Ground Area Basis Research Articles

Direct effects of atmospheric carbon dioxide concentration on whole canopy dark respiration of rice

SummaryThe purpose of this study was to test for direct inhibition of rice canopy apparent respiration by elevated atmospheric carbon dioxide concentration ([CO2]) across a range of short‐term air temperature treatments. Rice (cv. IR‐72) was grown in eight naturally sunlit, semiclosed, plant growth chambers at daytime [CO2] treatments of 350 and 700 μmol mol−1. Short‐term night‐time air temperature treatments ranged from 21 to 40 °C. Whole canopy respiration, expressed on a ground area basis (Rd), was measured at night by periodically venting the chambers with ambient air. This night‐time chamber venting and resealing procedure produced a range of increasing chamber [CO2] which we used to test for potential inhibitory effects of rising [CO2] on Rd. A nitrous oxide leak detection system was used to correct Rd measurements for chamber leakage rate (L) and also to determine if apparent reductions in night‐time Rd with rising [CO2] could be completely accounted for by L. The L was affected by both CO2 concentration gradient between the chamber and ambient air and the inherent leakiness of each individual chamber. Nevertheless, after correcting Rd for L, we detected a rapid and reversible, direct inhibition of Rd with rising chamber [CO2] for air temperatures above 21 °C. This effect was larger for the 350 compared with the 700 μmol mol−1 daytime [CO2] treatment and was also increased with increasing short‐term air temperature treatments. However, little difference in Rd was found between the two daytime [CO2] treatments when night‐time [CO2] was at the respective daytime [CO2]. These results suggest that naturally occurring diurnal changes in both ambient [CO2] and air temperature can affect Rd. Because naturally occurring diurnal changes in both [CO2] and air temperature can be expected in a future higher CO2 world, short‐term direct effects of these environmental variables on rice Rd can also be expected.

Analysis of Cotton Yield Stability Across Population Densities

Final lint yield in cotton (Gossypium hirsutum L.) is relatively stable across a wide range of population densities. This study was conducted to determine (i) which components of final lint yield impart this yield stability across plant populations and (ii) how yield distribution is influenced by population density. Studies were conducted in 1997 and 1998 on a Tifton loamy sand (Fine‐loamy, kaolinitic, thermic Plinthic Kandiudults). Cotton was planted in each study on 91‐cm row widths at seeding rates ranging from 3.5 to 25.1 seeds m−2. At harvest, each plot was hand picked and boll numbers and weights were recorded at each monopodial branch and sympodial branch fruiting position. Lower population densities led to plants with more mainstem nodes and monopodial branches with increased fruit retention, resulting in greater fruit production per plant. Boll size was inversely related to population density. Mean net assimilation rate from first flower to peak bloom also was related inversely to population density. The mainstem node of peak boll set increased with population density. Fruit production on a ground area basis was greater in the first sympodial position as population density increased, while fruit production on a ground area basis in third positions and monopodial branches was greater as population density decreased. Accumulative seedcotton from sympodial branches also increased with population density. Total fruit number and seedcotton yield per area were not influenced by population density in these studies. Yield stability across population densities was achieved through manipulation of boll occurrence and weight.

A field study of the effects of elevated CO2 and plant species diversity on ecosystem‐level gas exchange in a planted calcareous grassland

AbstractThe relationship between plant species diversity and ecosystem CO2 and water vapour fluxes was investigated for planted calcareous grassland communities composed of 5, 12, or 32 species assembled from the native plant species pool. These diversity manipulations were done in factorial combination with a CO2 enrichment experiment in order to investigate the degree to which ecosystem responses to elevated CO2 are altered by a loss of plant diversity. Ecosystem CO2 and H2O fluxes were measured over several 24‐h periods during the 1994 and 1995 growing seasons. Ecosystem CO2 assimilation on a ground area basis decreased with decreasing plant diversity in the first year and this was related to a decline in above‐ground plant biomass. In the second year, however, CO2 assimilation was not affected by diversity, and this corresponded to the disappearance of a diversity effect on above‐ground biomass. Irrespective of diversity treatment, CO2 assimilation on a ground area basis was linearly related to peak above‐ground biomass in both years. Elevated CO2 significantly increased ecosystem CO2 assimilation in both years with no interaction between diversity and CO2 treatment, and no corresponding increase in above‐ground biomass. There were no significant effects of diversity on water vapour flux, which was measured only in the second year. There were indications of a small CO2 effect on water vapour flux (3–9% lower at elevated CO2 depending on the light level). Our findings suggest that decreasing plant species diversity may substantially decrease ecosystem CO2 assimilation during the establishment of such planted calcareous grassland communities, but also suggest that this effect may not persist. In addition, we find no evidence that plant species diversity alters the response of ecosystem CO2 assimilation to elevated CO2.

