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Ecology |
Department of Biology, Bucknell University, Lewisburg, Pennsylvania, USA; and Archbold Biological Station, Lake Placid, Florida, USA
Received for publication August 4, 2006. Accepted for publication June 6, 2007.
ABSTRACT
Plants of nutrient-poor, arid environments often have leaf traits that include small size, sclerophylly, long life span, low nutrient concentration, and low photosynthetic rate. Hence, the success of two large-leaved palmettos in peninsular Florida's seasonally xeric, nutrient-impoverished uplands seems anomalous, given that their leaves are orders of magnitude larger than the leaves of sympatric species. An examination of a 16-yr data set of leaf traits and leaf life spans across four vegetative associations differing in available light showed that Serenoa repens and Sabal etonia had low rates of leaf production coupled with long leaf life spans reaching 3.5 yr in heavily shaded plants. The adaptation of these palmettos to xeric, nutrient-poor habitats has generated dwarf statures, diminished leaf sizes and numbers, increased leaf life spans, and reduced rates of leaf production relative to other palms and congeners of more mesic sites. Leaf and petiole size, plant leaf canopy area, and leaf life span increased in both palmettos with decreasing available light, helping to compensate for reduced photosynthetic rates under shaded conditions and for the high leaf construction costs of the large, thick palmetto leaves. Large leaf size in these palmettos, likely due to phylogenetic conservatism, is compensated by other leaf traits (e.g., heavily cutinized epidermises, thick laminas) that increase survival in seasonally xeric, nutrient-impoverished environments.
Key Words: Arecaceae • Florida • leaf life span • leaf number • leaf size • light availability • plant size • Sabal etonia • Serenoa repens • xeric scrub vegetation
Vascular plant species differ markedly in the sizes, numbers, and life spans of their leaves, and such differences are thought to contribute to the coexistence of sympatric plant species (Sterck et al., 2006
). Leaf numbers and sizes range from numerous, tiny scale leaves in some species such as those of Thuja, to plants with few, but massive leaves as in Raphia regalis, an African palm with leaves that can exceed 25 m in length (Henderson, 2002
). Leaf life spans also vary from as little as 15 d to at least 45 yr (Chabot and Hicks, 1982
; Kikuzawa, 1991
).
Attempts during the past three decades to explain such differences in leaf design have focused on regulation of leaf temperature and water-use efficiency (e.g., Parkhurst and Loucks, 1972
), dry mass, transpirational costs, and photosynthetic gains (e.g., Givnish and Vermeij, 1976
; Givnish, 1979
, 1986
; Chabot and Hicks, 1982
; Thornley, 1991
; Kikuzawa, 1995
), trade-offs of leaf size and leaf support (e.g., Pearcy et al., 2004
, 2005
; Sun et al., 2006
; Niinemets et al., 2006
), influences of environment and climate (e.g., Karlsson, 1992
; Reich et al., 1992
; Ackerly et al., 2002
; Pickup et al., 2005
), and evolutionary adaptation and convergence (e.g., Ackerly and Reich, 1999
; Ackerly et al., 2000
; Ackerly, 2004
). These efforts have identified a spectrum of leaf-trait strategies, which produce varying rates of return on nutrient and biomass investments in leaves (e.g., Reich et al., 1997
; Wright et al., 2004
, 2005
; Shipley et al., 2006a
, b
; Whitfield, 2006
).
Among the several factors known to influence leaf-trait strategies, climate appears to have only a modest influence when considered across biomes (Reich et al., 1997
; Wright et al., 2004
). Even so, a distinctive pattern in leaf-trait variation occurs under arid or semi-arid climatic conditions, especially when arid climates are coupled with nutrient-impoverished soils. Plant adaptation in such environments has produced leaf traits that often include small size, sclerophylly, high mass per unit area, low concentration of nutrients, low photosynthetic rate, and long life span (e.g., Turner, 1994
; Reich et al., 1997
; Cunningham et al., 1999
; Wright et al., 2004
, 2005
). As expected, peninsular Florida's seasonally xeric, nutrient-impoverished, upland associations are characterized by plants that possess such leaves as evidenced by the abundant small-leaved, sclerophyllous oaks (e.g., Quercus geminata, Q. minima, Q. myrtifolia, Q. inopina), ericaceous shrubs (e.g., Lyonia ferruginia, L. fruticosa, Vaccinium myrsinites, Gaylussacia dumosa), and narrow endemics (e.g., Ceratiola ericoides, Prunus geniculata, Bumelia tenax) (Abrahamson et al., 1984
; Abrahamson and Hartnett, 1990
; Myers, 1990
; Menges, 1999
).
