Elevated CO2 reduces sap flux in mature deciduous forest trees

The effect of elevated CO2 on aboveground and belowground carbon allocation and eco-physiology of four species of angiosperm and gymnosperm forest trees

Although atmospheric CO2 concentration ([CO2]) continues to rise, the question of how tree carbon (C) allocation is affected by this change remains. Studies show that C assimilation increases under elevated CO2 (eCO2). Yet, no detailed study has determined the fate of the surplus C, i.e., its compartment and physiological process allocation, nor in multiple species together. In this project, we grew 2-year-old saplings of four key Mediterranean tree species (the conifers Cupressus sempervirens L. and Pinus halepensis Mill., and the broadleaf Quercus calliprinos Webb. and Ceratonia siliqua L.) to [CO2] levels of 400 or 700 p.p.m. for 6 months. We measured the allocation of C to below and aboveground growth, respiration, root exudation, storage and leaf litter. In addition, we monitored intrinsic water-use efficiency (WUE), soil moisture, soil chemistry and nutrient uptake. Net assimilation, WUE and soil nitrogen uptake significantly increased at eCO2 across the four species. Broadleaf species showed soil water savings, which were absent in conifers. All other effects were species-specific: Cupressus had higher leaf respiration, Pinus had lower starch in branches and transiently higher exudation rate and Quercus had higher root respiration. Elevated CO2 did not affect growth or litter production. Our results are pivotal to understanding the sensitivity of tree C allocation to the change in [CO2] when water is abundant. Species-specific responses should be regarded cautiously when predicting future changes in forest function in a higher CO2 world.

Temporal variability in tree responses to elevated atmospheric CO2

At leaf level, elevated atmospheric CO₂ concentration (eCO₂ ) results in stimulation of carbon net assimilation and reduction of stomatal conductance. However, a comprehensive understanding of the impact of eCO₂ at larger temporal (seasonal and annual) and spatial (from leaf to whole-tree) scales is still lacking. Here, we review overall trends, magnitude and drivers of dynamic tree responses to eCO₂ , including carbon and water relations at the leaf and the whole-tree level. Spring and early season leaf responses are most susceptible to eCO₂ and are followed by a down-regulation towards the onset of autumn. At the whole-tree level, CO₂ fertilization causes consistent biomass increments in young seedlings only, whereas mature trees show a variable response. Elevated CO₂ -induced reductions in leaf stomatal conductance do not systematically translate into limitation of whole-tree transpiration due to the unpredictable response of canopy area. Reduction in the end-of-season carbon sink demand and water-limiting strategies are considered the main drivers of seasonal tree responses to eCO₂ . These large temporal and spatial variabilities in tree responses to eCO₂ highlight the risk of predicting tree behavior to eCO₂ based on single leaf-level point measurements as they only reveal snapshots of the dynamic responses to eCO₂ .

Seasonal responses of maize growth and water use to elevated CO2 based on a coupled device with climate chamber and weighing lysimeters

The increase in atmosphere carbon dioxide (CO₂) concentrations has been the most important environmental change experienced by agricultural systems. It is still uncertain whether grain yield of the global food crop of maize will remain unchanged under a future elevated CO₂ (eCO₂) environment. A coupled device with climate chamber and weighing lysimeters was developed to explore the water-related yield responses of maize to eCO₂. Two experiments were conducted via this device under eCO₂ (700 ppm) and current CO₂ (400 ppm) concentrations. Seasonal changes in multiple growth indicators and related hydrological processes were compared between these two experiments. The results showed that the eCO₂ effects were not significant on several indicators, i.e., the leaf carbon (C) content, nitrogen (N) content, chlorophyll content, C/N ratio, net photosynthesis rate, and leaf area index over the entire growing season (p > 0.05). Nevertheless, the transpiration rate (T_r) significantly reduced during the seedling to filling stages but notably increased at the maturity stage due to eCO₂ (p < 0.05). Significant reduction in crop height (mean of 15.9%, p < 0.05) associated with notable increases in stem diameter (mean of 14.9%, p < 0.05) were found throughout the growing season. Dry matter per corncob at the final harvest decreased slightly under eCO₂ (mean of 7.7 g, p > 0.05). Soil water storage was not significantly conserved by the decline of T_r except during the filling stage. Soil evaporation was likely promoted by eCO₂ that the total evapotranspiration changed little (1.2%) over the entire growing season. Although the leaf water use efficiency increased significantly at every growth stage (mean of 27.3%, p < 0.05), the grain yield and water productivity were not improved noticeably by eCO₂. This study could provide significant insight into predicting future crop yield and hydrological changes under climate change.

