Relating coarse root respiration to root diameter in clonal Eucalyptus stands in the Republic of the Congo

Root respiration is an important component of the carbon balance of a forest ecosystem. We measured CO2 efflux of excised fine roots and intact coarse roots in 3-, 4- and 13-year-old Eucalyptus stands in the region of Pointe-Noire, Republic of the Congo. A transportable and adaptable closed chamber gas exchange system directly measured CO2 efflux of roots from 0.5 to 32 mm in diameter. Fluxes were corrected for measurement system leaks and normalized to a reference temperature of 30 degrees C. Mean fine root respiration rates at the reference temperature varied between 8.5 and 10.8 micromol CO2 kg(-1) s(-1) depending on the stand. Coarse root respiration was strongly negatively correlated to root diameter. We propose a model based on a radial gradient of respiratory activity within the root to simulate the exponential decrease in respiration with diameter. Although many sources of uncertainty in the measurements remain, as discussed in this paper, these results provide a basis for scaling up organ-level root respiration measurements to the tree and stand levels.     

Elevational change in woody tissue CO2 efflux in a tropical mountain rain forest in southern Ecuador

Much uncertainty exists about the magnitude of woody tissue respiration and its environmental control in highly diverse tropical moist forests. In a tropical mountain rain forest in southern Ecuador, we measured the apparent diurnal gas exchange of stems and coarse roots (diameter 1-4 cm) of trees from representative families along an elevational transect with plots at 1050, 1890 and 3050 m a.s.l. Mean air temperatures were 20.8, 17.2 and 10.6 degrees C, respectively. Stem and root CO(2) efflux of 13 to 21 trees per stand from dominant families were investigated with an open gas exchange system while stand microclimate was continuously monitored. Substantial variation in respiratory activity among and within species was found at all sites. Mean daily CO(2) release rates from stems declined 6.6-fold from 1.38 micromol m(-2) s(-1) at 1050 m to 0.21 micromol m(-2) s(-1) at 3050 m. Mean daily CO(2) release from coarse roots decreased from 0.35 to 0.20 micromol m(-2) s(-1) with altitude, but the differences were not significant. There was, thus, a remarkable shift from a high ratio of stem to coarse root respiration rates at the lowest elevation to an apparent equivalence of stem and coarse root CO(2) efflux rates at the highest elevation. We conclude that stem respiration, but not root respiration, greatly decreases with elevation in this transect, coinciding with a substantial decrease in relative stem diameter increment and a large increase in fine and coarse root biomass production with elevation.     

Foliage, fine-root, woody-tissue and stand respiration in Pinus radiata in relation to nitrogen status

We measured respiration of 20-year-old Pinus radiata D. Don trees growing in control (C), irrigated (I), and irrigated + fertilized (IL) stands in the Biology of Forest Growth experimental plantation near Canberra, Australia. Respiration was measured on fully expanded foliage, live branches, boles, and fine and coarse roots to determine the relationship between CO(2) efflux, tissue temperature, and biomass or nitrogen (N) content of individual tissues. Efflux of CO(2) from foliage (dark respiration at night) and fine roots was linearly related to biomass and N content, but N was a better predictor of CO(2) efflux than biomass. Respiration (assumed to be maintenance) per unit N at 15 degrees C and a CO(2) concentration of 400 micro mol mol(-1) was 1.71 micro mol s(-1) mol(-1) N for foliage and 11.2 micro mol s(-1) mol(-1) N for fine roots. Efflux of CO(2) from stems, coarse roots and branches was linearly related to sapwood volume (stems) or total volume (branches + coarse roots) and growth, with rates for maintenance respiration at 15 degrees C ranging from 18 to 104 micro mol m(-3) s(-1). Among woody components, branches in the upper canopy and small diameter coarse roots had the highest respiration rates. Stem maintenance respiration per unit sapwood volume did not differ among treatments. Annual C flux was estimated by summing (1) dry matter production and respiration of aboveground components, (2) annual soil CO(2) efflux minus aboveground litterfall, and (3) the annual increment in coarse root biomass. Annual C flux was 24.4, 25.3 and 34.4 Mg ha(-1) year(-1) for the C, I and IL treatments, respectively. Total belowground C allocation, estimated as the sum of (2) and (3) above, was equal to the sum of root respiration and estimated root production in the IL treatment, whereas in the nutrient-limited C and I treatments, total belowground C allocation was greater than the sum of root respiration and estimated root production, suggesting higher fine root turnover or increased allocation to mycorrhizae and root exudation. Carbon use efficiency, the ratio of net primary production to assimilation, was similar among treatments for aboveground tissues (0.43-0.50). Therefore, the proportion of assimilation used for construction and maintenance respiration on an annual basis was also similar among treatments.     

