac-companied by the induction of a state of dormancy.This is an advantage for seeds that mature in latesummer to early fall, since immediate germination

would leave vulnerable seedlings exposed to harshwinter conditions. In nature, dormant seeds remaininactive until favourable growing conditions occurthe following spring. Some may remain dormant fortwo growing seasons or more. Seeds can maintainviability for many years in a dormant state.

Seeds are released from dormancy throughchanges that occur during their exposure to cold, wetconditions over winter, and they (usually) germinate

as temperatures rise in the following spring. Some-times seeds do not germinate because water andgases cannot permeate the seed coats. In nature, suchseed coat dormancy may be removed by chemical ac-tion in the soil solution, which breaks down resistantcoats or leaches chemical inhibitors from the seeds.Dormancy may also be broken by seeds passingthrough the guts of birds or other animals.

. Germination

Germination is the reactivation of physiologicalprocesses in the seed that result in the developmentof an embryo into an independent seedling. Thethree phases of germination are hydration, activa-tion of growth processes, and emergence of theembryo (Figure )... HydrationWhen the moisture content of a mature seed fallsbelow %, it can survive extended storage periods.With this degree of dehydration, however, metabolicactivity is virtually non-existent. Seeds must be rehy-drated before germination can proceed.

Dry seeds take up water rapidly, but early hydra-tion is essentially a passive process. Thus dead and liveseeds cannot be distinguished from one another onthe basis of their initial water uptake; physical prop-erties of the seed coat, such as waxiness, hairiness,and thickness, appear to more important factorsgoverning the entry of water into the seed.

Comparison of the major steps in the natural and artificial regeneration sequences of forest tree seedlings.

Biological Stage Dormancy

Germination

Hydration Activation Emergence

Artificial regeneration Storage(-18°C, <10% mc)

Seed banks

Soak in water

Seeds soakedby fall rains

Stratification(2–5°C, >25% mc)

Overwinter in soil

Sow in nursery

Warm conditions in spring

Natural regeneration

Mature seeds Germinants

Seed membranes are not fully operational duringthe early phase of hydration, and substances are eas-ily leached from the seeds. Within a few minutes toseveral hours, however, membranes resume full func-tion. At this time, water is taken actively intothe seed, and respiration and other physiologicalprocesses increase to characteristic metabolic levels... Activation of growth processesOnce seeds are fully hydrated, moisture content andrespiration remain relatively constant as the essentialgrowth processes of germination take place. Duringthis phase, physiological activity is high as storedreserves are mobilized, (the genetic coding) isactivated, and cell repair and cell division begin. Thestorage compounds broken down during respirationrelease their energy to drive the processes of germin-ation. Other raw materials will be used to formproteins, membranes, and other cellular structuresof the developing seedling... EmergenceThe embryo grows primarily through cell divisionand elongation of existing cells. Cell elongation ispromoted by the transport of sugars, which increasesthe tendency of the embryo cells to take up water.The increased water pressure assists in the growth ofthe radicle, enabling it to break through the seed coat(Figure ). Once the radicle emerges, water uptake re-sumes. Oxygen is now more readily available to theembryo, and respiration rises sharply to supply theenergy needs of the new seedling.

Leaf

Epicotyl

Root

Cotyledon

Hypocotyl

Root

Stages of germinant development: a) Garry oak, an angiosperm, illustrates hypogeal germination, in which cotyledonsremain below the ground; b) white spruce, a gymnopserm, exhibits epigeal germination in which cotyledons are raisedabove the ground.

b)a)

.. Environmental factorsMoisture, oxygen, and favourable temperatures areall essential for germination. Water is needed to ac-tivate physiological membranes; oxygen is requiredfor respiration that fuels the germination process.The The environmental cues affecting germinationoperate in various and interrelated ways. Satisfactionof a single requirement is generally not sufficient totrigger germination.