Training and Pruning System Effects on Vegetative Growth Potential, Light Interception, and Cropping Efficiency in Peach Trees

Plant dry matter production is proportional to light interception, but fruit production does not always increase with increased light interception. Vegetative growth potential, the effect of cropping on vegetative growth, light interception and cropping efficiency of a clingstone peach [Prunus persica (L.) Batsch `Ross' on `Nemaguard' rootstock] were assessed in four production systems differing in tree density and training system. The four systems were a perpendicular V (KAC-V) system, a high-density perpendicular V (HiD KAC-V) system, a cordon system, and an open vase system. Vegetative growth potential, assessed on defruited trees, was higher in the cordon system and lower in the open vase system compared to the V systems. Cropping reduced leaf growth on the V and cordon systems and stem growth on the KAC-V and cordon systems. On a ground area basis, the HiD KAC-V system had the highest crop yields and the open vase system had the lowest. The cordon and HiD KAC-V systems intercepted more light and produced more fruit, stem, and leaf biomass than the open vase system. However, the modified harvest increment, the ratio of fruit dry mass to the sum of fruit, leaf, and stem dry mass, was lower in the cordon system than in the other systems. Thus, on this basis, the cordon system was the least efficient. On a trunk cross-sectional area basis, there were no significant differences in fruit production among any of the four training systems. For current year production, crop production per unit ground area is the best measure of economic efficiency. However, when planning the spacing, training and pruning of orchard trees, the most appropriate goal seems to be a system that increases light interception without increasing vegetative growth potential, such as the HiD KAC-V system.

Influence of fertilization and atmospheric CO 2 enrichment on ecosystem CO 2 and H 2O exchanges in single- and multiple-species grassland microcosms

This paper reports on measurements of net CO 2 and H 2O exchange from single- and multiple-species microcosms composed of California annual grassland species grown at either ambient or elevated (ambient+36 Pa) CO 2. Microcosms consisted of grassland species grown in PVC tubes (∼0.95 m deep×0.2 m diameter) containing ∼45 kg of either serpentine or sandstone derived soil or parent material in open-top enclosures under ambient meteorological conditions. Half of the microcosms were left unfertilized (low nutrient) while the other half received an intermediate level of a slow-release (N,P,K) fertilizer (high nutrient). Gas exchange was performed by sealing individual microcosms within a transparent chamber (on clear sunny days) and coupling this to an open gas-exchange system. In fertilized single-species microcosms, elevated CO 2 consistently enhanced net `ecosystem' CO 2 exchange (NCE) on a ground area basis in both early and late spring. Among unfertilized single-species microcosms, no significant trends or differences were observed in NCE between those grown at ambient versus elevated CO 2. The NCE in sandstone and serpentine multiple-species microcosms was monitored seasonally over a majority of the 1993–1994 growing season. Rates were largely unaffected by growth CO 2 or fertilization until after mid-February, 1994. Water-use efficiency (WUE=NCE/evapotranspiration (ET)) was generally enhanced by elevated CO 2, but this was primarily a result of enhancements in NCE as opposed to decreases in ET. Enhancements in NCE by elevated CO 2 in fertilized single-species microcosms at the growth-CO 2 concentration were partially explained by higher above-ground biomass in elevated CO 2 microcosms. However, ecosystem-level `acclimation' occurred such that microcosms grown at elevated CO 2 consistently had lower NCE than ambient CO 2 treatments at a single measurement CO 2 concentration (ambient or elevated). The reduction in apparent ecosystem-level photosynthetic capacity in elevated CO 2 microcosms was accompanied by decreases in foliar Rubisco activity, such that NCE measured at ambient CO 2 was highly correlated ( r=0.98) with foliar Rubisco activity across the three single-species microcosms in which it was measured.

