| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
1 Department of Biological Sciences, Henson School of Science and Technology, Salisbury State University,Salisbury, Maryland 21801
Received for publication September 4, 1998. Accepted for publication March 30, 1999.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Key Words: Aralia spinosa; • Araliaceae • biomechanics • devil's walking stick • specific gravity • trunk anatomy
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
; McMahon and Kronauer, 1976
; King and Loucks, 1978
; Norberg, 1988
). Recent evidence suggests that rather than being fixed, the mechanical architecture of woody stems varies as the tree grows and ages. For example, the physical and mechanical characteristics of wood have been reported to vary from pith to bark (Rueda and Williamson, 1992
; de Castro, Williamson and de Jesus, 1993
; McDonald, Williamson, and Wiemann, 1995
; Wiemann and Williamson, 1988, 1989a, b
), between heartwood and sapwood (Niklas, 1997a, b, c
), along the length of the trunk (Rueda and Williamson, 1992
; Niklas, 1997b
), and between different growth phases (Seino, 1998
). This variation can be attributed in part to anatomical and possibly chemical changes in the stem tissues during ontogeny (McDonald, Williamson, and Wiemann, 1995
; Rowe and Speck, 1996
; Mencuccini, Grace, and Fioravanti, 1997
). Niklas (1997a)
concluded that " ... the mechanical architecture of arborescent species is highly complex and temporally dynamic."
The trunk of a self-supporting woody plant (tree/shrub) performs a number of functions, including transport, storage, and support. Mechanically, a tree trunk must support both static and unpredictable dynamic loadings. Static loadings include those imposed by leaves, branches, and reproductive structures, while the most common dynamic loading experienced by the trunk is wind pressure (Speck, Spatz, and Vogellehner, 1990
; Telewski, 1995
; Vogel, 1995
; Niklas, 1997c
).
The growth of Aralia spinosa L. can be divided into two broad stages: the trunk-building and crown-building phases. The trunk-building phase lasts until first flowering, when ramets reach an average age of 3.5 yr and a height of 260 cm. The production of the terminal inflorescence consumes the apical meristem, and the following year, on average, 2.7 branches are formed from overwintering axillary buds (White, 1984
). During the trunk-building phase, the trunk supports the static loading of the leaves and at some point the first inflorescence. Subsequently during the crown-building phase the trunk must support the mass of leaves, branches, and additional inflorescences.
The mechanical properties of the trunk can be investigated by performing bending tests, which allow the calculation of variables such as the elastic modulus (E), flexural stiffness (EI), and modulus of rupture (MR) (Niklas, 1997a, b, c
), or by determining specific gravity (Panshin and de Zeeuw, 1980
; Wiemann and Williamson, 1988, 1989a, b;
Rueda and Williamson, 1992
; de Castro, Williamson, and de Jesus, 1993
; McDonald, Williamson, and Wiemann, 1995
; Niklas, 1997c
). Specific gravity is positively correlated with both the elastic modulus and the modulus of rupture (Niklas, 1992
). The main advantage of using specific gravity is that it can be determined economically with a minimum of equipment. Our present investigation focuses on the biomechanics of the trunk of Aralia spinosa during the crown-building phase. The trunk is a composite structure composed of an outer cylinder of bark and an inner cylinder of wood surrounding a central pith. The primary objectives of this study were: (1) to determine whether specific gravity varied along the length of the trunk; (2) to determine whether the cross-sectional anatomy of the trunk varied along its length; and (3) to determine whether specific gravity was related to trunk anatomy. The implications of trunk design on crown architecture and tree size will also be explored. This is the first attempt to quantify the biomechanical characteristics of the trunk for this species and is part of a larger investigation into the biomechanics and allometry of Aralia spinosa (Briand et al., 1998
).
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
).
|
= 0.05. The validity of linear regression was determined using the experimental lack-of-fit test ( = 0.10) (Burns and Ryan, 1983
) and by examining the relationship between the standardized residuals and the fitted values. Statistical analyses were performed using Minitab (Minitab, 1996
). |
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
|
|
|
Trunk anatomy
Percent bark varied among the ramets sampled (P < 0.001; one-way ANOVA), ranging from a low of 15.2% to a high of 28.3% (mean ± 95% CI: 19.7 ± 1.9%). There was no relationship between % bark and either basal diameter or age (Table 2). Bark area declined along the length of the trunk for all trunks sampled, while 60% of the trunks exhibited a significant decrease in % bark (Table 1). Position explained, on average, 69% of the variation in area, and 35% of the variation in % bark. At POS1, 23.8 ± 3.2% of of the trunk was composed of bark; decreasing to 15.8 ± 2.2% at POS11 (Fig. 4).
|
Percent pith varied among the ramets sampled (P = 0.002; Kruskal-Wallis test), ranging from a low of 3.7% to a high of 15.2% (mean ± 95% CI: 10.0 ± 2.2%). Percent pith declined with increasing basal diameter and age (Table 2). Pith area as well as % pith increased acropetally along the length of the trunk, for all of the trunks sampled (Table 1). At POS1 1.5 ± 0.5% of the trunk was composed of pith. This increased to 20.1 ± 5.2% at POS11 (Fig. 4). Position explained, on average, 69% of the variation in pith area, and 81% of the variation in % pith.
