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Interspecific variation in the ability to establish under shade is a fundamental determinant of temporal and spatial patterns in the distribution of woody plants (Smith & Huston 1989; Niinemets & Valladares 2006). Physiological ecologists have explored the possible roles of a range of seedling and juvenile traits in determining this variation (Givnish 1988; Walters & Reich 1999). Early work focused on leaf-level gas exchange (Grime 1965; Loach 1967), and this emphasis has found renewed support in recent reports that shade-tolerant species tend to have lower respiration rates and light-compensation points than light-demanding associates, suggesting more favourable leaf-level carbon balance in low light (Lusk 2002; Craine & Reich 2005). In addition, advocacy of a whole-plant perspective (Givnish 1988) has led to increased interest in biomass distribution, foliage display and tissue turnover as determinants of light interception and carbon balance (King 1994; Walters & Reich 1999; Lusk 2004; Pearcy et al. 2004).
The balance of carbon allocation between storage and growth could also play an important role in shade tolerance (Kitajima 1994; Kobe 1997). It has been argued that the difficulty in recovering from defoliation and other damage in low-energy understorey environments, and the low opportunity cost of storage in low light, will select for ample storage of carbohydrates in roots and stems of seedlings and juvenile trees (Kobe 1997). The selective importance of defoliation in the understorey is given credence by evidence that shade leaves suffer higher herbivory rates than sun leaves (Niesenbaum 1992; Dudt & Shure 1994). Carbohydrate storage would serve as a buffer against defoliation in low light, and might at least partly explain why pioneer and late-successional species differ in survival at a given growth rate (Kobe et al. 1995). Allocation to storage might also help explain biomass-distribution patterns of small seedlings. Large root-mass fraction has been widely reported in young seedlings of shade-tolerant trees (Kitajima 1994; Paz 2003; Lusk 2004), which might indicate early allocation to below-ground storage in these taxa (Kitajima 1994).
Despite the plausibility of the argument linking shade tolerance to carbohydrate storage, empirical support to date has been patchy. Non-structural carbohydrates (NSC) of seedlings and juvenile trees have been measured in few comparative studies involving more than two species (Kobe 1997; DeLucia et al. 1998; Canham et al. 1999), and these have produced quite diverse results. These discrepancies could be partly attributable to age and size differences between plants studied by different authors, as well as variation in leaf habit. Large seedlings and saplings of shade-tolerant evergreens are probably exposed to a low risk of catastrophic defoliation, at least by insect herbivores. These taxa often accumulate five or more foliage cohorts in shaded environments (King 1994; Williams, Field & Mooney 1989; Lusk 2002); as mature, hardened foliage is generally much less attractive than young, expanding leaves to chewing and sucking insects (Coley 1983), established juveniles of evergreens with slow foliage turnover expose only a small fraction of their photosynthetic surface area to a high risk of herbivory at a given time. In contrast, deciduous species are, by implication, highly vulnerable during leaf-out, which might constitute a stronger selection pressure for maintenance of extensive reserves in low light. First-year seedlings of evergreens are in a position analogous to deciduous species, in that they depend on a single cohort of leaves that are vulnerable to herbivory during expansion. If herbivory is a major selective pressure on carbohydrate storage in rainforest understoreys, we might thus expect NSC concentrations of evergreens to show close relationships with shade-tolerance variation in very young seedlings, but not necessarily in older established juveniles.
The distribution of carbohydrate-storage pools could also change during juvenile ontogeny. Root-mass fraction of shade-tolerant evergreens is initially high, but in some cases declines later to the point where shaded juveniles approaching 1 m tall have only ≈20% of their biomass below ground (Lusk 2004; Machado & Reich 2006). Roots therefore seem unlikely to be the main storage organs of established juveniles of these taxa. Because of uncertainties associated with the daily NSC dynamics of leaves, leaf tissues are often omitted from studies of carbohydrate storage. In rainforest understoreys, however, the large leaf-mass fraction of established juveniles of shade-tolerant species is likely to contain a large fraction of these plants’ total NSC reserves.
