Changes in leaf morphology and anatomy with tree age and height in the broadleaved evergreen species, Eucalyptus regnans F. Muell

Relatively little is known about changes in leaf attributes over the lifespan of woody plants. Knowledge of such changes may be useful in interpreting physiological changes with age. This study investigated changes in leaf morphology and anatomy with tree age and height in the broadleaved evergreen species, Eucalyptus regnans. Fully expanded leaves were sampled from the upper canopy of tree ages ranging from 6 to 240 years, and tree heights ranging from about 10–80 m. There were significant changes in leaf form with increasing tree age and height. Leaf size and specific leaf area (SLA; leaf area/leaf mass) decreased, leaf thickness increased, and leaves became narrower relative to their length, with increasing tree age and height. Cuticle thickness and leaf waxiness, including wax occlusion of the stomatal antechamber, increased with increasing age and height. By comparison, there were no clear trends in stomatal frequency or stomatal length with tree age, although there were curvilinear relationships between an index of total stomatal pore area per leaf lamina and both tree age and tree height. The results support the hypothesis that leaves of E. regnans become more xeromorphic with tree age and height. The results are discussed in relation to their significance for changes in water relations in the canopy with age.     

Light intensity, gibberellin content and the resolution of shoot growth in Brassica

The role of gibberellins (GAs) in the regulation of shoot elongation is well established but the phytohormonal control of dry-matter production is poorly understood. In the present study, shoot elongation and dry-matter production were resolved by growing Brassica napus L. seedlings under five light intensities (photon flux densities) ranging from 25 to 500 μmol m⁻² s⁻¹. Under low light, plants were tall but produced little dry weight; as light intensity was increased, plants were progressively shorter but had increasing dry weights. Endogenous GAs in stems of 16- and 17-d-old plants were analyzed by gas chromatography-selected ion monitoring with [²H₂] internal standards. The contents of GAs increased dramatically with decreasing light intensity: GA₁, GA₃, GA₈ and GA₂₀ were 62, 15, 16 and 32 times higher, respectively, under the lowest versus highest light intensities. Gibberellin A₁₉ was not measured at 25 μmol m⁻² s⁻¹ but was 9␣times greater in the 75 compared to 500 μmol m⁻² s⁻¹ treatment. Shoot and hypocotyl lengths were closely positively correlated with (log) GA concentration (for example: r ² = 0.93 for GA₁ and hypocotyl length) but shoot dry matter was negatively correlated with GA concentration. The application of gibberellic acid (GA₃) produced elongation of plants grown under high light, indication that their low level of endogenous GA was limiting shoot elongation. Although endogenous GA₂₀ showed the greatest influence of light treatment, metabolism of [³H]GA₂₀ and of [³H]GA₁ was only slightly influenced by light intensity, suggesting that neither 2β- nor 3β-hydroxylation were points of metabolic regulation. The results of this study indicate that GAs control shoot elongation but are not directly involved in the regulation of shoot dry weight in Brassica. The study also suggests a role of GAs in photomorphogenesis, serving as an intermediate between light condition and shoot elongation response. Received: 18 June 1998 / Accepted: 29 July 1998     

Light,leaf age,and leaf nitrogen concentration in a tropical vine

Summary Tropical vines in the Araceae family commonly exhibit alternating periods of upward and downward growth, decoupling the usual relationship between decreasing light environment with increasing age among the leaves on a shoot. In this study I examined patterns of light, leaf specific mass, and leaf nitrogen concentration in relation to leaf position, a measure of developmental age, in field collected shoots of Syngonium podophyllum. These data were analyzed to test the hypothesis that nitrogen allocation parallels within-shoot gradients of light availability, regardless of the relationship between light and leaf age. I found that leaf nitrogen concentration, on a mass basis, was weakly correlated with leaf level light environment. However, leaf specific mass, and consequently nitrogen per unit leaf area, were positively correlated with gradients of light within the shoot, and either increased or decreased with leaf age, providing support for the hypothesis that nitrogen allocation parallels gradients of light availability.     

