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2Department of Plant Ecology and Evolutionary Biology, Utrecht University, P.O. Box 80084, 3508 TB Utrecht, The Netherlands; and 3Programa Manejo de Bosques de la Amazonía Boliviana (PROMAB), Casilla 107, Riberalta, Bolivia
Received for publication August 31, 1998. Accepted for publication February 23, 1999.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: architecture • Bolivia • leaf display • light environment • shade tolerance • tropical rain forest
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). In the early 1970s, Horn (1971)
addressed the functional relationship among light environment, crown form, and light capture. He postulated that in a high-light environment several leaf layers can be stapled above each other before light interception by the lowest leaf layer is so low that its photosynthetic light compensation point would be reached. Hence, pioneer trees adapted to a high-light environment were hypothesized to have deep, multilayered crown forms, with leaves scattered throughout the crown. In contrast, climax species adapted to regenerate in a low-light environment would reduce self-shading by having shallow, monolayered crown forms, with a single layer of leaves at the periphery. Due to its simplicity and mathematical base, Horn's theory quickly found its way into the ecological textbooks, although it has rarely been formally tested. In the late 1970s, Honda and Fisher (Honda and Fisher, 1978
; Fisher and Honda, 1979
) made a cost-benefit analysis of crown architecture. They simulated the effect of variation in branch length and branch angle on light interception and compared the calculated optimal crown form with the actual crown form of the trees. Only recently, Pearcy and Yang (1996)
took this approach much further by developing a model that allowed for estimation of light interception and photosynthetic rates of leaves in a three-dimensional crown.
Kohyama (1987)
and King (1990)
emphasized the difference between the static function of tree architecture (i.e., optimality of leaf display given the current light conditions) and the dynamic function of tree architecture (i.e., what architecture allows the tree to attain a position in the canopy at the lowest costs for construction and support?). In this respect the difference between adults of understory species and juveniles of canopy species has been emphasized. For adults of understory species the static function should be most important. They are hypothesized to have wide crowns, which should lead to an increased leaf area, a lower leaf area index, or a larger area explored. In contrast, similar-sized saplings of canopy species are still heading for the canopy and are hypothesized to have narrow crowns and slender trunks, thus reducing the biomass requirements per unit height growth. Using the same line of reasoning, it can be argued that light-demanding species should invest more in height growth, thus avoiding being shaded by their neighbors. Shade-tolerant species should invest more in horizontal crown expansion to increase light interception in the light-limited understory. Much of architectural theory (Horn, 1971
; Kohyama, 1987
; King, 1990
) and many architectural studies (e.g., Rich et al., 1986
; King, 1996
; Thomas, 1996
) focus on interspecific differences in architecture, embarking from the point of view that different architectural models are beneficial in different light environments. Less attention has been paid to the benefits of intraspecific acclimation of tree architecture to the light environment. Yet, the adaptive value of a certain crown form may be more easily inferred by comparing intraspecific plant adjustments to light environment than by comparing different species groups. Intraspecific acclimation to light environment has been evaluated for several crown traits. Saplings growing in large clearings have a higher branching ratio (Steingraeber, Lawrence, and Franck, 1979
) and leaf area index (Canham, 1988
; Sterck, 1997
) than saplings in the understory. Whether saplings show such a response in a gap environment as well depends on the species.
In this study, sapling architecture was compared in six rain forest tree species differing in shade tolerance. Naturally growing saplings were selected along the whole light range encountered in the forest environment, from the shaded understory to the high-light environment of large treefall gaps. Dimensions and orientation of leaves, stems, and branches were measured in detail to be able to evaluate the efficiency of height growth, crown extension, and leaf display. A high efficiency is here defined as a maximal extension of tree height or crown area, at the lowest costs of support. Often the efficiency is evaluated by comparing trees at a similar diameter (e.g., King, 1990
), but from a functional point of view it is better to compare them at a similar biomass (Kohyama and Hotta, 1990
). A certain crown architecture is the consequence of a large number of shoot characteristics. We therefore analyzed how different species attain a different efficiency. Comparisons were made between species (pioneers vs. shade tolerants) and within species (sun vs. shade plants).
