Spatial and seasonal variability of temperature responses of biochemical photosynthesis parameters and leaf nitrogen content within a Pinus densiflora crown

We measured seasonal variation in area-based nitrogen concentration (N), maximum rate of carboxylation (Vcmax) and maximum rate of electron transport (Jmax) in 1-year-old needles along four first-order branches within a Pinus densiflora Sieb. & Zucc. crown, and analyzed their relationships to growth irradiance and temperature. Each leaf light environment was expressed as a ratio of the monthly mean of daily integrated photosynthetically active irradiance (Iint) for the particular needle to Iint above the canopy (Irel). Needle N decreased in the upper crown during the development of new needles, whereas it remained fairly constant in the lower crown, reflecting differences between upper and lower crown needles in their contribution to the nitrogen of new needles. Gradients of N within the crown were correlated with Irel in all seasons (r2 = 0.40-0.78). Seasonal variation in N was weakly correlated with mean daily air minimum temperatures. Both Vcmax and Jmax showed seasonal variation in all first-order branches, and decreased to their lowest values in winter. The gradients of Vcmax and Jmax within the crown were not correlated with Irel in some seasons, but were correlated with changes in N in most months (r2 = 0.33-0.75), except in the winter. Furthermore, the regression slope of the relationship between N and Vcmax and the temperature response of Vcmax and Jmax exhibited seasonal variation.     

Vertical, horizontal and azimuthal variations in leaf photosynthetic characteristics within a Fagus crenata crown in relation to light acclimation

An understanding of spatial variations in gas exchange parameters in relation to the light environment is crucial for modeling canopy photosynthesis. We measured vertical, horizontal and azimuthal (north and south) variations in photosynthetic capacity (i.e., the maximum rate of carboxylation: Vcmax), nitrogen content (N), leaf mass per area (LMA) and chlorophyll content (Chl) in relation to relative photosynthetic photon flux (rPPF) within a Fagus crenata Blume crown. The horizontal gradient of rPPF was similar in magnitude to the vertical gradient of rPPF from the upper to the lower crown. The rPPF in the north quadrant of the crown was slightly lower than in the south quadrant. Nitrogen content per area (Narea), LMA and Vcmax were strictly proportional to rPPF, irrespective of the vertical direction, horizontal direction and crown azimuth, whereas nitrogen content per dry mass, Chl per area and photosynthetic capacity per dry mass (Vm) were fairly constant. Statistical analyses separating vertical trends from horizontal and azimuthal trends indicated that, although horizontal and vertical light acclimation of leaf properties were similar, there were two significant azimuthal variations: (1) Vcmax was lower in north-facing leaves than in south-facing leaves for a given Narea, indicating low photosynthetic nitrogen-use efficiency (PNUE) of north-facing leaves; and (2) Vcmax was lower in north-facing leaves than in south-facing leaves for a given LMA, indicating low Vm of the north-facing leaves. With respect to the low PNUE of the north-facing leaves, there were no significant azimuthal variations in leaf CO2 conductance from the stomata to the carboxylation site. Biochemical analysis indicated that azimuthal variations in nitrogen allocation to ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and in nitrogen allocation between carboxylation (Rubisco and other Calvin cycle enzymes) and light harvesting machinery (Chl pigment-protein complexes) were not the main contributor to the difference in PNUE between north- and south-facing leaves. Lower specific activity of Rubisco may be responsible for the low PNUE of the north-facing leaves. Anatomical analysis indicated that not only high leaf density, which is compatible with a greater fraction of non-photosynthetic tissue, but also thick photosynthetic tissue contributed to the low Vm in the north-facing leaves. These azimuthal variations may need to be considered when modeling canopy photosynthesis based on the Narea-Vcmax or LMA-Vcmax relationship.     

