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.     

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.     

Separating rhizosphere respiration from total soil respiration in two larch plantations in northeastern China

The potential capacity of soil to sequester carbon in response to global warming is strongly regulated by the ratio of rhizosphere respiration to respiration by soil microbial decomposers, because of their different temperature sensitivities. To quantify relative contributions of rhizosphere respiration to total soil respiration as influenced by forest stand development, we conducted a trenching study in two larch (Larix gmelini (Rupr.) Rupr.) plantations, aged 17 and 31 years, in northeastern China. Four plots in each plantation were randomly selected and trenched in early May 2001. Soil surface CO2 effluxes both inside and outside the plots were measured from May 2001 to August 2002. Soil respiration (i.e., the CO2 effluxes outside the trenched plots) varied similarly in the two plantations from 0.8 micromol m(-2) s(-1) in winter to 6.0 micromol m(-2) s(-1) in summer. Rhizosphere respiration (i.e., CO2 efflux outside the trenched plots minus that inside the plots) varied from 0.2 to 2.0 micromol m(-2) s(-1) in the old forest and from 0.3 to 4.0 micromol m(-2) s(-1) in the young forest over the seasons. Rhizosphere respiration, on average, accounted for 25% of soil respiration in the old forest and 65% in the young forest. Rhizosphere and soil respiration were significantly correlated with soil temperature but not with soil water content. We conclude that the role forests play in regulating climate change may depend on their age.     

Coarse and fine root respiration in aspen (Populus tremuloides)

Coarse and fine root respiration rates of aspen (Populus tremuloides Michx.) were measured at 5, 15 and 25 degrees C. Coarse roots ranged from 0.65 to 4.45 cm in diameter, whereas fine roots were less than 5 mm in diameter. To discriminate between maintenance and growth respiration, root respiration rates were measured during aboveground growing periods and dormant periods. An additional measurement of coarse root respiration was made during spring leaf flush, to evaluate the effect of mobilization of resources for leaf expansion on root respiration. Fine roots respired at much higher rates than coarse roots, with a mean rate at 15 degrees C of 1290 micromol CO2 m-3 s-1 during the growing period, and 660 micromol CO2 m-3 s-1 during the dormant period. The temperature response of fine root respiration rate was nonlinear: mean Q10 was 3.90 for measurements made at 5-15 degrees C and 2.19 for measurements made at 15-25 degrees C. Coarse root respiration rates measured at 15 degrees C in late fall (dormant season) were higher (370 micromol CO2 m-3 s-1) than rates from roots collected at leaf flush and early summer (200 micromol CO2 m-3 s-1). The higher respiration rates in late fall, which were accompanied by decreased total nonstructural carbohydrate (TNC) concentrations, suggest that respiration rates in late fall included growth expenditures, reflecting recent radial growth. Neither bud flush nor shoot growth of the trees caused an increase in coarse root respiration or a decrease in TNC concentrations, suggesting a limited role of coarse roots as reserve storage organs for spring shoot growth, and a lack of synchronization between above- and belowground growth. Pooling the data from the coarse and fine roots showed a positive correlation between nitrogen concentration and respiration rate.     

The effect of aqueous transport of CO(2) in xylem sap on gas exchange in woody plants

The influence of CO(2) transported in the transpiration stream on measurements of leaf photosynthesis and stem respiration was investigated. Measurements were made on trees in a temperate forest in Scotland and in a tropical rain forest in Cameroon, and on shrubs in the Sahelian zone in Niger. A chamber was designed to measure the CO(2) partial pressure in the gas phase within the woody stems of trees. High CO(2) partial pressures were found, ranging from 3000 to 9200 Pa. Henry's Law was used to estimate the CO(2) concentration of xylem sap, assuming that it was in equilibrium with the measured gas phase partial pressures. The transport of CO(2) in the xylem sap was calculated by multiplying sap CO(2) concentration by transpiration rate. The magnitude of aqueous transport in the studied species ranged from 0.03 to 0.35 &mgr;mol CO(2) m(-2) s(-1), representing 0.5 to 7.1% of typical leaf photosynthetic rates. These values strongly depend on sap pH. To examine the influence of aqueous transport of CO(2) on stem gas exchange, we made simultaneous measurements of stem CO(2) efflux and sap flow on the same stem. After removing the effect of temperature, stem CO(2) efflux was positively related to sap flow. The apparent effect on measurements of stem respiration was up to 0.7 &mgr;mol m(-2) s(-1), representing ~12% of peak stem respiration rates.     