Effects of leaf,shoot and fruit development on photosynthesis of lychee trees (Litchi chinensis)

Changes in gas exchange with leaf age and fruit growth were determined in lychee trees (Litchi chinensis Sonn.) growing in subtropical Queensland (27 degrees S). Leaves expanded in a sigmoid pattern over 50 days during spring, with net CO2 assimilation (A) increasing from -4.1 +/- 0.9 to 8.3 +/- 0.5 micromol m-2 s-1 as the leaves changed from soft and red, to soft and light green, to hard and dark green. Over the same period, dark respiration (Rd) decreased from 5.0 +/- 0.8 to 2.0 +/- 0.1 micromol CO2 m-2 s-1. Net CO2 assimilation was above zero about 30 days after leaf emergence or when the leaves were half fully expanded. Chlorophyll concentrations increased from 0.7 +/- 0.2 mg g-1 in young red leaves to 10.3 +/- 0.7 mg g-1 in dark green leaves, along with stomatal conductance (gs, from 0.16 +/- 0.09 to 0.47 +/- 0.17 mol H2O m-2 s-1). Fruit growth was sigmoidal, with maximum values of fresh mass (29 g), dry mass (6 g) and fruit surface area (39 cm2) occurring 97 to 115 days after fruit set. Fruit CO2 exchange in the light (Rl) and dark (Rd) decreased from fruit set to fruit maturity, whether expressed on a surface area (10 to 3 micromol CO2 m-2 s-1 and 20 to 3 micromol CO2 m-2 s-1, respectively) or on a dry mass basis (24 to 2 nmol CO2 g-1 s-1 and 33 to 2 nmol CO2 g-1 s-1, respectively). Photosynthesis never exceeded respiration, however, the difference between Rl and Rd was greatest in young green fruit (4 to 8 micromol CO2 m-2 s-1). About 90% of the carbon required for fruit growth was accounted for in the dry matter of the fruit, with the remainder required for respiration. Fruit photosynthesis contributed about 3% of the total carbon requirement of the fruit over the season. Fruit growth was mainly dependent on CO2 assimilation in recently expanded dark green leaves.     

Effects of soil temperature and time of decapitation on sucker initiation of intact Populus tremuloides root systems

Abstract

In a growth chamber experiment, root suckering of aspen (Populus tremuloides Michx.) was assessed in relation to timing of cutting and soil temperature. Aspen seedlings were grown in large pots for 3 years before experimentation. In a 2×2 factorial experiment, 3-year-old seedlings were cut at the end of the dormant period or after leaf flush and grown at two soil temperatures (8 or 20°C) for 39 days. Root systems were evaluated for suckering response and carbohydrate reserve status. There were no differences between the two soil temperatures and times of cut in the number of sucker buds initiated on the roots, but the number of buds that developed into suckers was much greater at 20°C. Cutting the dormant seedlings delayed suckering by nearly a week, resulting in smaller suckers at the time of harvest. However, cutting the seedlings when dormant produced almost twice the number of suckers than when cutting occurred after leaf-out. Total non-structural carbohydrates (TNC) of roots declined from 35.6% of dry weight at the end of the dormant season to 21.6% at the time of leaf-out, but there were no differences between the soil temperature and timing of cut treatments. After the 39 day growth period, root systems had 7% lower root TNC in the 20°C treatment than in the 8°C treatment, likely to support the development of the emerging suckers and higher respiration demands.     

Patterns of root respiration rates and morphological traits in 13 tree species in a tropical forest

