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Plant size and form in the understory palm genus : are species variations on a theme?

Chazdon, Robin L https://research.usc.edu.au/discovery/delivery/61USC_INST:ResearchRepository/12126526940002621?l#13126944680002621

Chazdon, R. L. (1991). size and form in the understory palm genus Geonoma: are species variations on a theme? American Journal of Botany, 78(5), 680–694. https://doi.org/10.1002/j.1537-2197.1991.tb12592.x Document Type: Published Version

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Plant Size and Form in the Understory Palm Genus Geonoma: Are Species Variations on a Theme? Author(s): Robin L. Chazdon Source: American Journal of Botany, Vol. 78, No. 5 (May, 1991), pp. 680-694 Published by: Botanical Society of America, Inc. Stable URL: http://www.jstor.org/stable/2445089 Accessed: 23-05-2017 01:17 UTC

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This content downloaded from 203.29.104.10 on Tue, 23 May 2017 01:17:51 UTC All use subject to http://about.jstor.org/terms American Journal of Botany 78(5): 680-694. 1991.

PLANT SIZE AND FORM IN THE UNDERSTORY PALM GENUS GEONOMA: ARE SPECIES VARIATIONS ON A THEME?1

ROBIN L. CHAZDON

Department of Ecology and Evolutionary Biology, The University of Connecticut, Storrs, Connecticut 06268

This study addressed the hypothesis that phylogenetic changes in plant size at reproductive maturity may have facilitated adaptive radiation of Geonoma species within rain forest un- derstory habitats. Leaf size, leaf form, plant size, and growth form were compared within and among 23 species of Geonoma from lowland and montane rain forest areas of Costa Rica and Colombia. Leaf size was significantly correlated with crown height in 18 of the 21 species examined, and with stem diameter in 17 of the species. In species characterized by a gradual ontogenetic transition from bifid to dissected leaves, shoots with bifid leaves were significantly smaller than shoots with dissected leaves with respect to rachis length, number of plications, and stem diameter. Among species, stem diameter below the crown explained 74% of the variation in leaf size (rachis length). Crown height and stem diameter were positively correlated among clustered species, but not among solitary species or all species combined. Leaf dissection was correlated with crown height among the 17 species with dissected leaves; species with bifid leaves were significantly smaller than species with dissected leaves with respect to leaf size and stem diameter. Solitary species had larger leaves and larger stem diameters than clustered species at the same crown heights. Morphological patterns among species generally followed within- species trends. These patterns suggest that Geonoma species are variants on a generic theme: within and among species, leaf size and complexity of form increase with stem diameter and crown height. Solitary and clustered growth forms appear to be morphologically convergent; within each of these architectural groups, the generic theme still applies. Evolutionary changes in leaf size, leaf form, and plant size, however, have clearly involved other factors in addition to variation in plant size.

Ecological and evolutionary studies intersect is variation in one morphological feature in- in the study of morphological relationships dependent of other aspects of morphology? within and among related species. Many as- In this paper I focus on patterns of variation pects of variation may be viewed as adaptive in leaf size, leaf form, and plant size within evolutionary responses to enviromental con- and among species in the diverse palm genus ditions; these same characters may also be Geonoma. This genus presents a striking ex- useful in differentiating closely related taxa. ample of ecological diversification within the Analyses of adaptive radiations within circum- understory of moist tropical forests, yet species scribed taxa require a careful synthesis of eco- are confined to two relatively simple architec- logical and evolutionary considerations. To tural models. Species following Corner's model what extent do observed patterns of variation have a single, unbranched axis (solitary), reflect underlying developmental patterns or whereas species described by Tomlinson's phylogenetic history? To what extent do these model branch at the base, producing multiple patterns represent uniquely evolved solutions shoots (clustered) (Halle, Oldeman, and Tom- to diverse selection pressures? To what extent linson, 1978). Geonoma species range in stat-

I Received for publication 30 May 1990; revision ac- ure from dwarf palms less than 1 m tall to cepted 9 January 1991. subcanopy trees up to 10 m tall. Leaf mor- The author thanks Gloria Galeano Garces for hospital- phology also varies widely within and among ity, encouragement, and assistance in Colombia; Maria species. Seedlings of all species are bifid, yet Marta Chavarria Diaz, Reynaldo Aguilar Femandes, and during ontogeny many species exhibit a gradual Virgilio Alvarado for assistance with field work in Costa Rica; Donald Kaplan and Lewis Feldman for support dur- transition from bifid to irregularly or regularly ing my fellowship tenure at Berkeley and for their interest divided leaves. Other species retain bifid leaf in my research; Natalie Uhl, Donald Kaplan, Paul Rich, morphology throughout their life cycle. The Carl Schlichting, and Robert Colwell for helpful comments objective of this study was to determine the on earlier versions of this manuscript; and Peter Gogarten extent to which patterns of variation in leaf for help with German translations. This research was fund- ed by an NSF Postdoctoral Fellowship in Environmental size and form of Geonoma species are asso- Biology (BSR-8502449). ciated with phyletic or ontogenetic variation

