Seasonal variation in biomass and carbohydrate partitioning of understory sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) seedlings

We investigated seasonal patterns of biomass and carbohydrate partitioning in relation to shoot growth phenology in two age classes of sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britt.) seedlings growing in the understory of a partially harvested forest. The high root:shoot biomass ratio and carbohydrate concentration of sugar maple are characteristic of species with truncated growth patterns (i.e., cessation of aboveground shoot growth early in the growing season), a conservative growth strategy and high shade tolerance. The low root:shoot biomass ratio and carbohydrate concentration of yellow birch are characteristic of species with continuous growth patterns, an opportunistic growth strategy and low shade tolerance. In both species, starch represented up to 95% of total nonstructural carbohydrates and was mainly found in the roots. Contrary to our hypothesis, interspecific differences in shoot growth phenology (i.e., continuous versus truncated) did not result in differences in seasonal patterns of carbohydrate partitioning. Our results help explain the niche differentiation between sugar maple and yellow birch in temperate, deciduous understory forests.     

Crown architecture of stand-grown sugar maple (Acer saccharum Marsh.) in the Adirondack Mountains

Leaf and crown morphology of shade-tolerant sugar maple (Acer saccharum Marsh.) were examined to test the hypotheses (1) that leaf area exhibits significant plasticity both within and between crown classes and individual tree crowns and (2) that leaf area is accurately predicted from estimates of crown volume. A total of 18 trees, ranging from 3.3 to 43.4 cm dbh, were felled and dissected into upper, middle, lower, and below-crown layers, for measurements of leaf, bark, and xylem dimensions. For dominant trees only, bark thickness and xylem radii were higher within the crown than below the crown. Cumulative leaf area index increased with decreasing stratum height at similar rates in all trees, except for two trees that were located in the understory. Area leaf weight declined with decreasing stratum height within the crown of all except four overstory trees. These four trees showed an increase with decreasing stratum height, i.e., leaves were heavier per unit area in the lower crown stratum and below the crown than they were at mid-crown. Within-tree leaf area density was usually higher in the upper crown of overstory trees and in the lower crown of understory trees. Total crown volume was the best predictor of whole-tree leaf area, but it was only slightly better than dbh.     

Influence of light availability on growth, leaf morphology and plant architecture of beech ( Fagus sylvatica L.), maple ( Acer pseudoplatanus L.) and ash ( Fraxinus excelsior L.) saplings

In a field study, we measured saplings of beech, ash and maple growing in a fairly even-aged mixed-species thicket established by natural regeneration beneath a patchy shelterwood canopy with 3–60% of above canopy radiation reaching the saplings. Under low light conditions, maple and ash showed a slight lead in recent annual length increment compared with beech. With increasing light, ash and maple constantly gained superiority in length increment, whereas beech approached an asymptotic value above 35% light. A suite of architectural and leaf morphological attributes indicated a more pronounced ability of beech to adapt to shade than ash and maple. Beech displayed its leaves along the entire tree height (with a concentration in the middle crown), yielding a higher live crown ratio than ash and maple. It allocated biomass preferentially to radial growth which resulted in low height to diameter ratios, and expressed marked plagiotropic growth in shade indicating a horizontal light-foraging strategy. In addition, beech exhibited the highest specific leaf area, a greater total leaf area per unit tree height, a slightly greater leaf area index, and a greater plasticity to light in total leaf area. Ash and maple presented a “gap species” growth strategy, characterized by a marked and constant response in growth rates to increasing light and an inability to strongly reduce their growth rates in deep shade. In shade, they showed some plasticity in displaying most of their leaf area at the top of the crown to minimize self-shading and to enhance light interception. Through this, particularly, maple developed an “umbrella” like crown. These species-specific responses may be used for controlling the development of mixed-species regeneration in shelterwood systems.     

