Maintenance and growth respiration of the aboveground parts of young field-grown hinoki cypress (Chamaecyparis obtusa)

Aboveground respiration of five 8-year-old trees of field-grown hinoki cypress (Chamaecyparis obtusa (Sieb. et Zucc.) Endl.) was nondestructively measured at monthly intervals over 1 year with an enclosed standing tree method. The relationship between monthly specific respiration rate and monthly mean relative growth rate at the individual tree level was described by a linear equation. During the dormant season, respiration was used mainly for maintenance purposes, whereas during the growing season, more than 40% of the respiration was used for growth purposes, i.e., 60 to 70% in May. We conclude that annual maintenance and growth respiration of a tree are directly proportional to the aboveground phytomass and its annual increment, respectively. The maintenance coefficient was estimated to be 0.504 +/- 0.039 (SE) kg kg(-1) year(-1), indicating that the amount respired for maintaining already existing phytomass was equivalent to about half of the existing phytomass. The growth coefficient was estimated to be 0.772 +/- 0.043 (SE) kg kg(-1), indicating that the amount respired for constructing new phytomass was equivalent to about three-fourths of the annual phytomass increment. The annual stand maintenance and growth respiration were, respectively, 8.8 Mg ha(-1) year(-1) for an aboveground biomass of 17.4 Mg ha(-1) and 5.0 Mg ha(-1) year(-1) for an annual stand aboveground biomass increment of 6.5 Mg ha(-1) year(-1). About two-thirds of the total respiration was used to maintain already existing biomass, and about one-third was used to construct new biomass.     

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.     

Changes in the relationship between tree size and aboveground respiration in field-grown hinoki cypress (Chamaecyparis obtusa) trees over three years

Respiration measurements of aerial parts of 18-year-old hinoki cypress (Chamaecyparis obtusa (Sieb. et Zucc.) Endl.) trees were made under field conditions over three years to study changing relationships with tree age between respiration and phytomass, phytomass increment, and leaf mass. The relationship between annual respiration (r(a)) and phytomass (w(T)) was approximated by a proportional function (r(a) = aw(T)), where the proportional constant (a) decreased year by year. The effect of time on the relationship between annual respiration and phytomass of each sample tree was fitted by a power function. Respiration of the tree suppressed by the canopy decreased year by year, but respiration of the other trees increased slightly with age. The relationship between annual respiration and leaf mass was also approximated by a generalized power function. Excluding the suppressed tree, the relationship between annual respiration (r(a)) and the annual increment of aboveground phytomass (Deltaw(T)) was described by a proportional function (r(a) = 2.27Deltaw(T)), where the proportional constant, 2.27, was independent of sample tree and year, indicating that about 2.3 times of the annual aboveground phytomass increment equivalent was respired annually. For any tree, the time constant relationships between annual respiration and leaf mass and phytomass increment for different-sized trees were similar to the corresponding time continuum relationships. In contrast, the time continuum relationship between annual respiration and phytomass differed from the time constant relationship, indicating that respiration of less active woody tissue contributed significantly to aboveground respiration. Based on the relationship between tree size and annual respiration, annual aboveground stand respiration was estimated to be 25.0, 26.9, and 25.8 Mg(dm) ha(-1) year(-1) for the three consecutive years, respectively, and the corresponding aboveground stand biomass was 60.0, 69.0, and 76.8 Mg(dm) ha(-1).     

Gas exchange of the lowest branches of young Scots pine: a cost-benefit analysis of seasonal branch carbon budget

A cost-benefit approach was developed to analyze the carbon budget of the lowest Scots pine (Pinus sylvestris L.) branches subject to abscission. In addition to within-branch growth and respiratory costs, the budget included an estimation of a branch's share of the maintenance respiration of the stem and root. A branch was considered productive if the budget was positive. Foliar gas exchange and woody-tissue respiration were non-destructively measured at monthly intervals during the growing season on the six lowest branches of 10-year-old Scots pine trees, to the moment when the branches died naturally. Photosynthetic light response and temperature response of respiration, together with measurements of canopy light conditions and meteorological data, were used to calculate seasonal carbon budgets for the branches. Maintenance respiration of stems and roots was estimated from published data. All but one of the branches studied were found to be nonproductive over the growing season. Following a decrease in photosynthetic capacity in July, the cumulative budget became negative and the branches died, indicating that a negative carbon budget corresponds with the onset of abscission of the lowest branches.     

