Effects of elevated ozone and low light on diurnal and seasonal carbon gain in sugar maple

The long‐term interactive effects of ozone and light on whole‐tree carbon balance of sugar maple (Acer saccharum Marsh.) seedlings were examined, with an emphasis on carbon acquisition, foliar partitioning into starch and soluble sugars, and allocation to growth. Sugar maple seedlings were fumigated with ambient, 1·7 × ambient and 3·0 × ambient ozone in open‐top chambers for 3 years under low and high light (15 and 35% full sunlight, respectively). Three years of ozone fumigation reduced the total biomass of seedlings in the low‐ and high‐light treatments by 64 and 41%, respectively, but had no effect on whole‐plant biomass allocation. Ozone had no effect on net photosynthesis until late in the growing season, with low‐light seedlings generally exhibiting more pronounced reductions in photosynthesis. The late‐season reduction in photosynthesis was not due to impaired stomatal function, but was associated more with accelerated senescence or senescence‐like injury. In contrast, the 3·0 × ambient ozone treatment immediately reduced diurnal starch accumulation in leaves by over 50% and increased partitioning of total non‐structural carbohydrates into soluble sugars, suggesting that injury repair processes may be maintaining photosynthesis in late spring and early summer at the expense of storage carbon. The results in the present study indicate that changes in leaf‐level photosynthesis may not accurately predict the growth response of sugar maple to ozone in different light environments. The larger reduction in seedling growth under low‐light conditions suggests that seedlings in gap or closed‐canopy environments are more susceptible to ozone than those in a clearing. Similarly, understanding the effects of tropospheric ozone on net carbon gain of a mature tree will require scaling of leaf‐level responses to heterogeneous light environments, where some leaves may be more susceptible than others.     

Phenological responses in maple to experimental atmospheric warming and CO2 enrichment

Evidence that global warming has altered the phenology of the biosphere, possibly contributing to increased plant production in the northern hemisphere, has come from a diversity of observations at scales ranging from the view of the back yard to satellite images of the earth. These observations, coupled with an understanding of the effects of temperature on plant phenology, suggest that future changes in the atmosphere and climate could alter plant phenology with unknown or unpredictable consequences. We assessed the effects of simulated climatic warming and atmospheric CO₂ enrichment on the spring and autumn phenology of maple trees (Acer rubrum and A. saccharum) growing for four years in open‐top field chambers. CO₂ enrichment (+300 ppm) had no consistent effects on the timing of budbreak and leaf unfolding in the spring or leaf abscission in the autumn. Warming (+4°C) usually had predictable effects: in two of the three years of assessment, budbreak occurred earlier in warm chambers than in ambient temperature chambers, and leaf abscission always occurred later. The lengthening of the growing season could contribute to increased productivity, although effects of temperature on other physiological processes can concurrently have negative effects on productivity. In 1995, budbreak was unexpectedly delayed in the warmer chambers, apparently the result of advanced budbreak leading to injury from a late‐spring frost. Likewise, there was increased risk associated with longer leaf retention in the autumn: in 1994, leaves in the warm chambers were killed by freezing temperatures before they had senesced. These observations support the premise that global warming could increase the length of the growing season. Phenological responses should, therefore, be part of any assessment of the possible consequences of global change, but our results also suggest that those responses may not always have positive effects on production.     

Interaction of ozone pollution and light effects on photosynthesis in a forest canopy experiment

