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Tight coupling between below-ground autotrophic respiration and the availability of recently assimilated carbon (C) has become a paradigm in the ecophysiological literature. Here, we show that stored carbohydrates can decouple respiration from assimilation for prolonged periods by mobilizing reserves from transport roots to absorptive roots. We permanently disrupted the below-ground transfer of recently assimilated C using stem girdling and root trenching and measured soil CO2 efflux for over 1 yr in longleaf pine (Pinus palustris), a species that has large reserves of stored carbohydrates in roots. Soil CO2 efflux was not influenced by girdling or trenching through the 14-month observation period. Stored carbohydrate concentrations in absorptive roots were not affected by the disrupted supply of current photosynthate for over 1 yr; however, carbohydrate concentrations in transport roots decreased. Our results indicate that root respiration can be decoupled from recent canopy assimilation and that stored carbohydrates can be mobilized from transport roots to absorptive roots to maintain respiration for over 1 yr. This refines the current paradigm that canopy assimilation and below-ground respiration are tightly coupled and provides evidence of the mechanism and dynamics responsible for decoupling the above- and below-ground processes.

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.

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.

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 CO₂ 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 CO₂ 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 CO₂ that moved upward from the root system into the stem via the xylem stream (0.26 mol CO₂ m⁻² d⁻¹) rivalled that which diffused from the soil surface to the atmosphere (0.27 mol CO₂ m⁻² d⁻¹). We estimated that twice the amount of CO₂ 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 CO₂ diffuses into the soil atmosphere.

Developing approaches to simultaneously maximize short‐rotation woody crop (SRWC) productivity while minimizing footprints associated with intensive management is imperative to profitable and sustainable bioenergy production systems. Intercropping nitrogen (N)‐fixing cover crops in SRWC systems is an overlooked approach to sustainably intensify SRWC production by increasing N availability using less environmentally costly inputs. Here, we discuss how functional traits (e.g., seasonal activity, lifespan, leaf habit, soil exploration) of cover crops and SRWCs may interact through space and time influencing access to light, water, and nutrients to provide a framework for successful integration of cover crops into SRWCs. Next, we summarize the literature on intercropping forest plantations with N‐fixing cover crops to identity research gaps and outline future research needs and opportunities. And then, using empirical N demand and productivity data from SRWCs and cover crop N inputs from the literature, we illustrate how SRWC leaf habit (conifer evergreens and deciduous hardwoods) would influence successful integration of cover crops and potential N fixation. We estimate that integrating cover crops into SRWCs could supply 27% and 72% of the N demand across a 10‐year rotation for an evergreen and a deciduous hardwood, respectively. These figures suggest these integrated SRWC systems may approach a virtual minimal external N input when other biogeochemical cycles are considered. The guiding principles presented here are grounded in ecological theory and provide a framework for sustainable intensification of forest production. Our study discussed the potential of integrating nitrogen (N)‐fixing cover crops into short‐rotation woody crop (SRWC) systems as an alternative for ecological intensification of bioenergy production systems. Using ecological theory and numerical simulations, we explored how functional traits (e.g., seasonal activity, lifespan, leaf habit, soil exploration) of cover crops and SRWCs can influence competition for resources in space and time to provide a framework for successful integration of cover crops into SRWCs. Our simulations suggest integrated SRWC systems increase whole‐system N cycling potential and reduce external N demand, ultimately maximizing SRWC productivity while reducing greenhouse gas emissions and N pollution.

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 α and β 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 species-specific calibrations can substantially increase the accuracy of the thermal dissipation technique.

Aims Trees regulate water availability among their rooting strata through a nocturnal, passive transference of water known as hydraulic redistribution (HR). This study investigates differences in HR and groundwater use among common canopy species in longleaf pine ( Pinus palustris Mill., Pinaceae) woodlands and explores environmental factors influencing HR. Methods HR was estimated by sap flux of lateral roots and main stems of three mature canopy species ( P. palustris , Quercus laevis Walter., Fagaceae and Quercus margarettae Ashe., Fagaceae). We used δ 18 O and δD of xylem water, soil water, and groundwater to determine water source. Finally, we related HR to environmental factors (Temperature, VWC, VPD) to better understand controls of HR dynamics. Results Pinus palustris had higher water use than either Quercus species, and also redistributed significantly more water as a nocturnal subsidy. HR fluxes were inversely related with mean nightly temperature and independent of shallow soil moisture. Stable isotope mixing models, based on δ 18 O and δD, indicated that all species have access to groundwater, but utilized shallow soil water in differing amounts when available. Conclusions In systems with strong water potential gradients among soil strata, any species with access to a groundwater source is likely capable of HR; however, the magnitude of HR varies significantly by species, even among closely related taxa.

Recent interest in restoring longleaf pine ecosystems highlights the need for an improved understanding, appreciation, and consideration of longleaf pine’s unique life history compared to other southern pines. Longleaf pine ecosystems are considered more resilient than other southern pine ecosystems and the attributes that confer this resilience may be related to belowground carbon allocation patterns that occur during the seedling stage of its life history where it exists for years in a grass stage. The long residence time spent in the grass stage has impeded adoption of longleaf pine by landowners for decades, which has resulted in efforts to expedite emergence from the grass stage through silviculture, nursery production, and selection breeding techniques; however, expedited emergence may come at the expense of root and belowground carbohydrate storage reserve development—traits that purportedly enhance longleaf pine tree and ecosystem resilience relative to other southern pine species. Here, I caution that efforts to expedite longleaf pine emergence from the grass stage also consider the potential implications on belowground carbon allocation and identify research priorities to help facilitate the improvement of longleaf pine seedling silviculture, nursery production, and breeding in a way that ensures the maintenance of attributes linked to resilience.

The increasing demand for plant‐derived bioenergy is projected to expand tree plantations with intensive silviculture and improved tree genetics. These silvicultural practices result in faster stand development and canopy closure, which may also influence the systems' water dynamics. Here, we studied the evapotranspiration (ET) of a young (5 years old) intensively managed loblolly pine (Pinus taeda) stand and investigated the components of ET to determine its contribution to overall water use. We also compared ET with plantations that received less intensive management to determine whether our stand used more water. We used the eddy covariance method to estimate ecosystem‐level total ET (ETEC), while plot‐level estimates of ET (ETP) were obtained via soil lysimeters, sap flow sensors, and throughfall collectors, enabling measurement of the components of ET. Soil evaporation (Es) was the largest component of ETP (36%) over the course of the study, while transpiration and canopy interception accounted for 27% and 22%, respectively. Es decreased with stand development, while transpiration and canopy interception increased. Leaf area index (LAI) and precipitation were the most significant factors controlling ET and its components. Compared to previous studies in different sites that have similar age but lower LAI, our stand had higher water use. This high water use in the early stages of stand development was primarily due to high Es before the canopy was fully developed. While there are potential sources of uncertainty when comparing ETEC and the component fluxes in ETP, results from the two methods were not significantly different. This study had the advantage of using multiple methods to understand and verify the component processes that contribute to ET. Therefore, we recommend that multiple measurement techniques be used in the long‐term observation of ET, and in particular for the evaluation of the impact that intensively managed forests have on water resources in the southeastern United States.

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.