Specific gravity and trunk anatomy
Sixty-seven percent of the trunks sampled exhibited positive correlations between SG and % wood, while 80% of the trunks exhibited significant negative correlations between SG and % pith (Table 3). On average, 47% of the variation in SG could be attributed to % wood and 77% to % pith. There was little association between SG and % bark, with only 20% of the trunks exhibiting a significant association between these two variables (Table 3). Percent bark accounted for, on average, only 14% of the variation in SG.
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
; Niklas, 1992
). During the crown-building phase in Aralia spinosa the stiffness of the basal region of the trunk is predicted to increase with ramet age, and on the basis of trunk taper, to decrease acropetally, i.e., stiffer at the base and less stiff at the top. Taper was not invariant between ramets but was size and age dependent, with larger and older ramets exhibiting a greater degree of taper. In other words, the base of the trunk was relatively stiffer in relation to more acropetal regions in older larger trunks than in younger smaller trunks. A tapered trunk is thought to be mechanically advantageous for two reasons: (1) maximum stress occurs at the base of the trunk during bending (dynamic loading) where diameter is greatest, and (2) distal portions can deflect without breaking (Mosbrugger, 1990
; Speck, Spatz, and Vogellehner, 1990
; Niklas, 1997a
). Also, a tapered trunk is more economical to construct than a nontapered trunk with the same basal diameter (Niklas, 1997d
). Briand et al. (1998)
reported that during the trunk-building phase, only about half of the sampled trunks exhibited continuous taper along the length of the trunk. Taper was also unrelated to ramet size. This may represent real adaptive differences between trunks during different ontogenetic stages. Speck, Spatz, and Vogellehner (1990)
suggested that taper may be less important for smaller individuals that occupy a region of relatively low wind velocity than for larger individuals that occupy a region of relatively high wind velocity (wind velocity increases with distance from the ground).
Specific gravity
Specific gravity can be used as an indicator of the mechanical properties of wood, as SG is positively correlated with both the elastic modulus (E) and the modulus of rupture (MR) (Niklas, 1992
). Thus as SG increases, wood generally becomes stiffer and less prone to fracture. Overall trunk SG in Aralia spinosa was both size and age dependent; SG increased with increasing ramet size and age. Thus as Aralia spinosa becomes older and larger, the trunk becomes stiffer, more able to handle increased static and dynamic loadings resulting from increased crown size: the number of live branches in the crown of Aralia spinosa increases with ramet age (White, 1984
). Specific gravity varied along the length of the trunk, with 73% of the ramets exhibiting continuous acropetal decline in SG. Thus, irrespective of diameter, younger more acropetal portions of the trunk are predicted to be less stiff than older more basipetal portions. Wood SG was generally unrelated to ramet basal diameter, age, and position along the length of the trunk. Wiemann and Williamson (1989a) have reported that SG in a single large (dbh = 23 cm) specimen of Aralia spinosa exhibited a 10% decline from pith to bark. In light of our data, further investigation of radial variation in wood SG in this species is warranted.
Anatomy
The primary stiffening tissue in the trunks of most arborescent plants is secondary xylem or wood. In the majority of angiosperms and gymnosperms the trunk is composed of a rigid core of wood, surrounding a small central pith, the conifer type. In some species, however, a cylinder of wood surrounds a much larger central pith, the Calamites type (Mosbrugger, 1990, 1991
; Niklas, 1997a
). Rather than fitting one of these models of trunk construction, the trunk of Aralia spinosa exhibits a gradient from one model to the other. At the base of the trunk of Aralia spinosa, 75% of the cross-sectional area is composed of wood and only 2% is pith, while just below the crown, the amount of wood decreases to 64% and pith increases to 20% (Fig. 4). In other words, the percentage of the cross-sectional area composed of wood decreased, while the percentage of pith increased acropetally. The percentage of the cross-sectional area composed of bark declined in only 60% of the ramets. As wood is the stiffest material in the trunk, stiffness would be highest at the base and lowest in the distal region. This situation is mechanically advantageous, because as indicated previously the majority of stress in the trunk during dynamic loading is at the base.