Spatial and temporal variations in soil respiration in relation to stand structure and soil parameters in an unmanaged beech forest

Soil CO2 efflux (soil respiration) plays a crucial role in the global carbon cycle and efflux rates may be strongly altered by climate change. We investigated the spatial patterns of soil respiration rates in 144 measurement locations in a 0.5-ha plot and the temporal patterns along a 300-m transect in the 0.5-ha plot. Measurements were made in an unmanaged, highly heterogeneous beech forest during 2000 and 2001. We investigated the effects of soil, roots and forest stand structure on soil respiration, and we also assessed the stability of these spatial patterns over time. Soil temperature alone explained between 68 and 95% of the temporal variation in soil respiration; however, pronounced spatial scatter of respiration rates was not explained by soil temperature. The observed spatial patterns stayed remarkably stable throughout the growing season and over 2 years. The most important structural parameter of the stand was the mean diameter at breast height of trees within a distance of 4 m of the measurement locations (m-dbh4), which explained 10-19% of the variation in soil respiration throughout the growing season. Among the soil chemical parameters, carbon content (bulk as well as dissolved) and magnesium content explained 62% of the spatial variation in soil respiration. The final best model combining soil, root and stand structural parameters (fine root biomass, soil carbon content, m-dbh4 and soil water content) explained 79% of the variation in soil respiration, illustrating the importance of both biotic and abiotic factors.     

Sapwood temperature gradients between lower stems and the crown do not influence estimates of stand-level stem CO(2) efflux

Temperature plays a critical role in the regulation of respiration rates and is often used to scale measurements of respiration to the stand-level and calculate annual respiratory fluxes. Previous studies have indicated that failure to consider temperature gradients between sun-exposed stems and branches in the crown and shaded lower stems may result in errors when deriving stand-level estimates of stem CO(2) efflux. We measured vertical gradients in sapwood temperature in a mature lowland podocarp rain forest in New Zealand to: (1) estimate the effects of within-stem temperature variation on the vertical distribution of stem CO(2) efflux; and (2) use these findings to estimate stand-level stem CO(2) efflux for this forest. Large within-stem gradients in sapwood temperature (1.6 +/- 0.1 to 6.0 +/- 0.5 degrees C) were observed. However, these gradients did not significantly influence the stand-level estimate of stem CO(2) efflux in this forest (536 +/- 42 mol CO(2) ha(-1) day(-1)) or the vertical distribution of stem CO(2) efflux, because of the opposing effects of daytime warming and nighttime cooling on CO(2) efflux in the canopy, and the small fraction of the woody biomass in the crowns of forest trees. Our findings suggest that detailed measurements of within-stand temperature gradients are unlikely to greatly improve the accuracy of tree- or stand-level estimates of stem CO(2) efflux.     

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.     

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.     