680

This content downloaded from 203.29.104.10 on Tue, 23 May 2017 01:17:51 UTC All use subject to http://about.jstor.org/terms May 1991] CHAZDON-PLANT SIZE AND FORM IN GEONOMA 681 in plant size. Other diverse understory palm floor to heights of at least 4 m in the understory genera, such as Chamaedorea and Bactris in (Chazdon, 1986b). The ability of some species tropical America, Neophloga and Dypsis in to reproduce while still at a small size creates Madagascar, and Pinanga in Malaysia, exhibit the evolutionary potential for shade adapta- patterns of morphological variation quite sim- tion, given the appropriate selective regime. ilar to those observed in Geonoma (Tomlinson, Conversely, delaying reproductive maturity 1960; Dransfield, 1978, 1989). until attain larger sizes enables these The third largest neotropical palm genus, plants to exploit conditions of greater light Geonoma contains from 75 to 100 species availability. (Wessels Boer, 1968; Uhl and Dransfield, 1987). Phylogenetic variation in shoot size at re- Species are distributed in understory habitats productive maturity in these palms may be of wet lowland and montane forests from correlated with variation in leaf size and com- southern Mexico throughout Central America plexity among species. Relationships between and the West Indies, reaching their southern stem diameter, leaf size, and leaf morphology limit in southeastern Brazil (Wessels Boer, within individual shoots reflect a syndrome of 1968). With the exception of G. megalospatha ontogenetic changes correlated with changes in (Wessels Boer, 1968) and G. trigona (Gentry, shoot size, as described by Troll (1937, 1939). 1986)-species of xeromorphic, high altitude The phenomenon of Erstarkungswachstum, vegetation- Geonoma species are restricted to literally "strengthening growth" (Troll, 1937, forested areas receiving more than 1,000 mm 1939; Troll and Rauh, 1950), is clearly ob- of annual rainfall. Areas with the greatest num- served in arborescent monocots, where onto- ber of species generally receive from 2,000 to genetic changes in shoot diameter result from 5,000 mm of annual rainfall (Wessels Boer, primary thickening growth. Coincident with 1968). increases in shoot diameter are increases in Geonoma is considered a relatively ad- both leaf size and morphological complexity vanced palm genus (Uhl and Dransfield, 1987). (Troll and Rauh, 1950). Shoot ontogeny re- Because of their strong association with moist, veals a series of integrated developmental fea- shaded, understory habitats, Geonoma species tures ultimately correlated with the size of the have presumably evolved and diversified with- shoot apical meristem. Although the concept in rain forest understory conditions. Based on of Erstarkungswachstum applies generally to ecological studies of G. cuneata, G. congesta, all plants, it is most evident in shoots lacking and the related geonomoid species secondary growth, which masks the obconic martiana, Chazdon (1985, 1986a-c) conclud- form of the primary plant body. Yet no studies ed that, given similar photosynthetic capacities have examined the extent to which these cor- per unit leaf area, relative shade tolerance was related developmental changes may be asso- greatest in plants that maximized the amount ciated with morphological variation among of leaf area displayed on a whole-plant basis, species within the same taxon. while minimizing the biomass costs of leaf- In this study, I present preliminary results support structures. Within and among the three from ongoing investigations to evaluate rela- species studied, small plants had higher effi- tionships between size and morphology within ciencies of light interception (Chazdon, 1985), and among Geonoma species. In this initial lower biomass costs of leaf support, greater leaf approach, I characterize shoot diameter using safety, and more total leaf area per plant bio- measurements of the diameter of the stem in- mass (Chazdon, 1986a). Shade adaption may ternode just below the crown, rather than by be viewed as a scaling problem, with plant size examining developmental relationships be- at reproductive maturity determining the de- tween shoot apex diameter and leaf primor- gree of shade that can be exploited by a par- dium size. ticular species. Thus, G. cuneata, which re- produces at heights of less than 1 m, exploits MATERIALS AND METHODS the most heavily shaded microsites within the understory. Species and study area -A total of 23 Geon- These ecological studies suggest that phy- oma species were studied in their natural rain logenetic changes in plant size at reproductive forest understory habitats. According to the maturity may have facilitated the adaptive ra- classification scheme of Wessels Boer (1968), diation of Geonoma species within rain forest the described species represent nine of the 17 understory habitats (Chazdon, 1986c). Mea- subgeneric groups of Geonoma. Ten of these surements of photosynthetically active radia- species were studied in Costa Rica at La Selva tion in closed-canopy rain forest habitats show Biological Field Station in the Atlantic low- that light availability increases from the forest lands and in the adjacent Braulio Carrillo Na-