The root systems of forest trees are composed of different diameters and heterogeneous physiological traits. However, the pattern of root respiration rates from finer and coarser roots across various tropical species remains unknown. To clarify how respiration is related to the morphological traits of roots, we evaluated specific root respiration and its relationships to mean root diameter (D) of various diameter and root tissue density (RTD; root mass per unit root volume; gcm(-3)) and specific root length (SRL; root length per unit root mass; mg(-1)) of the fine roots among and within 14 trees of 13 species from a primary tropical rainforest in the Pasoh Forest Reserve in Peninsular Malaysia. Coarse root (2-269mm) respiration rates increased with decreasing D, resulting in significant relationships between root respiration and diameter across species. A model based on a radial gradient of respiration rates of coarse roots simulated the exponential decrease in respiration with diameter. The respiration rate of fine roots (<2mm) was much higher and more variable than those of larger diameter roots. For fine roots, the mean respiration rates for each species increased with decreasing D. The respiration rates of fine roots declined markedly with increasing RTD and increased with increasing SRL, which explained a significant portion of the variation in the respiration among the 14 trees from 13 species examined. Our results indicate that coarse root respiration in tree species follows a basic relationship with D across species and that most of the variation in fine root respiration among species is explained by D, RTD and SRL. We found that the relationship between root respiration and morphological traits provides a quantitative basis for separating fine roots from coarse roots and that the pattern holds across different species.     

Exposure to elevated carbon dioxide concentration in the dark lowers the respiration quotient of Vitis cane wood

Cane cuttings of the grapevine rootstock Vitis rupestris Scheele x V. riparia Michx. cv. 3309 Couderc were brought out of endodormancy by warming at 30 degrees C. Cane pieces (12 to 13 cm long) with nodes containing a primary bud were placed in a gas exchange system and monitored for net respiratory fluxes of CO2 and O2. Grapevine respiration rates expressed on a wood volume basis were 1.4 to 3.4 mmol CO2 or O2 m-3s-1, which is higher than stem respiration rates reported for many other woody taxa but similar to rates measured for ecodormant buds of other Vitis species. Passive water loss from canes was 0.7 to 1.2 mmol H2O m-3s-1. During a 7-day period, nonstructural carbohydrate concentrations in cane wood declined only slightly, whereas sucrose was nearly completely consumed. When ambient CO2 concentration ([CO2]) was raised from 300 to 750 micro molmol-1 and then 2000 micromol mol-1, net CO2 exchange rates declined by 5.9 +/- 0.6 and then 11.0 +/- 0.6%, whereas net O2 consumption rates remained about constant. The mean respiration quotient (net CO2/O2 flux) for canes with intact ecodormant buds was 0.99 +/- 0.03 when the [CO2] was 300 micromol mol-1, and decreased to 0.87 +/- 0.03 and 0.088 +/- 0.02 when the [CO2] was increased to 750 and 2000 micromol mol-1, respectively. The results support the hypothesis that, in Vitis canes, inhibition of respiratory CO2 efflux in response to high [CO2] is an indirect consequence of non-photosynthetic carboxylation reactions, and not a result of inhibition of respiratory metabolism.     

Spatial distribution of leaf morphological and physiological characteristics in relation to local radiation regime within the canopies of 3-year-old Populus clones in coppice culture

Spatial distributions of leaf characteristics relevant to photosynthesis were compared within high-density coppice canopies of Populus spp. of contrasting genetic origin. We studied three clones representative of the range in growth potential, leaf morphology, coppice and canopy structure: Clone Hoogvorst (Hoo) (Populus trichocarpa Torr. & Gray x Populus deltoides Bartr. & Marsh), Clone Fritzi Pauley (Fri) (Populus trichocarpa Torr. & Gray) and Clone Wolterson (Wol) (Populus nigra L.). Leaf area index ranged from 2.7 (Fri and Wol) to 3.8 (Hoo). The clones exhibited large vertical variation in leaf area density (0.02-1.42 m2 m-3). Leaf dry mass per unit leaf area (DM(A)) increased with increasing light in Clones Hoo and Fri, from about 56 g m-2 at the bottom of the canopy to 162 g m-2 at the top. In Clone Wol, DM(A) varied only from 65 to 100 g m-2, with no consistent relationship with respect to light. Conversely, nitrogen concentration on a mass basis was nearly constant (around 1.3-2.1%) within the canopies of Clones Hoo and Fri, but increased strongly with light in Clone Wol, from 1.4% at the bottom of the canopy to 4.1% at the top. As a result, nitrogen per unit leaf area (N(A)) increased with light in the canopies of all clones, from 0.9 g m-2 at the bottom to 2.9 g m-2 at the top. Although a single linear relationship described the dependence of maximum carboxylation rate (17-93 micromol CO2 m-2 s-1) or electron transport capacity (45-186 micromol electrons m-2 s-1) on N(A), for all clones, Clone Wol differed from Clones Hoo and Fri by exhibiting a higher dark respiration rate at low N(A) (1.8 versus 0.8 micromol CO2 m-2 s-1).     