Disentangling the role of photosynthesis and stomatal conductance on rising forest water-use efficiency

Forests remove about 30% of anthropogenic CO₂ emissions through photosynthesis and return almost 40% of incident precipitation back to the atmosphere via transpiration. The trade-off between photosynthesis and transpiration through stomata, the water-use efficiency (WUE), is an important driver of plant evolution and ecosystem functioning, and has profound effects on climate. Using stable carbon and oxygen isotope ratios in tree rings, we found that WUE has increased by a magnitude consistent with estimates from atmospheric measurements and model predictions. Enhanced photosynthesis was widespread, while reductions in stomatal conductance were modest and restricted to moisture-limited forests. This result points to smaller reductions in transpiration in response to increasing atmospheric CO₂, with important implications for forest–climate interactions, which remain to be explored.

Multiple lines of evidence suggest that plant water-use efficiency (WUE)—the ratio of carbon assimilation to water loss—has increased in recent decades. Although rising atmospheric CO₂ has been proposed as the principal cause, the underlying physiological mechanisms are still being debated, and implications for the global water cycle remain uncertain. Here, we addressed this gap using 30-y tree ring records of carbon and oxygen isotope measurements and basal area increment from 12 species in 8 North American mature temperate forests. Our goal was to separate the contributions of enhanced photosynthesis and reduced stomatal conductance to WUE trends and to assess consistency between multiple commonly used methods for estimating WUE. Our results show that tree ring-derived estimates of increases in WUE are consistent with estimates from atmospheric measurements and predictions based on an optimal balancing of carbon gains and water costs, but are lower than those based on ecosystem-scale flux observations. Although both physiological mechanisms contributed to rising WUE, enhanced photosynthesis was widespread, while reductions in stomatal conductance were modest and restricted to species that experienced moisture limitations. This finding challenges the hypothesis that rising WUE in forests is primarily the result of widespread, CO₂-induced reductions in stomatal conductance.

Elevated CO2 did not affect the hydrological balance of a mature native Eucalyptus woodland

Elevated atmospheric CO₂ concentration (eC_a ) might reduce forest water-use, due to decreased transpiration, following partial stomatal closure, thus enhancing water-use efficiency and productivity at low water availability. If evapotranspiration (E_t ) is reduced, it may subsequently increase soil water storage (ΔS) or surface runoff (R) and drainage (D_g ), although these could be offset or even reversed by changes in vegetation structure, mainly increased leaf area index (L). To understand the effect of eC_a in a water-limited ecosystem, we tested whether 2 years of eC_a (~40% increase) affected the hydrological partitioning in a mature water-limited Eucalyptus woodland exposed to Free-Air CO₂ Enrichment (FACE). This timeframe allowed us to evaluate whether physiological effects of eC_a reduced stand water-use irrespective of L, which was unaffected by eC_a in this timeframe. We hypothesized that eC_a would reduce tree-canopy transpiration (E_tree ), but excess water from reduced E_tree would be lost via increased soil evaporation and understory transpiration (E_floor ) with no increase in ΔS, R or D_g . We computed E_t , ΔS, R and D_g from measurements of sapflow velocity, L, soil water content (θ), understory micrometeorology, throughfall and stemflow. We found that eC_a did not affect E_tree , E_floor , ΔS or θ at any depth (to 4.5 m) over the experimental period. We closed the water balance for dry seasons with no differences in the partitioning to R and D_g between C_a levels. Soil temperature and θ were the main drivers of E_floor while vapour pressure deficit-controlled E_tree , though eC_a did not significantly affect any of these relationships. Our results suggest that in the short-term, eC_a does not significantly affect ecosystem water-use at this site. We conclude that water-savings under eC_a mediated by either direct effects on plant transpiration or by indirect effects via changes in L or soil moisture availability are unlikely in water-limited mature eucalypt woodlands.

Stomatal CO2 responsiveness and photosynthetic capacity of tropical woody species in relation to taxonomy and functional traits

