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Ecology |
Laboratory of Forest Ecology, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan
Received for publication December 17, 2003. Accepted for publication August 26, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: biomass allocation • crown development • leaf size • relative PFD • specific leaf area • stem structure • tree height
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Patterns of crown development directly influence the leaf arrangement and light-capture efficiency and also restrict future growth directions. Trees have crown architectures that are typical for a species as a result of genetically determined growth rules and different responses to environmental factors (Hallé et al., 1978
; Bell, 1991
). Recent studies have shown that the relative importance of light availability and tree height to structural traits differs across different organizational levels within species (Sterck and Bongers, 2001
; Kawamura and Takeda, 2002
). Accordingly, whole crown architecture is regulated by various factors, depending on the organizational level.
Among the organizational levels of tree crowns, the demography and architecture of current-year shoots (e.g., Maillette, 1982
; Takenaka, 1997
) and overall crown architecture (e.g., King, 1994
; Aiba and Kohyama, 1997
) have been the focus of most studies. The structure of the current-year shoot (or growth unit in tropical trees), a fundamental unit of growth in trees, directly determines the growth strategy of the species (Remphrey and Powell, 1984
; Koike, 1989
; Wilson, 1989
; Room et al., 1994
; Takenaka, 1997
; Kimura et al., 1998
; Yagi, 2000
; Sterck and Bongers, 2001
; Kawamura and Takeda, 2002
; Osada et al., 2002
). For example, the biomass allocation between stem mass and total leaf mass within a current-year shoot and the structure of stems and of leaves, strongly influence the shoot growth rate and light-capture efficiency. Thus, the relative importance of growth and light-capture efficiency may influence the structure of the current-year shoot.
Environmental light generally increases with increasing tree height inside forests (Parker, 1995
; Osada et al., 2001
, 2004
). This light gradient is expected to affect the structure of current-year shoots on trees of different heights. Osada et al. (2002)
showed that, with increasing tree height, stems became thicker and shorter and that, concomitantly, individual leaf mass and area became smaller within a growth unit of comparable mass in a tropical canopy species, Elateriospermum tapos Blume. They suggested that these changes in the structures of growth units are adaptive under the different environments of small and tall trees. Although Osada et al. (2002)
did not distinguish the confounding effects of the light environment and tree height, these two factors may affect the structures of current-year shoots differently. That is, the leaf structure of a given species is known to change with changes in environmental light; leaves become thicker with a smaller specific leaf area (SLA) as the light level increases (Oberbauer and Strain, 1986
; Pearcy, 1987
; Ellsworth and Reich, 1992
, 1993
; King, 1994
; Kitajima, 1994
; Thomas and Bazzaz, 1999
; Osada et al., 2001
). We therefore predict that the changes in leaf structure with tree height are mainly induced by changes in the light environment. In contrast, stems develop sequentially, according to a geometric configuration, with yearly increases in both diameter and length (Room et al., 1994
). Thus, we predict that stem structure is strongly related to tree height (developmental constraints) rather than to the light environment.
Crown width and depth have often been investigated as indices of overall crown architecture (e.g., Aiba and Kohyama, 1997
; Bongers and Sterck, 1998
). Recent studies have indicated that crown architecture is greatly influenced by tree height, rather than by light (Bongers and Sterck, 1998
; Kawamura and Takeda, 2002
; Osada et al., 2004
). Small trees cannot produce crowns as large as those of tall trees, and tall trees would not survive with crowns as small as those of small trees. This suggests that the big differences in crown architecture between small and tall trees may mask the relatively small differences in crown architecture between trees that are due to different light environments. Therefore, different results may be found if crown parts of similar size are compared among trees of various heights and light levels.
