We revisit the relationship between plant water use efficiency and carbon isotope signatures (δ¹³C) of plant material. Based on the definitions of intrinsic, instantaneous and integrated water use ...efficiency, we discuss the implications for interpreting δ¹³C data from leaf to landscape levels, and across diurnal to decadal timescales. Previous studies have often applied a simplified, linear relationship between δ¹³C, ratios of intercellular to ambient CO₂ mole fraction (C i/C a), and water use efficiency. In contrast, photosynthetic ¹³C discrimination (Δ) is sensitive to the ratio of the chloroplast to ambient CO₂ mole fraction, C c/C a (rather than C i/C a) and, consequently, to mesophyll conductance. Because mesophyll conductance may differ between species and over time, it is not possible to determine C c/C a from the same gas exchange measurements as C i/C a. On the other hand, water use efficiency at the leaf level depends on evaporative demand, which does not directly affect Δ. Water use efficiency and Δ can thus vary independently, making it difficult to obtain trends in water use efficiency from δ¹³C data. As an alternative approach, we offer a model available at http://carbonisotopes.googlepages.com to explore how water use efficiency and ¹³C discrimination are related across leaf and canopy scales. The model provides a tool to investigate whether trends in Δ indicate changes in leaf functional traits and/or environmental conditions during leaf growth, and how they are associated with trends in plant water use efficiency. The model can be used, for example, to examine whether trends in δ¹³C signatures obtained from tree rings imply changes in tree water use efficiency in response to atmospheric CO₂ increase. This is crucial for predicting how plants may respond to future climate change.
Net photosynthesis is the largest single flux in the global carbon cycle, but controls over its variability are poorly understood because there is no direct way of measuring it at the ecosystem ...scale. We report observations of ecosystem carbonyl sulfide (COS) and CO ₂ fluxes that resolve key gaps in an emerging framework for using concurrent COS and CO ₂ measurements to quantify terrestrial gross primary productivity. At a wheat field in Oklahoma we found that in the peak growing season the flux-weighted leaf relative uptake of COS and CO ₂ during photosynthesis was 1.3, at the lower end of values from laboratory studies, and varied systematically with light. Due to nocturnal stomatal conductance, COS uptake by vegetation continued at night, contributing a large fraction (29%) of daily net ecosystem COS fluxes. In comparison, the contribution of soil fluxes was small (1–6%) during the peak growing season. Upland soils are usually considered sinks of COS. In contrast, the well-aerated soil at the site switched from COS uptake to emissions at a soil temperature of around 15 °C. We observed COS production from the roots of wheat and other species and COS uptake by root-free soil up to a soil temperature of around 25 °C. Our dataset demonstrates that vegetation uptake is the dominant ecosystem COS flux in the peak growing season, providing support of COS as an independent tracer of terrestrial photosynthesis. However, the observation that ecosystems may become a COS source at high temperature needs to be considered in global modeling studies.
For the past decade, observations of carbonyl sulfide (OCS or COS) have been investigated as a proxy for carbon uptake by plants. OCS is destroyed by enzymes that interact with CO2 during ...photosynthesis, namely carbonic anhydrase (CA) and RuBisCO, where CA is the more important one. The majority of sources of OCS to the atmosphere are geographically separated from this large plant sink, whereas the sources and sinks of CO2 are co-located in ecosystems. The drawdown of OCS can therefore be related to the uptake of CO2 without the added complication of co-located emissions comparable in magnitude. Here we review the state of our understanding of the global OCS cycle and its applications to ecosystem carbon cycle science. OCS uptake is correlated well to plant carbon uptake, especially at the regional scale. OCS can be used in conjunction with other independent measures of ecosystem function, like solar-induced fluorescence and carbon and water isotope studies. More work needs to be done to generate global coverage for OCS observations and to link this powerful atmospheric tracer to systems where fundamental questions concerning the carbon and water cycle remain.
