Photosynthetic and transpirational responses of red spruce understory trees to light and temperature

Understory red spruce (Picea rubens Sarg.) trees, between 20 and 50 cm in height and 12 years or more in age, were collected from mid- and high-elevation stands in north-central Vermont and placed in a closed-cuvette system to measure photosynthetic and transpirational responses to photosynthetic photon flux density (PPFD) and temperature. Photosynthesis, dark respiration, transpiration and water-use efficiency of trees from both stands responded to changes in PPFD and temperature in similar ways. Trees from both stands exhibited maximum rates of net photosynthesis at temperatures between 15 and 20 degrees C, and exposure to higher temperatures resulted in reduced rates of photosynthesis and increased rates of respiration. Net photosynthetic rates generally increased with increasing light intensity but began to level off at 250 micro mol m(-2) s(-1). Water-use efficiency was maximal when temperature and PPFD were at 15 degrees C and above 400 micro mol m(-2) s(-1), respectively.     

Photosynthetic induction responses of Pinus koraiensis seedlings grown in different light environments

The time processes of photosynthetic induction responses to various irradiances in Korean pine (Pinus koraiensis) seedlings grown in open-light environments and in understory of forest were studied in an area near the Research Station of Changbai Mountain Forest Ecosystems, Jilin Province, China from July 15 to August 5, 1997. The results showed that at 200 μmol·m⁻²·s⁻¹ photosynthetic photon flux density (PPFD) and 500 μmol·m⁻²·s⁻¹ PPFD, the induction time for the photosynthetic rates of understory-grown seedlings to reach 50% and 90% steady-state net photosynthetic rates was longer than that of the open-grown seedlings. The induction responses of open-growth seedlings at 500 μmol·m⁻²·s⁻¹ PPFD were slower than those at 200 μmol·m⁻²·s⁻¹ PPFD, but it was the very reverse for understory-growth seedlings, which indicates that the photosynthetic induction times of Korean pine seedlings grown in the understory depended on the sunfleck intensity. Biograph: ZHOU Yong-bin (1970-), female, associate professor of Shenyang Agricultural University, Shenyang 110161, P.R. China. Responsible editor: Song Funan     

Seasonal photosynthetic responses to light and temperature in white spruce (Picea glauca) seedlings planted under an aspen (Populus tremuloides) canopy and in the open

Photosynthetic light and temperature response curves were measured seasonally in seedlings of white spruce (Picea glauca (Moench.) Voss) grown for two years in the understory of aspen (Populus tremuloides Michx.) or in the open in central Alberta. Light-saturated rate of net photosynthesis, the optimum temperature for net photosynthesis, transpiration rate, photochemical efficiency, and stomatal and mesophyll conductances increased from spring to summer and declined thereafter, whereas dark respiration rate and compensation and saturation points were highest in spring. Depression of photosynthetic parameters was greater in open-grown seedlings than in understory seedlings during the periods in spring and autumn when night frosts were common. Net photosynthetic rates were similar in understory and open-grown seedlings in summer, but they were significantly lower in open-grown seedlings in spring and autumn. Significantly lower transpiration rates and stomatal conductances in open-grown seedlings than in understory seedlings were also observed at 15 and 25 degrees C in the autumn. Shoot and needle growth were less in open-grown seedlings than in understory seedlings. In summer, when irradiances were low in the aspen understory, understory white spruce seedlings maintained a positive carbon balance by decreasing their compensation and saturation points and increasing their photochemical efficiency compared to spring and autumn.     

