The physiological status of Douglas fir seedlings and the field performance of freshly lifted and cold stored stock

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The physiological status of Douglas fir seedlings and the

field performance of freshly lifted and cold stored stock

Conor O’Reilly, Nick Mccarthy, Michael Keane, Charles P. Harper, John J.

Gardiner

To cite this version:

Original

article

The

_{physiological}

status

of

_Douglas

fir

_seedlings

and

the

field

_performance

of

_freshly

lifted

and

cold

stored stock

Conor

_O’Reilly

_a

Nick

_McCarthy

_b

Michael Keane’

Charles

P.

_Harper

John J.

Gardiner

a

Department

of

_Crop

Science, Horticulture and

_{Forestry, Faculty}

of

_{Agriculture, University College}

Dublin, Belfield, Dublin 4, Ireland

b

Waterford Institute of

_Technology,

Waterford, Ireland c

Research and

_Development

Division, CoiliteTeo,

Newtownmountkennedy,

Co. Wicklow, Ireland

(Received 17 December 1997;

accepted

2 March 1999)

Abstract - The

_{physiological}

status of

_Douglas

fir

_seedlings

in Ireland was followed from October to

_April

each year from 1991-1995 and examined in relation to field

_performance

in a farm-field trial established

concurrently

with the

_physiology

work. A

precise

cold hardiness _pattern was defined: the

_seedlings

were less cold

_hardy

_{in 1994/1995 than in other years, but the}

_period

of

highest

cold hardiness was from November to

_February

_{each year. The} roots of the

_seedlings

were

_mitotically

active

_throughout

the winter in some _{years. The best time} to

_{plant freshly}

lifted stock was from November to _{December/January.}

_Following

cold _storage until June, field

_performance

was

_acceptable

for

_seedlings

lifted to the store from December/

_January

to

_February.

(© Inra/Elsevier, Paris.)

plant quality

/

_{plant handling}

/ cold hardiness / mitotic index / cold _storage /

_Pseudotsuga

meuziesii

Résumé -

_{État physiologique}

et

_performance

en

_plantation

de _plants de

_douglas

frais ou conservés au froid. L’état

physiolo-gique

de semis de

_douglas

en Irlande a été suivi d’octobre à avril,

chaque

année de 1991 à 1995. Il a été mis en relation avec _la

per-formance des

_{plants après plantation}

sur terre

_agricole.

L’évolution de l’endurcissement au froid des semis a _{pu être défini: les semis}

étaient moins résistants au froid en 1994/1995 que les autres années, mais la

_période

de résistance maximale au froid allait

_toujours

de novembre à debut février. Les racines des semis ont eu une activité

_mitotique

durant _l’hiver, certaines années.

_{L’époque optimale}

de

_plantation

des

_plants

frais allait de novembre à

_{décembre-janvier. Après stockage}

au froid

_{jusqu’en juin,}

la

_performance

des

_plants

était correcte

_{lorsque l’arrachage}

avait été effectué de

_{décembre-janvier}

à février. (© Inra/Elsevier, Paris.)

qualité

des plants / manutention des

_plants

/ résistance au froid / indice

_mitotique

/ _stockage au froid /

_Pseudostuga

menziesii

1.

Introduction

The use of

_freshly

lifted bare-root

_seedlings

between November and March is still the most common

_practice

followed in the

_planting

_{programme in} Ireland. The

planting

stock is

_usually

_sufficiently

dormant at this time

to resist the stresses of

_lifting

and

_handling.

This

_practice

has been

_relatively

successful with _many

_species

(espe-*

Correspondence

and

_reprints

[email protected]

cially

Picea sitchensis

_(Bong.)

_Carr.),

but low survival and/or _poor

_growth

_{following planting}

was revealed to

be a common

_problem

with

_Douglas

fir

_(Pseudotsuga

menziesii

(Mirb.) Franco).

It has

_long

been

_suspected

that variation in the

have been shown to be

_key

determinants of

_planting

stock

_quality

in

_Douglas

fir and other

_species

[21, 24].

