An open-air system for exposing forest-canopy branches to ozone pollution

We developed a chamberless system to expose branches to elevated concentrations of ozone with little alteration of micro-meteorological conditions. In a 35-year-old stand of sugar maple (Acer saccharum Marsh.), scaffolding and a platform (14 m in height) provided access to 10 branches and ten paired controls within the canopy. Ozone was delivered to the canopy through a manifold and an array of loops (38 cm in diameter) of teflon tubing individually fitted to each branch. Ozone-enriched air was discharged through numerous small holes in each loop positioned beneath the exposed foliage. A sampling system controlled by a microcomputer monitored ozone concentrations for each loop by means of composite air samples from 12 leaves, drawn through small teflon tubes (1.65 mm diameter) attached to the petioles. On average, coefficients of variation for ozone concentrations for the sample points within each branch loop were less than 50%. Between 0900 and 1700 h for 68 d of exposure, the mean hourly ozone concentrations among the branches averaged 95nmol mol⁻¹ (±13SD), about twice the ambient mean. Frequency distributions of mean hourly concentrations during exposure were unimodal and approximately log-normal, comparable to ambient ozone concentrations. The open-air loop system enables exposure of branches to gaseous pollutants under relatively natural conditions.     

Effects of elevated ozone on CO2 uptake and leaf structure in sugar maple under two light environments

The interactive effects of ozone and light on leaf structure, carbon dioxide uptake and short-term carbon allocation of sugar maple ( Acer saccharum Marsh.) seedlings were examined using gas exchange measurements and ¹⁴C-macroautoradiographic techniques. Two-year-old sugar maple seedlings were fumigated from budbreak for 5 months with ambient or 3 × ambient ozone in open-top chambers, receiving either 35% (high light) or 15% (low light) of full sunlight. Ozone accelerated leaf senescence, and reduced net photosynthesis, ¹⁴CO₂ uptake and stomatal conductance, with the effects being most pronounced under low light. The proportion of intercellular space increased in leaves of seedlings grown under elevated ozone and low light, possibly enhancing the susceptibility of mesophyll cells to ozone by increasing the cumulative dose per mesophyll cell. Indeed, damage to spongy mesophyll cells in the elevated ozone × low light treatment was especially frequent. ¹⁴C macroautoradioraphy revealed heterogeneous uptake of ¹⁴CO₂ in well defined areole regions, suggesting patchy stomatal behaviour in all treatments. However, in seedlings grown under elevated ozone and low light, the highest ¹⁴CO₂ uptake occurred along larger veins, while interveinal regions exhibited little or no uptake. Although visible symptoms of ozone injury were not apparent in these seedlings, the cellular damage, reduced photosynthetic rates and reduced whole-leaf chlorophyll levels corroborate the visual scaling of whole-plant senescence, suggesting that the ozone × low light treatment accelerated senescence or senescence-like injury in sugar maple.     

Fertilization effects on fineroot biomass, rhizosphere microbes and respiratory fluxes in hardwood forest soils

Fertilizer-induced reductions in CO(2) flux from soil ((F)CO(2)) in forests have previously been attributed to decreased carbon allocation to roots, and decreased decomposition as a result of nitrogen suppression of fungal activity. Here, we present evidence that decreased microbial respiration in the rhizosphere may also contribute to (F)CO(2) reductions in fertilized forest soils. Fertilization reduced (F)CO(2) by 16-19% in 65-yr-old plantations of northern red oak (Quercus rubra) and sugar maple (Acer saccharum), and in a natural 85-yr-old yellow birch (Betula allegheniensis) stand. In oak plots, fertilization had no effects on fine root biomass but reduced mycorrhizal colonization by 18% and microbial respiration by 43%. In maple plots, fertilization reduced root biomass, mycorrhizal colonization and microbial respiration by 22, 16 and 46%, respectively. In birch plots, fertilization reduced microbial respiration by 36%, but had variable effects on root biomass and mycorrhizal colonization. In plots of all three species, fertilization effects on microbial respiration were greater in rhizosphere than in bulk soil, possibly as a result of decreased rhizosphere carbon flux from these species in fertile soils. Because rhizosphere processes may influence nutrient availability and carbon storage in forest ecosystems, future research is needed to better quantify rhizo-microbial contributions to (F)CO(2).     

