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There is evidence that mosses with miniature foliage elements have extremely large leaf area index (LAI) values, but it is unclear what canopy traits are responsible for these high LAI values in architecturally divergent mosses, and how the inherent trade-offs limiting maximum LAI in vascular plants can be overcome in mosses. To determine the quantitative significance of different traits in determining LAI, we developed a method to dissect LAI into underlying functionally dependent constituent traits at leaf, shoot and canopy scales. The suites of structural traits were studied altogether for 43 moss canopies from 11 species with contrasting light and water requirements along gap-understory gradients to obtain as large a range of variation in moss architecture as possible and evaluate the differentiation in moss LAI in relation to species ecology. Extensive variation in moss structural traits, 11- (shoot length) to 77-fold (shoot number per area, N S ¯ ), was observed at all structural scales from leaf to canopy. However, LAI only varied nine-fold, as the result of two key trade-offs: leaf size vs number trade-off and shoot leaf area vs shoot density trade-off. Owing to these negative relationships, and greater variability in N S ¯ , LAI primarily scaled with N S ¯ . N S ¯ and LAI increased with site light availability, and LAI was greater in open and dry habitat species. This study highlights a huge structural diversity among moss canopies, but indicates that canopies converge to a much narrower range of LAI due to trait trade-offs such that, counterintuitively, minute leaf size and densely leafed stems are not necessarily responsible for high LAI in mosses.

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.

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.

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.

Leaf phenology varies markedly across tree species of temperate deciduous forests. Early leafing in spring may increase light capture and carbon gain prior to canopy closure, allowing saplings to survive in understory sites deeply shaded in midsummer. We quantified sapling leaf phenology for 18 tree species and seasonal variation in understory light availability at three sites along a ridge-slope-cove landform gradient in the Great Smoky Mountains National Park. Early leafing species (e.g., Aesculus flava, Carpinus caroliniana) broke bud an average of 24 d before late leafers (e.g., Magnolia fraseri, Nyssa sylvatica). Canopy closure occurred 14-18 d earlier and summer understory light was on average 63-74% lower on intermediate and mesic sites than on the xeric site. Early leafing species intercepted 45-80% of their growing season photon flux before canopy closure vs. 8-15% for late leafers. However, earlier leafing increased exposure to freezing temperatures by 5.5% per week near the mean time of bud break. Early leafing is strongly correlated with midsummer shade, risk of freezing temperatures, and distribution on mesic sites across a \"main spectrum\" of 15 deciduous species. Differences in leaf phenology and resultant impacts on spring carbon gain may help determine tree shade tolerance and distribution in southern Appalachian forests.

Background and aims - The leaf size/number trade-off has been recently established as a wide-spread and highly predictable relationship associated with between-species leaf size variation. In this study, we examine whether this trade-off relationship also applies at the between-plant (within-species), and at the between-shoot (within-plant) levels associated with spatial variation in incident light availability within tree canopies. Methods - Replicate current-year shoots were sampled from north-facing (shaded) and south-facing (sun-exposed) canopy sides of sixteen broadleaf tree species in eastern Ontario, Canada. For each shoot, measurements were recorded for mean individual leaf dry mass, number of leaves, number of side branches, and stem length, girth, and tissue dry mass. Leafing intensity was calculated as the number of leaves produced per unit of supporting stem tissue dry mass. Key results - All of the direct trait measurements had generally larger values for shoots collected from south-facing canopy sides (as expected). However, negative isometric relationships between leaf size and leafing intensity were found at the between-plant level (for Acer saccharum) and the between-shoot (within-tree) level for at least some individuals of most species. The predominant trend at the within-tree level, however, was allometric - i.e. north-facing (light-limited) shoots generally had lower individual leaf dry mass but disproportionately higher leafing intensity compared with south-facing shoots. Conclusions - The results confirm that there is a fundamental leaf size/number trade-off at the between-plant (within-species) level and also at the between-shoot (within-plant) level, as previously reported at the between-species level. But more specifically, the results reveal distinctly different leaf deployment strategies in response to spatial light variability within tree canopies: Under high light exposure, larger leaves are favoured (with lower leafing intensity imposed as a trade-off), but in deeply shaded portions of the canopy, smaller leaves result, we suggest, for two reasons: (i) they are favoured directly (because they minimize overlap of closely spaced adjacent leaves); (ii) they are imposed as a trade-off of selection favouring high leafing intensity, which in turn maximizes the size of the reserve bud bank (number of axillary meristems per unit of supporting stem tissue) available for initiating continued growth or reproduction in the following year.