Effects of light availability on leaf gas exchange and expansion in lychee (Litchi chinensis)

Effects of photosynthetic photon flux density (PPFD) on leaf gas exchange of lychee (Litchi chinensis Sonn.) were studied in field-grown "Kwai May Pink" and "Salathiel" orchard trees and young potted "Kwai May Pink" plants during summer in subtropical Queensland (27 degrees S). Variations in PPFD were achieved by shading the trees or plants 1 h before measurement at 0800 h. In a second experiment, potted seedlings of "Kwai May Pink" were grown in a heated greenhouse in 20% of full sun (equivalent to maximum noon PPFD of 200 micromol m(-2)xs(-1)) and their growth over three flush cycles was compared with seedlings grown in full sun (1080 micromol m(-2)xs(-1)). Young potted plants of "Kwai May Pink" were also grown outdoors in artificial shade that provided 20, 40, 70 or 100% of full sun (equivalent to maximum PPFDs of 500, 900, 1400 and 2000 micromol m(-2)xs(-1)) and measured for shoot extension and leaf area development over one flush cycle. Net CO2 assimilation increased asymptotically in response to increasing PPFD in both orchard trees and young potted plants. Maximum rates of CO2 assimilation (11.9 +/- 0.5 versus 6.3 +/- 0.2 micromol CO2 m(-2) s(-1)), dark respiration (1.7 +/- 0.3 versus 0.6 +/- 0.2 micromol CO2 m(-2) s(-1)), quantum yield (0.042 +/- 0.005 versus 0.027 +/- 0.003 mol CO2 mol(-1)) and light saturation point (1155 versus 959 micromol m(-2) s(-1)) were higher in orchard trees than in young potted plants. In potted seedlings grown in a heated greenhouse, shoots and leaves exposed to full sun expanded in a sigmoidal pattern to 69 +/- 12 mm and 497 +/- 105 cm(2) for each flush, compared with 27 +/- 7 mm and 189 +/- 88 cm(2) in shaded seedlings. Shaded seedlings were smaller and had higher shoot:root ratios (3.7 versus 3.1) than seedlings grown in full sun. In the potted plants grown outdoors in 20, 40, 70 or 100% of full sun, final leaf area per shoot was 44 +/- 1, 143 +/- 3, 251 +/- 7 and 362 +/- 8 cm(2), respectively. Shoots were also shorter in plants grown in shade than in plants grown in full sun (66 +/- 5 mm versus 101 +/- 2 mm). Photosynthesis in individual leaves of lychee appeared to be saturated at about half full sun, whereas maximum leaf expansion occurred at higher PPFDs. We conclude that lychee plants can persist as seedlings on the forest floor, but require high PPFDs for optimum growth.     

Changes in photosynthesis and water status of developing leaves of Brachystegia spiciformis Benth

Changes in net carbon assimilation and water status were studied during leaf development in the deciduous, tropical species Brachystegia spiciformis Benth. In this upland savanna African tree, bud-burst and leaf development occur approximately two months before the rainy season. The newly formed leaves synthesize anthocyanin until the fully expanded leaves of the whole canopy are red. This foliage is referred to as "spring flush" foliage. Subsequently, the anthocyanins are metabolized and the pre-rain leaves become green. Carbon dioxide assimilation exhibited a bimodal diurnal pattern and was similar for pre-rain green leaves and fully expanded flushing leaves, although pre-rain green leaves showed a net uptake of carbon throughout the daylight period, whereas flushing leaves exhibited only brief periods of net photosynthesis in the morning and early afternoon. Measurements of leaf water potential and relative water content showed a diurnal pattern with considerable variation throughout the day. Leaf water potential and relative water content values decreased soon after sunrise reaching a minimum at a time corresponding to the afternoon peak in CO(2) assimilation. Stomatal conductance was closely related to transpiration rate in both flushing and pre-rain green leaves, although flushing leaves had lower stomatal conductances than pre-rain green leaves. Pre-rain green leaves exhibited a compensation irradiance of approximately 180 micro mol m(-2) s(-1), whereas flushing leaves had positive net photosynthesis only at PPFDs greater than 300 micro mol m(-2) s(-1). Rate of photosynthesis (expressed per leaf area or chlorophyll unit) increased as anthocyanin concentration decreased, although the photosynthetic rate continued to increase long after the leaf anthocyanins had been degraded to low, visually undetectable amounts. Post-rain green leaves had chlorophyll concentrations, transpiration rates and stomatal conductances similar to those of pre-rain green leaves; however, photosynthetic rates in post-rain leaves were more than three times higher. Thus, during the early stages of the spring flush, carbon asimilation rates of the flushing leaves were inversely related to leaf anthocyanin concentrations. In pre-rain green leaves, photosynthesis was limited by other non-stomatal factors.     