The frequency with which small leaves occur among the plants native to these xeric, nutrient-impoverished uplands makes the success of the native, large-leaved palmettos (low-growing palms with fan-shaped leaves) seem anomalous. Serenoa repens (hereafter referred as Serenoa) and Sabal etonia (hereafter Sabal) have few leaves compared to sympatric species, and those leaves are exceptionally large compared to the small-leaved flora.
This paper examines the leaf traits of sympatric Serenoa and Sabal populations in four upland, nutrient-impoverished vegetative associations of peninsular Florida's Lake Wales Ridge that differ markedly in overstory canopies and hence in light availabilities. The relationships between Serenoa and Sabal leaf traits and light availability is assessed given that competition for light is vital to the coexistence of sympatric plants in light-limited environments and trade-offs in leaf traits may facilitate persistence in different light environments. Such trade-offs, in turn, can influence plant performance and the ability of plant species to coexist (e.g., Pearcy et al., 2004
, 2005
; Passarge et al., 2006
; Poorter et al., 2006
; Sterck et al., 2006
).
Using a 16-yr data set of leaf sizes, numbers, and life spans, this paper examines the seeming anomaly of large leaves among a small-leaved flora and explores possible reasons why large-leaved palmettos are so successful in arid, nutrient-impoverished Florida uplands. In doing so, four fundamental questions concerning patterns of leaf-trait variation are asked. (1) How do palmetto leaf traits differ from those of sympatric plants? (2) Has palmetto adaptation to seasonally xeric, nutrient-impoverished habitats entailed a decrease in leaf size and an increase in leaf life span? (3) How do palmetto leaf traits vary across differing light environments? (4) How do palmetto leaf traits allow these plants to persist in seasonally xeric, nutrient-impoverished environments?
MATERIALS AND METHODS
Study organisms
Leaf traits were examined in two shrublike, low-growing, fan-leaved palms (palmettos): Serenoa repens (Bartr.) Small (saw palmetto) and Sabal etonia Swingle ex Nash (scrub palmetto). Both species are members of the subfamily Coryphoideae within the family Arecaceae (Uhl and Dransfield, 1987
; Dransfield et al., 2005
). Serenoa, a 1–3 m tall plant with palmate leaves and creeping stems that sometimes become erect, is North America's most abundant palm, occurring on the coastal plain from South Carolina to Louisiana (Corner, 1966
; Hilmon, 1969
; Henderson et al., 1995
). Sabal, an up to 1.2 m tall plant with costapalmate (palmate with a short rachis) leaves and subterranean stem, is a narrow, "insular" endemic that is sympatric with Serenoa on the white or yellow sands of uplands within peninsular Florida's central and Atlantic coast ridges (Zona and Judd, 1986
; Zona, 1987
, 1990
). Individuals of both species are long lived; time-series growth-rate measurements on Serenoa indicate that many Serenoa are greater than 500 yr old (Abrahamson, 1995
).
Serenoa and Sabal have several overlapping characteristics including habitat preference, low, shrublike growth form, reproductive phenology, and recovery after fire. Both tolerate severe drought and thrive in fire-prone environments (Abrahamson and Abrahamson, 2002
). Each has strong postfire flowering responses and a rapid postfire regeneration of leaf canopies (Abrahamson, 1995
, 1999
; Abrahamson and Abrahamson, 2006
; Carrington and Mullahey, 2006
).
Study site
The Archbold Biological Station (ABS) (27°12' N, 81°21' W) is located near the southern terminus of the Florida peninsula's Lake Wales Ridge, 12 km south of the town of Lake Placid. The climate is characterized by hot, wet summers and mild, dry winters. The highest monthly mean temperature (27.5°C) occurs in August and the lowest (16°C) in January. Long-term mean annual rainfall is 1335 mm, of which 796 mm (60%) falls during a 4-mo wet season (June through September) (Layne and Abrahamson, 2004
). Paleo sandhills, beach ridges, and sand dunes create a rolling topography (Brooks, 1981
) with elevations within the study area ranging from 38 to 61 m above mean sea level (U.S.G.S. Childs, Florida, 7.5' quadrangle). The vegetation of the study area is appropriately considered as "old growth" because it has experienced little anthropogenic disturbance.