Results from studies carried out in Britain have indicated that

_Douglas

fir

planting

stock is

_highly

sensitive to the

stresses of

_lifting,

_handling

and cold

_storing

_{[4, 19, 32].}

Douglas

fir is native to north-western North America and it has

_evolved

in areas where the climate is

_relatively

mild,

but summer

_droughts

are common. For this reason,

it is

_thought

that

dormancy development

in

_Douglas

fir

responds

to

_specific

environmental cues, such as

_drought

stress in late summer followed

_{by chilling}

in autumn and winter

_{[28]. However,}

there is a

_widespread

belief in

forestry

in

Ireland

that

_Douglas

fir does not become

_fully

dormant in the winter. Ireland

_{rarely experiences}

droughts

and winters are often _very mild.

Thus,

the annual

_cycle

of

_{dormancy development}

in this

_species

may differ in Ireland from that

reported

elsewhere.

Therefore,

the first

_objective

of this

_study

was to

describe the

_{physiological}

status of

_Douglas

fir

_seedlings

during

the

_period

_spanning

the

_lifting

season. While bud

dormancy

stage

or

_days

to bud break

_[12]

was not esti-mated in this

_study,

the

_dormancy

status and stress

resis-tance levels of the

_seedlings

were assessed

_indirectly

using

physiological

parameters such as mitotic index and cold hardiness levels. The

_performance

of

_seedlings

in a

farm-field trial established

_concurrently

with the

physiol-ogy work was evaluated to determine if there was an

association between

_{physiological}

status and

perfor-mance.

Although

most forests in Ireland are established

_by

using freshly

lifted

_stock,

an

_increasing

_proportion

(10-20 %)

of

_seedlings

is now cold stored. As mentioned

above,

some results from the UK indicate that

_Douglas

fir is

_highly

sensitive to cold

_storage,

_perhaps

not

tolerat-ing

more than 2-3 months of _storage without

_significant

deterioration

[4, 19, 32].

The most

_likely

reason for the

poor field

performance

of cold stored stock may be because this

_species

does not become

_sufficiently

dor-mant in the British climate

_[19].

The second

_objective

of this

_study

was to evaluate the

_potential

for cold

_storing

Douglas

fir in Ireland.

2. Materials and methods 2.1. Plant material and

_sampling

Douglas

fir

_{transplants (2 + 1)}

_{grown in}

_Ballintemple

Nursery,

Co. Carlow

_(lat.

52° 44’

N,

long.

6° 42’

W,

100

m

elevation)

were

_sampled

at 2-4 week intervals from

September/October

until

_April

in

_{1991/1992, 1992/1993,}

1993/1994 and 1994/1995. The

_seedlings

were of a

Washington

provenance each year

(seed

zone identities

and elevations: 1991/1992:

042,

450 m;

1992/1993,

1994/1995:

041,

300 m; and 1993/1994:

412,

600

m);

the exact _{location of each provenance is} not known. The

seedlings

were lined out in

_July/August

_{of the year}

_prior

to

_lifting.

In each _year, sections or whole beds of

seedlings

were chosen from the commercial _crop and set

aside for

_study.

The

_seedlings

received identical

treat-ments to those used

_{operationally}

in the

_planting

pro-gramme. The mean

_heights

_(and

standard

errors)

of the

transplants

were 66

_(1.3),

62

_(0.9),

57

_(1.2)

and 65

(1.5)

cm, and the mean root collar diameters were 10

(0.3),

9

_(0.2),

9

_(0.3)

and 11

_(0.4)

mm in

1991/1992,

1992/1993,

1993/1994 and

1994/1995,

respectively.

These values were based _upon measurements of 60

seedlings

each year.

The soil in this _nursery is a

_sandy

loam of about

_pH

5.7,

having

an

_organic

matter content of 8-12 %, and

sand,

silt and

_clay

fractions of

66,

19 and 15

%,

respec-tively.

The cultural

_practices

used each _year were

broad-ly

similar,

_although

some

_changes

in the nutrition

_regime

were

_implemented

over the _years of

_study.

The nutrition

prescriptions

used in this nursery

rely

heavily

on the use

of tissue

_analyses,

and _steps are taken to correct _any

potential

deficiencies as

_they

arise. The

_target

foliar

con-centrations are

_1.4-2.2,

0.1-0.4 and 0.4-1.5 % for

_N,

P

and

_K,

_{respectively.}

It is not

_possible

to describe the

exact nutrient

_regime

_used,

but the most common

_regime

in the _{final year of}

_growth

is to

_apply

14

_kg

N

ha

-1

once

a month from

_April

to

_July.

Potassium and

_magnesium

are

_applied

as _necessary,

_by

_top

_dressings

in June.

