Doug P. Aubrey

Root‐derived CO2 efflux via xylem stream rivals soil CO2 efflux

Respiration consumes a large portion of annual gross primary productivity in forest ecosystems and is dominated by belowground metabolism. Here, we present evidence of a previously unaccounted for internal CO(2) flux of large magnitude from tree roots through stems. If this pattern is shown to persist over time and in other forests, it suggests that belowground respiration has been grossly underestimated. Using an experimental Populus deltoides plantation as a model system, we tested the hypothesis that a substantial portion of the CO(2) released from belowground autotrophic respiration remains within tree root systems and is transported aboveground through the xylem stream rather than diffusing into the soil atmosphere. On a daily basis, the amount of CO(2) that moved upward from the root system into the stem via the xylem stream (0.26 mol CO(2) m(-2) d(-1)) rivalled that which diffused from the soil surface to the atmosphere (0.27 mol CO(2) m(-2) d(-1)). We estimated that twice the amount of CO(2) derived from belowground autotrophic respiration entered the xylem stream as diffused into the soil environment. Our observations indicate that belowground autotrophic respiration consumes substantially more carbohydrates than previously recognized and challenge the paradigm that all root-respired CO(2) diffuses into the soil atmosphere.

Transport of root‐respired CO2 via the transpiration stream affects aboveground carbon assimilation and CO2 efflux in trees

Upward transport of CO₂ via the transpiration stream from belowground to aboveground tissues occurs in tree stems. Despite potentially important implications for our understanding of plant physiology, the fate of internally transported CO₂ derived from autotrophic respiratory processes remains unclear. We infused a ¹³CO₂-labeled aqueous solution into the base of 7-yr-old field-grown eastern cottonwood (Populus deltoides) trees to investigate the effect of xylem-transported CO₂ derived from the root system on aboveground carbon assimilation and CO₂ efflux. The ¹³C label was transported internally and detected throughout the tree. Up to 17% of the infused label was assimilated, while the remainder diffused to the atmosphere via stem and branch efflux. The largest amount of assimilated ¹³C was found in branch woody tissues, while only a small quantity was assimilated in the foliage. Petioles were more highly enriched in ¹³C than other leaf tissues. Our results confirm a recycling pathway for respired CO₂ and indicate that internal transport of CO₂ from the root system may confound the interpretation of efflux-based estimates of woody tissue respiration and patterns of carbohydrate allocation.

A Simple Calibration Improved the Accuracy of the Thermal Dissipation Technique for Sap Flow Measurements in Juvenile Trees of Six Species

The thermal dissipation technique is widely used to estimate transpiration of individual trees and forest stands, but there are conflicting reports regarding its accuracy. We compared the rate of water uptake by stems of six tree species in potometers with sap flow (FS) estimates derived from thermal dissipation sensors to evaluate the accuracy of the technique. To include the full range of xylem anatomies (i.e., diffuse-porous, ring-porous, and tracheid), we used saplings of sweetgum (Liquidambar styraciflua), eastern cottonwood (Populus deltoides), white oak (Quercus alba), American elm (Ulmus americana), shortleaf pine (Pinus echinata), and loblolly pine (Pinus taeda). In almost all instances, estimated FS deviated substantially from actual FS, with the discrepancy in cumulative FS ranging from 9 to 55%. The thermal dissipation technique generally underestimated FS. There were a number of potential causes of these errors, including species characteristics and probe construction and installation. Species with the same xylem anatomy generally did not show similar relationships between estimated and actual FS, and the largest errors were in species with diffuse-porous (Populus deltoides, 34%) and tracheid (Pinus taeda, 55%) xylem anatomies, rather than ring-porous species Quercus alba (9%) and Ulmus americana (15%) as we had predicted. New species-specific α and β parameter values only modestly improved the accuracy of FS estimates. However, the relationship between the estimated and actual FS was linear in all cases and a simple calibration based on the slope of this relationship reduced the error to 1–4% in five of the species, and to 8% in Liquidambar styraciflua. Our calibration approach compensated simultaneously for variation in species characteristics and sensor construction and use. We conclude that species-specific calibrations can substantially increase the accuracy of the thermal dissipation technique.