Foliar morphological and physiological plasticity in Picea abies and Abies alba saplings along a natural light gradient

The role of morphological versus physiological foliar plasticity in the capacity for, and mechanisms of, photosynthetic acclimation was assessed in Picea abies (L.) Karst. and Abies alba Mill. saplings in a forest gap-understory light gradient (relative irradiance, RI, ranging from 0.02 to 0.32). The species investigated showed a similar foliar morphological plasticity along the light gradient, at both the needle level (through alteration in leaf dry mass per area) and the shoot level (through alteration in the silhouette area ratio, e.g., shoot silhouette to projected needle area ratio). In both species chlorophyll (Chl) concentration on a mass basis decreased at increasing RI, but was independent of RI when expressed on an area basis. In contrast, leaf N concentration on a mass basis was independent of RI, but was positively influenced by RI when expressed on an area basis. The parameters describing photosynthetic performance at low light (dark respiration rate, apparent quantum yield and light compensation point) suggest that Abies alba was better suited to maintain a positive carbon balance in shaded conditions. By contrast, parameters describing biochemical capacity at high light (maximum electron transport rate, Jmax and maximum ribulose-1,5-biphosphate carboxylation capacity, Vcmax) indicate that only Picea abies was capable of acclimating physiologically to high photosynthetic photon flux densities (PPFDs) by increasing nitrogen partitioning to Rubisco and Vcmax/mass by increasing RI. These results support the hypothesis that interspecific differences in nitrogen partitioning within the photosynthetic apparatus may provide a mechanistic basis for species separation along a light gradient. The differences in photosynthetic plasticity observed are likely to influence regeneration patterns and habitat breadth of the species investigated. The limited ability of Abies alba saplings to exploit high-light conditions may be a competitive disadvantage in large canopy gaps and thus limit recruitment of this species to small gaps.     

Seasonal and temperature dependence of photosynthesis and respiration for two co-occurring broad-leaved tree species with contrasting leaf phenology

We measured the seasonal and temperature responses of leaf photosynthesis and respiration of two co-occurring native New Zealand tree species with contrasting leaf phenology: winter-deciduous fuchsia (Fuchsia excorticata J. R. Forst & G. Forst) and annual evergreen wineberry (Aristotelia serrata J. R. Forst & G. Forst). There was no difference in the amount of nitrogen per unit leaf area (Narea, range 40-160 mmol m-2, P = 0.18) or specific leaf area (S, range 8-27 m2 kg-1, P = 0.87) in summer leaves of wineberry or fuchsia. The amount of nitrogen per unit leaf area and S varied significantly with height of leaves in the canopy for both species (r2 range 0.61-0.87). Parameters describing the maximum rates of rubisco carboxylation (Vcmax) and electron transport (Jmax) were related significantly to Narea, and were 60% higher on average in spring and summer leaves than in autumn and winter leaves for both species. The seasonal effect remained significant (P < 0.001) when Narea was included in a regression model, indicating that seasonal changes were not only due to changes in Narea. Values for Vcmax and Jmax were 30% lower in wineberry leaves than in fuchsia leaves on average, although the difference ranged from 15% in summer leaves to 39% in autumn leaves. Activation energies describing the temperature dependence of Vcmax and Jmax in wineberry were 111 and 114% of corresponding values for fuchsia (Ea (Vcmax) = 39.1 kJ mol-1, Ea (Jmax) = 32.9 kJ mol-1). Respiration at night was the same (P = 0.34) at 10 degrees C for both species (R10 = 0.7 micromol m-2 s-1), although activation energies (E0) were higher in wineberry than in fuchsia (47.4 and 32.9 kJ mol-1 K-1, respectively). These results show that rates of photosynthesis are higher in winter-deciduous fuchsia than in annual evergreen wineberry.     

Photosynthetic capacity and foliar nitrogen distribution in Eucalyptus nitens is altered by high-intensity thinning

The changes in photosynthetic rates, light environment and foliar nutrient concentrations following thinning were examined in an 8-year-old Eucalyptus nitens (Deane and Maiden) Maiden plantation. The objectives of the study were to: (1) determine the extent to which maximum photosynthetic rates (Amax) of E. nitens are affected by stand thinning; (2) relate the spatial pattern of Amax within the crown to the changes in light environment caused by thinning; and (3) establish if the responses of Amax to thinning are driven by changes in area-based foliar nitrogen (Na) or phosphorus (Pa) concentrations. Photosynthetic rates measured under saturating light availability increased throughout the crown after thinning. The greatest increases were observed in the lower and middle crown zones. Photosynthetic rate was positively related to foliar N concentration. Thinning increased Na and Pa because of a significant decrease in specific leaf area (SLA) after thinning. Changes in photosynthetic rates, SLA and foliar nutrient distributions with thinning were well correlated with changes in incident solar irradiance throughout the tree crown.     