Based on these predictions, we examined the architecture of the leading sections of the main tree trunks (a 100-cm length of the main trunk, hereafter termed "leader crowns"), and the current-year shoot structures within these leader crowns, for a canopy species, Fagus crenata Blume (Fagaceae). We studied the leader crown because it is directly determined by the number, structure, and arrangement of current-year shoots, and thus determines, in turn, the development and growth in height of the upper crown. The light environment is better and productivity is greater in upper crowns (Ellsworth and Reich, 1993
), emphasizing the importance of effective biomass allocation patterns in the leader crowns. Moreover, the architecture of leader crowns can be directly compared to those previously studied in saplings of a similar size (e.g., Niinemets, 1996
; Poorter and Werger, 1999
).
Forest stand structure often changes along forest slopes, and the forest canopy is shorter on ridge tops than in valley bottoms (Tanner, 1980
; Takyu et al., 2002
; Tateno and Takeda, 2003
). In accordance with this, the vertical gradient of light levels also changes along forest slopes; on average, trees on the ridge top receive more environmental light than do trees of a similar height on the valley bottom (Osada et al., 2004
). Fagus crenata is dominant in cool-temperate forests in Japan and is a species that regenerates in a wide range of environments, from valley bottoms to ridge tops (Masaki et al., 1992
; Tateno and Takeda, 2003
). In previously studying this species at different slope positions, we were able to distinguish the effects of tree height and competition for light on traits of the overall crown architecture, such as crown width and depth (Osada et al., 2004
). In this study, we used the same method to study the architecture of leader crowns and the structure of current-year shoots.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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We established a 200 x 30-m2 (0.6-ha) transect plot from the valley bottom to the ridge top of a northwest-facing slope. Every tree greater than 5 cm in diameter at breast height (DBH) was identified to species level, and the DBH was measured. Tateno and Takeda (2003)
have reported the details of the forest structure and the species composition of this study plot.
Fagus crenata is the dominant shade-tolerant canopy tree species in this forest. In the plot, its density was 163 trees/ha and total basal area was 7.4 m2/ha, accounting for 10.4% of the density and 23.4% and total basal area of all species (Tateno and Takeda, 2003
). This species has alternate, short-petioled leaves with a distichous (planar, two-ranked) phyllotaxis with all axes being plagiotropic ((??)Millett et al., 1998
; Kikuzawa, 2003
).
Measurement of current-year shoot structure
The ridge top and valley bottom of the transect (ridge and valley subplots) were chosen for the study of leader crown architecture and current-year shoot structure of F. crenata. Each subplot was 80 x 30 m2; the 40-m-long central portion of the transect between the subplots was not used in this study. Based on a generalized allometric equation (Thomas, 1996
), the asymptotic maximum height of F. crenata was estimated to be 12.9 m in the ridge subplots and 21.6 m in the valley (Osada et al., 2004
).
We selected seven trees of F. crenata in the ridge subplots and 10 trees in the valley. These trees represented a continuous range of height classes from small to tall trees in each subplot. In August 2001, the leader crown (the leading section of the main trunk, 100 cm in length) was harvested from each of the 17 trees. For tall trees, we harvested leader crowns with a pruning hook and by climbing with alpinist ropes or with ladders. Each leader crown contained more than 40 current-year shoots. At the time of harvest, the leaves of all current-year shoots within these leader crowns were fully expanded and toughened, but had not yet senesced.
We measured the width and depth of each leader crown by fixing it in a natural orientation. Widths were measured at the widest part and perpendicular to it. The projected area of the leader crown was calculated as an ellipse with these two widths. At the same time, the numbers of current-year shoots and leaves were counted and the length of each current-year shoot was measured for each leader crown. We then randomly chose and cut from each leader crown 39 to 40 current-year shoots that had no obvious damage. To investigate the biomass allocation patterns, the stem length, stem basal diameter, and leaf area of each current-year shoot were measured shortly after harvesting. For selected current-year shoots of various lengths (10–12 shoots for each individual tree), the diameter was also measured at 1-cm intervals along the stems to derive the equation estimating the stem volume, including stem taper, from (basal diameter)2 x (length of stems) (r2 > 0.99). The leaf area was calculated using the image analysis software, NIH image (National Institute of Health, Bethesda, Maryland, USA) with digitized data from photocopies of the leaves. All the stems and leaves were then oven-dried at 70°C, and the dry masses were measured.