2.2. Measurements and observations

Physiological development

of the

_plants

was followed

using

a

_variety

of

_techniques,

but not all assessments were carried out in _{each year. A} different

_sample

of

seedlings

was used for each test.

2.2.1. Shoot and root

_apical

mitotic

_activity

On most

_sampling

occasions until

_March/April,

the

tips

of terminal shoots and

_{vigorous long-roots}

were

removed for

_study

from 15

_randomly

selected

_seedlings,

fixed in 10 % neutral formalin and stored in a

refrigera-tor

_[9].

Before

_fixing,

each shoot

_tip

was dissected to remove the bud scales. At a later

date,

ten fixed shoot

apices

per lift date were excised from these

_tips,

then

squashed

and stained to determine the _percentage of

dividing

cells or mitotic index

_(MI),

_using

a

_technique

similar to that described

_by

Grob and Owens

_[9].

Shoot

prepared

by

’peeling’

off the root cap and some

epider-mal cells with the aid of tweezers, then

_excising

the more

_deeply

stained root _{apex from other tissue.} As for shoot

_apices,

ten fixed root

_apices

_per lift date were

excised and

_squashed

to determine MI.

_Sampling

was

performed by sequentially scanning

and

_counting

all

mitotic

_figures

under a

_{compound microscope}

at a

mag-nification of x400.

_Sampling

was aided

_by

the use of a 10 x 10 mm ocular micrometer with divisions at 1 mm. Cell counts were carried out in a similar manner at a

magnification

of x 100.

2.2.2. Cold hardiness

On each

_sampling

occasion,

15 first-order lateral shoots

_(10-15

cm

_long,

2-4 mm base

_diameter)

from the current

_{year’s growth}

were

_subjected

to one of a series of

three to five minimum

_freezing

_temperatures

in the _range - 3 to -21 °C

_(1992/1993)

or -3 to -35 °C

_(1993/1994,

1994/1995)

in a

_programmable

freezer. The freezer first

used in 1993/1994 had a lower limit than the one used in 1992/1993. Cold hardiness assessments were not made

in 1991/1992. The freezer cooled the air from 5 °C at

5 °C

h

-1

until the desired minimum _temperature was

reached

_[4].

The shoots were held at the minimum

tem-perature for 3 h and then warmed at 10 °C

h

-1

to the

holding

temperature

of 5 °C. After

_freezing,

the shoots

were

_placed

in beakers

_containing

_tap

water and held in a

heated

_{(18-23 °C}

_day/15-18

°C

_{night) greenhouse}

for 2

weeks. Cold hardiness was determined

_by

the extent of

damage

of needle tissue. Needle

_damage

was scored

_[4]:

0,

no

_damage;

1, < 50 % of needles

killed; 2,

> 50 % killed but less than 100 %

killed; 3,

all needles dead. The temperature at which 50 % of the needles

_(LT

₅₀

₎

died

was

_interpolated

from these

data,

assuming

that these scores

_represented

_{0, 33,}

66 and 100 %

_damage,

respec-tively.

2.3. Cold

_storage

treatments

Cold _storage treatments were

applied

in 1993/1994

and 1994/1995

_only.

Because

_Douglas

fir is considered

to be _very sensitive to cold _storage

_[19],

_seedlings

were

lifted to the store on

_only

four occasions between

November and

_January

each _{year. The} stock was

_placed

in co-extruded

_polyurethane

_bags

and stored at 1-2 °C.

Seedlings

from each lift date were removed from the

store for

_study

in

_early

June

_(various

durations of

_storage

depending

on lift

_date).

The field

_performance

of the

seedlings

was evaluated

_following

_storage.

2.4. Field

_performance

At

_{approximately}

4-5 week intervals in

1992/1993,

1993/1994 and

_1994/1995,

_coinciding

with _every second

test occasion

above,

an additional 100

_seedlings

were

dispatched

for field

_planting

at the Tree

_Improvement

Centre,

Kilmacurra,

Co. Wicklow

(lat.

52° 56’ N,

long.

6° 09’

_W,

120 m

_elevation).

The soil characteristics are:

pH

5.7,

7 %

_organic

matter, and

_sand,

silt and

_clay

frac-tions of

40,

32 and 27

%,

respectively.

The trial was laid out as a randomised block

(3),

split-plot design.

Lifting

date was the main

_plot

and _storage

(freshly

lifted and cold stored until

June)

was the

_(split)

subplot.