Ozone pollution may reduce net carbon gain in forests, yet data from mature trees are rare and the effects of irradiance on the response of photosynthesis to ozone remain untested. We used an open-air system to expose 10 branches within the upper canopy of an 18-m-tall stand of sugar maple (Acer saccharum Marsh.) to twice-ambient concentrations of ozone (95nmol mol^?1, 0900 to 1700, 1 h mean) relative to 10 paired, untreated controls (45nmol mol^?1) over 3 months. The branch pairs were selected along a gradient from relatively high irradiance (PPFD 14.5 mol m^?2 d^?1) to deep shade (0.7mol m^?2 d^?1). Ozone reduced light-saturated rates of net photosynthesis (A_sat) and increased dark respiration by as much as 56 and 40%, respectively. Compared to sun leaves, shade leaves exhibited greater proportional reductions in A_sat and had lower chlorophyll concentrations, quantum efficiencies, and leaf absorptances when treated with ozone relative to controls. With increasing ozone dose over time, A_sat became uncoupled from stomatal conductance as ratios of internal to external concentrations of carbon dioxide increased, reducing water-use efficiency. Ozone reduced net photosynthesis and impaired stomatal function, with these effects depending on the irradiance environment of the canopy leaves. Increased ozone sensitivity of shade leaves compared to sun leaves has consequences for net carbon gain in canopies.     

Effects of elevated CO2 and temperature-grown red and sugar maple on gypsy moth performance

Few studies have investigated how tree species grown under elevated CO₂ and elevated temperature alter the performance of leaf‐feeding insects. The indirect effects of an elevated CO₂ concentration and temperature on leaf phytochemistry, along with potential direct effects on insect growth and consumption, may independently or interactively affect insects. To investigate this, we bagged larvae of the gypsy moth on leaves of red and sugar maple growing in open‐top chambers in four CO₂/temperature treatment combinations: (i) ambient temperature, ambient CO₂; (ii) ambient temperature, elevated CO₂ (+ 300 μL L^?1 CO₂); (iii) elevated temperature (+ 3.5°C), ambient CO₂; and (iv) elevated temperature, elevated CO₂. For both tree species, leaves grown at elevated CO₂ concentration were significantly reduced in leaf nitrogen concentration and increased in C: N ratio, while neither temperature nor its interaction with CO₂ concentration had any effect. Depending on the tree species, leaf water content declined (red maple) and carbon‐based phenolics increased (sugar maple) on plants grown in an enriched CO₂ atmosphere. The only observed effect of elevated temperature on leaf phytochemistry was a reduction in leaf water content of sugar maple leaves. Gypsy moth larval responses were dependent on tree species. Larvae feeding on elevated CO₂‐grown red maple leaves had reduced growth, while temperature had no effect on the growth or consumption of larvae. No significant effects of either temperature or CO₂ concentration were observed for larvae feeding on sugar maple leaves. Our data demonstrate strong effects of CO₂ enrichment on leaf phytochemical constituents important to folivorous insects, while an elevated temperature largely has little effect. We conclude that alterations in leaf chemistry due to an elevated CO₂ atmosphere are more important in this plant–folivorous insect system than either the direct short‐term effects of temperature on insect performance or its indirect effects on leaf chemistry.     

Effect of elevated CO2 on the stomatal distribution and leaf physiology of Alnus glutinosa

Variation in stomatal development and physiology of mature leaves from Alnus glutinosa plants grown under reference (current ambient, 360 μmol mol⁻¹ CO₂) and double ambient (720 μmol mol⁻¹ CO₂) carbon dioxide (CO₂) mole fractions is assessed in terms of relative plant growth, stomatal characters (i.e. stomatal index and density) and leaf photosynthetic characters. This is the first study to consider the effects of elevated CO₂ concentration on the distribution of stomata and epidermal cells across the whole leaf and to try to ascertain the cause of intraleaf variation. In general, a doubling of the atmospheric CO₂ concentration enhanced plant growth and significantly increased stomatal index. However, there was no significant change in relative stomatal density. Under elevated CO₂ concentration there was a significant decrease in stomatal conductance and an increase in assimilation rate. However, no significant differences were found for the maximum rate of carboxylation ( V _cmax) and the light saturated rate of electron transport ( J _max) between the control and elevated CO₂ treatment.     

