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Ecology |
2Department of Botany, University of Toronto, Toronto, Ontario, Canada, M5S 3B2; 3Faculty of Forestry, University of Toronto, Toronto, Ontario, Canada, M5S 3B2
Received for publication November 6, 2001. Accepted for publication May 16, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Abutilon theophrasti • biomass allocation • biomechanical stability • drag • flexural stiffness • lateral shade • Malvaceae • stem allometry • thigmomorphogenesis • wind
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Weiner and Thomas, 1992
; Niklas, 1995
; Henry and Aarssen, 1999
). Neighbor effects can be attributed in large part to the effects of shade, both in terms of light quality (low red : far red ratios) (Ballaré, 1994
; Smith and Whitelam, 1997
) and quantity (low quantum flux density) (Mitchell and Woodward, 1988
). Plants grown in the presence of neighbors typically have low root : shoot ratios, slender stems, and strong apical dominance with low branching intensity relative to individually grown plants, traits that favor stem height growth, allowing for the overtopping of neighboring branches (Smith, 1982
, 1994
; Weiner, Berntson and Thomas, 1990
). Stem height growth represents a cost to resource acquisition through reductions in either root or leaf allocation (Henry and Aarssen, 1997
).
From a biomechanical perspective, stem diameter, stem stiffness, and root anchoring must be sufficiently high relative to stem height to ensure the mechanical stability of plants against stem buckling or uprooting (see review by Niklas, 1998
). Under field conditions, plants must be strong enough to resist stem buckling and uprooting under additional loadings such as wind and precipitation (King and Loucks, 1978
; Sterck and Bongers, 1998
). In addition to reducing light levels and altering light quality, neighbors may also promote altered allocation patterns and plant architecture as a consequence of the reduction in wind exposure (King, 1986
; Holbrook and Putz, 1989
). Mechanical stimuli such as wind exposure tend to result in increased stem diameter growth relative to height growth (Biro et al., 1980
; Latimer, Pappas, and Mitchell, 1986
; Telewski, 1990
; Jaffe and Forbes, 1993
) and high root : shoot ratios (Crook and Ennos, 1994
; Goodman and Ennos, 1996
), both traits that increase the mechanical strength of plants against stem failure or uprooting (Niklas, 1998
). Wind exposure generally increases with plant height due to increasing leaf ("sail") area and also reduced wind-buffering effects by neighbors. Thus, as plants grow taller relative to their neighbors, the effects of increased wind exposure and decreased shade on plant allocation patterns and architecture are confounded (Holbrook and Putz, 1989
; Sterck and Bongers, 1998
).
In dense stands plant stems can approach or even exceed their critical buckling heights predicted by biomechanical models (King, 1986
; Holbrook and Putz, 1989
; Thomas and Weiner, 1989
; Sterck and Bongers, 1998
), which suggests that shade responses may be favored at the expense of mechanical stability. However, studies of the effects of shade and wind on plant growth have largely remained separate, due in part to the difficulties of applying shade and wind treatments in a factorial manner. Few studies, with the exception of Ashby et al. (1979)
, Heuchert and Mitchell (1983)
, Pappas and Mitchell (1985)
, Holbrook and Putz (1989)
, and Jones et al. (1990)
, have attempted to quantify the relative effects of shade and mechanical stimulation and the extent to which they interact. Of these studies, only Holbrook and Putz (1989)
tested mechanical strength of stems in relation to stem allometry and none has quantified the contributions of root anchoring and leaf area to mechanical stability. Thus, interactions between shade and air movement on whole-plant allocation patterns and in relation to mechanical stability remain untested.
In this study, we examined the relative effects of lateral shade and wind on whole-plant allocation patterns and aboveground architecture in a glasshouse experiment using the herbaceous annual Abutilon theophrasti. Stem allometry and allocation patterns among stem, leaves, and roots were assessed in the context of mechanical stability, as determined by stem flexure of clamped and rooted stems. Flexural stiffness data were combined with measures of leaf area to estimate the contributions of mechanical stability and drag to stem deflection. We specifically sought to quantify the relative effects of lateral shading by neighboring conspecifics and wind exposure and to examine the nature of the interaction between lateral shading and wind on plant morphometric and biomechanical responses.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Seeds of A. theophrasti (F and J Seed Service, Champaign, Illinois, USA) were sown in 4 x 20 cm cylindrical tree planting tubes containing a sandy-loam soil on 20 February 2000 (three seeds per tube) and thinned to a density of one seedling per tube following germination to obtain individuals of uniform size and vigor. In a glasshouse, individuals were assigned randomly to a combination of wind and shade treatments in a 2 x 2 factorial design (12 replicates per treatment combination). Lateral shading was imposed by surrounding target plants with eight conspecific neighbors from the same cohort at a density equivalent to 625 individuals/m2. Because plants were grown in individual tubes, we restricted interactions to aboveground plant parts. Control (individually grown) plants were grown in the absence of neighbors, at a distance of 35 cm from the nearest experimental replicate.
