Dendrochronologia 31 (2013) 187–191
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Dendrochronologia
journal homepage: www.elsevier.com/locate/dendro
PERSONAL NOTE
What is ‘wood’ – An anatomical re-definition
Fritz Hans Schweingruber a , Ulf Büntgen a,b,c,∗
a
Swiss Federal Institute for Forest, Snow and Landscape (WSL), 8903 Birmensdorf, Switzerland
Oeschger Centre for Climate Change Research (OCCR), University of Bern, 3012 Bern, Switzerland
c
Global Change Research Centre AS CR, v.v.i., Bělidla 986/4a, CZ-60300 Brno, Czech Republic
b
a r t i c l e
i n f o
Article history:
Received 1 February 2013
Accepted 22 April 2013
Keywords:
Dendrochronology
Wood anatomy
Life form
a b s t r a c t
The technical definition of ‘wood’ is well accepted, but its botanical understanding remains vague. Different degrees and amounts of lignification in plants and their imprecise description, together with a
conceptually doubtful life form catalog including trees, shrubs and herbs further complicate our understanding of ‘wood’. Here, we use permanent micro sections to demonstrate that the xylem and bark of
terrestrial plants can vary from one tissue with a few lignified cells to an almost fully lignified tissue.
This universal principle of plant growth and stabilization, accounting for all taxonomic units within vascular plants, suggests that the classical life form separation into herbs, shrubs and trees is not valid.
An anatomical-based differentiation between ‘wood’, ‘woody’ and ‘woodiness’ is also only meaningful if
supplemented by insight on the particular plant section and its lignified proportion. We therefore recommend utilizing the botanically more neutral term ‘stem anatomy’ instead of ‘wood anatomy’, which
further implies integration of the xylem and bark of all terrestrial plants. Since dendrochronology considers shrubs, dwarf shrubs and perennial herbs in addition to trees, its semantic expansion toward
‘xylemchronology’ might be worthwhile considering.
© 2013 Published by Elsevier GmbH.
Introduction
‘Wood’ has been well defined as a raw material for many years
(Trendelenburg and Mayer-Wegelin, 1955), and also recognized as
an energetic resource with many technical and artistic applications
(Vorreiter, 1949). The technical meaning of ‘wood’ is scientifically
well established (Sell, 1987), and also accepted by foresters and
carpenters, as well as a broader public. ‘Wood science’, including dendrochronology and wood anatomy, generally focuses on
trees and the assessment of ‘wood’. The plant anatomical definition of ‘wood’, however, remains diffuse, because ‘wood’ is related
to a complex interplay of ontogenetic processes of primary and
secondary meristems (Evert, 2006; Dulin and Kirchoff, 2010), cell
formation and chemical composition, as well as phylogenetic origin (Schweingruber et al., 2012). Precise categories of lignified,
non-lignified and partly lignified plants and their conceptual classification into trees, shrubs and herbs also remain debatable (see
also references herein). Explicit descriptions and boundaries of the
theoretically separated dendrochronological and wood anatomical
communities are still limited, and their associated disciplinary foci
are expected to further diminish in academia.
∗ Corresponding author at: Swiss Federal Institute for Forest, Snow and Landscape
(WSL), 8903 Birmensdorf, Switzerland. Tel.: +41 44 739 2281; fax: +41 44 739 2215.
E-mail address: [email protected] (U. Büntgen).
1125-7865/$ – see front matter © 2013 Published by Elsevier GmbH.
http://dx.doi.org/10.1016/j.dendro.2013.04.003
At the same time are distinct terminologies key requirements
for any unequivocal communication, within and between all kinds
of communities. Terms and definitions within the broad field of
‘wood science’, however, often can have varying meanings. ‘Wood’,
‘woody’ and ‘woodiness’ are frequently used but rarely explained
(Dulin and Kirchoff, 2010). In fact, ‘wood’ is often equally used
as ‘xylem’ of plants with secondary radial thickening (Carlquist,
2001). The term ‘wood’ was traditionally also employed if most
cell walls per unit were lignified. Nevertheless, this rather simplistic assumption contradicts anatomical evidence of differently
lignified intensities and patterns that can range from the cell to
the plant. ‘Woody’ and ‘woodiness’ may therefore describe various transitional forms of lignified parts within a plant, such as
those cell walls that are impregnated with lignin (Evert, 2006).