Stomatal CO2 responsiveness and photosynthetic capacity vary greatly among plant species, but the factors controlling these physiological leaf traits are often poorly understood. To explore if these traits are linked to taxonomic group identity and/or to other plant functional traits, we investigated the short-term stomatal CO2 responses and the maximum rates of photosynthetic carboxylation (V cmax) and electron transport (J max) in an evolutionary broad range of tropical woody plant species. The study included 21 species representing four major seed plant taxa: gymnosperms, monocots, rosids and asterids. We found that stomatal closure responses to increased CO2 were stronger in angiosperms than in gymnosperms, and in monocots compared to dicots. Stomatal CO2 responsiveness was not significantly related to any of the other functional traits investigated, while a parameter describing the relationship between photosynthesis and stomatal conductance in combined leaf gas exchange models (g 1) was related to leaf area-specific plant hydraulic conductance. For photosynthesis, we found that the interspecific variation in V cmax and J max was related to within leaf nitrogen (N) allocation rather than to area-based total leaf N content. Within-leaf N allocation and water use were strongly co-ordinated (r 2 = 0.67), such that species with high fractional N investments into compounds maximizing photosynthetic capacity also had high stomatal conductance. We conclude that while stomatal CO2 responsiveness of tropical woody species seems poorly related to other plant functional traits, photosynthetic capacity is linked to fractional within-leaf N allocation rather than total leaf N content and is closely co-ordinated with leaf water use.

Industrial-age changes in atmospheric [CO2] and temperature differentially alter responses of faster- and slower-growing Eucalyptus seedlings to short-term drought

Climate change may alter forest composition by differentially affecting the responses of faster- and slower-growing tree species to drought. However, the combined effects of rising atmospheric CO2 concentration ([CO2]) and temperature on drought responses of trees are poorly understood. Here, we examined interactive effects of temperature (ambient, ambient + °C) and [CO2] (290, 400 and 650mu;l l(-1)) on drought responses of Eucalyptus saligna Sm. (faster-growing) and E. sideroxylon A. Cunn. ex Woolls (slower-growing) seedlings. Drought was imposed via a controlled reduction in soil water over 1-2 weeks, re-watering seedlings when leaves visibly wilted. In ambient temperature, the effect of drought on the light-saturated net photosynthetic rate (Asat) in E. saligna decreased as [CO2] increased from pre-industrial to future concentrations, but rising [CO2] did not affect the response in Eucalyptus sideroxylon. In contrast, elevated temperature exacerbated the effect of drought in reducing Asat in both species. The drought response of Asat reflected changes in stomatal conductance (gs) associated with species and treatment differences in (i) utilization of soil moisture and (ii) leaf area ratio (leaf area per unit plant dry mass). Across [CO2] and temperature treatments, E. saligna wilted at higher soil water potentials compared with E. sideroxylon. Photosynthetic recovery from drought was 90% complete 2 days following re-watering across all species and treatments. Our results suggest that E. saligna (faster-growing) seedlings are more susceptible to drought than E. sideroxylon (slower-growing) seedlings. The greater susceptibility to drought of E. saligna reflected faster drawdown of soil moisture, associated with more leaf area and leaf area ratio, and the ability of E. sideroxylon to maintain higher gs at a given soil moisture. Inclusion of a pre-industrial [CO2] treatment allowed us to conclude that susceptibility of these species to short-term drought under past and future climates may be regulated by the same mechanisms. Further, the beneficial effects of rising [CO2] and deleterious effects of elevated temperature on seedling response to drought were generally offsetting, suggesting susceptibility of seedlings of these species to short-term drought in future climates that is similar to pre-industrial and current climate conditions.

Experimental vs. modeled water use in mature Norway spruce (Picea abies) exposed to elevated CO(2)

Rising levels of atmospheric CO(2) have often been reported to reduce plant water use. Such behavior is also predicted by standard equations relating photosynthesis, stomatal conductance, and atmospheric CO(2) concentration, which form the core of dynamic global vegetation models (DGVMs). Here, we provide first results from a free air CO(2) enrichment (FACE) experiment with naturally growing, mature (35 m) Picea abies (L.) (Norway spruce) and compare them to simulations by the DGVM LPJ-GUESS. We monitored sap flow, stem water deficit, stomatal conductance, leaf water potential, and soil moisture in five 35-40 m tall CO(2)-treated (550 ppm) trees over two seasons. Using LPJ-GUESS, we simulated this experiment using climate data from a nearby weather station. While the model predicted a stable reduction of transpiration of between 9% and 18% (at concentrations of 550-700 ppm atmospheric CO(2)), the combined evidence from various methods characterizing water use in our experimental trees suggest no changes in response to future CO(2) concentrations. The discrepancy between the modeled and the experimental results may be a scaling issue: while dynamic vegetation models correctly predict leaf-level responses, they may not sufficiently account for the processes involved at the canopy and ecosystem scale, which could offset the first-order stomatal response.