Tree allometry, leaf size and adult tree size in old-growth forests of western Oregon

Relationships between tree height and crown dimensions and trunk diameter were determined for shade-tolerant species of old-growth forests of western Oregon. The study included both understory and overstory species, deciduous and evergreen angiosperms and evergreen conifers. A comparison of adult understory species with sapling overstory species of similar height showed greater crown width and trunk diameter in the former, whether the comparison is made among conifers or deciduous trees. Conifer saplings had wider crowns than deciduous saplings, but the crown widths of the two groups converged with increase in tree height. Conifer saplings had thicker trunks than deciduous saplings of similar crown width, possibly because of selection for resistance to stem bending under snow loads. The results suggest that understory species have morphologies that increase light interception and persistence in the understory, whereas overstory species allocate their biomass for efficient height growth, thereby attaining the high-light environment of the canopy. The greater crown widths and the additional strength requirements imposed by snow loads on conifer saplings result in less height growth per biomass increment in conifer saplings than in deciduous saplings. However, the convergence in crown width of the two groups at heights greater than 20 m, and the proportionately smaller effect of snow loads on large trees, may result in older conifers equalling or surpassing deciduous trees in biomass allocation to height growth.     

Functional traits and plasticity in response to light in seedlings of four Iberian forest tree species

We investigated the differential roles of physiological and morphological features on seedling survivorship along an experimental irradiance gradient in four dominant species of cool temperate-Mediterranean forests (Quercus robur L., Quercus pyrenaica Willd., Pinus sylvestris L. and Pinus pinaster Ait.). The lowest photochemical efficiency (F(v)/F(m) in dark-adapted leaves) was reached in deep shade (1% of full sunlight) in all species except Q. robur, which had the lowest photochemical efficiency in both deep shade and 100% of full sunlight. Species differed significantly in their survival in 1% of full sunlight but exhibited similar survivorship in 6, 20 and 100% of full sunlight. Shade-tolerant oaks had lower leaf area ratios, shoot to root ratios, foliage allocation ratios and higher rates of allocation to structural biomass (stem plus thick roots) than shade-intolerant pines. Overall phenotypic plasticity for each species, estimated as the difference between the minimum and the maximum mean values of the ecophysiological variables studied at the various irradiances divided by the maximum mean value of those variables, was inversely correlated with shade tolerance. Observed morphology, allocation and plasticity conformed to a conservative resource-use strategy, although observed differences in specific leaf area, which was higher in shade-tolerant species, supported a carbon gain maximization strategy. Lack of a congruent suite of traits underlying shade tolerance in the studied species provides evidence of adaptation to multiple selective forces. Although the study was based on only four species, the importance of ecophysiological variables as determinants of interspecific differences in survival in limiting light was demonstrated.     

Effects of light and nutrients on seedlings of tropical Bauhinia lianas and trees

Lianas differ from trees in many life history characteristics, and we predicted that they are phenotypically more responsive to environmental variation than trees. We analyzed responsiveness to light and nutrient availability of five Bauhinia species (three lianas and two trees). Seedlings were grown in a shade house in two light regimes (5 and 25% of full sunlight) and two nutrient supply regimes (field soil and N fertilization equivalent to 100 kg ha(-1)), and important growth-related physiological and morphological plant parameters were measured. Light availability affected most of the measured variables, whereas N addition had only weak effects. In the four light-demanding species (two lianas and two trees), relative plant biomass growth rate increased and specific leaf area (SLA) decreased with increased light availability, whereas a shade-tolerant liana did not respond. Leaf N concentration and light-saturated photosynthetic rate per unit leaf area increased in response to increased irradiance or soil N in the light-demanding tree species and the shade-tolerant liana, but not in the two light-demanding lianas. The light-demanding lianas also had higher SLA and leaf mass ratio, resulting in a higher leaf area ratio (LAR) in high light, whereas the light-demanding trees did not. Across all treatments, mean plasticity indices of physiological and morphological traits, and all traits combined were similar among the studied species. Plasticity was higher in response to light than to N, indicating that light is the main factor controlling seedling responses of the studied species. Although lianas and trees did not differ in mean plasticity in response to light and N, the light-demanding lianas were phenotypically less plastic in LAR and in photosynthetic rates and biomass allocation than the trees. Light and N interacted in their effects on most physiological variables, but the consequences for relative growth rate differed little among species. We conclude that, contrary to our predictions, lianas were no more responsive to variation in light and N availability than trees.     