Interspecific variability of stem photosynthesis among tree species

The photosynthetic characteristics of current-year stems of six deciduous tree species, two evergreen tree species and ginkgo (Ginkgo biloba L.) were compared. Gas exchange, chlorophyll concentration, nitrogen concentration and maximum quantum yield of PSII were measured in stems in summer and winter. A light-induced decrease in stem CO(2) efflux was observed in all species. The apparent gross photosynthetic rate in saturating light ranged from 0.72 micromol m(-2 )s(-1) (ginkgo, in winter) to 3.73 micromol m(-2) s(-1) (Alnus glutinosa (L.) Gaertn., in summer). Despite this variability, a unique correlation (slope = 0.75), based on our results and those reported in the literature, was found between gross photosynthetic rate and dark respiration rate. Mass-based gross photosynthetic rate decreased with stem mass per area and correlated to chlorophyll concentration and nitrogen concentration, both in summer and winter. The radial distribution of stem chlorophyll differed among species, but all species except ginkgo had chlorophyll as deep as the pith. In summer, the maximum quantum yield of stem PSII (estimated from the ratio of variable to maximal fluorescence; F (v)/F (m)) of all species was near the optimal value found for leaves. By contrast, the values were highly variable in winter, suggesting large differences in sensitivity to low-temperature photoinhibition. The winter values of Fv/Fm were only 31-60% of summer values for the deciduous species, whereas the evergreen conifer species maintained high F (v)/F (m) in winter. The results highlight the interspecific variability of gross photosynthesis in the stem and its correlation with structural traits like those found for leaves. The structural correlations suggest that the selection of photosynthetic traits has operated under similar constraints in stems and leaves.     

Carbon dioxide efflux from roots of calamodin and apple

The specific rate of CO(2) efflux (respiration) from roots of intact fruiting calamodin plants (Citrus madurensis Lour.) showed no diel trend, and did not respond significantly to short-term (2 day) changes in shoot irradiance. Mean root respiration rate was about 8.4 nmol CO(2) g(-1) s(-1) at 20 degrees C, and increased with temperature with a Q(10) of about 2. In calamodin plants, the proportion of total root length that was white averaged 6.0 mm m(-1). Respiration of roots of apple plants (Malus domestica Borkh.), planted in spring as rootstocks and grown at high irradiance and N supply, declined from about 5.3 to 2.8 nmol CO(2) g(-1) s(-1) between 46 and 138 days after bud burst. At 50% irradiance, root respiration was reduced more than 25% at 46 and 92 days after bud burst, but was not significantly affected later in the season. Reducing shoot irradiance reduced the proportion of total root length that was white, e.g., from 217 to 146 mm m(-1) at 46 days after bud burst. For plants previously grown at low irradiance, increasing shoot irradiance for 6 days increased the rate of root respiration by 5 to 10%. For plants previously grown at high irradiance, reducing shoot irradiance for 6 days reduced root respiration by about 20% early in the season, but had no significant effect later in the season. For plants grown with low-N supply (5% of the high-N), root respiration was reduced early in the season, but was not significantly affected later. Reducing the N supply increased slightly the proportion of total root length that was white. For plants previously grown with low-N, increasing the N supply for 6 days reduced further the rate of root respiration. For plants previously grown with high-N, reducing the N supply for 6 days did not significantly affect the rate of root respiration. Specific respiration rates of root systems excised from mature trees growing outdoors peaked in June, at about 2.4 nmol CO(2) g(-1) s(-1), and then declined for the remainder of the growing season.     

Belowground carbon allocation in unfertilized and fertilized red pine plantations in northern Wisconsin

We estimated carbon allocation to belowground processes in unfertilized and fertilized red pine (Pinus resinosa Ait.) plantations in northern Wisconsin to determine how soil fertility affects belowground allocation patterns. We used soil CO(2) efflux and litterfall measurements to estimate total belowground carbon allocation (root production and root respiration) by the carbon balance method, established root-free trenched plots to examine treatment effects on microbial respiration, estimated fine root production by sequential coring, and developed allometric equations to estimate coarse root production. Fine root production ranged from 150 to 284 g m(-2) year(-1) and was significantly lower for fertilized plots than for unfertilized plots. Coarse root production ranged from 60 to 90 g m(-2) year(-1) and was significantly lower for fertilized plots than for unfertilized plots. Annual soil CO(2) fluxes ranged from 331 to 541 g C m(-2) year(-1) and were significantly lower for fertilized plots than for unfertilized plots. Annual foliage litterfall ranged from 110 to 187 g C m(-2) year(-1) and was significantly greater for fertilized plots than for unfertilized plots. Total belowground carbon allocation ranged from 188 to 395 g C m(-2) year(-1) and was significantly lower for fertilized than for unfertilized plots. Annual soil CO(2) flux was lower for trenched plots than for untrenched plots but did not differ between fertilized and unfertilized trenched plots. Collectively, these independent estimates suggest that fertilization decreased the relative allocation of carbon belowground.     

