Robert O. Teskey

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.

Water and Forest Productivity

Abstract Water availability is a major factor influencing the distribution and productivity of the earths vegetation, but details of the mechanisms by which its effects are felt are not well understood. This is due in large part to the interactions between water and vegetation, such as through interception and change in leaf-area, which affect rates of canopy photosynthesis and transpiration. Physiological differences among species are not always directly translated to differences among stands, emphasizing the importance of climate and microclimate as controls. Leaf-area index ( L ) is a critical integrator of water availability and productivity, and changes in leaf-area, such as occur through thinning and understory control, may have dramatic effects on both. There is increasing evidence that L changes significantly within an annual cycle and from year to year, even in closed-canopy conifer stands. Consequently, the season and year in which a measurement of L is made may explain much of the variability noted before in its response to water availability and effects on productivity. Because carbon, water, and nutrient cycles are so closely coupled, simulation models that represent both direct and indirect relationships are useful tools for understanding and managing forest ecosystems.

Responses of tree species to heat waves and extreme heat events

The number and intensity of heat waves has increased, and this trend is likely to continue throughout the 21st century. Often, heat waves are accompanied by drought conditions. It is projected that the global land area experiencing heat waves will double by 2020, and quadruple by 2040. Extreme heat events can impact a wide variety of tree functions. At the leaf level, photosynthesis is reduced, photooxidative stress increases, leaves abscise and the growth rate of remaining leaves decreases. In some species, stomatal conductance increases at high temperatures, which may be a mechanism for leaf cooling. At the whole plant level, heat stress can decrease growth and shift biomass allocation. When drought stress accompanies heat waves, the negative effects of heat stress are exacerbated and can lead to tree mortality. However, some species exhibit remarkable tolerance to thermal stress. Responses include changes that minimize stress on photosynthesis and reductions in dark respiration. Although there have been few studies to date, there is evidence of within-species genetic variation in thermal tolerance, which could be important to exploit in production forestry systems. Understanding the mechanisms of differing tree responses to extreme temperature events may be critically important for understanding how tree species will be affected by climate change.

Seasonal Changes in Tissue Water Relations of Three Woody Species of the Quercus-Carya Forest Type

Tissue water relations of white and northern red oaks and mockernut hickory were studied during the growing season of 1979. In all species, osmotic potentials at full saturation and turgor loss point decreased during the period of leaf maturation; subsequently, osmotic potential responded to soil moisture availability. Estimates of the bulk modulus of elasticity and the relative water content at the turgor loss point revealed that white oak possessed less elastic leaf tissue than did either northern red oak or hickory. The bulk leaf pressure potential associated with the initiation of stomatal closure was lower in white oak and hickory (0.2 MPa) than in northern red oak (0.4 MPa) and remained seasonally constant, while total leaf water potential associated with stomatal closure was lower during periods of drought as osmotic potential decreased. The drought-tolerating behavior of white oak is consistent with its frequent occurrence and success in xeric habitats. Northern red oak and mockernut hickory exhibited responses more typical of drought-avoiding species, which would result in sustained turgor-mediated processes essential for growth and high competitive ability at moderate moisture stresses characteristic of more mesic habitats.

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.

Stem respiration and carbon dioxide efflux of young Populus deltoides trees in relation to temperature and xylem carbon dioxide concentration

Oxidative respiration is strongly temperature driven. However, in woody stems, efflux of CO2 to the atmosphere (EA), commonly used to estimate the rate of respiration (RS), and stem temperature (Tst) have often been poorly correlated, which we hypothesized was due to transport of respired CO2 in xylem sap, especially under high rates of sap flow (fs). To test this, we measured EA, Tst, fs and xylem sap CO2 concentrations ([CO2*]) in 3-year-old Populus deltoides trees under different weather conditions (sunny and rainy days) in autumn. We also calculated RS by mass balance as the sum of both outward and internal CO2 fluxes and hypothesized that RS would correlate better with Tst than EA. We found that EA sometimes correlated well with Tst, but not on sunny mornings and afternoons or on rainy days. When the temperature effect on EA was accounted for, a clear positive relationship between EA and xylem [CO2*] was found. [CO2*] varied diurnally and increased substantially at night and during periods of rain. Changes in [CO2*] were related to changes in fs but not Tst. We conclude that changes in both respiration and internal CO2 transport altered EA. The dominant component flux of RS was EA. However, on a 24-h basis, the internal transport flux represented 9–18% and 3–7% of RS on sunny and rainy days, respectively, indicating that the contribution of stem respiration to forest C balance may be larger than previously estimated based on EA measurements. Unexpectedly, the relationship between RS and Tst was sometimes weak in two of the three trees. We conclude that in addition to temperature, other factors such as water deficits or substrate availability exert control on the rate of stem respiration so that simple temperature functions are not sufficient to predict stem respiration.