This content downloaded from 203.29.104.10 on Tue, 23 May 2017 01:17:51 UTC All use subject to http://about.jstor.org/terms 682 AMERICAN JOURNAL OF BOTANY [Vol. 78 tional Park (Heredia Province) at elevations whole. Ecologically meaningful measurements from 35 to 2,000 m. I studied six species from must be made in relatively undisturbed rain lowland forest (60 to 70 m elevation) near forest understory habitats using wild popula- Quibdo in Choco Province, Colombia, and nine tions, so that natural patterns of variation can species from lowland forest (150 to 250 m el- be observed. In many cases, species are rela- evation) near Araracuara (Rio Caqueta) in tively rare and are protected from cutting. Amazonas Province, Colombia. Two of the Moreover, related studies involve repeated species from Araracuara are currently being measurements to determine growth rates and described (G. Galeano Garces, personal com- ontogenetic changes. Consequently, whole plant munication). One species, G. deversa, occurs dry weight is not an appropriate size measure in all three locations. When possible, at least for my purposes. Measurements of the size of 20 reproductive stems of each species were the apical meristem are generally not appro- measured with respect to the parameters de- priate because of the need to destroy shoots. scribed below. In 1 1 of the species, fewer than Second, physical measurements defining the 20 reproductive stems were measured because space occupied by an individual, such as height, of plant scarcity or lack of time available at can be misleading. If palm stems are not ver- the field site. For six species, fewer than ten tically oriented, stem length may be a better reproductive stems were found (Table 1). Stems indicator of shoot size than crown height. were classified as reproductive if inflores- Crown height, however, is an ecologically rel- cences, infructescences, or developing inflo- evant measure of plant size because of its as- rescence buds were visible. In the case of clus- sociation with light availability (Yoda, 1974; tered species, all reproductive stems of each Chazdon, 1986b; Torquebiau, 1988). Third, individual were measured. When possible, at measures such as the number of nodes on the least ten nonreproductive stems of each species stem may not be comparable among individ- were also measured, although in some cases, uals or species because of environmentally in- stems were not yet visible above ground and duced or ontogenetic variations in rates of leaf could not be measured. All measurements were production and internodal elongation. made in relatively undisturbed, closed-canopy, Using stem diameter measurements as mea- mature forest habitats, with the exception of sures of plant size presents further problems. G. maxima, for which some individuals were Stem diameter in palms and many other mono- measured in a secondary forest near Arara- cotyledonous taxa is initially determined by cuara. primary thickening growth of the seedling axis (Ball, 1941; DeMason, 1979). Duing this pro- Measurements ofplant size-In morpholog- cess, the axis increases in diameter through the ical studies of animals, measurement of body progressive widening of successive internodes, size is relatively straightforward (Calder, 1984; producing an obconical seedling axis (Troll and Schmidt-Nielsen, 1984). Total mass is usually Rauh, 1950; DeMason, 1979). Once a char- the preferred measure, when available. For acteristic diameter is attained, successive elon- plants, however, no standardized measures of gation of internodes leads to vertical extension body size exist, although height and diameter of the axis (Tomlinson, 1970). The ontogenetic at breast height are common measures of tree process of primary thickening growth during size used by foresters. Early studies of plant the establishment phase should be differenti- growth were based on plant dry weight as mea- ated from the organographic process of axis sures of plant size (Blackman, 1919). In his thickening following leaf initiation in mature studies of relationships between body size and shoots that have already attained their char- organ size in plants, Sinnott (1921) used only acteristic diameter (DeMason, 1979). Diffuse shoot dry weight as a measure of plant size. secondary growth (termed sustained primary Although useful, these quantitative biomass growth by Waterhouse and Quinn [1978] and measures do not provide any information on sustained stem thickening by Rich [1987]) may morphological or developmental features of lead to further increases in stem diameter at plants in relation to size. later developmental stages (Schoute, 1912; Clearly, measurements of plant size relevant Tomlinson, 1961). Diffuse secondary thick- to studies of plant morphology and ecology will ening has been observed in G. pinnatifrons, a depend on the structural and developmental relatively tall species, but is lacking in smaller features of particular taxa. In seeking appro- Geonoma species with cane-like stems (Wes- priate measures of plant size in this study of sels Boer, 1968). 23 Geonoma species, several considerations In acaulescent species and seedlings of cau- apply. First, I require nondestructive field mea- lescent species, stem diameter can be accu- surements of the size of the shoot system as a rately measured only if plants are removed

This content downloaded from 203.29.104.10 on Tue, 23 May 2017 01:17:51 UTC All use subject to http://about.jstor.org/terms May 1991] CHAZDON-PLANT SIZE AND FORM IN GEONOMA 683 from the soil. Moreover, stem diameter may dissection, was compared among species by change dramatically along the length of the dividing the number of divisions per lamina stem. Measurements of stem diameter, if they by the number of plications. are to be comparable among individuals and Bivariate correlations of log-transformed among species, must be made at a standard measures of lamina size and morphology with position along the stem (such as the most re- measures of plant size were used to compare cently exposed internode below the crown), size relationships observed among reproduc- rather than at a standard height. Diameter at tive individuals of the 23 Geonoma species breast height means little for palm stems that with relationships observed within individual are leaning, bent, or otherwise contorted (King, species. Linear regression analysis and analysis 1987). of covariance were used to compare size re- Given the problems associated with using lationships between solitary and clustered spe- any one measure of plant size, in this study I cies. Principal components analysis was used rely on several different size-related measure- to examine patterns of covariation and cor- ments. Some of these parameters, such as the relation among species. Nine log-transformed number of nodes and stem length, are estimates variables describing leaf and plant size were of relative shoot age as well as size. Others, used in the analysis: crown height, crown size, such as crown height, also serve as environ- rachis length, leaf width, stem length, stem di- mental indices. Measurements of stem diam- ameter, number of plications, node length, and eter below the crown and crown size (number number of nodes. The principal component of living leaves in the crown) are perhaps the analysis was based on the correlation matrix. most appropriate indicators of the size of the shoot system. RESULTS

Measurements of leaf size and leaf form - Species characteristics -The sample of 23 This study focuses on morphological charac- species (Table 1) included 1 1 solitary (Corner's teristics of the lamina for several reasons. First, model) and 12 multiple-stemmed or clustered the lamina exhibits impressive variation in size species (Tomlinson's model). Four of the spe- and form among species, as well as within many cies, G. cuneata, G. gracilis, G. leptospadix, and species. Second, petiole elongation continues G. pycnostachys, exhibited population or geo- long after the lamina has expanded, making it graphic variation in growth form; for these spe- difficult to measure the final length of the entire cies, the predominant growth form was used. leaf axis for all but the oldest leaves within the Among species, crown size and the number of crown. These older leaves are often damaged plications per leaf showed the lowest coeffi- and, in many cases, unsuitable for measure- cients of variation (CV = 22.2% and 46.6%, ment. Third, measurements of the entire leaf respectively), whereas the number of divisions axis require destructive sampling of the crown per leaf (CV = 1 19.3%) showed the greatest to reach the sheathing leaf bases. variation. Mean crown height ranged from 95 Two major developmental features give rise cm in G. cuneata to over 5 m in G. longeva- to the dissected, pinnate palm leaf. First, pli- ginata; whereas mean stem diameter below the cations (laminar folds or pleats) form by a pro- crown ranged from 0.6 cm in G. leptospadix cess of differential growth within the devel- to over 5 cm in G. interrupta. One species, G. oping lamina (Dengler, Dengler, and Kaplan, macrostachys, is acaulescent (lacking an above- 1982). Once plications have formed, a process ground stem), and one species, G. linearis, is of tissue separation cleaves the laminar pli- rheophytic, confined to running watercourses cations into distinct segments (Kaplan, Den- (Galeano Garces and Skov, 1989). Six of the gler, and Dengler, 1982). In most Geonoma species produced only bifid leaves at repro- species, however, tissue separation does not ductive maturity, three species exhibited both uniformly occur between all laminar plica- bifid and irregularly dissected leaves at repro- tions; the pattern of leaf division often varies ductive stages (G. chococola, G. cuneata, and greatly within and among species. Rachis length G. gracilis), and 14 species consistently pro- (excluding petiole), lamina width at the widest duced dissected leaves at reproductive matu- point, and the number of plications (as mea- rity (Table 1). sured by the number of adaxial primary veins) were the three measures of lamina size used in Intraspecific variation in size and morphol- this study. The total number of divisions per ogy-Patterns of variation in leaf size, leaf form, lamina and the ratio of rachis length: lamina and plant size were examined within 21 Geon- width were two measures of lamina shape. A oma species; two species (G. camana and G. third measure of leaf shape, the degree of leaf pycnostachys) were represented by fewer than

