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Because leaf size scales negatively and isometrically with leaf number per shoot size (leafing intensity) in woody species, and because most tree and shrub species have small leaves, Kleiman and Aarssen (J Ecol 95:376-382, 2007) recently proposed that natural selection favors high leafing intensity resulting in small leaves, i.e., the leafing-intensity-premium hypothesis. However, empirical evidence for or against this hypothesis is still lacking. In addition, this hypothesis has not been examined in the context of how leaf size varies among habitats. To fill this void, we investigated leaf size frequency distributions of woody species from temperate China and explored the relationships among leaf mass, leaf number, and stem mass of current-year shoots of 133 woody species at low and high altitudes of three mountain ranges. The scaling relationships between leaf size and leafing intensity (leaf number per stem mass) were determined using both standardized major axis regression analyses and phylogenetically independent comparative techniques. In light of the leafing-intensity-premium hypothesis, we made three predictions: (1) leaf size frequency distributions should be right-skewed for each local study area and for the entire study region, (2) leafing intensities at different altitudes at different sites should differ while leafing intensities at comparable altitudes should be similar baring large taxonomic differences among sites, and (3) that leafing intensity should be higher for any given leaf size in habitats with small-leaved species. Significant negative and isometric scaling relationships between leaf size and leafing intensity were found to be consistently conserved independent of habitat type, both across species and across correlated evolutionary divergences. Within each mountain range or across the entire study region, leaf size frequency distributions were right-skewed, in accordance with our prediction. However, leafing intensity was smaller for any given leaf size at the altitude with smaller leafed species than for altitudes characterized by large leafed species, i.e., altitudes characterized by species with small leaves did not have consistently higher leafing intensities than other altitudes on each mountain range. Our analyses therefore indicate the direct adaptive value of leaf size but not the selective advantage in high leafing intensity as posited by the leafing-intensity-premium hypothesis. We suggest that this hypothesis explains less about the variation of leaf size among different habitats as it does about variation within habitats, i.e., the relative importance of the adaptive significance of leafing intensity and leaf size can and does vary with habitats.

Recent studies have reported a consistent trade‐off between leaf size (mass) and leafing intensity (the number of leaves produced per unit of supporting stem tissue volume); however, a theoretical basis for this trade‐off has not been described. We explore the mechanistic basis for this trade‐off and assess the relationship in the light of other prominent theories for allometric biomass partitioning. We show algebraically how the allocation of mass to leaves versus stems and the density of stem tissue can potentially influence this trade‐off. To assess these possible effects, we compared the relationship between leaf size and leafing intensity, expressed on both mass and volume basis, at the level of a single branch as well as the entire above‐ground plant in 61 forbs over a 3‐year period. Our results support the idea that the trade‐off between leaf size and volume‐based leafing intensity depends on both biomass investment (leaves vs. stems) and stem bulk density (mass vs. volume), whereas the trade‐off between leaf size and mass‐based leafing intensity only depends on the biomass investment. Similar exponents in the scaling of leaf mass vs. stem mass and stem mass vs. stem volume at branch and whole plant levels lead to similar trade‐offs. An isometric trade‐off between leaf size and volume‐based leafing intensity is consistent with a constant biomass partitioning between leaves and stems as well as constant stem tissue density. Conversely, an allometric trade‐off between leaf size and volume‐based leafing intensity arises when biomass allocation is allometric and stem bulk density varies with plant size.

Understanding the scaling between leaf size and leafing intensity (leaf number per stem size) is crucial for comprehending theories about the leaf costs and benefits in the leaf size–twig size spectrum. However, the scaling scope of leaf size versus leafing intensity changes along the twig leaf size variation in different leaf habit species remains elusive. Here, we hypothesize that the numerical value of scaling exponent for leaf mass versus leafing intensity in twig is governed by the minimum leaf mass versus maximum leaf mass (Mmin versus Mmax) and constrained to be ≤−1.0. We tested this hypothesis by analyzing the twigs of 123 species datasets compiled in the subtropical mountain forest. The standardized major axis regression (SMA) analyses showed the Mmin scaled as the 1.19 power of Mmax and the ‐α (−1.19) were not statistically different from the exponents of Mmin versus leafing intensity in whole data. Across leaf habit groups, the Mmax scaled negatively and isometrically with respect to leafing intensity. The pooled data's scaling exponents ranged from −1.14 to −0.96 for Mmin and Mmax versus the leafing intensity based on stem volume (LIV). In the case of Mmin and Mmax versus the leafing intensity based on stem mass (LIM), the scaling exponents ranged from −1.24 to −1.04. Our hypothesis successfully predicts that the scaling relationship between leaf mass and leafing intensity is constrained to be ≤−1.0. More importantly, the lower limit to scaling of leaf mass and leafing intensity may be closely correlated with Mmin versus Mmax. Besides, constrained by the maximum leaf mass expansion, the broad scope range between leaf size and number may be insensitive to leaf habit groups in subtropical mountain forest. Our hypothesis successfully predicts that the scaling relationship between leaf mass and leafing intensity is constrained to be ≤−1.0. The broad scope range between leaf size and number may not be sensitive to leaf habit groups in the subtropical mountain forest.