Individuals of both palmettos occur in a range of vegetative associations from low-elevation flatwoods with poorly drained to somewhat poorly drained, sandy soils to high-elevation southern ridge sandhill (hereafter referred to as sandhill) with excessively well-drained sands. Between these extremes, palmettos occur in scrubby flatwoods, a transitional association between flatwoods and sand pine scrub (hereafter referred to as scrub), and, at slightly higher elevations relative to the water table, in scrub (Abrahamson, 1995
, 1999
).
Flatwoods range from open savanna-like associations dominated by Serenoa and scattered pines (Pinus elliottii var. densa or P. palustris) to associations of dense pine stands with thick understories of Serenoa and other shrubs (most notably ericaceous shrubs). Sabal is an occasional element of the flatwoods flora. The overstory canopies above palmettos in the flatwoods are generally open with high light availability. Mean stand overstory coverage of examined flatwoods varied from a low of 7% to a high of 28%. Overstory coverage data are from Abrahamson (1999)
, who measured canopy area coverage in the north, east, south, and west horizons at the top of each individual palmetto's canopy using a Paul Lemmon Model C forest densiometer (Robert E. Lemmon Forest Densiometers, Bartlesville, Oklahoma, USA). These four values were averaged for each palmetto, then averaged for each association.
Scrubby flatwoods are a low (1–2 m) shrubby association dominated by evergreen, xeromorphic oaks, ericaceous shrubs, and abundant Serenoa and Sabal. Canopy-forming pines (P. elliottii var. densa and P. clausa) are widely scattered, creating open overstories (3–19% cover), that allow a high amount of light into palmetto canopies. In contrast, long-unburned scrub has a nearly closed overstory (85–90% cover) of even-aged sand pine (P. clausa) and small trees and shrubs. Recently burned scrubs at ABS averaged 51–56% overstory cover. Sandhills have an overstory of scattered pine (P. elliottii var. densa and P. clausa) and shrubby trees, with overstory cover above palmettos averaging 67–71% in long-unburned sandhill but only 14–18% in recently burned stands.
These four vegetative associations have acidic, nutrient-impoverished, sandy soils that seasonally have low water availability (Table 2). The percentage of water by mass in the upper 25 cm of soil can reach very low levels, for example, ranging from only 0.1 to 2.3% in the extremely well-drained soils of sandhills, scrub, and scrubby flatwoods during the latter parts of some dry seasons (April). More details of these vegetative associations are available in Abrahamson et al. (1984)
, Abrahamson and Hartnett (1990)
, Myers (1990)
, and Menges (1999)
.
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, Chazdon (1986)
, and Cunningham (1997)
as Y = L/N, where L = the number of living, fully expanded leaves in the crown and N = the number of new leaves produced during that year. This leaf life span estimate does not include "nonworking" leaf development time before leaf expansion, which according to Corner (1966)
would roughly double the life spans reported in this paper.
Dry mass (including all aboveground organs as well as all living belowground rhizome and adjacent roots, but excluding deep roots) and leaf and petiole dimensions of each of the 940 "census" palmettos were estimated using regressions developed from 33 destructively harvested plants (Serenoa = 17 and Sabal = 16; Table 3). These harvested palmettos, which represented a range of adult sizes, were collected from scrubby flatwoods and scrub, the associations that typically harbor the smallest and largest adult palmettos, respectively (Abrahamson, 1995
, 1999
). Individual leaf areas for each "census" palmetto were approximated as a triangle, based on leaf length and width. This approximation slightly overestimates leaf area. Nevertheless, because leaf shapes are similar across a range of sizes and between species, the estimate is useful and reliable for comparative purposes.
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) and the old-growth nature of the study area (Abrahamson et al., 1984
). Consequently, plant sizes and leaf traits in this study represent ranges that are, for the most part, in equilibrium with the environmental conditions.
Data analyses
SPSS version 14 was used for analyses (SPSS Inc., Chicago, Illinois, USA). Regressions were performed using either the linear regression or curve-fitting procedures. Analyses of variance used the general linear model procedure, and two-way ANOVA models used species and association as fixed factors. The Student–Newman–Keuls (SNK) post-hoc procedure was used to determine significance among associations when association was significant. When multiple correlations were performed, a sequential Bonferroni test was used to ensure that the table-wide significance of correlations was at least P = 0.05 (Rice, 1989
).