Each of the three blocks contained one

_replicate

of each of the

_lifting

date

_by

treatment

combinations,

as

a row

_plot

of 30

_seedlings.

_Spacing

was

approximately

50 cm between rows and 30 cm within rows.

Survival _per

_subplot,

initial

_height,

_height

increment and lammas

_growth

increment were recorded at the end

of the first

_growing

season of each year

_only.

Because

there was some variation in initial

_planting

stock

size,

the

_height

increment data were

_analysed

as

_percentage

of initial

_{height. Subplot}

means were used in all data

analy-ses.

2.5.

_{Meteorological}

data

Because

_dormancy,

cold hardiness

_development

and

growth

are

_heavily

influenced

_by

weather conditions

[12],

air _temperature and rainfall data were obtained

from the weather station in

_Oakpark,

Co.

Carlow,

approximately

18 km from

_Ballintemple

_nursery

_(figure

1).

Unfortunately,

similar data were not available for the nursery itself.

Complete

weather data for the

_planting

site were not available either.

_Comparison

of

_partial

data for Kilmacurra with data for

_Oakpark

(50

km

_apart)

showed small differences in _temperatures and rainfall.

2.6. Data

_analysis

and

_presentation

Because the exact time of

_sampling

varied from _year

to _year,

_comparison

of calendar date effects on _response

data were difficult to _carry out.

Therefore,

the least

sig-nificant differences of the means

_(P

<

_0.05)

are

present-ed in most cases. The standard errors are also

_presented

for the

_height

increment

data,

but to maintain

_clarity

of

presentation they

are not shown in other cases. For the mitotic index

data,

dates

_having

a

_high

_frequency

of

zeros

_(mainly

in the case of shoot

_apices)

were excluded

To _{compare cold hardiness levels between 1993/1994}

and

1994/1995,

linear functions

_(other

non-linear func-tions did not

_improve

the

_fit)

were fitted to the

_LT

₅₀

data for each year. Because few data were available for

1992/1993,

and the trend in cold hardiness

_development

was almost identical to that of

_1993/1994,

these data were not

_{analysed. Separate}

functions were fitted to the acclimation or

_{hardening phase (September/October}

to

the lines for the acclimation and deacclimation

_phases

of each _year were

_{compared using}

the GLM

_procedure

in

SAS [29].

The survival

_(after

arc sine _square root

transforma-tion)

and _percentage

_height

increment data for _{each year}

were

_subjected

to an ANOVA

_[29]

to test for block and lift date

_separately

for each treatment

_(freshly

lifted/cold

stored).

Means for each lift date within each treatment were

_{compared using}

Duncan’s

_multiple

_range test. The

performance

of cold stored stock was

_compared

with that

of the

_{freshly planted}

stock of same lift date

_using

a t-test.

3. Results

3.1.

_Physiology

3.1.1. Shoot mitotic index

Although

the

_general

_pattern of shoot

_{apical activity}

in

_Douglas

fir was similar each year, there were some

differences among years

(figure

2).

Shoot MI was

high-est in November in 1991/1992 and

1992/1993,

and

low-est at this time the

_following

2 _years. The

_apices

became inactive in mid

November, 1993/1994,

the coolest year,

and in

_early

December in _{the other years.}

_Resumption

of

activity

occurred in

_early

to mid March each _year.

3.1.2. Root mitotic index

The _pattern of root MI was almost identical in

1991/1992 and

1992/1993,

although

fewer data

_points

are available for the former _year

_(figure

_3).

The root

apices

were

_highly

active

_throughout

the winters of both

years. In

1992/1993,

for

example,

root MI was

_relatively

low from November to

_early

December

_{(1-3 %),}

then increased to a maximum in

_January

to

_early

March

(4-6

%).

MI declined in mid March

_{(1-2 %)}

and increased

_again

in

_April.

_Apices

became inactive

_by

mid November in 1993/1994 and

_{early January}

in the milder

1994/1995

_lifting

season. MI

_generally

increased

_rapidly

in

_February

_{of these years.} MI declined

_again

in late March 1994/1995.

3.1.3. Shoot cold hardiness

The

_pattern

of cold hardiness

_development

in

_Douglas

fir was almost identical in 1992/1993 and

1993/1994,

although

no data are available for cold hardiness below

- 21 °C in 1992/1993

_(figure

_4).