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).     

Respiratory responses of Scots pine stems to 5 years of exposure to elevated CO2 concentration and temperature

Stem respiration in 20-year-old Scots pine (Pinus sylvestris L.) trees was examined following 5 years of exposure to ambient conditions (CON), elevated atmospheric carbon dioxide concentration ([CO2]) (ambient + 350 micromol mol(-1), (EC)), elevated temperature (ambient + 2-6 degrees C, (ET)) or a combination of elevated [CO2] and elevated temperature (ECT). Stem respiration varied seasonally regardless of the treatment and displayed a similar trend to temperature, with maximum rates occurring around Day 190 in summer and minimum rates in winter. Respiration normalized to 15 degrees C (R15) was higher in the growing season than in the non-growing season, whereas the temperature coefficient (Q10) was lower in the growing season. Annually averaged R15 was 0.36, 0.43, 0.40 and 0.44 micromol m(-2) s(-1) under CON, EC, ET and ECT conditions, respectively, whereas the corresponding values for total stem respiration were 6.55, 7.69, 7.50 and 7.90 mol m(-2) year(-1). The EC, ET and ECT treatments increased R15 by 18, 11 and 22%, respectively, relative to CON, and increased the modeled annual total stem respiration by 18, 15 and 21%. The increase in modeled annual stem respiration under EC and ECT conditions was caused mainly by higher maintenance respiration (22 and 25%, respectively, whereas the increase in growth respiration was 9 and 12%). Growth respiration was unaltered by ET. The treatments did not significantly affect the respiratory response to stem temperature; the mean Q10 value was 2.04, 2.10, 1.99 and 2.12 in the CON, EC, ET and ECT treatments, respectively. It is suggested that the increase in stem respiration was partly a result of the increased growth rate. We conclude that elevated [CO2] increased the maintenance component of respiration more than the growth component.     

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.     

Influences of canopy photosynthesis and summer rain pulses on root dynamics and soil respiration in a young ponderosa pine forest

Our first objective was to link the seasonality of fine root dynamics with soil respiration in a ponderosa pine (Pinus ponderosa P. & C. Lawson) plantation located in the Sierra Nevada of California. The second objective was to examine how canopy photosynthesis influences fine root initiation, growth and mortality in this ecosystem. We compared CO2 flux measurements with aboveground and belowground root dynamics. Initiation of fine root growth coincided with tree stem thickening and shoot elongation, preceding new needle growth. In the spring, root, shoot and stem growth occurred simultaneously with the increase in canopy photosynthesis. Compared with the other tree components, initial growth rate of fine roots was the highest and their growing period was the shortest. Both above and belowground components completed 90% of their growth by the end of July and the growing season lasted approximately 80 days. The period for optimal growth is short at the study site because of low soil temperatures during winter and low soil water content during summer. High photosynthetic rates were observed following unusual late-summer rains, but tree growth did not resume. The autotrophic contribution to soil respiration was 49% over the whole season, with daily contributions ranging between 18 and 87%. Increases in soil and ecosystem respiration were observed during spring growth; however, the largest variation in soil respiration occurred during summer rain events when no growth was observed. Both the magnitude and persistence of the soil respiration pulses were positively correlated with the amount of rain. These pulses accounted for 16.5% of soil respiration between Days 130 and 329.     

A growth model of a single sugi (Cryptomeria japonica) tree based on the dry matter budget of its aboveground parts

Growth of a single sugi (Cryptomeria japonica (L.f.) D. Don.) tree was analyzed on the basis of a dry matter budget. The aboveground net production rate and death rate were defined as the anabolic rate and catabolic rate, respectively. Growth rate of aboveground tree weight, v(w) (kg(dw) year(-1)), was defined as follows: v(w) = v(p) - v(d) (1) where v(p) (kg(dw) year(-1)) is the aboveground net production rate and v(d) (kg(dw) year(-1)) is the aboveground death rate. The value of v(d) is obtained by measuring the monthly clippings of new dead leaves and branches attached to a sample tree. The value of v(w) was calculated as the annual difference in the estimated aboveground tree weight, w(T) (kg(dw)). Finally, the value of v(p) was estimated as the sum of the values of v(d) and v(w). The following allometric relationships were found between v(p) and w(T) and between v(d) and w(T): v(p) = aw(T) (alpha), v(d) = bw(T) (beta) (2). Combining Equations 1 and 2 gives a growth equation, Bertalanffy's equation, of the sample tree. dw(T)/dt = v(w) = aw(T) (alpha) - bw(T) (beta) (3). Because the growth curve of w(T) was derived from Equation 3, the analysis of the growth of w(T) is based on direct measurement of the dry matter budget.     