Stem girdling affects the quantity of CO2 transported in xylem as well as CO2 efflux from soil

There is recent clear evidence that an important fraction of root-respired CO2 is transported upward in the transpiration stream in tree stems rather than fluxing to the soil. In this study, we aimed to quantify the contribution of root-respired CO2 to both soil CO2 efflux and xylem CO2 transport by manipulating the autotrophic component of belowground respiration. We compared soil CO2 efflux and the flux of root-respired CO2 transported in the transpiration stream in girdled and nongirdled 9-yr-old oak trees (Quercus robur) to assess the impact of a change in the autotrophic component of belowground respiration on both CO2 fluxes. Stem girdling decreased xylem CO2 concentration, indicating that belowground respiration contributes to the aboveground transport of internal CO2 . Girdling also decreased soil CO2 efflux. These results confirmed that root respiration contributes to xylem CO2 transport and that failure to account for this flux results in inaccurate estimates of belowground respiration when efflux-based methods are used. This research adds to the growing body of evidence that efflux-based measurements of belowground respiration underestimate autotrophic contributions.

Overlap in Roosting Habits of Indiana Bats (Myotis sodalis) and Northern Bats (Myotis septentrionalis)

Abstract Roosts are an integral habitat component for species of bats and may affect their survival and fitness. Conversion of forests to agricultural and urban areas may decrease available roosting habitat for the endangered Indiana bat (Myotis sodalis). Furthermore, the effects of habitat loss could be exacerbated if sympatric species favor and compete for similar roost-sites. We used radio-telemetry to study roosting habits of two species of Myotis in northeastern Missouri. We did not directly test for competition between these species for maternity roosts; rather, our goal was to determine if similarities in roost site characteristics were strong enough to warrant further investigation into competition for maternity roosts. Of 118 roosts located throughout the study, 79 were used by Indiana bats and 39 by northern bats (M. septentrionalis). Differences in roost structure (natural vs. manmade), tree status (live vs. snag), roost type (bark vs. cavity/crevice) and canopy coverage indicate that subtle, but biologically important differences exist in roost selection. Northern and Indiana bats both relied heavily on trees as roosts sites; however, Indiana bats roosted in trees with lower canopy cover and less often in cavities and live trees than northern bats. Our results suggest that niche separation in roost selection exists between northern and Indiana bats.

Assimilation of xylem-transported CO2 is dependent on transpiration rate but is small relative to atmospheric fixation

The effect of transpiration rate on internal assimilation of CO2 released from respiring cells has not previously been quantified. In this study, detached branches of Populus deltoides were allowed to take up (13)CO2-labelled solution at either high (high label, HL) or low (low label, LL) (13)CO2 concentrations. The uptake of the (13)CO2 label served as a proxy for the internal transport of respired CO2, whilst the transpiration rate was manipulated at the leaf level by altering the vapour pressure deficit (VPD) of the air. Simultaneously, leaf gas exchange was measured, allowing comparison of internal CO2 assimilation with that assimilated from the atmosphere. Subsequent (13)C analysis of branch and leaf tissues revealed that woody tissues assimilated more label under high VPD, corresponding to higher transpiration, than under low VPD. More (13)C was assimilated in leaf tissue than in woody tissue under the HL treatment, whereas more (13)C was assimilated in woody tissue than in leaf tissue under the LL treatment. The ratio of (13)CO2 assimilated from the internal source to CO2 assimilated from the atmosphere was highest for the branches under the HL and high VPD treatment, but was relatively small regardless of VPD×label treatment combination (up to 1.9%). These results showed that assimilation of internal CO2 is highly dependent on the rate of transpiration and xylem sap [CO2]. Therefore, it can be expected that the relative contribution of internal CO2 recycling to tree carbon gain is strongly dependent on factors controlling transpiration, respiration, and photosynthesis.

Internal Recycling of Respired CO 2 May Be Important for Plant Functioning under Changing Climate Regimes

Recent studies have provided evidence of a large flux of root-respired CO2 in the transpiration stream of trees. In our study, we investigated the potential impact of this internal CO2 transport on aboveground carbon assimilation and CO2 efflux. To trace the transport of root-respired CO2, we infused a 13C label at the stem base of field-grown Populus deltoides Bartr. ex. Marsh trees. The 13C label was transported to the top of the stem and throughout the crown via the transpiration stream. Up to 17% of the 13C label was assimilated by chlorophyll-containing tissues. Our results provide evidence of a mechanism for recycling respired CO2 within trees. Such a mechanism may have important implications for how plants cope with predicted increases in intensity and frequency of droughts. Here, we speculate on the potential significance of this recycling mechanism within the context of plant responses to climate change and plants currently inhabiting arid environments.