Photosynthetic responses to lightflecks ofFagus crenata seedlings grown in a gap and understory of a deciduous forest

Photosynthetic responses to a series of 1-min lightflecks (1,000μmol m⁻² s⁻¹) superimposed on a background with different duration (1, 5, and 10 min) and intensity (25 and 50μmol m⁻² s⁻¹) of low background photosynthetic photon flux density (PPFD) were measured in the leaves ofFagus crenata grown in a gap and understory of aFagus crenata forest in the Naeba Mountains. The two background PPFD intensities most frequently occurred in understory and gap sites respectively. The maximum net photosynthetic rate (P _Nmax) and maximum stomatal conductance (g _smax) were higher in the gap seedlings than in the understory seedlings. However, when the background PPFD was 25μmol m⁻²s⁻¹, the net photosynthetic rate (P ₂₅) and stomatal conductance (g _s25) were almost the same between the gap and understory. When the background PPFD duration was 1-min, the net photosynthetic rate (P _N ) at the end of each lightfleck increased progressively. When the background PPFD duration was 5- and 10-min, the increase inP _N at the end of each lightfleck was less. This indicates that background PPFD duration is important to photosynthetic responses to lightflecks. The higher ratios ofP ₂₅/P _Nmax andg _s25/g _smax in the understory seedlings indicate that the understory seedlings can maintain relatively lower levels of biochemical and stomatal limitations than the gap seedlings under low light conditions. The ratios ofP _N /P _Nmax at the end of each lightfleck (IS) and light utilization efficiency of single lightflecks (LUE _s) that showed the influence of lightflecks on carbon gain were higher in the understory seedlings than in the gap seedlings when the background PPFD was 25μmol m⁻² s⁻¹. This means that understory seedling are capable of utilizing fluctuating light more efficiently under low light conditions than the gap seedlings although the net carbon gain of single lightflecks (CG _s) in the understory seedlings was not higher than that in the gap seedlings. There were no significant differences inIS andLUE _s between understory seedlings at a background PPFD of 25μmol m⁻² s⁻¹ and gap seedlings at a background PPFD of 50μmol m⁻² s⁻¹. However,CG _s in gap seedlings was higher than in understory seedlings. These results provide more evidence thatF. crenata acclimate to a natural light environment in respect to relative induction state at low background PPFD and can capture the fluctuating light at the same efficiency in both the gap and understory seedlings under natural light environments. This study was funded by the research project, Evaluation of Total CO₂ Budget in Forest Ecosystems, coordinated by the Ministry of Agriculture, Forestry and Fisheries of Japan.     

Influence of photosynthetic photon flux density on growth and transpiration in seedlings of Fagus sylvatica

