The Tree Habit in Land Plants A Functional Comparison of Trunk Constructions with a Brief Introduction into the Biomechanics of Trees

165 Pages·1990·3.649 MB·English

by  Dr. Volker Mosbrugger (auth.)

Lecture Notes ni
Earth Sciences
Edited by Somdev Bhattacharji, Gerald M. Friedman,
Horst .J Neugebauer and Adolf Seilacher
28
rekloV Mosbrugger
The Tree Habit ni Land stnalP
A lanoitcnuF Comparison of knurT Constructions
with a Brief Introduction into eht scinahcemoiB fo seerT
galreV-regnirpS
Berlin Heidelberg NewYork London ParisTokyo Hong Kong
Author
Dr. Volker Mosbrugger
Institute of Paleontology, University of Bonn
Nussallee 8, D-5300 Bonn ,1 FRG
ISBN 3-540-52374-X Sprmger-Verlag Berlin Heidelberg New York
ISBN 0-387-52374-X Sprlnger-Verlag NewYork Berlin Hetdelberg
This work is subject to copyright rights All are reserved, whether the owrh ole part of the material
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Loafw September 9, version its in 1965, of June 24, a and 1985, copynght fee always must be
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I suggest that as a more general principle natural selection is
continually trying to economize every part of the organization.
Charles Darwin
You now see how, from the things demonstrated thus far, there clearly
follows the impossiblllty (not only for art, but for nature herself)
of increasing machines to immense size. Thus it is imposslble to bulld
enormous ships, palaces or temples ...; nor could nature make trees of
immeasurable size because their branches would eventually fail of
their own weight.
Galileo Galilei
Contents
Page
1 Introduction .................................................. 1
2 Historical Remarks ............................................ 4
3 Trees as Living Systems ....................................... 7
3.1 Why Do Trees Exist? ........................................... 7
3.2 Systemic Conditions .......................................... 14
3.3 Biomechanical Aspects of the Tree Habit ...................... 19
3.3.1 Mechanical Properties of Wood ................................ 19
3.3.2 Trees as Free-Standing Columns ............................... 23
3.3.3 Trees as Cantilevers ......................................... 30
3.3.4 The Form of Branches and Tree Boles .......................... 36
3.3.5 Cracks and Fracture Energy ................................... 42
3.3.6 Prestressing ................................................. 45
3.3.7 Stability and Flexibility Strategists ........................ 46
4 Constructional Principles of Tree Trunks ..................... 49
4.1 Constructional Principle i: Supporting Tissue
Forming a Woody or Sclerenchymatic Cylinder .................. 49
4.1.1 Constructional Principle la: the Conifer Type ................ 50
4.1,2 Constructional Principle ib: the Calamites Type .............. 55
4.1.3 Constructional Principle ic: the Lepidodendron Type .......... 61
4.2 Constructional Principle 2: Support Provided
by Isolated Strengthening Elements ........................... 74
4.2.1 Constructional Principle 2a: the Tree-Fern Typ ............... 74
4.2.2 Constructional Principle 2b: the Medullosa Type .............. 85
4.2.3 Constructional Principle 2c: the Palm Type ................... 93
4.2.4 Constructional Principle 2d: the Cacti Type ................. 109
4.3 Constructional Principle 3: the Musa Type ................... 114
5 General Discussion .......................................... 119
5.1 Strengthening System in Tree Trunks ......................... 119
5.2 Trunk Design and Growth Habit ............................... 122
5.3 f- and s-Strategy ........................................... 126
5.4 General Aspects ............................................. 129
6 Summary ..................................................... 134
Acknowledgements .................................................. 137
Appendix .......................................................... 138
References ........................................................ 139
Index ............................................................. 158
1 Introduction
In botany, functional analysis traditionally dates from the late
19th century. Well-known milestones in its history are Simon SCHWENDE-
NER's classical dissertation "Das mechanische Prinzip im anatomi-
schen Bau der Monokotylen" (1874) and the textbook, "Physiologische
Pflanzenanatomie" (1884) of Gottlieb HABERLANDT, a former student of
SCHWENDENER. In contrast to HABERLANDT's rather physiological
studies, SCHWENDENER's biomechanical approach has experienced a fairly
changeable fate. In Germany in particular, the country of its origin,
it has generally found relatively little attention during the last 50
years. In a few fields of research, however, plant biomechanics have
always been of some importance. This is true for pollination and
propagation biology and particularly for the research on the
secondary wood of gymnospermous and dicotyledonous trees. Wood has
always been one of the most important structural materials and
therefore is, structurally and functionally, the best-known plant
tissue. As a consequence of this interest in wood, the specific
functional problems of the tree habit (transport of water, stability
etc.) are also well studied. Much information about tree architecture
is due to the recent works of F. HALLE, R.A.A. OLDEMAN, P.B.
