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.     

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.     

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.     

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.     

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.     

Dependence of the aboveground respiration of hinoki cypress (Chamaecyparis obtusa) on tree size

Nighttime respiration was measured at monthly intervals over one year on the aboveground parts of five sample trees in an 8-year-old hinoki cypress (Chamaecyparis obtusa (Sieb. et Zucc.) Endl.) stand, by an enclosed standing-tree method. The respiration rate rose rapidly from early spring to a maximum in June, and decreased abruptly in July and then gradually toward autumn and winter. The seasonal change in the respiration rate was synchronized with stem volume increment rather than with monthly mean air temperature. The respiration rate, r, of individual trees increased with increasing tree dimensions, such as stem volume, v(S), and stem girth at the base of the live crown, G(B). The dependence of respiration rate on tree size was successfully represented by a power function. The r - v(S) dependence was rather stronger than the r - G(B) (2) dependence, especially toward the end of the growing season (from July to September). The observed respiration rate was almost the same as the respiration rate corrected for the monthly mean air temperature. The annual respiration of individual trees was directly proportional to their phytomass or to its increment. Although the annual respiration of individual trees decreased proportionally to the square root of the leaf mass, it decreased abruptly in the range close to the smallest sample tree. Combining the monthly relationship between respiration rate and stem volume with the tree size distribution in the stand, the stand aboveground annual respiration was estimated to be 20.4 Mg CO(2) ha(-1) year(-1) (= 12.5 Mg dry mass ha(-1) year(-1)) for an aboveground biomass of 17.4 Mg ha(-1) with an annual increment of 6.51 Mg ha(-1) year(-1), i.e., the stand aboveground annual respiration amounted to the equivalent of 72% of the biomass or to almost twice the biomass increment.     

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.     

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.     

Effects of temperature and tissue nitrogen on dormant season stem and branch maintenance respiration in a young loblolly pine (Pinus taeda) plantation

We measured dormant season (November through February) maintenance respiration rates (R(m)) in stems and branches of 9-year-old loblolly pine (Pinus taeda L.) growing in plots under conditions of controlled nutrient and water supply in an effort to determine the relationships between R(m) and tissue size (surface area, sapwood volume, sapwood dry weight), tissue nitrogen content and temperature. Dormant season R(m) per unit size (i.e., surface area, &mgr;mol m(-2) s(-1); sapwood volume, &mgr;mol m(-3) s(-1); or sapwood dry weight, nmol g(-1) s(-1)) varied with tissue size, but was constant with respect to tissue nitrogen content (&mgr;mol mol(-1) N s(-1)). Cambium temperature accounted for 61 and 77% of the variation in stem and branch respiration, respectively. The basal respiration rate (respiration at 0 degrees C) increased with tissue nitrogen content, however, the Q(10) did not. Improved nutrition more than doubled stem basal respiration rate and increased branch basal respiration by 38%. Exponential equations were developed to model stem and branch respiration as a function of cambium temperature and tissue nitrogen content. We conclude that failure to account for tissue nitrogen effects on respiration rates will result in serious errors when estimating annual maintenance costs.     

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.     

Seasonal variation of soil respiration in a Pinus cembra forest at the upper timberline in the Central Austrian Alps

Soil respiration (R) of a 95-year-old Pinus cembra L. forest at the alpine timberline was measured continuously from October 2001 to January 2003 with an automated multiplexing gas exchange system. There was significant spatial variability in soil respiration, and R at a soil temperature of 10 degrees C (R10) decreased by about 20% m(-1) with increasing distance from the trunk. Needle litter and fine root density also decreased. The spatially averaged annual soil CO2 efflux was 35 g C m(-2) year(-1) in 2002. About 70% of the temporal variation in soil respiration could be explained by variations in soil temperature, whereas the influence of soil water potential and thus soil water content was negligible because soil water availability was supra-optimal.     

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.     