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TABLE 2. Bivariate product-moment correlations between rachis length and plant size parameters. All variables were log-transformeda

Species N Crown height Stem length Stem diameter Crown size Number of nodes

G. chococola 20 0.512* ns 0.530* 0.518* ns G. congesta 270 0.402** 0.328** 0.355** 0.607** ns G. cuneata 25 0.499** 0.598** 0.747** 0.466* ns G. deversa 121 0.847** 0.316** 0.353** 0.772** 0.224* G. divisa 37 0.707** 0.453** 0.748** 0.534** ns G. epetiolata 25 ns ns 0.643* ns -0.425* G. ferruginea 26 ns ns ns 0.604** 0.447* G. gracilis 25 0.904** ns 0.713** 0.721** ns G. hoffmanniana 25 0.757** 0.727** 0.549* 0.637** 0.745** G. interrupta 38 0.927** 0.623** 0.955** 0.819** ns G. leptospadix 27 0.772** ns 0.467* 0.504** ns G. linearis 19 0.659** ns ns ns ns G. longevaginata 34 0.913** 0.481** 0.479** 0.927** ns G. macrostachys 33 0.667** - ns 0.355* G. maxima 48 ns ns 0.428** ns ns G. oxycarpa 10 0.895** 0.867* ns 0.738* ns G. piscicauda 42 0.783** ns 0.572** 0.824** ns G. aff. leptospadix 25 0.787** ns ns 0.752** ns G. aff. oxycarpa 13 0.575* ns 0.907** ns -0.597* G. sp. nova 1 22 0.734** ns 0.596** 0.532* -0.468* G. sp. nova 2 33 0.775** ns ns 0.567** ns a * P < 0.05; ** P < 0.01; ns = P > 0.05.

ten individuals, an insufficient sample size for analysis. In 18 of the 21 species examined, rachis length was significantly correlated with C_ crown height, whereas 17 species showed a 05 significant correlation between rachis length CD-z / and crown size (Table 2). Rachis length was C_, positively correlated with stem diameter in 15 0 4 of the species, whereas only eight species Cl showed a significant correlation between rachis length and stem length (Table 2). Three species 3 0 showed a significant positive correlation be- tween rachis length and node number (P < 0.05): and three species showed a significant z2 . . .S 0 1 2 3 negative correlation between these variables (P Ln (Stem Diameter) < 0.05; Table 2). Correlations between different measures of 5 leaf size and plant size varied greatly within a species as well as for the same measure among species (Table 2). In G. interrupta, G. longeva- ginata, and G. gracilis, correlation coefficients

0) for two different measures of plant size ex- U) X ceeded 90%, suggesting a close integration of leaf size with plant size (Fig. 1A, B). Crown height was the strongest correlate of rachis 3 length in nine of the species; stem diameter was most closely correlated in eight species, whereas crown size was the closest correlate of rachis length in only four of the species. Despite S.5 1.0 1.5 2.0 2.5 3.0 the lack of consistency in these relationships, Ln (Crown Size) rachis length was significantly correlated (P < 0.05) with at least one measure of plant size Fig. 1. Within-species variation in (A) rachis length in all of the species examined (Table 2). (cm) and stem diameter (cm) in G. interrupta, and (B) rachis length (cm) and crown size in G. longevaginata. All No consistent pattern relating the number variables were log-transformed. of leaf divisions to leaf or plant size was ob-

This content downloaded from 203.29.104.10 on Tue, 23 May 2017 01:17:51 UTC All use subject to http://about.jstor.org/terms May 1991] CHAZDON-PLANT SIZE AND FORM IN GEONOMA 687

TABLE 3. Bivariate correlations between leaf division (number of divisions), rachis length, and plant size parameters. All variables were log-transformeda

Number Species N Rachis length Crown height Stem length Stem diameter Crown size of nodes

G. congesta 270 0.548** 0.432** 0.236** 0.355** 0.318** ns G. deversa 91 0.451** ns ns ns 0.364* ns G. ferruginea 26 ns ns ns ns ns ns G. hoffmanniana 24 0.595** ns 0.644** ns ns 0.782* G. interrupta 38 0.324* ns 0.414* ns ns 0.376* G. linearis 19 0.773** ns ns ns ns ns G. longevaginata 34 0.765** 0.716* ns ns 0.674* ns G. macrostachys 33 0.424* 0.354* - 0.377* ns - G. maxima 48 ns ns ns ns ns ns G. piscicauda 34 ns ns ns ns ns ns G. oxycarpa 10 0.908** 0.858** 0.796** ns 0.709** ns G. aff. oxycarpa 13 0.702** 0.709** 0.675* 0.679* ns ns G. sp. nova 2 33 0.824** 0.839** 0.378* ns 0.532** ns a * P < 0.05; ** P < 0.01; ns = P > 0.05.