TOMLINSON and M.H. ZIMMERMANN, but the functional interpretation of
the different architectural tree models is still in its formative
stages (see FISHER 1984 for a review).
Relatively little attention, however, has been paid to the fact
that the tree habit has been developed independently and with
different trunk constructions in several groups of plants. SCHWENDE-
NER and his students have applied "the mechanical principle" to some
exotic tree types (especially in monocotyledons and ferns) but their
results appear incomplete or even incorrect by today's knowledge.
More recently, too, some alien tree forms, like bamboos and palms,
have been investigated but again, primarily from a technical and
economical viewpoint. The fossil trees, however, have found nearly no
attention at all. (See Chap. 2 for references).
The present study will help to close these remaining gaps. It will
provide an introduction into the biomechanics of trees and will give a
critical survey of the phylogeny and the constructional principles of
the tree habit. Since the trunk is considered the basic and crucial
element of a tree, the analysis is largely restricted to a functional
comparison of the stem anatomy of the various tree forms. It is based
on the concept of constructional morphology (see SEILACHER 1970, REIF
et al. 1985), thus considering simultaneously the functional aspect
and the ontogenetical and phylogenetical development of the various
trunk types. The main questions to be answered in this study are; Why
do trees exist? - Which are the constructional principles of tree
trunks and when and in which group of plants do they appear? - How
important are internal (phylogenetic) and external (functional,
constructional) constraints? - What are the specific properties of
the different constructional principles and does a correlation between
trunk design and growth habit exist? - Is there a tendency in
phylogeny to a better performance? (This latter question was addressed
for the first time by H. POTONI~ 1901 and led to a vigorous dis-
cussion; cf. WESTERMAIER 1902, POTONI~ 1902). The study does not (and
cannot) intend to provide a detailed biophysical analysis of individ-
ual cases because experimental data on the mechanical properties of
the structural elements of the different kinds of trees are still
lacking. Instead, it will he the task to evaluate in a comprehensive
and qualitative or semi-quantitative manner the available data of the
morphology, anatomy and phylogeny of fossil and recent trees by using
concepts of biomechanics and constructional morphology. Thus a
somewhat holistic approach is used, which is becoming increasingly
more acceptable today (cf. MAYR 1983: 66).
As a whole, this study aims at understanding the basic principles
of trunk design in arborescent land plants. For this purpose generali-
zations are necessary which, however, should not be expected to be
valid in every particular case. The systems considered here (i.e.
trees) are so complex that all relevant parameters can never be taken
into consideration and hence no strict laws exist, but only rules with
many exceptions. Even though the author is fully aware that numerous
limitations and exceptions exist to the generalizations made in this
study, they are not discussed in length because this would disrupt the
chain of reasoning.
To further delimit the scope of the study, it is necessary to make
a brief remark on the use of the term "tree". Intuitively it appears
to be rather clear what a "tree" is. Yet it is virtually impossible
to give a precise and meaningful definition because there exist all
intermediate states between "non-trees" (herbs, shrubs) and "trees".
Indeed, foresters use very rigid definitions. However, from a
biological viewpoint, they are all rather arbitrary and cannot serve
the purpose of this present study, which expressively encompasses the
whole spectrum of "trees", from giant herbs (like bananas) to "false
trees" (like some fossil ferns). Here plants are considered "tree-
like", if they reach a certain height (of about 2 - 3 m, at least)
and follow an acrotonic growth, resulting in the formation of crowns
and one or several true or false trunks.
Finally, it must be added that it would be impossible to take all
slightly differing types of trees into consideration. It is intended,
however, to make a selection in such a way that all basic construc-
tional principles are sufficiently documented. Thereby main emphasis
is laid on fossil forms, but gigantic aquatic plants like Prototaxites
(cf. SCHWEITZER 1983) are ignored although some authors consider
Prototaxites to be a small- to medium-sized land plant.
2 Historical Remarks
Apparently, SCHWENDENER (1874) was the first to consider plant
structure consistently from an engineer's viewpoint. In its time,
SCHWENDENER's dissertation, "Das mechanische Prinzip im anatomischen
Bau der Monokotylen" was supposed to be fairly complete, so that
POTONI~ (1882: 173) wrote, "SCHWENDENER hat die Sache in so eingehen-
der Weise hehandelt, dass wesentlich Neues kaum hinzugef~gt werden
kann". Indeed, SCHWENDENER's students did not really expand his
biomechanical concept (cf. RASDORSKY 1911), but rather applied it to
different organs and groups of plants. Only the papers of GREENHILL
(1881) and METZGER (1893) are outstanding in this period. The latter
postulated a constant stress hypothesis which predicted that tree
boles taper in such a way that the maximum stress, due to bending,
remains constant along the trunk; GREENHILL could show that the
theoretical maximum height of a tree depended on the stem diameter at
the base, the Young's modulus and the specific weight of the trunk.