Wind treatments used electric fans that provided a turbulent air velocity of 
10 m/s. Plants were always orientated in the same direction relative to the wind source. Treatments commenced 12 d after germination to avoid damage to fragile seedlings and were thereafter applied for 1 h every second day. Treatments were administered on a separate bench to avoid disturbing control (no wind) plants and were applied after sundown when photosynthetic activity was low. Both target plants and their neighbors were wind-treated; however, target plants were not surrounded by neighbors during wind treatments to avoid wind buffering effects and mechanical bracing between plants. For all individuals, soil was kept moist by immersing the planting tubes in trays of water. When necessary, top watering was applied directly to the soil surface (using a wash bottle to avoid thigmic stimulation of stems). A top-dressing of slow-release fertilizer (Osmokote 10:10:10) was applied at a rate of 450 kg N·ha–1·yr–1 10 d post-germination to minimize nutrient limitation. The experiment was conducted from late February until late April in Toronto, Ontario, Canada (43°41' N, 79°38' W). Due to the low peak light intensity and short daylength at this time of the year relative to peak summer values, natural sunlight with a maximum photon flux densities ranging from 1100 to 1500 µmol photons·m–2·s–1 over the course of the experiment, was supplemented with 16 h/d of artificial light using sodium halide lamps that provided an additional approximately 400 µmol photons·m–2·s–1 at bench height.
Stem height and stem diameter at 1.5 cm were measured at 20-d intervals, including the final harvest at 60 d post-germination. At final harvest, stem diameter measurements were also taken at the base and one-quarter, one-half, and three-quarters the height of the stem to assess taper and to obtain an estimate of stem volume for stem density measures. For each plant, stem deflection under known loadings was measured using a 100-g Pesola spring scale attached at 80% the height of the stem, a position roughly coincident with the center of mass of the leaves (sensu Milne, 1991
; see Fig. 1). For stem flexure measures, plants were held horizontally and measures were taken with plants alternatively rooted in the soil and clamped at the base of the stem, allowing for an assessment of the role of root anchorage in biomechanical support. Deflection measurements were made both in the directions facing and opposite to the direction of wind treatment. Total leaf area was estimated using a leaf-area meter (LI-COR Model LI-3000, Lincoln, Nebraska, USA). Plants were then separated into roots, stem, leaves, and petioles, dried at 60°C for 4 d, and weighed. Prior to drying, thin cross sections were cut at the base, 1.5 cm and one-quarter, one-half, and three-quarters the height of each stem. Cross sections were stained for lignin using phloroglucinol, and images of stained cross sections were scanned using a flatbed scanner. The proportion of cross-sectional area of stem occupied by the lignified central core (primary and secondary xylem) was determined at a height of 1.5 cm using CorelDRAW 7 (Corel Corporation, Ottawa, Ontario, Canada).
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 | (1) |
for untapered columns:
 | (2) |
).
Stem deflection data were used to estimate flexural stiffness based upon a modification by Niklas (1992)
of the Euler Bernoulli formula:
 | (3) |
d4/64), p is the point load, measured in Newtons (N) (9.8 kg·m–1·s–2), L is the distance between the fixed end of the stem and the position of the point load in metres, and x is the vertical deflection under the point load in metres (Fig. 1). Flexural stiffness estimates of rooted stems were expressed as a proportion of the flexural stiffness of clamped stems to obtain a measure of the contribution of root anchorage to mechanical stability. Total lignified area was expressed as a fraction of the total cross-sectional area of the stem.
