Respiration and CO2 Fluxes in Trees. In: Tcherkez G, Ghashghaie J, editors. Plant Respiration: Metabolic Fluxes and Carbon Balance. Cham: Springer International Publishing; 2017. pp. 181-207. [DOI: 10.1007/978-3-319-68703-2_9]

The quandary of sources and sinks of CO2 efflux in tree stems-new insights and future directions

Stem respiration (RS) substantially contributes to the return of photo assimilated carbon to the atmosphere and, thus, to the tree and ecosystem carbon balance. Stem CO2 efflux (ECO2) is often used as a proxy for RS. However, this metric has often been challenged because of the uncertain origin of CO2 emitted from the stem due to post-respiratory processes. In this Insight, we (i) describe processes affecting the quantification of RS, (ii) review common methodological approaches to quantify and model RS and (iii) develop a research agenda to fill the most relevant knowledge gaps that we identified. Dissolution, transport and accumulation of respired CO2 away from its production site, reassimilation of respired CO2 via stem photosynthesis and the enzyme phosphoenolpyruvate carboxylase, axial CO2 diffusion in the gas phase, shifts in the respiratory substrate and non-respiratory oxygen (O2) consumption are the most relevant processes causing divergence between RS and measured stem gas exchange (ECO2 or O2 influx, IO2). Two common methodological approaches to estimate RS, namely the CO2 mass balance approach and the O2 consumption technique, circumvent some of these processes but have yielded inconsistent results regarding the fate of respired CO2. Stem respiration modelling has recently progressed at the organ and tree levels. However, its implementation in large-scale models, commonly operated from a source-driven perspective, is unlikely to reflect adequate mechanisms. Finally, we propose hypotheses and approaches to advance the knowledge of the stem carbon balance, the role of sap pH on RS, the reassimilation of respired CO2, RS upscaling procedures, large-scale RS modelling and shifts in respiratory metabolism during environmental stress.

Nitrogen concentration and physical properties are key drivers of woody tissue respiration

BACKGROUND AND AIMS

Despite the critical role of woody tissues in determining net carbon exchange of terrestrial ecosystems, relatively little is known regarding the drivers of sapwood and bark respiration.

METHODS

Using one of the most comprehensive wood respiration datasets to date (82 species from Australian rainforest, savanna and temperate forest), we quantified relationships between tissue respiration rates (Rd) measured in vitro (i.e. 'respiration potential') and physical properties of bark and sapwood, and nitrogen concentration (Nmass) of leaves, sapwood and bark.

KEY RESULTS

Across all sites, tissue density and thickness explained similar, and in some cases more, variation in bark and sapwood Rd than did Nmass. Higher density bark and sapwood tissues had lower Rd for a given Nmass than lower density tissues. Rd-Nmass slopes were less steep in thicker compared with thinner-barked species and less steep in sapwood than in bark. Including the interactive effects of Nmass, density and thickness significantly increased the explanatory power for bark and sapwood respiration in branches. Among these models, Nmass contributed more to explanatory power in trunks than in branches, and in sapwood than in bark. Our findings were largely consistent across sites, which varied in their climate, soils and dominant vegetation type, suggesting generality in the observed trait relationships. Compared with a global compilation of leaf, stem and root data, Australian species showed generally lower Rd and Nmass, and less steep Rd-Nmass relationships.

CONCLUSIONS

To the best of our knowledge, this is the first study to report control of respiration-nitrogen relationships by physical properties of tissues, and one of few to report respiration-nitrogen relationships in bark and sapwood. Together, our findings indicate a potential path towards improving current estimates of autotrophic respiration by integrating variation across distinct plant tissues.