Here we examine the NSC reserves of shade-grown seedlings of six temperate rainforest evergreens shown to differ widely in shade tolerance. We asked if concentrations of starch and soluble sugars, and the distribution of NSC reserves between different organs, vary systematically in relation to shade-tolerance level, and how these patterns change during seedling ontogeny. These questions were addressed by quantifying the biomass distribution of two different seedling size classes, and by measuring starch and sugar concentrations of roots, stems and leaves.
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- Materials and methods
In both size classes, seedlings of all species had substantial NSC reserves. Machado & Reich (2006), working with larger juveniles of two evergreen and one deciduous species, reported NSC levels that were highly variable, but generally lower than those reported here. A study of 2-year-old seedlings growing in low light in a deciduous forest understorey (Canham et al. 1999) reported root and stem NSC concentrations somewhat higher than those found in small seedlings (40–60 mm tall) of our evergreen species, but only slightly higher on average than those seen in our large (400–600 mm tall) seedlings. Even in low light, juveniles of both evergreen and deciduous temperate trees therefore appear to accumulate substantial amounts of NSC. Non-structural carbohydrates have other functions in addition to energy storage, such as the involvement of sugars in cold resistance of plants exposed to freezing temperatures (Sakai & Larcher 1987). However, this function cannot explain the large amounts of insoluble starch accumulated by the plants we studied (Fig. 2), as starch is not known to have a role in cold-resistance mechanisms. We sampled towards the end of the growing season, when conditions were still favourable for carbon gain but when the strength of growth sinks was probably waning. This partial asynchrony of supply and demand (Chapin, Schulze & Mooney 1990) is probably an important contributor to the high NSC levels we report here, although overestimation of starch by perchloric acid extraction could also be involved (Rose et al. 1991).
NSC concentrations of large seedlings were higher than those of small seedlings, especially in the light-demanding taxa (Figs 2 and 3). This pattern was entirely due to differences in starch concentration (Fig. 2), and indicates a progressive accumulation of carbohydrate reserves during seedling development. As far as we are aware, no previous studies have compared carbohydrate reserves of different juvenile size classes for a comparable number of species. However, average NSC levels of seedlings in a tropical rainforest at Panama (Wurth, Winter & Korner 1998) were much lower than the average NSC content of adult trees (Wurth et al. 2005). As light environment appears to have a relatively weak long-term effect on tissue NSC levels (Kobe 1997), at least part of this difference reflects an ontogenetic increase in the relative magnitude of NSC reserves in trees.
We found no evidence that shade-tolerant species stored more carbohydrate than similarly sized light-demanders growing in the same light environment. Average NSC levels of small seedlings showed no trend in relation to species light requirements, and those of large seedlings were actually higher in light-demanding species (Figs 2 and 3). This contrasts with results obtained from saplings of four northern temperate species: in separate comparisons of deciduous and evergreen pairs, Kobe (1997) reported that shade-tolerant Acer saccharum and Tsuga canadensis had higher low-light NSC concentrations than intolerant Fraxinus americana and Pinus strobus, respectively, although the effect size was minimal in the evergreen comparison. A study of four northern temperate deciduous species found that low-light survival of young seedlings was correlated with NSC variation across species and across experimental treatments (Canham et al. 1999). However, neither survival, nor NSC concentrations, nor NSC pool size was related to species’ reputed shade tolerance. In contrast, a study of germinants of seven neotropical rainforest species found that shade tolerance was positively correlated with NSC pool size in low light, but not with tissue concentrations (J. A. Myers & K. Kitajima, unpublished data). Machado & Reich (2006) found that understorey saplings of shade-tolerant Abies balsamea and relatively intolerant P. strobus had similar whole-plant NSC concentrations: although leaf NSC was higher in A. balsamea, P. strobus had the higher concentration in roots. The limited evidence available to date therefore does not indicate any consistent relationship between light requirements and low-light NSC concentrations.