Measurement of light environment
The integrated light level was measured for five and seven trees of the ridge and valley subplots, respectively, using Oil Red-O film (Taisei Chemical Industries, Tokyo, Japan), which fades in proportion to the integrated solar radiation. The maximum absorption wavelength of this film is 521 nm. The light transmission coefficient was determined with a portable light meter (TS450, Taisei Chemical Industries) before and after exposure. Three pieces of film were taped to the top of an extension pole (6–15 m long), and the pole was extended and fixed so as to place the film horizontally at the top of the leader crown. The integrated solar radiation was obtained on three consecutive clear days in August 2001. Moreover, 22 pieces of film and one quantum sensor (LI-190SA, Li-Cor, Lincoln, Nebraska, USA) were placed in an open area about 100 m from the transect. These pieces were sampled at different times during 5 days, including the three clear days described in August 2001, and a calibration equation was obtained (r2 > 0.99, N = 22). Photosynthetic photon flux density (PFD) was calculated for each piece of film based on the equation. The relative PFD (PFD relative to PFD in the open area) was calculated for each piece of film, and the average of the three pieces was then calculated for each tree. Although we could not measure the light environment of some of the tall trees, the leader crowns of these trees were exposed to full light above the canopy, and their relative PFD was assumed to be one.
Using the same trees of F. crenata, we had found from the carbon isotope composition of the leaves of leader crowns that the leaf-level water-use efficiency was higher in ridge trees than in valley trees of similar heights and that this difference could be explained by the differences in the light environments (Osada et al., 2004
). For trees with similar light availabilities, the leaf-level water-use efficiency was similar between the ridge and valley trees, irrespective of differences in tree height (Osada et al., 2004
). Thus, if water availability influences the results, it would be explained indirectly by the effects of the light environment.
Analysis of current-year shoot structure
The total biomass, stem mass, total leaf mass, and leaf mass ratio (LMR, ratio of leaf mass to total biomass) within a current-year shoot were determined as follows
 | (2) |
W and vS are wood density (g cm–3) and stem volume (cm3), respectively. Total leaf mass was divided into leaf number and mean individual leaf mass, whereas mean individual leaf mass and SLA determine mean individual leaf area: 
; Sokal and Rolf, 1995
).
We analyzed the effects of tree height and environmental light separately because these factors are strongly correlated. We used analysis of covariance (ANCOVA) to test whether the traits of leader crowns and current-year shoots were different among the trees in the ridge and valley subplots and whether these traits changed in association with tree height or environmental light. First, the homogeneity of the slopes was tested; if no difference was found among slopes, an ANCOVA was conducted to test for differences among the adjusted means. If tree height was the main factor affecting the traits, the regression lines would have been similar for the trees in the ridge and valley subplots when ANCOVA was conducted with tree height as a covariable, while the regression lines would have been different for these trees when ANCOVA was conducted with the light environment as a covariable (Fig. 1a, b). On the other hand, if the light environment constituted the main factor influencing the traits, the regression lines would have been similar for the trees in the ridge and valley subplots when ANCOVA was conducted with the light environment as a covariable, and the regression lines would have been different for these trees when ANCOVA was conducted with tree height as a covariable (Fig. 1c, d; Osada et al., 2004
). Based on these analyses, we distinguished the effects of the light environment and tree height on the specified traits of leader crowns and current-year shoots.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Because stem structure was mainly related to height, whereas leaf structure and leaf biomass allocations were related primarily to relative PFD, the relationships between stems and leaves within current-year shoots changed with both height and relative PFD (Table 4). Stems with similar diameters supported smaller total leaf areas with increasing height and/or relative PFD (Table 5). The differences in the slopes of log (total leaf area) to log (stem diameter) indicate that, with increasing stem diameter, total leaf area increased more slowly on trees of greater height and/or relative PFD. Similarly, stems of similar lengths supported smaller total leaf areas with increasing height and/or relative PFD (Table 5).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Because the asymptotic maximum height was lower among the ridge trees than among the valley trees at both the stand level (Tateno and Takeda, 2003
) and the species level (Osada et al., 2004
), the vertical gradient in light availability differed among the ridge and valley subplots. Accordingly, we were able to distinguish the effects of light and tree height on traits of leader crowns and current-year shoots. In this study, although we selected trees of different heights, relative PFD was clearly distinguishable between tall and small trees; relative PFD was quite high among tall trees and quite low among small trees (Fig. 2). The leader crowns of canopy trees were not shaded by the surrounding trees and thus received high levels of light. On the contrary, shading by surrounding trees considerably reduced the relative PFD of small trees. As a consequence, we could not obtain sample trees with 0.2–0.7 relative PFD (Fig. 2). However, the effects of height and relative PFD were clearly distinguishable for most traits, suggesting that the lack of a continuous range of relative PFD values does not affect the results.