Beech seedlings (Fagus sylvatica L.) were grown in various combinations of three photosynthetic photon flux densities (PPFD, 0.7, 7.3 or 14.5 mol m(-2) day(-1)) for two years in a controlled environmental chamber. Dry mass of leaves, stem and roots, leaf area and number of leaves, and unit leaf rate were affected by both previous-year and current-year PPFD. Number of shoots and length of the main shoot were affected by previous-year PPFD but not by current-year PPFD. Number of leaves per shoot did not change with PPFD, whereas leaf dry mass/leaf area ratio was mainly affected by current-year PPFD. During the first 10 days that newly emerged seedlings were grown at a PPFD of 0.7 or 14.5 mol m(-2) day(-1), transpiration rate per unit leaf area declined. Thereafter, transpiration increased to a constant new rate. Transpiration rate per seedling was closely related to leaf area but the relationship changed with time. In two-year-old seedlings grown at various PPFD combinations of 0.7, 7.3 and 14.5 mol m(-2) day(-1) during Years 1 and 2, leaf area and transpiration rate per seedling were closely correlated at Weeks 7 and 11 after bud burst. Weak correlations were found between root dry mass and transpiration rate per seedling. During Year 2, transpiration rate per leaf area was higher at a particular PPFD in seedlings grown at a previous-year PPFD of 0.7 mol m(-2) day(-1) than in seedlings grown at a previous-year PPFD of 14.5 mol m(-2) day(-1). After transfer of two-year-old seedlings at the end of the experiment to a new PPFD (7.3 or 14.5 mol m(-2) day(-1)) for one day, transpiration rates per leaf area, measured at the new PPFD, were correlated with leaf area and root dry mass, irrespective of former PPFD treatment.     

Leaf gas exchange and cholorphyll fluorescence in relation to leaf angle,azimuth, and canopy position in the tropical pioneer tree,Macaranga conifera

We tested the hypothesis that, in tropical pioneer tree species, vertical leaf angle contributes to high carbon gain because it minimizes damage caused by high irradiances. Diurnal changes in leaf gas exchange and chlorophyll fluorescence were measured in east-facing (EL), west-facing (WL) leaves, and in leaves artificially held horizontal (HL) in the uppermost canopy of Macaranga conifera (Zoll.) Muell. Arg. Maximum values of net photosynthetic rate (P(n)) for EL and HL reached 12 &mgr;mol m(-2) s(-1), whereas maximum P(n) for WL was only 6 &mgr;mol m(-2) s(-1). Midday depressions of P(n) and stomatal conductance occurred at high photosynthetic photon flux densities (PPFD), especially for HL. Photosystem II quantum yield (DeltaF/F(m)') of HL for a given PPFD at the leaf surface was lower in the afternoon than in the morning. Values of DeltaF/F(m)' for HL measured at dusk were lower than those measured just before dawn, suggesting that HL suffered from high light and heat load. Variations in the morphology and physiology of the canopy leaves were associated with different light environments, and there was circumstantial evidence of a transitional point at a PPFD of about 20-30% of full sunlight. Maximum P(n) and nitrogen (N) content were higher in upper canopy leaves than in lower canopy leaves, and the differences were mainly associated with differences in lamina thickness. We conclude that the vertical leaf angle and thick lamina of the top canopy leaves contributed to enhance total carbon gain of the whole plant.     

Photosynthetic induction responses to variable light under field conditions in three species grown in the gap and understory of a Fagus crenata forest

Photosynthetic induction responses to abrupt increases in photon flux density (PFD) to 800 and 1500 &mgr;mol m(-2) s(-1) from either darkness or 100 &mgr;mol m(-2) s(-1) were examined in situ in leaves of Fagus crenata Blume, Daphniphyllum humile Maxim., and Acer rufinerve Siebold & Zucc. growing in a gap and the understory of an F. crenata forest. Among the species studied, F. crenata exhibited the highest assimilation rate (A(100)), stomatal conductance (g(s100)) at the background PFD of 100 &mgr;mol m(-2) s(-1), and A(100)/A(max) (A(max) = maximum assimilation rate), in both the gap and the understory. Time required for full induction depended on both background PFD and maximum PFD. The induction period was 2-4-fold shorter at a background PFD of 100 &mgr;mol m(-2) s(-1) than in darkness. For the three understory species, time required to full induction was 2-3-fold longer when irradiance was increased from darkness to 800 &mgr;mol m(-2) s(-1) than when irradiance was increased from darkness to 1500 &mgr;mol m(-2) s(-1). Acer rufinerve showed higher initial stomatal conductance (g(s0)) and a shorter induction period in the understory than in the gap. Fagus crenata exhibited a similar g(s0) and induction period in both habitats. Daphniphyllum humile demonstrated lower g(s0) and a longer induction period in the understory than in the gap. These findings indicate that initial stomatal conductance is closely correlated with the photosynthetic induction response. We conclude that the photosynthetic induction response is affected by the light conditions experienced by plants before the sudden increase in irradiance and by the extent of the increase in irradiance.     