Cold hardiness

_(LT

₅₀

₎

increased from about -8 °C in late

_September

or

_early

October to about -20 °C in

_early

November of these 2

years. In

1993/1994,

cold hardiness increased more

slowly

after this

time,

reaching

a maximum hardiness of

about -31 °C in mid

_January.

Thereafter,

the shoots dehardened

_gradually

and reached -25 °C

_by

mid

Cold hardiness

_{developed significantly}

more

_slowly

from

_September

to December in 1994/1995 than in 1993/1994

(P

<

_0.05)

_(figure

_4).

Shoots were

_nearly

10 °C more

_hardy

in November 1994/1995 than in

1993/1994.

_During

_January

and

_February,

there were

small differences between _years. Deacclimation occurred

a little later in 1994/1995 than in other _years,

_although

the

_magnitude

of the differences was small and not

sig-nificant.

3.2. Field

_performance

3.2.1.

_{Freshly planted}

stock

Survival of

_{seedlings planted}

soon after

_lifting

was

generally

excellent

_(table

_I).

Survival was _greater than

95 % for most

_planting

dates in most _years.

However,

survival was

_{significantly}

lower for stock

_planted

in

January

1994

_{(81 %),}

and in

_May

1995

_{(71 %).}

The

_height

increment of

_{freshly planted}

stock varied somewhat from _year to year

(figure

5).

The effect of

planting

date on _percentage

_height

increment was

_highly

significant

for each year

(P

<

_0.001),

the best increment

being

achieved

_by

those

_{planted early}

in the season,

especially

for 1992/1993. A

_slight

increase in

_height

increment occurred for

_seedlings

_planted

between late

February

and

_early

March. Growth rates were more vari-able at each

_planting

date in 1992/1993.

Lammas

_growth

in 1993 was also

_frequently

observed

in stock

_{planted early}

in the

_planting

season of

_1992/1993,

but few

plants

produced

lammas shoots in other _years. About 60-70 % of the

_seedlings

had lammas

_growth,

and it accounted for as much as 33 % of the total

_height

incre-ment. The

_frequency

of lammas

_growth

varied

signifi-cantly

among

planting

dates

(P

<

_0.01),

and was most fre-quent in

_{seedlings planted}

from October to

_January.

3.2.2. Cold stored stock

For the

_seedling

lifted in

_1993/1994,

the survival of stock cold stored to June 1994 was _very

_good,

_varying

signifi-cant)

(table

I).

For the 1994/1995

stock,

seedlings

from the two first

_lifting

dates had _{poor survival}

₍₁₂

and 56

%,

in November and

December,

respectively),

while those lifted in

_January

and

_February

had

_nearly

100 %

sur-vival. Rainfall in the month of

_planting

_(June)

in 1995

was _very low and _temperatures were

_{high (figure 1).}

The

_height

increment of cold stored stock

_(figure

₅₎

was _greater for those

_planted

in 1994 than for those

planted

in the warmer, drier season of 1995

_{(figure 1).}

For those

_planted

in

1994,

the best

_growth

was achieved

by seedlings

lifted in late

_January

and

_February

1994. The

_height

increment of stock cold stored in

_January

was

significantly

better than that achieved

_by

the

_freshly

planted

stock of same lift date

_(P

<

_0.01),

but not for other lift dates. For

_{seedlings planted}

in

1995,

the

_height

increment of stock cold stored in

_January

was lower

(P

<

_0.01)

than that achieved

_by

the

_freshly

_planted

stock of same lift

_date,

but there was no

_significant

difference

for

_seedlings

lifted in

_February.

Survival was _poor for

the earlier lift dates of

1994/1995,

so

_height

increment

comparisons

may not be

_meaningful.

When both survival and

_height

increment

_percentage

are

_considered,

_only

those lifted to the store in

_January,

and

_February

to a

_slightly

lesser extent,

_performed

well

both _years, even if

_growth

decreased for the

January

lift in 1995

_(table

_{I; figure 6).}

The

_drought

and

_high

temper-atures that occurred in 1995

_(figure

₁₎

must be taken into

consideration when

_evaluating

the

_performance

of the 1994/1995 stock.

4. Discussion

There are two main

_aspects

to the results of the

_study.

First,

information is

_provided

on the

physiological

status

of

_Douglas

fir

_{seedlings during}

the

_{period spanning}

the

lifting

season.