Interactive effects of elevated CO2 concentration and nitrogen supply on partitioning of newly fixed 13C and 15N between shoot and roots of pedunculate oak seedlings (Quercus robur)

Pedunculate oak (Quercus robur L.) seedlings were grown for 3 or 4 months (second- and third-flush stages) in greenhouses at two atmospheric CO2 concentrations ([CO2]) (350 or 700 micromol mol(-1)) and two nitrogen fertilization regimes (6.1 or 0.61 mmol N l(-1) nutrient solution). Combined effects of [CO2] and nitrogen fertilization on partitioning of newly acquired carbon (C) and nitrogen (N) were assessed by dual 13C and 15N short-term labeling of seedlings at the second- or third-flush stage of development. In the low-N treatment, root growth, but not shoot growth, was stimulated by elevated [CO2], with the result that shoot/root biomass ratio declined. At the second-flush stage, overall seedling biomass growth was increased (13%) by elevated [CO2] regardless of N fertilization. At the third-flush stage, elevated [CO2] increased growth sharply (139%) in the high-N but not the low-N treatment. Root/shoot biomass ratios were threefold higher in the low-N treatment relative to the high-N treatment. At the second-flush stage, leaf area was 45-51% greater in the high-N treatment than in the low-N treatment. At the-third flush stage, there was a positive interaction between the effects of N fertilization and [CO2] on leaf area, which was 93% greater in the high-N/elevated [CO2] treatment than in the low-N/ambient [CO2] treatment. Specific leaf area was reduced (17-25%) by elevated [CO2], whereas C and N concentrations of seedlings increased significantly in response to either elevated [CO2] or high-N fertilization. At the third-flush stage, acquisition of C and N per unit dry mass of leaf and fine root was 51 and 77% greater, respectively, in the elevated [CO2]/high-N fertilization treatment than in the ambient [CO2]/low-N fertilization treatment. However, there was dilution of leaf N in response to elevated [CO2]. Partitioning of newly acquired C and N between shoot and roots was altered by N fertilization but not [CO2]. More newly acquired C and N were partitioned to roots in the low-N treatment than in the high-N treatment.     

Stem growth and respiration in loblolly pine plantations differing in soil resource availability

Stem respiration and growth in 10-year-old loblolly pine (Pinus taeda L.) plantations were measured monthly during the third year of fertilization and irrigation treatments to determine whether soil resource availability differentially altered growth and respiration in stem tissue. Fertilized trees had significantly greater stem biomass, stem nitrogen concentration ([N]) and growth rate than unfertilized trees. Stem respiration (Rt) was significantly greater in fertilized trees when expressed on a per unit surface area (Rt,a, micromol CO2 m-2 s-1), sapwood volume (Rt,v, micromol CO2 m-3 s-1), or mass (Rt,w, nmol CO2 g-1 s-1) basis; however, there was no difference between treatments when expressed as a function of stem N content (Rt,n, micromol CO2 (mol N)-1 s-1). Irrigation had no significant effect on Rt or annual stem growth. Daily total respiration (Rd, mol CO2 m-2 day-1) and stem diameter growth both had a seasonal bimodal pattern with peaks in early spring and midsummer. Stem [N] declined significantly during the growing season. Stem growth rate and [N] explained 75% of the seasonal variation in temperature-normalized Rt,a. The mature tissue method was used to partition total stem respiration (Rt) into maintenance (Rm) and growth (Rg) components. There was a linear correlation between winter Rt,v, a measure of basal Rm, and sapwood N content; however, Rt,v per unit N was greater in January before diameter growth started than in the following December after growth ceased, indicating that Rt,v declined as stem diameter increased. Consequently, estimates of annual maintenance respiration (RM) based on January data were 44% higher than estimates based on December data. Growth respiration was correlated with stem growth rate (r2 = 0.55). The growth respiration coefficient (rg)-the slope of the relationship between Rg and stem growth rate-was 0.24. Respiration accounted for 37% of annual stem carbon budget. Stem carbon-use efficiency (CUE)-the ratio of stem growth to stem growth plus respiration-averaged 0.63 and was unaffected by fertilization.     