Mechanistic drivers of stem respiration: A modelling exercise across species and seasons

Stem respiration (R_S ) plays a crucial role in plant carbon budgets. However, its poor understanding limits our ability to model woody tissue and whole-tree respiration. A biophysical model of stem water and carbon fluxes (TReSpire) was calibrated on cedar, maple and oak trees during spring and late summer. For this, stem sap flow, water potential, diameter variation, temperature, CO₂ efflux, allometry and biochemistry were monitored. Shoot photosynthesis (P_N ) and nonstructural carbohydrates (NSC) were additionally measured to evaluate source-sink relations. The highest R_S and stem growth was found in maple and oak during spring, both being seasonally decoupled from P_N and [NSC]. Temperature largely affected maintenance respiration (R_M ) in the short term, but temperature-normalized R_M was highly variable on a seasonal timescale. Overall, most of the respired CO₂ radially diffused to the atmosphere (>87%) while the remainder was transported upward with the transpiration stream. The modelling exercise highlights the sink-driven behaviour of R_S and the significance of overall metabolic activity on nitrogen (N) allocation patterns and N-normalized respiratory costs to capture R_S variability over the long term. These insights should be considered when modelling plant respiration, whose representation is currently biased towards a better understanding of leaf metabolism.

Efflux and assimilation of xylem-transported CO2 in stems and leaves of tree species with different wood anatomy

Determining the fate of CO₂ respired in woody tissues is necessary to understand plant respiratory physiology and to evaluate CO₂ recycling mechanisms. An aqueous ¹³ C-enriched CO₂ solution was infused into the stem of 3-4 m tall trees to estimate efflux and assimilation of xylem-transported CO₂ via cavity ring-down laser spectroscopy and isotope ratio mass spectrometry, respectively. Different tree locations (lower stem, upper stem and leafy shoots) and tissues (xylem, bark and leaves) were monitored in species with tracheid, diffuse- and ring-porous wood anatomy (cedar, maple and oak, respectively). Radial xylem CO₂ diffusivity and xylem [CO₂ ] were lower in cedar relative to maple and oak trees, thereby limiting label diffusion. Part of the labeled ¹³ CO₂ was assimilated in cedar (8.7%) and oak (20.6%) trees, mostly in xylem and bark tissues of the stem, while limited solution uptake in maple trees hindered the detection of label assimilation. Little label reached foliar tissues, suggesting substantial label loss along the stem-branch transition following reductions in the radial diffusive pathway. Differences in respiration rates and radial xylem CO₂ diffusivity (lower in conifer relative to angiosperm species) might reconcile discrepancies in efflux and assimilation of xylem-transported CO₂ so far observed between taxonomic clades.

Spatiotemporal variability and origin of CO2 and CH4 tree stem fluxes in an upland forest

The exchange of multiple greenhouse gases (i.e., CO₂ and CH₄ ) between tree stems and the atmosphere represents a knowledge gap in the global carbon cycle. Stem CO₂ and CH₄ fluxes vary across time and space and are unclear, which are their individual or shared drivers. Here we measured CO₂ and CH₄ fluxes at different stem heights combining manual (biweekly; n = 678) and automated (hourly; n > 38,000) measurements in a temperate upland forest. All trees showed CO₂ and CH₄ emissions despite 20% of measurements showing net CH₄ uptake. Stem CO₂ fluxes presented clear seasonal trends from manual and automated measurements. Only automated measurements captured the high temporal variability of stem CH₄ fluxes revealing clear seasonal trends. Despite that temporal integration, the limited number of automated chambers made stand-level mean CH₄ fluxes sensitive to "hot spots," resulting in mean fluxes with high uncertainty. Manual measurements provided better integration of spatial variability, but their lack of temporal variability integration hindered the detection of temporal trends and stand-level mean fluxes. These results highlight the potential bias of previous studies of stem CH₄ fluxes solely based on manual or automated measurements. Stem height, temperature, and soil moisture only explained 7% and 11% of the stem CH₄ flux variability compared to 42% and 81% for CO₂ (manual and automated measurements, respectively). This large unexplained variability, in combination with high CH₄ concentrations in the trees' heartwood, suggests that stem CH₄ fluxes might be more influenced by gas transport and diffusivity through the wood than by drivers of respiratory CO₂ flux, which has crucial implications for developing process-based ecosystem models. We postulate that CH₄ is likely originated within tree stems because of lack of a consistent vertical pattern in CH₄ fluxes, evidence of CH₄ production in wood incubations, and low CH₄ concentration in the soil profile but high concentrations within the trees' heartwood.

