Effects of light spectra and 15N pulses on growth, leaf morphology, physiology, and internal nitrogen cycling in Quercus variabilis Blume seedlings.

Light spectra of sunlight transmittance can generate an interactive effect with deposited nitrogen (N) on regenerated plants across varied shading conditions. Total N content in understory plants can be accounted for by both exogeneous and endogenous sources of derived N, but knowledge about the response of inner N cycling to interactive light and N input effects is unclear. We conducted a bioassay on Chinese cork oak (Quercus variabilis Blume) seedlings subjected to five-month N pulsing with 15NH4Cl (10.39 atom %) at 120 mg 15N plant-1 under the blue (48.5% blue, 33.7% green, and 17.8% red), red (14.6% blue, 71.7% red, 13.7% green), and green (17.4% blue, 26.2% red, 56.4% green) lighting-spectra. Half of the seedlings were fed twice a week using a 250 ppm N solution with micro-nutrients, while the other half just received distilled water. Two factors showed no interaction and neither affected growth and morphology. Compared to the red-light spectrum, that in blue light increased chlorophyll and soluble protein contents and glutamine synthetase (GS) activity, root N concentration, and N derived from the pulses. The green-light spectrum induced more biomass allocation to roots and a higher percentage of N derived from internal reserves compared to the red-light spectrum. The 15N pulses reduced the reliance on N remobilization from acorns but strengthened shoot biomass, chlorophyll content, GS activity, and N concentration. In conclusion, light spectrum imposed an independent force from external N pulse to modify the proportion of N derived from internal sources in total N content in juvenile Q. variabilis.

Leaf biochemical adjustments in two Mediterranean resprouter species facing enhanced UV levels and reduced water availability before and after aerial biomass removal.

Effects of supplemented UV radiation and diminished water supply on the leaf concentrations of phenols and antioxidants of two Mediterranean resprouter species, Arbutus unedo and Quercus suber, were assessed before and after entire aerial biomass removal. Potted seedlings of both species were grown outdoors for 8 months with enhanced UV-A + UV-B, enhanced UV-A or ambient UV, in combination with two watering conditions (field capacity or watering reduction). After this period, all aerial biomass was removed and new shoots (resprouts) developed for a further 8 months under the two treatments. In general, the investment in leaf phenols was substantially greater in A. unedo than in Q. suber, while Q. suber allocated more resources to non-phenolic antioxidants (ascorbate and glutathione). In response to enhanced UV-B radiation, Q. suber leaves rose their UV-screening capacity mainly via accumulation of kaempferols, accompanied by an increased concentration of rutins, being these effects exacerbated under low-watering conditions. Conversely, A. unedo leaves responded to UV-B radiation reinforcing the antioxidant machinery by increasing the overall amount of flavonols (especially quercetins) in seedlings, and of ascorbate and glutathione, along with catalase activity, in resprouts. Nevertheless, UV effects on the amount/activity of non-phenolic antioxidants of A. unedo resprouts were modulated by water supply. Indeed, the highest concentration of glutathione was found under the combination of enhanced UV-B radiation and reduced watering, suggesting an enlargement of the antioxidant response in A. unedo resprouts. Different biochemical responses to enhanced UV and drier conditions in seedlings and resprouts of these two species might modulate their competitive interactions in the near future.

Linking fine root morphology, hydraulic functioning and shade tolerance of trees.

Background and Aims: Understanding root traits and their trade-off with other plant processes is important for understanding plant functioning in natural ecosystems as well as agricultural systems. The aim of the present study was to determine the relationship between root morphology and the hydraulic characteristics of several orders of fine roots (<2 mm) for species differing in shade tolerance (low, moderate and high). Methods: The morphological, anatomical and hydraulic traits across five distal root orders were measured in species with different levels of shade tolerance and life history strategies. The species studied were Acer negundo, Acer rubrum, Acer saccharum, Betula alleghaniensis, Betula lenta, Quercus alba, Quercus rubra, Pinus strobus and Pinus virginiana. Key Results: Compared with shade-tolerant species, shade-intolerant species produced thinner absorptive roots with smaller xylem lumen diameters and underwent secondary development less frequently, suggesting that they had shorter life spans. Shade-tolerant species had greater root specific hydraulic conductance among these roots due to having larger diameter xylems, although these roots had a lower calculated critical tension for conduit collapse. In addition, shade-intolerant species exhibited greater variation in hydraulic conductance across different root growth rings in woody transport roots of the same root order as compared with shade-tolerant species. Conclusions: Plant growth strategies were extended to include root hydraulic properties. It was found that shade intolerance in trees was associated with conservative root hydraulics but greater plasticity in number of xylem conduits and hydraulic conductance. Root traits of shade-intolerant species were consistent with the ability to proliferate roots quickly for rapid water uptake needed to support rapid shoot growth, while minimizing risk in uncertain environments.