Photosynthesis, water relations, and growth of planted Pinus strobus L. on burned sites in the southern Appalachians

We measured net photosynthesis, leaf conductance, xylem water potential, and growth of Pinus strobus L. seedlings two years after planting on two clear-cut and burned sites in the southern Appalachians. Multiple regression analysis was used to relate seedling net photosynthesis to vapor pressure deficit, seedling crown temperature, photosynthetically active radiation (PAR), needle N, xylem water potential, and soil water, and to relate seedling size and growth to physiological measurements (average net photosynthesis, leaf conductance, and cumulative xylem water potential), soil water, needle N, seedling temperature, and PAR. Seedling net photosynthesis was significantly related to vapor pressure deficit, midday water potential, crown temperature, and PAR (r(2) = 0.70) early in the growing season (May 1992) with vapor pressure deficit alone explaining 42% of the variation. As neighboring vegetation developed, light became more limiting and significantly reduced seedling net photosynthesis later in the growing season (July, August, and September). Final seedling diameter was significantly related to competitor biomass, average photosynthetic rate, and needle N (r(2) = 0.68).     

Growth response of Douglas-fir seedlings to nitrogen fertilization: importance of Rubisco activation state and respiration rates

High foliar nitrogen concentration ([N]) is associated with high rates of photosynthesis and thus high tree productivity; however, at excessive [N], tree productivity is reduced. Reports of excessive [N] in the Douglas-fir forests of the Oregon Coast Range prompted this investigation of growth and needle physiological responses to increasing foliar N concentrations in 1-year-old Douglas-fir seedlings. After 1 year of N fertilization, total seedling biomass increased with each successive increase in N fertilizer concentration, except in the highest N fertilization treatment. Of the many physiological responses that were analyzed, only photosynthetic capacity (i.e., Vcmax), respiration rates and leaf specific conductance (KL) differed significantly between N treatments. Photosynthetic capacity showed a curvilinear relationship with foliar [N], reaching an apparent maximum rate when needle N concentrations exceeded about 12 mg g(-1). In vitro measurements of ribulose-1,5-bisphosphate carboxylase (Rubisco) activity suggested that photosynthetic capacity was best related to activated, not total, Rubisco content. Rubisco activation state declined as foliar [N] increased, and based on its significant correlation (r2= 0.63) with foliar Mn:Mg ratios, it may be related to Mn inactivation of Rubisco. Respiration rates increased linearly as foliar N concentration increased (r2= 0.84). The value of K(L) also increased as foliar [N] increased, reaching a maximum when foliar [N] exceeded about 10 mg g(-1). Changes in K(L) were unrelated to changes in leaf area or sapwood area because leaf area to sapwood area ratios remained constant. Cumulative effects of the observed physiological responses to N fertilization were analyzed by modeling annual net CO2 assimilation (Anet) based on treatment specific values of Vcmax, dark respiration (Rdark) and KL. Estimates of Anet were highly correlated with measured total seedling biomass (r2= 0.992), suggesting that long-term, cumulative effects of maximum Rubisco carboxylation, Rdark and KL responses to N fertilization may limit seedling production when foliar N exceeds about 13 mg g(-1) or is reduced to less than about 11 mg g(-1).     

Leaf-age effects on seasonal variability in photosynthetic parameters and its relationships with leaf mass per area and leaf nitrogen concentration within a Pinus densiflora crown

