ABSTRACT
Carbon uptake and transpiration in plant leaves occurs through stomata that open and close. Stomatal action is usually considered a response to environmental driving factors. Here we show ...that leaf gas exchange is more strongly related to whole tree level transport of assimilates than previously thought, and that transport of assimilates is a restriction of stomatal opening comparable with hydraulic limitation. Assimilate transport in the phloem requires that osmotic pressure at phloem loading sites in leaves exceeds the drop in hydrostatic pressure that is due to transpiration. Assimilate transport thus competes with transpiration for water. Excess sugar loading, however, may block the assimilate transport because of viscosity build‐up in phloem sap. Therefore, for given conditions, there is a stomatal opening that maximizes phloem transport if we assume that sugar loading is proportional to photosynthetic rate. Here we show that such opening produces the observed behaviour of leaf gas exchange. Our approach connects stomatal regulation directly with sink activity, plant structure and soil water availability as they all influence assimilate transport. It produces similar behaviour as the optimal stomatal control approach, but does not require determination of marginal cost of water parameter.
Requiring stomata to open such that assimilate transport from leaves is maximized reproduces known plant responses of stomatal conductance to various environmental conditions, above and below ground, in a single theoretical framework. The model agrees with field observations and produces similar behavior than the optimal stomatal control approach. However, here no marginal cost of water is needed but the stomatal behavior results from axial and radial water flows and the osmotic competition for water between phloem and the transpiration stream.
Climate change mitigation requires, besides reductions in greenhouse gas emissions, actions to increase carbon sinks in terrestrial ecosystems. A key measurement method for quantifying such sinks and ...calibrating models is the eddy covariance technique, but it requires imputation, or gap-filling, of missing data for determination of annual carbon balances of ecosystems. Previous comparisons of gap-filling methods have concluded that commonly used methods, such as marginal distribution sampling (MDS), do not have a significant impact on the carbon balance estimate. By analyzing an extensive, global data set, we show that MDS causes significant carbon balance errors for northern (latitude Formula: see text) sites. MDS systematically overestimates the carbon dioxide (COFormula: see text) emissions of carbon sources and underestimates the COFormula: see text sequestration of carbon sinks. We also reveal reasons for these biases and show how a machine learning method called extreme gradient boosting or a modified implementation of MDS can be used to substantially reduce the northern site bias.
Intrinsic water‐use efficiency (iWUE) characterizes the physiological control on the simultaneous exchange of water and carbon dioxide in terrestrial ecosystems. Knowledge of iWUE is commonly gained ...from leaf‐level gas exchange measurements, which are inevitably restricted in their spatial and temporal coverage. Flux measurements based on the eddy covariance (EC) technique can overcome these limitations, as they provide continuous and long‐term records of carbon and water fluxes at the ecosystem scale. However, vegetation gas exchange parameters derived from EC data are subject to scale‐dependent and method‐specific uncertainties that compromise their ecophysiological interpretation as well as their comparability among ecosystems and across spatial scales. Here, we use estimates of canopy conductance and gross primary productivity (GPP) derived from EC data to calculate a measure of iWUE (G1, “stomatal slope”) at the ecosystem level at six sites comprising tropical, Mediterranean, temperate, and boreal forests. We assess the following six mechanisms potentially causing discrepancies between leaf and ecosystem‐level estimates of G1: (i) non‐transpirational water fluxes; (ii) aerodynamic conductance; (iii) meteorological deviations between measurement height and canopy surface; (iv) energy balance non‐closure; (v) uncertainties in net ecosystem exchange partitioning; and (vi) physiological within‐canopy gradients. Our results demonstrate that an unclosed energy balance caused the largest uncertainties, in particular if it was associated with erroneous latent heat flux estimates. The effect of aerodynamic conductance on G1 was sufficiently captured with a simple representation. G1 was found to be less sensitive to meteorological deviations between canopy surface and measurement height and, given that data are appropriately filtered, to non‐transpirational water fluxes. Uncertainties in the derived GPP and physiological within‐canopy gradients and their implications for parameter estimates at leaf and ecosystem level are discussed. Our results highlight the importance of adequately considering the sources of uncertainty outlined here when EC‐derived water‐use efficiency is interpreted in an ecophysiological context.
The intrinsic water‐use efficiency (iWUE) metric G1 inferred from eddy covariance measurements is affected by confounding physical and methodological factors (1–6) to a different extent. Meteorological deviations between measurement height and canopy surface (4), the NEE‐partitioning approach (5) and possible physiological within‐canopy gradients (6) are generally less critical for the accurate estimation of intrinsic WUE at the ecosystem level than non‐transpirational water fluxes (1), aerodynamic resistance (2) or the energy balance non‐closure (3).