Carbon translocation patterns associated with new root proliferation during episodic growth of transplanted Quercus rubra seedlings

Patterns of carbon allocation in northern red oak (Quercus rubra L.), characterized by episodic growth through recurrent single-season flushing, vary by growth stage. To examine post-transplant timing and carbohydrate sources for new root growth, dormant, bare-root, half-sibling northern red oak seedlings were transplanted to pots and placed in a favorable growth chamber environment. Unlabeled seedlings were harvested at transplant and at the bud swell stage. After leaf emergence, seedlings were exposed to (14)CO(2) at the linear shoot, linear leaf or lag growth stages. Seedlings were then placed in a growth room for 48 h to allow for translocation of (14)C-labeled current photosynthate and its stabilization in sink component plant parts. Seedlings were subsequently harvested and tissue (14)C:(12)C ratio analyzed. New root growth began during the linear shoot growth stage. However, no increase in (14)C:(12)C ratio was found in new roots until the linear leaf and lag growth stages, indicating a downward shift in translocation of current photosynthate to fuel new root growth. In old roots, (14)C:(12)C ratio increased at the lag stage. Our results indicate that both stored carbohydrates and current photosynthate contribute to new root growth of transplanted northern red oak seedlings; stored carbohydrates promote initial new root proliferation, whereas current photosynthate assumes a greater role as new leaves mature and the flush terminates. Optimizing nursery practices to increase carbohydrate reserves may reduce the time required to establish root-soil contact and facilitate early post-planting survival.     

Influence of tree internal N status on uptake and translocation of C and N in beech: a dual 13C and 15N labeling approach

Influence of plant internal nitrogen (N) stocks on carbon (C) and N uptake and allocation in 3-year-old beech (Fagus sylvatica L.) was studied in two 15N- and 13C-labeling experiments. In the first experiment, trees were grown in sand and received either no N nutrition (-N treatment) or 4 mM unlabeled N (+N treatment) for 1 year. The -N- and +N-pretreated trees were then supplied with 4 mM 15N and grown in a 13CO2 atmosphere for 24 weeks. In the second experiment, trees were pretreated with 4 mM 15N for 1 year and then supplied with unlabeled N for 24 weeks and the remobilization of stored 15N was monitored. On the whole-plant level, uptake of new C was significantly reduced in -N-pretreated trees; however, partitioning of new C was not altered, although there was a trend toward increased belowground respiration. The amount of N taken up was not influenced by N nutrition in the previous year. In +N-pretreated trees, partitioning of new N was dominated by the fine roots (59.7% at Week 12), whereas in -N-pretreated trees, partitioning of new N favored stem, coarse roots and fine roots (24, 21 and 31.9%, respectively, at Week 12), indicating the formation of N stores. The contribution of previous-year N to leaf N was about 15%. The N remobilized for leaf formation had been stored in stem and coarse roots. We conclude that, within a growing season, the growth of beech is strongly determined by the availability of tree internal N stores, whereas the current N supply is of less importance.     

Temperature responses of growth and wood anatomy in European beech saplings grown in different carbon dioxide concentrations

Effects of temperature on growth and wood anatomy were studied in young European beech (Fagus sylvatica L.) grown in 7-l pots for 2.5 years in field-phytotron chambers supplied with an ambient (approximately 400 micromol mol-1) or elevated (approximately 700 micromol mol-1) carbon dioxide concentration ([CO2]). Temperatures in the chambers ranged in increments of 2 degrees C from -4 degrees C to +4 degrees C relative to the long-term mean monthly (day and night) air temperature in Berlin-Dahlem. Soil was not fertilized and soil water and air humidity were kept constant. Data were evaluated by regression analysis. At final harvest, stem diameter was significantly greater at increased temperature (Delta1 degrees C: 2.4%), stems were taller (Delta1 degrees C: 8.5%) and stem mass tree-1 (Delta1 degrees C: 10.9%) and leaf area tree-1(Delta1 degrees C: 6.5%) were greater. Allocation pattern was slightly influenced by temperature: leaf mass ratio and leaf area ratio decreased with increasing temperature (Delta1 degrees C: 2.3% and 2.2% respectively), whereas stem mass/total mass increased (Delta1 degrees C: 2.1%). Elevated [CO2] enhanced height growth by 8.8% and decreased coarse root mass/total mass by 10.3% and root/shoot ratio by 11.7%. Additional carbon was mainly invested in aboveground growth. At final harvest a synergistic interaction between elevated [CO2] and temperature yielded trees that were 3.2% taller at -4 degrees C and 12.7% taller at +4 degrees C than trees in ambient [CO2]. After 2.5 seasons, cross-sectional area of the oldest stem part was approximately 32% greater in the +4 degrees C treatment than in the -4 degrees C treatment, and in the last year approximately 67% more leaf area/unit tree ring area was produced in the highest temperature regime compared with the lowest. Elevated [CO2] decreased mean vessel area of the 120 largest vessels per mm2 by 5.8%, causing a decrease in water conducting capacity. There was a positive interaction between temperature and elevated [CO2] for relative vessel area, which was approximately 38% higher at +4 degrees C than at -4 degrees C in elevated [CO2] compared with ambient [CO2]. Overall, temperature had a greater effect on growth than [CO2], but elevated [CO2] caused quantitative changes in wood anatomy.     