It was Wladimir RASDORSKY who pointed out in several papers (1911,
1926, 1928, 1930) the specific misconceptions in SCHWENDENER's
biomechanical theory. SCHWENDENER assumed that all plants tend to be
as stiff as possible and thus overlooked that a high flexibility may
also be of some advantage. Furthermore, his theory is based on the
concept that plants are supported by more or less isolated beams,
roughly comparable to the truss system in railway bridges, whereas
real plant axes consist of composite material. Likewise, no special
attention has been paid to the fact that an isometric change of the
abso]ute dimensions of a structure will also affect its mechanical
behaviour (e.g. bending under its own weight, safety factor against
global buckling etc.). This neglect of the "principle of similitude"
(THOMPSON ]917; cf. Chap. 3.3.2) is particularly evident in popular
books comparing plant and technical structures (e.g. FRANC~ 1907,
1919). A classical example, which has entered botanical textbooks,
compares the slenderness (length/diameter) of a grass stem and a 140-
m-high chimney to demonstrate the supremacy of natural over technical
constructions (e.g. NEGER 1913). It must be emphasized that RASDORSKY
not only stressed these different weaknesses of SCHWENDENER's theory
but he also gave quite a comprehensive survey of plant biomechanics
which is still worth reading.
Subsequent research on the biomechanics of trees was done
primarily by the applied sciences. Wood technology became established
as a science and special textbooks were published (e.g. KOLLMANN 1936,
BROWN, PANSHIN & FORSAITH 1949). Investigations, however, remained
largely restricted to economically important timber, although purely
biological problems have been analyzed, too. For instance~ RASHEVSKY
(1943aj b), OPATOWSKI (1944a, b, 1945) and ESSER (1946a, b), con-
tinuing the concept of METZGER (1893), tried to find a biomechanical
explanation for the form of branches, tree boles and whole trees,
whereby RASHEVSKY also considered physiological factors. Another
important field of research was the structure and ultrastructure of
wood, as a key to the understanding of its physical and mechanical
properties (COT~ 1965, MARK 1967).
Due to these and other more recent works (which will later be
dealt with in more detail), we now have an ample, but by no means
complete, knowledge of the function and biomechanics of gymnospermous
and dicotyledonous trees. On the other hand, many questions still
remain unanswered since the technologically oriented wood science did
not stimulate the functional analysis and interpretation of unusual
and economically unimportant types of trees. Of course quite a number
of anatomical studies exist on arborescent plants other than gymno-
sperms and dicotyledons (for example, SCHOUTE 1903, MONOYER 1925 on
palms, TAKENOUCHI 1931 on bamboos), but these are typically compara-
tive and descriptive investigations, as were customary in the days
before SCHWENDENER, de BARY (1877) being the last and most prominent
representative of that early period. More recently, the physical and
mechanical properties of some alien tree forms have also been investi-
gated (for instance, ATROPS 1969, JANSSEN 1981, KILLMANN 1983, RICH
1987) but elementary data a?e still lacking for quite a number of
tree-like plants such as cycads, tree ferns, bananas etc. Even less
information is available concerning fossil trees and until about 1980,
only SCHWENDENER (1874) and POTONI~ (1901) briefly discussed the
mechanical design of some palaeozoic plants.
During the last I0 to 15 years, however~ constructional morphology
and biomechanics have attracted increasing interest, first in zoology
(e.g. the textbooks of WAINWRIGHT et al. 1976, VOGEL 1981 and VINCENT
1982) and somewhat later in botany and palaeobotany. For instance,
NIKLAS with various co-authors (e.g. NIKLAS 1982, NIKLAS & O'ROURKE
1982, NIKLAS & KERCHNER 1984) has concentrated on the biophysical
aspects of branching morphologies and plant shape, in particular in
early land plants; and in several papers SPECK (1986, unpubl. Thesis)
and SPECK & VOGELLEHNER (1988a, h, e, 1989, in press) have analyzed
and compared the bending stability of the basic stele types, including
the tree and liana strategy but with the main emphasis on the
evolution of the upright axes of early vascular plants. There also
exists the Sonderforschungsbereich 'Nat~rliche Konstruktionen'
(Universities of Stuttgart and T~bingen, FRG) stimulating and
supporting research projects on biomechanics and constructional
morphology of plants (e.g. BLUM & FOBO 1985, FOBO 1986, NACHTIGALL et
al. 1986, WESSOLY 1988, this study). It is hoped that this process
will continue and that the present study will have such a stimulating
effect.