Structural correlates of increased adhesive efficiency with adult size in the toe pads of hylid tree frogs

Tree frogs are able to climb smooth, vertical substrates using specialised toe pads which adhere via an area-based wet adhesive mechanism. Although the link between pads and arboreality in frogs is well-established, few studies have investigated the influence of morphology on adhesion. Trinidadian tree frogs from the genus Hyla are geometrically similar. There is a tendency towards comparatively reduced mass in larger species, but toe pad area increases as expected with isometry. As adhesion is area-dependent, forces are affected directly by the increase in mass relative to pad area, and there is a decrease in the ability of larger species to adhere to smooth rotation platforms. However, there is an increase in force per unit area that suggests larger species have more efficient toe pads. Toe pad structure is very similar though there are variations in the details of a number of features. Crucially, although differences in morphology appeared small they had demonstrable effects on adhesive efficiency of the pads. Epithelial cell area correlates positively with frog length and adhesive efficiency, related features of cell density and intercellular channel length correlate negatively. These findings are discussed in relation to the different forces involved in the tree frogs’ wet adhesive system.     

Frugivore gape size and the evolution of fruit size and shape in southern hemisphere floras

Abstract The present study uses differences among frugivore faunas of the southern hemisphere landmasses to test whether frugivore characteristics have influenced the evolution of fruit traits. Strong floristic similarities exist among southern landmasses; for example, 75% of New Zealand vascular plant genera also have species in Australia. However, plants in Australia and South America have evolved in the presence of a range of mammalian frugivores, whereas those in New Zealand, New Caledonia and the Pacific Islands have not. In addition, the avian frugivores in New Zealand and New Caledonia are generally smaller than those of Australia. If frugivore characteristics have influenced the evolution of fruit traits, predictable differences should exist between southern hemisphere fruits, particularly fruit size and shape. Fruit dimensions were measured for 77 New Zealand species and 31 Australian species in trans‐Tasman genera. New Zealand fruits became significantly more ellipsoid in shape with increasing size. This is consistent with frugivore gape size imposing a selective pressure on fruit ingestability. This result is not a product of phylogenetic correlates, as fruit length and width scaled isometrically for Australian species in genera shared with New Zealand. Within‐genus contrasts between New Zealand and Australian species in 20 trans‐Tasman genera showed that New Zealand species have significantly smaller fruits than their Australian counterparts. Within‐genus contrasts between New Zealand and South American species in nine genera gave the same result; New Zealand species had significantly smaller fruits than their South American counterparts. No difference was found in fruit size or shape between New Zealand and New Caledonia congeneric species from 12 genera. These results are consistent with the broad characteristics of the frugivore assemblage influencing the evolution of fruit size and shape in related species. The smaller‐sized New Zealand frugivore assemblage has apparently influenced the evolution of fruit size of colonizing taxa sometimes within a relatively short evolutionary timeframe.     

Influence of leaf size on tree architecture: first branch height and crown dimensions in tropical rain forest trees

 First branch height is an important attribute of sapling architecture, as it defines the height at which prolonged lateral growth is possible. First branch height, measured on saplings of 70 species in tropical rain forests of Australia, Costa Rica, Panama, and Sabah, Malaysia, was highly correlated with leaf blade and petiole length. The observed relationship, first branch height ∝ blade length × (petiole length)^0.5, implies that the ratio of first branch height to blade length increases somewhat with increasing leaf size, among species with a given ratio of petiole to blade length. Orthotropic species, with more or less radially symmetric arrangements of leaves on ascending axes, had a mean first branch height of 7x that observed for plagiotropic species, with planar leaf arrangements. The greater first branch height of orthotropic species was associated with their larger leaves and longer petioles. Plagiotropic species had wider crowns than orthotropic species in the sapling stage, as assessed at the Costa Rican site. Thus, leaf dimensions influence the dynamics of crown construction (or visa versa), as well as affecting leaf energy balance and gas exchange. Received: 5 September 1997 / Accepted: 3 March 1998     

Tree-rings and transportation in the New Zealand kauri (Agathis australis) timber industry: Investigating the time lag from tree to building