Additional complexity originates from diverse lignified proportions within one individual specimen (Lens et al., 2012). Different
degrees of plant-internal lignification, together with continuous
and/or abrupt transitions of lignified cell walls are semantically
not yet deliberated in a sufficient way. A straightforward scientific
depiction is missing.
Recent endeavors at the (traditional) interface of dendrochronology and wood anatomy analyze annual growth
increments of arctic shrubs (Myers-Smith et al., 2011), alpine herbs
(Schweingruber et al., 2006) and temperate meadows (Mitlacher
et al., 2002), for instance. These pioneering examples underline the
importance of microscopic investigations at the level of cellular
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Fig. 1. Microscopically magnified thin sections of lycopods, conifers and monocots that visualize various plant-specific lignification degrees (red). (A) Perennial herb, 40 cm
in length, lycopodiaceae without cambium, Lycopodium alpinum, shoot. A thick-walled and intensively lignified cortex surrounds a non-lignified parenchymatic tissue and
irregularly formed vascular bundles consisting of lignified tracheids. 40×. (B) Tree, conifer with cambium, Picea abies, twig. A large cortex and a small phloem surround the
intensively lignified xylem with distinct annual rings (20×). (C) Herb, grass, 10 cm in height, monocotyledonae without cambium. Trisetum distichophyllum, culm. Vascular
bundles with thin surrounding sheaths and triangular peripheral thick-walled groups of fibers (40×). (D) Tree, palm, 15 m in height, monocotyledonae without cambium.
Phoenix dactylifera, stem. Vascular bundles with lignified vessels and thick-walled unilateral sheaths of fibers between two lignified radial leaf traces (40×).
F.H. Schweingruber, U. Büntgen / Dendrochronologia 31 (2013) 187–191
189
Fig. 2. Microscopically magnified thin sections with a cambium of dicotyledonous angiosperms that visualize various plant-specific lignification degrees (red). (A) Annual
herb, 20 cm in height, dicotyledonous angiosperm with cambium. Arabidopsis thaliana, root collar. A large cortex and phloem surrounds the xylem. The peripheral xylem
belt is intensively lignified (40×). (B) Perennial herb, 3 cm in height, dicotyledonous angiosperm with cambium. Antennaria sornborgeri, shoot. The totally lignified xylem
contains 5 annual rings, surrounded by an un-lignified phloem, cortex and phellem (100×). (C) Dwarf shrub, 20 cm in length, Dicotyledonous angiosperm with cambium.
Cassiope tetragona, stem. The totally lignified xylem contains ∼45 annual rings, surrounded by an un-lignified phloem, cortex and phellem (100×). (D) Tree, 25 m in height,
dicotyledonous angiosperm with cambium. Fagus sylvatica, short shoot. The totally lignified xylem contains ca. 8 annual rings. It is surrounded by a partially lignified phloem
and cortex (100×).
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tissues, and also question the conceptual separation between dendrochronology (tree-ring chronology) and wood anatomy (wood of
trees). Wood anatomical and botanical textbooks describe ‘wood’
as the product of secondary radial thickening (Fengel and Wegener,
2003). Many authors use the term ‘xylem’ to characterize vascular
bundles, but do not distinguish between life forms of dicotyledons
(Fahn, 1990; Eschrich, 1995; Evert, 2006). Monocotyledonous
plants such as palms and bamboo as whole plant bodies, exclusively produced by the primary meristem, are generally not even
considered. Although a fundamental understanding of axial and
radial stem growth has existed for several centuries (Sanio, 1873),
many dendrochronologists and wood anatomists still exclusively
examine the radial growth of gymnosperms and dicotyledons.
Here, we seek to overcome terminological gaps between the
technical and botanical meaning of ‘wood’. We than discuss possible implications for the existing life form paradigm in ‘wood
science’, and introduce additional semantic changes at the interface
of wood anatomy and dendrochronology.
Materials, methods and results
We created permanent micro sections from thousands of terrestrial plant stems, from which we selected examples of trees,
shrubs and herbs among six taxonomic clades: lycopods horsetails, ferns, conifers, monocotyledons and dicotyledons All samples
were double stained with cellulose and lignin specific dyes (astrablue and safranin), dehydrated and embedded in Canada balsam.
Xylem, phloem and cortex structure of each sample was analyzed
under normal and polarized light.