臭氧和气溶胶复合污染对杨树叶片光合作用的影响

由于经济的快速发展, 中国大部分地区正面临着严峻的复合型大气污染, 其中臭氧和气溶胶是两种主要污染物。已有的研究表明臭氧对叶片的氧化性伤害能够抑制光合作用, 而气溶胶可通过增加散射辐射比例或缓解高温抑制促进光合作用。但复合污染下, 臭氧和气溶胶如何共同调控叶片光合作用, 仍缺乏研究。该研究利用北京及周边地区之间的污染梯度, 选择加杨(Populus × canadensis)作为实验对象, 于2012-2013年生长季期间对叶片光合速率进行连续观测, 并同时监测臭氧浓度(AOT40)、气溶胶光学厚度(AOD)、空气温度和冠层内外光合有效辐射(PAR)等环境因子, 以期探讨大气复合污染下臭氧和气溶胶变化对植物叶片光合作用的影响及相关机制。结果表明: (1)臭氧浓度与空气温度、气溶胶浓度之间均呈显著正相关关系, 但气溶胶浓度与空气温度没有显著相关关系; (2)臭氧浓度增加显著抑制了阳生叶片的光合作用, 但气溶胶浓度上升促进了阳生叶片的光合作用; 臭氧浓度升高对阴生叶片光合作用的影响较小, 但气溶胶浓度上升促进了阴生叶片的光合作用; (3)标准化后的结果显示, 臭氧对阳生叶片光合作用的影响最大, 此时气溶胶的促进作用一定程度上补偿了臭氧浓度上升所带来的抑制效应。对于阴生叶片光合作用而言, 气溶胶则是最重要的影响因素。该研究发现复合污染下阴生叶和阳生叶光合响应不同, 这表明冠层结构可能通过影响阴生叶和阳生叶的比例, 从而对植物生长产生不同影响。该研究对理解大气复合污染如何影响光合作用提供了的机理支持, 同时也表明, 为了维持生态系统生产力及功能, 需要同时控制气溶胶和臭氧污染。     

Drought during canopy development has lasting effect on annual carbon balance in a deciduous temperate forest

* Climate change projections predict an intensifying hydrologic cycle and an increasing frequency of droughts, yet quantitative understanding of the effects on ecosystem carbon exchange remains limited. * Here, the effect of contrasting precipitation and soil moisture dynamics were evaluated on forest carbon exchange using 2 yr of eddy covariance and microclimate data from a 50-yr-old mixed oak woodland in northern Ohio, USA. * The stand accumulated 40% less carbon in a year with drought between bud-break and full leaf expansion (354 +/- 81 g C m(-2) yr(-1) in 2004 and 252 +/- 45 g C m(-2) yr(-1) in 2005). This was caused by greater suppression of gross ecosystem productivity (GEP; 16% = 200 g) than of ecosystem respiration (ER; 11% = 100 g) by drought. Suppressed GEP was traced to lower leaf area, lower apparent quantum yield and lower canopy conductance. The moisture sensitivity of ER may have been mediated by GEP. * The results highlight the vulnerability of the ecosystem to even a moderate drought, when it affects a critical aspect of development. Although the drought was preceded by rain, the storage capacity of the soil seemed limited to 1-2 wk, and therefore droughts longer than this are likely to impair productivity in the region.     

