Stem respiration (RS) 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 (PN) and nonstructural carbohydrates (NSC) were additionally measured to evaluate source−sink relations. The highest RS and stem growth was found in maple and oak during spring, both being seasonally decoupled from PN and NSC. Temperature largely affected maintenance respiration (RM) in the short term, but temperature‐normalized RM 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 RS and the significance of overall metabolic activity on nitrogen (N) allocation patterns and N‐normalized respiratory costs to capture RS 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.
• 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.
Daytime decreases in temperature-normalised stem CO2 efflux (E
A_D) are commonly ascribed to internal transport of respired CO2 (F
T) or to an attenuated respiratory activity due to lowered turgor ...pressure. The two are difficult to separate as they are simultaneously driven by sap flow dynamics.
To achieve combined gradients in turgor pressure and F
T, sap flow rates in poplar trees were manipulated through severe defoliation, severe drought, moderate defoliation and moderate drought. Turgor pressure was mechanistically modelled using measurements of sap flow, stem diameter variation, and soil and stem water potential. A mass balance approach considering internal and external CO2 fluxes was applied to estimate F
T.
Under well-watered control conditions, both turgor pressure and sap flow, as a proxy of F
T, were reliable predictors of E
A_D. After tree manipulation, only turgor pressure was a robust predictor of E
A_D. Moreover, F
T accounted for < 15% of E
A_D.
Our results suggest that daytime reductions in turgor pressure and associated constrained growth are the main cause of E
A_D in young poplar trees. Turgor pressure is determined by both carbohydrate supply and water availability, and should be considered to improve our widely used but inaccurate temperature-based predictions of woody tissue respiration in global models.
Tree stem respiration (RS) is a substantial component of the forest carbon balance. The mass balance approach uses stem CO2 efflux and internal xylem fluxes to sum up RS, while the oxygen‐based ...method assumes O2 influx as a proxy of RS. So far, both approaches have yielded inconsistent results regarding the fate of respired CO2 in tree stems, a major challenge for quantifying forest carbon dynamics. We collected a data set of CO2 efflux, O2 influx, xylem CO2 concentration, sap flow, sap pH, stem temperature, nonstructural carbohydrates concentration and potential phosphoenolpyruvate carboxylase (PEPC) capacity on mature beech trees to identify the sources of differences between approaches. The ratio of CO2 efflux to O2 influx was consistently below unity (0.7) along a 3‐m vertical gradient, but internal fluxes did not bridge the gap between influx and efflux, nor did we find evidence for changes in respiratory substrate use. PEPC capacity was comparable with that previously reported in green current‐year twigs. Although we could not reconcile differences between approaches, results shed light on the uncertain fate of CO2 respired by parenchyma cells across the sapwood. Unexpected high values of PEPC capacity highlight its potential relevance as a mechanism of local CO2 removal, which merits further research.
Summary Statement
Our field experiment in mature beech trees, measuring CO2 and O2 fluxes simultaneously, showed that 30% of the respired CO2 is retained in the stem. However, CO2 internal fluxes could not explain the difference between CO2 efflux and O2 influx. The internal carbon recycling mechanism mediated by PEPC is active in mature trees and can be considered as a missing C sink.
Hydraulic modelling is a primary tool to predict plant performance in future drier scenarios. However, as most tree models are validated under non‐stress conditions, they may fail when water becomes ...limiting. To simulate tree hydraulic functioning under moist and dry conditions, the current version of a water flow and storage mechanistic model was further developed by implementing equations that describe variation in xylem hydraulic resistance (RX) and stem hydraulic capacitance (CS) with predawn water potential (ΨPD). The model was applied in a Mediterranean forest experiencing intense summer drought, where six Quercus ilex trees were instrumented to monitor stem diameter variations and sap flow, concurrently with measurements of predawn and midday leaf water potential. Best model performance was observed when CS was allowed to decrease with decreasing ΨPD. Hydraulic capacitance decreased from 62 to 25 kg m−3 MPa−1 across the growing season. In parallel, tree transpiration decreased to a greater extent than the capacitive water release and the contribution of stored water to transpiration increased from 2.0 to 5.1%. Our results demonstrate the importance of stored water and seasonality in CS for tree hydraulic functioning, and they suggest that CS should be considered to predict the drought response of trees with models.
In this work, we investigate the importance of implementing both variable hydraulic conductance and capacitance in mechanistic models to describe tree hydraulic functioning in drier scenarios. Best model performance was observed when the hydraulic capacitance of the stem was allowed to decrease with decreasing predawn water potential suggesting that seasonality in stored water and hydraulic capacitance should be considered to predict the drought response of trees with mechanistic models.