In the temperate zone of Japan, Pinus densiflora Sieb. et Zucc. bears needles of up to three age classes in the upper crown and up to five age classes in the lower crown. To elucidate the effects of leaf age on photosynthetic parameters and its relationships with leaf mass per unit area (LMA) and leaf nitrogen (N(l)) concentration on an area (N(a)) and mass (N(m)) basis, we measured seasonal variations in LMA, N(l), light-saturated photosynthetic rate (A(max)), stomatal conductance (g(s)), maximum rate of carboxylation (V(cmax)) and maximum rate of electron transport (J(max)) in leaves of all age classes in the upper and lower crown. Leaf mass per unit area increased by 27% with increasing leaf age in the lower crown, but LMA did not depend on age in the upper crown. Leaf age had a significant effect on N(m) but not on N(a) in both crown positions, indicating that decreases in N(m) resulted from dilution. Photosynthetic parameters decreased significantly with leaf age in the lower crown (39% for A(max) and 43% for V(cmax)), but the effect of leaf age was not as great in the upper crown, although these parameters exhibited seasonal variation in both crown positions. Regression analysis indicated a close relationship between LMA and N(a), regardless of age class or when each age class was pooled (r(2) = 0.57-0.86). Relationships between LMA and N(a) and among A(max), V(cmax) and J(max) were weak or not significant when all age classes were examined by regression analysis. However, compared with older leaves, relationships among LMA, N(a) and A(max) were stronger in younger leaves. These results indicate that changes in LMA and N(l) mainly reflect light acclimation during leaf development, but they are only slightly affected by irradiance in mature leaves. In conclusion, LMA and N(l) are useful parameters for estimating photosynthetic capacity, but age-related effects need to be taken into account, especially in evergreen conifers.     

Seasonal and yearly variations in light use and nitrogen use by seedlings of four deciduous broad-leaved tree species invading larch plantations

Several deciduous broad-leaved tree species, differing in leaf phenology, invade larch (Larix kaempferii (Lamb.) Carrière) plantations in Japan. The understory light environment of larch forests changes drastically between the leafy and leafless periods. To determine how the invading seedlings exploit the changing light environment, and if phenological differences reflect the light- and nitrogen-use traits of the seedlings, we measured leaf phenology, seasonal changes in light-saturated photosynthetic rate (P(sat)), leaf nitrogen (N) content (N(area)), chlorophyll/nitrogen ratio (Chl/N), specific leaf area (SLA) and N remobilization rate (NRMR) over 3 years. The mid-successional or gap-phase species, Magnolia hypoleuca Siebold & Zucc., had a short leafy period and high P(sat) and NRMR. In contrast, two late-successional tree species, Prunus ssiori Friedr. Schmidt, which undergoes leaf flush before larch, and Carpinus cordata Blume, which maintains green leaves until frost, both had low P(sat) and NRMR but exploited the opportunity for growth during the period when the larch canopy trees were leafless. Quercus mongolica Fisch. ex Ledeb. var. crispula (Blume) Ohashi, a mid-late-successional species that underwent leaf flush at the same time as the overstory larch, had values of photosynthetic parameters between those of the gap-phase and late-successional species. Among species, M. hypoleuca and Q. mongolica had higher photosynthetic rates and photosynthetic N-use efficiencies. In all species, the relationship between N(area) and P(sat) showed species-specific yearly fluctuations; however, there was no yearly fluctuation in the relationship between N(area) and P(sat) at CO2 saturation. Yearly fluctuations in the N(area)-P(sat) relationship appeared to be induced by changes in SLA and N-use characteristics, which in turn are affected by climatic variations.     

Interrelationships among light,photosynthesis and nitrogen in the crown of mature Pinus contorta ssp. latifolia

Scaling leaf-level measurements to estimate carbon gain of entire leaf crowns or canopies requires an understanding of the distribution of photosynthetic capacity and corresponding light microenvironments within a crown. We have compared changes in the photosynthetic light response and nitrogen (N) content (per unit leaf area) of Pinus contorta Dougl. ssp. latifolia Engelm. (lodgepole pine) leaves in relation to their age and light microenvironment. The vertical gradient in integrated daily photosynthetic photon flux density (PPFD) from the upper to the lower crown of lodgepole pine was similar in magnitude to the horizontal gradient in daily PPFD along shoots from young to old leaves. The relationship between light-saturated net photosynthesis (A(max)) and daily PPFD was significant for both young and old leaves. However, old leaves had a lower A(max) than young leaves in a similar daily irradiance regime. For leaves of all ages from throughout the crown, A(max) was linearly related to the estimated daily net carbon gain that leaves could achieve in their natural PPFD environment (estimated A(day)) (r(2) = 0.84, P < 0.001, n = 39), indicating that estimated A(day) may be dominated by carbon fixed when leaves are light-saturated and operating at A(max). Comparison of the PPFD required to achieve A(max) and the PPFD available to the leaves showed that all of the measured leaves (n = 39), regardless of their position in the crown or age, were in light environments that could light-saturate photosynthesis for a similar proportion of the day. For all data pooled, foliar N was weakly correlated with daily PPFD. Analyzing each leaf age class separately showed that foliar N was significantly related to daily PPFD, A(max), and estimated A(day) for the youngest leaves but not for middle-aged or old leaves. Therefore, the general theory that foliar N is allocated within a crown according to total daily light availability was supported only for young (1-4 years old) leaves in this study.     