Crown architecture of understory and open-grown white pine (Pinus strobus L.) saplings

Crown architecture and growth allocation were studied in saplings of eastern white pine (Pinus strobus L.), a species classified as intermediate in shade tolerance. A comparison was made of 15 understory saplings and 15 open-grown saplings that were selected to have comparable heights (mean of 211 cm, range of 180-250 cm). Mean ages of understory and open-grown trees were 25 and 8 years, respectively. Understory trees had a lower degree of apical control, shorter crown length, and more horizontal branch angle, resulting in a broader crown shape than that of open-grown trees. Total leaf area was greater in open-grown saplings than in understory saplings, but the ratio of whole-crown silhouette (projected) leaf area to total leaf area was significantly greater in understory pine (0.154) than in open-grown pine (0.128), indicating that the crown and shoot structure of understory trees exposed a greater percentage of leaf area to direct overhead light. Current-year production of understory white pine was significantly less than that of open-grown white pine, but a higher percentage of current-year production was allocated to foliage in shoots of understory saplings. These modifications in crown structure and allocation between open-grown and understory white pine saplings are similar to those reported for more shade-tolerant fir (Abies) and spruce (Picea) species, but the modifications were generally smaller in white pine. As a result, white pine did not develop the flat-topped "umbrella" crown structure observed in understory fir and spruce, which approaches the idealized monolayer form that maximizes light interception. The overall change to a broader crown shape in understory white pine was qualitatively similar, but much more limited than the changes that occurred in fir and spruce. This may prevent white pine from persisting in understory shade as long as fir and spruce saplings.     

Functional relationships between crown morphology and within-crown characteristics of understory saplings of three codominant conifers in a subalpine forest in central Japan

Light-related plasticity of crown morphology and within-crown characteristics were investigated in understory sun and shade saplings of three codominant subalpine conifers, Abies mariesii M.T. Mast., Abies veitchii Lindl. and Picea jezoensis var. hondoensis (Mayr) Rehd. Compared with those of sun saplings, current-year shoots of shade saplings allocated less biomass to needles, resulting in less dense needle packing and hence less mutual needle shading. The proportion of lateral branch biomass in foliage was either similar in sun and shade saplings or greater in shade saplings, depending on the species, suggesting that, over the lifetime of a branch, greater needle longevity in shade compensates for reduced biomass investment in needles of current-year shoots of shade saplings. Saplings with slower-growing branches tended to have greater needle life spans, suggesting that plasticity of branch growth rate and plasticity of needle life span are interdependent. Both Abies species showed greater light-related plasticity of needle life span and branch growth than P. jezoensis. The greater shade tolerance of the Abies species derives from their broad flattened crowns with slow-growing branches. This type of crown development incurs substantial support costs, but the long needle life span of shade saplings of the Abies species compensates, at least in part, for their low annual investment in foliage, especially in the case of A. mariesii, which has a longer needle life span and slower-growing and stouter branches than A. veitchii. Compared with the Abies species, P. jezoensis had a less plastic crown morphology, and less variability of needle life span and branch growth in response to light, resulting in lower shade tolerance. However, compared with the flattened crown of Abies shade saplings, the conical crown of P. jezoensis saplings imposes a smaller support cost, making this species better adapted to rapid height growth than to survival in shade.     