Seasonal courses of CO(2) exchange and carbon balance in fruits of Cinnamomum camphora

Carbon dioxide exchange in fruits of Cinnamomum camphora Sieb. was followed over a growing season from July to December 1992. Dark respiration was exponentially related to temperature, with a Q(10) value near 2. Light dependence of photosynthetic CO(2) refixation, i.e., the ratio of gross photosynthesis to dark respiration, was approximated by a hyperbolic function. Seasonal variation in maximum CO(2) refixation capacity ranged between 52 and 174%, reaching a maximum in early August. Daily photosynthetic CO(2) refixation ranged between 17 and 51% over the growth period. We evaluated seasonal variation in translocation rate to the fruit on the basis of the seasonal rates of gross photosynthesis, dark respiration and increase in fruit dry weight, and used the results to develop a simple carbon flow model of fruit development. Seasonal changes in translocation rate paralleled those in fruit growth rate, with two peaks during the periods before and after September. Seed formation took place in the period between the two peaks. The relationship between fruit growth rate and translocation rate was approximated by a linear function. The carbon flow model estimated that, over the reproductive period, the amount of assimilate translocated to each fruit was 377.2 mg dry weight, of which 58.5% was accounted for by weight growth and 41.5% was consumed by net respiration. Carbon dioxide refixation accounted for 22.9% of the carbon balance of the fruit.     

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.     

Seasonal change in the temperature coefficientQ 10 for respiration of field-grown hinoki cypress (Chamaecyparis obtusa) trees

The effect of temperature upon nighttime respiration was examined on four different sized sample trees in a 17-year-old hinoki cypress (Chamaecyparis obtusa (Sieb. et Zucc.) Endl.) stand over two years. Seasonal changes inQ ₁₀ values and their responses to mean temperature were investigated. On the basis of the monthly relationships between nighttime respiration (r) and temperature inside a chamber (θ),r=r ₀exp (kθ), theQ ₁₀ value (=exp(10k)) was calculated. TheQ ₁₀ values were high (Q ₁₀≥3.0) in winter when mean air temperature was low, and gradually decreased toward summer (Q ₁₀≤1.5) through spring with increasing temperature. TheQ ₁₀ values were negatively correlated with mean air temperature. The response ofQ ₁₀ values to mean air temperature was described by a single equation, regardless of tree size. This result, which might be characteristic of this species, shows that respiration ofC. obtusa trees is promoted by slight increases of air temperature in winter season. On the other hand, temperature sensitivity of total respiration reduced during growing season when ambient temperature was high. These chaning temperature sensitivity according to seasons may depend on the seasonal change of the ratio of growth respiration to total respiration. It is concluded that changes in temperature due to changing seasons not only change respiration rate, but also change the response of respiration rate to temperature by shiftingQ ₁₀ values.     

鄂尔多斯高原油蒿灌丛群落土壤呼吸日变化和季节变化特征

本研究对鄂尔多斯高原沙化灌丛群落油蒿土壤呼吸日变化和季节变化进行了野外定位观测,并对其环境驱动因子进行了初步的探讨.结果表明:油蒿群落两个不同生长期土壤呼吸日变化及其对温度因子的响应存在差异.营养生长期,土壤呼吸日变化不明显,且土壤呼吸速率和温度日变化无显著的相关关系;而在生殖生长期,土壤呼吸日变化非常明显,气温及0-10 cm土壤温度日变化与土壤呼吸速率相关显著(P<0.05).整个生长季期间,土壤呼吸高峰期出现在7-8月,与该段时间水热因子条件最佳且配置较好密切相关.荒漠灌丛生态系统中,降雨是土壤呼吸出现激发现象的控制因素.降雨对土壤产生的干湿交替作用能够显著提高土壤呼吸速率.生长季期间,土壤呼吸速率变化与气温及0-10 cm土壤含水量变化的相关性显著(P<0.05).通过逐步回归发现,0-10 cm土壤含水量的变化能够说明生长季土壤呼吸速率变化的41.9% (P<0.05).图3表2参34.     

