Short-term carbon cycling responses of a mature eucalypt woodland to gradual stepwise enrichment of atmospheric CO2 concentration

Microbial competition for phosphorus limits the CO2 response of a mature forest

The capacity for terrestrial ecosystems to sequester additional carbon (C) with rising CO2 concentrations depends on soil nutrient availability1,2. Previous evidence suggested that mature forests growing on phosphorus (P)-deprived soils had limited capacity to sequester extra biomass under elevated CO2 (refs. 3-6), but uncertainty about ecosystem P cycling and its CO2 response represents a crucial bottleneck for mechanistic prediction of the land C sink under climate change7. Here, by compiling the first comprehensive P budget for a P-limited mature forest exposed to elevated CO2, we show a high likelihood that P captured by soil microorganisms constrains ecosystem P recycling and availability for plant uptake. Trees used P efficiently, but microbial pre-emption of mineralized soil P seemed to limit the capacity of trees for increased P uptake and assimilation under elevated CO2 and, therefore, their capacity to sequester extra C. Plant strategies to stimulate microbial P cycling and plant P uptake, such as increasing rhizosphere C release to soil, will probably be necessary for P-limited forests to increase C capture into new biomass. Our results identify the key mechanisms by which P availability limits CO2 fertilization of tree growth and will guide the development of Earth system models to predict future long-term C storage.

Do rice growth and yield respond similarly to abrupt and gradual increase in atmospheric CO2?

Crops have been well studied at abruptly elevated CO2 (e[CO2]). In fact, atmospheric CO2 concentration is rising gradually, but its ecological effect is little known. Thus, rice growth and yield were investigated under gradual e[CO2] (GE) and abrupt e[CO2] (AE) using open-top chambers. Gradual e[CO2] involved an ambient CO2 (a[CO2]) + 40 μmol mol-1 per year in 2016 until a[CO2] + 200 μmol mol-1 in 2020, while AE maintained a[CO2] + 200 μmol mol-1 from 2016 to 2020. We found that steady-state photosynthetic rates responded similarly and increased significantly under GE and AE, however, photosynthetic induction time in dynamic photosynthesis was reduced by AE. Gradual e[CO2] had little effect on biomass before the grain filling stage, while AE significantly stimulated biomass because of the stronger tillering ability and faster photosynthetic induction rate. Neither e[CO2] increased biomass at maturity, however, a significant increase in panicle density was observed under AE. Surprisingly, rice yield was not promoted by both e[CO2], possibly resulting from the reduced carbon assimilation caused by accelerated phenology from grain filling to maturity. These results promote a new understanding of the CO2 fertilization effect with small and slow increases in CO2 concentration, closer to what happens in nature. This may partly challenge the classic view of elevated CO2 fertilization effects from AE.

Soil Respiration in Planted and Naturally Regenerated Castanopis carelesii Forests during Three Years Post-Establishment

Reforestation through assisted natural regeneration usually accumulates more biomass carbon than through tree planting, but its effects on soil respiration (Rs) and its components, autotrophic respiration (Ra) and heterotrophic respiration (Rh), are poorly understood despite the importance in forest carbon cycling. In this study, we clear-cut part of a 35-year-old secondary Castanopsis carelesii (C. carelesii) forest and reforested the logged land with C. carelesii via two approaches—active tree planting and assisted natural regeneration—and measured Rs, Ra, and Rh as well as soil temperature and moisture in these forests. In the first two years following reforestation, Rs, Ra and Rh rates were mostly reduced in the two young forests compared to the secondary forest, likely due to reduced photosynthate production and thus carbon substrate input associated with the clear-cut. However, the Rh:Rs ratio was significantly greater in the young plantation than in the other two forests in the first two years, suggesting a greater loss of soil organic carbon from the young plantation. In the third year, the mean Rs, Rh, and Ra rates of the young forest established via assisted natural regeneration were similar to those of the secondary forest, but significantly greater than those of the young plantation. The rates of Rs, Rh, and Ra mostly increased exponentially with increasing soil temperature in all forests, but mostly lack significant relationships with soil moisture. These findings indicate that, compared with reforestation via tree plantation, assisted natural regeneration not only reduced the loss of soil organic carbon via soil respiration, but also had a more rapid recovery of soil respiration to the level of the secondary forest. Our study highlights that, in addition to temperature, carbon substrate availability is also important in regulating soil respiration following reforestation.