丘陵山地针阔混交林光合作用-光响应特性研究

为探讨丘陵山地针阔混交林中杉木、马尾松、红榉、闽楠、南酸枣5个混交树种对光环境的适应及生理响应,对5个混交树种树冠上、下层叶绿素含量、净光合速率、光响应曲线等光合特性进行了研究。结果表明:混交林中各树种净光合速率与树冠部位、叶绿素含量均达到显著相关性。闽楠叶绿素a/b最低,并具有较高的表观量子效率,为耐阴树种;南酸枣与红榉的叶绿素a/b较高,表观量子效率较低,为喜光树种;杉木、马尾松叶绿素a/b和表观量子效率居中,为喜光树种,略耐阴。闽楠、红榉、杉木光适应幅度较大,且兼具不同需光特性,是适宜的针阔混交造林配置树种。     

Relative importance of photosynthetic physiology and biomass allocation for tree seedling growth across a broad light gradient

Studies of tree seedling physiology and growth under field conditions provide information on the mechanisms underlying inter- and intraspecific differences in growth and survival at a critical period during forest regeneration. I compared photosynthetic physiology, growth and biomass allocation in seedlings of three shade-tolerant tree species, Virola koschynii Warb., Dipteryx panamensis (Pittier) Record & Mell and Brosimum alicastrum Swartz., growing across a light gradient created by a forest-pasture edge (0.5 to 67% diffuse transmittance (%T)). Most growth and physiological traits showed nonlinear responses to light availability, with the greatest changes occurring between 0.5 and 20 %T. Specific leaf area (SLA) and nitrogen per unit leaf mass (N mass) decreased, maximum assimilation per unit leaf area (A area) and area-based leaf N concentration (N area) increased, and maximum assimilation per unit leaf mass (A mass) did not change with increasing irradiance. Plastic responses in SLA were important determinants of leaf N and A area across the gradient. Species differed in magnitude and plasticity of growth; B. alicastrum had the lowest relative growth rates (RGR) and low plasticity. Its final biomass varied only 10-fold across the light gradient. In contrast, the final biomass of D. panamensis and V. koschynii varied by 100- and 50-fold, respectively, and both had higher RGR than B. alicastrum. As light availability increased, all species decreased biomass allocation to leaf tissue (mass and area) and showed a trade-off between allocation to leaf area at a given plant mass (LAR) and net gain in mass per unit leaf area (net assimilation rate, NAR). This trade-off largely reflected declines in SLA with increasing light. Finally, A area was correlated with NAR and both were major determinants of intraspecific variation in RGR. These data indicate the importance of plasticity in photosynthetic physiology and allocation for variation in tree seedling growth among habitats that vary in light availability.     

Sugar maple and yellow birch regeneration in response to canopy opening,liming and vegetation control in a temperate deciduous forest of Quebec

We examined how the density, growth and survival of sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britton) regeneration are influenced by gap size, soil nutrient availability and understory vegetation. We used a factorial combination of (1) three gap sizes (small: <100 m²; medium: 100–300 m²; large: ∼1000 m²); (2) presence/absence of liming (92% CaCO₃ at 500 kg ha⁻¹, 1st year post-harvest); and (3) presence/absence of vegetation control (weeding twice a year; 1st to 3rd year post-harvest). We monitored height increment and survival of 1500 seedlings and saplings of both species from the 3rd to the 6th year post-harvest, and assessed density 6 years post-harvest. Both species exhibited a complex set of density, growth and survival responses across the combination of treatments. Compared to sugar maple, yellow birch had an overall lower density, greater growth, and similar survival rate; the two species attained maximum values in different gap size for density, and similar gap size for growth and survival. Liming had very little or no effect on the species. The growth of yellow birch was slightly but significantly greater when understory vegetation was controlled, particularly in medium and large gaps. These results suggest that a variety of canopy gap sizes can provide the right combination of understory conditions for regenerating these two functionally different tree species.     