Canopy photosynthesis and respiration of kiwifruit (Actinidia deliciosa var. deliciosa) vines growing in the field

Net CO(2) assimilation (A) for canopies of kiwifruit (Actinidia deliciosa var. deliciosa) vines enclosed in a whole-canopy cuvette was measured continuously for three periods of 15-20 days during late summer, near Hamilton, New Zealand (latitude 38.2 degrees S). Canopy A showed an asymptotic response to incident radiation (PAR), saturating at about 1300 micromol m(-2) s(-1) for one vine and about 800 micromol m(-2) s(-1) for two other vines. Radiation interception at low solar angles and low leaf area apparently limited the response of A to PAR. Radiation saturated rates of A were 25-30 micromol CO(2) m(-2) s(-1) for one vine, and 12-18 micromol CO(2) m(-2) s(-1) for two other vines. At any PAR, canopy A was often lower in the afternoon than in the morning. Canopy respiration averaged 8.9 micromol CO(2) m(-2) s(-1) at 12 degrees C, but increased only 24-34% over the range 7-17 degrees C. Net daily C gains for the whole canopy, calculated as the temporal integral of A, ranged from -0.8 g C m(-2) for a cloudy day (PAR 相似文献   

Belowground carbon pools and processes in different age stands of Douglas-fir

Forest floor material and soil organic matter may act as both a source and a sink in global CO2 cycles. Thus, the ecosystem processes controlling these pools are central to understanding the transfers of carbon (C) between the atmosphere and terrestrial systems. To examine these ecosystem processes, the effect of stand age on temporal carbon source-sink relationships was examined in 20-year-old, 40-year-old and old-growth stands of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in the Cascade Mountains of south-central Washington State. Belowground C and nitrogen (N) storage and soil respiration were measured. In addition, nylon mesh bags containing homogenized soils from each site were buried at the respective sites to quantify root ingrowth and potential C sequestration and loss. The sites supporting the 20- and 40-year-old stands had soil C stores reflecting the C contributions from logging residue, coarse woody debris and stumps left after harvest. Because the N-fixer red alder (Alnus rubra Bong.) comprised 33% of the 40-year-old stand, this site had significantly greater concentrations and pools of N in the forest floor than sites without red alder. This N-rich site had consistently lower soil CO2 efflux rates during the growing season than the sites supporting the 20-year-old and old-growth stands. Estimated annual soil C efflux was 1367, 883 and 1194 g m-2 for the sites supporting the 20-, 40- and old-growth stands, respectively. These values are higher than previously reported values. Root ingrowth was significantly less in the 40-year-old stand than in the 20-year-old stand, and both young stands showed markedly less fine root growth than the old-growth stand. At the sites supporting the young stands, C and N were lost from the soil bags, whereas there was an increase in C and N in the soil bags at the site supporting the old-growth stand. The fine root growth and soil respiration data support the hypothesis that belowground C allocation decreases with increasing fertility. Quantification of the source-sink relationship of soil C at the three stands based on litterfall, relative root ingrowth and soil respiration measurements was compromised because of significant CO2 flux from decaying organic matter in the young stands.     