Elevated CO2 Impacts on Plant-Pollinator Interactions: A Systematic Review and Free Air Carbon Enrichment Field Study

Simple Summary

Climate change is having a profound impact on pollination systems, yet we still do not know to what extent increasing concentrations of carbon dioxide (CO₂) will directly affect the interactions between plants and their pollinators. We review all the existing published literature on the effect of elevated CO₂ (eCO₂) on flowering time, nectar and pollen production and plant–pollinator interactions. We also conduct a field experiment to test the effect of eCO₂ on bluebells and their pollinators. We found that few studies have assessed the impact of eCO₂ on pollination, and our field data found that bluebells flowered on average 6 days earlier under eCO₂ conditions. Hoverflies and bumble bees were the main visitors to bluebell flowers, but insect activity was low early in the flowing period. Although we did not find a difference in the number of visits made by insects to bluebell flowers under eCO₂, or the amount of seeds those flowers produced, the change in the timing of flowering could mean that a mismatch could develop between bluebells and their pollinators in the future, which would affect pollination success.

Abstract

The impact of elevated CO₂ (eCO₂) on plant–pollinator interactions is poorly understood. This study provides the first systematic review of this topic and identifies important knowledge gaps. In addition, we present field data assessing the impact of eCO₂ (150 ppm above ambient) on bluebell (Hyacinthoides non-scripta)–pollinator interactions within a mature, deciduous woodland system. Since 1956, only 71 primary papers have investigated eCO₂ effects on flowering time, floral traits and pollination, with a mere 3 studies measuring the impact on pollination interactions. Our field experiment documented flowering phenology, flower visitation and seed production, as well as the abundance and phenology of dominant insect pollinators. We show that first and mid-point flowering occurred 6 days earlier under eCO₂, but with no change in flowering duration. Syrphid flies and bumble bees were the dominant flower visitors, with peak activity recorded during mid- and late-flowering periods. Whilst no significant difference was recorded in total visitation or seed set between eCO₂ and ambient treatments, there were clear patterns of earlier flowering under eCO₂ accompanied by lower pollinator activity during this period. This has implications for potential loss of synchrony in pollination systems under future climate scenarios, with associated long-term impacts on abundance and diversity.

Integrating the evidence for a terrestrial carbon sink caused by increasing atmospheric CO2

Atmospheric carbon dioxide concentration ([CO₂ ]) is increasing, which increases leaf-scale photosynthesis and intrinsic water-use efficiency. These direct responses have the potential to increase plant growth, vegetation biomass, and soil organic matter; transferring carbon from the atmosphere into terrestrial ecosystems (a carbon sink). A substantial global terrestrial carbon sink would slow the rate of [CO₂ ] increase and thus climate change. However, ecosystem CO₂ responses are complex or confounded by concurrent changes in multiple agents of global change and evidence for a [CO₂ ]-driven terrestrial carbon sink can appear contradictory. Here we synthesize theory and broad, multidisciplinary evidence for the effects of increasing [CO₂ ] (iCO₂ ) on the global terrestrial carbon sink. Evidence suggests a substantial increase in global photosynthesis since pre-industrial times. Established theory, supported by experiments, indicates that iCO₂ is likely responsible for about half of the increase. Global carbon budgeting, atmospheric data, and forest inventories indicate a historical carbon sink, and these apparent iCO₂ responses are high in comparison to experiments and predictions from theory. Plant mortality and soil carbon iCO₂ responses are highly uncertain. In conclusion, a range of evidence supports a positive terrestrial carbon sink in response to iCO₂ , albeit with uncertain magnitude and strong suggestion of a role for additional agents of global change.