Responses of deciduous broadleaf trees to defoliation in a CO2 enriched atmosphere

Relatively little is known about the implications of atmospheric CO2 enrichment for tree responses to biotic disturbances such as folivory. We examined the combined effects of elevated CO2 concentration ([CO2]) and defoliation on growth and physiology of sugar maple (Acer saccharum Marsh.) and trembling aspen (Populus tremuloides Michx.). Seedlings were planted in the ground in eight open-top chambers. Four chambers were ventilated with CO2-enriched air (ambient + 283 micromol mol-1) and four chambers were supplied with ambient air. After 6 weeks of growth, half of the leaf area was removed on a subset of seedlings of each species in each CO2 treatment. We monitored subsequent biomass gain and allocation, along with leaf gas exchange and chemistry. Defoliation did not significantly affect final seedling biomass in either species or CO2 treatment. Growth recovery following defoliation was associated with increased allocation to leaf mass in maple and a slight enhancement of mean photosynthesis in aspen. Elevated [CO2] did not significantly affect aspen growth, and the observed stimulation of maple growth was significant only in mid-season. Correspondingly, simulated responses of whole-tree photosynthesis to elevated [CO2] were constrained by a decrease in photosynthetic capacity in maple, and were partially offset by reductions in specific leaf area and biomass allocation to foliage in aspen. There was a significant interaction between [CO2] and defoliation on only a few of the measured traits. Thus, the data do not support the hypothesis that atmospheric CO2 enrichment will substantially alter tree responses to folivory. However, our findings do provide further indication that regeneration-stage growth rates of certain temperate tree species may respond only moderately to a near doubling of atmospheric [CO2].     

Growth and photosynthetic responses of four Virginia Piedmont tree species to shade

To determine the effects of shade on biomass, carbon allocation patterns and photosynthetic response, seedlings of loblolly pine (Pinus taeda L.), white pine (Pinus strobus L.), red maple (Acer rubrum L.), and yellow-poplar (Liriodendron tulipifera L.) were grown without shade or in shade treatments providing a 79 or 89% reduction of full sunlight for two growing seasons. The shade treatments resulted in less total biomass for all species, with loblolly pine showing the greatest shade-induced growth reduction. Yellow-poplar was the only species to show increased stem height growth in the 89% shade treatment. The shade treatments increased specific leaf area of all species. Quantum efficiency, dark respiration and light compensation point were generally not affected by the shade treatments. Quantum efficiency, dark respiration, maximum photosynthesis and light compensation point did not change consistently between the first and second growing seasons. We conclude that differences in shade tolerance among these species are not the result of changes in the photosynthetic mechanism in response to shade.     

Interspecific variation in nighttime transpiration and stomatal conductance in a mixed New England deciduous forest

Transpiration is generally assumed to be insignificant at night when stomata close in response to the lack of photosynthetically active radiation. However, there is increasing evidence that the stomata of some species remain open at night, which would allow for nighttime transpiration if there were a sufficient environmental driving force. We examined nighttime water use in co-occurring species in a mixed deciduous stand at Harvard Forest, MA, using whole-tree and leaf-level measurements. Diurnal whole-tree water use was monitored continuously with Granier-style sap flux sensors in paper birch (Betula papyrifera Marsh.), red oak (Quercus rubra L.) and red maple (Acer rubrum L.). An analysis was conducted in which nighttime water flux could be partitioned between refilling of internal water stores and transpiration. Substantial nighttime sap flux was observed in all species and much of this flux was attributed to the refilling of depleted water stores. However, in paper birch, nighttime sap flux frequently exceeded recharge estimates. Over 10% of the total daily sap flux during the growing season was due to transpiration at night in paper birch. Nighttime sap flux was over 8% of the total daily flux in red oak and 2% in red maple; however, this flux was mainly associated with recharge. On nights with elevated vapor pressure deficit, sap flux continued through the night in paper birch, whereas it reached zero during the night in red oak and red maple. Measurements of leaf-level gas exchange on a night with elevated vapor pressure deficit showed stomatal conductance dropping by only 25% in paper birch, while approaching zero in red oak and red maple. The study highlighted differences in ecophysiological controls on sap flux exerted by co-occurring species. Paper birch is a fast-growing, shade-intolerant species with an earlier successional status than red oak and red maple. Risking water loss through nighttime transpiration may provide paper birch with an ecological advantage by enabling the species to maximize photosynthesis and support rapid growth. Nighttime transpiration may also be a mechanism for delivering oxygen to respiring cells in the deep sapwood of paper birch.     