The following questions were addressed: (1) Do pioneer species have a higher efficiency of vertical stem extension than shade-tolerant species and do shade plants have a higher efficiency of vertical stem extension than sun plants? (2) Do shade-tolerant species have a higher efficiency of lateral crown extension than pioneer species and are parallel patterns found for shade plants compared to sun plants? It is often tacitly assumed that a larger crown area can be translated into a lower leaf area index, which facilitates light interception in a low light environment. We therefore also tested whether shade-tolerant species have a lower leaf area index. (3) Do pioneer species have a multilayer leaf arrangement and shade-tolerant species a monolayer leaf arrangement? Are parallel patterns found for sun and shade plants? We first tested whether pioneer species/sun plants have a deeper crown at a given height and subsequently whether they have a different vertical leaf distribution.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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1780 mm, with a distinct dry period lasting from May until September. Vegetation in the region can be classified as lowland moist forest, with a canopy of 25–35 m high and a tree species richness (trees >10 cm diameter at breast height) of 75 species/ha. Some canopy trees are deciduous during the dry season.
Six tree species were selected for the study on the basis of their light requirements and architecture (Table 1). Classification of the species in pioneer, intermediate, and shade-tolerant species was based on the relative abundance of saplings in gap and understory habitats, and on their whole-plant light compensation point (Table 1). Cecropia ficifolia and Bellucia pentamera are typical pioneer species, which establish in large gaps, quickly after disturbance. Tachigali sp. is a monocarpic species with intermediate light requirements (a nonpioneer light demander, sensu Hawthorne, 1993
), whereas Cariniana micrantha, Capirona decorticans, and Theobroma speciosum are shade-tolerant species, which can regenerate in the understory. The species differ largely in adult stature (Table 1) and sapling architecture. Cecropia and Tachigali have a monopodial orthotropic stem. Lateral crown expansion of saplings takes place through the formation of large petioles, and rachae, respectively. Bellucia and Capirona form orthotropic branches. Bellucia starts branching when it attains a height of 1 m, whereas Capirona commences branching when it attains a height of 2 m. Generally, Bellucia saplings branch quite regularly and Capirona saplings branch quite sparsely. Cariniana has a stocky growth form with many fine branches and starts branching as early as the seedling stage. Theobroma produces horizontal tiers with three branches per node. Henceforth the species will be referred to by their generic names only.
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) they were measured nondestructively in the period from October until December 1996. Stem diameters were determined at 3–5 positions along the stem axis, including one position at 10 cm from the stem base. Branch diameter was measured for each branch at 5 cm from the branch's base. Stem and branch diameters were measured in two perpendicular directions, using a caliper. Length of the stem and branches and spacer organs like petioles (Cecropia) and rachae (Tachigali) were determined. For all species but Cariniana, which has very small-sized leaves, height insertion point of the leaves at the stem base and length and width of leaves were measured and leaf damage was estimated in 10% area interval classes. For Cariniana the leaf number was counted. Other parameters measured were crown width in two perpendicular directions and height of the lowest leaf. To estimate stem biomass and leaf area from nondestructive measurements, ten additional, similar-sized saplings per species were searched for in the area. Five saplings were sampled from gaps, and five saplings from the understory. For each sapling three leaves were sampled, and a 10-cm section was taken from the upper and lower parts of the stem. Leaf areas and length and width of the leaf blades were measured, and volume of the stem sections was determined. Afterwards stem parts were oven-dried as described above. Specific stem mass was calculated as the stem mass per unit stem volume (SSM, in grams per cubic centimetre). By using multiple regression, an allometric relation was established for each species among ln(leaf area), ln(leaf length), and ln(leaf width). Mean coefficient of determination for the regression equations was 0.96 (range 0.91–0.99). The potential leaf area of the monitored saplings was calculated on the basis of the nondestructive leaf measurements. The actual leaf area was obtained by subtracting the leaf area lost to herbivory from the potential leaf area.
Stem volume of the study saplings was determined as l x 
x 0.25 x dh2, where l is the stem length and dh is the diameter at half stem height (Philip, 1994
); dh was calculated from a sapling specific regression equation. For each sapling the 3–5 measured stem diameters were regressed against their corresponding height position along the stem. Branch volume for each branch was determined as l x x 0.25 x da2 where l is the branch length and da is the average of the diameter measured at the beginning and the end of the branch. Cariniana had a somewhat different branch morphology, and for this species branch volume was calculated assuming a conic branch shape of the primary branches (1/3 x l x x 0.25 x db2) and a cylindric branch shape for the secondary branches (l x x 0.25 x db2 ), where db is the diameter at the beginning of the branch. Stem mass was obtained by multiplying stem volume by SSM and branch mass by mutiplying branch volume by SSM.