The interpretation of felling dates established by dendrochronological dating requires understanding of sample context and building practices, as well as knowledge of processes and timescales within the timber supply chain. In New Zealand (N.Z.), tree ring analysis of kauri from colonial-era (1840–1906) and early Dominion-era (1907–1920) buildings has been undertaken since 2000 and felling dates have been identified from several structures, providing a terminus post quem date for construction or modification of the building. What has been lacking is a good understanding of how long it took for a kauri tree to become timber used in a building or other structure. As a first estimate, felling dates from a small set of buildings were compared to documented construction dates, suggesting a delay of up to 5 years. To improve interpretation of felling dates, we investigated timescales associated with two stages of timber production within the 19th and early 20th century kauri timber industry: (a) transportation from stump to mill, and (b) seasoning of sawn timber. Documentary sources for ∼1850 until the 1920s were analysed, covering the development, expansion and decline of industrialised kauri timber production. The available evidence suggests that transportation to the sawmill was the most important variable. Logs could arrive within a few weeks of felling or potentially experience a delay of a year or more. Transportation times were affected by geography, weather, spatial differences and temporal changes in transportation methods and economic fluctuations. Seasoning of sawn timber probably contributed little to the time lag, as the balance of evidence suggests framing timber was not usually seasoned. In general, any delay associated with transportation and seasoning is unlikely to be more than a couple of years. The findings support the earlier use-date range of up to 5 years, but suggest it is generous. The dating of waney-edge timbers from buildings with accurate construction dates would help refine a use date range for kauri, as would further research into other stages of the production process including conversion of logs, transportation to market, and stockpiling in the timber yard and/or building site. The current study is also a starting point for further research in three related areas: (a) biography of timber; (b) technological change; and (c) trade networks.     

Growth and leaf production in the tropical palm Euterpe edulis: Light conditions versus developmental constraints

We studied developmental and environmental constraints on leaf dynamics, morphology and physiology in the monopodial tropical palm of the Atlantic Forest biome, Euterpe edulis. Plastic responses to light environments in terms of photosynthesis, leaf size, leaf life span, patterns of biomass allocation and growth were analysed. Plants were grown during 14 months in a shade house under four different growth irradiances. Plants of Euterpe edulis were able to adjust leaf demography and biomass allocation in the different light treatments. Leaf life span increased by 100 days with decreasing light levels while the rate of leaf production decreased, consistent with lower electron transport rates. At low light levels, adjustments in biomass allocation to leaf components allowed E. edulis to reduce self-shading and increase light interception. At high light plants allocated more biomass to roots, and the plants exhibited small leaf sizes when leaves were compared using an explicit ontogenetic analysis. Ontogeny constrained the maximum size that each consecutive leaf could achieve, while growth irradiance determined the rate of leaf production and other leaf traits. Consequently, there were both, developmental constraints and environmental determinants influencing leaf demography and morphology in E. edulis. The findings of this ecophysiological and demographic study are relevant to palms growing under natural conditions and help to explain the success of E. edulis in the forest understory and its absence from large gap openings. Our results not only confirm that E. edulis is a shade tolerant species, but also show that palms are able to acclimate to different growing condition as well as trees.     

Early effects of the mutation laurina on the functioning and size of the shoot apex in coffee tree and analysis of the plastochron phases: relationships with the dwarfism of leaves

The current article presents the investigations into the effect of the laurina mutation on the functioning and size of the shoot apical meristem (SAM) in Coffea arabica. This monolocus and Mendelian mutation is known to have pleiotropic effects on tree shape and dwarfism. A comparison between the wild type C. arabica var. Bourbon and its natural dwarf mutant C. arabica var. laurina, also called Bourbon pointu, was carried out leading to three main results: (1) the effects appeared immediately after the emergence of the buttress but did not affect the dome-shaped SAM (size and shape); (2) the effects were located at the peripheral zone and maintained subsequently within the leaf primordia; (3) the effects consisted of reduction in both the size of primordia and the height of incipient internode, consequently resulting in dwarfism of mature leaves and internodes. By contrast, the laurina mutation had no effect on the relationship between the phyllochron and the plastochron, the decussate and opposite phyllotaxis, and the relative timing of SAM functioning within the plastochron.     

Light and nutrient availability affect the size-scaling of growth in phytoplankton

Communities of marine phytoplankton consist of cells of many different sizes. The size-structure of these communities often varies predictably with environmental conditions in aquatic systems. It has been hypothesized that physiological differences in nutrient and light requirements and acquisition efficiencies contribute to commonly observed correlations between phytoplankton community size structure and resource availability. Using physiological models we assess how light and nutrient availability can alter the relative growth rates of phytoplankton species of different cell sizes. Our models predict a change in the size dependence of growth rate depending on the severity of limitation by light and nutrient availability. Under conditions of growth-saturated resource supply, phytoplankton growth rate (mol C ) scales with cell volume with a size-scaling exponent of ; light limitation reduces the size-scaling exponent to approximately , and nutrient limitation decreases the exponent to as a consequence of the size-scaling of resource acquisition. Exponents intermediate between and occur under intermediate availability of light and nutrients and depend on the size-scaling of pigment photoacclimation and the size range examined.     