Leader crown architecture
For leader crowns, the number of current-year shoots and leaves was mainly related to tree height. The number of current-year shoots is controlled by the branching intensity of the tree crowns, and therefore the branching intensity may be strongly influenced by tree height. The number of leaves in a leader crown is a product of the number of current-year shoots and the mean number of leaves per current-year shoot. The number of leaves per current-year shoot generally increases with increasing stem length within a species (e.g., Takenaka, 1997
; Kimura et al., 1998
; Osada et al., 2002
). In our results, the mean length of current-year shoots was related to relative PFD (Table 1). This suggests that the effect of branching (i.e., number of current-year shoots) is more important in determining the number of leaves in a leader crown than is the number of leaves per current-year shoot. This is probably because of the limited range of the number of leaves per current-year shoot, in contrast to the wide variation in the number of current-year shoots. Alvarez-Buylla and Martinez-Ramos (1992)
also reported the importance of branching with respect to increasing the leaf number in the crown in a tropical pioneer species, Cecropia obtusifolia Bertol.
Crown area decreased and crown depth and LAI increased with increasing relative PFD. The changes in leader crown architecture with light in this study are similar to those of whole crowns for the saplings of temperate (Niinemets, 1996
) and tropical trees (Poorter and Werger, 1999
). Wide crowns with small LAI would be important for capturing light efficiently under low light levels. This resulted mainly from the different stem angles within the leader crowns, because the length of the main axis was fixed to 100 cm. However, a greater biomass investment in the lateral branches is required to produce wide crowns, and thus such crowns may not be effective for vertical growth under high light levels (Poorter and Werger, 1999
). It is interesting to note that overall crown architecture traits, such as whole crown width and depth, were regulated not by the competition for light but by tree height in this species and in Quercus crispula Blume at this study site (Osada et al., 2004
). Moreover, similar results have been reported for tropical canopy trees (Sterck and Bongers, 2001
) and temperate shrub species (Kawamura and Takeda, 2002
). Thus, the architecture of leader crowns and that of overall crowns should be affected differently by the light environment and by tree height in these trees. In leader crowns, sensitive responses to light may be important for determining efficient light capture and crown development. Overall crown architecture is, on the other hand, related mainly to tree height, probably because the big differences in crown architecture between small and tall trees may mask the relatively small differences in crown architecture that result from different light environments.
Biomass allocation and structure of current-year shoots
The mass of individual current-year shoots increased with relative PFD. Thus, shoot growth is regulated by light availability in the leader crowns. However, this total mass was allocated to stems and leaves differently, depending on tree height; with increasing tree height, the relative allocation to stems increased. Furthermore, stem structures within current-year shoots also changed with height, and taller trees produced thicker and shorter stems of the same volume. This change in stem structure with height is consistent with that of the tropical canopy species Elateriospermum tapos (Osada et al., 2002
), further suggesting the importance of such changes. As predicted, stem growth is regulated by the geometric configuration, and thus greater biomass is needed for the unit extension of stems in taller trees. A decrease in LMR with increasing tree height may be important with regard to compensating for increasing stem costs, as in F. crenata.