The following variables were derived: the crown area ( x 0.25 x average crown diameter2, in square metres), the relative crown depth (RCRD: 100 x crown depth/plant height, in percentage points), the leaf area index (LAI: total leaf area/projected crown area, in square metre per square metre), the support mass (stem + branch + petiole mass, in grams), and the spacer to support mass ratio [SSMR: 100(branch + petiole mass)/support mass, in percentage points]. Stem tapering index was calculated as the stem diameter at half stem height divided by the stem diameter at a tenth of the stem height. Diameters at these reference points were derived from the sapling-specific regression equations of diameter against height. The vertical leaf area distribution along the main stem was analyzed for four species only. Due to their complex branching pattern, such an analysis was not feasible for Cariniana and Theobroma. The vertical leaf area distribution was obtained by dividing each plant, independent of its size, into ten equal-sized sections. Each leaf was assigned to a certain stem section on the basis of the height of its insertion point on the stem. In case the leaf was inserted on a branch, the leaf was assigned to the corresponding vertical section along the main stem based on the branch angle and the branch length from stem to leaf base. Subsequently the leaf area per stem section was calculated. Although this gives a fairly good approximation, the calculated vertical leaf area distribution may differ somewhat from the real one, as the effect of petiole length and leaf angle on vertical leaf area distribution is not taken into account. For the same species it was calculated at what percentage of the stem length 50% of the cumulative leaf area was attained (the median leaf area height, MLAH, in percentage points).
To quantify the light environment, hemispherical fish-eye photographs were taken above the saplings in December 1996. Photographs were made under overcast sky conditions, using a Canon AE-1 camera with a 7.5 mm fish-eye lens, mounted on a pole. The camera was kept in horizontal position by a leveling device. Negatives were scanned using a Sony XC-77CE black and white CCD camera attached to a VIDAS image analysis system (Kontron/Zeiss, Eching, Germany) and analyzed with Winphot 5 (ter Steege, 1997
) for the direct site factor (DSF, in percentage points). The DSF estimates what percentage of the sunlight available above the forest canopy reaches the sapling at the forest floor.
Saplings of species should be of similar sizes and experience similar light conditions to be able to make straightforward comparisons between species regarding their architecture and their responses to light. Mean sapling height ranged from 1.6 to 1.8 m and was similar for all species (Table 1; one-way ANOVA, F5,84 = 0.29, P > 0.05). There was no significant relationship between height and DSF for five out of six species, Theobroma being the only exception (Pearson's r = 0.77, P < 0.001, N = 15). It is therefore not likely that plant height will be a confounding factor for the evaluation of plant responses to light. The DSF differed considerably between species (Table 1; one-way ANOVA, F5,84 = 6.1, P < 0.001) despite the effort made to select saplings over the same range of light environments; the mean DSF of the two pioneer species (24–25%) was significantly higher compared to the DSF of the other species (5.5–9.2%). The species experience similar light conditions when saplings from large gaps (with a DSF > 30%) are excluded from the analysis (one-way ANOVA, F5,73 = 1.4, P > 0.05). Therefore, throughout the paper, two types of analysis are done. For intraspecific comparisons all saplings will be included, and for interspecific comparisons only the restricted data set will be used, thus assuring that all species experience, on average, similar light conditions. Interspecific differences in architectural characteristics were analyzed with an ANCOVA, including species as a factor. Many architectural traits are size dependent, and plant size (i.e., support mass or height) was therefore included as a covariable. It was first tested whether there was a significant species x plant size interaction. If this interaction was not significant, then the analysis was repeated excluding the interaction effect. If the covariable had a significant effect, then differences between species were evaluated using species contrasts with joined Bonferroni confidence intervals. If the covariable was not significant, then a one-way ANOVA was carried out, and differences between species were analyzed with a least significant difference post hoc test. Intraspecific responses to light environment were analyzed using multiple regression analysis with the architectural trait under concern as dependent and light and plant size as independent variables. If necessary, the dependent and independent variables were ln-transformed. All statistical analyses were carried out, using SPSS 6.0 (Noru
is, 1993
).
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RESULTS |
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Tapering index varies from 0.58 for Cariniana to 0.95 for Cecropia (Table 3). A low tapering index indicates that a species rapidly diminishes in diameter with increasing height. Hence, Cariniana has a more conic stem form and Cecropia a more cylindric stem form. Light-demanding species have, on average, a higher tapering index than shade-tolerant species.
SSM varies from 0.20 for Cecropia to 0.73 for Cariniana. The low SSM of Cecropia is mainly due to its hollow stem.
