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 distinguish, 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. species, 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 conditions [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 embolism [47].

In ring-porous species, the big vessels of the earlywood 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 vessel density in a single season [14]. Thus, initial wood provides conductive efficiency, final wood conductive safety.

Gilbert [23] considers the ring-porous feature as anomalous 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 species 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 margin 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 reconstruction with the help of statistics on leaf characteristics [50], [51], [52] and [53]. Leaf habits (deciduous vs. evergreen) seem also to be of great significance in the geographic distribution 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 characterize 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 continents 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] and [40]. All these hypotheses were already summarized [15] and [24]. Evergreen trees can photosynthesize during a longer period, including parts of the unfavourable season; they can begin photosynthesizing 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 transpiration rate during the unfavourable season.

Phenological phenomena such as swelling, elongation 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 cambial 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 period 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] and [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 angiosperm record, especially from the very abundant fossil wood. We have first verified this potential relationship between ring-porous structure and deciduousness by drawing up a list of ring-porous species from temperate and tropical environments and by comparing this anatomical 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?

For this study, we mostly used data on wood anatomy from the Insidewood database [29], but also from wood Atlases [28] and [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 numerous published – or online – floras [38] and herbaria (‘Muséum 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.

Regarding the obtained results, it first appears that it is not always easy to distinguish among all the porosity 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] and [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 Rosmarinus 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 feature 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 relationship between the percentage of deciduous trees in the forest and the number of dry months exists [8]. Axelrod [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 tropical areas, we found several species that show both 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 patterns can explain, in some extent, our results. Indeed, a few tropical species are known as evergreen species (Cinnamomum camphora (L.) J. Presl., Magnolia grandiflora 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 tropical species do not present any ring-porous structure of their wood.

Foliage abscission patterns appear to be a beneficial 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 developed 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 establishment of a strongly seasonal climate.

The annual ring formation began in the Late Carboniferous 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 vessels are present. The earliest known ring-porous wood is described from the Cretaceous of Antarctica [37]. According to these palaeobotanical observations, it is advisable to think that both deciduousness and ring-porous structure 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] and [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 vessels loose their ability to transport water each winter and no refilling with water occurs in them [43] and [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] and [48].

In ring-porous angiosperms, the large earlywood vessels 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 formed throughout the remaining portions of the growth season [3] and [22]. These results are consistent with the observation 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 sylvatica L., wood formation begins later, a few weeks after that leaves had already been expanded. Several authors [11], [25] and [26] proved that the early formation of a new vessel in ring-porous species is a strategy to encompass winter 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] and [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 pressurization. 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] and [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.

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 define more precisely this ring-porous feature and examine 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 seasonality 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.