Interactive effects of elevated CO2 and drought on nocturnal water fluxes in Eucalyptus saligna

Nocturnal water flux has been observed in trees under a variety of environmental conditions and can be a significant contributor to diel canopy water flux. Elevated atmospheric CO(2) (elevated [CO(2)]) can have an important effect on day-time plant water fluxes, but it is not known whether it also affects nocturnal water fluxes. We examined the effects of elevated [CO(2)] on nocturnal water flux of field-grown Eucalyptus saligna trees using sap flux through the tree stem expressed on a sapwood area (J(s)) and leaf area (E(t)) basis. After 19 months growth under well-watered conditions, drought was imposed by withholding water for 5 months in the summer, ending with a rain event that restored soil moisture. Reductions in J(s) and E(t) were observed during the severe drought period in the dry treatment under elevated [CO(2)], but not during moderate- and post-drought periods. Elevated [CO(2)] affected night-time sap flux density which included the stem recharge period, called 'total night flux' (19:00 to 05:00, J(s,r)), but not during the post-recharge period, which primarily consisted of canopy transpiration (23:00 to 05:00, J(s,c)). Elevated [CO(2)] wet (EW) trees exhibited higher J(s,r) than ambient [CO(2)] wet trees (AW) indicating greater water flux in elevated [CO(2)] under well-watered conditions. However, under drought conditions, elevated [CO(2)] dry (ED) trees exhibited significantly lower J(s,r) than ambient [CO(2)] dry trees (AD), indicating less water flux during stem recharge under elevated [CO(2)]. J(s,c) did not differ between ambient and elevated [CO(2)]. Vapour pressure deficit (D) was clearly the major influence on night-time sap flux. D was positively correlated with J(s,r) and had its greatest impact on J(s,r) at high D in ambient [CO(2)]. Our results suggest that elevated [CO(2)] may reduce night-time water flux in E. saligna when soil water content is low and D is high. While elevated [CO(2)] affected J(s,r), it did not affect day-time water flux in wet soil, suggesting that the responses of J(s,r) to environmental factors cannot be directly inferred from day-time patterns. Changes in J(s,r) are likely to influence pre-dawn leaf water potential, and plant responses to water stress. Nocturnal fluxes are clearly important for predicting effects of climate change on forest physiology and hydrology.

Water relations of climbing ivy in a temperate forest

Ivy (Hedera helix) is the most important liana in temperate European forests. We studied water relations of adult ivy in a natural, 35 m tall mixed deciduous forest in Switzerland using a construction crane to access the canopy. Predawn leaf water potential at the top of climbing ivy ranged from -0.4 to -0.6 MPa, daily minima ranged from -1.3 to -1.7 MPa. Leaf water potentials as well as relative sap flow were held surprisingly constant throughout different weather conditions, suggesting a tendency to isohydric behaviour. Maximum stomatal conductance was 200 mmol m⁻² s⁻¹. The use of a potometer experiment allowed us to measure absolute transpiration rates integrated over a whole plant of 0.23 mmol m⁻² s⁻¹. Nightly sap flow of ivy during warm, dry nights accounted for up to 20% of the seasonal maximum. Maximum sap flow rates were reached at ca. 0.5 kPa vpd. On the other hand, the host trees showed a less conservative stomatal regulation, maximum sap flow rates were reached at vpd values of ca. 1 kPa. Sap flow rates of ivy decreased by ca. 20% in spring after bud break of trees, suggesting that ivy profits strongly from warm sunny days in early spring before budbreak of the host trees and from mild winter days. This species may benefit from rising winter temperatures in Europe and thus become a stronger competitor against its host trees.

Sustained enhancement of photosynthesis in mature deciduous forest trees after 8 years of free air CO(2) enrichment

Carbon uptake by forests constitutes half of the planet's terrestrial net primary production; therefore, photosynthetic responses of trees to rising atmospheric CO(2) are critical to understanding the future global carbon cycle. At the Swiss Canopy Crane, we investigated gas exchange characteristics and leaf traits in five deciduous tree species during their eighth growing season under free air carbon dioxide enrichment in a 35-m tall, ca. 100-year-old mixed forest. Net photosynthesis of upper-canopy foliage was 48% (July) and 42% (September) higher in CO(2)-enriched trees and showed no sign of down-regulation. Elevated CO(2) had no effect on carboxylation efficiency (V (cmax)) or maximal electron transport (J (max)) driving ribulose-1,5-bisphosphate (RuBP) regeneration. CO(2) enrichment improved nitrogen use efficiency, but did not affect leaf nitrogen (N) concentration, leaf thickness or specific leaf area except for one species. Non-structural carbohydrates accumulated more strongly in leaves grown under elevated CO(2) (largely driven by Quercus). Because leaf area index did not change, the CO(2)-driven stimulation of photosynthesis in these trees may persist in the upper canopy under future atmospheric CO(2) concentrations without reductions in photosynthetic capacity. However, given the lack of growth stimulation, the fate of the additionally assimilated carbon remains uncertain.