Characteristics of free air carbon dioxide enrichment of a northern temperate mature forest

In 2017, the Birmingham Institute of Forest Research (BIFoR) began to conduct Free Air Carbon Dioxide Enrichment (FACE) within a mature broadleaf deciduous forest situated in the United Kingdom. BIFoR FACE employs large-scale infrastructure, in the form of lattice towers, forming 'arrays' which encircle a forest plot of ~30 m diameter. BIFoR FACE consists of three treatment arrays to elevate local CO₂ concentrations (e[CO₂ ]) by +150 µmol/mol. In practice, acceptable operational enrichment (ambient [CO₂ ] + e[CO₂ ]) is ±20% of the set point 1-min average target. There are a further three arrays that replicate the infrastructure and deliver ambient air as paired controls for the treatment arrays. For the first growing season with e[CO₂ ] (April to November 2017), [CO₂ ] measurements in treatment and control arrays show that the target concentration was successfully delivered, that is: +147 ± 21 µmol/mol (mean ± SD) or 98 ± 14% of set point enrichment target. e[CO₂ ] treatment was accomplished for 97.7% of the scheduled operation time, with the remaining time lost due to engineering faults (0.6% of the time), CO₂ supply issues (0.6%) or adverse weather conditions (1.1%). CO₂ demand in the facility was driven predominantly by wind speed and the formation of the deciduous canopy. Deviations greater than 10% from the ambient baseline CO₂ occurred <1% of the time in control arrays. Incidences of cross-contamination >80 µmol/mol (i.e. >53% of the treatment increment) into control arrays accounted for <0.1% of the enrichment period. The median [CO₂ ] values in reconstructed three-dimensional [CO₂ ] fields show enrichment somewhat lower than the target but still well above ambient. The data presented here provide confidence in the facility setup and can be used to guide future next-generation forest FACE facilities built into tall and complex forest stands.

Elevated CO2 does not affect stem CO2 efflux nor stem respiration in a dry Eucalyptus woodland, but it shifts the vertical gradient in xylem [CO2 ]

To quantify stem respiration (R_S ) under elevated CO₂ (eCO₂ ), stem CO₂ efflux (E_A ) and CO₂ flux through the xylem (F_T ) should be accounted for, because part of respired CO₂ is transported upwards with the sap solution. However, previous studies have used E_A as a proxy of R_S , which could lead to equivocal conclusions. Here, to test the effect of eCO₂ on R_S , both E_A and F_T were measured in a free-air CO₂ enrichment experiment located in a mature Eucalyptus native forest. Drought stress substantially reduced E_A and R_S , which were unaffected by eCO₂ , likely as a consequence of its neutral effect on stem growth in this phosphorus-limited site. However, xylem CO₂ concentration measured near the stem base was higher under eCO₂ , and decreased along the stem resulting in a negative contribution of F_T to R_S , whereas the contribution of F_T to R_S under ambient CO₂ was positive. Negative F_T indicates net efflux of CO₂ respired below the monitored stem segment, likely coming from the roots. Our results highlight the role of nutrient availability on the dependency of R_S on eCO₂ and suggest stimulated root respiration under eCO₂ that may shift vertical gradients in xylem [CO₂ ] confounding the interpretation of E_A measurements.

Nitrogen and Phosphorus Retranslocation of Leaves and Stemwood in a Mature Eucalyptus Forest Exposed to 5 Years of Elevated CO2

Elevated CO2 affects C cycling processes which in turn can influence the nitrogen (N) and phosphorus (P) concentrations of plant tissues. Given differences in how N and P are used by plants, we asked if their stoichiometry in leaves and wood was maintained or altered in a long-term elevated CO2 experiment in a mature Eucalyptus forest on a low P soil (EucFACE). We measured N and P concentrations in green leaves at different ages at the top of mature trees across 6 years including 5 years in elevated CO2. N and P concentrations in green and senesced leaves and wood were determined to evaluate both spatial and temporal variation of leaf N and P concentrations, including the N and P retranslocation in leaves and wood. Leaf P concentrations were 32% lower in old mature leaves compared to newly flushed leaves with no effect of elevated CO2 on leaf P. By contrast, elevated CO2 significantly decreased leaf N concentrations in newly flushed leaves but this effect disappeared as leaves matured. As such, newly flushed leaves had 9% lower N:P ratios in elevated CO2 and N:P ratios were not different in mature green leaves (CO2 by Age effect, P = 0.02). Over time, leaf N and P concentrations in the upper canopy slightly declined in both CO2 treatments compared to before the start of the experiment. P retranslocation in leaves was 50%, almost double that of N retranslocation (29%), indicating that this site was P-limited and that P retranslocation was an important mechanism in this ecosystem to retain P in plants. As P-limited trees tend to store relatively more N than P, we found an increased N:P ratio in sapwood in response to elevated CO2 (P < 0.01), implying N accumulation in live wood. The flexible stoichiometric ratios we observed can have important implications for how plants adjust to variable environmental conditions including climate change. Hence, variable nutrient stoichiometry should be accounted for in large-scale Earth Systems models invoking biogeochemical processes.