Drag (D) was estimated based on total leaf area and windspeed according to Vogel (1989)
:
 | (4) |
is air density in kilograms per square metre and U is windspeed in metres per second. For Cd, the dimensionless coefficient of drag, a value of 0.1 was used based on estimates by Vogel (1989)
for clusters of broad leaves. For the leaf area of reference (S), the total planar area of all leaves on a plant in square metres was used (Vogel, 1981
). Estimates of drag were inserted as a point load into Eq. 3 to estimate the predicted deflection of stems under a windspeed of 10 m/s. Differences in response variables between treatments were assessed using two-way ANOVA or two-way ANCOVA with the logarithm of height as a covariate when height was correlated with the response variable. Normality of data was assessed prior to analysis and highly right-skewed variables (height, diameter, leaf dry mass, leaf area, and flexural stiffness) were log-transformed to improve normality. Residual plots from ANOVA and ANCOVA models were also examined to check for influential points or deviations from normality.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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Leaf allocation
Leaf dry mass and total leaf area, both controlled for height, were lower in shaded plants than in unshaded plants (P < 0.001 for both leaf dry mass and area) and lower in wind-treated plants than in untreated plants (P = 0.002 and P = 0.057, respectively) (Table 1B). No significant interactions between shade and wind for leaf dry mass and area were present. Variation in total leaf area was explained largely by average area/leaf (r2 = 0.85, P < 0.001) and less so by leaf number (r2 = 0.17, P = 0.002). Leaf dry mass per unit area was lower in shaded plants than in unshaded plants (P < 0.001) and lower in wind-treated plants than in untreated plants (P = 0.010), with no significant shade by wind interaction (Table 1A). Estimated stem deflection based on drag was lower in unshaded plants than shaded plants (P < 0.0001) and marginally lower in wind-treated plants than in untreated plants (P = 0.089) when controlled for height. Estimated stem deflections (using Eqs. 2 and 4) ranged from 1 to 9 cm, which was consistent with observed values.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Jurik, 1991
; Aarssen, 1995
; Berntson and Wayne, 2000
). Rapid stem elongation under crowded conditions may be especially pronounced in colonizing annual species such as Abutilon, which possesses low shade tolerance (Regnier and Harrison, 1993
). In the present study, Abutilon stem height increased by 33% in the presence of lateral shade due to neighbors, despite a reduction in total plant biomass (Table 1A). In contrast to this relatively large effect of lateral shade on stem height, intermittent wind exposure reduced stem height by only 9%. The effect of wind on stem height was weakest in the shade, where stem height was reduced by only 3% relative to untreated plants.
A similar interaction between shade and wind on stem height has been reported for seedlings of Acer saccharinum (Ashby et al., 1979
), although an interaction opposite in direction, in which larger wind effects were present in the shade, has been reported for Glycine max (Pappas and Mitchell, 1985
). A strong shade response regardless of wind disturbance may generally be adaptive for plants, as lateral shade and mechanical bracing by neighbors typically coincide (Holbrook and Putz, 1989
). Here, we experimentally isolated these effects by applying wind to plants removed from neighbors. Thus, the lack of a response of plant height to wind in the presence of neighbors cannot be attributed to aerodynamic buffering effects, but rather must be due to some physiological interaction. Although plants had not reached their full height after 60 d, the experiment was ended to assess plants during their exponential phase of growth and to avoid the confounding effects of reproduction. Termination of the experiment was also necessary to minimize root binding in the narrow container tubes used. The mean stem diameter of shaded plants was lower than that of unshaded plants, indicating a trade-off between stem height and diameter growth. Such a result is consistent with most literature on shade effects (see Henry and Aarssen, 1997
). However, the mean diameter of wind-treated plants was lower than that of untreated plants, even with diameter statistically controlled for differences in height (Table 1A, B). This result was unusual, given that stem diameter typically increases in response to wind (Telewski, 1995
). Due to this unusual response in diameter growth, diameter : height ratios of wind-treated plants were lower than those of control plants. Therefore, in Abutilon, changes in diameter–height allometry actually imparted lower mechanical strength to stems of wind-treated plants.
Dynamic changes in stem diameter-height allometry were characterized by increasingly rapid rates of height growth relative to rates of diameter growth from day 20 until the final harvest at day 60 (Fig. 2). These results are contrary to those predicted by simple biomechanical models for stem allometry (e.g., elastic similarity, constant stress, and geometric similarity), which predict that diameter scales as a power of height equal to or greater than one in order to maintain a constant safety margin against buckling (Dean and Long, 1986
; O'Brien et al., 1995
). Such models assume a stem material with a constant density and strength, an assumption not met in Abutilon stems, which were herbaceous for the first few weeks after germination but became increasingly woody over time. Stem density was not measured formally over the course of the experiment due to the requirement for destructive harvesting, but at final harvest the proportion of stem area lignified was positively correlated with stem diameter (Fig. 3).
While the proportion of stem area lignified increased with stem diameter within treatments, it did not differ between treatments (Fig. 3). Stem density (which was not significantly correlated with the proportion of stem area lignified) was lower in shaded plants than in unshaded plants but did not differ between wind treatments. Therefore, differences in stem density cannot be invoked as an explanation for how wind-treated plants may have compensated for low diameters relative to untreated plants. Likewise, stem taper, which increases the mechanical strength of stems by reducing the length of the lever arm of a column (Holbrook and Putz, 1989
; Niklas, 1998
), only differed between shaded and unshaded plants, whereas wind had no significant effect on stem taper (Table 1A).