Temperature is one of the most important factorsaffecting seeds. Water uptake, gas diffusion, respira-tion, and other metabolic processes all proceed fasterat higher temperatures. Germination is dependenton all these processes, and thus is strongly affectedby temperature.

All seeds have an optimum temperature or tem-perature range for germination. Some species havea fairly narrow optimum temperature range, whileothers germinate over a wide range. For mostBritish Columbia conifers, optimal temperaturesfor germination are between and °C. Generally,the rate of germination is inhibited when tempera-tures fall below °C. Some species are reportedly ca-pable of germinating in the snowpack, but do soslowly over several months. Prolonged exposure totemperatures of °C or higher is usually lethal togerminating seeds.

Some conifer seeds require light to stimulate ger-mination, but seeds must be fully hydrated in order

to respond. The light stimulus is received throughthe phytochrome system, which operates as anon/off switch for many physiological processes inplants (Figure ). Germination is usually stimulatedby exposure to red light ( nm) and inhibited byexposure to far-red light ( nm). The intensity oflight required to activate the phytochrome system islow, and – lx (comparable to bright moonlight) isgenerally sufficient.

Absorption of far-red light converts the pigmentphytochromefar-red (ususally the active form) backto phytochromered (the inactive form). This reactionis reversible, depending on the relative amounts ofred and far-red light. In sunlight, red light is pre-dominant, whereas far-red light is predominant incanopy-filtered light.

Far-red light

Pfar-red

Pred

Germination

Dormancy

Red light

under a variety of conditions. They are potentiallya more sensitive indicator of seed performance be-cause vigour declines more rapidly than viability.As yet, no single test has been found to adequatelyquantify seed vigour, but most attempts have beenbased on characteristics that distinguish vigorousfrom non-vigorous seeds. These characteristicsinclude:

Rapid germination Seeds that germinate rapidlyare better able to compete for available water,light, and nutrients (Figure ).

Germination under various temperaturesAll seeds germinate well under optimumtemperatures, but vigorous seeds germinatewell under a wide range of temperatures.

Respiration rate Respiration may vary, dependingon the seed’s moisture content or stage of germi-nation, but higher than normal rates signalimpaired physiological activity.

Stress tests Seeds are incubated under very lowtemperatures, or under very high temperatureand humidity conditions. Vigorous seeds germi-nates better, and are more resistant to attack byfungi or moulds.

. Tree Seed Biology and Reforestation

A successful reforestation program depends on acontinuous supply of healthy seedlings. This processbegins with successful seed collection, storage, pro-cessing, and sowing operations, then continues withthe careful growing, lifting, and storage of seedlings.

Various treatments, based on the physiologicalrequirements of seeds, are used to stimulate andenhance germination. It is vital to understand thebiological and environmental conditions associatedwith germination and to know when and why specialtreatments may be needed to enhance the process. Withthis knowledge, it is possible to realize the maximumpotential of seeds and produce high-quality seedlings.

. Seed Quality and Vigour

Viable seeds may vary widely in their ability toproduce vigorous, healthy seedlings. The potentialof a seed to develop into an independent seedling isreferred to as seed quality, and is usually assessed us-ing germination and vigour tests. Germination testsare the most often used; they are standardized andrelatively easy to perform, but the results will dependon seed preconditioning and the test environment.

Vigour tests attempt to predict performance

3 APPLYING THE PRINCIPLES OF TREE SEED BIOLOGY

Vigorous seeds complete germination first.

. Seed Collection and Storage

The health of the female parent directly affects seedquality because the nutritive tissue of conifers isderived entirely from the female parent. A healthy,vigorously growing female parent tree will be able tocontribute more resources to the megagametophytictissue. The heavy investment of resources by thefemale parent during seed production is one of thereasons why good seed crops are generally followedby poorer ones. In seed orchards, cultural treatmentssuch as watering or fertilization of parent treesfavour the development of seed storage tissues.