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.

Understorey productivity in temperate grassy woodland responds to soil water availability but not to elevated [CO2 ]

Rising atmospheric [CO₂ ] and associated climate change are expected to modify primary productivity across a range of ecosystems globally. Increasing aridity is predicted to reduce grassland productivity, although rising [CO₂ ] and associated increases in plant water use efficiency may partially offset the effect of drying on growth. Difficulties arise in predicting the direction and magnitude of future changes in ecosystem productivity, due to limited field experimentation investigating climate and CO₂ interactions. We use repeat near-surface digital photography to quantify the effects of water availability and experimentally manipulated elevated [CO₂ ] (eCO₂ ) on understorey live foliage cover and biomass over three growing seasons in a temperate grassy woodland in south-eastern Australia. We hypothesised that (i) understorey herbaceous productivity is dependent upon soil water availability, and (ii) that eCO₂ will increase productivity, with greatest stimulation occurring under conditions of low water availability. Soil volumetric water content (VWC) determined foliage cover and growth rates over the length of the growing season (August to March), with low VWC (<0.1 m³  m^-3 ) reducing productivity. However, eCO₂ did not increase herbaceous cover and biomass over the duration of the experiment, or mitigate the effects of low water availability on understorey growth rates and cover. Our findings suggest that projected increases in aridity in temperate woodlands are likely to lead to reduced understorey productivity, with little scope for eCO₂ to offset these changes.

Effects of competition and herbivory over woody seedling growth in a temperate woodland trump the effects of elevated CO2

A trend of increasing woody plant density, or woody thickening, has been observed across grassland and woodland ecosystems globally. It has been proposed that increasing atmospheric [CO₂] is a major driver of broad scale woody thickening, though few field-based experiments have tested this hypothesis. Our study utilises a Free Air CO₂ Enrichment experiment to examine the effect of elevated [CO₂] (eCO₂) on three mechanisms that can cause woody thickening, namely (i) woody plant recruitment, (ii) seedling growth, and (iii) post-disturbance resprouting. The study took place in a eucalypt-dominated temperate grassy woodland. Annual assessments show that juvenile woody plant recruitment occurred over the first 3 years of CO₂ fumigation, though eCO₂ did not affect rates of recruitment. Manipulative experiments were established to examine the effect of eCO₂ on above-ground seedling growth using transplanted Eucalyptus tereticornis (Myrtaceae) and Hakea sericea (Proteaceae) seedlings. There was no positive effect of eCO₂ on biomass of either species following 12 months of exposure to treatments. Lignotubers (i.e., resprouting organs) of harvested E. tereticornis seedlings that were retained in situ for an additional year were used to examine resprouting response. The likelihood of resprouting and biomass of resprouts increased with lignotuber volume, which was not itself affected by eCO₂. The presence of herbaceous competitors and defoliation by invertebrates and pathogens were found to greatly reduce growth and/or resprouting response of seedlings. Our findings do not support the hypothesis that future increases in atmospheric [CO₂] will, by itself, promote woody plant recruitment in eucalypt-dominated temperate grassy woodlands.

Acclimation of light and dark respiration to experimental and seasonal warming are mediated by changes in leaf nitrogen in Eucalyptus globulus

Quantifying the adjustments of leaf respiration in response to seasonal temperature variation and climate warming is crucial because carbon loss from vegetation is a large but uncertain part of the global carbon cycle. We grew fast-growing Eucalyptus globulus Labill. trees exposed to +3 °C warming and elevated CO2 in 10-m tall whole-tree chambers and measured the temperature responses of leaf mitochondrial respiration, both in light (RLight) and in darkness (RDark), over a 20-40 °C temperature range and during two different seasons. RLight was assessed using the Laisk method. Respiration rates measured at a standard temperature (25 °C - R25) were higher in warm-grown trees and in the warm season, related to higher total leaf nitrogen (N) investment with higher temperatures (both experimental and seasonal), indicating that leaf N concentrations modulated the respiratory capacity to changes in temperature. Once differences in leaf N were accounted for, there were no differences in R25 but the Q10 (i.e., short-term temperature sensitivity) was higher in late summer compared with early spring. The variation in RLight between experimental treatments and seasons was positively correlated with carboxylation capacity and photorespiration. RLight was less responsive to short-term changes in temperature than RDark, as shown by a lower Q10 in RLight compared with RDark. The overall light inhibition of R was ∼40%. Our results highlight the dynamic nature of leaf respiration to temperature variation and that the responses of RLight do not simply mirror those of RDark. Therefore, it is important not to assume that RLight is the same as RDark in ecosystem models, as doing so may lead to large errors in predicting plant CO2 release and productivity.