In general, crown area increases with support mass, although slopes and intercepts vary between species (Table 2; Fig. 1B). Capirona has the smallest crown area (0.39 m2) and Tachigali the largest crown area (1.03 m2) when compared at the same support mass (Table 3). Despite the large interspecific differences, again there is no relation with adult height (Spearman's r = -0.29, P > 0.05) or shade tolerance (see Table 3, results of species contrasts).
The horizontal and vertical positioning of leaves in the environment are facilitated by spacer organs such as branches, petioles, and rachae. Patterns in crown area parallel patterns in biomass allocation to spacers. Capirona allocates the smallest fraction of its support mass to spacers (2%) and Tachigali the largest fraction (16%). Consequently, Capirona has the lowest total spacer length (0.1 m) when compared at a common support mass and Cariniana, with its stocky growth form, the largest (7 m).
The relative crown depth gives a first indication of how leaves are vertically distributed along the stem. It increases with plant height and varies among species (Table 2). Cecropia has the most shallow crown (17%) and Cariniana the deepest (56%). Analysis of the vertical leaf distribution shows that by far the largest share of leaf area is concentrated in the upper 30% of the stem (Fig. 2). For Tachigali, Bellucia, and Capirona the amount of leaf area increases gradually with stem height, although they differ in their median leaf area height (Table 3). In contrast, Cecropia has most of its leaf area confined to a small zone just below the top of the stem. Despite the large interspecific variation in RCRD, there are no significant interspecific differences in LAI (Table 3).
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DISCUSSION |
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70% higher than that of Tachigali when compared at the same support mass (Fig. 1A; Table 3). There are several ways in which a species can increase its height gain. A high stem length per unit support mass might be caused by a slender stem, a low tapering index, a reduced amount of horizontal spacers, a low specific stem mass, or a combination of these. Cecropia has a very efficient height gain, due to its low specific stem mass; its stem is constructed as a hollow cylinder. This is a mechanically efficient design, which provides good strength at the lowest construction costs (Niklas, 1992
). Cariniana realizes a high stem length, due to its extremely low tapering index. This is at the expense of a rapidly declining stem diameter with plant height. The small diameter does not provide sufficient support to maintain the stem in an erect position. The upper part of the leader shoot is nearly horizontal, which reduces the absolute height gain of the plant and makes it vulnerable to damage by falling debris. Crown damage is regularly observed for saplings of this species in the field. A similar observation has been made for saplings of another rain forest tree species (King, 1990
). Tachigali is one of the species with the lowest stem height per unit support mass. This is partly caused by a relatively large investment of support mass in spacers, which increases its crown area but reduces its height gain. Theobroma produced a short stem, mainly because of its extremely thick trunk. A thick trunk provides structural support to the sapling to bear its own mass with a large safety margin against additional stresses like falling debris, wind, or rain load (Niklas, 1992
; Sterck and Bongers, 1998)
.
Interspecific variation in efficiency of height gain could not readily be related to shade tolerance or adult stature. Given the species and the height range (1–3 m) evaluated here, there is little evidence for the hypothesis that pioneer species or canopy species have a higher efficiency of height gain than shade-tolerant species or understory species. Kohyama and Hotta (1990)
found no significant differences in sapling height per unit support mass for nine tree species differing in adult stature. Efficiency of height gain is best evaluated when expressed as realized plant height per unit support mass. Often stem diameter is taken as an estimator of support mass (e.g., King, 1990, 1996;
Thomas, 1996
) as it is a parameter that is easy to measure. Given the fact that species differ largely in specific stem mass, stem tapering, and branch to support mass ratio (Table 3), stem diameter may only be a coarse approximation of support mass; in this study 73% of the variation in support mass could be explained by variation in diameter.
Another important issue is the size range over which species are compared, as this may have a substantial effect on the outcome of the results. King (1990)
studied the sapling allometry of three canopy and three understory species. The understory species tended to have a smaller plant height at a given diameter than the canopy species, but differences were only significant for the two smallest shrub species. Sapling height of the other understory species was comparable to or taller than that of the canopy species. Here it becomes critical at what diameter saplings are compared, as height growth ceases at a smaller stem diameter in understory species than in canopy species (Thomas, 1996
). Consequently, saplings of understory species may be comparable to similar-sized saplings of canopy species, but as adults they may be different. In a study on allometric relationships of 38 Malaysian rain forest species, no correlation was found between height at 1 cm diameter and adult height (Thomas, 1996
).