Climatic classification and ordination of the Spanish Sistema Central: relationships with potential vegetation

Comparisons among European, Japanese and North-American temperate deciduous woody floras revealed that there is no difference in shade-tolerance or in successional position between the compound- and simple-leaved species. Given that the compound-leaved species usually have greater biomass investments in non-productive throwaway supporting structures, it remained unclear how they could be as shade-tolerant as the simple-leaved analogues. To find out the role of the variability in leaf structure and composition in shade-tolerance of these species, foliar morphology and chemistry were analysed in 15 Estonian temperate compound-leaved deciduous woody taxa.Both foliar morphological and chemical parameters influenced the fractional investment of foliar biomass in petioles. The proportion of leaf biomass in petioles was independent of leaf size, but it increased with increasing leaflet number per leaf, suggesting that spacing rather than support requirements determined the biomass investment in petioles. The leaves with greater nitrogen concentrations also had larger foliar biomass investments in petioles. The latter effect possibly resulted from a greater water demand of functionally more active protein-rich leaves. Though the proportion of leaf biomass invested in petioles was high (for the whole material on average 15.9±0.4%), petioles were considerably cheaper to construct in terms of mineral nutrients than leaflets. e.g., petioles contained on average only 5.55±0.14% of total leaf nitrogen. Since in many cases the availability of mineral nutrients such as nitrogen rather than organic carbon sets limits to total leaf biomass on the plant, I suggested, contrary to previous claims, that the costs for foliage formation should not necessarily be different between compound- and simple-leaved species. Compound-leaved species also fit the basic relationships previously observed in simple-leaved analogues. Leaf size increased and leaf dry mass per area (LMA) decreased with increasing shade-tolerance. Thus, more shade-tolerant species construct a more effective foliar display for light interception at low irradiance with similar biomass investment in leaves. Species shade-tolerance was independent of biomass investment in petioles. However, due to the genotypic plasticity in LMA, more shade-tolerant species supported more foliar area at a constant leaf biomass investment in petioles.     

Distribution patterns of foliar carbon and nitrogen as affected by tree dimensions and relative light conditions in the canopy of Picea abies

 Variations in the partitioning of foliar carbon and nitrogen in combination with changes in needle and shoot structure were studied in trees of Picea abies along a vertical gradient of relative irradiance (RI). RI was the major determinant of needle morphology, causing all needle linear parameters – width, thickness and length – to increase. Due to the different responsiveness of needle thickness and width in respect of RI, the ratio of total to projected needle area increased with RI. Furthermore, shoot structure was also influenced by RI, and the ratio of shoot silhouette area to total needle area, which characterises the packing of needles and needle area within the shoot, was greater at lower values of irradiance. Needle dry weight per total needle area (LWA_t) was also increased by RI. Similarly, irrespective of the measure for surface area, needle nitrogen content per area, as the product of needle dry weight per area and nitrogen content per needle dry weight (N_m), scaled quasi-linearly with needle weight per area. Thus, the changes in needle and shoot morphology made it possible to invest more photosynthesising weight per unit light-intercepting surface there, where the pay-back due to elevated irradiances was the highest. However, N_m behaved in an entirely different manner, decreasing hyperbolically with LWA_t. Since non-structural (carbon in non-structural carbohydrates), and structural (total minus non-structural) needle carbon per dry weight also increased with LWA_t, N_m was inversely correlated with both non-structural and structural carbon. Total tree height, increasing significantly LWA_t, also influenced needle structure. It appeared that total height did not affect needle thickness or width, but larger trees had greater needle density (dry weight per volume). Because needle density was positively correlated with needle carbon content per dry weight, it was assumed that the greater values of needle carbon content can be attributed to increased lignification and thickening of needle cell walls. Thus, it appeared that the proportion of supporting structures was greater in needles of larger trees. Inasmuch as an increased fraction of supporting structures dilutes other leaf substances, including also leaf compounds responsible for CO₂-assimilation, enhanced requirement for supporting structures may be responsible for lower rates of carbon assimilation per foliage dry weight observed in large trees. Increasing water limitation with increasing tree size is discussed as a possible cause for increased needle supporting costs in large trees. Received: 2 April 1995 / Accepted: 16 February 1996     