Suitability of a combined stomatal conductance and photosynthesis model for calculation of leaf-level ozone fluxes

Currently, the most important source of uncertainty in stomatal ozone flux ( FO3) modelling is the stomatal conductance ( gst) factor. Hence FO3 model accuracy will strongly depend on the gst model being implemented. In this study the recently developed semi-empirical Gst model of Dewar was coupled to the widely known biochemical photosynthesis ( An) model of Farquhar. The Gst performance of this model combination was evaluated with a 4-month time series of beech ( Fagus sylvatica L.) measurements. The Gst model was hereto optimized in two steps to a 4-day and a 8-day period. A comparison between the modelled and measured gst to O(3) (gstO3) revealed a rather good overall performance (R(2)=0.77). Errors between the model combination and the measurements are thought to be largely caused by a moderate performance of the AN model, due to poor parameterization. Two 2-day periods with distinctly differing soil and meteorological conditions were chosen to give a picture of the daily gst performance. Although instant relative differences between modelled and measured gstO3 are sometimes high, the model combination is able to simulate the rough daily courses of gstO3 and hence FO3 reasonably well. Further improvement on full parameterization of the gst model and a well-parameterized An model to be linked to are needed to draw founded conclusions about its performance. Future efforts hereto are certainly justified since the model's mechanistic nature makes it a tool able to model gst variation in space and time, O(3) effects on gst, and effective FO3.     

Functional responses of Acer species to two simulated forest gap environments: leaf-level properties and photosynthesis

Seedlings of eight forest maple (Acer L.) species were grown outdoors through a full season under two irradiation treatments: (a) “gap edge” with a photosynthetic photon flux density of 30 μmol m-² s-¹ and a red:far-red ratio of 0.55, and (b) “gap centre” with 400 μmol m-² s-¹ and a red:far-red ratio of 1.12. Area-based leaf nitrogen concentration was greater in gap centre-grown seedlings, whereas, except for A. saccharum, area-based chlorophyll (Chl) (a+b) was higher in gap edge-grown plants. There was also a significantly lower Chl a/b ratio in gap edge-grown plants. Maximum photosynthetic rate (P _max ) was 60 % higher in the gap-centre treatment. These results are consistent with the functional expectation that shade-acclimated plants will increase their radiant-energy harvesting capacity as a result of limited photon input while gap-acclimated plants will operate more efficiently under bright irradiance by increasing their carboxylation capacity. This inverse relationship between the capacity of the light-harvesting component and the carboxylation component is, however, only partially supported by Chl fluorescence measurements of intact leaves. Compared to gap centre-grown plants, the lower total fluorescence quenching in gap edge-grown plants indicated a lower carboxylation capacity that was in accord with the observed P _max . However, edge-grown seedlings did not show the expected improvement in light-harvesting efficiency and reduction in electron transport of photosystem 2 inferred from their marginally greater t_1/2 and lower F_v/F_m, respectively. Hence while maples acclimated to different irradiation levels by adjusting leaf N and Chl contents, they showed limited acclimation potential at the photosystem level. Variations in the leaf traits examined had only minor effect on low irradiance photosynthesis and sunfleck utilization. This revised version was published online in August 2006 with corrections to the Cover Date.     

Coping with branch excision when measuring leaf net photosynthetic rates in a lowland tropical forest

Measuring leaf gas exchange from canopy leaves is fundamental for our understanding of photosynthesis and for a realistic representation of carbon uptake in vegetation models. Since canopy leaves are often difficult to reach, especially in tropical forests with emergent trees up to 60 m at remote places, canopy access techniques such as canopy cranes or towers have facilitated photosynthetic measurements. These structures are expensive and therefore not very common. As an alternative, branches are often cut to enable leaf gas exchange measurements. The effect of branch excision on leaf gas exchange rates should be minimized and quantified to evaluate possible bias. We compared light-saturated leaf net photosynthetic rates measured on excised and intact branches. We selected branches positioned at three canopy positions, estimated relative to the top of the canopy: upper sunlit foliage, middle canopy foliage, and lower canopy foliage. We studied the variation of the effects of branch excision and transport among branches at these different heights in the canopy. After excision and transport, light-saturated leaf net photosynthetic rates were close to zero for most leaves due to stomatal closure. However, when the branch had acclimated to its new environmental conditions—which took on average 20 min—light-saturated leaf net photosynthetic rates did not significantly differ between the excised and intact branches. We therefore conclude that branch excision does not affect the measurement of light-saturated leaf net photosynthesis, provided that the branch is recut under water and is allowed sufficient time to acclimate to its new environmental conditions.     