Nitrogen deposition does not affect the impact of shade on Quercus acutissima seedlings.

Light and atmospheric nitrogen (N) deposition are among the important environmental factors influencing plant growth and forest regeneration. We used Quercus acutissima, a dominant broadleaf tree species native to the deciduous forests of Northern China, to study the combined effects of light exposure and N addition on leaf physiology and individual plant growth. In the greenhouse, we exposed Quercus acutissima seedlings to one of two light conditions (8% and 80% of full irradiation) and one of three N treatments (0, 6, and 12 g N m-2 y-1). After 87 d, we observed that nitrogen deposition had no significant effects on the seedlings regardless of light exposure. In addition, shade significantly reduced plant height, basal diameter, leaf number, total biomass, gas exchange capacity, and carbohydrate content. In contrast, however, shade significantly increased the amount of photosynthetic pigment, above-ground biomass allocation, and specific leaf area. There was also a hierarchical plasticity among the different seedling characteristics. Compared to traits of growth, biomass, biomass allocation and leaf morphology, the leaf physiology, including photosynthetic pigment, gas exchange, carbohydrate, and PUNE, is more sensitive to light conditions. Among the biomass allocation parameters, the leaf and root mass ratios had a relatively low phenotypic plasticity. The seedlings had high foliar physiological plasticity under various light conditions. Nevertheless, we recommend high irradiance to maintain vigorous seedling growth and, in turn, promote the restoration and reconstruction of vegetation.

Light energy partitioning, photosynthetic efficiency and biomass allocation in invasive Prunus serotina and native Quercus petraea in relation to light environment, competition and allelopathy.

This study addressed whether competition under different light environments was reflected by changes in leaf absorbed light energy partitioning, photosynthetic efficiency, relative growth rate and biomass allocation in invasive and native competitors. Additionally, a potential allelopathic effect of mulching with invasive Prunus serotina leaves on native Quercus petraea growth and photosynthesis was tested. The effect of light environment on leaf absorbed light energy partitioning and photosynthetic characteristics was more pronounced than the effects of interspecific competition and allelopathy. The quantum yield of PSII of invasive P. serotina increased in the presence of a competitor, indicating a higher plasticity in energy partitioning for the invasive over the native Q. petraea, giving it a competitive advantage. The most striking difference between the two study species was the higher crown-level net CO2 assimilation rates (Acrown) of P. serotina compared with Q. petraea. At the juvenile life stage, higher relative growth rate and higher biomass allocation to foliage allowed P. serotina to absorb and use light energy for photosynthesis more efficiently than Q. petraea. Species-specific strategies of growth, biomass allocation, light energy partitioning and photosynthetic efficiency varied with the light environment and gave an advantage to the invader over its native competitor in competition for light. However, higher biomass allocation to roots in Q. petraea allows for greater belowground competition for water and nutrients as compared to P. serotina. This niche differentiation may compensate for the lower aboveground competitiveness of the native species and explain its ability to co-occur with the invasive competitor in natural forest settings.

Association between radionuclides (210Po and 210Pb) and antioxidant enzymes in oak (Quercus coccifera) and mastic tree (Pistacia lentiscus).