The relationship between leaf composition and morphology at elevated CO2 concentrations

The composition and morphology of leaves exposed to elevated [CO₂] usually change so that the leaf nitrogen (N) per unit dry mass decreases and the leaf dry mass per unit area increases. However, at ambient [CO₂], leaves with a high leaf dry mass per unit area usually have low leaf N per unit dry mass. Whether the changes in leaf properties induced by elevated [CO₂] follow the same overall pattern as that at ambient [CO₂] has not previously been addressed. Here we address this issue by using leaf measurements made at ambient [CO₂] to develop an empirical model of the composition and morphology of leaves. Predictions from that model are then compared with a global database of leaf measurements made at ambient [CO₂]. Those predictions are also compared with measurements showing the impact of elevated [CO₂]. In the empirical model both the leaf dry mass and liquid mass per unit area are positively correlated with leaf thickness, whereas the mass of C per unit dry mass and the mass of N per unit liquid mass are constant. Consequently, both the N:C ratio and the surface area:volume ratio of leaves are positively correlated with the liquid content. Predictions from that model were consistent with measurements of leaf properties made at ambient [CO₂] from around the world. The changes induced by elevated [CO₂] follow the same overall trajectory. It is concluded that elevated [CO₂] enhances the rate at which dry matter is accumulated but the overall trajectory of leaf development is conserved.     

Genetic variation in germination, growth, and survivorship of red maple in response to subambient through elevated atmospheric CO2

Genetic variation in plant response to atmospheric carbon dioxide (CO₂) may have influenced paleo‐vegetation dynamics and could determine how future elevated CO₂ drives plant evolution and ecosystem productivity. We established how levels of relatedness – the maternal family, population, and provenance – affect variation in the CO₂ response of a species. This 2‐year growth chamber experiment focused on the germination, growth, biomass allocation, and survivorship responses of Acer rubrum to four concentrations of CO₂: 180, 270, 360, and 600 μL L^?1– representing Pleistocene through potential future conditions. We found that all levels of relatedness interacted with CO₂ to contribute to variation in response. Germination responses to CO₂ varied among families and populations, growth responses depended on families and regions of origin, and survivorship responses to CO₂ were particularly affected by regional identities. Differences among geographic regions accounted for 23% of the variation in biomass response to CO₂. If seeds produced under subambient CO₂ conditions behave similarly, our results suggest that A. rubrum may have experienced strong selection on seedling survivorship at Pleistocene CO₂ levels. Further, this species may evolve in response to globally rising CO₂ so as to increase productivity above that experimentally observed today. Species responses to future atmospheric CO₂ and the accompanying biotic effects on the global carbon cycle will vary among families, populations, and provenances.     

Increased leaf reflectance in tropical trees under elevated CO2

Globally increasing atmospheric CO₂ concentrations are known to affect many aspects of plant physiology and development; however, little attention has been given to leaf and canopy optical properties. Three tropical trees in the Leguminosae, an important canopy tree family in many tropical forests, responded similarly to an experimental doubling of CO₂ partial pressure with a 9–23% increase in spectral leaf reflectance to light in the visible (400–700 nm) waveband. Decreased leaf chlorophyll content under elevated CO₂ may explain part of the observed increase in reflectance. However, analyses that statistically corrected for chlorophyll content effects on reflectance still indicated a significant CO₂ effect. This results, in conjunction with the spectral pattern of the response, suggests that the primary mechanism is increased optical masking of chlorophyll under elevated CO₂. The magnitude of the increase in leaf reflectance is sufficient to suggest that increased canopy reflectance of tropical forests (and possibly other terrestrial ecosystems) may be an important negative feedback in the response of global net radiative climate forcing to increasing atmospheric CO₂.     