Abstract
Remote sensing is a powerful tool for understanding and scaling measurements of plant carbon uptake via photosynthesis, gross primary productivity (GPP), across space and time. The success ...of remote sensing measurements can be attributed to their ability to capture valuable information on plant structure (physical) and function (physiological), both of which impact GPP. However, no single remote sensing measure provides a universal constraint on GPP and the relationships between remote sensing measurements and GPP are often site specific, thereby limiting broader usefulness and neglecting important nuances in these signals. Improvements must be made in how we connect remotely sensed measurements to GPP, particularly in boreal ecosystems which have been traditionally challenging to study with remote sensing. In this paper we improve GPP prediction by using random forest models as a quantitative framework that incorporates physical and physiological information provided by solar-induced fluorescence (SIF) and vegetation indices (VIs). We analyze 2.5 years of tower-based remote sensing data (SIF and VIs) across two field locations at the northern and southern ends of the North American boreal forest. We find (a) remotely sensed products contain information relevant for understanding GPP dynamics, (b) random forest models capture quantitative SIF, GPP, and light availability relationships, and (c) combining SIF and VIs in a random forest model outperforms traditional parameterizations of GPP based on SIF alone. Our new method for predicting GPP based on SIF and VIs improves our ability to quantify terrestrial carbon exchange in boreal ecosystems and has the potential for applications in other biomes.
Land surface modellers need measurable proxies to
constrain the quantity of carbon dioxide (CO2) assimilated by
continental plants through photosynthesis, known as gross primary production
(GPP). ...Carbonyl sulfide (COS), which is taken up by leaves through their
stomates and then hydrolysed by photosynthetic enzymes, is a candidate GPP
proxy. A former study with the ORCHIDEE land surface model used a fixed
ratio of COS uptake to CO2 uptake normalised to respective ambient
concentrations for each vegetation type (leaf relative uptake, LRU) to
compute vegetation COS fluxes from GPP. The LRU approach is known to have
limited accuracy since the LRU ratio changes with variables such as
photosynthetically active radiation (PAR): while CO2 uptake slows under
low light, COS uptake is not light limited. However, the LRU approach has
been popular for COS–GPP proxy studies because of its ease of application
and apparent low contribution to uncertainty for regional-scale
applications. In this study we refined the COS–GPP relationship and
implemented in ORCHIDEE a mechanistic model that describes COS uptake by
continental vegetation. We compared the simulated COS fluxes against
measured hourly COS fluxes at two sites and studied the model behaviour and
links with environmental drivers. We performed simulations at a global scale,
and we estimated the global COS uptake by vegetation to be −756 Gg S yr−1,
in the middle range of former studies (−490 to −1335 Gg S yr−1). Based
on monthly mean fluxes simulated by the mechanistic approach in ORCHIDEE, we
derived new LRU values for the different vegetation types, ranging between
0.92 and 1.72, close to recently published averages for observed values of
1.21 for C4 and 1.68 for C3 plants. We transported the COS using the monthly
vegetation COS fluxes derived from both the mechanistic and the LRU
approaches, and we evaluated the simulated COS concentrations at NOAA sites.
Although the mechanistic approach was more appropriate when comparing to
high-temporal-resolution COS flux measurements, both approaches gave similar
results when transporting with monthly COS fluxes and evaluating COS
concentrations at stations. In our study, uncertainties between these two
approaches are of secondary importance compared to the uncertainties in the
COS global budget, which are currently a limiting factor to the potential of
COS concentrations to constrain GPP simulated by land surface models on the
global scale.
The uptake of carbonyl sulfide (COS) by terrestrial plants is linked to
photosynthetic uptake of CO2 as these gases partly share the same
uptake pathway. Applying COS as a photosynthesis tracer in ...models requires an
accurate representation of biosphere COS fluxes, but these models have not
been extensively evaluated against field observations of COS fluxes. In this
paper, the COS flux as simulated by the Simple Biosphere Model, version 4
(SiB4), is updated with the latest mechanistic insights and evaluated with site
observations from different biomes: one evergreen needleleaf forest, two
deciduous broadleaf forests, three grasslands, and two crop fields spread over
Europe and North America. We improved SiB4 in several ways to improve its
representation of COS. To account for the effect of atmospheric COS mole
fractions on COS biosphere uptake, we replaced the fixed atmospheric COS mole
fraction boundary condition originally used in SiB4 with spatially and
temporally varying COS mole fraction fields. Seasonal amplitudes of COS mole
fractions are ∼50–200 ppt at the investigated sites with a
minimum mole fraction in the late growing season. Incorporating seasonal
variability into the model reduces COS uptake rates in the late growing
season, allowing better agreement with observations. We also replaced the
empirical soil COS uptake model in SiB4 with a mechanistic model that
represents both uptake and production of COS in soils, which improves the
match with observations over agricultural fields and fertilized grassland
soils. The improved version of SiB4 was capable of simulating the diurnal and
seasonal variation in COS fluxes in the boreal, temperate, and Mediterranean
region. Nonetheless, the daytime vegetation COS flux is underestimated on
average by 8±27 %, albeit with large variability across sites. On a
global scale, our model modifications decreased the modeled COS terrestrial
biosphere sink from 922 Gg S yr−1 in the original SiB4 to
753 Gg S yr−1 in the updated version. The largest decrease in
fluxes was driven by lower atmospheric COS mole fractions over regions with
high productivity, which highlights the importance of accounting for
variations in atmospheric COS mole fractions. The change to a different soil
model, on the other hand, had a relatively small effect on the global
biosphere COS sink. The secondary role of the modeled soil component in the
global COS budget supports the use of COS as a global photosynthesis tracer. A
more accurate representation of COS uptake in SiB4 should allow for improved
application of atmospheric COS as a tracer of local- to global-scale
terrestrial photosynthesis.