The activity levels of naturally occurring radionuclides Polonium-210 and lead-210 in different subjects including plant species have direct or indirect impact on human beings. High levels of ionising radiation cause oxidative stress and the interaction between antioxidative defense and radionuclides is not well established in plant systems. In this study, we aimed to understand the impact of oxidative stress caused by 210Po and 210Pb in two Mediterranean plants; Quercus coccifera and Pistacia lentiscus. We analysed the constitutive and seasonal levels of 210Po, 210Pb, lipid peroxidation levels, superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities in the field-collected samples. The highest activity concentrations of 210Po and 210Pb were detected in both plants in summer and Q. coccifera had higher levels than that of P. lentiscus. SOD and APX activity trends were different between oak and mastic; as compared to P. lentiscus, Q. coccifera efficiently used the two major components of antioxidative defense. Lipid peroxidation levels were low in both plants in all seasons except that of spring which were in good agreement with high antioxidant enzyme activities. In conclusion, we found that high 210Po and 210Pb activity concentrations in oak and mastic did not interfere with their growth and life cycles. The ability of both plants for survival and adaptation to Mediterranean environmental constraints provided an additional advantage for coping radionuclide induced oxidative stress as well.

Leaf functional plasticity decreases the water consumption without further consequences for carbon uptake in Quercus coccifera L. under Mediterranean conditions.

The accumulation of epicuticular waxes over stomata in Quercus coccifera L. contributes to a severe reduction in maximum stomatal conductance (g s,max) under Mediterranean (MED) conditions. However, this phenomenon was not observed in this species under temperate (TEM) conditions, which could lead to differences in the ability to assimilate CO2 between the sites. We hypothesise that the overall importance of such a reduction in gs,max on photosynthesis is modulated by other factors affecting carbon gain, mainly mesophyll conductance to CO2 (g m), through a plastic response to changes in environmental conditions (i.e., vapour pressure deficit, VPD, and mean daily quantum flux density, Q int). The results reveal that leaves grown at the TEM site did not show an increased ability for net CO2 assimilation (A N), mainly due to an equal gm at both sites. This fact is explained by a trade-off between an increased conductance of the gas phase (g ias) and a reduced conductance of the liquid phase (g liq) at the TEM site compared with the MED site. In spite of the reduction in gs,max at the MED site, transpiration (E) did not diminish during midsummer to the levels of the TEM site due to a higher VPD found at the MED site, yielding a higher water use efficiency (AN/E) at the TEM site. Moreover, photosynthetic nitrogen use efficiency was also higher at the TEM site, indicating these leaves can reach similar values of AN with lower nitrogen investment that those at the MED site. These results suggest that Q. coccifera does not always use the main resources (water and nutrients) at leaf level as efficiently as possible. Moreover, the different patterns of resource use (in particular N), together with the functional plasticity, cannot overcome the morpho-functional constraints that limit photosynthetic activity, even under potentially favourable conditions.

Light compensation points in shade-grown seedlings of deciduous broadleaf tree species with different successional traits raised under elevated CO2.

We measured leaf photosynthetic traits in shade-grown seedlings of four tree species native to northern Japan, raised under an elevated CO2 condition, to investigate the effects of elevated CO2 on shade tolerance of deciduous broadleaf tree species with different successional traits. We considered Betula platyphylla var. japonica and Betula maximowicziana as pioneer species, Quercus mongolica var. crispula as a mid-successional species, and Acer mono as a climax species. The plants were grown under shade conditions (10% of full sunlight) in a CO2 -regulated phytotron. Light compensation points (LCPs) decreased in all tree species when grown under elevated CO2 (720 µmol·mol(-1) ), which were accompanied by higher apparent quantum yields but no photosynthetic down-regulation. LCPs in Q. mongolica and A. mono grown under elevated CO2 were lower than those in the two pioneer birch species. The LCP in Q. mongolica seedlings was not different from that of A. mono in each CO2 treatment. However, lower dark respiration rates were observed in A. mono than in Q. mongolica, suggesting higher shade tolerance in A. mono as a climax species in relation to carbon loss at night. Thus, elevated CO2 may have enhanced shade tolerance by lowering LCPs in all species, but the ranking of shade tolerance related to successional traits did not change among species under elevated CO2 , i.e. the highest shade tolerance was observed in the climax species (A. mono), followed by a gap-dependent species (Q. mongolica), while lower shade tolerance was observed in the pioneer species (B. platyphylla and B. maximowicziana).

Reducing the infectivity and richness of ectomycorrhizal fungi in a calcareous Quercus ilex forest through soil preparations for truffle plantation establishment: A bioassay study.