Stomatal behaviour, photosynthesis and transpiration under rising CO2

Definitions of the variables used and the units are given in Table 1

The literature reports enormous variation between species in the extent of stomatal responses to rising CO₂. This paper attempts to provide a framework within which some of this diversity can be explained. We describe the role of stomata in the short-term response of leaf gas exchange to increases in ambient CO₂ concentration by developing the recently proposed stomatal model of Jarvis & Davies (1998 ). In this model stomatal conductance is correlated with the functioning of the photosynthetic system so that the effects of increases in CO₂ on stomata are experienced through changes in the rate of photosynthesis in a simple and mechanistically transparent way. This model also allows us to consider the effects of evaporative demand and soil moisture availability on stomatal responses to photosynthesis and therefore provides a means of considering these additional sources of variation. We emphasize that the relationship between the rate of photosynthesis and the internal CO₂ concentration and also drought will have important effects on the relative gains to be achieved under rising CO₂.  

  Table 1 . Abbreviations     

20.

Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO₂ and/or O₃   总被引：3，自引：2，他引：3  

A. Noormets  A. Sôber  E. J. Pell  R. E. Dickson  G. K. Podila  J. Sôber  J. G. Isebrands  & D. F. Karnosky 《Plant, cell & environment》2001,24(3):327-336

Leaf gas exchange parameters and the content of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of two 2‐year‐old aspen (Populus tremuloides Michx.) clones (no. 216, ozone tolerant and no. 259, ozone sensitive) were determined to estimate the relative stomatal and mesophyll limitations to photosynthesis and to determine how these limitations were altered by exposure to elevated CO₂ and/or O₃. The plants were exposed either to ambient air (control), elevated CO₂ (560 p.p.m.) elevated O₃ (55 p.p.b.) or a mixture of elevated CO₂ and O₃ in a free air CO₂ enrichment (FACE) facility located near Rhinelander, Wisconsin, USA. Light‐saturated photosynthesis and stomatal conductance were measured in all leaves of the current terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age‐related variation in relative stomatal limitation (leaf age is described as a function of leaf plastochron index). Photosynthesis was increased by elevated CO₂ and decreased by O₃ at both control and elevated CO₂. The relative stomatal limitation to photosynthesis (l_s) was in both clones about 10% under control and elevated O₃. Exposure to elevated CO₂ + O₃ in both clones and to elevated CO₂ in clone 259, decreased l_s even further – to about 5%. The corresponding changes in Rubisco content and the stability of C_i/C_a ratio suggest that the changes in photosynthesis in response to elevated CO₂ and O₃ were primarily triggered by altered mesophyll processes in the two aspen clones of contrasting O₃ tolerance. The changes in stomatal conductance seem to be a secondary response, maintaining stable C_i under the given treatment, that indicates close coupling between stomatal and mesophyll processes.     

Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and/or O3

Leaf gas exchange parameters and the content of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of two 2‐year‐old aspen (Populus tremuloides Michx.) clones (no. 216, ozone tolerant and no. 259, ozone sensitive) were determined to estimate the relative stomatal and mesophyll limitations to photosynthesis and to determine how these limitations were altered by exposure to elevated CO₂ and/or O₃. The plants were exposed either to ambient air (control), elevated CO₂ (560 p.p.m.) elevated O₃ (55 p.p.b.) or a mixture of elevated CO₂ and O₃ in a free air CO₂ enrichment (FACE) facility located near Rhinelander, Wisconsin, USA. Light‐saturated photosynthesis and stomatal conductance were measured in all leaves of the current terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age‐related variation in relative stomatal limitation (leaf age is described as a function of leaf plastochron index). Photosynthesis was increased by elevated CO₂ and decreased by O₃ at both control and elevated CO₂. The relative stomatal limitation to photosynthesis (l_s) was in both clones about 10% under control and elevated O₃. Exposure to elevated CO₂ + O₃ in both clones and to elevated CO₂ in clone 259, decreased l_s even further – to about 5%. The corresponding changes in Rubisco content and the stability of C_i/C_a ratio suggest that the changes in photosynthesis in response to elevated CO₂ and O₃ were primarily triggered by altered mesophyll processes in the two aspen clones of contrasting O₃ tolerance. The changes in stomatal conductance seem to be a secondary response, maintaining stable C_i under the given treatment, that indicates close coupling between stomatal and mesophyll processes.