Relations hydriques chez l’épicéa commun (Picea abies (L) Karst) soumis à une sécheresse édaphique dans les Vosges : conductance hydraulique totale, embolie du xylème et régulation des pertes en eau

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Water relations of adult Norway spruce (Picea abies (L)

Karst) under soil drought in the Vosges mountains :

whole-tree hydraulic conductance, xylem embolism and

water loss regulation

Ping Lu, P. Biron, André Granier, Hervé Cochard

To cite this version:

Ping Lu, P. Biron, André Granier, Hervé Cochard. Water relations of adult Norway spruce (Picea

abies (L) Karst) under soil drought in the Vosges mountains : whole-tree hydraulic conductance, xylem

embolism and water loss regulation. Annales des sciences forestières, INRA/EDP Sciences, 1996, 53

(1), pp.113-121. �hal-02693434�

Original

article

Water relations of adult

_Norway

_spruce

(Picea

abies

_{(L) Karst)}

under soil

_drought

in the

_Vosges

mountains: whole-tree

_hydraulic

conductance,

_xylem

embolism

and

water

loss

_regulation

P Lu

1

P

Biron

A

Granier

H

Cochard

1 _Laboratoire

_{d’écophysiologie}

_et

_{bioclimatologie,}

_{INRA, 54280}

_Champenoux;

2_{CEREG, ULP,}

3, rue de

l’Argonne,

67000

Strasbourg

cedex, France

(Received

27

_February

1995;

accepted

22

_{August 1995)}

Summary — Drought-induced changes

in whole-tree

hydraulic

conductances

_(gL)

were monitored

throughout

a

_growing

season in a

_30-year-old

stand of Picea abies.

_gL

was derived from concurrent

mea-surements of leaf water

_potentials

and _sap flux densities

_through

the trunk. Soil water deficits

_clearly

reduced

_gL,

the reduction

_being

most

_likely

located in the soil-root _compartment of the

_soil-plant

_sap

pathway.

The decreases in

gL

did not result in

large

decreases in

midday

leaf water

_potentials

because

midday

sap flux densities were reduced

proportionally

to

gL.

We therefore

hypothesized

that maximum sap flux densities in Picea abies are

adjusted

under

_dry

conditions

_according

to

_changes

in whole-tree

hydraulic

conductances with effect of

_{maintaining midday}

water

_potentials

above the _point of

_xylem

dys-function caused

by

water stress-induced cavitations.

hydraulic

conductance / cavitation / stomatal conductance

/ drought / sap

flux / water

potential

/ Picea abies

Résumé — Relations

_hydriques

chez

l’épicéa

commun

_(Picea

abies

(L) Karst)

soumis à une

sécheresse

édaphique

dans les

_{Vosges :}

conductance

_hydraulique

totale, embolie du

xylème

et

_régulation

des _pertes en eau. Les variations de conductances

_hydrauliques

totales induites par une

*

Correspondence

and _reprints.

Abbreviations: F: water flow; dF: sap flux

density;

dF

midday

and

_dF

_predawn

_:

dF at

_midday

and

_predawn,

respectively;

GL: whole-tree _apparent

_hydraulic

conductance;

gL: sapwood-area-specific

GL; gs:

stom-atal conductance for

_H

₂

_O;

K:

hydraulic

conductance of a

_xylem

_segment;

_K

_init

_:

initial K;

K

max

:

Kat

sat-uration; PLC: percent loss of

conductivity;

Ψ: water

_{potential; Ψ}

_soil

_:

soil Ψ;

Ψ

leaf

:

leaf Ψ;

_Ψ

_midday

and

Ψ

predawn

sécheresse ont été suivies tout au

_long

d’une saison de

_végétation

dans une

_parcelle

de Picea abies

âgés

de 30 ans.

_gL

a été calculé à _partir de mesures simultanées de

potentiels hydriques

foliaires et de densités de flux de sève dans les troncs. Le déficit

_hydrique

dans le sol a réduit nettement

_gL,

cette réduction étant _probablement localisée dans le compartiment sol-racine. La chute de

_gL

n’a _pas induit de diminution

_importante

du _potentiel

_hydrique

minimum parce que les densités de flux de sève maximales ont été réduites _{proportionnellement} à

gL.

Nous faisons

_{l’hypothèse}

_{que les valeurs}

maxi-males journalières

de flux de sève chez

l’épicéa

sont

_ajustées

en fonction de la conductance

hydrau-lique

totale, ceci ayant pour effet de maintenir le _potentiel

_hydrique

minimum au dessus du

point

de

dys-fonctionnement xylémique

causé _{par la} cavitation des trachéïdes.

conductance

hydraulique

/ cavitation / conductance

stomatique

/ sécheresse / flux de sève /

potentiel hydrique / Picea

abies

INTRODUCTION

Severe

_drought

events since the mid-1970s are

_probably

_responsible

for

_Norway

_spruce forest decline observed in the

_Vosges

moun-tains

_{(eastern France)}

_during

the 1980s

(Lévy

and

Becker, 1987;

Probst et

_al,

_1990).

However,

the existence of a causal

rela-tionship

between

_drought

_{and spruce decline} is still an _open

_question.

Water deficits

develop

in forest soils as a result of an unbalance between water

_input

(precipita-tion)

and water

_output

_(mostly

tree

transpi-ration).

The responses of tree and stand

transpiration

to

_long-term

soil water deficits are therefore

_{key points}

in the

understand-ing

of spruce decline. It has

recently

been

suggested

that

_{xylem dysfunctions}

due to

catastrophic

cavitation events

_(Tyree

and

Sperry,

1988)

may be

_responsible

for crown desiccation and tree dieback

_(Tributsch,

1992; Auclair,

1993).

Cumulative tracheid cavitation

_impairs

the

_xylem

water

_transport

capacity

which,

eventually,

can lead to a

complete disruption

of the water

_supply

to the leaves.

_Xylem

embolism

_develops

when the

_xylem

tension becomes

_higher

than a threshold value

_specific

of an organ and of a

_species

_(Sperry

and

_{Tyree, 1988).}

For Picea

abies,

this critical tension is around -2.5 MPa when estimated

_by

the leaf water

potential (Cochard, 1992).

Our

_{understanding}

of

plant

water rela-tions is based on the "tension-cohesion"

theory

initially

developed by

Dixon

₍₁₉₁₄₎

and on its "Ohm’s

_analogy"

_formalism

pro-posed by

Van den Honert

_(1948).

Water moves from the soil to the leaves

_along

a

negative potential gradient

caused

_by

hydraulic

resistances. The sap mass flow

(Fi, kg

s

-1

)

through

any

segment

i of sap

pathway

will,

at

_steady

_state,

_{only depend}

on the

_dynamic

water

_{potential drop}

across the

segment

(dΨi, Pa)

and on its

_hydraulic

con-ductance

_{(Ki, kg}

s-1

_Pa

_-1

_):

Successful

attempts

have been made to

simplify

and

_generalize

this

_equation

to the whole water

_pathway

_(eg

for

_{woody plants,}

Landsberg

et

al, 1976; Cohen

et

al, 1983;

Granier et al, 1989):

where F

represents

the water flow

_through

the whole

_{soil-plant continuum,}

GL the total

_apparent

_hydraulic

conductance from the soil to the

leaves,

_Ψ

_soil

the mean soil water

_potential

in the root zone and

_Ψ

_leaves

the mean leaf water

_potential.

When a

water flux

_density

is measured

_(sap

flux per unit conductive

sapwood area),

then a

_{specific hydraulic}

conductance

_gL

can be

_computed.

Equation [1] gives

a

_simple

functional

the sap flux

through

the

_plant,

the soil water status and the total

_hydraulic

conductance from the soil to the leaves. It is therefore necessary to

analyze

the concurrent

changes

in

_Ψ

_soil

_,

GL and Fto understand the

_changes

in

_Ψ

_leaves

and to assess the

possible

risk of

catastrophic xylem

cavita-tion.

In the framework of the French Forest Decline Research

_Program

_(DEFORPA),

a stand of Picea abies was chosen in the Vos-ges mountains where intensive

ecophysio-logical investigations

were undertaken

dur-ing

the 1990

_growing

season. The seasonal and

_drought

effects on water

_potential,

stom-atal conductance and

_{transpiration}

have been

_published

in a

_previous

paper

(Lu

et

al,

1995).

This second paper

reports

results on the

_hydraulic

_functioning

and

dysfunc-tioning

of spruce under

drought

conditions and its

_{possible implications}

for

_regulation

of water loss.

MATERIALS AND METHODS

Study

site

Measurements were conducted from June to

November 1990 in a

30-year-old

Picea abies

(L)

Karsten

_plantation

in the

_Vosges

mountains

_(NE

France, 7°15’E, 48°15’N, 1 050 m

_elevation).

Stand

density

was 2 343 stems.ha-1, mean

_height

12.6 m and

projected

leaf area around

5.8 m2.m-2. Two

_{representative adjacent plots}

of about 30 trees each were selected in the

plan-tation and

_equipped

with 12-m

_{high scaffoldings}

to

_give

access to the crown of the trees. The

summer

_drought

was increased in the

"dry"

treat-ment

_{by restraining}

external

inputs

of water from

10

_July

to 7

_{September by}

means of a 1-m _deep circular trench all around the

_plot

and a

water-proof plastic

roof located 2 m above the

ground.

At the end of this

period,

this

plot

was

rehydrated

by

a 40 mm

_irrigation,

and allowed to

_dehydrate

anew. The control "watered"

plot

was

_repeatedly

irrigated throughout

the summer

₍₆

times for a

total of 58

_mm)

but limited soil water deficits could not be

_fully

avoided.

Ecophysiological

measurements

Sap

flux

density (dF, kg.dm

-2

.h

-1

)

was measured

continually throughout

the _{study period} on four trees of each _plot with _sap flowmeters

_(Granier,

1987)

inserted in the trunk at breast

height.

Total sap flow

through

the trunk can then be derived

by multiplying the sap flux density by the

sap-wood area at breast

height.

More details about

this

technique

are

_given

in the

previous

paper

(Lu

et al,

1995).

Leaf water

_potential

_(Ψ

_leaf

₎

was

mea-sured on _one-year-old

_{leafy twigs}

with a _pressure

chamber. For each measurement, three or four

sun

_exposed

and shaded

twigs

were _sampled in the upper half of the crown in order to get a

_good

estimation of the average canopy

twig

water

potential.

Daily

courses of

_Ψ

_leaf

were assessed on two

different trees in each

_plot

on seven sunny

days

throughout

the

_{study period. Ψ}

_leaf

was measured

every 2 h from sunrise to sunset. On the same

day, midday

stomatal conductances

_(g

_s

₎

were

measured on the same trees between 12:00 and 13:00 solar time with a Li-Cor 1600 porometer

(Lincoln, NE, USA)

on four sunlit and shaded

twigs

in the _upper half of the crown. Predawn water

_potential

_(Ψ

_predawn

₎

and

midday

water poten-tial

_(Ψ

_midday

₎

were measured more

_extensively

during

sunny

days

every 2 weeks on all the

_eight

trees

_equipped

with sap flowmeters.

Whole-tree

_{specific hydraulic}

conductance

_(gL)

gL

was calculated

_i)

as the

_slope

of the least-squares linear

regression

between the

_daily

courses of

twig

water

_potential

_{and sap} flux

den-sity,

and

_ii)

in a

_simpler

_way

_according

to

_equation

[1],

based

only

on the

predawn

and

midday twig

water

potential

and the

_midday

sap flux.

Seasonal course

_{of xylem}

embolism

and

_{vulnerability}

to cavitation

The

degree

of

xylem

embolism in

leafy

branches

was measured with the

_technique

described

_by

Sperry

et al

₍₁₉₈₈₎

and Cochard

_(1992).

One to

four-year-old

branches from two trees of each

plot were _{sampled early} in the

morning,

wrapped

losses, and

_brought

to the

_laboratory

where

_they

were

_analyzed

the next

day.

In the

laboratory,

branches were

_rehydrated

in tap water and 8 to 15 2-3 cm

_long

_segments were

_randomly

excised

under water. The

hydraulic

conductance

(K

init

)

of each segment was determined

_{by forcing}

dis-tilled water

_through

the samples with a 6 kPa

pressure head and

measuring

the

_resulting

flux

rate with an

_analytical

balance. The embolism

was then resorbed

by

a series of 30 min 100 kPa

pressurization

with

_degassed

distilled water. The

maximum

_conductivity

_(K

_max

₎

was then measured

as described earlier and the

_degree

of embolism

estimated as a _percent loss of conductance:

100*(1

-K

init

/K

max

).

Measurements were per-formed 7 times

throughout

the

growing

season.

Xylem vulnerability

to cavitation was assessed

as described

by

Cochard

(1992).

Seven 1- to

3-year-old

branches were

_{randomly sampled}

in the

crowns of the well-watered trees and

dehydrated

in the

_laboratory

under controlled conditions. After

a few hours to a few

_days

of

dehydration,

1

branch was chosen, its

_xylem

water

_potential

was

measured on

_{leafy twigs}

with a _{pressure chamber}

and the

_degree

of embolism was estimated as

described earlier. The percent loss of

conduc-tance versus minimum

xylem

water

_potential

rep-resents the

_{"vulnerability}

curve" of this

_xylem.

RESULTS

gL

estimations derived from the

_daily

sap flux

_density

versus leaf water

_potential

rela-tionships

were in close

_agreement

with

_gL

values based on the

predawn

and

_midday

values alone

_(n

=

_24,

r2 =

0.91,

slope

not

different from one at P =

0.05) (fig 1). The

agreement

between the 2 methods resulted

from the

_linearity

of the

_dF/Ψ

_leaf

relation-ships (see

fig

2)

observed for most of the trees.

_Therefore,

we include with confidence in this paper the values of

gL computed

with the second

_technique.

The

_changes

in the

_{flux/potential}

rela-tionships during

the summer for one tree from the control and one from the

_{dry plot}

are shown in

_figure

2. The

_slope

of the

regression

lines

represents

1/gL by

defini-tion.

_gL,

_Ψ

_predawn

_,

_Ψ

_midday

and

_dF

_midday

remained

_high

for the watered trees

through-out the summer

_{although they}

could be reduced when limited water deficits

devel-oped.

In contrast, for the nonwatered

trees,

the water

_shortage

and the

drop

in

_Ψ

_predawn

induced a clear reduction in

_gL.

Concur-rently,

with the decrease in

_gL,

an

impor-tant reduction in

_dF

_midday

was observed: from about 2.0

_kg.dm

_-2

_.h

_-1

to less than 0.5

kg.dm

-2

.h

-1

at the end of the

_{drought period.}

It can also be seen in

_figure

2 that the decline in

_Ψ

_midday

was limited and that

_Ψ

mid-day remained above -2.5 MPa all

through

the

_{drought period.}

These

_general

trends noted for the two trees in

_figure

2 are shown for all the studied trees in more detail in the

subsequent

figures.

In

_figure

3 we

_plotted

_dF

_midday

and

_gL

as a function of

_Ψ

_predawn

_.

The decreases of

_gL

and

_dF

_midday

for the

_droughted

trees were of an

_{exponential type,}

_ie,

the most

_significant

decrease was noted at the

_beginning

of the

drought

when

_Ψ

_predawn

was still

_high.

The

first

_day

after the

_rehydration

of the

_{dry plot,}

Ψ

predawn

came back to _very

_high

values but both

_gL

and

_dF

_midday

remained low.

Water-ing

of the upper

layers

of the soil was

prob-ably

enough

to

_rapidly

restore

_Ψ

_predawn

but because roots in the

_{deeper layers}

were not

yet watered,

gL

remained low. Thirteen

_days

after

_rewatering,

when the

_drought

was

developing

anew,

_gL

and

_dF

_midday

recov-ered, but for

two _trees, values for a same

Ψ

predawn

were

_higher

than

_during

the first

drought cycle.

Data for the control trees

were much more scattered than for the

droughted

trees. This

_probably

resulted from the successive

_{dehydration/rehydration}

episodes

that the trees

_{experienced during}

the

_{study period}

that _may have caused

pat-terns similar to those described earlier for the

_droughted

trees.

A linear

_relationship

was found between

gL

and

_dF

_midday

_(r

₂

=

0.78,

n =

83) (fig 4).

A

unique

relation was observed for

_dry,

control and

_rehydrated

trees. The

_midday

leaf stom-atal conductance

_(g

_s

₎

was not correlated with

_gL

_(r

₂

=

_0.04,

n =

_{29) (fig 5),}

but a bet-ter

_relationship

was found

_(r

₂

=

0.51,

n =

29)

when g

s

values were

_{multiplied by}

the

midday

vapor pressure deficit

(ie

the con-ductance converted to a flux

_density).

How-ever, the correlation remained

_weak,

prob-ably

because _gswas _{measured in the upper}

part

of the crown _{and may} not _be repre-sentative of the whole tree.

The

_{vulnerability}

of Picea abies tracheids to cavitation is shown in

_figure

6. On this same

_graph,

we

_replotted

data from Cochard

(1992)

on the same

_species.

We also added the data on the seasonal evolution of

embolism,

the water

_potential

values

_being

the

_midday

leaf water

_potentials

recorded on the

_days

the

_samples

were collected. The

degree

of embolism in

_leafy

branches of Picea abies submitted to natural

_drought

always

remained below 10%

_throughout

the

study period.

Cavitation events in the tra-cheids were not

_{provoked by}

the

develop-ment of the

_drought

nor

_by

the first winter frost. Embolism

_{significantly developed}

in bench

_dehydrated

branches of Picea abies when

_Ψ

_leaf

became less than a threshold

potential

of ca -2.5

MPa, 50%

loss of con-ductance

_being

noted for

_Ψ

_leaf

close to -3.5 MPa. It is clear from this

_graph

that embolism did not

_develop

in the branches of the field

droughted

trees because their minimum water

_{potentials always}

remained above the threshold

potential.

DISCUSSION

Whole-tree

_hydraulic

conductances of Picea abies under

_good

soil water status were

comparable

to that

_{reported by}

other authors for conifer

_(Granier

et

al, 1989; Loustau

et

al,

1990)

or broadleaved trees

_(Bréda

et

al,

1993)

using

similar methods. When water

availability

is reduced in the

soil,

an

impor-tant decrease of

_gL

is observed. Our results

suggest

that the

_change

in conductance was located in the soil-trunk

_compartment

because no

_xylem

embolism was detected in the terminal branches. This is consistent with the fact that the minimum water

poten-tial remained above the threshold water

potential inducing

cavitation. It is also

unlikely

that cavitation occurred in the

upstream part

of the

_xylem

tissue because water

_potential

is

_higher

in the trunk and the roots. This supposes that the

vulnerability

of these organs is

comparable

to that of the

branch,

which may not be the case

_(Sperry

and

Saliendra,

1994).

Tracheids in conifers are known to be

_irreversibly

embolized because

_pit

membranes are sealed to the

_pit

pores after cavitation

(Sperry

and

_Tyree,

1990).

The fact that

_gL

was

_rapidly

restored after

_{rehydration suggests}

that if cavitation did occur in the roots, it was

_probably

_very limited. The

_changes

of

_gL

were therefore not due to

_changes

in

_{xylem hydraulic}

prop-erties. These reversible modifications in

hydraulic

conductance were most

_likely

located in the root

cortex,

in the soil-root interface and in the soil itself

_(Nobel

and

Cui, 1992).

An

_{important objective}

of this

_study

was

to

_analyze

_{the stomatal responses of spruce} to soil water deficits. Stomata are known to close in the presence of a

_{drought, thereby}

limiting

leaf water stress.

_According

to equa-tion

_{[1], leaf}

water stress

_(estimated

_{by Ψ}

_leaf

₎

results from a static water stress

_(soil

water

potential

estimated

_by

_Ψ

_predawn

₎

and a

dynamic

water stress

_equal

to

_{gL*F. Drought}

is known to affect water

_transport

in the

soil-plant

continuum

_{by increasing}

the static water stress

_(decrease

in the soil water

potential).

Stomatal responses to

_Ψ

_predawn

have been discussed in the

_previous

_paper

(Lu

et

al,

1995)

and we concluded that

Ψ

predawn

was a _{poor indicator of} water stress

actually experienced by

trees. The conse-quences of an increase in static water stress may therefore be rather limited. On the other

hand, the

important

variation in

_soil-plant

hydraulic

conductance

_(gL)

found in this

study implies

that a more

_significant

effect of

drought

would be a

_potential

increase in the

dynamic

water stress caused

_by

the water flow. The linear

_relationship

between

_gL

and

dF

midday

found in spruce and other

species

(Reich

and

_Hinckley,

1989; Meinzer and

Grantz,

1990;

Sperry

and

Pockman, 1993;

Brisson

et al, 1993;

Cochard et

al,

1996)

suggests

that

_gL

may

actually

be a critical

parameter

of the

_soil-plant

continuum

lim-iting

maximum

_{transpiration}

rates. It will be noted that

_{although gL}

is derived from

dF

midday

values, this

relationship

is more than

_apparent

because

_i)

the

_dF/Ψ

_leaf

_daily

variations were linear in our

_study,

which proves that

gL

is

_independent

of

_dF

_midday

and

_{ii) gL}

was also

_linearly

related to

inde-pendent

measurements of water flow in the gas

phase

at the leaf level

_(g

_s

_*dsat).

_Spruce

trees _{cope with the}

_drop

in

_gL

_by

_actively

controlling

their water losses and hence

lim-iting

the

_dynamic

water stress.

How stomata may

respond

to

changes

in

_gL

remains an _open

_question.

Stomatal conductance is known to _{be very}

depen-dent on _{air vapor pressure deficit and}

_light,

but these factors cannot

_explain

alone the stomatal behavior in our

_study.

Meinzer and Grantz

_{(1990) suggested}

that in _sugarcane

in roots and that their

_production

and

com-position

are modified

_{by changes}

in

_gL.

Sperry

and Pockman

_(1993),

_{by inducing}

embolism in

branches,

demonstrated the in

Betula,

stomata were

_capable

of

_responding

to variations in

_{gL independently}

of

_changes

in soil water status. Our data

_suggest

that it is not the stomatal conductance which is

regulated

but more

_precisely

the water flux

through

the stomata

_g

_s

_*dsat

In other

words,

transpiration,

not stomatal conductance, is

being

balanced

_{against gL.}

This result is in

agreement

with the

_findings

of Meinzer and

Grantz

₍₁₉₉₁₎

_{in sugarcane}

_(see

also Mott and

Parkhurst,

1991).

Stomatal closure reduces assimilation rate in the short

_{term, which may lower}

_plant

growth

and

_competition

in the

_long

term.