In contrast to the stem structure, leaf structure and leaf biomass allocations changed with relative PFD. As expected, SLA decreased with increasing relative PFD. In addition, leaf number increased more rapidly with increasing individual leaf mass for trees of greater relative PFD. Under low relative PFD, self-shading may be reduced through the production of a smaller number of leaves with larger SLA. Self-shading would not be important, however, under high relative PFD. Here, individual leaf area decreased with increases in both relative PFD and tree height. Individual leaf area would be regulated not at the branch level but at the whole crown level, and branching density may be strongly related to individual leaf area (Osada et al., 2002
). Accordingly, current-year shoot density (number/cm3) increased (data not shown) and individual leaf area decreased with increasing height. Thus, the complexity of tree crowns increases with increasing size as the tree matures and approaches maximum crown dimensions. In addition, SLA (or leaf mass per area; inverse of SLA) has been shown to change with tree height, independently of light, in various tree species (King, 1999
; Cavender-Bares and Bazzaz, 2000
; Rijkers et al., 2000
). It is important to note that our results do not suggest that SLA is determined completely by light, but that the relative importance of light is greater than that of tree height in F. crenata.
As a consequence of stem and leaf structures, the relationships between stems and leaves were related to both relative PFD and tree height. The total leaf area supported by a stem of a given diameter decreased with increasing height and relative PFD. This is important because both mechanical and hydraulic stresses may be reduced in trees with high relative PFD and greater height. Similar results were obtained by Osada et al. (2002)
, although they considered the effect of tree height only. The total leaf area supported by a given length of stem also decreased with increasing tree height and relative PFD. This parameter has been considered an index of the relative importance of stem extension and efficient light capture for the current-year shoots (Takenaka, 1997
; Yagi, 2000
) because longer stems enhance stem extension and greater leaf area enhances the capture of light. According to this view, the current-year shoots of smaller trees, or of trees with lower relative PFD, capture light more efficiently at the expense of stem extension than do current-year shoots of tall trees in F. crenata. However, due to self-shading, the light availability for individual leaves might be reduced if shorter stems support greater leaf area. Actual light availability for the leaves within current-year shoots is determined by the complex structure of current-year shoots, including phyllotaxy (Niklas, 1988
; Takenaka, 1994
), the length : width ratio of a leaf blade (Takenaka, 1994
), petiole length (Takenaka, 1994
), directional changes in leaf size along a stem (Osada et al., 2002
), and stem angle (Kikuzawa, 2003
). In particular, this species produces plagiotropic branches, and therefore the effects of stem angle on the efficiency of light capture and photosynthesis are considered to be important (Kikuzawa, 2003
). It is interesting to note that the relationships between total leaf area and stem length did not change with tree height in E. tapos (Osada et al., 2002
); this species produces orthotropic branches, and thus the results may differ.
Our results suggest that the different effects of light and tree height on stems and leaves totally determine the architecture of leader crowns and the whole structure of current-year shoots. Tree height mainly restricts the developmental patterns of crown architecture, such as branching intensity (number of current-year shoots per leader crown) and stem structure (diameter–length relationship). Under such constraints, the light environment modulates crown architecture by inducing changes in the area and depth of leader crowns (probably through changing branch angles) and in leaf structure, such as SLA, to allow the efficient capture of light. Further extension of this work, i.e., investigating the difference of shoot structure between the saplings of high- and low-light environments and among the trees of different ontogenetic stages but similar light environment, is of great interest because the length of current-year shoots may differ from this study.
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FOOTNOTES |
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2 Author for correspondence (present address): Graduate School of Life Sciences, Tohoku University, Aoba, Sendai 980-8578, Japan (osadada{at}biology.tohoku.ac.jp
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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