辽西北沙地5种针叶树光合特征比较

2015年,在辽宁省彰武县应用Li-6400光合仪,对樟子松(Pinus sylvestris var.mongolica Litv.)、彰武松(Pinus densiflora var.zhanguensis)、油松(Pinus tabulaeformis Carr.)、赤松(Pinus densiflora Sieb.et Zucc.)和长白松(Pinus sylvestriformis)光合作用的日变化进行观测。结果表明:5种针叶树在8月光合速率的日变化趋势基本相同,均表现为双峰型,并有明显的光合"午休"现象,其中彰武松光合速率最高、长白松光合速率较低。通过光响应曲线的分析得出:在0~2 000μmol m~(-2)s~(-1)光强下,光合能力顺序为长白松赤松樟子松油松彰武松;赤松和樟子松的光补偿点较高,为110μmol m~(-2)s~(-1);樟子松的暗呼吸速率最大,为2.338μmol m~(-2)s~(-1)。     

Photosynthetic response to green crown pruning in young plantation-grown Eucalyptus pilularis and E. cloeziana

The loss of foliage through pruning of live branches may reduce tree growth or it may be compensated by photosynthetic up-regulation of the remaining crown. Here, the changes in light-saturated photosynthesis following pruning to remove 50% of green crown length were examined in 4-year-old Eucalyptus pilularis Sm. and Eucalyptus cloeziana F. Muell. trees. The objectives of the study were to: (1) compare leaf-level physiological (light-saturated photosynthesis (A_max), stomatal conductance (g), transpiration (T), dark respiration (R_d), quantum yield (Φ), light compensation point (Γ), water-use efficiency (WUE), nitrogen-use efficiency (NUE)) traits in species with contrasting crown dynamics and structure, (2) examine the effect of crown position on these traits, and (3) examine the effect of pruning on A_max, g, T, WUE, NUE, leaf N and P concentrations and specific leaf area (SLA). Prior to pruning there were no differences in R_d, Γ and Φ between E. pilularis and E. cloeziana but differences in A_max, T, g, leaf N, leaf P, WUE, NUE and SLA. Whereas the rate of physiological processes (A_max, T, and g) and leaf N and P concentrations increased with crown height, R_d, Γ, Φ and SLA declined along this vertical gradient, except in the upper crown of E. cloeziana where A_max, T and g were not different to the lower crown. No up-regulation of photosynthesis or changes in leaf physiology occurred between 6 and 13 months after pruning in either species. The results provide an important basis for modelling pruning effects in process-based tree growth models.     

Needle longevity, photosynthetic rate and nitrogen concentration of eight spruce taxa planted in northern Japan

Growth characteristics of Picea glehnii Masters, P. jezoensis (Sieb. et Zucc) Carr., P. jezoensis var. hondoensis (Mayr) Rehder and P. shirasawae Hayashi from Japan, P. abies (L.) Karst. from Europe and P. glauca Voss, P. mariana Britt., Sterns and Pogg. and P. rubens Sarg. from North America were compared. The trees were grown in similar conditions at the Tomakomai Experimental Forest of Hokkaido University in northern Japan. Tree growth, needle biomass, longevity, photosynthetic rate, nitrogen concentration and specific leaf area (SLA) were measured, and photosynthetic nitrogen-use efficiency was calculated. Picea jezoensis, P. jezoensis var. hondoensis, P. abies and P. glauca had high growth rates, high photosynthetic rates in young needles, high needle nitrogen concentrations and short needle life spans. In contrast, P. glehnii, P. shirasawae, P. mariana and P. rubens had low growth and photosynthetic rates, low needle nitrogen concentrations, long needle life spans and maintained a high photosynthetic nitrogen-use efficiency in older needles. Examination of relationships between several growth parameters of the eight taxa revealed positive correlations between SLA and mass-based photosynthetic rate and between SLA and mass-based nitrogen concentration, whereas mass-based photosynthetic rate and mass-based nitrogen concentration were negatively correlated with needle longevity. The species differed greatly in growth characteristics despite being grown in similar conditions.     