Our selection, together with the analyses of >3000 stems
(Schweingruber et al., 2012), demonstrates that the xylem and
bark of terrestrial plants can vary from a single tissue with few
lignified cells to an almost totally lignified tissue. This universal principle accounts for all taxonomic units within vascular
plants (Figs. 1 and 2). Lignified cell walls occur in different patterns and amounts in the cortex, xylem and phloem of lycopods,
ferns, conifers, monocotyledons and dicotyledons. Lignified vessels, fibers, sclereids and parenchyma cells occur in spore plants
with tracheids (Fig. 1a), in seed plants, in taxa exclusively with primary (Fig. 1c and d) and secondary meristems (Figs. 1b and 2a–d),
in all terrestrial life forms between tiny annual (Fig. 2a) and perennial herbs (Figs. 1a and 2b), in shrubs (Fig. 2c), as well as in large
trees of various growth forms (Figs. 1d and 2d).
Comparison between the genetically important Arabidopsis
thaliana (Brassicaceae) and the economically important Fagus
sylvatica (Fagaceae) further emphasizes the universal principle
of shoot growth and stabilization in angiosperms of different
longevity and size. Lifetime of Arabidopsis is restricted to few
months, whereas Fagus can grow for several centuries. Height of
Arabidopsis and Fagus roughly reaches 40 cm and 40 m, respectively
(Fig. 2a and d). The xylem of both herbs and trees originates from
a secondary meristem (vascular cambium), which is characterized
by a majority of fibers, vessels and parenchyma cells with lignified
cell walls.
Discussion and conclusions
Our anatomical observations suggest that ‘wood’ of different
traditional life form categories is macroscopically not clearly distinguished, and microscopically not dependent on the amount
and distribution of lignified xylem, phloem and cortex elements.
Although the common terms ‘woody’ and ‘non-woody’ appear
suitable for daily use and macro-ecological description (phytosociology), they are inappropriate for any anatomical and dendrochronological classification. The term ‘non-woody’ is generally
misleading since many aquatic plants and high alpine terrestrial
plants are free of any lignified cell walls. We therefore recommend
semantically changing ‘wood anatomy’ to ‘stem anatomy’, and to
conceptually broaden its application to all life forms. This redefinition would further allow the integration of ferns and monocots.
Moreover appears plant size alone to be an inadequate criterion for
any robust life form separation. A theoretical disciplinary boundary
between dendrochronology and wood anatomy will likely rapidly
dissolve in a growing cross-disciplinary research arena.
The above argumentation implies rethinking of the existing
life form paradigm in ‘wood science’, which traditionally classified all plants concerning their size and shape, as well as their
stem-specific hibernating shoot location and degree of lignification
(Raunkiaer, 1937). Our anatomical examples, however, contradict
the prevailing life form classification and reveal a generally different texture of stems, roots and rhizomes. It appears obvious that
the conventional perception of life forms is not compatible with
the concept of secondary radial thickening. Annual herbs of 5 cm
height and 500-year old trees of 40 m height can, for instance, be
both characterized by a majority of lignified xylem cells.
The terminological differentiation between ‘wood’, ‘woody’
and ‘woodiness’ is only meaningful if anatomical knowledge
is accompanied by information on the particular plant section,
its lignified proportion and the overall life form. When relating
stem structure to plant size and shape, we advocate using height
measures as a life form surrogate (Schweingruber et al., 2012).
Trees, shrubs, perennial liana and woody chamaephytes may
be grouped as plants with intensively lignified xylem, phloem
and cortex, whereas hemicryptophyte, geophyte and therophyte
herbs, together with annual liana may be aggregated as plants
with variably lignified xylem, phloem and cortex. Nevertheless,
semi-woody chamaephytes must be allocated to one of these
clusters following their specific anatomical features.
We propose using the botanically neutral term ‘stem anatomy’
instead of ‘wood anatomy’. This semantic modification further permits the integration of xylem and bark of trees, shrubs and herbs in
all taxonomic units among conifers, monocotyledons and dicotyledons. Our simple but straightforward recommendation of a ‘stem
anatomical’ separation would further allow the unambiguous division of all terrestrial plants. Moreover, we propose a semantic
broadening of ‘dendrochronology’ toward ‘xylemchronology’ to
include both trees as traditionally understood, and also shrubs,
dwarf shrubs and perennial herbs.
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