Root respiration in Chamaecyparis obtusa trees

We examined the respiration rate of root segments, which had a constant length in relation to their diameter, from three small and two large 26-year-old Chamaecyparis obtusa (Siebold & Zucc.) Endl. trees. The dependence of respiration rate on segment diameter was described by a power function with an exponent of about 1.5, except for the smallest sample tree, for which the exponent was 1.74. Unlike stem segments, root segments of similar diameter showed similar rates of respiration regardless of the tree from which the root segments had been taken. On the basis of the power function, we propose a new equation to estimate the total root respiration rate of a tree. The relationship between root respiration rate per tree and root weight can be expressed by a power function with an exponent of 1.11. The ratio of the specific respiration rate of stems to that of roots was 0.7 for the three smaller trees, and 1.1 to 1.3 for the two larger trees. In November, the stand respiration rate of roots was estimated to be 0.36 kg CO(2) ha(-1) h(-1) for a root biomass (dry weight) of 28 Mg ha(-1).     

Estimating stem respiration in trees by a mass balance approach that accounts for internal and external fluxes of CO2

The respiration rate of a tree stem has commonly been estimated from measurements of CO2 efflux to the atmosphere. These estimates assume that all CO2 efflux originates from respiration of local tissues and that all CO2 produced by local tissues escapes to the atmosphere through the bark. However, dissolved CO2 can be transported in the xylem stream, and CO2 concentration ([CO2]) in xylem can be up to three orders of magnitude greater than that of the atmosphere, suggesting that measurements of CO2 efflux do not account for all CO2 produced by respiration. Here, we propose a new mass balance approach for estimating the respiration rate of tree stems that accounts for both external and internal fluxes of CO2. We demonstrate this approach using measurements of CO2 efflux, sap flux and internal [CO(2)] to calculate the rate of CO2 production of a segment of stem tissue in situ. At different times of the day, CO2 produced by respiration of stem tissues followed different flux pathways. During daylight hours when sap was flowing, a large proportion of respired CO2 was carried away in the xylem stream, whereas at night, most respiratory CO2 escaped to the atmosphere through the bark. Our calculations showed errors in efflux-based estimates of respiration of up to 76% compared with estimates that include both internal and external fluxes.     

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.     

Seasonal dynamics of soil carbon dioxide efflux and simulated rhizosphere respiration in a beech forest

Respiration of the rhizosphere in a beech (Fagus sylvatica L.) forest was calculated by subtracting microbial respiration associated with organic matter decomposition from daily mean soil CO2 efflux. We used a semi-mechanistic soil organic matter model to simulate microbial respiration, which was validated against "no roots" data from trenched subplots. Rhizosphere respiration exhibited pronounced seasonal variation from 0.2 g C m(-2) day(-1) in January to 2.3 g C m(-2) day(-1) in July. Rhizosphere respiration accounted for 30 to 60% of total soil CO2 efflux, with an annual mean of 52%. The high Q10 (3.9) for in situ rhizosphere respiration was ascribed to the confounding effects of temperature and changes in root biomass and root and shoot activities. When data were normalized to the same soil temperature based on a physiologically relevant Q10 value of 2.2, the lowest values of temperature-normalized rhizosphere respiration were observed from January to March, whereas the highest value was observed in early July when fine root growth is thought to be maximal.     