Is Climate Change a Possible Explanation for Woody Thickening in Arid and Semi-Arid Regions?

Increased woody plant density (woody encroachment or woody thickening) is a globally observed phenomenon. Similarly, increased atmospheric carbon dioxide concentrations and decreased pan evaporation rates are globally observed phenomena. In this paper, we propose that the former (increased woody plant density) is a product of the latter. We propose that decreased stomatal conductance and increased rates of carbon fixation arising from an enriched atmospheric carbon dioxide concentration, in conjunction with reduced rates of pan evaporation, result in increased woody plant density. We suggest that this is analogous to the increased woody plant density that is observed along rainfall gradients that span arid to mesic environments. From this conceptual model, we make three predictions, namely, that (a) long-term trends in tree water-use-efficiency should reveal increased values; (b) run-off data should show an increase where woody thickening is occurring; (c) enrichedCO2experiments should reveal an enhanced plant water status. These three predictions are discussed and shown to be supported by experimental data.

Canopy CO2 enrichment permits tracing the fate of recently assimilated carbon in a mature deciduous forest

How rapidly newly assimilated carbon (C) is invested into recalcitrant structures of forests, and how closely C pools and fluxes are tied to photosynthesis, is largely unknown. A crane and a purpose-built free-air CO2 enrichment (FACE) system permitted us to label the canopy of a mature deciduous forest with 13C-depleted CO2 for 4 yr and continuously trace the flow of recent C through the forest without disturbance. Potted C4 grasses in the canopy ('isometers') served as a reference for the C-isotope input signal. After four growing seasons, leaves were completely labelled, while newly formed wood (tree rings) still contained 9% old C. Distinct labels were found in fine roots (38%) and sporocarps of mycorrhizal fungi (62%). Soil particles attached to fine roots contained 9% new C, whereas no measurable signal was detected in bulk soil. Soil-air CO2 consisted of 35% new C, indicating that considerable amounts of assimilates were rapidly returned back to the atmosphere. These data illustrate a relatively slow dilution of old mobile C pools in trees, but a pronounced allocation of very recent assimilates to C pools of short residence times.

No down-regulation of leaf photosynthesis in mature forest trees after three years of exposure to elevated CO2

The photosynthetic responses of six species of mature forest trees to long-term exposure to elevated CO2 (ca. 530 ppm) were determined at the Swiss Canopy Crane (SCC) site near Basel, Switzerland. In the third year of growth in elevated CO2, using web-FACE technology, net photosynthesis (As) in fully sunlit, upper canopy foliage was stimulated by ca. 40% compared to ambient controls. This enhancement did not differ from the instantaneous increase in As found in ambient-grown leaves that were temporarily measured at elevated CO2. A complete lack of down-regulation of photosynthesis was found in all species and in both the early and the late growing season. Neither was leaf nitrogen content significantly affected by long-term exposure to elevated CO2. Our results document a persistent enhancement in leaf level photosynthesis in response to elevated CO2 in mature forest trees over a period of three years. Circumstantial evidence suggests that the additional assimilates feed into large sinks other than stem and shoot growth.

Stomatal conductance and not stomatal density determines the long-term reduction in leaf transpiration of poplar in elevated CO2

Using a free-air CO2 enrichment (FACE) experiment, poplar trees (Populus x euramericana clone I214) were exposed to either ambient or elevated [CO2] from planting, for a 5-year period during canopy development, closure, coppice and re-growth. In each year, measurements were taken of stomatal density (SD, number mm(-2)) and stomatal index (SI, the proportion of epidermal cells forming stomata). In year 5, measurements were also taken of leaf stomatal conductance (gs, micromol m(-2) s(-1)), photosynthetic CO2 fixation (A, mmol m(-2) s(-1)), instantaneous water-use efficiency (A/E) and the ratio of intercellular to atmospheric CO2 (Ci:Ca). Elevated [CO2] caused reductions in SI in the first year, and in SD in the first 2 years, when the canopy was largely open. In following years, when the canopy had closed, elevated [CO2] had no detectable effects on stomatal numbers or index. In contrast, even after 5 years of exposure to elevated [CO2], gs was reduced, A/E was stimulated, and Ci:Ca was reduced relative to ambient [CO2]. These outcomes from the long-term realistic field conditions of this forest FACE experiment suggest that stomatal numbers (SD and SI) had no role in determining the improved instantaneous leaf-level efficiency of water use under elevated [CO2]. We propose that altered cuticular development during canopy closure may partially explain the changing response of stomata to elevated [CO2], although the mechanism for this remains obscure.