Photosynthesis and light-use efficiency by plants in a Canadian boreal forest ecosystem

Measurements of the photosynthetic response to midsummer irradiance were made for 11 species representing the dominant trees, understory shrubs, herbaceous plants and moss species in an old black spruce (Picea mariana (Mill.) B.S.P.) boreal forest ecosystem. Maximum rates of photosynthesis per unit foliage area at saturating irradiance, A(max), were highest for aspen (Populus tremuloides Michx.), reaching 16 micromol m(-2) s(-1). For tamarack (Larix laricina (Du Roi) K. Kock) and P. mariana, Amax was only 2.6 and 1.8 micromol m(-2) s(-1), respectively. Values of A(max) for understory shrubs and herbaceous plants were clustered between 9 and 11 micromol m(-2) s(-1), whereas A(max) of feather moss (Pleurozium schreberi (Brid.) Mitt.) reached only 1.9 micromol m(-2) s(-1). No corrections were made for differences in shoot structure, but values of photosynthetic light-use efficiency were similar for most species (70-80 mmol CO2 mol(-1)); however, they were much lower for L. laricina and P. mariana (15 mmol CO2 mol(-1)) and much higher for P. schreberi (102 m;mol CO2 mol(-1)). There was a linear relationship between Amax and foliage nitrogen concentration on an area basis for the broad-leaved species in the canopy and understory, but the data for P. mariana, L. laricina and P. schreberi fell well below this line. We conclude that it is not possible to scale photosynthesis from leaves to the canopy in this ecosystem based on a single relationship between photosynthetic rate and foliage nitrogen concentration.     

Estimates of net photosynthetic parameters for twelve tree species in mature forests of the southern Appalachians

Leaf gas exchange, temperature, and incident radiation were measured in situ for 20 mature trees of 12 deciduous species spanning a range of heights from 7.9 to 30.1 m and growing in the southern Appalachian Mountains. Air temperature, water vapor pressure, total radiation, photosynthetically active radiation, and carbon dioxide concentration were also measured. Estimated mean, light-saturated net assimilation rates ( micro mol m(-2) s(-1)) were: Quercus coccinea Muenchh. (10.3), Q. prinus L. (9.9), Q. rubra L. (8.9), Betula lenta L. (8.1), Liriodendron tulipifera L. (7.9), Q. alba L. (7.6), Carya glabra Mill. (7.2), Acer rubrum L. (5.6), Nyssa sylvatica Marsh. (3.9), Cornus florida L. (3.5), and Acer pensylvanicum L. (1.7). There were significant differences in both net assimilation rates and quantum yield efficiencies between species, with the understory species C. florida and A. pensylvanicum exhibiting lower net assimilation rates at saturation and higher estimated quantum yield efficiencies than the other species. Average temperature and light decreased from the canopy top to bottom, whereas ambient CO(2) concentration increased, and vapor pressure and vapor pressure deficits were inconsistent. We observed curvilinear effects of temperature and vapor pressure deficit on net assimilation response to light, and these effects varied by species. Errors in predicted net assimilation ranged from 1 to 3 micro mol m(-2) s(-1) under the environmental conditions prevailing during the study.     