RESULTS
Overstory canopies, leaf-blade size, and leaf number
The canopies created by the shrubs and trees that overtopped palmettos varied significantly by vegetative association (F3,432 = 475.1; P < 0.001). Canopy above individual palmettos averaged only 10% ± 1.6 (SE) in scrubby flatwoods but increased to 20% ± 1.6 in flatwoods, 69% ± 1.8 in sandhill, and reached a high of 88% ± 1.8 in scrub. A SNK post-hoc procedure showed that the overstory of each association was significantly different from those of the other three associations. Overall, there was no significant difference in the overstory canopies of Serenoa and Sabal (F1,432 = 2.4; P = 0.121), and there was no significant interaction of species and association (F3,432 = 0.5; P = 0.669).
A two-way ANOVA using vegetative association and palmetto species as main effects found that the largest leaves, measured by individual leaf-blade area, length, or width, for both palmettos occurred in nearly closed-canopied scrub and the smallest occurred in open-canopied scrubby flatwoods (Fig. 1; Table 4). A SNK post-hoc procedure showed three significant subsets of associations for leaf-blade area, length, and width; scrubby flatwoods and scrub palmettos were each significantly different from all other associations for these three leaf traits, but the intermediate-sized leaves of palmettos in flatwoods and sandhill did not differ significantly from one another. Leaf blades of Sabal were significantly larger than those of Serenoa (Tables 4 and 5). The association by species interaction was significant for individual leaf area and leaf width because of differences in the rank order of mean widths for the two species in sandhill and flatwoods.
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Palmetto leaf canopies and plant mass
Leaf areas per palmetto varied significantly across associations such that canopy leaf areas per palmetto were highest for both species in nearly closed-canopied scrub and lowest in open-canopied scrubby flatwoods (Fig. 1; Tables 4 and 5). A SNK post-hoc procedure found that canopy leaf areas per palmetto for scrub and scrubby flatwoods palmettos were each significantly different from canopy leaf areas of palmettos in all other associations; however, canopy leaf areas of palmettos in flatwoods and sandhill were not significantly different. There was a significant species by association interaction due to the differences between species in the rank order of canopy areas for sandhill and flatwoods palmettos. In spite of the striking across-association variation, canopy leaf areas per palmetto were not significantly different between the two species because the larger leaf-blade areas of Sabal were counterbalanced by canopies composed of fewer leaves (Fig. 1; Tables 4 and 5).
Combined aboveground and belowground dry mass of palmettos differed significantly across vegetative association and between species (Fig. 1; Tables 4 and 5). Both species attained their largest mean dry mass in heavily shaded scrub and their smallest mean mass in open-canopied scrubby flatwoods. Serenoa species were larger than Sabal across all associations, and the increase in mean mass from scrubby flatwoods to scrub was greater for Serenoa than Sabal. Mean masses of scrub and scrubby flatwoods palmettos were each significantly different from the mean mass of palmettos in other associations; however, mean masses of palmettos in flatwoods and sandhill did not differ significantly according to a SNK post-hoc procedure. The masses of the two species differed in the rank order of sandhill and flatwoods, which resulted in a significant species by association interaction.
In the stepwise multiple regression, three variables were significant predictors of the total leaf-canopy area of a palmetto: (1) the palmetto's dry mass, (2) the amount of tree and shrub canopy cover above a given palmetto, and (3) the vegetative association in which the palmetto occurred. The Serenoa model (r2 = 0.87) predicted total leaf-canopy area per palmetto = 4.3 plant dry mass + 65.5 overhead canopy + 957.9 association – 2245, while the Sabal model (r2 = 0.76) predicted total leaf-canopy area per palmetto = 14.6 plant dry mass + 99.7 overhead canopy + 1346.0 association – 16 196. Association was categorized as scrubby flatwoods (5), flatwoods (6), sandhills (2), and scrub (4). Sites 1 and 3 for burned sandhills and scrub, respectively, were not used in the regression to avoid overweighting the regression with palmettos from sandhill and scrub and because of the marked alteration of canopy cover due to fire.
