Plant Structure-Function Relationships and Woody Tissue Respiration: Upscaling to Forests from Laser-Derived Measurements

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.

Global climate and nutrient controls of photosynthetic capacity

There is huge uncertainty about how global exchanges of carbon between the atmosphere and land will respond to continuing environmental change. A better representation of photosynthetic capacity is required for Earth System models to simulate carbon assimilation reliably. Here we use a global leaf-trait dataset to test whether photosynthetic capacity is quantitatively predictable from climate, based on optimality principles; and to explore how this prediction is modified by soil properties, including indices of nitrogen and phosphorus availability, measured in situ. The maximum rate of carboxylation standardized to 25 °C (Vcmax25) was found to be proportional to growing-season irradiance, and to increase-as predicted-towards both colder and drier climates. Individual species' departures from predicted Vcmax25 covaried with area-based leaf nitrogen (Narea) but community-mean Vcmax25 was unrelated to Narea, which in turn was unrelated to the soil C:N ratio. In contrast, leaves with low area-based phosphorus (Parea) had low Vcmax25 (both between and within communities), and Parea increased with total soil P. These findings do not support the assumption, adopted in some ecosystem and Earth System models, that leaf-level photosynthetic capacity depends on soil N supply. They do, however, support a previously-noted relationship between photosynthesis and soil P supply.

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.

Drought stress and tree size determine stem CO2 efflux in a tropical forest

CO₂ efflux from stems (CO_{2_stem} ) accounts for a substantial fraction of tropical forest gross primary productivity, but the climate sensitivity of this flux remains poorly understood. We present a study of tropical forest CO_{2_stem} from 215 trees across wet and dry seasons, at the world's longest running tropical forest drought experiment site. We show a 27% increase in wet season CO_{2_stem} in the droughted forest relative to a control forest. This was driven by increasing CO_{2_stem} in trees 10-40 cm diameter. Furthermore, we show that drought increases the proportion of maintenance to growth respiration in trees > 20 cm diameter, including large increases in maintenance respiration in the largest droughted trees, > 40 cm diameter. However, we found no clear taxonomic influence on CO_{2_stem} and were unable to accurately predict how drought sensitivity altered ecosystem scale CO_{2_stem} , due to substantial uncertainty introduced by contrasting methods previously employed to scale CO_{2_stem} fluxes. Our findings indicate that under future scenarios of elevated drought, increases in CO_{2_stem} may augment carbon losses, weakening or potentially reversing the tropical forest carbon sink. However, due to substantial uncertainties in scaling CO_{2_stem} fluxes, stand-scale future estimates of changes in stem CO₂ emissions remain highly uncertain.