Our leaf NSC data are likely closely to reflect net assimilation rates. Against initial expectations, NSC per unit leaf mass was actually lower in shade-tolerant species than in light-demanding taxa (Fig. 3; Table 3). A study of saplings of four deciduous species (Niinemets 1997) reported that the more light-demanding taxa tended to accumulate more leaf NSC in high light, but that all four species had similar leaf NSC levels in low light. This presumably reflects an intimate link with rates of carbon gain, inasmuch as the greater photosynthetic capacity of the light-demanding species is fully expressed only in high light, species of differing shade tolerance having similar net assimilation rates in low light (Poorter 1999). A consideration of species differences in specific leaf area suggests that an essentially similar mechanism may explain our leaf NSC data. In contrast to the four deciduous species studied by Niinemets (1997), our six evergreens differed substantially in specific leaf area: as a result, light-demanders and shade-tolerant taxa showed a similar range of NSC concentrations per unit leaf area (Fig. 4). This may indicate that the light-demanding species had higher assimilation rates per unit mass of leaf tissue, but similar rates per unit area. Marenco, Gonçalves & Vieira (2001) report a similar pattern in their comparison of two evergreens of differing light requirements: in both high and low light, leaf starch and sugar concentrations of shade-tolerant Dipteryx odorata and mid-tolerant Swietenia macrophylla were different on a mass basis, but almost identical on an area basis.
Although tissue NSC concentrations did not conform to the adaptive pattern hypothesized by Kobe (1997), partitioning of the NSC pool between leaves, stems and roots of small seedlings did show an interesting relationship with species’ shade tolerance. Small seedlings of intolerant and mid-tolerant species had, on average, about half their NSC pool in leaves (Fig. 1c), largely because of their large leaf-mass fraction (Fig. 1a). They therefore risk a major depletion of reserves if defoliated. In contrast, small seedlings of shade-tolerant species had, on average, only about one-third of their NSC pool in leaves: this ensures the retention of the greater part of the NSC pool even in the event of extensive defoliation, and availability of reserves to subsequently replace lost leaves. This pattern was partially reversed in large seedlings: the accretion of many leaf cohorts by shade-tolerant Myrceugenia, Aextoxicon and Laureliopsis (Lusk 2002), plus a probable ontogenetic decline in allocation to roots (Lusk 2004), gives large seedlings of these taxa a large leaf-mass fraction in low light (Fig. 1). As a result, large seedlings of the shade-tolerant species had 36–54% of their NSC in foliage (Fig. 1), despite relatively low NSC concentrations in leaf tissues (Fig. 3). There was thus some support for our initial prediction that any relationship of carbohydrate-storage traits with shade tolerance of evergreens will be more evident in small seedlings than in larger juveniles. The latter, having several cohorts of hardened leaves, are probably less vulnerable to defoliation than first-year seedling evergreens, or deciduous species of any size. Relationships of NSC storage with shade tolerance might therefore be a more persistent life-history trait in deciduous species than in evergreens.
Differences in total NSC pool size may also influence early seedling survival. J. A. Myers & K. Kitajima (unpublished data) showed that survival of germinants of seven tropical tree species was correlated with interspecific variation in NSC pool size (although not with NSC concentrations), reflecting the ability of large-seeded, shade-tolerant species to supply young seedlings with abundant carbohydrate reserves. On the other hand, we did not find a significant correlation of light requirements with NSC pools of young seedlings of our six species (P = 0·13; data not shown), but as we standardized seedling height across species, our study was not likely to be very sensitive to interspecific variation in this trait.
We conclude that the importance and precise nature of any relationship of carbohydrate storage with variations in shade tolerance may depend on species’ leaf habit and on ontogenetic stage. Although our study of evergreens did not show the postulated relationship of NSC concentrations with shade tolerance, the partitioning of NSC reserves between the organs of small seedlings was broadly consistent with an adaptive explanation invoking defoliation as a major selective pressure on early ontogenetic stages (Kitajima 1994). No such pattern was evident in large seedlings, which had a biomass distribution and NSC partitioning traits very different from small seedlings. This study, the first to examine low-light carbohydrate-storage patterns in more than one size class, adds to recent evidence that consideration of multiple life-history stages may help further understanding of the determinants of variations in shade tolerance in humid forests (Lusk 2004; Niinemets 2006).