14C Allocation in tree-soil systems

We studied whole-tree C allocation with special emphasis on the quantification of C allocation to roots and root respiration. To document seasonal patterns of C allocation, 2-year-old hybrid poplar trees greater than 3 m tall were labeled with (14)CO(2) in a large Plexiglas chamber in the field, in July and September. Climate and CO(2) concentration were controlled to track ambient conditions during labeling. Individual tree canopy CO(2) assimilation averaged 3.8 micromol CO(2) m(-2) s(-1) (12.9 g C day(-1) tree(-1)) in July and 6.2 micromol CO(2) m(-2) s(-1) (9.8 g C day(-1) tree(-1)) in September. Aboveground dark respiration was 12% of net daytime C fixation in July and 15% in September. Specific activity of root-soil respiration peaked 2 days after labeling and stabilized to less than 5% of maximum 2 weeks later. Low specific activity of root-soil respiration and a labeled pool of root C demonstrated that current photosynthate was the primary source of C for root growth and maintenance during the growing season. Root respiration averaged 20% of total soil respiration in both July and September based on the proportion of labeled C respired to labeled C fixed. In July, 80% of the recovered (14)C was found above ground and closely resembled the weight distribution of the growing shoot. By September, 51% of the recovered (14)C was in the root system and closely resembled the weight distribution of different size classes of roots. The finding that the distribution of biomass and (14)C were similar verified that the C introduced during labeling followed normal seasonal translocation pathways. Results are compared to smaller scale labeling studies and the suitability of the approach for studying long-term C fluxes is discussed.     

Interspecific and environmentally induced variation in foliar dark respiration among eighteen southeastern deciduous tree species

We measured variations in leaf dark respiration rate (Rd) and leaf nitrogen (N) across species, canopy light environment, and elevation for 18 co-occurring deciduous hardwood species in the southern Appalachian mountains of western North Carolina. Our overall objective was to estimate leaf respiration rates under typical conditions and to determine how they varied within and among species. Mean dark respiration rate at 20 degrees C (Rd,mass, micromol CO2 per kg leaf dry mass per s) for all 18 species was 7.31 micromol per kg per s. Mean Rd,mass of individual species varied from 5.17 micromol per kg per s for Quercus coccinea Muenchh. to 8.25 micromol per kg per s for Liriodendron tulipifera L. Dark respiration rate varied by leaf canopy position and was higher in leaves collected from high-light environments. When expressed on an area basis, dark respiration rate (Rd,area, micromol CO2 per kg leaf dry area per s) showed a strong linear relationship with the predictor variables leaf nitrogen (Narea, g N per square m leaf area) and leaf structure (LMA, g leaf dry mass per square m leaf area) (r squared = 0.62). This covariance was largely a result of changes in leaf structure with canopy position; smaller thicker leaves occur at upper canopy positions in high-light environments. Mass-based expression of leaf nitrogen and dark respiration rate showed that nitrogen concentration (Nmass, mg N per g leaf dry mass) was only moderately predictive of variation in Rd,mass for all leaves pooled (r squared = 0.11), within species, or among species. We found distinct elevational trends, with both Rd,mass and Nmass higher in trees originating from high-elevation, cooler growth environments. Consideration of interspecies differences, vertical gradients in canopy light environment, and elevation, may improve our ability to scale leaf respiration to the canopy in forest process models.     

Fine root respiration in the mangrove Rhizophora mangle over variation in forest stature and nutrient availability

Root respiration uses a significant proportion of photosynthetically fixed carbon (C) and is a globally important source of C liberated from soils. Mangroves, which are an important and productive forest resource in many tropical and subtropical countries, sustain a high ratio of root to shoot biomass which may indicate that root respiration is a particularly important component in mangrove forest carbon budgets. Mangroves are often exposed to nutrient pollution from coastal waters. Here we assessed the magnitude of fine root respiration in mangrove forests in Belize and investigated how root respiration is influenced by nutrient additions. Respiration rates of excised fine roots of the mangrove, Rhizophora mangle L., were low (4.01 +/- 0.16 nmol CO(2) g(-1) s(-1)) compared to those measured in temperate tree species at similar temperatures. In an experiment where trees where fertilized with nitrogen (N) or phosphorus (P) in low productivity dwarf forests (1-2 m height) and more productive, taller (4- 7 m height) seaward fringing forests, respiration of fine roots did not vary consistently with fertilization treatments or with forest stature. Fine roots of taller fringe trees had higher concentrations of both N and P compared to dwarf trees. Fertilization with P enhanced fine root P concentrations in both dwarf and fringe trees, but reduced root N concentrations compared to controls. Fertilization with N had no effect on root N or P concentrations. Unlike photosynthetic C gain and growth, which is strongly limited by P availability in dwarf forests at this site, fine root respiration (expressed on a mass basis) was variable, but showed no significant enhancements with nutrient additions. Variation in fine root production and standing biomass are, therefore, likely to be more important factors determining C efflux from mangrove sediments than variations in fine root respiration per unit mass.     