served among the 13 species with dissected genetic transition from bifid to dissected leaves, leaves, although leaf division was significantly the bifid morphology is almost universally as- correlated with rachis length in ten of the spe- sociated with smaller leaf sizes (regardless of cies (Table 3). Three of the species (G. ferru- clustered or solitary habit). This trend is well ginea, G. maxima, and G. piscicauda) showed expressed within G. congesta, where bifid leaves no significant variation in leaf division with were significantly smaller than dissected leaves any measures of leaf or plant size. Correlation with respect to rachis length, leaf width, and coefficients were generally low, suggesting that number of plications (P < 0.001; Fig. 2). In- factors other than leaf and plant size influence dividual shoots with bifid leaves also had sig- patterns of leaf dissection. nificantly smaller stem diameters, shorter In species characterized by a gradual onto- crown heights, and shorter stems (P < 0.001; Fig. 2). In species that exhibit both bifid and irregularly dissected leaves at reproductive 80 maturity, bifid leaves did not differ in size from 0 Dissected Leaves dissected leaves (P > 0.05). * Bifid Leaves Crown height within species correlated pos-

CD 60 TABLE 4. Correlations between crown height and stem E diameter within species. Both variables were log-trans- formeda

0 Species r P T40 G. chococola 0.343 ns G. congesta 0.402 <0.001 G. cuneata 0.358 ns G. deversa -0.347 <0.01 E G. divisa 0.314 ns Z 20 G. gracilis 0.499 <0.05 G. epetiolata -0.271 ns G. ferruginea -0.664 <0.01 G. hoffmanniana 0.665 <0.01 G. interrupta 0.887 <0.01

0 G. leptospadix -0.352 ns E E c E E E G. linearis 0.464 ns oC C o E G. longevaginata 0.188 ns G. maxima -0.352 <0.05 e-c c~~o* 2 G. oxyvcarpa -0.013 ns Fig. 2. Differences G. piscicauda -0.515 inleafandplantze<0.01 G. aff. leptospadix -0.318 ns G. aff. oxyvcarpa 0.322 ns of 0G. cogsawthbfd) z 0rdsetd()lae.Broa G. sp. nova 1 0.570 <0.01 Fig. 2. Differences in leaf and plant size between stems G.sp.nova2 -0.313 ns of G. congesta with bifid (51) or dissected (M) leaves. Bars indicate one standard deviation of the mean. ans = P > 0.05.

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CO .S' 4 o S

4 5 6 7 0 0 Ln (Crown Height) 0 co0

00 0

0 0

0 3 -1 0 1 2 G. l .pt.pa.x. o 0 Ln (Stem Diameter) is G. l e Fig. 3. The relationship between rachis length (cm) and stem diameter (cm) among 23 Geonoma species. Values are means for reproductive individuals of each species. Solitary species are indicated by closed circles; open circles are clustered species.

4 5 6 7 itively with stem diameter in only four of the Ln (Crown Height) 20 species examined (P < 0.05; Table 4). In four cases, crown height was negatively cor- Fig. 4. Comparisons between solitary (0) and clustered related with stem diameter, suggesting a re- (O) species with respect to (A) rachis length (cm) and crown height (cm) and (B) stem diameter (cm) and crown height duction in stem diameter associated with later (cm). Values are means for reproductive individuals. In stages of ontogeny or with increasing light lev- both cases, regression lines differ only in their y-intercept. els.

Phyletic variation in size and morphology- species, crown support at increasing heights is Among species, rachis length was highly cor- associated with greater allocation to stem tis- related with stem diameter (r = 0.849; P < sue. In contrast, solitary species did not show 0.00 1; Fig. 3), but only weakly correlated with a significant correlation between crown height crown height (r = 0.413; P = 0.05; Fig. 4A). and stem diameter (r = 0.590; P > 0.05). Simple regression analysis revealed that stem Analysis of covariance for the two growth diameter explained 74% of the variation in forms with crown height as a covariate revealed rachis length, whereas crown height explained that solitary species had significantly larger stem only 17%. Solitary and clustered species did diameters than clustered species for a given not differ with respect to the slope or intercept crown height (F = 38.0; P < 0.005; Fig. 4B; of the regression of rachis length on stem di- Table 5). One exception to this trend is the ameter (Table 4; P > 0.05). These two groups clustered rheophytic species G. linearis, which did differ significantly, however, with respect was intentionally omitted from this regression to relationships between rachis length and analysis because of its unusual habit. This spe- crown height (Fig. 4A). Although the slope of cies is confined to the beds of rapidly flowing the regression lines did not differ significantly, streams and rivers, often growing to flood-level the intercepts did (F = 26.4; P < 0.005; Table (Dransfield, 1978; Galeano Garces and Skov, 5). Solitary species had significantly larger 1989). G. linearis has relatively little inter- leaves than clustered species with the same nodal elongation compared to other clustered crown heights (Fig. 4A). Geonoma species. Another exception is G. lep- Crown height and stem diameter were not tospadix, classified here as a solitary species, significantly correlated among species as a but described as being either solitary or clus- whole (r = 0.21 1; P > 0.05; Fig. 2B). Among tered (Wessels Boer, 1968). Among species, clustering species, however, crown height and rachis length was not correlated with node stem diameter were positively correlated (r = number (P = 0.308), crown size (P = 0.434), 0.842; P < 0.01), suggesting that among these or stem length (P = 0.962).

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TABLE 5. Regression statistics and analysis of covariance among solitary and clustered Geonoma species. All variables were log-transformeda

Dependent variable Independent variable Sample N Slope Intercept

Rachis length Stem diameter All species 23 0.631 3.507 solitary 11 0.650 3.490 clustered 12 0.597 3.512 F 0.048ns 0.000ns

Rachis length Height All species 23 0.415 1.586 solitary 11 0.642 0.731 clustered 12 0.620 0.142 F 0.006ns 26.437*

Stem diameter Height All species 22 0.324 -1.251 solitary 11 0.719 -2.830 clusteredb 11 0.873 -4.832 F 0.141ns 37.990*

ans =P > 0.05; * =P < 0.005. b G. linearis removed from sample.