Second,

the association between

physio-logical

status and the field

_performance

of

_freshly

lifted and cold stored

_plants

is examined.

_However,

no

_attempt

is made to

_predict

field

_{performance using physiological}

parameters. Provenance _{differences among years} were

unlikely

to be a

_major

factor in

_explaining

_response dif-ferences. For

_example,

_{provenances from the} same seed zone were used in 1992/1993 and

1994/1995,

but the

physiological

responses of the

seedlings

differed between years.

4.1.

_{Physiological}

status of

_seedlings

at

_lifting

No clear

_relationship

was found between the

develop-ment of cold hardiness from

_{September/October}

to

December

_{(figure 4)}

and shoot mitotic index

(figure

2).

The

_seedlings

were

_considerably

less cold

_hardy

in the

relatively

warm _year of 1994/1995 than in the cool years

of 1992/1993 and 1993/1994.

However,

the MI data did

not _appear to follow the same trend. For

_example,

shoot

MI was lower in October/November 1994/1995 than in 1991/1992

_(figure

_2),

but the _{former year} was warmer

than the latter year at this time

(figure

1). Nevertheless,

shoot

_apices

_{became inactive earliest in the coldest year}

(1993/1994).

Perhaps

these results underline the fact that

although

cold hardiness and shoot _{MI may}

_change

in response to similar environmental

stimuli,

the

_pattern

of

change

can not be

_expected

to be identical. Similar results have been

_{reported by}

Burr

_[2]

and Cannell et al.

[4].

Colombo et al.

_[5]

found a _very close

_relationship

between shoot MI and the

_early

_stages of cold hardiness

development

in Picea mariana

_(Mill)

B.S.P.

_growing

in

Cold hardiness of shoots increased from -4 °C in

September

to about -15 °C in October

_1994,

but

increased little from November to

_early

December that year

(figure 4).

Burr et al.

_[3]

found that the

_early

_stages

of cold hardiness

_development

in

_Douglas

fir were influ-enced

_{primarily by photoperiod,}

but further

_hardening

required

low

_{temperatures.}

There was little

_chilling

until

December in 1994

_(figure

1),

consistent with this

inter-pretation.

In contrast in

_1993,

there was a continuous accumulation of

_chilling

from October

onward,

and

chilling

temperatures accumulated

_rapidly

in

_November;

cold hardiness also

_developed

more

_rapidly

in 1993.

The

_pattern

of

_early

cold hardiness

_development

in the

cool years of 1992/1993 and 1993/1994

(figure

4)

was

similar to that

_reported

for

_Douglas

fir

_growing

in Scotland

_{[4]. However,}

maximum hardiness levels were

achieved earlier in

Scotland,

and

_high

levels of hardiness

were maintained until

_early

March. In Ireland

deharden-ing

began slowly

in

_{early February.}

In the mild

_lifting

season of

_1994/1995,

_seedlings

were less cold

_hardy

than

in other years. Cold hardiness also

developed

late in

Douglas

fir in a _very mild season in another

_study

car-ried out in Britain

_[19].

Cold hardiness levels _appear to

be

_heavily

influenced

_by

climatic

factors,

so the _pattern

of cold hardiness

_development

can be

_expected

to _vary

from year to _year.

Cold hardiness

_development

followed the clearest

sea-sonal

_pattern

over the 3 year

_period

_(1992-1995),

and

this _may be the most useful _parameter for

_describing

the

physiological

status of

_Douglas

fir

_growing

in Ireland

(see

below).

Cold hardiness is

_routinely

used in the

mon-itoring

of the

_{physiological}

status of

_seedlings

_[24].

_High

cold hardiness levels are

_generally

associated with

_good

stress resistance levels

_{[2, 27].}

The

_{physiological}

measurements showed that the shoot

_system

had a clear annual

_cycle

of

_dormancy

development.

However, the roots of the

_seedlings

were

mitotically highly

active

_throughout

the winter in the

first _{2 years} of observation. In

fact,

mean root MI was

higher

in the colder

_period

of

_January

and

_February,

than

in the warmer October to December

_period.

The reason

for this

_high

MI is difficult to

_explain.

Nevertheless,

root

growth

potential

of

seedlings

in a warm controlled

envi-ronment often increases at _{this time of the year}

_[28],

and

perhaps

root MI can

_respond

in a similar manner in the

nursery

provided

that ambient environmental conditions

are not

_limiting.