Measures of stem flexural stiffness integrate the combined biomechanical effects of stem diameter, density, and taper. When height was controlled in the analysis, stiffness was lower in shaded plants than in unshaded plants (Fig. 4), consistent with the low diameter : height ratio and stem density of shaded plants. Among shaded plants, flexural stiffness was higher in wind-treated plants than in untreated plants (Fig. 4). However, among unshaded plants, flexural stiffness was lower in wind-treated plants than in untreated plants. Although the latter result may appear counterintuitive, it is consistent with the observation that wind-treated plants had lower diameter : height ratios than untreated plants while retaining equal stem tissue densities. While stem properties did not provide wind-treated plants with increased mechanical strength in the shade treatments (Fig. 5), lower stiffness may impart more flexibility, providing the plant with the strategy of load avoidance (bending in the wind) rather than load tolerance (absorbing the loading force) (Telewski, 1995
).
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; Gartner, 1994
; Stokes et al., 1997
). The importance of root anchorage was evident in the present study, where all plants uprooted before stem buckling occurred (although this tendency to uproot may have depended in part on the integrity of the soil substrate). The strength of root anchorage is dependent on root : shoot ratio, the spatial arrangement of roots, and root strength (Ennos, 1991
; Niklas, 1998
). Among shaded plants, root : shoot ratios were higher in wind-treated plants than untreated plants (Table 1A). Therefore, although stem flexural stiffness was low in wind-treated shade plants due to low diameter : height ratios, stem flexural stiffness measured on rooted stems was higher for wind-treated shade plants than for untreated shade plants due to the contribution of high root : shoot ratios to rooting strength (Table 1B). Root : shoot ratios and the ratio of flexural stiffness between rooted and clamped stems did not differ between wind treatments for unshaded plants. Thus, root anchorage was equivalent between wind treatments for unshaded plants and did not compensate for the relatively low stem stiffness of wind-treated plants relative to untreated plants (Fig. 5).
Leaf allocation
Although plant stems may equal or even exceed their theoretical buckling heights under deep shade (Holbrook and Putz, 1989
), stems of healthy plants typically fail as a result of dynamic loadings, rather than failing under their own mass (Putz et al., 1983
; Mattheck, 1995
). In the present study, Abutilon stems only attained from 5.8 to 9.1% of their maximum theoretical buckling heights (Table 1A). Hence, it was particularly important to assess the biomechanical strength of stems and roots in the context of drag, which is directly related to leaf area (Vogel, 1989
). In Abutilon, both shade and wind resulted in decreases in leaf dry mass and leaf area (Table 1B). Low total leaf area is commonly observed in plants exposed to either shade (Henry and Aarssen, 1997
) or wind or mechanical stimulation (Grace et al., 1982
; Heuchert and Mitchell, 1983
; Telewski and Pruyn, 1998
). Due to the positive relationship between leaf area and drag, wind-treated plants experience lower wind loadings than untreated plants at any given windspeed. As a result, stem deflections of wind-treated plants were equal to (for unshaded plants) or lower than (for shaded plants) those of untreated plants at a given windspeed, despite their low diameter : height ratios (Fig. 5; Table 1B).
Differences between plants in total leaf area were attributed primarily to differences in average leaf area as opposed to leaf number, which suggests that the low total leaf area of wind-treated plants did not result from the detachment of petioles from the stem due to wind damage. Leaf dry mass per unit leaf area was lower in shade plants than in unshaded plants. Such a result is consistent with the low leaf thickness typically exhibited by shade leaves and is thought to minimize internal shading of chloroplasts (Boardman, 1977
). Total leaf dry mass divided by total leaf area was also lower in wind-treated plants than in untreated plants, indicating possible thigmomorphogenic reductions in leaf cell expansion.
Conclusions
Results indicate that (1) the relative strengths of lateral shade and wind effects differ between variables and (2) that shade and wind effects are generally not additive, but rather that wind effects are diminished by lateral shading of neighbors. This is not a result of wind buffering by neighbors, because wind treatments were applied to plants removed from neighbors. Our results thus point to an overriding physiological effect of neighbors through lateral shading. The direction and strength of such an interaction may be specific to weedy, herbaceous species like Abutilon; the question of how they may differ for other growth forms, such as large trees, remains to be explored. The relationship between stem allometry and mechanical stability should be viewed in the context not only of height–diameter relationships, but also of root anchorage and leaf display, despite the difficulties that such a whole-plant approach may present for analyses of larger plants.
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FOOTNOTES |
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4 Author for reprint requests (hhenry{at}botany.utoronto.ca
; phone: 416-978-3534; fax: 416-978-5878)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONLITERATURE CITED |
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Ashby W. C. C. A. Kolar T. R. Hendricks R. E. Phares 1979 Effects of shaking and shading on growth of three hardwood species. Forest Science 25: 212-216
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Crook M. J. A. R. Ennos 1994 Stem and root characteristics associated with lodging resistance in four winter wheat cultivars. Journal of Agricultural Science 123: 167-174
Dean T. J. J. N. Long 1986 Validity of constant-stress and elastic-instability principles of stem formation in Pinus contorta and Trifolium pratense. Annals of Botany 58: 833-840
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