The number of plications, another measure significantly smaller and narrower leaves and of leaf size, was strongly correlated with stem had smaller stem diameters than species with diameter (r = 0.542; P < 0.01) and with crown dissected leaves (P < 0.01; Table 6). These two height (r = 0.553; P < 0.01). As with rachis groups did not differ with respect to the number length, plication number was not correlated of plications, crown height, stem length, in- significantly with stem length or crown size. A ternode length, or number of nodes (P > 0.05; significant correlation was found, however, be- Table 6). tween plication number and number of nodes (r = 0.525; P < 0.05). Multivariate relationships among species- Leaf form, as described by the number of Results of the bivariate correlation analyses leaf divisions, was weakly correlated with only described above suggest that many leaf and one measure of plant size, crown height, among plant characteristics covary in size. Principal the 17 species with divided leaves (r = 0.594; component analysis was used to examine mor- P < 0.05; Fig. 5). Neither the number of leaf phological patterns of covariation and corre- divisions nor the degree of leaf dissection var- lation among species. Means of reproductive ied significantly with rachis length (P > 0.05). individuals were used to form the correlation Species with bifid leaves, however, did produce matrix. G. macrostachys was omitted from this analysis because it lacks an above-ground stem. 4 Among species, the first principal axis ex- plained 42% of the variation among species. 0 Crown height, number of nodes, number of 0 plications, number of leaf divisions, rachis 3 0 length, and leaf width showed high factor load- 0~~~~~~~~~ ings on the first prinicipal component. Tall >5 2 0 species with large leaves have high, positive scores, whereas short species with small leaves o0 0 have large, negative scores (Fig. 6). The first CD 1 principal axis, therefore, appears to reflect E overall plant size. Z 0 The second principal axis explained an ad- C5o ditional 31% of the total variation among spe-

0 0 cies. Mean internode length and stem length made large negative contributions to the sec- -1. . . . ond principal component, whereas stem di- 4 5 6 7 ameter and rachis length made large positive Ln (Crown contributions. Species with relatively long in- Height) ternodes, narrow stem diameters, and small Fig. 5. Leaf dissection (number of divisions) vs. crown height (cm) among 17 Geonoma species with dissected leaf sizes had a large, negative score on axis 2. leaves at reproductive maturity. Solitary species are in- With the exception of G. linearis, all species dicated by closed circles; open circles are clustered species. with positive scores on axis 2 are solitary.

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TABLE 6. Comparisons between species with only bifid leaves at reproductive stages and species with dissected or bifid leaves. Values are means ? I SD for reproductive individuals, based on species means. Variables were not log- transformed

Variable Bifid (N = 6) Dissected (N 17) P

Lamina width (cm) 15.6 ? 6.8 40.5 ? 15.1 0.001 Stem diameter (cm) 1.1 ? 0.6 2.4 ? 1.4 0.003 Rachis length (cm) 35.2 ? 12.6 58.3 ? 28.4 0.015 Rachis:width ratio 2.4 ? 0.6 1.5 ? 0.6 0.015 Crown height (cm) 186.1 ? 77.0 273.8 ? 126.3 0.066 Number of plications 27.8 ? 7.4 33.2 ? 14.0 0.254 Stem length (cm) 125.6 ? 82.7 171.7 ? 114.1 0.311 Crown size 10.5 ? 2.2 9.8 ? 2.3 0.560 Internode length (cm) 2.7 ? 2.0 3.0 ? 2.1 0.738 No. nodes 50.8 ? 11.8 50.6 ? 26.9 0.973

Principal component axis 2, therefore, effec- gesting that size variation explains a greater tively separates Geonoma species into two ma- amount of total morphological variation with- jor groups; those with unbranched (solitary) in each group separately than among all species vs. branched (clustered) axes. As a group, sol- combined. Factor loadings on axis 1 were sim- itary species have relatively short intemodes ilar in both groups of species; crown height, and larger stem diameters compared to clus- rachis length, stem diameter, and internode tered species, even though crown heights do length had high, positive loadings. Factor load- not differ significantly (Table 7). ings on axis 2 differed, however. Among sol- The third principal axis explained an addi- itary species, crown size, stem length, and tional 10.6% of the total variation. Crown size, number of nodes had large, positive loadings, number of nodes, and stem length had high, whereas stem diameter loaded negatively. negative loadings on axis 3, whereas the num- Among clustered species, crown size and stem ber of leaf divisions and leaf width had high, diameter had large positive loadings, whereas positive loadings. stem length and number of nodes had large, Principal component analysis was also per- negative loadings. formed within each of the two major archi- tectural groups. The percentage of total vari- DISCUSSION ation explained by the first principal axis increased from 42% to 56% and 55% for clus- These analyses portray Geonoma species as tered and solitary species, respectively, sug- variants on a generic theme. The theme is best described as follows: leaf size increases with stem diameter and crown height; bifid-leaved species have relatively small stem diameters and leaf sizes; and in species with dissected

CMi 2 leaves, the number of leaf divisions increases

0 with crown height (Figs. 1, 3, 4A, B; 5; Tables G. lInearls * 2, 3, 6). Within species, this morphological 1 0 theme is an expression of correlated devel- Cli) 0 CD opmental changes in shoot morphology during ontogeny (Troll and Rauh, 1950). Among spe- 0 Coo 0~~~~~ cies, however, the recurrence of these size-re- E lated morphological correlations suggests that 0 1 0 o ? o morphological diversification has been chan- -B 0 0 nelled by the constraints imposed by close de- C.Q o~~~~ 0 velopmental integration within growing shoots. Q -2 0 For example, in 22 of this sample of 23 species,