In the final 2 _years of the

_study,

the

roots had a _{very low MI} or were inactive in December to

January,

but MI increased in

_{February. Changes}

in

nurs-ery

practices during

the

study period

may have

influ-enced the differences among years. In

_general,

root MI

declined

_again

in March /

_{April, coinciding}

with the time that shoot

_growth

resumes. A similar decline has been

noted at _{this time in Sitka spruce}

_[22].

_McKay

and Mason

_[19]

also noted

_high

root MI

_during

the winter months in

_Douglas

fir,

with a

_rapid

rise in

_activity

occur-ring

in

_January.

Winter root

_activity

may have

implica-tions for the

_storability

of

_Douglas

fir

_[19],

and this issue

is discussed

_separately

below.

4.2.

_{Physiological}

status and field

_performance

The field

_performance

of

_Douglas

fir

_seedlings

plant-ed on various dates over several seasons in this

farm-field trial

_probably

reflected the

_biological

_potential

of the

_seedlings

on this site. The term

_{’biological potential’}

is used because all

_seedlings

were handled with _great care after

_lifting

and/or after removal from the cold store, and

_planted

soon after. Under

_{forestry operational}

condi-tions in

Ireland,

this

_potential

_may not be realised because the

_plants

_may be handled

_differently.

For

example,

it is not uncommon for

_seedlings

to remain in

temporary

storage for some weeks before

_planting

and

post-planting

conditions

_(e.g.

weed

_competition)

_may be

more stressful. Some of the other stresses that can occur

during handling

include

_desiccation,

_{rough handling,}

high

or low

_temperatures

and lack of

_light

_{[16, 20].}

In

this

_{study, planting}

dates that resulted in

_good

field per-formance in the farm-field trial are

_recommended,

but

only

if the

_dormancy

status/stress resistance levels were

relatively high

as indicated

_by

the

_{physiological}

data. 4.2.1.

_Freshly

_lifted

stock

Survival of

_seedlings

_planted

from all

_planting

dates

was

_{extremely good}

_(>

90 % most

_dates),

with two

exceptions: planting

in

_January

1994 and in

_May

1995.

The low survival of the

_January

_{stock may be} a result of

the low soil temperatures

being

unfavourable for root

growth

[17,

31,

32],

or due to

_damage

to the roots

_during

lifting

under wet conditions. The

_{seedlings planted}

in

May

probably

had low survival because

_they

were lifted

when stress resistance levels were _{very low}

_(as

indicated

by

cold hardiness

_levels),

and

_probably

also because of

the

_dry,

warm weather that occurred after

_planting

that

year

(figure

1).

The best

height

increment was

_generally

achieved

_by

_{seedlings planted}

from October until

December/January.

Lammas

_growth

was a

_major

con-tributing

factor to this for those

_planted

in 1992/1993. In

studies conducted in the North York Moors in Britain

[17, 18],

there was no clear

_relationship

between

planti-ng date and

early height growth

for

_freshly

_planted

Douglas

fir

_seedlings,

but survival was better for those

planted

in the

_spring

in another

_study

_[18].

Other studies conducted on

_Douglas

fir in North America found no

[35]

or found that those

_planted

in

_{December/January}

performed

best

[6, 26].

The

_generally

_{good performance}

of

_seedlings

_planted

early

in the

_lifting

season _{may be} due to three factors.

First,

the root

_{growth potential}

of the stock was

relative-ly

high (unpublished

results),

and

_second,

soil

tempera-tures were

_probably

more favourable for root

_growth

[17, 31, 32]. Third,

root

_growth

in

_Douglas

fir is

_thought

to be

_{highly dependent}

on the

_availability

of current

pho-tosynthate

[23],

and

_{light intensity}

levels would be more

favourable for this _process in October and

_early

November than in late November to

_January.

The

_period

of

_high

stress resistance when

_{lifting and}

handling

of

_planting

stock is recommended is

_usually

characterised

_by

a

_period

of low mitotic

_activity,

and

high

cold hardiness levels

_[2],

which is from November

to

_February

for

_Douglas

fir in this

_{study (figures}

_2-4).

However, when field conditions are also

considered,

only

the November to December

_period

_may be suitable for

planting

this

_species

under

_operational

conditions in Ireland.

_Planting

_{in October may also be}

_possible,

but

only

if the

_seedlings

are

_planted

within 2-4

_days

of

lift-ing,

as occurred here.