Effects of light availability on leaf gas exchange and expansion in lychee (Litchi chinensis)

Effects of photosynthetic photon flux density (PPFD) on leaf gas exchange of lychee (Litchi chinensis Sonn.) were studied in field-grown "Kwai May Pink" and "Salathiel" orchard trees and young potted "Kwai May Pink" plants during summer in subtropical Queensland (27 degrees S). Variations in PPFD were achieved by shading the trees or plants 1 h before measurement at 0800 h. In a second experiment, potted seedlings of "Kwai May Pink" were grown in a heated greenhouse in 20% of full sun (equivalent to maximum noon PPFD of 200 micromol m(-2)xs(-1)) and their growth over three flush cycles was compared with seedlings grown in full sun (1080 micromol m(-2)xs(-1)). Young potted plants of "Kwai May Pink" were also grown outdoors in artificial shade that provided 20, 40, 70 or 100% of full sun (equivalent to maximum PPFDs of 500, 900, 1400 and 2000 micromol m(-2)xs(-1)) and measured for shoot extension and leaf area development over one flush cycle. Net CO2 assimilation increased asymptotically in response to increasing PPFD in both orchard trees and young potted plants. Maximum rates of CO2 assimilation (11.9 +/- 0.5 versus 6.3 +/- 0.2 micromol CO2 m(-2) s(-1)), dark respiration (1.7 +/- 0.3 versus 0.6 +/- 0.2 micromol CO2 m(-2) s(-1)), quantum yield (0.042 +/- 0.005 versus 0.027 +/- 0.003 mol CO2 mol(-1)) and light saturation point (1155 versus 959 micromol m(-2) s(-1)) were higher in orchard trees than in young potted plants. In potted seedlings grown in a heated greenhouse, shoots and leaves exposed to full sun expanded in a sigmoidal pattern to 69 +/- 12 mm and 497 +/- 105 cm(2) for each flush, compared with 27 +/- 7 mm and 189 +/- 88 cm(2) in shaded seedlings. Shaded seedlings were smaller and had higher shoot:root ratios (3.7 versus 3.1) than seedlings grown in full sun. In the potted plants grown outdoors in 20, 40, 70 or 100% of full sun, final leaf area per shoot was 44 +/- 1, 143 +/- 3, 251 +/- 7 and 362 +/- 8 cm(2), respectively. Shoots were also shorter in plants grown in shade than in plants grown in full sun (66 +/- 5 mm versus 101 +/- 2 mm). Photosynthesis in individual leaves of lychee appeared to be saturated at about half full sun, whereas maximum leaf expansion occurred at higher PPFDs. We conclude that lychee plants can persist as seedlings on the forest floor, but require high PPFDs for optimum growth.     

Root order-dependent seasonal dynamics in the carbon and nitrogen chemistry of poplar fine roots

Fine roots play an important role in above- and belowground carbon (C) allocation in forest ecosystems. However, few studies have focused on the seasonal dynamics of fine roots with different branching orders. The objective of this study is to provide insight to the seasonal heterogeneity in roots of different orders within root hierarchies of poplar trees under different soil conditions. Three plots were established in high (plantation A) and low (plantation B) soil nutrient conditions. Fine roots were sampled in each of four seasons throughout one year. All sampled roots were classified into one to five groups depending on their branching order, and the dry biomass of living roots and the concentrations of C, nitrogen (N) and total non-structural carbohydrate (TNC) were examined. Low order (first- to second-order) roots demonstrated more significant seasonal dynamics than high order roots, and the biomass of first-order fine roots was positively influenced by soil temperature and moisture while the biomass of second-order fine roots was negatively affected by soil nutrient conditions. The different responses of fine roots to environmental fluctuations implied a high division of root function, even within low order roots. The C and N chemistry of poplar fine roots also differed significantly with branching order; element concentrations were lower in low order roots. Principal component analysis indicated that root order explained 98.2% of the variation in fine root chemistry. Moreover, the first-order roots in plantation A had greater C but less TNC concentrations than those in plantation B, suggesting that C allocation in low order roots may be more responsive to soil nutrient conditions. The allocation of C and N also exhibited significant seasonal dynamics (p < 0.05); the TNC concentration was highest in winter, whereas C:N ratios were significantly lower in the summer and fall in each order of fine roots (p < 0.05). All these results suggest that branching order may be related to root growth and photoassimilate allocation, which should receive greater attention in future studies on C and N fluxes in forest ecosystems.