In the early years of a black truffle plantation, the field proliferation of the nursery-inoculated fungi can be hampered by native ectomycorrhizal fungi colonising the seedling roots. Reducing the soil ectomycorrhizal infectivity in the planting hole before introducing the inoculated seedling could be an effective strategy to reduce this problem. Three bioassays were conducted to evaluate the impact of several soil preparations on the ectomycorrhizal infectivity and richness of a Quercus ilex soil in a truffle-producing region. Microwaves, quicklime, and acetic acid significantly decreased the percent root colonisation and morphotype richness of the native ectomycorrhizal fungi. However, they also decreased seedling survival or growth. Peracetic acid, hydrogen peroxide, and sodium hypochlorite did not show a significant negative effect on the soil ectomycorrhizal community. The results support the potential of soil preparation for reducing the ectomycorrhizal infectivity of forest soils, thus being a promising strategy to reduce the early colonisation by native fungi in truffle plantations. However, the indications of damage to the seedling development must be addressed.

Do Small Canopy Gaps Created by Japanese Black Bears Facilitate Fruiting of Fleshy-Fruited Plants?

Japanese black bears often break branches when climbing trees and feeding on fruit in canopies, thereby creating small canopy gaps. However, the role of black bear-created canopy gaps has not been evaluated in the context of multiple forest dynamics. Our hypothesis was that small canopy gaps created by black bears improve light conditions, which facilitates fruiting of adult fleshy-fruited plants located beneath the gaps, and also that this chain interaction depends on interactions among the size of gaps, improved light conditions, forest layers, and life form of plants. The rPPFD, size of black bear-created canopy gaps, and fruiting/non-fruiting of fleshy-fruited plants were investigated in five forest layers beneath black-bear-created canopy gaps and closed canopies of Mongolian oak (Quercus crispula). We found that light conditions improved beneath black bear-disturbed trees with canopy gaps of large size, and the effect of improvement of light conditions was reduced with descending forest layers. Fruiting of fleshy-fruited plants, especially woody lianas and trees, was facilitated by the improvement of light conditions accompanied by an increase in the size of black-bear-created gaps. Data from this study revealed that canopy disturbance by black bears was key for improving light conditions and accelerating fruiting of fleshy-fruited trees and woody lianas in the canopy layers in particular. Therefore, our hypothesis was mostly supported. Our results provide evidence that Japanese black bears have high potential as ecosystem engineers that increase the availability of resources (light and fruit in this study) to other species by causing physical state changes in biotic materials (branches of Q. crispula in this study).

Atmospheric direct uptake and long-term fate of radiocesium in trees after the Fukushima nuclear accident.

Large areas of forests were radioactively contaminated by the Fukushima nuclear accident of 2011, and forest decontamination is now an important problem in Japan. However, whether trees absorb radioactive fallout from soil via the roots or directly from the atmosphere through the bark and leaves is unclear. We measured the uptake of radiocesium by trees in forests heavily contaminated by the Fukushima nuclear accident. The radiocesium concentrations in sapwood of two tree species, the deciduous broadleaved konara (Quercus serrata) and the evergreen coniferous sugi (Cryptomeria japonica), were higher than that in heartwood. The concentration profiles showed anomalous directionality in konara and non-directionality in sugi, indicating that most radiocesium in the tree rings was directly absorbed from the atmosphere via bark and leaves rather than via roots. Numerical modelling shows that the maximum (137)Cs concentration in the xylem of konara will be achieved 28 years after the accident. Conversely, the values for sugi will monotonously decrease because of the small transfer factor in this species. Overall, xylem (137)Cs concentrations will not be affected by root uptake if active root systems occur 10 cm below the soil.

Photoperiod and temperature responses of bud swelling and bud burst in four temperate forest tree species.

Spring phenology of temperate forest trees is optimized to maximize the length of the growing season while minimizing the risk of freezing damage. The release from winter dormancy is environmentally mediated by species-specific responses to temperature and photoperiod. We investigated the response of early spring phenology to temperature and photoperiod at different stages of dormancy release in cuttings from four temperate tree species in controlled environments. By tracking bud development, we were able to identify the onset of bud swelling and bud growth in Acer pseudoplatanus L., Fagus sylvatica L., Quercus petraea (Mattuschka) Liebl. and Picea abies (L.) H. Karst. At a given early stage of dormancy release, the onset and duration of the bud swelling prior to bud burst are driven by concurrent temperature and photoperiod, while the maximum growth rate is temperature dependent only, except for Fagus, where long photoperiods also increased bud growth rates. Similarly, the later bud burst was controlled by temperature and photoperiod (in the photoperiod sensitive species Fagus, Quercus and Picea). We conclude that photoperiod is involved in the release of dormancy during the ecodormancy phase and may influence bud burst in trees that have experienced sufficient chilling. This study explored and documented the early bud swelling period that precedes and defines later phenological stages such as canopy greening in conventional phenological works. It is the early bud growth resumption that needs to be understood in order to arrive at a causal interpretation and modelling of tree phenology at a large scale. Classic spring phenology events mark visible endpoints of a cascade of processes as evidenced here.