The ability to evaluate seed maturity is essentialto the correct timing of cone collections, since seedquality is at its peak in fully mature seeds. Immatureseeds are more difficult to process and more suscep-tible to damage during extraction, and they do notstore well. Cones must be handled carefully aftercollection and during field storage. Freshly collectedcones must be kept well ventilated and dry. Thishelps prevent the growth of moulds and otherpathogens and minimizes heat accumulation fromphysiological activity.

As cones and seeds mature, they usually take ona characteristic colour. However, colour alone is notalways sufficient to establish seed maturity. Seedsshould be examined for embryo development, butsince embryos often reach their full length beforeseeds are fully mature, the condition of nutritivetissue should also be noted (Figure ). Nutritive tis-sues contain simple compounds that are transformedto more complex products as seeds mature, andthese changes in the chemical nature of the nutritivetissues are reflected in their appearance. Nutritivetissues appear watery or translucent in immatureseeds, changing to a more opaque appearance as theseed matures. In the fully mature seed, nutritivetissues look similar to the meat of a coconut.

The conditions under which seeds are stored arecritical to maintaining seed quality. The amount ofseed reserves is fixed when the connection to theparent tree is broken (i.e., when the cones open andrelease the seeds, or the cones are picked for seedcollection). These reserves will have to support allmetabolic activities of the seed until the embryodevelops into an independent seedling.

The key concern is that physiological activity,especially respiration, is kept to a minimum so thatseed resources are not depleted during storage. Seedsdamaged during handling, or those stored underconditions that permit elevated respiration, willconsume valuable resources, leaving fewer resourcesavailable for germination and subsequent seedlinggrowth.

b)

a)

Mature and immature embryos of Douglas-fir:a)␣ The storage tissue of a mature seed completelyfills the interior of the seed coat, and the embryoextends at least 90% of the length of the embryocavity.b)␣ In an immature seed, the embryo does notextend the full length of the embryo cavity, andafter cutting, the storage tissues tend to pull awayfrom the inside of the seed coat.

Physiological activity is minimal in dehydratedseeds kept at low temperatures. Conifer seeds may besuccessfully stored for many years at –°C and –%moisture content (Figure ). Under these storageconditions, germination in some collections ofDouglas-fir, white spruce, lodgepole pine, and yellowpine seeds has remained high (more than %), evenafter years. If either seed moisture or storage tem-perature is increased, seed quality may be adverselyaffected.

. Dormancy

When seeds are used for growing seedlings in aforest nursery, dormancy presents a major problem.Unless dormancy is removed, the seeds may germ-inate haphazardly, or not at all. Seed performance inthe nursery can be maximized only if the appropri-ate treatments are used to promote germination.

For nursery use, dormant seeds can be artificiallystimulated to germinate using treatments that emu-late natural conditions (Table ). The choice of asuitable dormancy-release treatment can increasegermination rates, and broaden the range of envi-ronmental conditions under which germination canoccur.

Dormancy-release treatments for tree seeds

Treatment Description

Stratification Moist chilling at –°C; removesmetabolic blocks, weakens seedcoats, increases germinationpromoter levels

Light Exposure to specific wavelengths;stimulates the phytochrome system

Leaching Soaking in water; removes inhibitorsfrom seed coats

Scarification Chemical (sulphuric acid) ormechanical (abrasion) treatment:breaks down seed coats

Plant growth Enhance natural levels in favourregulators of germination

High oxygen Supply respiration; removeconcentrations metabolic blocks

The longevity of seeds increases as seed moisturecontent and storage temperature decreases.

Storage temperature / seed moisture content

HighLow

+

+

Ger

min

atio

n (%

) af

ter

stor

age

Stratification is the most consistently effectivedormancy-release treatment for British Columbiaconifer seeds. The treatment simulates winter condi-tions by exposing hydrated seeds to cold temperatures(–°C). Seeds are soaked in water (hydrated) usuallyfor hours, drained, then placed in a plastic bag andrefrigerated for several weeks. With the exception ofwestern redcedar seeds (not considered to be dor-mant), all British Columbia conifer seeds requirestratification for best germination (Table ).