Boreal forests comprise 73% of the world's coniferous forests. Based on forest floor measurements, they have been considered a significant natural sink of methane (CH4) and a natural source of ...nitrous oxide (N2O), both of which are important greenhouse gases. However, the role of trees, especially conifers, in ecosystem N2O and CH4 exchange is only poorly understood. We show for the first time that mature Scots pine (Pinus sylvestris L.) trees consistently emit N2O and CH4 from both stems and shoots. The shoot fluxes of N2O and CH4 exceeded the stem flux rates by 16 and 41 times, respectively. Moreover, higher stem N2O and CH4 fluxes were observed from wet than from dry areas of the forest. The N2O release from boreal pine forests may thus be underestimated and the uptake of CH4 may be overestimated when ecosystem flux calculations are based solely on forest floor measurements. The contribution of pine trees to the N2O and CH4 exchange of the boreal pine forest seems to increase considerably under high soil water content, thus highlighting the urgent need to include tree-emissions in greenhouse gas emission inventories.
Temperate and boreal forests in the Northern Hemisphere cover an area of about 2 x 10(7) square kilometres and act as a substantial carbon sink (0.6-0.7 petagrams of carbon per year). Although forest ...expansion following agricultural abandonment is certainly responsible for an important fraction of this carbon sink activity, the additional effects on the carbon balance of established forests of increased atmospheric carbon dioxide, increasing temperatures, changes in management practices and nitrogen deposition are difficult to disentangle, despite an extensive network of measurement stations. The relevance of this measurement effort has also been questioned, because spot measurements fail to take into account the role of disturbances, either natural (fire, pests, windstorms) or anthropogenic (forest harvesting). Here we show that the temporal dynamics following stand-replacing disturbances do indeed account for a very large fraction of the overall variability in forest carbon sequestration. After the confounding effects of disturbance have been factored out, however, forest net carbon sequestration is found to be overwhelmingly driven by nitrogen deposition, largely the result of anthropogenic activities. The effect is always positive over the range of nitrogen deposition covered by currently available data sets, casting doubts on the risk of widespread ecosystem nitrogen saturation under natural conditions. The results demonstrate that mankind is ultimately controlling the carbon balance of temperate and boreal forests, either directly (through forest management) or indirectly (through nitrogen deposition).
The photochemical reflectance index (PRI) is regarded as a promising proxy to track the dynamics of photosynthetic light use efficiency (LUE) via remote sensing. The implementation of this approach ...requires the relationship between PRI and LUE to scale not only in space but also in time. The short-term relationship between PRI and LUE is well known and is based on the regulative process of non-photochemical quenching (NPQ), but at the seasonal timescale the mechanisms behind the relationship remain unclear. We examined to what extent sustained forms of NPQ, photoinhibition of reaction centres, seasonal changes in leaf pigment concentrations, or adjustments in the capacity of alternative energy sinks affect the seasonal relationship between PRI and LUE during the year in needles of boreal Scots pine. PRI and NPQ were highly correlated during most of the year but decoupled in early spring when the foliage was deeply downregulated. This phenomenon was attributed to differences in the physiological mechanisms controlling the seasonal dynamics of PRI and NPQ. Seasonal adjustments in the pool size of the xanthophyll cycle pigments, on a chlorophyll basis, controlled the dynamics of PRI, whereas the xanthophyll de-epoxidation status and other xanthophyll-independent mechanisms controlled the dynamics of NPQ at the seasonal timescale. We conclude that the PRI leads to an underestimation of NPQ, and consequently overestimation of LUE, under conditions of severe stress in overwintering Scots pine, and most likely also in species experiencing severe drought. This severe stress-induced decoupling may challenge the implementation of the PRI approach.
Nighttime vegetative uptake of carbonyl sulfide (COS) can exist due to the incomplete closure of stomata and the light independence of the enzyme carbonic anhydrase, which complicates the use of COS ...as a tracer for gross primary productivity (GPP). In this study we derived nighttime COS fluxes in a boreal forest (the SMEAR II station in Hyytiälä, Finland; 61°51′ N, 24°17′ E; 181 m a.s.l.) from June to November 2015 using two different methods: eddy-covariance (EC) measurements (FCOS-EC) and the radon-tracer method (FCOS-Rn). The total nighttime COS fluxes averaged over the whole measurement period were −6.8 ± 2.2 and −7.9 ± 3.8 pmol m−2 s−1 for FCOS-Rn and FCOS-EC, respectively, which is 33–38 % of the average daytime fluxes and 21 % of the total daily COS uptake. The correlation of 222Rn (of which the source is the soil) with COS (average R2 = 0.58) was lower than with CO2 (0.70), suggesting that the main sink of COS is not located at the ground. These observations are supported by soil chamber measurements that show that soil contributes to only 34–40 % of the total nighttime COS uptake. We found a decrease in COS uptake with decreasing nighttime stomatal conductance and increasing vapor-pressure deficit and air temperature, driven by stomatal closure in response to a warm and dry period in August. We also discuss the effect that canopy layer mixing can have on the radon-tracer method and the sensitivity of (FCOS-EC) to atmospheric turbulence. Our results suggest that the nighttime uptake of COS is mainly driven by the tree foliage and is significant in a boreal forest, such that it needs to be taken into account when using COS as a tracer for GPP.