Experimental and modeling studies of ultrasound-assisted release of phenolics from oak chips into model wine.

The enhancement of release of oak-related compounds from oak chips during wine aging with oak chips may interest the winemaking industry. In this study, the 25-kHz ultrasound waves were used to intensify the mass transfer of phenolics from oak chips into a model wine. The influences of acoustic energy density (6.3-25.8 W/L) and temperature (15-25 °C) on the release kinetics of total phenolics were investigated systematically. The results exhibited that the total phenolic yield released was not affected by acoustic energy density significantly whereas it increased with the increase of temperature during sonication. Furthermore, to describe the mechanism of mass transfer of phenolics in model wine under ultrasonic field, the release kinetics of total phenolics was simulated by both a second-order kinetic model and a diffusion model. The modeling results revealed that the equilibrium concentration of total phenolics in model wine, the initial release rate and effective diffusivity of total phenolics generally increased with acoustic energy density and temperature. In addition, temperature had a negative effect on the second-order release rate constant whereas acoustic energy density had an opposite effect.

A comparative study of physiological and morphological seedling traits associated with shade tolerance in introduced red oak (Quercus rubra) and native hardwood tree species in southwestern Germany.

Northern red oak (Quercus rubra L.), a moderately shade-tolerant tree species, is failing to regenerate throughout its native North American range, while successful recruitment in Central Europe has been observed since its introduction. To examine whether comparative photosynthetic performance could explain the regeneration success of this non-native species in Central Europe, we compared the physiological and morphological seedling traits of red oak with three co-occurring tree species under three canopy types in southwestern Germany. Native species included a moderately shade-tolerant native oak (Quercus robur L.) and two shade-tolerant species (Acer pseudoplatanus L. and Carpinus betulus L.). The photosynthetic traits of non-native red oak seedlings were similar to those reported for this species in the native range, where shade-tolerant competitors readily outperform red oak under low light conditions. However, compared with native shade-tolerant species in Europe, red oak seedlings photosynthesized efficiently, especially under closed canopies and in small canopy gaps, exhibiting high photosynthetic capacity, low leaf dark respiration and leaf-level light compensation points that were similar to the more shade-tolerant species. The superior net carbon gain of red oak seedlings at low and moderate light levels was likely facilitated by high leaf areas and reflected by seedling dry masses that were greater than the observed native European species. A competitive advantage for red oak was not evident because relative height growth was inferior to seedlings of co-occurring species. In North America, the inability of seedlings to compete with shade-tolerant tree species in deeply shaded understories is central to the problem of poor oak recruitment. Our study suggests that the ability of non-native red oak to perform equally well to native shade-tolerant species under a variety of light conditions could contribute to the consistent success of red oak regeneration in Europe.

Physiological strategies of co-occurring oaks in a water- and nutrient-limited ecosystem.