Stratification enables seeds to germinate morequickly 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 seedsshould 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 andthen stratified for weeks; seed moisture content ishigh, usually above %. The seeds are then dried to–% moisture content and chilled for an addi-tional weeks.

Stratification regimes commonly used for conifers grown in British Columbia

Common practice a Alternative practice b__________________________________________ __________________________________________

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 thestratification-redry treatment begin and completegermination quickly (Figure ). This is a significantadvantage in the nursery because crops that progressrapidly and uniformily from the germinant to theseedling stage are easier to cultivate and less costlyto handle.

Redrying may be effective because seeds exhibitlower respiration rates when they are chilled at alower moisture content (Figure ). Just as athletesrequire less oxygen during strenuous activity, seedsthat have received the redry treatment consume lessoxygen and respire less of their stored reserves. Thisleaves more resources available for germination andthe critical seedling establishment period. The strati-fication-redry treatment generally has not beenshown to improve the germination of other treespecies, but it has been found to be effective forsome seeds sources of Douglas-fir.

. Germination

.. HydrationMature seeds must be fully hydrated to germinate.This involves soaking the seeds in water, or placingthem in a highly humid environment.

Movement of water within the seed progressesslowly, and complete hydration of the inner tissuesmay take several days. The amount of time that seedsmust be soaked to completely hydrate the mega-gametophyte and embryo differs among species.

The ability of biological membranes to retainwater within cells has inspired a technique thatseparates high- from low-vigour seeds. This separa-tion is possible because, when dried in air, high-and low-vigour seeds have different water retentioncapabilities. Seeds are first soaked in water, kept at–oC for several days to ensure that tissues arephysiologically functional, and then dried for a fewhours. During this drying, low-vigour or damagedseeds lose their moisture more quickly than high-vigour seeds. When the partially dried seeds areplaced in water, low-vigour seeds tend to float,whereas high-vigour seeds tend to sink.

Seed moisture content is an indicator of thephysiological state of the seed. Moisture content canbe used as a guide to determine whether seeds are ina condition suitable for stratification or sowing, orwhether they can be stored or shipped without dam-age (Table ).

Time (days)

Ger

min

atio

n (%

)

5

20

0

40

60

80

100

10 15 20

12 6 3 0

Stratification (wk)

Ger

min

atio

n ra

te (

% p

er d

ay)

0

2

4

6

8

10

0 7 14 21 28Time (days)

Stratification (8 wk)

Stratification-redry

No stratification

Effects of stratification regime on the germinationrates of Pacific silver fir seeds (data from Leadem1986).

Respiration of subalpine fir seeds duringstratification (data from Leadem 1989).

0

2

4

6

8

10

12

14

16

Resp

irat

ion

(µL

O

g

min

)2

-1-1

4 8 12 16 20 24 280

Weeks at 2°C

Stratification(no moisture control)

Stratification-redry(moisture control)

Effects of stratification regime on the germinationof western hemlock seeds (data from Edwards1973).

Moisture content guidelines for tree seeds

Moisture Physiologicalcontent (%) status

<5 All water is chemically bound; removal may

be detrimental

5–10 Seeds may be stored for prolonged periodsat low temperatures (–°C)

<20 Seeds may revert to dormant state

25–30 Reduced risk for premature germinationduring stratification (–°C)

30–45 + Moisture level of fully imbibed seeds inpreparation for stratification or sowing

Germination of a) lodgepole pine, b) Sitka spruce,and c) Douglas-fir seeds at different temperaturesafter stratification for 0, 3, 6, and 18 weeks (datafrom Jones and Gosling 1994).

