SummaryUpward transport of CO 2 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 CO 2 derived from autotrophic respiratory processes remains unclear.We infused a 13 CO 2 -labeled aqueous solution into the base of 7-yr-old field-grown eastern cottonwood (Populus deltoides) trees to investigate the effect of xylem-transported CO 2 derived from the root system on aboveground carbon assimilation and CO 2 efflux. The 13 C 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 13 C was found in branch woody tissues, while only a small quantity was assimilated in the foliage. Petioles were more highly enriched in 13 C than other leaf tissues.Our results confirm a recycling pathway for respired CO 2 and indicate that internal transport of CO 2 from the root system may confound the interpretation of efflux-based estimates of woody tissue respiration and patterns of carbohydrate allocation.

Summary• 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) 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.

Increased forest productivity has been obtained by improving resource availability through water and nutrient amendments. However, more stress-tolerant species that have robust site requirements do not respond consistently to irrigation. An important factor contributing to robust site requirements may be the distribution of biomass belowground, yet available information is limited. We examined the accumulation and distribution of above-and below-ground biomass in sweetgum (Liqrridambar sfyrac$lua L.) and loblolly pine (Pinus taeda L.) stands receiving irrigation and fertilization. Mean annual aboveground production after 4 years ranged from 2.4 to 5.1 ~g.ha-'.year' for sweetgum and from 5.0 to 6.9 ~g.ha-l.year-l for pine. Sweetgum responded positively to irrigation and fertilization with an additive response to irrigation + fertilization. Pine only responded to fertilization. Sweetgum root mass fraction (RME) increased with fertilization at 2 years and decreased with fertilization at 4 years. There were no detectable treatment differences in loblolly pine RMF. Development explained from 67% to 98% of variation in shoot versus root allometry for ephemeral and perennial tissues, fertilization explained no more than 5% of the variation in for either species, and irrigation did not explain any. We conclude that shifts in allocation from roots to shoots do not explain nutrient-induced growth stimulations.Rdsumd : Une augmentation de la productiviti de la forh a it6 obtenue en amkliorant la disponibiliti des ressources i l'aide d'amendements en eau et en nutriments. Cependant, les espkces plus tolirantes au stress ayant des exigences Bcologiques robustes ne riagissent pas de la mgme faqon i I'inigation. La distribution de la biomasse souterraine pourrait btre un facteur important contribuant aux exigences 6cologiques robustes. mais peu d'information est disponible. Nous avons itudi6 l'accumulation et la distribution de la biomasse akrienne et souterraine dans des peuplements de copalme d'AmMque (Liquidambar styracijZua L.) et de pin 1 encens (Pinus taeda L.) irriguis et fertilisks. La production adrienne annuelle moyenne apres quatre ans variait de 2,4 ?I 5,l Mg.ha-'an-' dans le cas du copalme d3Amirique et de 5,O B 6,9 Mgha-'.an-' dans le cas du pin 5 encens. Le copalme d'Amirique a riagi positivement B l'inigation et 1 la fertilisation tout en ayant une r6action additive lorsque ces deux traitements itaient appliquis. Le pin 1 encens a riagi uniquement 1 la fertilisation.La fraction de masse racinaire (FMR) du copalme d'Amirique augmentait avec la fertilisation aprks d e w ans et dirninuait avec la fertilisation aprhs 4 ans. Nous n'avons pas dktectk de diffkrence significative entre les traitements dans le cas de la FMR du pin 1 encens. La croissance a expliqu6 de 67 % B 98 % de la variation de l'allom6trie entre les pousses et les racines dans le cas des tissus iph6mkres et pirennes alors que la fertilisation n'a pas expliqui plus de 5 % de la variation chez I'une ou I'autre des dew espkces et que I'irrig...

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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.

The dead foliage of scorched crowns is one of the most conspicuous signatures of wildland fires.Globally, crown scorch from fires in savannas, woodlands, and forests causes tree stress and death across diverse taxa. The term crown scorch, however, is inconsistently and ambiguously defined in the literature, causing confusion and conflicting interpretation of results. Furthermore, the underlying mechanisms causing foliage death from fire are poorly understood. The consequences of crown scorch-alterations in physiological, biogeochemical, and ecological processes and ecosystem recovery pathways-remain largely unexamined. Most research on the topic assumes the mechanism of leaf and bud death is exposure to lethal air temperatures, with few direct measurements of lethal heating thresholds. Notable information gaps include how energy transfer injures and kills leaves and buds, how nutrients, carbohydrates, and hormones respond, and what physiological consequences lead to mortality. We clarify definitions to encourage use of unified terminology for foliage and bud necrosis resulting from fire. We review the current understanding of the physical mechanisms driving foliar injury, discuss the physiological responses, and explore novel ecological consequences of crown injury from fire. From these elements, we propose research needs for the increasingly interdisciplinary study of fire effects.

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 (F S ) 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 F S deviated substantially from actual F S , with the discrepancy in cumulative F S ranging from 9 to 55%. The thermal dissipation technique generally underestimated F S . 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 F S , 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 a and b parameter values only modestly improved the accuracy of F S estimates. However, the relationship between the estimated and actual F S 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 speciesspecific calibrations can substantially increase the accuracy of the thermal dissipation technique.