Oak species are well suited to water-limited conditions by either avoiding water stress through deep rooting or tolerating water stress through tight stomatal control. In co-occurring species where resources are limited, species may either partition resources in space and/or time or exhibit differing efficiencies in the use of limited resources. Therefore, this study seeks to determine whether two co-occurring oak species (Quercus prinus L. and Quercus velutina Lam.) differ in physiological parameters including photosynthesis, stomatal conductance, water-use (WUE) and nitrogen-use efficiency (NUE), as well as to characterize transpiration and average canopy stomatal responses to climatic variables in a sandy, well-drained and nutrient-limited ecosystem. The study was conducted in the New Jersey Pinelands and we measured sap flux over a 3-year period, as well as leaf gas exchange, leaf nitrogen and carbon isotope concentrations. Both oak species showed relatively steep increases in leaf-specific transpiration at low vapor pressure deficit (VPD) values before maximum transpiration rates were achieved, which were sustained over a broad range in VPD. This suggests tight stomatal control over transpiration in both species, although Q. velutina showed significantly higher leaf-level and canopy-level stomatal conductance than Q. prinus. Average daytime stomatal conductance was positively correlated with soil moisture and both oak species maintained at least 75% of their maximum canopy stomatal conductance at soil moistures in the upper soil layer (0-0.3 m) as low as 0.03 m(3) m(3)(-3). Quercus velutina had significantly higher photosynthetic rates, maximum Rubisco-limited and electron-transport-limited carboxylation rates, dark respiration rates and nitrogen concentration per unit leaf area than Q. prinus. However, both species exhibited similar WUEs and NUEs. Therefore, Q. prinus has a more conservative resource-use strategy, while Q. velutina may need to exploit niches that are locally higher in nutrients and water. Likewise, both species appear to tap deep, stable water sources, highlighting the importance of rooting depth in modeling transpiration and stomatal conductance in many oak ecosystems.

Drivers of radial growth and carbon isotope discrimination of bur oak (Quercus macrocarpa†Michx.) across continental gradients in precipitation, vapour pressure deficit and irradiance.

Tree-ring characteristics are commonly used to reconstruct climate variables, but divergence from the assumption of a single biophysical control may reduce the accuracy of these reconstructions. Here, we present data from bur oaks (Quercus macrocarpa Michx.) sampled within and beyond the current species bioclimatic envelope to identify the primary environmental controls on ring-width indices (RWIs) and carbon stable isotope discrimination (Δ(13) C) in tree-ring cellulose. Variation in Δ(13) C and RWI was more strongly related to leaf-to-air vapour pressure deficit (VPD) at the centre and western edge of the range compared with the northern and wettest regions. Among regions, Δ(13) C of tree-ring cellulose was closely predicted by VPD and light responses of canopy-level Δ(13) C estimated using a model driven by eddy flux and meteorological measurements (R(2) = 0.96, P = 0.003). RWI and Δ(13) C were positively correlated in the drier regions, while they were negatively correlated in the wettest region. The strength and direction of the correlations scaled with regional VPD or the ratio of precipitation to evapotranspiration. Therefore, the correlation strength between RWI and Δ(13) C may be used to infer past wetness or aridity from paleo wood by determining the degree to which carbon gain and growth have been more limited by moisture or light.

Relationship between photochemical reflectance index and leaf ecophysiological and biochemical parameters under two different water statuses: towards a rapid and efficient correction method using real-time measurements.

The use of the photochemical reflectance index (PRI) as a promising proxy of light use efficiency (LUE) has been extensively studied, and some issues have been identified, notably the sensitivity of PRI to leaf pigment composition and the variability in PRI response to LUE because of stress. In this study, we introduce a method that enables us to track the short-term PRI response to LUE changes because of photosynthetically active radiation (PAR) changes. The analysis of these short-term relationships between PRI and LUE throughout the growing season in two species (Quercus robur L. and Fagus sylvatica L.) under two different soil water statuses showed a clear change in PRI response to LUE, which is related to leaf pigment content. The use of an estimated or approximated PRI0, defined as the PRI of perfectly dark-adapted leaves, allowed us to separate the PRI variability due to leaf pigment content changes and the physiologically related PRI variability over both daily (PAR-related) and seasonal (soil water content-related) scales. The corrected PRI obtained by subtracting PRI0 from the PRI measurements showed a good correlation with the LUE over both of the species, soil water statuses and over the entire growing season.

Light and competition gradients fail to explain the coexistence of shade-tolerant Fagus sylvatica and shade-intermediate Quercus petraea seedlings.