The magnitude of fractionation during photorespiration and the effect on net photosynthetic ¹³C discrimination (Δ) were investigated for three Senecio species, S. squalidus, S. cineraria, and S. ...greyii. We determined the contributions of different processes during photosynthesis to Δ by comparing observations (Δobs) with discrimination predicted from gas-exchange measurements (Δpred). Photorespiration rates were manipulated by altering the O₂ partial pressure (pO₂) in the air surrounding the leaves. Contributions from ¹³C-depleted photorespiratory CO₂ were largest at high pO₂. The parameters for photorespiratory fractionation (f), net fractionation during carboxylation by Rubisco and phosphoenolpyruvate carboxylase (b), and mesophyll conductance (gi) were determined simultaneously for all measurements. Instead of using Δobs data to obtain gi and f successively, which requires that b is known, we treated b, f, and gi as unknowns. We propose this as an alternative approach to analyze measurements under field conditions when b and gi are not known or cannot be determined in separate experiments. Good agreement between modeled and observed Δ was achieved with f = 11.6per thousand ± 1.5per thousand, b = 26.0per thousand ± 0.3per thousand, and gi of 0.27 ± 0.01, 0.25 ± 0.01, and 0.22 ± 0.01 mol m⁻² s⁻¹ for S. squalidus, S. cineraria, and S. greyii, respectively. We estimate that photorespiratory fractionation decreases Δ by about 1.2per thousand on average under field conditions. In addition, diurnal changes in Δ are likely to reflect variations in photorespiration even at the canopy level. Our results emphasize that the effects of photorespiration must be taken into account when partitioning net CO₂ exchange of ecosystems into gross fluxes of photosynthesis and respiration.
A comparative study has been made of the photosynthetic physiological ecology and carbon isotope discrimination characteristics for modern-day bryophytes and closely related algal groups. Firstly, ...the extent of bryophyte distribution and diversification as compared with more advanced land plant groups is considered. Secondly, measurements of instantaneous carbon isotope discrimination (Δ), photosynthetic CO2 assimilation and electron transport rates were compared during the drying cycles. The extent of surface diffusion limitation (when wetted), internal conductance and water use efficiency (WUE) at optimal tissue water content (TWC) were derived for liverworts and a hornwort from contrasting habitats and with differing degrees of thallus ventilation (as intra-thalline cavities and internal airspaces). We also explore how the operation of a biophysical carbon-concentrating mechanism (CCM) tempers isotope discrimination characteristics in two other hornworts, as well as the green algae Coleochaete orbicularis and Chlamydomonas reinhardtii. The magnitude of Δ was compared for each life form over a drying curve and used to derive the surface liquid-phase conductance (when wetted) and internal conductance (at optimal TWC). The magnitude of external and internal conductances, and WUE, was higher for ventilated, compared with non-ventilated, liverworts and hornworts, but the values were similar within each group, suggesting that both factors have been optimized for each life form. For the hornworts, leakiness of the CCM was highest for Megaceros vincentianus and C. orbicularis (approx. 30%) and, at 5%, lowest in C. reinhardtii grown under ambient CO2 concentrations. Finally, evidence for the operation of a CCM in algae and hornworts is considered in terms of the probable role of the chloroplast pyrenoid, as the origins, structure and function of this enigmatic organelle are explored during the evolution of land plants.
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