Woody tissue photosynthesis reduces stem CO2 efflux by half and remains unaffected by drought stress in young Populus tremula trees

A substantial portion of locally respired CO₂ in stems can be assimilated by chloroplast-containing tissues. Woody tissue photosynthesis (P_wt ) therefore plays a major role in the stem carbon balance. To study the impact of P_wt on stem carbon cycling along a gradient of water availability, stem CO₂ efflux (E_A ), xylem CO₂ concentration ([CO₂ ]), and xylem water potential (Ψ_xylem ) were measured in 4-year-old Populus tremula L. trees exposed to drought stress and different regimes of light exclusion of woody tissues. Under well-watered conditions, local P_wt decreased E_A up to 30%. Axial CO₂ diffusion (D_ax ) induced by distant P_wt caused an additional decrease in E_A of up to 25% and limited xylem [CO₂ ] build-up. Under drought stress, absolute decreases in E_A driven by P_wt remained stable, denoting that P_wt was not affected by drought. At the end of the dry period, when transpiration was low, local P_wt and D_ax offset 20% and 10% of stem respiration on a daily basis, respectively. These results highlight (a) the importance of P_wt for an adequate interpretation of E_A measurements and (b) homeostatic P_wt along a drought stress gradient, which might play a crucial role to fuel stem metabolism when leaf carbon uptake and phloem transport are limited.

TReSpire - a biophysical TRee Stem respiration model

Mechanistic models of plant respiration remain poorly developed, especially in stems and woody tissues where measurements of CO₂ efflux do not necessarily reflect local respiratory activity. We built a process-based model of stem respiration that couples water and carbon fluxes at the organ level (TReSpire). To this end, sap flow, stem diameter variations, xylem and soil water potential, stem temperature, stem CO₂ efflux and nonstructural carbohydrates were measured in a maple tree, while xylem CO₂ concentration and additional stem and xylem diameter variations were monitored in an ancillary tree for model validation. TReSpire realistically described: (1) turgor pressure to differentiate growing from nongrowing metabolism; (2) maintenance expenditures in xylem and outer tissues based on Arrhenius kinetics and nitrogen content; and (3) radial CO₂ diffusivity and CO₂ solubility and transport in the sap solution. Collinearity issues with phloem unloading rates and sugar-starch interconversion rates suggest parallel submodelling to close the stem carbon balance. TReSpire brings a breakthrough in the modelling of stem water and carbon fluxes at a detailed (hourly) temporal resolution. TReSpire is calibrated from a sink-driven perspective, and has potential to advance our understanding on stem growth dynamics, CO₂ fluxes and underlying respiratory physiology across different species and phenological stages.

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.

Isotope ratio laser spectroscopy to disentangle xylem-transported from locally respired CO2 in stem CO2 efflux

Respired CO2 in woody tissues radially diffuses to the atmosphere or it is transported upward with the transpiration stream, making the origin of CO2 in stem CO2 efflux (EA) uncertain, which may confound stem respiration (RS) estimates. An aqueous 13C-enriched solution was infused into stems of Populus tremula L. trees, and real-time measurements of 13C-CO2 and 12C-CO2 in EA were performed via Cavity Ring Down Laser Spectroscopy (CRDS). The contribution of locally respired CO2 (LCO2) and xylem-transported CO2 (TCO2) to EA was estimated from their different isotopic composition. Mean daily values of TCO2/EA ranged from 13% to 38%, evidencing the notable role that xylem CO2 transport plays in the assessment of stem respiration. Mean daily TCO2/EA did not differ between treatments of drought stress and light exclusion of woody tissues, but they showed different TCO2/EA dynamics on a sub-daily time scale. Sub-daily CO2 diffusion patterns were explained by a light-induced axial CO2 gradient ascribed to woody tissue photosynthesis, and the resistance to radial CO2 diffusion determined by bark water content. Here, we demonstrate the outstanding potential of CRDS paired with 13C-CO2 labelling to advance in the understanding of CO2 movement at the plant-atmosphere interface and the respiratory physiology in woody tissues.