Carbohydrate requirements for dark respiration by peach vegetative organs

The specific respiration rate at 20 degrees C (R(20)) of peach leaves and stems declined rapidly from a high value in the early spring (22.5 nmol CO(2) g(dw) (-1) s(-1)) to relatively constant rates by July (3.1 nmol CO(2) g(dw) (-1) s(-1)). Leaf R(20) declined more rapidly than current-year stem R(20), but leaf and current-year stem R(20)s were similar by July. The R(20) of current-year stems in July was approximately 2.5 times greater than that of one-year-old stems (1.3 nmol CO(2) g(dw) (-1) s(-1)), and about 30 times greater than that of the trunk R(20) (0.1 nmol CO(2) g(dw) (-1) s(-1)). The Q(10)s of leaves and stems were approximately 2 for a temperature increase between 20 and 30. The Q(10)s above 30 were 2.03 for leaves but only 1.61 for stems. Leaves and current-year stems accounted for 2 and 17% of the aboveground vegetative biomass in April and August, respectively, but accounted for 59-80% of total daily (24 h) respiration. Although trunk biomass accounted for 91 and 77% of aboveground vegetative biomass, in April and August, respectively, trunk respiration accounted for only 8-15% of daily aboveground respiration. Before harvest, during a period when fruit growth was source-limited, daily fruit respiration exceeded respiration by all aboveground vegetative organs.     

Carbon dioxide exchange of developing avocado (Persea americana Mill.) fruit

Net efflux of CO(2) from attached avocado (Persea americana Mill.) fruit was measured periodically from three weeks after anthesis to fruit maturity. Net CO(2) exchange was determined in daylight (light respiration, R(l)) at a photosynthetic photon flux (PPF) greater than 600 micromol m(-1) s(-1), and in the dark (dark respiration, R(d)). Dark respiration and R(l) were highest during the early cell division stage of fruit growth (about 25 and 22 nmol CO(2) g(dw) (-1) s(-1), respectively) and decreased gradually until fruit maturity to about 1 and 0.5 nmol CO(2) nmol CO(2) g(dw) (-1) s(-1), respectively. Fruit photosynthesis, calculated from the difference between R(d) and R(l), ranged from 0.5 to 3.1 nmol CO(2) g(dw) (-1) s(-1). Net rate of CO(2) assimilation on a fruit dry weight basis was highest during the early stages of fruit growth and reached the lowest rate at fruit maturity. Net rate of CO(2) assimilation of fruit exposed to light was 0.4 to 2.5% of that for fully expanded leaves. Although the relative amount of carbon assimilated by the fruit was small compared with the total amount of carbon assimilated by the leaves, the data indicate that avocado fruit contribute to their own carbon requirement by means of CO(2) assimilated in the light.     

Ecosystem respiration in a young ponderosa pine plantation in the Sierra Nevada Mountains, California

We estimated total ecosystem respiration from a ponderosa pine (Pinus ponderosa Dougl. ex Laws.) plantation in the Sierra Nevada Mountains near Georgetown, California, from June to October, 1998. We apportioned ecosystem respiration among heterotrophic, root, stem and foliage based on relationships for each component that considered microclimate and vegetation characteristics. We measured each respiration component at selected sampling points, and scaled the measurements up to the ecosystem based on modeled relationships. Over the study period, total mean ecosystem respiration was 5.7 +/- 1.3 mumol m-2 s-1 (based on daily mean), comprising about 67% from soil-surface CO2 efflux, 10% from stem and branch respiration and 23% from foliage respiration. Shrub leaves contributed about 24% to total foliage respiration, and current-year needles (1998 age class) accounted for 40% of total tree needle respiration. Root respiration accounted for 47% of soil-surface CO2 efflux. We conclude that ecosystem respiration can be estimated based on daily mean air and soil temperatures through exponential relationships with r2 values of 0.85 and 0.87, respectively. When based on both air and soil temperatures, about 91% of the variation in total ecosystem respiration could be explained by a linear regression.     

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.