BACKGROUND AND AIMS: The coexistence of forest tree species has often been linked to differences among species in terms of their response to light availability during the regeneration stage. From this perspective, species coexistence results from growth-growth or mortality-growth trade-offs along spatial light gradients. Experimental evidence of growth-growth trade-offs in natural conditions is sparse due to various confounding factors that potentially hinder the relationship. This study examined growth hierarchies along light gradients between two tree species with contrasting shade tolerance by controlling potential confounding factors such as seedling size, seedling status, seedling density and species composition. METHODS: Natural regenerated shade-tolerant Fagus sylvatica and shade-intermediate Quercus petraea seedlings were used, and growth rankings over a 4-year period were compared in 8- to 10-year-old tree seedlings. KEY RESULTS: No rank reversal occurs between the two species along the light gradient, or along the density, mixture or seedling size gradients. The shade-tolerant species was always the more competitive of the two. Pronounced effects of initial size on seedling growth were observed, whereas the effects of light and competition by neighbours were of secondary importance. The paramount effect of size, which results from the asymmetric nature of interseedling competition, gives a strong advantage to tall seedlings over the long term. CONCLUSIONS: This study extends previous efforts to identify potential drivers of rank reversals in young tree mixtures. It does not support the classical assumption that spatial heterogeneity in canopy opening explains the coexistence of the two species studied. It suggests that spatial variation in local size hierarchies among seedlings that may be caused by seedling emergence time or seedling initial performance is the main driver of the dynamics of these mixed stands.

Variation in leaf and twig CO2 flux as a function of plant size: a comparison of seedlings, saplings and trees.

Rates of tissue-level function have been hypothesized to decline as trees grow older and larger, but relevant evidence to assess such changes remains limited, especially across a wide range of sizes from saplings to large trees. We measured functional traits of leaves and twigs of three cold-temperate deciduous tree species in Minnesota, USA, to assess how these vary with tree height. Individuals ranging from 0.13 to 20 m in height were sampled in both relatively open and closed canopy environments to minimize light differences as a potential driver of size-related differences in leaf and twig properties. We hypothesized that (H1) gas-exchange rates, tissue N concentration and leaf mass per unit area (LMA) would vary with tree size in a pattern reflecting declining function in taller trees, yet maintaining (H2) bivariate trait relations, common among species as characterized by the leaf economics spectrum. Taking these two ideas together yielded a third, integrated hypothesis that (H3) nitrogen (N) content and gas-exchange rates should decrease monotonically with tree size and LMA should increase. We observed increasing LMA and decreasing leaf and twig Rd with increasing size, which matched predictions from H1 and H3. However, opposite to our predictions, leaf and twig N generally increased with size, and thus had inverse relations with respiration, rather than the predicted positive relations. Two exceptions were area-based leaf N of Prunus serotina Ehrh. in gaps and mass-based leaf N of Quercus ellipsoidalis E. J. Hill in gaps, both of which showed qualitatively hump-shaped patterns. Finally, we observed hump-shaped relationships between photosynthetic capacity and tree height, not mirroring any of the other traits, except in the two cases highlighted above. Bivariate trait relations were weak intra-specifically, but were generally significant and positive for area-based traits using the pooled dataset. Results suggest that different traits vary with tree size in different ways that are not consistent with a universal shift towards a lower 'return on investment' strategy. Instead, species traits vary with size in patterns that likely reflect complex variation in water, light, nitrogen and carbon availability, storage and use.

Three pools of zeaxanthin in Quercus coccifera leaves during light transitions with different roles in rapidly reversible photoprotective energy dissipation and photoprotection.

Under excess light, the efficient PSII light-harvesting antenna is switched into a photoprotected state in which potentially harmful absorbed energy is thermally dissipated. Changes occur rapidly and reversibly, enhanced by de-epoxidation of violaxanthin (V) to zeaxanthin (Z). This process is usually measured as non-photochemical quenching (NPQ) of chlorophyll (Chl) fluorescence. Using instrumentation for instantaneous leaf freezing, NPQ, spectral reflectance, and interconversions within the xanthophyll cycle with time resolution of seconds were recorded from Quercus coccifera leaves during low light (LL) to high light (HL) transitions, followed by relaxation at LL. During the first 30 s of both the LL to HL and HL to LL transitions, no activity of the xanthophyll cycle was detected, whereas 70-75% of the NPQ was formed and relaxed, respectively, by that time, the latter being traits of a rapidly reversible photoprotective energy dissipation. Three different Z pools were identified, which play different roles in energy dissipation and photoprotection. In conclusion, ΔpH was crucial to NPQ formation and relaxation in Q. coccifera during light transitions. Only a minor fraction of Z was associated to quenching, whereas the largest Z pool was not related to thermal dissipation. The latter is proposed to participate in photoprotection acting as antioxidant.