Poor recruitment is changing the structure and species composition of an old-growth hemlock-hardwood forest

Anthropogenic factors such as elevated deer populations, invasive earthworms or climate change may alter old-growth forests of the Upper Midwest region of the United States. We examined demographic trends of woody species across all size classes over 35 years in a late-successional forest dominated by hemlock (Tsuga canadensis), sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) in Michigan's Upper Peninsula using two sets of permanent plots. For the duration of the study period, species that were less-preferred white-tailed deer (Odocoileus virginianus) forage, especially sugar maple, comprised a much higher fraction of all seedlings and saplings compared to overstory trees. The density of small sugar maple declined across the study period, but no other species became more abundant, creating a more open forest understory. By the most recent census, preferred species for deer browse had been nearly eliminated from the understory, and declines in unpreferred species such as sugar maple were also apparent. We found small changes in temperature (<0.5-1 °C rise in minimum and maximum temperatures depending on season) and precipitation (±28 mm depending on season) and little evidence of invasive earthworms impacts. Our results suggest that the sustained elevated deer density is shifting the structure and composition of this old-growth forest. A demographic model showed that if current recruitment, growth and mortality rates were to continue for 500 years the forest would eventually reach a new equilibrium with virtually no hemlock or yellow birch remaining.     

Hydraulic responses to environmental perturbations in Tsuga canadensis and Betula lenta

Eastern hemlock (Tsuga canadensis (L). Carr.) is a late-successional species found across the northeastern United States of America that is currently threatened by the exotic pest, hemlock woolly adelgid (Adelges tsugae Annand). Because whole-tree physiological characteristics may scale to influence ecosystem processes, we considered whole-tree hydraulic controls in eastern hemlock and the replacement species black birch (Betula lenta L.). Through a series of misting perturbations, whole-tree resistances (R), capacitances (C) and time constants (tau) were determined from time series sap flux data in eastern hemlock and black birch. Black birch trees responded more rapidly to environmental perturbations than eastern hemlock. Utilizing the step function after applied treatments, whole-tree tau ranged between 9.4 and 24.8 min in eastern hemlock trees compared with 5.9 to 10.5 min in black birch. Species was not a significant predictor of R or C when controlling for tree size. In both species, R decreased with sapwood area and C increased. Our tau results indicate that the loss and replacement of eastern hemlock by black birch will decrease the lag between transpiration and absorption of water from the soil and potentially alter the diurnal pattern of carbon and water uptake.     

Species composition and dynamics in two hardwood stands in Vermont: a disturbance history

Stand composition and structure utilizing stem analysis was studied in two hardwood stands in Vermont. In a mixed hardwood stand with some white pine and hemlock, a major entry of new trees in the main canopy seems associated with harvesting coincident with land exchange. More recent partial cuttings have promoted establishment of new seedlings or development of suppressed advanced-growth shade-tolerant beech, (Fagus grandifolia, Ehrh.), hophornbeam (Ostrya virginiana, (Mill) K. Koch), and striped maple (Acer pensylvanicum, L.). Very few sugar maple and red oak seedlings and saplings are present.

In a northern hardwood stand some red spruce (Picea rubens, Sarg.), that were 240 to 306 years old, became established before any known harvest, and exhibited release following harvests of the mid-1800s. This major harvest, coupled with the differential growth between spruce and hardwoods, and seed/seedling availability, resulted in a major change in stand composition. Trees now in the main canopy of sampled stands appear to have either been released or newly established following various harvests. Harvests have been of such frequency that natural disturbances seem insignificant. Many of the competitive understory species have become abundant following harvests of the 1960s and 1980s and may have been present as advanced growth and responded to the release. Following the harvest of 1981–1982, abundant yellow birch became established on skid trails. Elsewhere in the stand, yellow birch seedlings and saplings are only in great abundance in areas that were possibly sizable gaps following earlier harvesting.