.. OxygenOxygen requirements differ by species, but sincemost tree seeds are able to germinate at concentra-tions well below atmospheric levels (% by vol-ume), oxygen is not generally considered to be alimiting factor. In flooded soils, however, availableoxygen can be limited because the air in soil porespaces is displaced by water. Seed coats also act asbarriers to oxygen during germination, as indicatedby the marked increase in respiration after radiclesprotrude through the coat... TemperatureIn nature, germination usually occurs over a rangeof temperatures that are higher during the day andlower at night. Thus, a diurnal temperature is oftenused during germination tests. A temperature regimeof °C day (-h light) and °C night (-h dark) iscommonly used for most British Columbia conifers.

Tree seeds usually germinate more quickly underhigh temperatures, but more is not necessarily better.For example, although Pacific silver fir seeds usuallygerminate quickly under warm conditions (°Cdays and °C nights), total germination is oftengreater when seeds are incubated under cool condi-tions (°C days and °C nights). Such a positiveresponse at low temperatures may reflect anadaption of true fir seeds to the cool environmentsthey would ordinarily encounter in nature.

Stratification broadens the temperature range forgermination (Figure ). Unstratified seeds tend to

0

20

40

60

80

100

100 15 20 25 30 35 40

0

20

40

60

80

100

10

0 wk

3 wk 18 wk

6 wk

0 15 20 25 30 35 40

Ger

min

atio

n (%

) at

42

days

Ger

min

atio

n (%

) at

42

days

Ger

min

atio

n (%

) at

42

days

0

20

40

60

80

100

a) Lodgepole pine

b) Sitka spruce

c) Douglas-fir

100 15 20 25 30 35 40

Temperature (°C)

Temperature (°C)

Temperature (°C)

Legend:

germinate well within a fairly narrow temperaturerange, but beyond that range, germination maydecline notably. It is impossible to predict what envi-ronmental conditions will exist when seeds are sownin forest tree nurseries. Stratification allows earliersowing and more reliable germination under unfa-vourable early season temperatures... LightSeeds lying on or near the soil surface receive enoughlight to trigger germination if all other conditionshave been satisfied. However, seeds buried too deeplyin the soil would not receive enough light forgermination. In the nursery, the light requirement isgenerally met during routine handling of hydratedseeds. The light requirement for germination maybe affected by treatments such as stratification. Forexample, unstratified seeds of species that requirelight for germination can be made to germinate indarkness once they have been stratified... Other factorsFailure to germinate is not always linked to dormancy.Poor germination may be caused by immaturity—seeds may have been picked too early or collectedfrom high elevation or high latitude areas that experi-ence shortened growing seasons. Such seeds maybenefit from artificial ripening in the cones after theyhave been collected and before they are extracted.Pests, either fungal or insect, may severely diminishthe quality of seeds (Figure ).

(a) (b)

(c)

X-rays are used to determine whether seeds arefully developed, damaged, or have been attackedby insects. a) mature: c = cotyledon, e = mega-metophyte, r = radicle, s = seed coat; b) immatureseed; c) insect larva; d) damaged seed.

(d)

. Natural Regeneration

The success of natural regeneration programsdepends on how well seedbed environments meetthe requirements for seed germination and seedlingestablishment. The suitability of different seedbedsvaries with their light, moisture, and temperaturecharacteristics.

Many conifer seeds germinate better on mineralsoils because they provide more available moistureand higher average temperatures than organic soils.However, in bright sunlight the surface temperaturesof mineral soils can become so high as to be lethalto germinating seeds. Rotting wood can be a goodsubstrate because it retains moisture well, whereassurfaces that have been burned tend to develop hightemperatures that reduce the moisture-holdingcapability of upper soil layers.

Seedbed environments can be enhanced byselecting appropriate silvicultural systems and sitepreparation methods. Site preparation can improvenatural regeneration by increasing the amount ofexposed mineral soil and by bringing buried seedsto the surface. Burning is probably detrimental tomost conifer species since seeds are generally onor close to the surface of the forest floor, and aretherefore vulnerable to mortality from even a low-severity fire.