The main advantage of a stem with a large central pith is that it requires less energy to construct than a solid trunk, allowing resources to be diverted into extension growth. Aralia spinosa is an early-successional species, which exhibits rapid stem elongation (White, 1984
; Sullivan, 1992
). Rapid stem elongation may allow this species to quickly erect a crown and shade out competitors. The positioning of wood in an outer cylinder also provides the best mechanical support when the quantity of stiffening tissue is limited (Niklas, 1997d
). The central pith may also provide some mechanical support through turgor pressure (hydrostatic skeleton), thus helping to prevent local buckling (Mosbrugger, 1990
; Niklas, 1992
).
The anatomy of the trunk may, however, impose severe restrictions on the growth habit of this species. Trunks that have a relatively large pith or are hollow are predicted to greatly constrain the growth of the crown that they support (Mosbrugger, 1990, 1991
). The stability of lateral branches is compromised, as they are supported by only a relatively thin cylinder of wood. Thus, in order to reduce the risk of branch failure and buckling of the trunk, the number of branches and branch angles must be minimized. The resulting small crown further constrains height by reducing photosynthate production. The growth habit of Aralia spinosa is consistent with Mosbrugger's predictions. Figure 1 depicts the sparse branching and relatively small branch angles described by White (1984)
. In fact, this species has fewer branches per unit of leaf area than all other species in White's (1983)
study of the eastern deciduous forest. The large bipinnately compound leaves (White, 1983, 1984
; Briand et al., 1998
) may, however, provide some compensation for the lack of branch production. Height growth also appears to be compromised, with maxima reported only between 7 and 10 m (Sullivan, 1992
). Stevens and Perkins (1992)
reported that trees like Aralia spinosa with relatively massive branches (large diameter) tend to be smaller, on average, than species with relatively small branches. They concluded that "crown architecture constrains or is constrained by plant life history." We propose that crown architecture, at least in Aralia spinosa, is constrained by the mechanical architecture of the trunk.
Anatomy and specific gravity
Trunk SG is positively correlated with the percentage of wood and negatively correlated with the percentage of pith. Changes in SG along the length of the stem and between ramets of differing size and age can be largely explained by changes in the ratio of wood to pith. As indicated previously, wood SG exhibited little longitudinal variation.
Conclusions
Our data support Niklas' (1997a)
contention that " ... the mechanical architecture of arborescent species is highly complex and temporally dynamic." During the crown-building phase the trunk of Aralia spinosa is constructed to maximize stiffness at the base where maximum stress occurs during dynamic loading and to bend without breaking in more distal regions. This is achieved by constructing a tapered trunk, whose anatomy varies such that the percentage of the trunk composed of wood increases, while the percentage of pith decreases basipetally. Consequently, stiffness as measured by SG increases basipetally and decreases acropetally. The internal architecture of the trunk (large pith surrounded by a cylinder of wood), especially in more distal regions, is economical and allows for resources to be diverted to extension growth, an important feature for a pioneer. This design, however, imposes severe constraints on crown size and the maximum size attainable by this species.
|
|
FOOTNOTES |
|---|
2 Author for correspondence (chbriand{at}ssu.edu
).
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Burns, D. A., and T. A. Ryan Jr. 1983 A diagnostic test for lack of fit in regression models. American Statistical Association, Statistical Computing Section, 1983, Proceedings of the Statistical Computing Section, American Statistical Association, Washington, DC. pp. 286–290.
de Castro, F., G. B. Williamson, and R. M. de Jesus. 1993 Radial variation in the wood specific gravity of Joannesia princeps: the roles of age and diameter. Biotropica 25: 176–182. [CrossRef][ISI]
Haygreen, J. G., and J. L. Bowyer. 1982 Forest products and wood science. The Iowa State University Press, Ames, IA.
King, D. A., and O. L. Loucks. 1978 The theory of tree bole and branch form. Radiation and Environmental Biophysics 15: 141–165. [CrossRef][ISI][Medline]
McDonald, S. S., G. B. Williamson, and M. C. Wiemann. 1995 Wood specific gravity and anatomy in Heliocarpus appendiculatus (Tiliaceae). American Journal of Botany 82: 855–861. [CrossRef][ISI]
Mattheck, C. 1991 Trees: the mechanical design. Springer-Verlag, Berlin.
McMahon, T. A. 1973 Size and shape in biology. Science 179: 1201–1204.
———. 1975 The mechanical design of trees. Scientific American 233: 92–102. [ISI][Medline]
———, and R. E. Kronauer. 1976 Tree structures: deducing the principle of mechanical design. Journal of Theoretical Biology 59: 443–466. [CrossRef][ISI][Medline]
Mencuccini, M., J. Grace, and M. Fioravanti. 1997 Biomechanical and hydraulic determinants of tree structure in Scots pine: anatomical characteristics. Tree Physiology 17: 105–113.