The amount of overstory was significantly related to leaf-blade area and total leaf-canopy area per palmetto (Fig. 2). Areas of palmetto leaves and canopies increased significantly for both species as shade increased in spite of the considerable variation in leaf and canopy sizes due to differences in plant dry mass within and across associations. Furthermore, the relationships between leaf-blade area and overstory, and between total leaf-canopy area per palmetto and overstory above palmettos, were similar for both species despite the greater average leaf-blade size of Sabal than Serenoa.
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Leaf petiole length and leaf size
The longest mean leaf petioles (52 ± 2 and 60 ± 2 cm for Serenoa and Sabal, respectively) occurred in heavily canopied scrub where they were associated with more massive palmettos possessing larger leaf blades. The shortest mean leaf petioles (27 ± 1 and 32 ± 1 cm for Serenoa and Sabal, respectively) were in open-canopied scrubby flatwoods and were linked to small palmettos with small leaf blades (Table 4). A SNK post-hoc procedure showed three significant subsets of associations for petiole lengths; scrubby flatwoods and scrub palmettos each differed significantly from all other associations, but palmettos in flatwoods and sandhill did not differ significantly from one another.
Petiole lengths were significantly longer for Sabal than Serenoa and mirrored the patterns for leaf traits across associations (Tables 4 and 5). Thus, as expected, petiole lengths were strongly and positively correlated with leaf lengths (r = 0.98 and r = 0.99; P < 0.001 for Serenoa and Sabal, respectively), leaf widths (r = 0.87 and r = 0.94; P < 0.001 for Serenoa and Sabal, respectively), and total leaf-canopy areas (r = 0.90 and r = 0.96; P < 0.001 for Serenoa and Sabal, respectively).
Leaf life span, overstory, and leaf production
Leaf life span was strongly and positively related to overstory. Leaf life spans ranged from less than 2 yr for palmettos with no overstory to 3.5 yr for palmettos growing under nearly complete overstory canopies (Fig. 3). Consequently, mean leaf life spans were shorter for palmettos growing in open-canopied flatwoods (2.2 ± 0.03 yr) and scrubby flatwoods (2.3 ± 0.03 yr) and longer for palmettos of greater-canopied sandhills (2.6 ± 0.03 yr) and scrub (2.8 ± 0.03 yr; Fig. 4). A SNK post-hoc test showed that the mean leaf life span within each association was significantly different from each of the other associations. The larger, and possibly more costly, leaves of Sabal (Zona, 1990
) lived significantly longer than those of Serenoa (Tables 4 and 5; Fig. 4).
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DISCUSSION
How do palmetto leaf traits differ from those of sympatric plants?
Palmetto leaves were roughly 10 times longer and had two to three orders of magnitude more area than the leaves of sympatric plants in peninsular Florida's seasonally xeric, nutrient-impoverished uplands. For example, the mean leaf-blade length and individual leaf-blade area was 
74 cm and 2300 cm2 for Serenoa and 92 cm and 4200 cm2 for Sabal. These values were strikingly greater than the 5–10 cm leaf lengths and 4–13 cm2 leaf areas for Asimina tetramera, 4–8 cm and 4–6 cm2 for Quercus inopina, 3–7 cm and 1–9 cm2 for Q. geminata, 2–7 cm and 3–8 cm2 for Q. minima, 2–5 cm and 2–5 cm2 for Q. myrtifolia, 2–7 cm and 1–7 cm2 for Lyonia fruticosa, 0.5–1.5 cm and 
1 cm2 for Ceratiola ericoides, 1.0–1.5 cm and <1–1 cm2 for Prunus geniculata, and <1.5 cm and <1 cm2 for Vaccinum myrsinites (Ward, 1979
; Clewell, 1985
; Flora of North America Editorial Committee [FNA], 1997
; W. G. Abrahamson, unpublished data). Furthermore, Serenoa and Sabal bore very few leaves (means of 7.8 and 4.1, respectively) compared to sympatric plants. Depending on the vegetative association and plant mass, Serenoa maintained 7–9 leaves and Sabal 3–5 leaves.