巴西半干旱地区自然条件和菌根菌接种条件下赤桉和大桉地上生物量营养成分分析(英文)

对巴西米纳斯吉拉斯州北部半干旱地区(15°09’S43°49’W)的赤桉(Eucalyptus camaldulensis)和大桉(Eucalyptus grandis)人工林的地上生物量、营养成分含量和菌根菌定植百分率进行了调查和分析。结果表明,赤按和大桉人工林的总地上生物量分别为33.6Mg·hm-2和153.1Mg·hm-2。赤桉树干、叶子、枝条和树皮的生物量分别占总生物量的64.4%,19.6%,15.4%,0.6%,大桉地上生物量的分配与赤按基本相同。赤桉叶子和枝条的干物质占其总生物量的35%,叶子和枝条中的N,P,K,Ca,Mg,and S的含量分别占总生物量这些营养元素的15.5%,0.7%,12.3%,22.6%,19%,1.4%。树干(包括树皮)中的营养成分累积相对比较低。与赤桉相比,大桉的营养含量变化较小。这2个树种的树干上部含有高浓度的磷,树皮也含有大量的营养物质,尤其是大桉;说明在半干旱地区,立地上脱落的植物性废物对降低树木生产力损失有重要意义。赤桉和大桉都有菌根营养。     

PEACH: A simulation model of reproductive and vegetative growth in peach trees

The hypothesis that carbohydrate partitioning is driven by competition among individual plant organs, based on each organ's growth potential, was used to develop a simulation model of the carbon supply and demand for reproductive and vegetative growth in peach trees. In the model, photosynthetic carbon assimilation is simulated using daily minimum and maximum temperature and solar radiation as inputs. Carbohydrate is first partitioned to maintenance respiration, then to leaves, fruits, stems and branches, then to the trunk. Root activity is supported by residual carbohydrate after aboveground growth. Verification of the model was carried out with field data from trees that were thinned at different times. In general, the model predictions corresponded to field data for fruit and vegetative growth. The model predicted that resource availability limited fruit and stem growth during two periods of fruit growth, periods that had been identified in earlier experimental studies as resource-limited growth periods. The model also predicted that there were two periods of high carbohydrate availability for root activity. The fit between model predictions and field data supports the initial hypothesis that plants function as collections of semiautonomous, interacting organs that compete for resources based on their growth potentials.     

Changes in fine root production and longevity in relation to water and nutrient availability in a Norway spruce stand in northern Sweden

The PEACH computer simulation model of reproductive and vegetative growth of peach trees (Grossman and DeJong 1994) was adapted to estimate seasonal nitrogen (N) dynamics in organs of mature peach (Prunus persica (L.) Batsch cv. O'Henry) trees grown with high and low soil N availability. Seasonal N accumulation patterns of fruits, leaves, stems, branches, trunk and roots of mature, cropping peach trees were modeled by combining model predictions of organ dry mass accumulation from the PEACH model with measured seasonal organ N concentrations of trees that had been fertilized with either zero or 200 kg N ha(-1) in April. The results provided a comparison of the N use of perennial and annual organs during the growing season for trees growing under both low and high N availability. Nitrogen fertilization increased tree N content by increasing organ dry masses and N concentrations during the fruit growing season. Dry mass of current-year vegetative growth was most affected by N fertilization. Whole-tree N content of fertilized trees was almost twice that of non-fertilized trees. Although N use was higher in fertilized trees, calculated seasonal N accumulation patterns were similar for trees in both treatments. Annual organs exhibited greater responses to N fertilization than perennial organs. Estimated mean daily N use per tree remained nearly constant from 40 days after anthesis to harvest. The calculations indicated that fertilized trees accumulated about 1 g N tree(-1) day(-1), twice that accumulated by non-fertilized trees. Daily N use by the fertilized orchard was calculated to be approximately 1 kg N ha(-1), whereas it was approximately 0.5 kg N ha(-1) for the non-fertilized trees. During the first 25-30 days of the growing season, all N use by growing tissues was apparently supplied by storage organs. Nitrogen release from storage organs for current growth continued until about 75 days after anthesis in both N treatments.     