Aims: The functional traits of twigs and leaves are closely related to the ability of plants to cope with heterogeneous environments. The analysis of the characteristics of twigs and leaves and leaf thermal dissipation in riparian plants is of great significance for exploring the light energy allocation and ecological adaptation strategies of plant leaves in heterogeneous habitats. However, there are few studies on the correlation between the twig–leaf characteristics of riparian plants and their heat dissipation in light heterogeneous environments. Methods: In this study, the riparian plant Hippophae rhamnoides in Taohe National Wetland Park was the research object. According to the differences in the canopy light environment of the H. rhamnoides population, three habitat gradients were set: I, the full sight zone; II, the moderate shade zone; and III, the canopy cover zone. We studied the relationship between the twig–leaf characteristics of H. rhamnoides and leaf thermal dissipation in a heterogeneous light environment. Important Findings: The results are as follows: from the full sight zone to the canopy cover zone, the population characteristics and the twig, leaf, and photosynthetic fluorescence physiological characteristics of H. rhamnoides demonstrated significant changes (p < 0.05). In the full sight zone, H. rhamnoides tended to have thick leaves with a smaller SLA on short and thick twigs, and the light energy absorbed by the leaves accounted for a higher proportion of thermal dissipation. In the moderate shade zone, H. rhamnoides tended to grow many thin leaves with high SLA on long and thick twigs, and the proportion of light energy absorbed by the leaves for heat dissipation was lower than that in the full sight zone. In the canopy cover zone, H. rhamnoides tended to grow a few large and thick leaves with a low SLA on slender and long twigs, and the proportion of light energy absorbed by the leaves for heat dissipation was the lowest. There was a significant correlation between the twig–leaf and leaf heat dissipation of H. rhamnoides in the three habitats (p < 0.05). The co-variation of plant branches and leaves and the timely adjustment of thermal dissipation in photoheterogeneous habitats reflect the phenotypic plasticity mechanism and self-protection strategy of riparian plants in adapting to heterogeneous environments.

1. According to the traditional \"Size Advantage\" (SA) hypothesis, plant species with larger body size are expected to be more successful when competition is intense, that is, within severely crowded vegetation. Recent studies in old-field habitats, however, have shown that those species with greater numerical abundance as resident plants generally have a relatively small minimum reproductive threshold size (MIN), not a relatively large maximum potential body size (MAX). 2. In this study, we test for a size advantage in terms of species abundance representation in the soil seed bank, and we extend the SA hypothesis to include two additional size metrics: leaf size and seed size. Specifically, we ask, for resident species within a crowded old-field meadow: is larger seed size, leaf size, and/or body size associated with greater reproductive / recruitment success (i.e. number of germinable seeds within—and establishing plants emerging from—the soil seed bank)? We collected soil cores for a greenhouse experiment to record relative species abundances of germinable seeds in the seed bank, and we used a field experiment to record local abundances of species emerging from the resident seed bank within denuded plant neighbourhoods over three subsequent field seasons. 3. We found no general support for the SA hypothesis involving any of the size metrics, and none of the latter was a strong predictor of the number of germinable seeds emerging from soil cores in the greenhouse experiment. However, for species establishing in the field experiment from the seed bank over the 3-year survey period, more abundant species in years 2 and 3 tended to be those with smaller MIN, and thus smaller MAX. In addition, within more crowded neighbourhoods, representation of reproductive plants was generally greater for species with relatively small MIN (and hence small MAX). 4. Synthesis. Our results extend the support for the \"Reproductive Economy Advantage\" hypothesis in old-field habitats, to include not just established, largely undisturbed vegetation, but also very early stages of recruitment from seed within locally crowded plant neighbourhoods. Specifically, more successful species here are not those with relatively large potential body size (MAX); they are species capable of producing at least some offspring despite severe body size suppression, because they have a relatively small MIN. We summarize our interpretation using a simple conceptual model for predicting selection effects of local neighbourhood crowding on variation in fecundity (fitness estimate) of resident plants, resulting from genetic covariation in MAX and MIN. Relatively large potential body size (MAX) should be expected to promote fitness here, not when competition from near neighbour effects is severe, but when its effects are locally more moderate, or virtually absent—and hence relatively rarely within the generally crowded vegetation of old-field habitats.