Atmospheric carbon dioxide enrichment (eCO
) can enhance plant carbon uptake and growth
, thereby providing an important negative feedback to climate change by slowing the rate of increase of the ...atmospheric CO
concentration
. Although evidence gathered from young aggrading forests has generally indicated a strong CO
fertilization effect on biomass growth
, it is unclear whether mature forests respond to eCO
in a similar way. In mature trees and forest stands
, photosynthetic uptake has been found to increase under eCO
without any apparent accompanying growth response, leaving the fate of additional carbon fixed under eCO
unclear
. Here using data from the first ecosystem-scale Free-Air CO
Enrichment (FACE) experiment in a mature forest, we constructed a comprehensive ecosystem carbon budget to track the fate of carbon as the forest responded to four years of eCO
exposure. We show that, although the eCO
treatment of +150 parts per million (+38 per cent) above ambient levels induced a 12 per cent (+247 grams of carbon per square metre per year) increase in carbon uptake through gross primary production, this additional carbon uptake did not lead to increased carbon sequestration at the ecosystem level. Instead, the majority of the extra carbon was emitted back into the atmosphere via several respiratory fluxes, with increased soil respiration alone accounting for half of the total uptake surplus. Our results call into question the predominant thinking that the capacity of forests to act as carbon sinks will be generally enhanced under eCO
, and challenge the efficacy of climate mitigation strategies that rely on ubiquitous CO
fertilization as a driver of increased carbon sinks in global forests.
is famous for its high-quality timber production all the way and is much more famous for its high value health-care nut oil production potential since 1990's, but the less understanding of its ...reproduction biology seriously hindered its nut productivity increase. Exploring the effects of reproduction on nutrient uptake, allocation and storage help to understand and modify reproduction patterns in masting species and high nut yield cultivar selection and breeding. Here, we compared seasonality in growth and in nitrogen (N) and phosphorus (P) concentrations in needles, branches and cones of reproductive (cone-bearing) and vegetative branches (having no cones) of
during a masting year. The growth of one- and two-year-old reproductive branches was significantly higher than that of vegetative branches. Needle, phloem and xylem N and P were lower in reproductive branches than in vegetative branches, although the extent and significance of the differences between branch types varied across dates. N and P in most tissues were high in spring, decreased during summer, and then recovered by the end of the growing season. Overall, N and P were highest in needles, lowest in the xylem and intermediate in the phloem. More than half of the N (73.5%) and P (51.6%) content in reproductive branches were allocated to cones. There was a positive correlation between cone number and N and P content in needles (R
= 0.64, R
= 0.73) and twigs (R
= 0.65, R
= 0.62) of two-year-old reproductive branches. High nutrient sink strength of cones and vegetative tissues of reproductive branches suggested that customized fertilization practices can help improve crop yield in
.
Abstract
Given the importance of carbon allocation for plant performance and fitness, it is expected that competition and abiotic stress influence respiratory costs associated with stem wood biomass ...production and maintenance. In this study, stem respiration (R) was measured together with stem diameter increment in adult trees of eight co-occurring species in a sub-Mediterranean forest stand for 2 years. We estimated growth R (Rg), maintenance R (Rm) and the growth respiration coefficient (GRC) using two gas exchange methods: (i) estimating Rg as the product of growth and GRC (then Rm as R minus Rg) and (ii) estimating Rm from temperature-dependent kinetics of basal Rm at the dormant season (then Rg as R minus Rm). In both cases, stem basal-area growth rates governed intra-annual variation in R, Rg and Rm. Maximum annual Rm occurred slightly before or after maximum Rg. The mean contribution of Rm to R during the growing season ranged from 56% to 88% across species using method 1 and from 23% to 66% using method 2. An analysis accounting for the phylogenetic distance among species indicated that more shade-tolerant, faster growing species exhibited higher Rm and Rg than less shade-tolerant, slower growing ones, suggesting a balance between carbon supply and demand mediated by growth. However, GRC was not related to species growth rate, wood density, or drought and shade tolerance across the surveyed species nor across 27 tree species for which GRC was compiled. The GRC estimates based on wood chemical analysis were lower (0.19) than those based on gas exchange methods (0.35). These results give partial support to the hypothesis that wood production and maintenance costs are related to species ecology and highlight the divergence of respiratory parameters widely used in plant models according to the methodological approach applied to derive them.
Spruce budworm (Choristoneura fumiferana Clem.) outbreaks cause extensive mortality and growth reductions throughout boreal forests in eastern North America. As tree vulnerability to defoliation ...remains partially unexplained by tree and stand attributes, we hypothesized that root grafting might attenuate the negative impact of severe defoliation in tree growth. Two experimental sites in the Abitibi-Témiscamingue region dominated by black spruce (Picea mariana Mill.) were harvested and hydraulically excavated to study tree growth in 36 trees in relation to root grafting and the last spruce budworm outbreak using dendroecological methods. Root grafts reduced the negative effects of defoliation by maintaining stable growth in connected trees during epidemic periods. Among dominant trees, growth releases immediately after the outbreak were uniquely observed in grafted trees. Among suppressed trees, grafted trees tended to grow more than non-grafted trees when defoliation severity was the highest. Carbohydrate transfers through root grafts and enhanced efficiency to acquire resources may explain the better performance of grafted trees under scenarios of limited carbon supply. This study reinforces the growing body of literature that suggests root grafting as a cooperative strategy to withstand severe disturbances and highlights the key role of root grafting in stand dynamics to cope with periodic outbreaks.
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.
PDF