Modeling transpiration and soil water content from a corn ( Zea Maize L.) field: 20 min vs. daytime integration step

Simulations of transpiration (T) and soil water depletion using daytime and 20 min interval models of similar general structure were compared. The performance of the models were evaluated using the measurements obtained from a full-canopy corn field subjected to wet and dry irrigation treatments. The wet treatment was well irrigated throughout the season while irrigation for the dry treatment was stopped when full canopy was reached. 20 min interval evapotranspiration (ET) in the wet treatment was determined using the Bowen ratio method and was used as a crop maximum ET input to the models. For both treatments, sap flow measurements averaged over 20 min intervals were performed continuously for 2 to 9-day periods, for a total of 40 days. These devices measure crop water uptake, which can be equated to T by neglecting plant storage. Sap flow per unit green leaf area was measured simultaneously on 7–10 plants per treatment and expressed on a per unit ground area basis. In addition, soil water content and root density were measured throughout the season in both treatments. The daytime and the 20 min (with output integrated for daytime periods) models performed similarly in the simulation of T for both irrigation treatments. Overall, the root mean square error (RMSE) fluctuated from 0.28 to 0.38 mm day −1, and the relative error (RE=100×RMSE/observed mean) from 7.2 to 10.5%. Also, both models performed similarly in simulating soil water content. No advantage was found from the use of the shorter time step model. The use of a daytime integration step appears sufficient unless short-time interval details are needed as model output or for use by other components of a larger model.

Leaf conductance and CO(2) assimilation of Larix gmelinii growing in an eastern Siberian boreal forest.

In July 1993, we measured leaf conductance, carbon dioxide (CO(2)) assimilation, and transpiration in a Larix gmelinii (Rupr.) Rupr. ex Kuzen forest in eastern Siberia. At the CO(2) concentration of ambient air, maximum values (mean of 10 highest measured values) for CO(2) assimilation, transpiration and leaf conductance for water vapor were 10.1 micro mol m(-2) s(-1), 3.9 mmol m(-2) s(-1) and 365 mmol m(-2) s(-1), respectively. The corresponding mean values, which were much lower than the maximum values, were 2.7 micro mol m(-2) s(-1), 1.0 mmol m(-2) s(-1) and 56 mmol m(-2) s(-1). The mean values were similar to those of Vaccinium species in the herb layer. The large differences between maximum and actual performance were the result of structural and physiological variations within the tree crowns and between trees that reduced maximum assimilation and leaf conductance by about 40 and 60%, respectively. Thus, maximum assimilation and conductance values averaged over the canopy were 6.1 micro mol m(-2) s(-1) and 146 mmol m(-2) s(-1), respectively. Dry air caused stomatal closure, which reduced assimilation by an additional 26%. Low irradiances in the morning and evening had a minor effect (-6%). Daily canopy transpiration was estimated to be 1.45 mm day(-1), which is higher than the value of 0.94 mm day(-1) measured by eddy covariance, but similar to the value of 1.45 mm day(-1) calculated from the energy balance and soil evaporation, and less than the value of 2.1 mm day(-1) measured by xylem flux. Daytime canopy carbon assimilation, expressed on a ground area basis, was 0.217 mol m(-2) day(-1), which is higher than the value measured by eddy flux (0.162 mol m(-2) day(-1) including soil respiration). We discuss the regulation of leaf gas exchange in Larix under the extreme climatic conditions of eastern Siberia (temperature > 35 degrees C and vapor pressure deficit > 5.0 kPa).