Seasonal CO(2) assimilation and stomatal limitations in a Pinus taeda canopy

Net CO(2) assimilation (A(net)) of canopy leaves is the principal process governing carbon storage from the atmosphere in forests, but it has rarely been measured over multiple seasons and multiple years. I measured midday A(net) in the upper canopy of maturing loblolly pine (Pinus taeda L.) trees in the piedmont region of the southeastern USA on 146 sunny days over 36 months. Concurrent data for leaf conductance and photosynthetic CO(2) response curves (A(net)-C(i) curves) were used to estimate the relative importance of stomatal limitations to CO(2) assimilation in the field. In fully expanded current-year and 1-year-old needles, midday light-saturated A(net) was constant over much of the growing season (5-6 &mgr;mol CO(2) m(-2) s(-1)), except during drought periods. During the winter season (November-March), midday A(net) of overwintering needles varied in proportion to leaf temperature. Net CO(2) assimilation at light saturation occurred when daytime air temperatures exceeded 5-6 degrees C, as happened on more than 90% of the sunny winter days. In both age classes of foliage, winter carbon assimilation accounted for approximately 15% of the daily carbon assimilation on sunny days throughout the year, and was relatively insensitive to year-to-year differences in temperature during this season. However, strong stomatal limitations to A(net) occurred as a result of water stress associated with freezing cycles in winter. During the growing season, drought-induced water stress produced the largest year-to-year differences in seasonal CO(2) assimilation on sunny days. Seasonal A(net) was more drought sensitive in current-year needles than in 1-year-old needles. Relative stomatal limitations to daily integrated A(net) were approximately 40% over the growing season, and summer drought rather than high temperatures had the largest impact on summer A(net) and integrated annual CO(2) uptake in the upper crown. Despite significant stomatal limitations, a long duration of near-peak A(net) in the upper crown, particularly in 1-year-old needles, conferred high seasonal and annual carbon gain.     

Radiation-use efficiency and dry matter partitioning of a young olive (Olea europaea) orchard

Radiation-use efficiency (RUE) relates biomass production to the photosynthetically active radiation (PAR) intercepted by a plant or crop. We determined RUE and biomass partitioning coefficients of young olive (Olea europaea) trees for use in a general growth model. In 1995, 1-year-old olive trees var. 'Picual' were planted at a density of either 0.5 or 2.0 trees m(-2) near Córdoba, Spain, at a site providing favorable growth conditions. During the experiment (1995-1997), both PAR interception by the canopy and plant area index (PAI) were measured with radiation sensors. Regular harvests were performed to determine leaf area and biomass accumulation in roots, wood (stem, branches and trunk) and leaves. Leaf, wood and root biomass partitioning coefficients were calculated. The leaf area partitioning coefficients were also estimated. Dry matter production was linearly related to cumulative intercepted PAR. Seasonal RUE, calculated as the slope of the regression of aboveground biomass and cumulative intercepted PAR, was 1.35 g (MJ PAR)(-1). Radiation-use efficiency appeared to respond to environmental conditions, but was independent of planting density and PAI. The young olive trees allocated 0.26 of their total biomass to roots. Partitioning of aboveground dry matter was 0.60 to wood and 0.37 to leaves. As competition increased, dry matter partitioning to wood increased to 0.70.     

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.     

Scaling foliar respiration to the stand level throughout the growing season in a Quercus rubra forest