Serenoa and Sabal's few, large leaves are clearly unusual amongst a flora composed of plants with numerous, small leaves. Hence, the success of palmettos in this seasonally xeric, nutrient-impoverished environment seems anomalous given their leaf traits and the expectations for leaf traits of plants adapted to such settings (e.g., Turner, 1994
; Reich et al., 1997
; Wright et al., 2004
, 2005
; Pickup et al., 2005
; Niinemets et al., 2006
; Whitfield, 2006
). So why are the leaves of Serenoa and Sabal so much larger than those of sympatric plants? The most likely explanation is the constraint imposed by their phylogeny. Palms have some of the largest leaves known (Henderson, 2002
), and studies such as those of Ackerly and Donoghue (1998)
and Ackerly (2004)
have demonstrated that leaf traits are phylogenetically conserved.
Has palmetto adaptation to seasonally xeric, nutrient-impoverished habitats entailed a decrease in leaf size and an increase in leaf life span?
As large as Serenoa and Sabal leaves are relative to sympatric species, both palmettos are small and bear only a few diminutive (for palms) leaves. For example, the leaves and stature of the monotypic Serenoa are markedly reduced relative to palms in general and relative to other coryphoid palms that are adapted to more mesic and/or more nutrient-rich sites (Tomlinson, 1961
, 1990
; Henderson, 2002
). Only three Sabal species of this 15- or 16-species, Caribbean-basin genus have dwarf statures: S. etonia, S. minor, and S. miamiensis (Zona, 1990
; FNA, 2000
; Henderson, 2002
). Sabal etonia is the smallest and fewest-leaved member of the genus (Zona, 1990
; Henderson, 2002
) and appears better adapted to seasonally xeric, nutrient-impoverished conditions than other Sabal species (Zona, 1990
). Sabal minor, which ranges from North Carolina to Texas in somewhat more mesic environments than S. etonia, bears larger leaves (Zona, 1990
) and more leaves (4–10 leaves; Zona and Judd, 1986
; Zona, 1987
, 1990
; FNA, 2000
) than S. etonia. Similarly, S. miamiensis (which may not be a distinct species from S. etonia; Zona, 1990
; FNA, 2000
) sustains more leaves and has larger petioles and leaf blades than S. etonia (Zona, 1985
; FNA, 2000
). Furthermore, the stems of the Florida native S. palmetto, the closest relative of S. etonia and S. miamiensis, grow to 20 m, and the leaves of this species reach 3 m in length (Zona, 1990
).
A number of authors (e.g., Chabot and Hicks, 1982
; Coley, 1988
; Williams et al., 1989
; Reich et al., 1991
, 2004
; Wright et al., 2004
; Poorter and Bongers, 2006
) have argued that leaves of plants in nutrient-impoverished and/or low-light environments should live longer than leaves of plants in nutrient-rich and/or high-light environments to compensate for inherent low rates of photosynthesis and relatively high leaf construction costs. Leaves of Serenoa in seasonally xeric, nutrient-impoverished Florida uplands of the Lake Wales Ridge had longer life spans (mean = 2.4 yr) than previously reported for S. repens growing in Florida's more mesic, coastal-plain flatwoods (1.5–2.0 yr; Hilmon, 1969
). Similarly, S. etonia had a much longer life span (mean = 2.5 yr) than the just over 1-yr life span reported for S. minor (Hesse and Conner, 1996
). The longer leaf life span of Sabal etonia vs. S. minor may compensate for higher leaf construction costs given that the leaves of S. etonia are considerably thicker and probably more costly than those of S. minor (Zona, 1990
).
However, the life spans of Serenoa and Sabal leaves are not exceptional among understory palms, suggesting that many palms can compensate for slow leaf production caused by low light and/or nutrient-impoverished soils. Three Costa Rican understory palms have leaf life spans that range from 2.0 yr in seedling leaves to 3.5 yr for leaves of mature plants (Chazdon, 1986
). The leaves of other understory palms have life spans of 2 yr (Chamaedorea bartlingiana, Ataroff and Schwarzkopf, 1992
), <3 yr (Geonoma congesta, Chazdon, 1992
), and up to 5 yr (Calyptrogyne ghiesbreghtiana, Cunningham, 1997
).
How do palmetto leaf traits vary across differing light environments?