Abstract The trade-off between leaf number and individual leaf size on current-year shoots (twigs) is crucial to light interception and thus net carbon gain. However, a theoretical basis for understanding this trade-off remains elusive. Here, we argue that this trade-off emerges directly from the relationship between annual growth in leaf and stem mass, a hypothesis that predicts that maximum individual leaf size (i.e. leaf mass, Mmax, or leaf area, Amax) will scale negatively and isometrically with leafing intensity (i.e. leaf number per unit stem mass, per unit stem volume or per stem cross-sectional area). We tested this hypothesis by analysing the twigs of 64 species inhabiting three different forest communities along an elevation gradient using standardized major axis (SMA) analyses. Across species, maximum individual leaf size (Mmax, Amax) scaled isometrically with respect to leafing intensity; the scaling constants between maximum leaf size and leafing intensity (based on stem cross-sectional area) differed significantly among the three forests. Therefore, our hypothesis successfully predicts a scaling relationship between maximum individual leaf size and leafing intensity, and provides a general explanation for the leaf size-number trade-off as a consequence of mechanical-hydraulic constraints on stem and leaf growth per year. The trade-off between leaf number and individual leaf size on current-year shoots (twigs) is crucial to light interception and thus net carbon gain. We present a model (stem-leaf growth hypothesis, SLGH) to provide a theoretical explanation for the trade-off between the maximum leaf size vs. leafing intensity. We found that the scaling exponents of maximum leaf size vs. the leafing intensity are close to −1.0 and are insensitive to forest types and different elevations. Therefore, our results successfully provide a general explanation for this trade-off as a consequence of mechanical-hydraulic constraints on stem and leaf growth per year.

PREMISE OF THE STUDY: The study of scaling examines the relative dimensions of diverse organismal traits. Understanding whether global scaling patterns are paralleled within species is key to identify causal factors of universal scaling. I examined whether the foliage-stem (Corner's rules), the leaf size-number, and the leaf mass-leaf area scaling relationships remained invariant and isometric with elevation in a wide-distributed treeline species in the southern Chilean Andes. METHODS: Mean leaf area, leaf mass, leafing intensity, and twig cross-sectional area were determined for 1-2 twigs of 8-15 Nothofagus pumilio individuals across four elevations (including treeline elevation) and four locations (from central Chile at 36°S to Tierra del Fuego at 54°S). Mixed effects models were fitted to test whether the interaction term between traits and elevation was nonsignificant (invariant). KEY RESULTS: The leaf-twig cross-sectional area and the leaf mass-leaf area scaling relationships were isometric (slope = 1) and remained invariant with elevation, whereas the leaf size-number (i.e., leafing intensity) scaling was allometric (slope ≠ - 1) and showed no variation with elevation. Leaf area and leaf number were consistently negatively correlated across elevation. CONCLUSIONS: The scaling relationships examined in the current study parallel those seen across species. It is plausible that the explanation of intraspecific scaling relationships, as trait combinations favored by natural selection, is the same as those invoked to explain across species patterns. Thus, it is very likely that the global interspecific Corner's rules and other leaf-leaf scaling relationships emerge as the aggregate of largely parallel intraspecific patterns.