Crown conductance and tree and stand transpiration in a second-growthAbies amabilisforest

We measured whole-tree sap flow in a 43-year-old Abies amabilis (Dougl. ex Loud.) Dougl. ex J. Forbes n Tsuga heterophylla (Raf.) Sarg. forest in western Washington, U.S.A. We calculated whole-tree crown conductance to water vapor (gcrown) by substituting the sap flow data and meteorological measurements into the inverted PenmannMonteith equation. Individual tree sap flow and crown conductance varied widely with tree size, with the smallest tree sampled having average gcrown (on a ground-area basis) of 0.57 mms n1 and transpiring up to 4.9 kgday n1 , while the largest tree measured had an average gcrown of 7.20 mms n1 and lost as much as 98 kgday n1 . Crown conductance responded linearly and positively to radiation, and had a negative exponential response to vapor pressure deficit. These response patterns were utilized to construct an empirical model of gcrown that explained from 52 to 73% (average 66%) of the variation in gcrown. The dominant and codominant trees in the stand transpired for longer periods during the day than trees in the smaller size classes, and contributed disproportionately to total stand transpiration. Daily total sap flow for individual trees was strongly correlated with tree basal area; we used the relationship between these variables to estimate daily stand transpiration. Stand transpiration calculated for 28 days between August 6 and October 12, 1993, ranged from 0.01 to 3.52 mmday n1 . Stand transpiration increased curvilinearly with increasing average daily vapor pressure deficit, reflecting the negative response of gcrown to vapor pressure deficit.

Nitrogen translocation via rhizome systems in monoclonal stands of Reynoutria japonica in an oligotrophic desert on Mt Fuji: Field experiments

AbstractMechanisms which enable Reynoutria japonica, a dominant pioneer herb, to be successful in maintaining large stands in an oligotrophic volcanic desert on Mt Fuji were investigated with special reference to its nitrogen acquisition. Reynoutria japonica forms circular stands, each of which comprises only one genet. As a stand develops outwards, the number of aerial shoots per unit area decreases in the center. Shoots grow vigorously in the peripheral area where the available nitrogen from soil and precipitation (about 2.4 g m−2 year−2) was much less than total nitrogen in the shoots (6.1–9.1 g m−2). Leaf nitrogen content per unit mass was also greater in the leaves of the peripheral shoots. When rhizomes extending radially from the center to the periphery were severed, the dry mass of shoots in the periphery diminished by 75% on a ground area basis. In the periphery, leaf nitrogen content also reduced significantly and no flowers were produced. When fertilizer was applied to the peripheral shoots with severed rhizomes, neither growth, survival nor flower production of the shoots was significantly smaller than the control levels. In these shoots, it is also found that the nitrogen content in the youngest leaves decreased for about 1 month and then increased to above that in the control leaves. These results suggest that (i) nitrogen accumulated in the central part is translocated to peripheral shoots via rhizomes, and that long‐distance translocation enables the stands to develop outwards, and (ii) aerial shoots in the periphery utilize the nitrogen translocated by rhizomes in the beginning of the growth season, whereas once the shoots have established, they begin to take up nitrogen with their own roots. Since the peripheral shoots are in sunnier environments than the shoots inside the stand, the acropetal nitrogen translocation via rhizomes will raise the production efficiency of a whole stand.

Growth indices and stomatal control of transpiration in Acacia koa stands planted at different densities.

We examined the influences of selected environmental variables on stomatal behavior and regulation of transpiration in 26-month-old Acacia koa Gray (koa) stands planted at spacings of 1 x 1 m or 2.5 x 2.5 m and grown without irrigation. Field measurements were made during recovery from an extended 60-day dry period with only 38 mm of precipitation. Biomass and leaf area were also measured at 3-month intervals over the first 24 months after planting and again following completion of the transpiration (T) and stomatal conductance (g(s)) measurements at about 26 months after planting. Transpiration was measured as sap flow through intact branches by a heat balance method. After a 22-day period during which 130 mm of rain were recorded, average T was substantially higher in the 2.5 x 2.5 m stand on both a leaf area and ground area basis even though leaf area index was about 3.5 times higher in the 1 x 1 m stand. After an additional 25 mm of rain during an 8-day period, T was still slightly higher on a leaf area basis in the 2.5 x 2.5 m stand but was about 3 times higher on a ground area basis in the 1 x 1 m stand. A strong stomatal response to humidity limited the increase in T with increasing evaporative demand. Values of g(s) in koa phyllodes were comparable to those reported for leaves of other mesic tropical forest trees, but were several times higher than those reported for Acacia species native to arid and semi-arid regions. The 1 x 1 m planting yielded three times more biomass per unit ground area than the 2.5 x 2.5 m planting. However, greater stand density, which resulted in more rapid depletion of soil water between rainfall inputs, was associated with lower growth efficiency and lower radiation conversion efficiency.

