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The land plant cover in the Devonian: a reassessment of the
evolution of the tree habit
B. MEYER-BERTHAUD
1,2
*, A. SORIA
1
& A.-L. DECOMBEIX
3
1
UM2, UMR AMAP (botAnique et bioinforMatique de l'Architecture des Plantes),
c/o CIRAD, TA-A51/PS2, Boulevard de la Lironde, 34398 Montpellier cedex 5, France
2
CNRS, UMR AMAP, F-34398 Montpellier
3
Department of Ecology and Evolutionary Biology, University of Kansas,
1200 Sunnyside Avenue, Haworth Hall, Lawrence, Kansas 66045-7534, US
*Corresponding author (e-mail: meyerberthaud@cirad.fr)
Abstract: This paper reviews information on the Devonian trees that evolved in the euphyllophyte
clade with special focus on the Middle Devonian Pseudosporochnales. The morphology of
pseudosporochnalean trees shows analogies with that of extant tree ferns, including the possession
of an adventitious root system of limited extent at the base of the trunk. Direct evidence on how
these trees were constructed is scarce. We propose a growth model integrating information from
younger representatives of the same class known to reach large diameters. According to this
model, trunk width in its aerial part results from the large size of its primary body where living
tissues are abundant, a condition reached early during growth. Secondary xylem contributes
little to trunk diameter. This model sharply diverges from that of the Late Devonian archae-
opteridalean trees characterized by an extended root system and where trunk diameter and mech-
anical support are achieved by the substantial development of secondary vascular tissues. These
differences suggest that pseudosporochnalean trees may have had a lesser impact on Devonian
environments than the Archaeopteridales. The important investment in living tissues in the
Pseudosporochnales probably made them vulnerable to drought and cold.
The diversification of early terrestrial plants has
been compared to the Cambrian explosion for
marine faunas in terms of intensity and importance
for shaping modern ecosystems (Kenrick & Crane
1997a; Bateman et al. 1998). It is characterized
by an intense phase of morphological innovation
during the Devonian that resulted in the evolution
of a large variety of growth forms. Several unrelated
taxa adopted the tree habit, a form characterized by
its extended lifetime and the possession of a tall,
vertical trunk (Barthelemy & Caraglio 2007). This
evolution was adaptive and provided large-sized
plants with functional advantages over smaller
ones in terms of reproduction and light interception
(Niklas 1997). From a biophysical point of view,
this increase in stature was a challenge as it created
tremendous new constraints in terms of biomechani-
cal support and water transport. The various groups
that evolved trees resolved these problems by
adopting specific strategies in relation to their own
evolutionary history and inherited traits (Donoghue
2005) (Fig. 1a).
The 'Devonian plant hypothesis' developed by
Algeo and his collaborators in the past 15 years
postulates a significant role of the vegetation in
the series of biogeochemical events occurring in
the Late Devonian (major biotic crises, extensive
deposition of black shale horizons, atmospheric
CO
2
level drop and global climate cooling) (Algeo
et al. 2001). These authors hypothesize that repro-
duction by seeds increased the geographical
distribution of plants by enabling the critical fertili-
zation phase to take place in most habitats including
dry ones. With the increase in maximal size of the
plants documented in the Devonian, and the evol-
ution of the tree habit, they postulate the formation
of thicker litters and the development of larger
root systems that penetrated the substrates more
extensively. This resulted in the extensive formation
of deeper and more mature palaeosols, a process that
uses atmospheric CO
2
.
The explicit network of feedbacks involving
plant traits, CO
2
and climate proposed by Beerling
& Berner (2005) is not specific to the Devonian.
It includes another factor thought to accelerate
the drawdown of atmospheric CO
2
, the amount of
organic carbon buried in the sediments such as that
derived from lignous plants. Lignin is a polymer of
phenylpropane that imparts rigidity to cell walls
and is found in plant tissues such as xylem and
sclerenchyma (Beck 2005). It is one of the most
resistant plant components to biodegradation and