Light-use properties in two sun-adapted shrubs with contrasting canopy structures

We investigated the impact of high solar irradiance and elevated temperature on carbon gain by two, co-occurring, sun-adapted, dwarf shrub species, Planchonella obovata var. dubia (Koidz.) Hatusima and Hibiscus glaber Matsumura, growing on sun-exposed ridges in the Bonin Islands, in the subtropical Pacific Ocean. Planchonella had steeply inclined, longer lived, sclerophyllous leaves, whereas Hibiscus has thinner, more horizontally oriented, and shorter lived leaves. We tested the hypothesis that leaf physiological tolerance to high light is lower in Planchonella than in Hibiscus. Under relatively high irradiances (photosynthetic photon flux density, PPFD, > 500 micromol m(-2) s(-1)), net photosynthetic rate (P(n)) was about 8.0 and 0.4 micromol m(-2) s(-1) in mature and young leaves of Planchonella, and about 12.4 and 10.3 micromol m(-2) s(-1) in mature and young leaves of Hibiscus, respectively. Both P(n) and photosystem II (PSII) quantum yield at a given PPFD were lower in Planchonella than in Hibiscus, whereas non-photochemical quenching (NPQ) at a given PPFD was higher in Planchonella. When leaf discs were exposed to high light (1900 micromol m(-2) s(-1) PPFD) at 37, 40 or 43 degrees C for 3 h, the recovery of PSII quantum yield (F(v)/F(m)) in the following 60-min dark period was slower in Planchonella than in Hibiscus, indicating that the ability of PSII to tolerate high light and high temperature was less in Planchonella than in Hibiscus. We postulate that there is a linkage between leaf display and leaf photochemical ability in sun-adapted shrub species.     

Vertical gradients in photosynthetic light response within an old-growth Douglas-fir and western hemlock canopy

We examined needle-level light response of photosynthesis across a vertical light gradient within 45-55-m-tall western hemlock (Tsuga heterophylla (Raf.) Sarg.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees growing in a 400-500-year-old mixed species stand. We determined: (1) whether light-saturated photosynthetic rates, light compensation points, and respiration rates varied from the upper to the lower canopy, and (2) if light-saturated photosynthetic rates, light compensation points, and respiration rates varied between Douglas-fir and western hemlock. Over a 25-m gradient from the canopy top to the lower canopy, mean light-saturated photosynthetic rates, light compensation points, and respiration rates declined in overstory Douglas-fir and western hemlock needles, paralleling a 65% decline in the mean daily photosynthetic photon flux density (PPFD). At the canopy top, increasing light-saturated photosynthetic rates relative to lower canopy needles increased carbon uptake at high PPFD. In the lower canopy, reduced respiration rates relative to upper canopy needles increased carbon uptake at low PPFD by reducing the light compensation point. At all canopy positions, western hemlock had lower mean light-saturated photosynthetic rates, light compensation points and respiration rates than Douglas-fir. As a result, western hemlock had higher net photosynthetic rates at low PPFD, but lower net photosynthetic rates at high PPFD compared with Douglas-fir.     

Foliage, fine-root, woody-tissue and stand respiration in Pinus radiata in relation to nitrogen status