Carbon assimilation and water-use efficiency by neighboring Mediterranean-climate oaks that differ in water access.

Quercus agrifolia Nee and Quercus durata Jeps. are sclerophyllous evergreens that co-occur in regions of northern California that have a serpentine-derived soil. The species are of similar stature and density where they grow together, even though late-drought xylem pressure potentials indicate that Q. durata has access to more soil water than Q. agrifolia. To assess how carbon uptake and water-use efficiency (WUE) are affected by water access, I monitored leaf and canopy gas exchange of neighboring Q. durata and Q. agrifolia trees over a 15-month period. Transpiration and photosynthesis by Q. agrifolia peaked in spring and declined through the summer, whereas transpiration and photosynthesis by Q. durata continued at a moderate rate year round. When summed over the study, Q. agrifolia transpired 25% less water on a ground-area basis than Q. durata, but assimilated 25% more carbon. Quercus agrifolia achieved a greater integrated WUE by: (1) maintaining a 20% advantage in instantaneous WUE as a result of lower leaf intercellular CO(2) concentrations; (2) responding rapidly to increased soil water following rain; and (3) assimilating carbon at high rates during periods of low evaporative demand.

Vegetation Effects on the Isotopic Composition of Atmospheric CO2 at Local and Regional Scales: Theoretical Aspects and a Comparison Between Rain Forest in Amazonia and a Boreal Forest in Siberia

In order to understand the factors influencing the isotopic composition of air above and within plant canopies, equations quantifying the effects of photosynthesis, respiration and turbulent transport on the isotopic composition of the surrounding CO2 are developed. These equations are then extended to the regional scale, allowing the average isotopic composition of CO2 within the convective boundary layer to be related to the isotopic composition of tropospheric CO2, and to isotopic fractionations during ecosystem carbon exchange. Equations presented have the potential to be inverted, allowing direct estimation of isotopic fractionations by vegetation at the local and regional scales. Equations allowing the estimation of the extent of refixation of respired CO2 ('recycling') at the regional scale are also presented. Using measurements of CO2 carbon isotopic composition in conjunction with ecosystem flux measurements, the theory is applied to a tropical rain forest in Amazonia and a boreal forest in Siberia. When examined on a ground area basis and over the course of a day it is observed that, by virtue of greater fluxes but similar isotopic fractionations, the tropical rainforest exerts much more influence over the isotopic composition of the surrounding air than does the boreal forest. Due to higher rates of ecosystem respiration, recycling of respired CO2 is modelled to be much greater for tropical rainforest, but values presented here are considerably lower than previously published estimates, the latter being based solely on the relationship between the isotopic composition and concentrations of CO2 within forest canopies. The reasons for these differences are examined.

Comparisons of xylem sap flow and water vapour flux at the stand level and derivation of canopy conductance for Scots pine

Simultaneous measurements of xylem sap flow and water vapour flux over a Scots pine (Pinus sylvestris) forest (Hartheim, Germany), were carried out during the Hartheim Experiment (HartX), an intensive observation campaign of the international programme REKLIP. Sap flow was measured every 30 min using both radial constant heating (Granier, 1985) and two types of Cermak sap flowmeters installed on 24 trees selected to cover a wide range of the diameter classes of the stand (min 8 cm; max 17.5 cm). Available energy was high during the observation period (5.5 to 6.9 mm.day−1), and daily cumulated sap flow on a ground area basis varied between 2.0 and 2.7 mm day−1 depending on climate conditions. Maximum hourly values of sap flow reached 0.33 mm h−1, i.e., 230 W m−2. Comparisons of sap flow with water vapour flux as measured with two OPEC (One Propeller Eddy Correlation, University of Arizona) systems showed a time lag between the two methods, sap flow lagging about 90 min behind vapour flux. After taking into account this time lag in the sap flow data set, a good agreement was found between both methods: sap flow = 0.745* vapour flux,r 2 = 0.86. The difference between the two estimates was due to understory transpiration. Canopy conductance (g c ) was calculated from sap flow measurements using the reverse form of Penman-Monteith equation and climatic data measured 4 m above the canopy. Variations ofg c were well correlated (r 2 = 0.85) with global radiation (R) and vapour pressure deficit (vpd). The quantitative expression forg c =f (R, vpd) was very similar to that previously found with maritime pine (Pinus pinaster) in the forest of Les Landes, South Western France.