CO2 transported in xylem sap affects CO2 efflux from Liquidambar styraciflua and Platanus occidentalis stems, and contributes to observed wound respiration phenomena

The [CO2] in the xylem of tree stems is typically two to three orders of magnitude greater than atmospheric [CO2]. In this study, xylem [CO2] was experimentally manipulated in saplings of sycamore (Platanus occidentalis L.) and sweetgum (Liquidambar styraciflua L.) by allowing shoots severed from their root systems to absorb water containing [CO2] ranging from 0.04% to 14%. The effect of xylem [CO2] on CO2 efflux to the atmosphere from uninjured and mechanically injured, i.e., wounded, stems was examined. In both wounded and unwounded stems, and in both species, CO2 efflux was directly proportional to xylem [CO2], and increased 5-fold across the range of xylem [CO2] produced by the [CO2] treatment. Xylem [CO2] explained 76–77% of the variation in pre-wound efflux. After wounding, CO2 efflux increased substantially but remained directly proportional to internal stem [CO2]. These experiments substantiated our previous finding that stem CO2 efflux was directly related to internal xylem [CO2] and expanded our observations to two new species. We conclude that CO2 transported in the xylem may confound measurements of respiration based on CO2 efflux to the atmosphere. This study also provided evidence that the rapid increase in CO2 efflux observed after tissues are excised or injured is likely the result of the rapid diffusion of CO2 from the xylem, rather than an actual increase in the rate of respiration of wounded tissues.

Resistance to radial CO2 diffusion contributes to between-tree variation in CO2 efflux of Populus deltoides stems

Rates of CO2 efflux of stems and branches are highly variable among and within trees and across stands. Scaling factors have only partially succeeded in accounting for the observed variations. In this study, the resistance to radial CO2 diffusion was quantified for tree stems of an eastern cottonwood (Populus deltoides Bartr. ex Marsh.) clone by direct manipulation of the CO2 concentration ([CO2]) of xylem sap under controlled conditions. Tree-specific linear relationships between rates of stem CO2 efflux (JO) and xylem [CO2] were found. The resistance to radial CO2 diffusion differed 6-fold among the trees and influenced the balance between the amount of CO2 retained in the xylem v. that which diffused to the atmosphere. Therefore, we hypothesised that variability in the resistance to radial CO2 diffusion might be an overlooked cause for the inconsistencies and large variations in woody tissue CO2 efflux. It was found that transition from light to dark conditions caused a rapid increase in JO and xylem [CO2], both in manipulated trees and in an intact tree with no sap manipulation. This resulted in an increased resistance to radial CO2 diffusion during the dark, at least for trees with smaller daytime resistances. Stem diameter changes measured in the intact tree supported the idea that higher actual respiration rates occurred at night owing to higher metabolism in relation to an improved water status and higher turgor pressure.

Tree size- and age-related changes in leaf physiology and their influence on carbon gain

Understanding how leaf-level physiology changes with tree size and age is important for scaling single leaf measurements to the whole plant and stand level and for quantifying carbon fluxes from forest ecosystems. This chapter reviews what is known about the influences of tree height and age on gas exchange and foliar structure in both gymnosperm and angiosperm trees. We address how the key physiological processes, photosynthesis, respiration and stomatal conductance vary with tree height and age. To help explain the observed patterns, the underlying factors that can be responsible for the changes in leaf physiology are assessed, including tree size- and age-related trends in foliar anatomy, morphology and chemistry. In addition to modifications in foliar morphology and chemistry, biochemical limitations to net assimilation rates associated with the diffusion of carbon dioxide from the atmosphere to the sites of carboxylation are examined. Our review emphasizes that a variety of factors collectively are responsible for tree height- and age-related decline in net photosynthetic rates, and that the importance of different limitations varies for different species and between gymnosperm and angiosperm trees. While there is still much to be learned, what is clear from our current understanding is that more integrated studies that consider the simultaneous roles of leaf structure, chemistry and stomatal and mesophyll factors are needed to disentangle and assign importance to the various factors responsible for decreases in carbon gain with tree age and size.

Higher growth temperatures decreased net carbon assimilation and biomass accumulation of northern red oak seedlings near the southern limit of the species range

If an increase in temperature will limit the growth of a species, it will be in the warmest portion of the species distribution. Therefore, in this study we examined the effects of elevated temperature on net carbon assimilation and biomass production of northern red oak (Quercus rubra L.) seedlings grown near the southern limit of the species distribution. Seedlings were grown in chambers in elevated CO(2) (700 µmol mol(-1)) at three temperature conditions, ambient (tracking diurnal and seasonal variation in outdoor temperature), ambient +3 °C and ambient +6 °C, which produced mean growing season temperatures of 23, 26 and 29 °C, respectively. A group of seedlings was also grown in ambient [CO(2)] and ambient temperature as a check of the growth response to elevated [CO(2)]. Net photosynthesis and leaf respiration, photosynthetic capacity (V(cmax), J(max) and triose phosphate utilization (TPU)) and chlorophyll fluorescence, as well as seedling height, diameter and biomass, were measured during one growing season. Higher growth temperatures reduced net photosynthesis, increased respiration and reduced height, diameter and biomass production. Maximum net photosynthesis at saturating [CO(2)] and maximum rate of electron transport (J(max)) were lowest throughout the growing season in seedlings grown in the highest temperature regime. These parameters were also lower in June, but not in July or September, in seedlings grown at +3 °C above ambient, compared with those grown in ambient temperature, indicating no impairment of photosynthetic capacity with a moderate increase in air temperature. An unusual and potentially important observation was that foliar respiration did not acclimate to growth temperature, resulting in substantially higher leaf respiration at the higher growth temperatures. Lower net carbon assimilation was correlated with lower growth at higher temperatures. Total biomass at the end of the growing season decreased in direct proportion to the increase in growth temperature, declining by 6% per 1 °C increase in mean growing season temperature. Our observations suggest that increases in air temperature above current ambient conditions will be detrimental to Q. rubra seedlings growing near the southern limit of the species range.