0 none had a large stem diameter with small leaves or a tall crown with poorly dissected -3 -4 -2 0 2 4 leaves. The one exception is G. linearis, one Principal ofComponent three palm species in the world with a truly Score, Axis 1 rheophytic habit (Dransfield, 1978; Galeano Fig. 6. Principal components analysis of 22 Geonoma species, based on nine log-transformed variables. Solitary Garces and Skov, 1989). This species is rela- species are indicated by closed circles; open circles are tively short with small, finely dissected leaves, clustered species. which offer less resistance to rapidly flowing

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TABLE 7. Comparisons between clustered and solitary Geonoma species. Values are means ? 1 SD for reproductive individuals, based on species means. Variables were not log-transformed

Variable Solitary (N = 11) Clustered (N = 12) P

Intemode length (cm) 1.4 ? 0.6 4.4 ? 1.7 0.001 Stem diameter (cm) 3.0 ? 1.4 1.2 ? 0.6 0.002 Stem length (cm) 103.1 ? 86.0 211.5 ? 100.3 0.011 Rachis length (cm) 66.8 ? 28.6 38.9 ? 17.7 0.013 Rachis: width ratio 2.0 ? 0.6 1.4 ? 0.7 0.044 Crown size 10.9 ? 1.9 9.2 ? 2.3 0.070 Number of plications 36.4 ? 14.3 27.6 ? 9.8 0.107 Crown height (cm) 217.1 ? 114.6 281.8 ? 122.0 0.205 Number of divisions 6.1 ? 8.1 8.5 ? 9.5 0.523 Lamina width (cm) 36.2 ? 17.5 31.9 ? 17.8 0.565 Numberofnodes 51.4? 29.3 49.9? 18.3 0.886

water (Galeano Garces and Skov, 1989; R. area per genetic individual. Moreover, clus- Chazdon, personal observation). Within the tered species support their crowns at the same genus, solitary and clustered species illustrate height as solitary species with a lower biomass two subthemes (Fig. 6). As a group, solitary investment in stem tissue per individual shoot. species have larger stem diameters, greater leaf As a consequence of producing cane-like stems, sizes, and longer stems (Table 7). Within each individual shoots of clustering species are often of these architectural groups, the generic theme mechanically unstable (King, 1987), and are still applies (Figs. 3, 4A, B, 5). Does this pattern most susceptible to stem breakage (Chazdon, reflect two distinct evolutionary lineages with- personal observation). Both growth forms ap- in Geonoma, or morphological convergence pear almost equally represented among Geon- within different phylogenetic lineages? oma species (Wessels Boer, 1968), suggesting Holttum (1955) considered palms with un- that either growth form can be successful in branched (monopodial) axes to be derived from light-limiting forest understory habitats. branched (sympodial) axes. He further noted The tendency for unbranched axes to be that in monocot genera where some species are thicker than branched axes, with relatively clustered and others solitary, the solitary spe- larger leaves, has become known as Corner's cies have thicker axes, made possible by the rule (Halle, Oldeman, and Tomlinson, 1978), suppression of basal lateral buds. Geonoma although Sinnott (1921) first proposed that leaf illustrates this pattern very well. Corner (1964) size should be correlated with the size of the pointed out that the thickest palm axes are primary axis. Large leaves require a larger axis unbranched, whereas more slender axes are for increased vascular supply as well as me- more likely to be branched. In contrast to Holt- chanical support (Givnish, 1979). Leaf size in tum's view, he proposed (Corner, 1966) that the South African shrub genus Leucadendron the evolution of branched palms from un- increased with stem diameter (Boyd and Midg- branched species was accompanied by decreas- ley, 1988). In extensive surveys of evergreen es in plant size. Although the derived or an- and deciduous woody plants, White (1983a, b) cestral nature of the solitary habit is not clear, showed that leaf size was positively correlated the evolutionary transition between the soli- with the cross-sectional area of first-year twigs. tary or clustered habit seems to have occurred The lack of above-ground branching in palms numerous times within Geonoma; solitary and focuses attention on the diameter (or cross- clustered species, including those in this study, sectional area) of the primary axis as the major are scattered throughout the subgeneric groups controller of leaf size and, potentially, form. defined by Wessels Boer (1968). Thus, con- Within and among species, the bifid leaf form vergence appears to be responsible for the sim- is strongly associated with small stem diameter ilarities found here among species within each and small leaf size (Table 6; Fig. 2). In Geon- growth form. oma and the closely allied genus Asterogyne, Several important ecological consequences bifid leaves become increasingly costly to sup- stem from the morphological differences be- port, in terms of biomass, as leaf size increases tween solitary and clustered species. Solitary (Chazdon, 1 986a). Larger, dissected leaves have species support a larger crown area than in- relatively short petioles, and the leaf center of dividual stems of clustered species at the same mass is located relatively closer to the leaf in- height within the forest understory. Clustered sertion point, thereby reducing bending mo- species, however, produce multiple crowns, and ments. Consequently, the maximum leaf size therefore may have a greater light intercepting that is mechanically stable is greater for dis-