Similar patterns of leaf-trait variation were identified in Serenoa and Sabal across vegetative associations of varying light availability. For both species, the largest leaf blades, longest petioles, and the greatest plant leaf canopy areas occurred in nearly closed-canopy scrub, while the smallest leaf blades, shortest petioles, and least plant leaf-canopy areas occurred in open-canopied scrubby flatwoods. Leaf-blade areas increased roughly 2.3-fold from scrubby flatwoods to scrub. These increases in leaf-blade size and plant canopy area with diminished light should increase plant photosynthetic gains in shaded environments. Indeed, both species had positive relationships between overstory cover and plant mass; mean masses were smallest in open-canopied scrubby flatwoods and greatest in heavily shaded scrub. Accordingly, Serenoa and Sabal had similar positive relationships of petiole length, leaf-blade size, and plant leaf canopy area with plant mass. For both species, the same three variables (plant dry mass, amount of overhead canopy cover, and the vegetative association) were the best predictors of palmetto leaf-canopy area.
For both palmettos, leaf life span was inversely related to light availability. Thus, leaf life spans were frequently <2 yr for palmettos under little to no overstory but reached 3.5 yr for palmettos under near-closed overstory, an increase of roughly 30%. Such enhanced leaf life spans when coupled with expanded leaf-blade areas and larger plant canopies suggest that Serenoa and Sabal effectively compensate for reduced light availability under increased overstory canopies. However, both species had a curvilinear relationship between overstory cover and leaf production, suggesting that partial overstory canopies may provide more optimal environments for these palmettos than either full-sun or full-shade environments.
A less obvious compensation for low light availability was the proportionate increase in petiole lengths with increases in the areas of the fan-shaped leaves. Both palmettos had strong positive relationships between petiole lengths and leaf areas, and mean petiole lengths for both species nearly doubled from open-canopied scrubby flatwoods to nearly closed-canopied scrub. Although such increases in petiole length with increased leaf area may result from genetic correlations of these traits, the consequence is that leaf overlap and self-shading are minimized, which may enhance the efficiency of light capture in shaded palmettos. Elongation of petioles with increased leaf area also may lengthen leaf life span given that leaf senescence is typically a function of leaf position within the canopy and hence shading rather than simply a function of leaf age (Ackerly, 1999
).
The relationship between petiole length and blade area is important in palmettos because petiole growth elevates the crowns of these acaulescent species above neighboring shrubs (DeCarvalho et al., 1999
). Leaf positions in Serenoa and Sabal change from nearly vertical in young, fully expanded leaves to horizontal in more mature leaves and subsequently to pendant in senescent leaves. Longer petioles increase the radius of the crown, which enables small understory species to intercept light over a larger area (Poorter et al., 2006
) and reduce shading by neighboring plants. Similar proportionate increases in petioles with leaf size reduced the overlap among adjacent leaves in the tropical understory palms Geonoma cuneata and Asterogyne maritana (Chazdon, 1985
, 1986
).
Serenoa may persist in heavily shaded environments because it can achieve light compensation (when the rate of photosynthesis equals the rate of respiration) at low light levels. Serenoa is a moderate heliophyte (i.e., a plant adapted to high light environments) based on its light-saturation level of 80 W/m2 (DeMoraes, 1980
). However, Serenoa achieved light compensation at a mere 1.8 W/m2, an irradiance level typical of shade-adapted plants. This ability to have net photosynthetic gains under low light and relatively high gains under high light may explain the wide tolerance of Serenoa to differing light conditions. Similar photosynthetic data are not available for Sabal, but its persistence and abundance in both heavily shaded and open-canopied environments suggests that it may also have a low compensation point coupled with a relatively high saturation level.
How do palmetto leaf traits allow the plants to persist in seasonally xeric, nutrient-impoverished environments?
In spite of their large size, the leaves of Serenoa and Sabal have several traits that are beneficial in seasonally xeric, nutrient-impoverished environments. For example, their leaves have thick, persistent, wholly (Sabal) to almost wholly (Serenoa) cutinized epidermises; the cuticles of leaves fill stomatal depressions; leaf stomata occur on both surfaces but are restricted to the intercostal regions; and leaf lamina are thick—each of these traits helps to conserve water (Tomlinson, 1961
; Corner, 1966
; Zona, 1990
).