Effect of nitrogen on the seasonal course of growth and maintenance respiration in stems of Norway spruce trees

To determine effects of stem nitrogen concentration ([N]) on the seasonal course of respiration, rates of stem respiration of ten control and ten irrigated-fertilized (IL), 30-year-old Norway spruce trees (Picea abies (L.) Karst.), growing in northern Sweden, were measured on seven occasions from June 1993 to April 1994. To explore sources of seasonal variation and mechanisms of fertilization effects on respiration, we separated total respiration into growth and maintenance respiration for both xylem and phloem bark. Stem respiration increased in response to the IL treatment and was positively correlated with growth rate, volume of living cells and stem nitrogen content. However, no significant effect of IL treatment or [N] in the living cells was found for respiration per unit volume of live cells. Total stem respiration during the growing season (June to September) was estimated to be 16.7 and 29.7 mol CO(2) m(-2) for control and IL-treated trees, respectively. Respiration during the growing season accounted for approximately 64% of total annual respiration. Depending on the method, estimated growth respiration varied between 40 and 60% of total respiration during the growing season. Between 75 and 80% of the live cell volume in the stems was in the phloem, and phloem maintenance accounted for about 70% of maintenance respiration. Because most of the living cells were found in the phloem, and the living xylem cells were concentrated in the outer growth rings, we concluded that the best base for expressing rates of stem growth and maintenance respiration in young Norway spruce trees is stem surface area.     

Hydraulic conductance, light interception and needle nutrient concentration in Scots pine stands and their relations with net primary productivity

Aboveground xylem hydraulic conductance was determined in Scots pine (Pinus sylvestris L.) trees and stands from 7 to about 60 years of age. At the stand scale, leaf area index and net primary productivity (NPP, above- plus belowground) increased and reached a plateau at about 25-30 and 15-20 years, respectively; both parameters declined in mature stands. Stand hydraulic conductance followed a similar trend to NPP, with a maximum at about 15-20 years and a pronounced reduction in old stands. At the tree scale, annual biomass growth per unit of leaf area (growth efficiency) declined with tree age, whereas aboveground sapwood volume per unit leaf area, which is linearly related to maintenance respiration costs, steadily increased. Radiation interception per unit leaf area increased significantly with reduced leaf area index of mature stands, despite increased foliage clumping in the canopies of mature trees. Needle nutrient concentration did not change in the chronosequence. Tree hydraulic conductance per unit leaf area was strongly and positively correlated with growth efficiency. We discuss our findings in the context of growth reductions in mature and old trees, and suggest that increased hydraulic resistance and maintenance respiration costs may be the main causes of reduced carbon gain in mature and old trees.     

Carbon and nitrogen cycling immediately following bark beetle outbreaks in southwestern ponderosa pine forests

Bark beetle infestation is a well-known cause of historical low-level disturbance in southwestern ponderosa pine forests, but recent fire exclusion and increased tree densities have enabled large-scale bark beetle outbreaks with unknown consequences for ecosystem function. Uninfested and beetle-infested plots (n = 10 pairs of plots on two aspects) of ponderosa pine were compared over one growing season in the Sierra Ancha Experimental Forest, AZ to determine whether infestation was correlated with differences in carbon (C) and nitrogen (N) pools and fluxes in aboveground biomass and soils. Infested plots had at least 80% of the overstory ponderosa pine trees attacked by bark beetles within 2 years of our measurements. Both uninfested and infested plots stored ∼9 kg C m⁻² in aboveground tree biomass, but infested plots held 60% of this aboveground tree biomass in dead trees, compared to 5% in uninfested plots. We hypothesized that decreased belowground C allocation following beetle-induced tree mortality would alter soil respiration rates, but this hypothesis was not supported; throughout the growing season, soil respiration in infested plots was similar to uninfested plots. In contrast, several results supported the hypothesis that premature needlefall from infested trees provided a pulse of low C:N needlefall that altered soil N cycling. The C:N mass ratio of pine needlefall in infested plots (∼45) was lower than uninfested plots (∼95) throughout the growing season. Mineral soils from infested plots had greater laboratory net nitrification rates and field resin bag ammonium accumulation than uninfested plots. As bark beetle outbreaks become increasingly prevalent in western landscapes, longer-term biogeochemical studies on interactions with other disturbances (e.g. fire, harvesting, etc.) will be required to predict changes in ecosystem structure and function.     