Limited effect of ozone reductions on the 20‐year photosynthesis trend at Harvard forest

Ozone (O₃) damage to leaves can reduce plant photosynthesis, which suggests that declines in ambient O₃ concentrations ([O₃]) in the United States may have helped increase gross primary production (GPP) in recent decades. Here, we assess the effect of long‐term changes in ambient [O₃] using 20 years of observations at Harvard forest. Using artificial neural networks, we found that the effect of the inclusion of [O₃] as a predictor was slight, and independent of O₃ concentrations, which suggests limited high‐frequency O₃ inhibition of GPP at this site. Simulations with a terrestrial biosphere model, however, suggest an average long‐term O₃ inhibition of 10.4% for 1992–2011. A decline of [O₃] over the measurement period resulted in moderate predicted GPP trends of 0.02–0.04 μmol C m^?2 s^?1 yr^?1, which is negligible relative to the total observed GPP trend of 0.41 μmol C m^?2 s^?1 yr^?1. A similar conclusion is achieved with the widely used AOT40 metric. Combined, our results suggest that ozone reductions at Harvard forest are unlikely to have had a large impact on the photosynthesis trend over the past 20 years. Such limited effects are mainly related to the slow responses of photosynthesis to changes in [O₃]. Furthermore, we estimate that 40% of photosynthesis happens in the shade, where stomatal conductance and thus [O₃] deposition is lower than for sunlit leaves. This portion of GPP remains unaffected by [O₃], thus helping to buffer the changes of total photosynthesis due to varied [O₃]. Our analyses suggest that current ozone reductions, although significant, cannot substantially alleviate the damages to forest ecosystems.     

Changes in effects of ozone exposure on growth,photosynthesis, and respiration of <Emphasis Type="Italic">Ginkgo biloba</Emphasis> in Shenyang urban area

An open-top chamber experiment was carried out from April through October 2006 to examine the effects of elevated (80 nmol mol⁻¹) atmospheric O₃ on Ginkgo biloba (4-years-old) in urban area. The air with ambient O₃ (AA, ≈ 45 nmol mol⁻¹) was used as control. The leaf mass and size, leaf area index, net photosynthetic rate (P _N), apparent quantum yield, transpiration rate, and stomatal conductance were decreased by elevated O₃ (EO) exposure. Visible foliar injury, which is light-brown flecks, was observed in the EO OTCs after 90 d of exposure. Carboxylation efficiency (Φ_CO2) and photorespiration and dark respiration rates were enhanced by EO exposure in the first half of the season, but all of them turned to be lower than those of the AA control at the end of experiment. Stomata limitation of photosynthesis was significantly higher than control in the whole season (p<0.05). Chlorophyll (Chl) content was lower in EO variant than in the control and the difference became more and more apparent through the season. Hence the decrease in P _N of G. biloba exposed to EO was the result of both stomatal and non-stomatal limitations. In the early season, the inhibition of photosynthesis was mainly caused by the stomatal limitation, and the earliest response was photoprotective down-regulation of photosynthesis but not photodamage. However, at the end of the season, the non-stomatal limiting factors such as decrease in Chl content, decrease in Φ_CO2, and anti-oxidative enzyme activity became more important.     

Free-air exposure systems to scale up ozone research to mature trees

Because seedlings and mature trees do not necessarily respond similarly to O(3) stress, it is critically important that exposure systems be developed that allow exposure of seedlings through to mature trees. Here we describe three different O(3) Free-Air Exposure Systems that have been used successfully for exposure at all growth stages. These systems of spatially uniform O(3) release have been shown to provide reliable O(3) exposure with minimal, if any, impact on the microclimate. This methodology offers a welcome alternative to chamber studies which had severe space constraints precluding stand or community-level studies and substantial chamber effects on the microclimate and, hence physiological tree performance.