Data on the adult marine and migratory phases in the life cycle of the southern hemisphere lamprey,Geotria australis Gray

Synopsis Large numbers of the Southern Hemisphere lamprey, Geotria australis, have been found in the regurgitated food of albatrosses breeding on South Georgia. This finding suggests that this lamprey is found in large groups at sea, presumably associated with its host, and can travel very large distances from its natal streams. The length and morphology of the individuals from South Georgia, which almost certainly represent a South American stock, were compared with those of representatives of the immediately pre- and post-marine trophic stages of G. australis caught in Western Australia. No significant differences could be detected either in the number of trunk myomeres or in the number and arrangement of the teeth. The mean length of the animals (± 95% confidence limits) from South Georgia was 45.9 ± 0.90 cm compared with 10.0 ± 0.23 cm and 62.5 ± 0.85 cm in G. australis collected from Western Australia just before they had entered and returned from the sea respectively.     

On the evolutionary consequences of increasing litter size with multiple paternity in wild boar (Sus scrofa scrofa)

Understanding how some species may be able to evolve quickly enough to deal with anthropogenic pressure is of prime interest in evolutionary biology, conservation, and management. Wild boar (Sus scrofa scrofa) populations keep growing all over Europe despite increasing hunting pressure. In wild boar populations subject to male‐selective harvesting, the initially described polygynous mating system may switch to a promiscuous/polyandrous one. Such a change in the mating system, where potentially more males sire a litter at one reproductive event, may be associated with the retention of high genetic diversity and an increase of litter size. We tested these hypotheses by estimating the number of sires per litter based on a six‐year long monitoring of a wild boar population subject to particularly high harvesting pressure. Our results show a high and stable genetic diversity and high rates of multiple paternity compared to other populations, thus depicting a promiscuous/polyandrous mating system in this population. We also show that litter size is positively linked to the number of sires, suggesting that multiple paternity increases fecundity. We finally discuss that multiple paternity may be one of the factors allowing rapid evolution of this population by maintaining both genetic and phenotypic diversity.     

Plant growth-form alters the relationship between foliar morphology and species shade-tolerance ranking in temperate woody taxa

Variation in leaf size (area per leaf) and leaf dry weight per area (LWA) in relation to species shade- and drought-tolerance, characterised by Ellenberg's light (ELD) and water demand (EWD) values, respectively, were examined in 60 temperate woody taxa at constant relative irradiance. LWA was independent of plant size, but leaf size increased with total plant height at constant ELD. Canopy position also affected leaf morphology: leaves from the upper crown third had higher LWA and were larger than leaves from the lower third. Leaf size and LWA were negatively correlated, and leaf size decreased and LWA increased with decreasing species shade-tolerance. Mean LWA was similar for trees and shrubs, but trees had larger leaves than shrubs. Furthermore, all relationships were altered by plant growth-form: none of the qualitative tendencies was significant for trees. This implies the considerably lower plasticity of foliar parameters in trees than those in shrubs. Accordingly, shade-tolerance of trees, having relatively constant leaf structure, may be most affected by the variability in biomass partitioning and crown geometry which influence foliage distribution and spacing and finally determine canopy light absorptance. Alteration of leaf form and investment pattern for construction of unit foliar surface area which change the efficiency of light interception per unit biomass investment in leaves, is a competitive strategy inherent to shrubs. EWD as well as wood anatomy did not control LWA and leaf size, though there was a trend of ring-porous tree species to be more shade-tolerant than diffuse-porous trees. Since ring-porous species are more vulnerable to cavitation than diffuse-porous species, they may be constrained to environments where irradiances and consequently evaporative demand is lower.     