Contrasting leaf and ?ecosystem? CO2 and H2O exchange in Avena fatua monoculture: Growth at ambient and elevated CO2

Elevated CO2 (ambient + 35 Pa) increased shoot dry mass production in Avena fatua by ∼ 68% at maturity. This increase in shoot biomass was paralleled by an 81% increase in average net CO2 uptake (A) per unit of leaf area and a 65% increase in average A at the 'ecosystem' level per unit of ground area. Elevated CO2 also increased 'ecosystem' A per unit of biomass. However, the products of total leaf area and light-saturated leaf A divided by the ground surface area over time appeared to lie on a single response curve for both CO2 treatments. The approximate slope of the response suggests that the integrated light saturated capacity for leaf photosynthesis is ∼ 10-fold greater than the 'ecosystem' rate. 'Ecosystem' respiration (night) per unit of ground area, which includes soil and plant respiration, ranged from-20 (at day 19) to-18 (at day 40) μmol m(-2) s(-1) for both elevated and ambient CO2 Avena. 'Ecosystem' below-ground respiration at the time of seedling emergence was ∼-10 μmol m(-2) s(-1), while that occuring after shoot removal at the termination of the experiment ranged from -5 to-6 μmol m(-2) s(-1). Hence, no significant differences between elevated and ambient CO2 treatments were found in any respiration measure on a ground area basis, though 'ecosystem' respiration on a shoot biomass basis was clearly reduced by elevated CO2. Significant differences existed between leaf and 'ecosystem' water flux. In general, leaf transpiration (E) decreased over the course of the experiment, possibly in response to leaf aging, while 'ecosystem' rates of evapotranspiration (ET) remained constant, probably because falling leaf rates were offset by an increasing total leaf biomass. Transpiration was lower in plants grown at elevated CO2, though variation was high because of variability in leaf age and ambient light conditions and differences were not significant. In contrast, 'ecosystem' evapotranspiration (ET) was significantly decreased by elevated CO2 on 5 out of 8 measurement dates. Photosynthetic water use efficiencies (A/E at the leaf level, A/ET at the 'ecosystem' level) were increased by elevated CO2. Increases were due to both increased A at leaf and 'ecosystem' level and decreased leaf E and 'ecosystem' ET.

Effects of Seeding Rate on Growth Duration and Accumulation and Partitioning of Dry matter in Oats

AbstractCrop management influences considerably the three components of grain yield, growth duration, growth rate, and harvest index (HI). Effects of seeding rate on these yield components in oats (Avena sativa L.) was assessed in field experiments at the Viikki Experimental Farm, University of Helsinki, Finland (60°13′N) in 1991 and 1992. Three Finnish oat genotypes were evaluated; a long‐strawed landrace cultivar, a moderately long‐strawed modern cultivar, and a semi‐dwarf breeding line. The following traits were measured: grain yield, days from sowing to yellow ripeness, number of tillers on main shoot, phytomass, vegetative phytomass, and their growth rates (PGR and VGR, respectively), panicle weight and its filling rate (PFR), HI, leaf area index (LAI), and at intervals, dry‐matter accumulation in leaves and straw.Increases in seeding rate significantly decreased growth duration and PGR of individual plants but increased PGR on a ground area basis. Seeding rate did not, however, affect HI. When seeding rate was increased from 200 seeds m−2 to 500 seeds m−2, reductions in vegetative phytomass, panicle weight, VGR, and PFR for individual plants ranged between 20 and 40 %, depending on genotype. At ≥600 seeds m−2 differences in these components between seeding rates were modest. However, PGR, VGR, and PFR per unit ground area increased with increasing seeding rates up to 600–700 seeds m−2. Moreover, the higher the seeding rate, the higher the peak LAI (2.7 maximum) and the earlier the canopy closure. