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C. R. Palevol 6 (2007) 385–391
A
vailable online at www.sciencedirect.com
Systematic Palaeontology (Palaeobotany)
Is porous wood structure exclusive of deciduous trees?
Ana¨
ıs Boura ∗, Dario De Franceschi
UMR CNRS–MNHN n◦5143 ‘Pal´eobiodiversit´e et pal ´eoenvironnements’, CP 38, 57, rue Cuvier, 75231 Paris cedex 05, France
Received 25 May 2007; accepted after revision 20 September 2007
Available online 19 November 2007
Written on invitation of the Editorial Board
Abstract
According to the IAWA committee (1989), the ring-porous wood is defined as a “wood in which the vessels in the earlywood
are distinctly larger than those in the latewood of the previous and of the same growth ring.” This ring-porous structure is mainly
present in regions with contrasted seasons. Some authors have mentioned the potential correlation between ring-porous structure
and the deciduousness of the trees, but no precise inventory of species has been provided until now to verify this hypothesis. We
compared, with the help of Insidewood (2004) and diverse floras, the wood porosity of 1176 species from temperate countries and
3886 from tropical countries as well as their foliage characteristics. As wood features are frequently preserved in fossil specimens,
they could constitute an interesting marker of vegetation type and be used to infer the seasonality of the palaeoclimate. To cite this
article: A. Boura, D. De Franceschi, C. R. Palevol 6 (2007).
© 2007 Acad´
emie des sciences. Published by Elsevier Masson SAS. All rights reserved.
R´
esum´
e
Les bois poreux sont-ils exclusifs des arbres d´
ecidus ?. D’apr`
es le IAWA Committee (1989), le bois `
a zone poreuse est d´
efini
comme un « bois dans lequel les pores du bois initial sont manifestement plus gros que ceux du bois final de l’ann´
ee pr´
ec´
edente
et du mˆ
eme cerne. » Ce type de porosit´
e du bois est principalement retrouv´
e dans les r´
egions `
a forte saisonnalit´
e. La corr´
elation
potentielle entre cette structure poreuse du bois et le caract`
ere caducifoli´
e des arbres a ´
et´
e mentionn´
ee par de nombreux auteurs,
mais aucun inventaire pr´
ecis n’a ´
et´
er
´
ealis´
e jusqu’`
apr
´
esent pour v´
erifier cette hypoth`
ese. Nous avons compar´
e, grˆ
ace `
a Insidewood
(2004) et `
a diverses flores, la porosit´
e de 1176 esp`
eces de pays temp´
er´
es et 3886 esp`
eces de pays tropicaux et le caract`
ere d´
ecidu
de leur feuillage. Les caract`
eres du bois sont fr´
equemment pr´
eserv´
es dans les sp´
ecimens fossiles ; ils pourraient donc constituer des
marqueurs fiables du type de v´
eg´
etation et pourraient ainsi ˆ
etre utilis´
es pour d´
eduire la saisonnalit´
e des pal´
eoclimats. Pour citer cet
article : A. Boura, D. De Franceschi, C. R. Palevol 6 (2007).
© 2007 Acad´
emie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Keywords: Ring-porous wood; Deciduousness; Leaves; Seasonality
Mots cl´es : Bois `
a zone poreuse ; D´
ecidu ; Caducit´
e ; Feuilles ; Saisonnalit´
e
∗Corresponding author.
E-mail address: boura@mnhn.fr (A. Boura).
Version franc¸aise abr´
eg´
ee
D’apr`
es le IAWA Committee [27], le bois `
a zone
poreuse est d´
efini comme un « bois dans lequel les pores
du bois initial sont manifestement plus gros que ceux
du bois final de l’ann´
ee pr´
ec´
edente et du mˆ
eme cerne. »
1631-0683/$ – see front matter © 2007 Acad´
emie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crpv.2007.09.009
386 A. Boura, D. De Franceschi / C. R. Palevol 6 (2007) 385–391
Un changement abrupt de taille et densit´
e des vaisseaux
permet de distinguer le bois `
a zone poreuse du bois semi-
poreux ou `
a pores diffus. Cependant, quelques esp`
eces
pr´
esentent un continuum de ces diff´
erents ´
etats de poro-
sit´
e, en fonction des conditions environnementales.
Dans les bois `
a zone poreuse, le bois initial assure
l’efficacit´
e de la conduction. L’embolisme, r´
esultant de
stress hydrique, induit un blocage des gros vaisseaux de
l’ann´
ee pr´
ec´
edente. Le bois final, comprenant de plus
petits vaisseaux, est moins efficace, mais pr´
esente une
meilleure s´
ecurit´
e hydraulique vis-`
a-vis du ph´
enom`
ene
d’embolisme. La structure en zone poreuse du bois est
donc consid´
er´
ee comme une adaptation aux climats `
a
forte saisonnalit´
e[14]. Dans ces r´
egions, les esp`
eces
`
a feuillage caduc dominent. Quelques auteurs ont d´
ej`
a
mentionn´
e l’existence d’une corr´
elation potentielle entre
le caract`
ere « bois `
a zone poreuse » et le caract`
ere cadu-
cifoli´
e des arbres [23,36,49], mais aucun inventaire
pr´
ecis n’a ´
et´
er
´
ealis´
e jusqu’`
apr
´
esent pour v´
erifier cette
hypoth`
ese.
Nous avons donc compar´
e, grˆ
ace `
a la base de donn´
ees
Insidewood (2004) [29] et `
a diverses flores, la porosit´
ede
1176 esp`
eces de pays temp´
er´
es et 3886 esp`
eces de pays
tropicaux, ainsi que le caract`
ere d´
ecidu de leur feuillage.
Les structures `
a bois poreux sont pr´
esentes dans environ
17 % des esp`
eces ´
etudi´
ees sous climat temp´
er´
e et environ
1 % sous climat tropical. Sur l’´
echantillonnage d’esp`
eces
temp´
er´
ees, 94 % des esp`
eces `
a bois poreux sont d´
ecidues.
Les esp`
eces tropicales `
a bois poreux le sont aussi, pour
la plupart. Les r´
esultats obtenus grˆ
ace `
a cette ´
etude nous
montrent que, malgr´
e certains probl`
emes de d´
efinition, il
existe bien un lien entre les esp`
eces `
a bois `
a zone poreuse
et le feuillage d´
ecidu. L’observation r´
eciproque n’est
cependant pas vraie : de nombreuses esp `
eces d´
ecidues
ont un bois `
a pores diffus. Il semble, en fait, y avoir des
diff´
erences de strat´
egie dans la reprise de la conduction
de la s`
eve brute au printemps, les esp`
eces `
a zone poreuse
mettant en place de nouveaux vaisseaux tr`
es tˆ
ot dans la
saison, les esp`
eces `
a pores diffus r´
etablissant la conduc-
tion dans les vaisseaux form´
es les ann´
ees precedents,
grˆ
ace `
a la pression racinaire.
Les caract`
eres anatomiques du bois li´
es au type de
porosit´
e sont fr´
equemment pr´
eserv´
es dans les sp´
ecimens
fossiles. Ils pourraient, grˆ
ace aux r´
esultats obtenus
dans cette ´
etude, ˆ
etre utilis´
es comme marqueurs du
type de vegetation, afin de d´
eduire la saisonnalit´
e des
pal´
eoclimats.
1. Introduction
According to the IAWA Committee [27], the ring-
porous wood was defined as a “wood in which the vessels
in the earlywood are distinctly larger than those in the
latewood of the previous and of the same growth ring.”
An abrupt change in the size and density of vessels
between earlywood and latewood enables us to distin-
guish, in some extent, species with ring-porous wood
from others with semi-ring-porous or diffuse-porous
wood. In ring-porous wood, such as Castanea sativa
Mill., Quercus robur L., Fraxinus excelsior L., annual
growth rings are relatively simple to identify, whereas in
diffuse-porous wood such as in Fagus sylvatica L., Acer
pseudoplatanus L. and Liriodendron tulipifera L. spe-
cies, the vessels have approximately the same diameter
throughout the ring from the earlywood to the late wood,
and thus growth rings are less marked. Nevertheless,
some species show a continuum between the different
states of porosity, depending on the environmental condi-
tions [12]. It is also possible to see a continuum from the
diffuse feature to the ring-porous one when looking at the
ontogeny of a ring-porous species. Indeed, many ring-
porous species show a diffuse-porous wood structure in
their first years of growth [12] (personal observations in
Castanea sativa Mill.).
In wood, the conduit diameter has a major impact
on conducting efficiency [42]. According to the
Hagen–Poiseuille law, wide conduits are more efficient
conductors of water than small ones [47], the lumen
conductivity increasing with the fourth power of the
lumen diameter [42]. Despite this efficiency to conduct
sap, large diameter vessels are more vulnerable to embo-
lism [47].
In ring-porous species, the big vessels of the ear-
lywood provide enhanced sap conduction during the
beginning of the growth season. These vessels are
usually embolised before the growth stops. Little vessels,
of less efficiency for sap conduction, provide conductive
safety during the end of the growth season. Ring porosity
is thus considered as an adaptation to seasonal climates,
providing the reversibility of vessel diameter and ves-
sel density in a single season [14]. Thus, initial wood
provides conductive efficiency, final wood conductive
safety.
Gilbert [23] considers the ring-porous feature as ano-
malous regarding to the preponderance of diffuse-porous
structure in world floras and outlines the restriction of
this feature to a limited geographical region, the North
Temperate Area. Since then, several ring-porous spe-
cies have been described from outside of this zone, but
ring-porous species seem to be more or less confined to
seasonal habitats.
Bailey and Sinnot [10] underlined a clear relationship
between leaf margin and environment in the distribution
of the Dicotyledons. Indeed, leaves with an entire mar-
A. Boura, D. De Franceschi / C. R. Palevol 6 (2007) 385–391 387
gin seem to be predominant in tropical mesic (lowland)
regions, whereas leaves with a dentate/non entire-margin
are more confined to temperate and cold habitat. This
observation was used for palaeoenvironmental recons-
truction with the help of statistics on leaf characteristics
[50–53]. Leaf habits (deciduous vs. evergreen) seem
also to be of great significance in the geographic dis-
tribution of taxa. Evergreen species occur in all parts
of the world, from the tropics to the Polar Regions,
from lowland to sub-Alpine altitudes [2]. More accurate
data from satellite imagery [17] or from Woodward [54]
and Givnish [24] show that evergreen broad-leaved trees
dominate tropical rain forests in aseasonal regions of
America, Africa, Madagascar, Australasia, and Islands
of the Pacific. Evergreen leathery-leaved trees characte-
rize temperate forests from the southern hemisphere, the
Mediteranean scrub, and the wetter temperate rain forest
in areas of winter rainfall, on the west sides of conti-
nents at mid latitude. Evergreen, needle leaved conifers
dominate boreal forests at high latitude in the northern
hemisphere. As for deciduous broad-leaved trees, they
characterize temperate forests at mid latitude in eastern
North America, eastern Asia, and northwestern Europe,
but they are also frequent in tropical and subtropical
areas, with a pronounced dry season.
Numerous authors have tried to explain patterns of
leaf life span [2,19,33,39,40]. All these hypotheses were
already summarized [15,24]. Evergreen trees can pho-
tosynthesize during a longer period, including parts of
the unfavourable season; they can begin photosynthe-
sizing earlier and continue later than deciduous ones.
Moreover, as they keep their leaves for more than one
year, evergreen species have a lower amortized cost of
constructing the carbohydrate skeleton. They also have a
lower amortized cost of replacing nutrients, making them
advantageous on nutrient poor sites. However, evergreen
leaves often have to be tougher and thicker.
Deciduous trees, as for them, have a higher rate
of photosynthesis per unit leaf mass during favourable
periods than evergreen ones have, and reduce their trans-
piration rate during the unfavourable season.
Phenological phenomena such as swelling, elonga-
tion and opening of buds or elongation of the stems
and leaves, maturation of the leaves, flowering, and
fruiting are often associated with specific stages in cam-
bial activity and certain structural variations in the ring
[22].
Bailey [9] outlined a link between the apparition of
ring porosity, the acquisition of a pronounced resting per-
iod and the beginning of the deciduous habit. Since then,
this potential correlation between ring-porous structure
and the deciduousness of the trees has been mentioned
by several authors [23,36,49], but no precise inventory
of species has been provided until now to verify this
hypothesis.
The aim of this study is to increase our knowledge of
the relationship between wood and leaf habit, in order
to provide more ecological deduction from fossil angio-
sperm record, especially from the very abundant fossil
wood. We have first verified this potential relationship
between ring-porous structure and deciduousness by dra-
wing up a list of ring-porous species from temperate and
tropical environments and by comparing this anatomi-
cal wood feature with their leaf habit. Then, we tried to
understand the divergences between these two features
by considering physiological processes.
Could some of the differences in life history, habit or
phenology of trees be explained in terms of differences
in hydraulic architecture?
2. Material and method
For this study, we mostly used data on wood anatomy
from the Insidewood database [29], but also from wood
Atlases [28,41]. We considered that the sampling of the
studied species present in the database reflects more or
less the global biodiversity occuring in nature.
In order to compare these wood data with foliage
characteristics data of the plants, we used nume-
rous published – or online – floras [38] and herbaria
(‘Mus´
eum national d’histoire naturelle’, Paris), where
descriptions of the leaves, particularly leaf habits, and
plants from various parts of the world are available.
We considered the geographical regions defined by
Brazier and Franklin [13].
We first tried to list all temperate European ring-
porous species. Then we compared their wood porosity
with their leaf habits. Following what, we listed on one
hand temperate species with ring-porous wood and on
the other hand tropical species with ring-porous wood.
We finally associated these tropical species with their
leaf habit and environment.
3. Results
3.1. European temperate species
We found 193 European species wood descriptions.
Among these species, nearly 15% are only ring porous,
10% are only semi-ring porous, and 27% are only
diffuse porous. The other species show two or three
features of porosity. The ring porosity feature can be
present in almost 27% of the European temperate species
(Fig. 1).
388 A. Boura, D. De Franceschi / C. R. Palevol 6 (2007) 385–391
Fig. 1. Growth ring patterns distribution in European temperate species
(RP, ring porous; SP, semi-porous; DP, diffuse porous).
Fig. 1. Distribution des diff´
erents types de porosit´
e au sein des esp`
eces
temp´
er´
ees europ´
eennes (RP, bois `
a zone poreuse ; SP, bois `
a zone semi-
poreuse ; DP, bois `
a pores diffus).
As regards the relationship between wood porosity
and leaf habit in the European temperate areas (Fig. 2),
it appears that 94% of the species that are ring porous are
also deciduous. Among these ring-porous species, only
two shrub species show at the same time a ring-porous
structure of their wood, as well as a semi-evergreen leaf
habit for the first one and an evergreen leaf habit with a
strong spring growth for the second one.
It is also clear that the reciprocal phenomenon is not
true. Some deciduous species have a semi-ring-porous
or a diffuse-porous wood.
3.2. Temperate vs. tropical species
Among the 5329 species with described wood, 1176
are temperate, whereas 3886 are tropical. The other ones
can be found in both temperate and tropical areas. Over
these studied species, 274 are ring porous.
In temperate areas, ring porous wood structure occurs
in 17.7% (208/1,176) of the studied species. Ring poro-
sity is more frequent in the northern hemisphere (24.8%
of the studied species) than in the southern one (3.5% of
the studied species).
In tropical countries, ring porosity occurs in only
1.1% of the total number of studied species (43/3886).
Fig. 2. Growth ring patterns and the associated leaf habit in European
temperate species (Eg, evergreen species; Dc, deciduous species; Sd,
semi-deciduous species; Nl, no leaf).
Fig. 2. Type de porosit´
e et feuillage associ´
e au sein des esp`
eces
europ´
eennes temp´
er´
ees (Eg, esp`
eces sempervirentes ; Dc, esp`
eces
d´
ecidues ; Sd, esp`
eces semi-d´
ecidues ; Nl, feuillage absent).
3.3. Tropical species
Among all the studied species, we found 43 tropi-
cal species with a ring-porous wood structure. Most of
them are deciduous, but a few appear to have evergreen
habit. Ninety percent of the ring-porous species have
also a wood with simple perforation, and 35% with spiral
thickenings. Both of these wood anatomical characters
are known to be an adaptation in cold and dry habitat
[14,30]: simple perforations provide a better conduc-
tion and spiral thickenings allow to refill embolised
vessels.
4. Discussion
Regarding the obtained results, it first appears that it
is not always easy to distinguish among all the poro-
sity features, most probably because definitions are
not very clear. Authors do not agree on the definition
of ring-porous, semi-ring-porous, and diffuse-porous
wood. Most of the definitions given to these three states
of porosity are based on a qualitative estimation made by
the observer [12,18,21,27]. Therefore, definitions vary
from one author to another and consequently there is not
always an agreement on the assigned categories [36].
Both temperate species Jasminum fruticans L. and Ros-
marinus officinalis L. that show a ring-porous structure
of their wood and a (semi) evergreen habit are a good
example of this phenomenon. Indeed, both of them can
be ring-porous, according to Insidewood [29], whereas
in Schweingruber [41], they both are only semi-ring
porous.
In temperate climates, ring porosity is frequent and
associated with autumnal deciduousness. In tropical
areas, the ring-porous structure is very rare. This fea-
ture happens in most cases, in species from regions of
alternate wet and dry seasons that present at the same
time a deciduous leaf habit.
In these species, the dry season triggers leaf fall
[1]. In tropical regions, the annual rainfalls condition
the forest type [7]: 2000 mm of annual rainfall usually
corresponds to the minimum required by the evergreen
forest, 1500 mm of annual rainfall is the limit between
moist deciduous and dry deciduous forests, and 900 mm
corresponds to a change in the structure and floristic
composition of the dry deciduous type [34]. A relation-
ship between the percentage of deciduous trees in the
forest and the number of dry months exists [8]. Axel-
rod [6] shows that in actual forest there is an increasing
defoliation pattern with increasing seasonal drought.
However, in our listing of ring-porous species from tro-
pical areas, we found several species that show both
A. Boura, D. De Franceschi / C. R. Palevol 6 (2007) 385–391 389
ring-porous structures and an evergreen foliage. Four
patterns of leaf phenology in tropical trees have been
distinguished [32]: (1) leaf fall before bud break, the
entire tree remaining leafless or nearly so for a few
weeks to several months; (2) leaf fall associated with
budbreak; (3) leaf-fall completed well after bud break;
(4) continuous production and loss of leaves. These pat-
terns can explain, in some extent, our results. Indeed,
a few tropical species are known as evergreen species
(Cinnamomum camphora (L.) J. Presl., Magnolia gran-
diflora L., Grevillea sp., and Persea Americana Mill.),
but have a complete change of foliage each year. The
abscission of the previous year’s leaves occurs as the
new growth develops on the tree [1]. The young leaves
are responsible for the increase of cambial activity, size,
and density of vessels through the production of auxin
[3]. The earlywood formation is thus mainly induced by
the leaf growth. This should be particularly true on trees
that loose leaves and then produce new leaves on the
overall crown simultaneously. Nevertheless, these tropi-
cal species do not present any ring-porous structure of
their wood.
Foliage abscission patterns appear to be a benefi-
cial adaptation to climates with alternate seasons. The
tree defoliates and becomes dormant during the period
of unfavourable weather. The majority of the ancient
fossil leaves seem to belong to the evergreen type,
though it remains quite uncertain. The earliest fossil
record of deciduous leaves is in the Glossopteridaceae
of the southern hemisphere during Carboniferous (about
300 Ma) [1]. Deciduousness of Angiosperm trees deve-
loped during Early Cretaceous (125 Ma) [6]. In both the
southern hemisphere during the Carboniferous and the
northern one during the Cretaceous, the deciduous habit
appeared and evolved in conjunction with the establish-
ment of a strongly seasonal climate.
The annual ring formation began in the Late Carboni-
ferous in the boreal regions [31]. Fossil wood from Early
Tertiary is mostly diffuse porous. The earliest known
woods of a definite Angiosperm nature show no signs of
a ring-porous arrangement, even if well-developed ves-
sels are present. The earliest known ring-porous wood is
described from the Cretaceous of Antarctica [37]. Accor-
ding to these palaeobotanical observations, it is advisable
to think that both deciduousness and ring-porous struc-
ture of the wood appeared at the same time in response
to a more seasonal climate.
The relationship between ring porosity and climate
has been enhanced by our study. Ring-porous species are
always deciduous. However, the reciprocal affirmation
is not true, deciduous species can have a ring-porous
wood, but also a semi-ring-porous or a diffuse one. The
presence of a root pressure in some species can explain
this phenomenon [4,25,35].
It seems that in dicot trees, the region that functions
in water transport varies among species [48]. In ring-
porous species, earlywood vessels of the current year
xylem are mainly involved in this transport. These ves-
sels loose their ability to transport water each winter and
no refilling with water occurs in them [43,46]. In diffuse
porous species, on the contrary, water transport occurs
in a large part of the sapwood thanks to the refilling of
vessels by root pressure [25,44,48].
In ring-porous angiosperms, the large earlywood ves-
sels are differentiated early in the season and mature
quickly, often prior to the full expansion of the new
leaves. Small vessels and thick-walled fibres are for-
med throughout the remaining portions of the growth
season [3,22]. These results are consistent with the obser-
vation we made last year [11]. We studied the wood
formation in the ring-porous species, Castanea sativa
Mill., and in the diffuse species Fagus sylvatica L. We
found that wood begins to be formed earlier in the ring-
porous species. Indeed, in Castanea sativa Mill., the
biggest vessels appear with the first leave. In Fagus syl-
vatica L., wood formation begins later, a few weeks after
that leaves had already been expanded. Several authors
[11,25,26] proved that the early formation of a new vessel
in ring-porous species is a strategy to encompass win-
ter embolism, and to supply the new leaves with more
efficiency. On the contrary, other species of trees like
Fagus sylvatica L., utilize root pressure to restore the
hydraulic capacity in early spring before the bud break
[25,44,48]. During spring, trees begin to take nutriments
and mineral elements from the soil through their roots.
For some of them, this intake induces root vessels pres-
surization. The pressure then propagates to the top of the
tree, causes “a rise of sap” and the dissolution of the gas in
the sap or pushes undissolved gas out of the vessels. This
root pressure was measured in several species, which
are all semi-ring-porous or diffuse species: woody vines
[20,45] Vitis labrusca L., Vitis riparia Michaux [44],
Acer pseudoplatanus L. [25],Acer saccharum Marsh.
[47],Betula pendula Roth. [25],Alnus [5],Juglans regia
L. [5],Fagus sylvatica L. [16], but seems to be absent in
other ones like Prunus persica (L) Batsch. [16],Fraxinus
[25], which are ring-porous.
5. Conclusion
As growth rings show diverse patterns according to
species from the tropics and temperate regions, further
studies are needed to analyse each observed structure
within a growth ring. Detailed studies are necessary to
390 A. Boura, D. De Franceschi / C. R. Palevol 6 (2007) 385–391
define more precisely this ring-porous feature and exa-
mine the tree phenology and leaf production, in order to
understand the different observed growth pattern.
Nevertheless, as the growth ring pattern is frequently
observable in fossil specimens, the ring-porous wood,
even as presently defined, could be an interesting marker
of the vegetation type. It could be used to infer the sea-
sonality of the palaeoclimate. Its frequency in different
species found in an outcrop could be quantified in order
to compare the woods with those of modern vegetations
under different climate conditions.
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