This content downloaded from 203.29.104.10 on Tue, 23 May 2017 01:17:51 UTC All use subject to http://about.jstor.org/terms 692 AMERICAN JOURNAL OF BOTANY [Vol. 78 sected leaves than for bifid leaves (Chazdon, were positively correlated (Table 4). Among 1 986a). Among Geonoma species studied here, clustered species, crown support at increasing the maximum mean rachis length for bifid- heights is associated with greater allocation to leaved species was 57 cm, whereas the maxi- stem tissue. In contrast, crown height and stem mum mean rachis length for species with dis- diameter were not correlated among solitary sected leaves was 103 cm. Within G. congesta, species. Whether Geonoma species differ phy- maximum rachis length for bifid leaves was 60 letically or ontogenetically in mechanical prop- cm, whereas maximum rachis length for dis- erties of stem tissue remains to be explored. sected leaves was 100 cm. These results suggest Based on the data presented here, phyletic that, within Geonoma, species capable of pro- and ontogenetic variation in leaf size and leaf ducing only bifid leaves may be more size- form is strongly associated with variation in limited than species capable of producing dis- plant size. To examine the developmental basis sected leaves. Morphological trends within for these integrated patterns of shoot devel- Geonoma, however, are not universal among opment, detailed studies of leaf and shoot de- palms. Marojejya darianii, a palm from mon- velopment are required. However, morpho- tane swamps in Madagascar, has bifid leaves logical differences among species cannot be 7-9 m in length (Dransfield and Uhl, 1984). explained on the basis of variation in shoot The species examined exhibited a great deal size alone. Interpretations of the principal of unexplained variation in leaf size and dis- component analysis indicate that overall plant section (Tables 2, 3). In some cases, low or size accounted for only 55% and 56% of the insignificant correlations may have resulted total variation observed among solitary and from under-sampling of seedlings and juvenile clustered species, respectively. Unlike the plants. In other cases, such as G. cuneata, and striking examples of heterochrony observed in G. chococola, variation in leaf dissection is a many vertebrate and invertebrate taxa (Gould, truly unexplained phenomenon; in these two 1977), variation among Geonoma species con- species, weakly dissected and bifid-leaved in- not easily be explained by acceleration or re- dividuals were often observed growing in prox- tardation of a postulated ancestral ontogenetic imity, under similar environmental condi- pathway. Tomlinson (1979) proposed that habit tions. The genetic basis for these variations in evolution of monocotyledonous taxa could leaf dissection has not been explored. have involved precocious flowering ofjuvenile Patterns of variation in size and morphology stages or phylogenetic modification of post- within species differed in several key respects seedling phases of development. Corner (1966) from patterns of variation among species. Lit- considered the evolution of dwarf palms with tle variation was observed in mean crown size bifid or poorly dissected leaves, such as many among species, yet crown size varied greatly Geonoma species, as a clear case of neoteny. within species (Table 2). Crown development Neoteny has also been discussed with respect occurs gradually during ontogeny; increases in to evolution within the endemic Madagascan the number of leaves in the crown are accom- palm genera Neophloga and Dypsis (Guillau- panied by primary thickening growth of the met, 1973). stem as well as increases in leaf size. It appears The results of this study do not address spe- that, phyletically, crown size is uncoupled from cifically the issue of heterochrony as an evo- leaf size and stem diameter. lutionary mechanism, but do suggest that evo- Crown height in relation to stem diameter lutionary changes in plant size can have major did not vary consistently within or among spe- implications for morphological variation with- cies. Within 1 1 Geonoma species, crown height in a taxon. Whether these changes in shoot size was not significantly correlated with stem di- are correlated with variation in the diameter ameter (Table 4). In the canopy palm Welfa of the shoot apex or the size of individual leaf georgii, another member of the Geonomeae, primordia within and among species remains stem diameter at breast height did not increase to be determined. These results offer partial with plant height (Rich et al., 1986). Rather, support for the initial hypothesis that phylo- mechanical stability of taller individuals in- genetic changes in plant size at reproductive creases via changes in stem tissue stiffness (Rich, maturity have facilitated adaptive radiation of 1987). Four species showed a negative corre- Geonoma species within rain forest understory lation between crown height and stem diam- habitats. A more conclusive test of this hy- eter; these are all clustered species that exhibit pothesis must wait until phylogenetic relation- pronounced taper in stem diameter along the ships among species are better understood. length of their stems. Not all clustered species Phylogenetic changes in leaf size and mor- shown pronounced stem tapering, however; in phology have clearly involved additional fac- G. congesta, crown height and stem diameter tors other than variation in plant size. The

This content downloaded from 203.29.104.10 on Tue, 23 May 2017 01:17:51 UTC All use subject to http://about.jstor.org/terms May 1991] CHAZDON-PLANT SIZE AND FORM IN GEONOMA 693 possibility exists that morphological features GENTRY, A. H. 1986. Notes on Peruvian palms. Annals independent of leaf and plant size evolved of the Missouri Botanical Garden 73: 158-165. GIVNISH, T. W. 1979. On the adaptive significance of within species following initial divergence in leaf form. In 0. T. Solbrig, S. Jain, B. G. Johnson, size. Throughout this paper, I have assumed and P. H. Raven [eds.], Topics in plant population that relationships between size and morphol- biology, 375-407. Columbia University Press, New ogy within species reflect ontogenetic patterns York. and underlying developmental constraints on GOULD, S. J. 1977. Ontogeny and phylogeny. Harvard form, rather than plastic responses to envi- University Press, Cambridge, MA. GUILLAUMET, J. L. 1973. Une nouvelle espece de palmier ronmental conditions. To the extent that this nain de Madagascar. Adansonia 13: 341-349 (Series assumption is warranted, size relationships can 2). provide insight into developmental constraints HALLt, F., R. A. A. OLDEMAN, AND P. B. TOMLINSON. on morphology at different taxonomic levels. 1978. Tropical trees and forests: an architectural analysis. Springer-Verlag, New York. HOLTTUM, R. E. 1955. Growth-habits of monocotyle- LITERATURE CITED dons-variations on a theme. Phytomorphology 5: 399- 413. KAPLAN, D. R., N. G. DENGLER, AND R. E. DENGLER. BALL, E. 1941. The development of the shoot apex and 1982. The mechanism of plication inception in palm ofthe primary thickening meristem in Phoenix canari- leaves: problem and developmental morphology. Ca- ensis Chaub. with comparisons to Washington filifera nadian Journal of Botany 60: 2939-2975. Wats. and Trachycarpus excelsa Wendl. American KING, D. A. 1987. 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