Crown area
In the present study, crown area of Tachigali was 160% higher than that of Capirona (Table 3). Vertically projected crown area is a function of support mass invested in spacer organs, their specific length, and their angle with the main stem. There is a positive correlation between the proportion of support mass a species invests in spacers and the projected crown area (Spearman's r = 0.83, P < 0.05, N = 6). Capirona, the species with the smallest crown area, branches only from 2 m height onwards, and below this threshold level the crown area is solely dependent on leaf size. In contrast, Tachigali does not branch at all over the height interval evaluated, but it forms large rachae, which function as "cheap, throw-away branches" (sensu Givnish, 1984
), which effectively expand the crown area.
There are various ways to realize a certain crown area. For example, Theobroma and Cariniana differ by a factor of 4 in their total branch length, but they have a similar crown area. Theobroma forms horizontal tiers with thick branches. In contrast, Cariniana forms thin, inclined branches. The result of this contrasting pattern in specific spacer length and branching angle is that both species realize a similar crown area.
Interspecific variation in sapling crown area could not be related to differences in shade tolerance or adult stature, thus providing little support for the contention that understory species and shade-tolerant species have larger crown areas at a given size than canopy species/shade-intolerant species. Similar results were obtained by other studies evaluating sapling allometry (Kohyama and Hotta, 1990
). In a study in Costa Rica including a large number of tree species (King, 1996
), it was found that 2.5 m tall saplings of understory species could have significantly lower, similar, or larger crown areas than saplings of canopy species. Only when species were compared at the adult height of the understory species (10 m), was it found that understory species could attain larger crown areas than canopy species. As is the case with the height–diameter relationships, the outcome of comparative studies depends largely on the plant size at which species are evaluated, and it is therefore best to study whole-plant development trajectories.
Leaf display
Relative crown depth (Table 3) and median leaf area height (Table 3; Fig. 2) varied between species but light-demanding species did not differ systematically from shade-tolerant species. Moreover, all species had a similar LAI. Hence, these findings for saplings do not provide any evidence for the idea that pioneer species are characterized by a multilayered leaf distribution, and shade-tolerant species by a monolayered leaf distribution. Horn developed his ideas on leaf layering, based on observations made in temperate forests on adult trees of early-successional species such as Betula and late-successional species such as Fagus. However, many pioneer species in the tropics are characterized by a sparsely branched tree crown, large-sized leaves at the tips of the branches, and therefore an umbrella-shaped, monolayered leaf distribution (Ashton, 1978
). Neotropical examples of such species are Cecropia, Ochroma, and Didymopanax, and paleotropical examples are Musanga and Macaranga. Tropical pioneer species have a highly variable crown architecture (Ackerly, 1996
); some species are sparsely branched and have large-sized leaves, others have many fine branches and many small-sized leaves. Typical examples of the latter group are Trema, Vismia, and Goupia. Probably this category comes closest to Horn's idea of multilayered, early-successional species.
It becomes clear from the present study and the current body of literature that there are many ways to make a living as a tropical tree. Tree species differ largely in efficiency of height gain, efficiency of crown expansion and their leaf display. However, observed characteristics were not tightly related to successional status or adult stature. Reason might be that the selection of study species and the ontogenetic trajectory over which species are evaluated, determine to a large extent the outcome of the results.
Intraspecific differences
Architecture of tree saplings in this study was dependent on plant size, light environment, or a combination of these (Table 4). Trees follow a fixed ontogenetic pattern, where many plant traits change with plant size (e.g., Bongers and Sterck, 1998
). Plant size might be an important confounding factor for the evaluation of plant responses to light environment, as plant height and light are often highly correlated. Evaluation of plant responses to light was facilitated by selecting the study saplings in such way that, with the exception of Theobroma, there was no correlation between height and light. Moreover, plant size was explicitly included as a factor in the regression analysis. The analysis showed that both plant height and crown area were highly correlated with support mass (Table 4), whereas light was only of additional importance in some cases. Similar results were found by Bongers and Sterck (1998) when analyzing tree architecture of two rain forest tree species in French Guiana. Using path analysis, they showed that plant height was a far more important determinant of tree architecture than light environment.