Comments on the through-space singlet energy transfers and energy migration (exciton) in the light harvesting systems

Recent findings on the photophysical investigations of several cofacial bisporphyrin dyads for through space singlet and triplet energy transfers raised several serious questions about the mechanism of the energy transfers and energy migration in the light harvesting devices, notably LH II, in the heavily studied purple photosynthetic bacteria. The key issue is that for simple cofacial or slipped dyads with controlled geometry using rigid spacers or spacers with limited flexibilities, the fastest possible rates for singlet energy transfer for three examples are in the 10 x 10(9)s(-1) (i.e. just in the 100 ps time scale) for donor-acceptor distances approaching 3.5-3.6 A. The reported time scale for energy transfers between different bacteriochlorophylls, notably B800*-->B850, is in the picosecond time scale despite the long Mg...Mg separation of approximately 18 A. Such a short rate drastically contrasts with the well accepted F?rster theory. This article reviews the modern knowledge of the structure, bacteriochlorophyll a transition moments, and photophysical processes and dynamics in LH II, and compares these parameters with the recently investigated model bisporphyrin dyads build upon octa-etio-porphyrin chromophores and rigid and semi-rigid spacers. The recently discovered role of the rhodopin glucoside residue called carotenoid will be commented as the possible relay for energy transfer, including the possibility of uphill processes at room temperature. In this context, the concept of energy migration, called exciton, may also be affected by relays and uphill processes. Also, it is becoming more and more apparent that the presence of an irreversible electron transfer reaction at the reaction center, i.e. electron transfer from the special pair to the phyophytin macrocycle and so on, renders the rates for energy transfer and migration more rapid precluding all possibility of back transfers.     

Light regulation of light‐harvesting antenna size substantially enhances photosynthetic efficiency and biomass yield in green algae†

One of the major factors limiting biomass productivity in algae is the low thermodynamic efficiency of photosynthesis. The greatest thermodynamic inefficiencies in photosynthesis occur during the conversion of light into chemical energy. At full sunlight the light‐harvesting antenna captures photons at a rate nearly 10 times faster than the rate‐limiting step in photosynthetic electron transport. Excess captured energy is dissipated by non‐productive pathways including the production of reactive oxygen species. Substantial improvements in photosynthetic efficiency have been achieved by reducing the optical cross‐section of the light‐harvesting antenna by selectively reducing chlorophyll b levels and peripheral light‐harvesting complex subunits. Smaller light‐harvesting antenna, however, may not exhibit optimal photosynthetic performance in low or fluctuating light environments. We describe a translational control system to dynamically adjust light‐harvesting antenna sizes for enhanced photosynthetic performance. By expressing a chlorophyllide a oxygenase (CAO) gene having a 5′ mRNA extension encoding a Nab1 translational repressor binding site in a CAO knockout line it was possible to continuously alter chlorophyll b levels and correspondingly light‐harvesting antenna sizes by light‐activated Nab1 repression of CAO expression as a function of growth light intensity. Significantly, algae having light‐regulated antenna sizes had substantially higher photosynthetic rates and two‐fold greater biomass productivity than the parental wild‐type strains as well as near wild‐type ability to carry out state transitions and non‐photochemical quenching. These results have broad implications for enhanced algae and plant biomass productivity.     

Changes in foliage distribution with relative irradiance and tree size: Differences between the saplings ofAcer platanoides andQuercus robur

Dependencies of foliage arrangement and structure on relative irradiance and total height (TH) were studied in saplings ofAcer platanoides andQuercus robur. The distribution of relative foliar area and dry weight (leaf area and weight in a crown layer per total tree leaf area and weight, respectively) were examined with respect to relative height (RH, height in the crown per TH) and characterized by the Weibull function. The distributions of relative area and weight were nearly identical, and the differences between them were attributable to a systematic decline in leaf dry weight per area with increasing crown depth. Foliage distribution was similarly altered by tree size in both species; RH at foliage maximum was lower and relative canopy size (RCS, length of live crown per TH) greater in taller trees. However, the distribution was more uniform inA. platanoides than inQ. robur. Apart from the size effects, relative irradiance also influenced canopy structure; RCS increased inQ. platanoides and decreased inQ. robur with increasing irradiance. As crown architecture was modified by irradiance, foliage distribution was shifted upwards with decreasing irradiance inA. platanoides, but it was independent of irradiance inQ. robur. Higher foliage maximum at lower irradiance in more shade-tolerantA. platanoides is likely to contribute towards more efficient foliar display for light interception and increase the competitive ability of this species in light-limited environments. Consequently, these differences in crown architecture and foliage distribution may partly explain the superior behavior ofA. platanoides in understory.