Furthermore,

stomatal closure may alter leaf

integrity by increasing

leaf surface

temper-ature.

Therefore,

there must be some

_strong

short-term

_{ecophysiological}

benefit for stom-atal closure. We

_suggest

_{that for spruce} trees in this

_study,

one of the

_major

benefits

of the observed stomatal closure was the maintenance of the

_{xylem integrity.}

We know from the

_xylem

_{vulnerability}

curve and the

midday twig

water

_potential

measurements that the

_droughted

trees were

_operating

close to the

_point

of

_{xylem dysfunction.}

We can

_{quantitatively}

assess this fact

_by

com-puting,

for each tree and

_given

any value of

_Ψ

_predawn

and

_gL,

the critical dF value that could

_experience

the

_xylem

without

devel-oping

embolism:

In

_figure

7,

we

_expressed

the actual

dF

midday

value versus the

_computed

critical

dF

cavitation

values. It is clear from this

_graph

that

_dF

_midday

was lower but close to

_dF

cavi-tation and that the

"safety

margin"

was

reduced when

_{drought developed.}

We cal-culated that for the driest trees

_(lowest

dF

midaay

values),

the difference between

dF

cavitation

and

_dF

_midday

could

represent

less than a few

_percent

of the observed

_dF

_midday

prior

to the onset of the

_drought.

The max-imum

_{transpiration}

rate seemed therefore

remarkably

regulated

for the control of

_xylem

embolism.

_{Straightforward}

_computations

(data

not

_shown)

also demonstrate that in the absence of water loss

_regulation

_(dF

midday of

the

_dry

_plot

set

_equal

for each

_day

to

_dF

_midday

of the control

_plot),

the

_Ψ

_midday

would have reached values far lower than

Ψ

cavitation

with

_predictable

shoot desicca-tion caused

_{by "runaway}

embolism"

_(Tyree

and Sperry,

1988).

Thus we conclude

that, because

Norway

spruce trees are

_operating

close to the

point

of

_{xylem dysfunction}

caused

_cavitation,

drought-induced

changes

in whole-tree

hydraulic

conductance

put

a

_{physiological}

limitation on

_midday

maximum

_{transpiration}

rate and hence on

_CO

₂

assimilation rates and

_growth.

A

_study

of water loss

regula-tion in the oak tree

_{(Quercus petraea)}

yielded

very similar conclusions

(Cochard

proves to be critical in the

understanding

of their water relations and

_growth,

but further research is needed for

_{assessing possible}

impacts

on forest decline.

REFERENCES

Auclair AND (1993) Extreme climatic fluctuations as a cause of forest dieback in the Pacific rim. Water Air and Soil Pollution 66, 207-229

Bréda N, Cochard H, Dreyer E, Granier A (1993) Water transfer in a mature oak stand (Quercus petraea): seasonal evolution and effects of a severe drought.

Can J For Res 23, 1136-1143

Brisson N, Olioso A, Clastre P (1993) Daily transpira-tion of field soybeans as related to hydraulic

con-ductance, root distribution, soil _potential and mid-day leaf potential. Plant and Soil 154, 227-237

Cochard H _{(1992) Vulnerability} of several conifers to air

embolism. Tree _{Physiol 11,} 73-83

Cochard H, Bréda N, Granier A ₍₁₉₉₆₎ Whole tree

hydraulic conductance and water loss regulation in

Quercus during drought: evidence for stomatal

con-trol of embolism? Ann Sci For (in press)