The dynamics of tree entry and growth in gaps of small or large size probably occur in a similar way in many other stands of the region. Though the sampling of this study is limited, there is no suggestion of continuous tree establishment at any particular location, the new age classes seem associated with either a gap or stand replacing disturbance attributed to harvesting.     

Crown plasticity reduces inter-tree competition in a mixed broadleaved forest

The objective of the study was to analyse to which extent horizontal crown plasticity reduces inter-tree competition at stand scale, and how it relates to species growth strategy. Two components of crown plasticity defined at the individual tree level (crown shape distortion, CSD and crown displacement relative to stem, CRD) were analysed and their relative importance in the reduction of competition was quantified. Inter-tree competition at stand scale was estimated using spatial pattern analysis and crown overlap estimation. Measurements were performed in a mixed broadleaved stand of Western Europe. Crown plasticity was shown to regularise the spatial distribution of crowns in comparison with the corresponding stems and to reduce inter-tree competition by optimising space occupation. A significant reduction in crown overlap was observed, mainly due to CRD and secondarily to CSD. At the species level, CSD and CRD were positively correlated. In addition, both were negatively correlated with species shade-tolerance scores. In particular, three European temperate tree species showed contrasting responses, which were related to their known specific ecological strategies. Fagus sylvatica, known to be a highly shade-tolerant species, showed large crowns, low CSD and CRD, indicating a low plasticity and suggesting a strong competitive ability. At the opposite, Quercus sp., known to be clearly less shade-tolerant, exhibited a reduced growth, associated with high CSD and CRD, indicating high crown plasticity and probably a lower competitive ability. For this species, plasticity could be described as passive. Last, Carpinus betulus, known to have a relatively good shade-tolerance, showed a contrasting behaviour with high CSD and CRD and a strong presence in the stand, suggesting high crown plasticity and a good competitive ability. In that case, plasticity was described as rather adaptive.     

Leaf morphology and photosynthetic adjustments among deciduous broad-leaved trees within the vertical canopy profile

Photosynthetic acclimation of deciduous broad-leaved tree species was studied along a vertical gradient within the canopy of a multi-species deciduous forest in northern Japan. We investigated variations in (1) local light regime and CO2 concentration ([CO2]), and (2) morphological (area, thickness and area per mass), biochemical (nitrogen and chlorophyll concentrations) and physiological (light-saturated photosynthetic rate) attributes of leaves of seven major species on three occasions (June, August and October). We studied early successional species, alder (Alnus hirsuta (Spach) Rupr.) and birch (Betula platyphylla var. japonica (Miq.) Hara); gap phase species, walnut (Juglans ailanthifolia Carrière) and ash (Fraxinus mandshurica var. japonica Rupr.); mid-successional species, basswood (Tilia japonica (Miq.) Simonk.) and elm (Ulmus davidiana var. japonica (Rehd.) Nakai); and the late-successional species, maple (Acer mono Bunge). All but maple initiated leaf unfolding from the lower part of the crown. The [CO2] within the vertical profile ranged from 320-350 ppm in the upper canopy to 405-560 ppm near the ground. The lowest and highest ambient [CO2] occurred during the day and during the night, respectively. This trend was observed consistently during the summer, but not when trees were leafless. Chlorophyll concentration was positively related to maximum photosynthetic rate within, but not among, species. Leaf senescence started from the inner part of the crown in alder and birch, but started either in the outer or top portion of the canopy of ash, basswood and maple. Chlorophyll (Chl) to nitrogen ratio in leaves increased with decreasing photon flux density. However, Chl b concentration in all species remained stable until the beginning of leaf senescence. Maximum photosynthetic rates observed in sun leaves of early successional species, gap phase or mid-successional species, and late successional species were 12.5-14.8 micromol m(-2) s(-1), 4.1-7.8 micromol m(-2) s(-1) and 3.1 micromol m(-2) s(-1), respectively.