1 Using a sample of 24 common deciduous angiosperm trees of the Eastern Deciduous Forest region of North America, we tested the hypothesis that leaf size variation across species can be interpreted in terms of a trade-off between individual leaf mass and the number of leaves produced. 2 The true nature of a resource allocation trade-off is detectable only if variation in the total amount of growth is accounted for. We controlled for this effect by measuring all of the components of annual growth associated with leaf production at the individual terminal shoot level. Hence, number of leaves produced was expressed as 'leafing intensity', i.e. the number of leaves produced by newly emerged (current year's) shoots, divided by the total volume of these shoots. 3 Ninety per cent (r² = 0.90) of the variation in mean individual leaf mass across species, spanning two orders of magnitude, could be accounted for by proportional variation in mean leafing intensity, i.e. representing an isometric trade-off, with a slope for log-transformed data that did not deviate significantly from -1.0. 4 We suggest that this isometric relationship may represent a generalized trade-off strategy for leaf deployment at the shoot level within temperate deciduous woody species. Following traditional interpretations, adaptation here may involve a fitness benefit associated with a particular leaf size. The present results also suggest an alternative, i.e. selection may instead favour high leafing intensity, with small leaf mass resulting not as a direct adaptation, but simply as a trade-off. 5 According to this 'leafing intensity premium' hypothesis, the fitness benefits of higher leafing intensity are associated primarily with the fitness benefits of a larger pool of lateral meristems, because each leaf is usually associated with an axillary bud. This may in turn provide greater facility for wide phenotypic plasticity in the allocation of these meristems to vegetative vs. reproductive functions. This represents a plausible hypothesis, we suggest, in accounting for why most woody deciduous angiosperms, even some of the largest/tallest ones, have relatively small leaves.

BACKGROUND AND AIMS: Trade-offs are fundamental to life-history theory, and the leaf size vs. number trade-off has recently been suggested to be of importance to our understanding leaf size evolution. The purpose of the present study was to test whether the isometric, negative relationship between leaf size and number found by Kleiman and Aarssen is conserved between plant functional types and between habitats. METHODS: Leaf mass, area and number, and stem mass and volume of current-year shoots were measured for 107 temperate broadleaved woody species at two altitudes on Gongga Mountain, south-west China. The scaling relationships of leaf size (leaf area and mass) vs. (mass- and volume-based) leafing intensity were analysed in relation to leaf habit, leaf form and habitat type. Trait relationships were determined with both a standardized major axis method and a phylogenetically independent comparative method. KEY RESULTS: Significant negative, isometric scaling relationships between leaf size and leafing intensity were found to be consistently conserved across species independent of leaf habit, leaf form and habitat type. In particular, about 99 % of the variation in leaf mass across species could be accounted for by proportional variation in mass-based leafing intensity. The negative correlations between leaf size and leafing intensity were also observed across correlated evolutionary divergences. However, evergreen species had a lower y-intercept in the scaling relationships of leaf area vs. leafing intensity than deciduous species. This indicated that leaf area was smaller in the evergreen species at a given leafing intensity than in the deciduous species. The compound-leaved deciduous species were observed usually to have significant upper shifts along the common slopes relative to the simple-leaved species, which suggested that the compound-leaved species were larger in leaf size but smaller in leafing intensity than their simple counterparts. No significant difference was found in the scaling relationships between altitudes. CONCLUSIONS: The negative, isometric scaling relationship between leaf size and number is largely conserved in plants, while the leaf size vs. number trade-off can be mediated by leaf properties. The isometry of the leaf size vs. number relationship may simply result from a biomass allocation trade-off, although a twig size constraint may provide an alternative mechanism.

1. Recent reports provide empirical evidence for a negative isometric scaling of (log)leaf number per unit shoot volume (i.e. leafing intensity) to (log)leaf size in woody plants. In this context, a theory of leaf size variation was anticipated, stating that natural selection should have favoured high leafing intensity, given that most tree and shrub species are small-leaved. Leaf size is thus proposed to be a mere correlate of leafing intensity. This was coined the leafing intensity premium hypothesis. 2. I investigated the generality of the leaf size-number trade-off by testing its extension to a set of 60 small-stature species, growing along an altitude gradient. Also, I compiled data from 224 species and re-analysed their leaf size and leafing intensity. Finally, I explored the validity of the leafing intensity premium hypothesis to account for the leaf size-number trade-off, if the latter was sustained, and examined patterns of coordinated evolution of leaf size and leafing intensity across seed plant lineages. 3. Negative isometric scaling was supported in all analyses, except when evergreens were considered in isolation. Frequency distribution of both leaf size and leafing intensity was severely right-skewed in the 224-species data set, which provided no evidence in support of the view that natural selection favoured species with high leafing intensity. 4. Node divergences in the phylogenetic tree showed a tendency for inverse coordinated evolution of leafing intensity and leaf size. However, interactions between environments and divergence patterns, and also node-wise peculiarities, were detected. 5.Synthesis. The leaf size-number trade-off is of general scope, although scaling exponents differ among evergreen and deciduous species. In addition, signals of coordinated evolution of both traits across the seed plant tree were detected. Still, the leafing intensity premium hypothesis cannot be supported, since leafing intensity of present-day species is not particularly biased towards high scores.