Logging may affect seed germination by alteringcritical environmental variables. Total canopyremoval may result in higher soil temperatures andextreme temperature fluctuations. In northern areaswhere moisture is not limiting, complete canopyremoval is sometimes beneficial because soil tem-peratures at high elevations or northern exposuresare often too cold for germination (Figure ). Theincreased exposure of logged areas may stimulatethe germination of species such as pines, but mayinhibit the germination of species adapted to shad-ed habitats. Partial canopy removal may createmore favourable conditions for germination be-cause of the moderating influence of forest coveron light, moisture, and temperature conditions ofthe seedbed.

The regeneration potential of a site cannot beevaluated on the basis of a single factor isolatedfrom other environmental or site variables. Carefulconsideration must be given to species, aspect, com-petition, soil, contour, and the other factors com-prising the physical and biological matrix uponwhich successful natural regeneration depends.

A young whitebark pine seedling struggles toestablish in a high alpine meadow. The seeds mustfirst escape predation by the Clark’s nutcracker,then overcome mechanical and physiologicaldormancy.

A handbook of this sort cannot provide answersto all of the questions that may arise with seed-related activities, but hopefully it explains someof the factors that govern tree seed biology.Understanding the biological principles shouldenable a prediction of the types of responses thatmight be expected under a particular set of circum-stances. It is our responsibility, as the present-daystewards of British Columbia’s natural resources, touse this information to best sustain the long-termcapabilities of our forests.

4 CONCLUSION

The seed is the startthe seed is the endand what takes place between the twois biochemical mystery

APPENDIX 1 Forest Tree Species Occurring in British Columbia

Scientific name/authority Common name

Angiosperms

Acer macrophyllum Pursh bigleaf mapleAlnus rubra Bong. red alderArbutus menziesii Pursh arbutusBetula neoalaskana Sarg. Alaska paper birchBetula papyrifera Marsh. paper birchCornus nuttallii Aud. ex T. & G. Pacific dogwoodFraxinus latifolia Benth. Oregon ashMalus fusca (Raf.) Schneid. Pacific crabapplePopulus balsamifera L. ssp. balsamifera balsam poplarPopulus balsamifera L. ssp. trichocarpa (T. & G.) Brayshaw black cottonwoodPopulus tremuloides Michx. trembling aspenPrunus emarginata (Dougl.) Walp. bitter cherryQuercus garryana Dougl. Garry oakRhamnus purshiana DC. cascaraSalix amygdaloides Anderss. peach-leaf willowSalix bebbiana Sarg. Bebb’s willowSalix discolor Muhlenb. pussy willowSalix exigua Nutt. sandbar willowSalix lucida Muhl. ssp. lasiandra (Benth.) E. Murray Pacific willowSalix scouleriana Barratt ex Hook. Scouler’s willow

Gymnosperms

Abies amabilis (Dougl. ex Loud.) Forbes Pacific silver firAbies grandis (Dougl. ex D. Don in Lamb.) Lindl. grand firAbies lasiocarpa (Hook.) Nutt. subalpine firChamaecyparis nootkatensis (D. Don in Lamb.) Spach yellow-cedarJuniperus scopulorum Sarg. Rocky Mountain juniperLarix laricina (Du Roi) K. Koch tamarackLarix lyallii Parl. in DC. subalpine larchLarix occidentalis Nutt. western larchPicea engelmannii (Parry ex Engelm.) Engelmann sprucePicea glauca (Moench) Voss white sprucePicea mariana (P. Mill.) B.S.P. black sprucePicea sitchensis (Bong.) Carr. Sitka sprucePinus albicaulis Engelm. whitebark pinePinus banksiana Lamb. jack pinePinus contorta Dougl. ex Loud. var. contorta shore pinePinus contorta Dougl. ex Loud. var. latifolia Engelm. lodgepole pinePinus flexilis James limber pinePinus monticola Dougl. ex D. Don in Lamb. western white pinePinus ponderosa Dougl. ex P. & C. Lawson ponderosa pinePseudotsuga menziesii (Mirb.) Franco var. glauca (Beissn.) Franco Rocky Mountain Douglas-firPseudotsuga menziesii (Mirb.) Franco var. menziesii coastal Douglas-firTaxus brevifolia Nutt. Pacific yewThuja plicata Donn ex D. Don in Lamb. western redcedarTsuga heterophylla (Raf.) Sarg. western hemlockTsuga mertensiana (Bong.) Carr. mountain hemlock