► We studied the seasonal variation in boreal pine forest albedo and the effects of canopy and ground snow cover on it. ► Canopy snow cover significantly influenced the forest albedo, but was ...depended on snow properties and illumination conditions. ► The amount of snow in the canopy and therefore the albedo were high in midwinter, when the solar irradiance was low and the albedo climate feedback negligible. ► Toward spring, the forest albedo decreased as the snow remained on the branches only for short periods of time and the reflectance of the ground snow decreased.
The low shortwave albedo of evergreen coniferous forests increases net radiation and thus the local temperature as compared to open areas. The difference between albedos of evergreen forests and open areas is most pronounced in winter and spring, when the forest canopies mask the reflective ground snow. However, also the albedo of coniferous evergreen forests changes as a function of the amount and optical properties of snow in the ground and canopy. In this study, we examined the shortwave radiation balance of a boreal Scots pine dominated forest. Canopy snow cover was observed to substantially influence the albedo, increasing it by about 0.2 as compared to snow free conditions. However, albedo varied greatly between and within the canopy snow classes, probably due to differences in illumination conditions and snow amount and properties. During midwinter months, most of the incident solar radiation was reflected during days with a lot of snow in the canopy. The difference in reflected radiation between the winters with least and most snow was 54MJm−2, which was also close to the difference (55MJm−2) between the reflectance of an average winter and that of a simulated snow free winter, corresponding to less than 2% of the annual solar irradiance. The albedo of the forest was greatest in midwinter, when the cold weather and low solar radiation caused the falling snow to remain on the tree branches. As the incident solar radiation in these months is very low, we concluded that the positive feedback to climate warming in case of a potentially decreased snow cover in this boreal pine forest would remain small.
► Modelling air/soil temperature effecton carbon and water cycles in boreal trees. ► 13-year eddy flux data and soil data was used to validate model. ► Air temperature triggered the onset of ...photosynthesis in spring. ► Low soil temperature inhibited recovery of photosynthesis in late spring. ► Both air and soil temperature acclimation functions are important for modelling.
Photosynthesis and transpiration in boreal forests are restricted by air temperature (Ta) and soil temperature (Ts), especially in spring after the dormant period, but the extent to which the recovery process is regulated these factors is still uncertain. To examine the role of air temperature and soil temperature, years with three types of typical patterns of temperature rises were identified from 13 years of continuous flux measurements for a Scots pine ecosystem in Hyytiälä, southern Finland. By combining a process-based ecosystem model (CoupModel) with an uncertainty estimation procedure (GLUE), the role of regulating factors was explored and 45 of 51 parameters were found to have reduced uncertainty after calibration. Air temperature was the major limiting factor for photosynthesis in early spring, autumn and winter, but soil temperature was a rather important limiting factor in late spring. Especially during warm spring with a large delay of soil temperature rise both water uptake and photosynthesis was strongly reduced due to low soil temperature. Soil moisture and nitrogen showed indications of being more important for regulating photosynthesis in the summer period. It proved possible to replace the soil temperature acclimation function on photosynthesis and transpiration with a corresponding air temperature function only during warm years with a small delay between Ta and Ts. Fluxes of photosynthesis and transpiration showed a sensitivity to the carbon footprint representation, as expected from the high spatial variability in soil temperature during the spring of a warm year with a large delay between Ta and Ts.
It is known that stem CO2 efflux differs somewhat both temporally and spatially from actual stem respiration, but relations between these two are not fully understood. A physical model of CO2 ...diffusion and advection by xylem sap flow is developed to interpret the CO2 flux signal from the stem. Model predictions are compared against measured CO2 efflux data from a field-grown 16-m Pinus sylvestris L. tree. The ratio of CO2 efflux to CO2 production is predicted to be much larger in the upper part of the tree than in the lower part as the xylem sap carries the respired CO2 upwards. The model also predicts the temperature dependency of real respiration to be higher than that of the CO2 efflux due to the slowness of diffusion. The relation between stem respiration and CO2 efflux depends strongly on the sap flow rate, radial diffusion resistance and stem geometry and size. The model may be used to scale individual CO2 efflux measurements to evaluate the respiration rate of whole trees and forests.
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