We measured respiration of 20-year-old Pinus radiata D. Don trees growing in control (C), irrigated (I), and irrigated + fertilized (IL) stands in the Biology of Forest Growth experimental plantation near Canberra, Australia. Respiration was measured on fully expanded foliage, live branches, boles, and fine and coarse roots to determine the relationship between CO(2) efflux, tissue temperature, and biomass or nitrogen (N) content of individual tissues. Efflux of CO(2) from foliage (dark respiration at night) and fine roots was linearly related to biomass and N content, but N was a better predictor of CO(2) efflux than biomass. Respiration (assumed to be maintenance) per unit N at 15 degrees C and a CO(2) concentration of 400 micro mol mol(-1) was 1.71 micro mol s(-1) mol(-1) N for foliage and 11.2 micro mol s(-1) mol(-1) N for fine roots. Efflux of CO(2) from stems, coarse roots and branches was linearly related to sapwood volume (stems) or total volume (branches + coarse roots) and growth, with rates for maintenance respiration at 15 degrees C ranging from 18 to 104 micro mol m(-3) s(-1). Among woody components, branches in the upper canopy and small diameter coarse roots had the highest respiration rates. Stem maintenance respiration per unit sapwood volume did not differ among treatments. Annual C flux was estimated by summing (1) dry matter production and respiration of aboveground components, (2) annual soil CO(2) efflux minus aboveground litterfall, and (3) the annual increment in coarse root biomass. Annual C flux was 24.4, 25.3 and 34.4 Mg ha(-1) year(-1) for the C, I and IL treatments, respectively. Total belowground C allocation, estimated as the sum of (2) and (3) above, was equal to the sum of root respiration and estimated root production in the IL treatment, whereas in the nutrient-limited C and I treatments, total belowground C allocation was greater than the sum of root respiration and estimated root production, suggesting higher fine root turnover or increased allocation to mycorrhizae and root exudation. Carbon use efficiency, the ratio of net primary production to assimilation, was similar among treatments for aboveground tissues (0.43-0.50). Therefore, the proportion of assimilation used for construction and maintenance respiration on an annual basis was also similar among treatments.     

The effect of defruiting at different stages of fruit development on leaf photosynthesis of "Golden Delicious" apple

Net photosynthetic rates (A) of leaves on 11-year-old, field-grown apple trees (Malus domestica Borkh. cv. Golden Delicious) were measured after removal of fruits at four different stages of development. Defruiting decreased A by 21, 42, 27 and 7% when fruits were growing at 311, 293, 229 and 113 mg(DW) day(-1), respectively. Photosynthesis was inhibited more in the afternoon than in the morning, but it was not affected during the first 8 h after fruit removal. Inhibition of A was positively correlated with crop sink strength, but it was not correlated with fruit relative growth rate or crop load. Defruiting decreased A at saturating irradiances (PPFD > 1000 micro mol m(-2) s(-1)), but did not modify the apparent quantum yield of single leaves. These results suggest that the overall effect of defruiting on carbon fixation is negligible in dense canopies, but it may be significant in sparse canopies and in single shoots.     

Short-term light and leaf photosynthetic dynamics affect estimates of daily understory photosynthesis in four tree species

Instantaneous measurements of photosynthesis are often implicitly or explicitly scaled to longer time frames to provide an understanding of plant performance in a given environment. For plants growing in a forest understory, results from photosynthetic light response curves in conjunction with diurnal light data are frequently extrapolated to daily photosynthesis (A(day)), ignoring dynamic photosynthetic responses to light. In this study, we evaluated the importance of two factors on A(day) estimates: dynamic physiological responses to photosynthetic photon flux density (PPFD); and time-resolution of the PPFD data used for modeling. We used a dynamic photosynthesis model to investigate how these factors interact with species-specific photosynthetic traits, forest type, and sky conditions to affect the accuracy of A(day) predictions. Increasing time-averaging of PPFD significantly increased the relative overestimation of A(day) similarly for all study species because of the nonlinear response of photosynthesis to PPFD (15% with 5-min PPFD means). Depending on the light environment characteristics and species-specific dynamic responses to PPFD, understory tree A(day) can be overestimated by 6-42% for the study species by ignoring these dynamics. Although these overestimates decrease under cloudy conditions where direct sunlight and consequently understory sunfleck radiation is reduced, they are still significant. Within a species, overestimation of A(day) as a result of ignoring dynamic responses was highly dependent on daily sunfleck PPFD and the frequency and irradiance of sunflecks. Overall, large overestimates of A(day) in understory trees may cause misleading inferences concerning species growth and competition in forest understories with < 2% full sunlight. We conclude that comparisons of A(day) among co-occurring understory species in deep shade will be enhanced by consideration of sunflecks by using high-resolution PPFD data and understanding the physiological responses to sunfleck variation.     