Seasonality of photosynthetic parameters in a multi-specific and vertically complex forest ecosystem in the Sierra Nevada of California

Understanding seasonal variations of photosynthetic parameters is critical for accurate modeling of carbon dioxide (CO2) uptake by ecosystems. Maximum carboxylation velocity (Vcmax), maximum rate of electron transport (Jmax), leaf respiration in the light (R(day)), light-saturated assimilation (Amax) and maximum quantum yield (Phi) were calculated from leaf gas exchange measurements made monthly throughout the year on leaves of three co-occuring evergreen species in a Pinus ponderosa Dougl. ex P. Laws. & C. Laws. forest with shrubs in the understory (Arctostaphylos manzanita Parry and Ceanothus cordulatus Kellogg.). The seasonality and relationships of the photosynthetic parameters with environmental and physiological variables differed among the species. The nitrogen-fixing species, C. cordulatus had the highest values of the parameters and the largest seasonal variation, whereas A. manzanita exhibited the lowest seasonality and weaker correlations with environmental variables. In general, variations in Vcmax were highly correlated with light, leaf mass per area and leaf nitrogen content on an area basis. Temporal scaling of the parameters with each other seemed possible for C. cordulatus and P. ponderosa. However, lags between these variables and Vcmax likely reflect the influences of other factors. The acclimation relationships found along vertical light gradients within canopies in other studies cannot be applied to seasonal variations. The Jmax to Vcmax ratio varied seasonally for P. ponderosa and A. manzanita, being lower at high light, high air temperature and low soil water content.     

Physiological basis of seasonal trend in leaf photosynthesis of five evergreen broad-leaved species in a temperate deciduous forest

The physiological basis of photosynthesis during winter was investigated in saplings of five evergreen broad-leaved species (Camellia japonica L., Cleyera japonica Thunb., Photinia glabra (Thunb.) Maxim., Castanopsis cuspidata (Thunb.) Schottky and Quercus glauca Thunb.) co-occurring under deciduous canopy trees in a temperate forest. We focused on temperature dependence of photosynthetic rate and capacity as important physiological parameters that determine light-saturated rates of net photosynthesis at low temperatures during winter. Under controlled temperature conditions, maximum rates of ribulose bisphosphate carboxylation and electron transport (Vcmax) and Jmax, respectively) increased exponentially with increasing leaf temperature. The temperature dependence of photosynthetic rate did not differ among species. In the field, photosynthetic capacity, determined as Vcmax and Jmax at a common temperature of 25 degrees C (Vcmax(25) and Jmax(25)), increased until autumn and then decreased in species-specific patterns. Values of Vcmax(25) and Jmax(25) differed among species during winter. There was a positive correlation of Vcmax(25) with area-based nitrogen concentration among leaves during winter in Camellia and Photinia. Interspecific differences in Vcmax(25) were responsible for interspecific differences in light-saturated rates of net photosynthesis during winter.     