The multiyear leaf life spans of these palmettos help recoup leaf construction costs in low light environments. Such extended leaf life spans may also conserve nutrients in nutrient-impoverished environments as suggested by several authors (Monk, 1966
; Chapin, 1980
; Chabot and Hicks, 1982
; Reich et al., 1991
, 1992
; Escudero et al., 1992
). Conservation of nutrients is likely important for Serenoa and Sabal given their acidic, nutrient-impoverished soil conditions (Table 2); their low leaf N concentrations (e.g., Serenoa: 0.99% ± 0.05; Sabal: 1.2% ± 0.1 percentage dry mass; Abrahamson, 1999
); and their increased growth rates and leaf N concentrations when fertilized (Abrahamson, 1999
; Gholz et al., 1999
). Typically, about 50% of leaf N and 33% of leaf P is recovered before a leaf is shed regardless of its life span. However, because nutrients are extracted over a longer time in long-lived, low nutrient-content leaves (Field and Mooney, 1983
), the rate of decline of photosynthetic capacity in such leaves is slower as leaves age (Chabot and Hicks, 1982
), which may enhance carbon gains.
Differences between Serenoa and Sabal
While the leaves of Serenoa and Sabal share major leaf traits and responses—they are long lived, slow to turnover, well protected against water loss, and respond similarly to light availability—these palmettos do differ in several ways. Serenoa had faster leaf turnover rates and somewhat shorter leaf life spans than did Sabal. From open-canopied to heavily shaded sites, the number of leaves in the palmetto canopy increased 3.3-fold with Serenoa compared to a 2.3-fold for Sabal. Future studies should determine whether this difference affects nutrient conservation or the efficiency of photosynthesis, given that photosynthetic efficiency declines with leaf age (Field and Mooney, 1983
). Light-capturing ability notwithstanding, Serenoa appears to require more light to initiate reproduction than does Sabal. While both species flowered more when growing in open overstory sites (e.g., gaps), Serenoa required more open overstory canopies to initiate flowering than did Sabal (Abrahamson, 1999
).
A key difference between Serenoa and Sabal is their size. The narrow endemic Sabal is considerably more dwarf than the widespread Serenoa. The greater masses of Serenoa compared to Sabal translate into higher levels of reproduction because plant mass along with stimulation by fire are critical determinates of whether a palmetto becomes reproductive (Abrahamson, 1999
; Carrington and Mullahey, 2006
). Over a 9-yr period, the largest Serenoa produced about twice as many inflorescences as did the largest Sabal (Abrahamson, 1999
).
The strong positive relationship between plant mass and leaf production for both species supports the notion that accumulation of resources by smaller individuals is essential to long-term success. However, the form of the relationship between plant size and new leaf production differed by species. Sabal had a linear relationship (perhaps owing to smaller masses for Sabal, which ranged to about 3 kg dry mass), whereas Serenoa had an asymptotic relationship in which the number of new leaves produced annually did not increase for plants above approximately 4 kg dry mass in spite of individual Serenoa approaching 8 kg.
The results of this study offer insights into our understanding of the expression of leaf size, especially relative to the leaf-trait strategies of plants adapted to seasonally xeric, nutrient-impoverished environments. Furthermore, this study adds to the few available studies that have examined variability in the life span of relatively long-lived leaves.
FOOTNOTES
1 The author thanks C. Abrahamson, J. Abrahamson, E. Anderson, S. Anderson, K. Ball Dobley, K. Bertram, D. Berry, L. Blazure, C. Bosio, D. Bresticker, E. Broderick, J. Brown, C. Campbell, N. Cao, D. Carleton, M. Ceh, A. Chartrand, M. Chipaloski, T. Craig, M. Domchek, R. Ellis, T. Enos, B. Fahey, K. Forosisky, S. Gebauer, K. Givens, D. Graves, D. Hoffman, J. Itami, A. Johnson, L. Kramme, J. Layne, S. MacKenzie, K. McCrea, G. Melika, S. Mutchler, D. Naugle, R. Packer, P. Peroni, G. Saunders, R. Scrafford, C. Shantz, E. Smalstig, C. Smith, M. Spiro, J. Stinchcombe, C. Winegarner, M. Winegarner, and P. Vitanzo for field assistance and/or valuable discussion. Earlier drafts were improved by comments from C. Abrahamson, C. Blair, M. Wise, and anonymous reviewers. The Archbold Biological Station and the David Burpee Plant Genetics Chair endowment of Bucknell University supported this work. 
2 abrahmsn{at}bucknell.edu
; mail address: Department of Biology, Bucknell University, Lewisburg, PA 17837 USA; phone: 570-577-1155; fax: 570-577-3537
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