Insect herbivory in a mature Eucalyptus woodland canopy depends on leaf phenology but not CO2 enrichment

Background

Climate change factors such as elevated atmospheric carbon dioxide concentrations (e[CO₂]) and altered rainfall patterns can alter leaf composition and phenology. This may subsequently impact insect herbivory. In sclerophyllous forests insects have developed strategies, such as preferentially feeding on new leaf growth, to overcome physical or foliar nitrogen constraints, and this may shift under climate change. Few studies of insect herbivory at elevated [CO₂] have occurred under field conditions and none on mature evergreen trees in a naturally established forest, yet estimates for leaf area loss due to herbivory are required in order to allow accurate predictions of plant productivity in future climates. Here, we assessed herbivory in the upper canopy of mature Eucalyptus tereticornis trees at the nutrient-limited Eucalyptus free-air CO₂ enrichment (EucFACE) experiment during the first 19 months of CO₂ enrichment. The assessment of herbivory extended over two consecutive spring—summer periods, with a first survey during four months of the [CO₂] ramp-up phase after which full [CO₂] operation was maintained, followed by a second survey period from months 13 to 19.

Results

Throughout the first 2 years of EucFACE, young, expanding leaves sustained significantly greater damage from insect herbivory (between 25 and 32 % leaf area loss) compared to old or fully expanded leaves (less than 2 % leaf area loss). This preference of insect herbivores for young expanding leaves combined with discontinuous production of new foliage, which occurred in response to rainfall, resulted in monthly variations in leaf herbivory. In contrast to the significant effects of rainfall-driven leaf phenology, elevated [CO₂] had no effect on leaf consumption or preference of insect herbivores for different leaf age classes.

Conclusions

In the studied nutrient-limited natural Eucalyptus woodland, herbivory contributes to a significant loss of young foliage. Leaf phenology is a significant factor that determines the level of herbivory experienced in this evergreen sclerophyllous woodland system, and may therefore also influence the population dynamics of insect herbivores. Furthermore, leaf phenology appears more strongly impacted by rainfall patterns than by e[CO₂]. e[CO₂] responses of herbivores on mature trees may only become apparent after extensive CO₂ fumigation periods.

Electronic supplementary material

The online version of this article (doi:10.1186/s12898-016-0102-z) contains supplementary material, which is available to authorized users.

Using models to guide field experiments: a priori predictions for the CO2 response of a nutrient- and water-limited native Eucalypt woodland

The response of terrestrial ecosystems to rising atmospheric CO2 concentration (Ca ), particularly under nutrient-limited conditions, is a major uncertainty in Earth System models. The Eucalyptus Free-Air CO2 Enrichment (EucFACE) experiment, recently established in a nutrient- and water-limited woodland presents a unique opportunity to address this uncertainty, but can best do so if key model uncertainties have been identified in advance. We applied seven vegetation models, which have previously been comprehensively assessed against earlier forest FACE experiments, to simulate a priori possible outcomes from EucFACE. Our goals were to provide quantitative projections against which to evaluate data as they are collected, and to identify key measurements that should be made in the experiment to allow discrimination among alternative model assumptions in a postexperiment model intercomparison. Simulated responses of annual net primary productivity (NPP) to elevated Ca ranged from 0.5 to 25% across models. The simulated reduction of NPP during a low-rainfall year also varied widely, from 24 to 70%. Key processes where assumptions caused disagreement among models included nutrient limitations to growth; feedbacks to nutrient uptake; autotrophic respiration; and the impact of low soil moisture availability on plant processes. Knowledge of the causes of variation among models is now guiding data collection in the experiment, with the expectation that the experimental data can optimally inform future model improvements.