Effect of elevated carbon dioxide concentration and root restriction on net photosynthesis, water relations and foliar carbohydrate status of loblolly pine seedlings

To determine the effects of CO(2)-enriched air and root restriction on photosynthetic capacity, we measured net photosynthetic rates of 1-year-old loblolly pine seedlings grown in 0.6-, 3.8- or 18.9-liter pots in ambient (360 micro mol mol(-1)) or 2x ambient CO(2) (720 micro mol mol(-1)) concentration for 23 weeks. We also measured needle carbohydrate concentration and water relations to determine whether feedback inhibition or water stress was responsible for any decreases in net photosynthesis. Across all treatments, carbon dioxide enrichment increased net photosynthesis by approximately 60 to 70%. Net photosynthetic rates of seedlings in the smallest pots decreased over time with the reduction occurring first in the ambient CO(2) treatment and then in the 2x ambient CO(2) treatment. Needle starch concentrations of seedlings grown in the smallest pots were two to three times greater in the 2x ambient CO(2) treatment than in the ambient CO(2) treatment, but decreased net photosynthesis was not associated with increased starch or sugar concentrations. The reduction in net photosynthesis of seedlings in small pots was correlated with decreased needle water potentials, indicating that seedlings in the small pots had restricted root systems and were unable to supply sufficient water to the shoots. We conclude that the decrease in net photosynthesis of seedlings in small pots was not the result of CO(2) enrichment or an accumulation of carbohydrates causing feedback inhibition, but was caused by water stress.     

Light environment alters ozone uptake per net photosynthetic rate in black cherry trees

Foliar ozone uptake rates of different-sized black cherry (Prunus serotina Ehrh.) trees were compared within a deciduous forest and adjacent openings in north-central Pennsylvania during one growing season. Study trees included open-grown seedlings and saplings, forest understory seedlings and saplings, and sunlit and shaded portions of mature canopy tree crowns. Instantaneous ozone uptake rates were highest in high-light environments primarily because of higher stomatal conductances. Low ozone uptake rates of seedlings and saplings in the forest understory could be attributed partially to lower average ambient ozone concentrations compared to the canopy and open environments. Among the tree size and light combinations tested, ozone uptake rates were highest in open-grown seedlings and lowest in forest-grown seedlings. Despite lower ozone uptake rates of foliage in shaded environments, ozone uptake per net photosynthesis of foliage in shaded environments was significantly higher than that of foliage in sunlit environments because of weaker coupling between net photosynthesis and stomatal conductance in shaded environments. The potential for greater ozone injury in shaded environments as a result of greater ozone uptake per net photosynthesis is consistent with previous reports of greater ozone injury in shaded foliage than in sunlit foliage.     

Effects of needle age, long-term temperature and CO(2) treatments on the photosynthesis of Scots pine

Naturally regenerated 20-25-year-old Scots pine (Pinus sylvestris L.) trees were grown in open-top chambers in the presence of an elevated temperature or CO(2) concentration, or both. The elevated temperature treatment was administered year-round for 3 years. The CO(2) treatment was applied between April 15 and September 15 for 2 years. The photosynthetic responses of 1- and 2-year-old needles to varying photon flux densities (0-1500 micro mol m(-2) s(-1)) and CO(2) concentrations (350, 700 and 1400 micro mol mol(-1)) during measurement were determined. The CO(2) treatment alone increased maximum photosynthetic rate and light-use efficiency, but decreased dark respiration rate, light compensation and light saturation regardless of needle age. In contrast, the temperature treatment decreased maximum photosynthetic rate and photosynthetic efficiency, but increased dark respiration rate, light compensation and light saturation. The aging of needles affected the photosynthetic performance of the shoots; values of all parameters except photosynthetic efficiency were less in 2- than in 1-year-old needles. The CO(2) treatment decreased and the temperature treatment enhanced the reduction in maximum photosynthesis due to needle aging.