Stand-level, canopy foliar carbon loss (R(can)) was modeled for a virtual Quercus rubra L. monoculture at two sites differing in soil water availability in a northeastern deciduous forest (USA) throughout the 2003 growing season. Previously reported foliar respiratory temperature responses of Q. rubra were used to parameterize a full distributed physiology model that estimates R(can) by integrating the effects of season, site and canopy position, and represents the best estimation of R(can). Model sensitivity to five simplified parameterization scenarios was tested, and a reasonable procedure of simplification was established. Neglecting effects of season, site or canopy position on respiration causes considerable relative error in R(can) estimation. By contrast, assuming a constant E(0) (a temperature response variable of the respiration model), or a constant night temperature (mean nighttime temperature) caused only a small relative error (< 10%) compared with the full model. From June 8 to October 28, 2003, modeled R(can) of the virtual Q. rubra monoculture was, on average, 45.3 mmol CO(2) m(-2) night(-1) on a ground-area basis (or 334 mmol CO(2) kg(-1) night(-1) on a biomass basis) and 101 mmol CO(2) m(-2) night(-1) (or 361 mmol CO(2) kg(-1) night(-1)) at the drier site and the more mesic site, respectively. To model R(can) of Q. rubra (or other Quercus species with similar respiratory properties), variations in the base respiration rate across season, site and canopy position need to be fully accounted for, but E(0) may be assumed constant. Modeling R(can) at the mean nighttime temperature would not strongly affect estimated canopy carbon loss.     

Effect of elevated [CO(2)] and varying nutrient application rates on physiology and biomass accumulation of Sitka spruce (Picea sitchensis)

Sitka spruce (Picea sitchensis (Bong.) Carr.) seedlings were supplied with solutions containing nitrogen (N) at 0.1 x or 2 x the optimum rate (low-N and high-N supply, respectively) and grown either outside in a control plot or inside open-top chambers and exposed to ambient (355 &mgr;mol mol(-1)) or elevated (700 &mgr;mol mol(-1)) CO(2) concentration ([CO(2)]). Gas exchange measurements, chlorophyll determinations and nutrient analysis were made on current-year (< 1-year-old) shoots of the upper whorl after the seedlings had been growing in the [CO(2)] treatments for 17 months and the nutrient treatments for 6 months. Total seedling biomass and biomass allocation were assessed at the end of the experiment. Nutrient treatment had a significant effect on the light response curves, irrespective of [CO(2)] or chamber treatment; seedlings supplied with high-N rates had higher net photosynthetic rates than seedlings supplied with low-N rates. The degree of photosynthetic stimulation in response to elevated [CO(2)] was larger in seedlings receiving high-N rates than in seedlings receiving low-N rates. Light-saturated net photosynthesis of seedlings grown and measured in elevated [CO(2)] was 26% higher than that of seedlings grown and measured in ambient [CO(2)]. There was no significant effect of [CO(2)] or chamber treatment on the CO(2) response curves of seedlings receiving High-N supply rates. In contrast, analysis of the CO(2) response curves of seedlings receiving Low-N supply rates showed acclimation to elevated [CO(2)]. Both maximum rate of carboxylation (V(cmax)) and maximum electron transport capacity (J(max)) were lower and J(max)/V(cmax) higher in seedlings in the elevated [CO(2)] treatment. There was no effect of elevated [CO(2)] on stomatal conductance, although it was highly dependent on foliar [N], ranging from ~60 mmol m(-2) s(-1) at ~1.5 g N m(-2) to 200 mmol m(-2) s(-1) at ~5 g N m(-2). In the high-N and low-N treatments, foliar N concentration was 10 and 28% lower in seedlings grown in elevated [CO(2)] than in seedlings grown in ambient [CO(2)], respectively. There was no [CO(2)] effect on foliar phosphorus concentration ([P]). Chlorophyll concentration increased with increasing N supply in all treatments. There was no significant effect of elevated [CO(2)] on specific leaf area. Chlorophyll concentration expressed either on an area or dry mass basis for a given foliar [N] was higher in seedlings grown in elevated [CO(2)] than in seedings grown in ambient [CO(2)]. Elevated [CO(2)] increased total biomass accumulation by 37% in seedlings in the high-N treatment but had no effect in seedlings in the low-N treatment. There was a proportionally bigger allocation of biomass to roots of seedlings in the elevated [CO(2)] + low-N supply rate treatment compared with seedlings in other treatments. This resulted in a reduction in aboveground biomass compared with corresponding seedlings grown in ambient [CO(2)].