Height
In the present study, the stem height of three species was negatively correlated with light environment; shade plants had a larger height at a given support mass than sun plants. Depending on the length and steepness of the vertical light profile in the vegetation, a plastic response in plant height can pay off in terms of an increased light interception. It is often suggested that such a plastic response is restricted to light-demanding species only, as they are adapted to regenerate in a short-statured gap vegetation where the light gradient is shorter and steeper than in the forest understory. However, in this study both pioneer and a shade-tolerant species showed such a response. Similar results have been obtained in other studies, where seedlings were grown under a range of light conditions (e.g., Sasaki and Mori, 1981
; Poorter, 1999). In the understory of our Bolivian forest mean canopy openness increased from 3.6 to 4.1% over a 1.2 m height interval (Arets, 1998
). From the regression equation it can be derived that plants growing in the shaded understory (DSF = 3%) are 32 cm taller than plants of a similar support mass growing in a gap environment (DSF = 25%) (Table 4). Such a plastic response in height may lead to a small increase in light interception and a somewhat enhanced growth.
Crown area
Light did not have any influence on the crown area of the saplings (Table 4). A large crown area may especially be of advantage in an understory environment, as in this way self-shading may be reduced. Yet, for saplings of this size there is only limited scope for such an acclimatory response in crown area, as variation in LAI is more determined by leaf area than by crown area. In a rain forest in French Guiana, the shade-tolerant canopy species Vouacapoua americana showed an increase in crown area with decreasing irradiance, whereas Dicorynia guianensis did not respond (Bongers and Sterck, 1998
). Although an increased crown area has the potential advantage of reducing self-shading, it also comes at an additional cost. An increase in crown area can be realized by a larger biomass investment in spacers, producing them at low specific costs, and by displaying them in a horizontal plane. In general, understory saplings increase light interception by allocating most of their biomass growth to leaves rather than to stem and spacers (King, 1991, 1994
; Poorter, 1998
). Investment in (woody) support mass is rather costly and, furthermore, it may be more rewarding to invest in vertical than in horizontal crown expansion (Bonser and Aarssen, 1994
). Changing spacer and leaf angles is the cheapest way to enhance leaf display; many understory saplings are characterized by plagiotropic branches and horizontal leaves (McMillen and McClendon, 1975
).
Leaf display
The relative crown depth and median leaf area height are the plant variables that are most influenced by light environment. For four species, saplings in a high-light environment were characterized by deeper crowns and a more evenly distributed leaf area along the stem (Fig. 3). Three species had a larger LAI at high irradiance. Few studies have evaluated crown responses to light. Acer saplings also had deeper crowns and a more evenly distributed foliage with increasing irradiance, whereas Quercus saplings did not respond (Niinemets, 1996
). In a tropical forest, treelets of Vouacapoua were found to increase their relative crown depth with increasing irradiance, whereas treelets of Dicorynia were not affected by light environment (Bongers and Sterck, 1998
). Saplings growing in large clearings have a higher LAI than understory saplings, but it depends on the species whether they show such a response in a gap environment as well (Canham, 1988
; Sterck, 1997
). These results are in line with the idea that high-light plants are characterized by a multilayered leaf arrangement and low-light plants by a monolayered leaf arrangement. It remains an intriguing question whether the observed differences in leaf arrangement are the result of a functional response of the plant to the light environment or simply the result of the interplay between leaf production rate and leaf loss rate. In a companion study (Poorter, 1998
) it was shown that leaf production rates of the gap plants were about seven times higher than the leaf production rates of understory plants. Leaf longevity was similar (Bellucia, Cariniana, Capirona, Theobroma) or lower (Cecropia, Tachigali) for gap plants compared to understory plants. Yet, even for the latter two species, leaf production still outweighed leaf losses, resulting in a higher standing leaf area for gap plants. Although the outcome (multilayered leaf arrangement in a high-light environment) is in line with the prediction of Horn, the mechanism by which it is brought about may be quite different. For saplings of a fast-growing pioneer species it was shown that the leaf position at which photosynthetic capacity was predicted to be zero was positively correlated with the number of leaves on the shoot (Ackerly and Bazzaz, 1995
). This supports the idea that the standing number of leaves is indeed regulated by self-shading. A definitive proof of Horn's hypothesis would be to compare light interception and carbon balance of the lowest leaf layer of sun and shade plants and to demonstrate that they are similar.
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FOOTNOTES |
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4 Author for correspondence and address: Lourens Poorter, Department of Plant Ecology and Evolutionary Biology, Utrecht University, P.O. Box 80084, 3508 TB Utrecht, The Netherlands (e-mail: L.Poorter{at}bio.uu.nl
).
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LITERATURE CITED |
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) between high- and low-light plants are indicated. LAI = leaf area index, MLAH = median leaf area height, RCRD = relative crown depth