Light,soil, and seedling characteristics associated with varying levels of competition in a red pine plantation

Red pine (Pinus resinosa Ait.) seedlings growing under differing levels of competition were evaluated during the fifth growing season following planting, and placed into low, moderate, or high tree classes, as a function of levels of competing vegetation. Tree growth, moisture status, and nutrition were monitored over the growing season. Additionally, site characteristics such as soil temperature and moisture, inorganic nitrogen concentration, mineralized soil nitrogen, and light were measured. Red pine seedlings growing under low competition had an absolute volume growth increase of 795% over the seedlings growing under the heaviest competition. The associated relative volume growth increase during the fifth growing season was 44%. Discriminant analysis was used to describe three classes of trees representing low, moderate, and high levels of competition. Trees growing under low competition had longer and heavier needles, but generally lower nutrient concentrations. Pre-dawn plant moisture status did not vary among competition levels. Soil variables indicated that, in general, the trees growing under low competition occupied warmer, drier, and less fertile microsites, as inorganic soil N and mean monthly mineralized NO₃-N and NH₄>-N tended to be lower on these microsites. The suite of independent variables was effective in classifying the model data into three tree competition classes, with percent correct classifications of 92, 75, and 100 for low (tree class one), moderate (tree class two), and heavy (tree class three) competition, respectively.     

Direct inhibition of maintenance respiration in western hemlock roots exposed to ambient soil carbon dioxide concentrations

Root respiration often exhibits a direct and immediate decline with increasing concentrations of ambient soil carbon dioxide concentration ([CO(2)]), and recent evidence suggests this decline may be attributable to a decline in maintenance respiration within the root. If true, this concept could provide a clue to the biochemical process underlying respiratory inhibition as well as improve our knowledge of the timing and degree to which this inhibition occurs in nature. To test the hypothesis that maintenance respiration exhibits a direct, negative response to increasing [CO(2)], we measured total respiration in intact root systems of western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings grown at different relative growth rates and exposed to soil [CO(2)]s ranging from 91 to 7008 &mgr;mol mol(-1). Analysis of covariance was used to separate maintenance from total respiration. Total respiration declined exponentially with increasing [CO(2)]. Maintenance respiration, which comprised 85% of total respiration over all treatments, also declined exponentially with increasing [CO(2)]. Growth respiration was not inhibited at any [CO(2)]. These findings may explain why roots of some fast-growing species do not show [CO(2)] inhibition.     

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.     

Evaluating a simple radiation/dry matter conversion model using data from Eucalyptus globulus plantations in Western Australia

A simple model that describes growth in terms of physical and physiological processes is needed to predict growth rates and hence the productivity of trees at particular sites. The linear relationship expected between absorbed photosynthetically active radiation (phi(pa), MJ m(-2)) and dry mass production (G(t)); i.e., G(t) = epsilonphi(pa), where epsilon is the radiation utilization coefficient, was fitted to three years' data from five Western Australian Eucalyptus globulus Labill. plantations for which monthly growth measurements, leaf area indices, weather data and soil water measurements were available. Reductions in growth efficiency relative to absorbed photosynthetically active radiation were associated with high vapor pressure deficits (D, kPa) so the relationship between monthly aboveground biomass increments and D was used to calculate utilizable phi(pa). Plotting cumulative aboveground growth against utilizable phi(pa) gave strong linear relationships with slope epsilon. Values of epsilon ranged from 0.93 to 2.23 g dry mass MJ(-1) phi(pa). The variation could not be explained either in terms of soil water content in the root zones, because all plantations appeared to have access to groundwater, or in terms of soil chemistry. A value of epsilon approximately 2.2 is considered near the maximum likely to be applicable to Eucalyptus plantations. An interesting peripheral finding was a strong relationship between allometric ratios and soil phosphorus; this, if confirmed elsewhere, will be of considerable value in converting biomass increments to wood production. There was also a strong negative relationship between the average ratio of leaf/total aboveground biomass and soil nitrogen content.