Within-crown structural variability of dwarfed mature Abies mariesii in snowy subalpine parkland in central Japan

We investigated mature dwarf Abies mariesii trees growing in conifer thicket–meadow parklands on a snowy subalpine plateau, where these dwarf trees are buried in the accumulated snow in winter. We focused on structural variation in the needles, shoots, and branchlets within different crown positions (leader crown vs lower crown) of the dwarf trees. In the leader crown, which appears above the snow surface earlier than the lower crown, current-year shoots and branchlets had greater total biomass, and foliage was more closely packed along the stem axis than in the lower crown, whereas current-year shoots in the leader crown had a lower needle mass ratio than in the lower crown. These results suggest that current-year shoots and branchlets in the leader crown have a specific structure that allows them to harvest more light, although construction and maintenance costs would be higher. In contrast, the structural characteristics of current-year shoots and branchlets in the lower crown efficiently concentrate incoming light by avoiding mutual shading within foliage, thus leading to increased biomass of photosynthetic needles within shoot and branchlet biomass. Such within-crown variability at various hierarchical levels from needles to branches in mature, but very dwarf, A. mariesii trees maintains the crown and allows survival within conifer clumps in areas of subalpine parklands that receive heavy snowfall.     

Leaf traits in relation to crown development, light interception and growth of elite families of loblolly and slash pine

Crown architecture and size influence leaf area distribution within tree crowns and have large effects on the light environment in forest canopies. The use of selected genotypes in combination with silvicultural treatments that optimize site conditions in forest plantations provide both a challenge and an opportunity to study the biological and environmental determinants of forest growth. We investigated tree growth, crown development and leaf traits of two elite families of loblolly pine (Pinus taeda L.) and one family of slash pine (P. elliottii Mill.) at canopy closure. Two contrasting silvicultural treatments -- repeated fertilization and control of competing vegetation (MI treatment), and a single fertilization and control of competing vegetation treatment (C treatment) -- were applied at two experimental sites in the West Gulf Coastal Plain in Texas and Louisiana. At a common tree size (diameter at breast height), loblolly pine trees had longer and wider crowns, and at the plot-level, intercepted a greater fraction of photosynthetic photon flux than slash pine trees. Leaf-level, light-saturated assimilation rates (A(max)) and both mass- and area-based leaf nitrogen (N) decreased, and specific leaf area (SLA) increased with increasing canopy depth. Leaf-trait gradients were steeper in crowns of loblolly pine trees than of slash pine trees for SLA and leaf N, but not for A(max). There were no species differences in A(max), except in mass-based photosynthesis in upper crowns, but the effect of silvicultural treatment on A(max) differed between sites. Across all crown positions, A(max) was correlated with leaf N, but the relationship differed between sites and treatments. Observed patterns of variation in leaf properties within crowns reflected acclimation to developing light gradients in stands with closing canopies. Tree growth was not directly related to A(max), but there was a strong correlation between tree growth and plot-level light interception in both species. Growth efficiency was unaffected by silvicultural treatment. Thus, when coupled with leaf area and light interception at the crown and canopy levels, A(max) provides insight into family and silvicultural effects on tree growth.     

Photosynthetic capacity and nitrogen partitioning in foliage of the evergreen shrub Daphniphyllum humile along a natural light gradient

We examined the effects of leaf age and mutual shading on the morphology, photosynthetic properties and nitrogen (N) allocation of foliage of an evergreen understory shrub, Daphniphyllum humile Maxim, growing along a natural light gradient in a deciduous Fagus crenata-dominated forest in Japan. Seedlings in high-light environments were subject to greater mutual shading and 1-year-old foliage survival was lower than in seedlings in low-light environments, indicating that the survival rates of foliage were related to the degree of mutual shading. Although specific leaf area (SLA) in current- and 1-year-old foliage was curvilinearly related to daily photosynthetic photon flux (PPF), SLA was unaffected by leaf age, indicating that foliage in D. humile may not acclimate morphologically to annual changes in light caused by mutual shading. Light-saturated net photosynthetic rates (Pmax) were correlated with daily PPF in current-year foliage. In addition, a strong, positive relationship was found between nitrogen concentration per unit leaf area and Pmax. In contrast, the relationship among PPF, N and photosynthetic parameters in 1-year old foliage was weak because of the strong remobilization of N from older leaves to current-year foliage in plants growing in high light. However, the relationship between daily PPF and both photosynthetic N-use efficiency and the ratio of maximum electron transport rate to maximum carboxylation rate did not differ between current-year and 1-year-old foliage, suggesting that these responses help maintain a high photosynthetic efficiency even in older foliage. We conclude that D. humile maximizes whole-plant carbon gain by maintaining a balance among photosynthetic functions across wide ranges of leaf ages and light environments.     