Appendix 1 Continued

5

2

3

18

6

7

Alnus rubrared alder

Betula glandulosascrub birch

Acer macrophyllumbigleaf maple

Alnus crispa ssp. sinuataSitka alder

Prunus emarginatabitter cherry

Betula papyriferapaper birch

Arbutus menziesiiarbutus

Cornus nuttalliiPacific dogwood

Quercus garryanaGarry oak

9

4

14

32

8

11

1513

14

12

Abies amabilisPacific silver fir

Tsuga mertensianamountain hemlock

Chamaecyparis nootkatensisyellow-cedar

Juniperus scopulorumRocky Mountain juniper

Pinus ponderosaponderosa pine

Pseudotsuga menziesii var.menziesii coastal Douglas-fir

Taxus brevifoliaPacific yew

Thuja plicatawestern redcedar

Pinus contorta var.latifolialodgepole pine

Abies grandisgrand fir

Picea sitchensisSitka spruce

Larix occidentaliswestern larch

Pinus albicauliswhitebark pine

Abies lasiocarpasubalpine fir

Pinus flexilislimber pine

5 7

6

10

9

Photographic tableau of forest tree seeds: angiosperms (top) and gymnosperms (bottom).

Activation Phase following hydration and ending withactual emergence of the radicle; includes mobilizationof stored reserves, repair of , and reactivation ofenzymes needed for the metabolic processes essential togermination.

Angiosperms Flowering plants; distinguished fromgymnosperms by having the ovules borne within theclosed cavity of the ovary; after fertilization the ovarybecomes a fruit, enclosing one or more seeds.

Chromosome A rod-shaped carrier of hereditary ma-terial (genes) inside the nucleus of cells.

Cotyledon First leaf produced by the embryo of a seedplant.

Dormancy Physical or physiological condition of aviable seed that prevents germination even in the pres-ence of otherwise favourable germination conditions.

Embryo Rudimentary plant within the seed; that partof a seed that develops from the union of the egg celland sperm cell, which after germination becomes theyoung plant.

Emergence Protrusion of the hypocotyl and cotyle-don above the soil surface.

Endosperm Nutritive tissue () of an angiospermseed, which surrounds and nourishes the embryo.(See also megagametophyte.)

Epicotyl That portion of the seedling stem above thecotyledons.

Fertilization Penetration of a pollen tube into theovule, in which the male sperm nucleus is dischargedinto the ovule to unite with the egg nucleus.

Germination Resumption of active growth in theembryo, which results in the protrusion of the embryofrom the seed and development of the embryo into anindependent plant.

Gymnosperms Conifers and their allies; distinguishedfrom angiosperms by having unprotected ovules (notenclosed in a fruit).

GLOSSARY

Hydration Uptake of water into the seed and incorpo-ration of water into seed tissues.

Hypocotyl Part of the axis of an embryo or stem of aseedling between the cotyledons and the radicle; usu-ally identifiable between the root collar and the base ofthe cotyledons.

Integument Outer cell layer or layers that surroundthe ovule and give rise to the seed coat.

Maturation Final stage of seed developmentcharacterized by dehydration of seed tissues and theinduction (in most British Columbia conifers) ofdormancy.

Megagametophyte The nutritive tissue () of gym-nosperm seeds, which surrounds and nourishes theembryo. Often incorrectly referred to as endosperm.(See also endosperm.)

Meristem Undifferentiated tissue that is capable ofundergoing cell division; located at the tips or growingpoints of vegetative or reproductive organs.