_Similarly,

Tabbush

_[32]

recom-mended

_planting

of

_Douglas

fir in October and November in the warmer areas of Britain. The

tempera-tures of the soil in _many forest sites in

_January

and

February

may not be favourable for root

_growth

and

absorption

of water

_{[17, 32].}

Such

_seedlings

_may

deterio-rate

_{slowly owing}

to the loss of moisture

_during

transpi-ration. In

_addition,

_Douglas

fir roots cannot tolerate frosts under -4 °C

_[15],

so the risk of frost

_damage

between the time of

_lifting

and field

_planting

is

_higher

also.

4.2.2. Cold stored stock

The results of this

_study

indicate that

_long-term

cold

storage of

_Douglas

fir is

_viable,

but the window of

opportunity

for

_placing

the

_plants

in _storage is narrow.

Field

_performance

was best for those

_placed

in store in

January

and

_February

_{of both years.}

_Seedlings

from the November and December lift dates did better in

1993/1994 than in

_1994/1995,

_probably

because

_they

were at a more advanced

_stage

of

_{dormancy/stress}

resis-tance

_development

at these times in 1993/1994

_(figures

2-4).

Cold hardiness levels may be a

_good

indicator of stress resistance levels

_[2],

and the

_seedlings

stored well

when shoot cold hardiness levels exceeded -20 °C.

Although

stock cold stored from

_January

or

_February

until June

_performed

well in the

field,

a shorter

_period

of storage may be desirable. The duration of the

growing

season is short for those

_planted

in June, and the risk of

drought

stress

_{following planting}

is

_higher.

Furthermore,

a shorter

_period

of _storage

_(perhaps

until

_April)

would

probably

widen the

_lifting

window.

_McKay

and Howes

[18]

make a similar recommendation for

_Douglas

fir

growing

in Britain.

In

_Douglas

_{fir grown} in northern

Britain,

Cannell et

al.

_[4],

_McKay

and Mason

_[19]

and

_McKay

_[14]

found that

_seedlings

deteriorated

_quickly

in cold _storage, while others

_{[30, 31] found}

some evidence that cold _storage may be

possible. Douglas

fir is

_routinely

cold or freezer

stored in North America and France

_{[1, 7,}

8, 10, 11, 25,

33,

34,

35].

McKay

and Mason

_[18]

_suggest that

Douglas

fir _may be difficult to store

_perhaps

because

i)

its roots remain active in the

winter,

and/or

_ii)

cultural

practices

in the _{nursery encourage} late season

_activity.

The

_seedlings

used in this

_study

were from an

opera-tional nursery, and the nursery has been

implementing

small

_changes

in the nutritional

_regimes

_(B.

_Thompson,

per.

comm.).

Root MI reached zero in the _very mild

win-ter of

1994/1995,

while

_they

remained active in the cool-er seasons of

_previous

_years,

_{perhaps partly}

in response to the new nutritional

_regimes.

This may be the reason

why Douglas

fir was

_successfully

cold stored. Cold

stor-age treatments were carried out

_only

in the last 2 _{years of}

study,

so the

_{hypothesis presented by McKay}

and Mason

[19]

can not be refuted.

Nevertheless,

Douglas

fir

seedlings

lifted from another nursery in Ireland have

been

_successfully

cold stored

_during

two

_subsequent

lift-ing

seasons

_(unpublished

results). Furthermore,

recent

research results from Britain

_[18]

_suggest that

_Douglas

fir can be stored until

_April

when lifted to the store from mid

_January

to mid March. Such results can be

_applied

also in Ireland.

Acknowledgements:

The authors would like to thank the

_following

for their assistance in

_carrying

out this work: N.

_Morrissey,

G. DeBrit and B.

_Thompson.

Special

thanks to J. Kilbride who

_supervised

most of the

lifting

operations.

Financial assistance was

_{provided by}

Coillte Teo.

_{(Irish Forestry}

_Board),

Forbairt

(Irish

National Science

_Agency),

the EU AIR

_Programme

(contract no. CT

_920143)

and COFORD

_(Council

for

Forest Research and

_{Development).}

B. Généré

(Cemagref,

Nogent-Sur-Vernisson,

France)

translated the summary. The

input

of two _{anonymous referees} to

improving

the paper is

appreciated.

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[35]

Winjum

J.K., Effects of

_lifting

date and _storage on 2-0