Temperature response of leaf photosynthetic capacity in seedlings from seven temperate tree species

Seedlings of seven temperate tree species (Acer pseudoplatanus L., Betula pendula Roth, Fagus sylvatica L., Fraxinus excelsior L., Juglans regia L., Quercus petraea Matt. Liebl. and Quercus robur L.) were grown in a nursery under neutral filters transmitting 45% of incident global irradiance. During the second or third year of growth, leaf photosynthetic capacity (i.e., maximal carboxylation rate, Vcmax, maximal photosynthetic electron transport rate, Jmax, and dark respiration, Rd) was estimated for five leaves from each species at five or six leaf temperatures (10, 18, 25, 32, 36 and 40 degrees C). Values of Vcmax and Jmax were obtained by fitting the equations of the Farquhar model on response curves of net CO2 assimilation (A) to sub-stomatal CO2 mole fraction (ci), at high irradiance. Primary parameters describing the kinetic properties of Rubisco (specificity factor, affinity for CO2 and for O2, and their temperature responses) were taken from published data obtained with spinach and tobacco, and were used for all species. The temperature responses of Vcmax and Jmax, which were fitted to a thermodynamic model, differed. Mean values of Vcmax and Jmax at a reference temperature of 25 degrees C were 77.3 and 139 micromol m(-2) s(-1), respectively. The activation energy was higher for Vcmax than for Jmax (mean values of 73.1 versus 57.9 kJ mol(-1)) resulting in a decrease in Jmax/Vcmax ratio with increasing temperature. The mean optimal temperature was higher for Vcmax than for Jmax (38.9 versus 35.9 degrees C). In addition, differences in these temperature responses were observed among species. Temperature optima ranged between 35.9 and above 45 degrees C for Vcmax and between 31.7 and 43.3 degrees C for Jmax, but because of data scatter and the limited range of temperatures tested (10 to 40 degrees C), there were few statistically significant differences among species. The optimal temperature for Jmax was highest in Q. robur, Q. petraea and J. regia, and lowest in A. pseudoplatanus and F. excelsior. Measurements of chlorophyll a fluorescence revealed that the critical temperature at which basal fluorescence begins to increase was close to 47 degrees C, with no difference among species. These results should improve the parameterization of photosynthesis models, and be of particular interest when adapted to heterogeneous forests comprising mixtures of species with diverse ecological requirements.     

Photosynthetic acclimation of overstory Populus tremuloides and understory Acer saccharum to elevated atmospheric CO2 concentration: interactions with shade and soil nitrogen

We exposed Populus tremuloides Michx. and Acer saccharum Marsh. to a factorial combination of ambient and elevated atmospheric CO2 concentrations ([CO2]) and high-nitrogen (N) and low-N soil treatments in open-top chambers for 3 years. Our objective was to compare photosynthetic acclimation to elevated [CO2] between species of contrasting shade tolerance, and to determine if soil N or shading modify the acclimation response. Sun and shade leaf responses to elevated [CO2] and soil N were compared between upper and lower canopy leaves of P. tremuloides and between A. saccharum seedlings grown with and without shading by P. tremuloides. Both species had higher leaf N concentrations and photosynthetic rates in high-N soil than in low-N soil, and these characteristics were higher for P. tremuloides than for A. saccharum. Electron transport capacity (Jmax) and carboxylation capacity (Vcmax) generally decreased with atmospheric CO2 enrichment in all 3 years of the experiment, but there was no evidence that elevated [CO2] altered the relationship between them. On a leaf area basis, both Jmax and Vcmax acclimated to elevated [CO2] more strongly in shade leaves than in sun leaves of P. tremuloides. However, the apparent [CO2] x shade interaction was largely driven by differences in specific leaf area (m2 g-1) between sun and shade leaves. In A. saccharum, photosynthesis acclimated more strongly to elevated [CO2] in sun leaves than in shade leaves on both leaf area and mass bases. We conclude that trees rooted freely in the ground can exhibit photosynthetic acclimation to elevated [CO2], and the response may be modified by light environment. The hypothesis that photosynthesis acclimates more completely to elevated [CO2] in shade-tolerant species than in shade-intolerant species was not supported.