Micropyle Opening in the integument of an ovulethrough which the pollen grain or pollen tube passes toreach the embryo sac, and through which the embryoradicle emerges during germination.

Ovule Structure in seed plants containing nutritivetissue and an egg cell, which is surrounded by one ortwo integuments; when the egg is fertilized, the ovuledevelops into a seed.

Pericarp In angiosperms, a fruit wall which developsfrom the ovary wall; it may be dry, hard, or fleshy.

Photosynthesis The combination of carbon dioxideand water by chlorophyll-containing plants, using sun-light as an energy source.

Phytochrome Protein pigment of plants that exists ineither of two forms; it changes from one form to theother by absorption of red or far-red light.

Pollen Spore body () of vascular plants thatcontains the male sex cells.

Pollination Process by which pollen is transferred inangiosperms from the anther, where it is produced, tothe stigma. In gymnosperms, pollen is dispersed bywind from male to female cones.

Radicle Portion of the axis of an embryo from whichthe root develops.

Respiration Metabolic reactions from which a plant oranimal derives energy.

Stratification Dormancy-breaking treatment in whichseeds are exposed to moist, cold ₍– °) conditions forseveral weeks (or months, depending on the species).

Testa Seed coat; protective covering of the embryoof seed plants formed from the integument; usuallyhard and dry.

Viability The state of being capable of germinationand subsequent growth and development of theseedling.

Vigour Seed properties that determine the potentialfor rapid emergence and development of normalseedlings under a wide range of field conditions.

REFERENCES

Edwards, D.G.W. . Effects of stratificationon western hemlock germination. Can. J. For.Res. ():–.

______. . Maturity and quality of tree seeds:a state-of-the-art review. Seed Sci. Technol.:–.

______ . . Collection, processing, testing, andstorage of true fir seeds: a review. In Biology andmanagement of true fir in the Pacific Northwest.C.D. Oliver and R.M. Kenady (editors). U.S. Dep.Agric. For. Serv., Pac. N.W. Range Exp. Sta., andUniv. Wash., Coll. For. Resour., Seattle, Wash.pp. –.

Eremko, R.D., D.G.W. Edwards, and D. Wallinger. .A guide to collecting cones of British Columbiaconifers. B.C. Min. For., Res. Br., Victoria, B.C.FRDA Rep. No. . pp.

Jones, S.F. and P.G. Gosling. . “Target moisturecontent” prechill overcomes the dormancy oftemperate conifer seeds. New For. :–.

Leadem, C.L. . Stratification of Abies amabilisseeds. Can. J. For. Res. ():–.

______. . Stratification and quality assessment ofAbies lasiocarpa seeds. B.C. Min. For., Res. Br.,Victoria, B.C. FRDA Rep. No. . pp.

______. . Respiration of tree seeds. In Dormancyand barriers to germination. D.G.W. Edwards(compiler and editor). For. Can., Pac. For. Cent.,Victoria, B.C. pp. –.

Leadem, C.L., R.D. Eremko, and I. Davis. .Seed biology, collection and post-harvesthandling. In Regenerating British Columbia’sforests. D. Lavender, R. Parish, C.M. Johnson,G. Montgomery, A. Vyse, R.A. Willis, andD. Winston (editors). Univ. B.C. Press,Vancouver, B.C. pp. –.

Osborne, Daphne J. . Physiological and biochemi-cal events in seed development. Adv. Res. Technol.Seeds :–.

Owens, J.N. and M. Molder. . The reproductivecycle of interior spruce. B.C. Min. For., Victoria,B.C. pp.

______ . . The reproductive cycle of lodgepolepine. B.C. Min. For., Victoria, B.C. pp.

______. . The reproductive cycles of westernand mountain hemlock. B.C. Min. For., Victoria,B.C. pp.

______. . The reproductive cycles of westernredcedar and yellow-cedar. B.C. Min. For.,Victoria, B.C. pp.

______. . The reproductive cycles of true firs.B.C. Min. For., Victoria, B.C. pp.