Root xylem CO2 flux: an important but unaccounted-for component of root respiration

Key messageIn tree roots, a large fractionof root-respired CO2remains within the root system rather than diffusing into the soil. This CO2is transported in xylem sap into the shoot, and because respiration is almost always measured as the flux of CO2into the atmosphere from plant tissues, it represents an unaccounted-for component of tree root metabolism.AbstractRoot respiration has been considered a large component of forest soil CO2 efflux, but recent findings indicate that it may be even more important than previous measurements have shown because a substantial fraction of root-respired CO2 remains within the tree root system and moves internally with the transpiration stream. The high concentration of CO2 in roots appears to originate mainly within the root. It has been suggested that plants can take up dissolved inorganic carbon (DIC) from soil, but under most conditions uptake from soil is minimal due to the root-to-soil diffusion gradient, which suggests that most of the CO2 in root xylem is derived from root respiration. Estimates of the internal flux of CO2 through root xylem are based on combined measurements of sap flow and internal [CO2]. Results quantifying root xylem CO2 flux, obtained for a limited number of species, have raised important concerns regarding our understanding of tree respiration. Taken together, the results of these studies call into question the partitioning of ecosystem respiration into its above- and belowground components, and redefine the energetic costs of tree root metabolism and hence estimates of belowground carbon allocation. Expanding our observations of root xylem CO2 flux to more species and at longer time scales, as well as improving the techniques used to study this process, could be fruitful avenues for future research, with the potential to substantially revise our understanding of root respiration and forest carbon cycles.

Poplar saplings exposed to recurring temperature shifts of different amplitude exhibit differences in leaf gas exchange and growth despite equal mean temperature

Most investigations of plant responses to changes in temperature have focused on a constant increase in temperature. However, changes in fluctuation in temperature, even if the mean temperature is the same, may affect plant growth. We tested the effects of weekly warm and then cool moderate (5°C) and large (10°C) fluctuation in temperature (with the same biweekly temperature sum) on plant growth. We found that, while the ratio of photosynthesis to respiration did not change, fluctuations in temperature did increase biomass accumulation and alter biomass allocation. Our findings suggest that, like mean temperature, fluctuation in temperature can significantly impact plant growth.

Global patterns and predictors of stem CO2 efflux in forest ecosystems.

Stem CO2 efflux (ES) plays an important role in the carbon balance of forest ecosystems. However, its primary controls at the global scale are poorly understood and observation-based global estimates are lacking. We synthesized data from 121 published studies across global forest ecosystems and examined the relationships between annual ES and biotic and abiotic factors at individual, biome, and global scales, and developed a global gridded estimate of annual ES . We tested the following hypotheses: (1) Leaf area index (LAI) will be highly correlated with annual ES at biome and global scales; (2) there will be parallel patterns in stem and root CO2 effluxes (RA) in all forests; (3) annual ES will decline with forest age; and (4) LAI coupled with mean annual temperature (MAT) and mean annual precipitation (MAP) will be sufficient to predict annual ES across forests in different regions. Positive linear relationships were found between ES and LAI, as well as gross primary production (GPP), net primary production (NPP), wood NPP, soil CO2 efflux (RS), and RA . Annual ES was correlated with RA in temperate forests after controlling for GPP and MAT, suggesting other additional factors contributed to the relationship. Annual ES tended to decrease with stand age. Leaf area index, MAT and MAP, predicted 74% of variation in ES at global scales. Our statistical model estimated a global annual ES of 6.7 ± 1.1 Pg C yr(-1) over the period of 2000-2012 with little interannual variability. Modeled mean annual ES was 71 ± 43, 270 ± 103, and 420 ± 134 g C m(2) yr(-1) for boreal, temperate, and tropical forests, respectively. We recommend that future studies report ES at a standardized constant temperature, incorporate more manipulative treatments, such as fertilization and drought, and whenever possible, simultaneously measure both aboveground and belowground CO2 fluxes.