Cohen Y, Fuchs M, Cohen S (1983) Resistance to water

uptake in a mature citrus tree. J Exp Bot 34, 451-460

Dixon HH (1914) Transpiration and the Ascent of Sap in

Plants. London, Macmillan

Granier A (1987) Evaluation of transpiration in a

Douglas-fir stand by means of sap flow measurements. Tree Physiol 3, 309-320

Granier A, Bréda N, Claustres JP, Colin F ₍₁₉₈₉₎ Vari-ation of _hydraulic conductance of some adult conifers

under natural conditions. Ann Sci For 46 (suppl), 357-360

Grantz DA, Meinzer FC ₍₁₉₉₀₎ Effect of soil deficit on

stomatal behaviour in sugarcane. In: Importance of

root to shoot communication in the responses to

environmental stress _(WJ Davies, B Jeffcoat, eds),

BSPGR _Monograph 21, 286-287

Landsberg JJ, Blanchard TW, Warrit B ₍₁₉₇₆₎ Studies on

the movement of water _{through apple} trees. J Exp

Bot 27, 579-596

Levy G, Becker M (1987) Le dépérissement du sapin

dans les Vosges : rôle primordial de deficits d’ali-mentation en eau. Ann Sci For 44, 403-416

Loustau D, Granier A, Moussa F El _{Hadj (1990)} Évolu-tion saisonnière du flux de sève dans un peuple-ment de pin maritime. Ann Sci For47, 599-618

Lu P, Biron P, Bréda N, Granier A ₍₁₉₉₅₎ Water

rela-tions of adult _Norway spruce (Picea abies (L) Karst) under soil drought in the Vosges mountains: water potential, stomatal conductance and transpiration. Ann Sci For 52, 117-129

Meinzer FC, Grantz DA (1990) Stomatal and hydraulic

conductance in growing sugarcane: stomatal adjust-ment to water transport capacity. Plant Cell Environ

13, 383-388

Meinzer FC, Grantz DA (1991) Coordination of

stom-atal, hydraulic, and canopy boundary layer properties: do stomata balance conductances by measuring transpiration? Physiol Plant 83, 324-329

Mott KA, Parkhurst DF (1991) Stomatal responses to

humidity in air and helox. Plant Cell Environ 14, 509-515

Nobel PS, Cui MY (1992) Hydraulic conductances of the

soil, the soil air gap, and the _{root - changes} for desert succulents in drying soil. J Exp Bot 43, 319-326

Probst A, Dambrine E, Viville D, Fritz B ₍₁₉₉₀₎ Influence of acid _{atmospheric inputs} on surface water _chemistry and mineral fluxes in a _{declining spruce} stand within

a small granitic catchment (Vosges massif, France). J Hydrol 126, 100-124

Reich PB, Hinckley TM ₍₁₉₈₉₎ Influence of pre-dawn water _potential and soil-to-leaf _hydraulic

conduc-tance on maximum daily leaf diffusive conductance

in two oak species. Funct Ecol 3, 719-726 Sperry JS, Tyree MT ₍₁₉₈₈₎ Mechanism of water

stress-induced _xylem embolism. Plant Physiol88, 581-587

Sperry JS, Tyree MT ₍₁₉₉₀₎ Water-stress-induced embolism in three _species of conifers. Plant Cell

Environ 13, 427-436

Sperry JS, Pockman WT (1993) Limitation of transpira-tion _{by hydraulic} conductance and xylem cavitation in Betula occidentalis. Plant Cell Environ 16, 279-287

Sperry JS, Saliendra NZ (1994) Intra- and inter-plant

variation in xylem cavitation in Betula occidentalis.

Plant Cell Environ 17, 1233-1241

Sperry JS, Donnelly JR, Tyree MT ₍₁₉₈₈₎ A method for

measuring hydraulic conductivity and embolism in xylem. Plant Cell Environ 11, 35-40

Tributsch H ₍₁₉₉₂₎ The water-cohesion-tension insuffi-ciency syndrome of forest decline. J Theor Biol 156,

235-267

Tyree MT, Sperry JS (1988) Do _woody plants operate

near the point of catastrophic xylem dysfunction

caused by dynamic water stress? Answers from a

model. Plant Physiol88, 574-580

Van den Honert TH ₍₁₉₄₈₎ Water transport in plants as