Minitab. 1996 Minitab reference manual, release 11. Minitab Inc., State College, PA.
Mosbrugger, V. 1990 The tree habit in land plants. Springer-Verlag, Berlin.
———. 1991 Trunk construction in extinct and living trees. A functional analysis. In C. Edelin [ed.], L'Arbre, biologie et développement. Naturalia Monspeliensia Numéro hors série A7.
Niklas, K. J. 1992 Plant biomechanics: an engineering approach to plant form and function. University of Chicago Press, Chicago, IL.
———. 1997a Mechanical properties of black locust (Robinia pseudoacacia L.) wood: size- and age-dependent variations in sap and heartwood. Annals of Botany 79: 265–272.
———. 1997b Size- and age-dependent variations in the properties of sap- and heartwood in black locust (Robinia pseudoacacia L.). Annals of Botany 79: 473–478. [CrossRef]
———. 1997c Mechanical properties of black locust (Robinia pseudoacacia) wood: correlations among elastic and rupture moduli, proportional limit, and tissue density and specific gravity. Annals of Botany 79: 479–485. [CrossRef]
———. 1997d The evolutionary biology of plants. University of Chicago Press, Chicago, IL.
Norberg, R. A. 1988 Theory of growth geometry of plants and self-thinning plant populations: geometric similarity, elastic similarity, and different modes of plant parts. American Naturalist 131: 220–256. [CrossRef][ISI]
Panshin, A. J., and C. de Zeeuw. 1980 Textbook of wood technology, 4th ed. McGraw-Hill, New York, NY.
Rowe, N. P., and T. Speck. 1996 Biomechanical characteristics of the ontogeny and growth habit of the tropical liana Condylocarpon guianense (Apocynaceae). International Journal of Plant Science 157: 406–417. [CrossRef][ISI]
Rueda, R., and G. B. Williamson. 1992 Radial and vertical wood specific gravity in Ochroma pyramidale (Cav. ex Lam.) Urb. (Bombaceae). Biotropica 24: 512–518. [CrossRef][ISI]
Seino, T. 1998 Intermittent shoot growth in saplings of Acanthopanax sciadophylloides (Araliaceae). Annals of Botany 81: 535–543.
Speck, T., H.-C. Spatz, and D. Vogellehner. 1990 Contributions to the biomechanics of plants. I. Stabilities of plant stems with strengthening elements of different cross-sections against weight and wind forces. Botanica Acta 103: 111–122. [ISI]
Stevens, G. C., and A. L. Perkins. 1992 The branching habits and life history of woody plants. American Naturalist 139: 267–275. [CrossRef][ISI]
Sullivan, J. 1992 Aralia spinosa. In W. C. Fisher [compiler], The Fire Effects Information System [Data base]. U.S. Department of Agriculture, Forest Service, Intermountain Research Station, Intermountain Fire Sciences Laboratory, Missoula, MT.
Telewski, F. W. 1995 Wind-induced physiological and developmental responses in trees. In M. P. Couts and J. Grace [eds.], Wind and trees, 237–263. Cambridge University Press, Cambridge.
Vogel, S. 1995 Twist-to-bend ratios of woody structures. Journal of Experimental Botany 46: 981–985.
White, P. S. 1983 Corner's rules in eastern deciduous trees: allometry and its implications for the adaptive architecture of trees. Bulletin of the Torrey Botanical Club 110: 203–212. [CrossRef][ISI]
———. 1984 The architecture of devil's walking stick, Aralia spinosa (Araliaceae). Journal of the Arnold Arboretum 65: 403–418. [ISI]
Wiemann, M. C., and G. B. Williamson. 1988 Extreme radial changes in wood specific gravity in some tropical pioneers. Wood and Fiber Science 20: 344–349. [ISI]
———, and ———. 1989a Radial gradients in the specific gravity of wood in some tropical and temperate trees. Forest Science 35: 197–210. [ISI]
———, and ———. 1989b Wood specific gravity gradients in tropical dry and montane rain forest trees. American Journal of Botany 76: 924–928. [CrossRef][ISI]
This article has been cited by other articles:
|
|
 |
|
  G. Jaouen, T. Almeras, C. Coutand, and M. Fournier How to determine sapling buckling risk with only a few measurements Am. J. Botany, October 1, 2007; 94(10): 1583 - 1593. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. A. Rosell and M. E. Olson Testing implicit assumptions regarding the age vs. size dependence of stem biomechanics using Pittocaulon (Senecio) praecox (Asteraceae) Am. J. Botany, February 1, 2007; 94(2): 161 - 172. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
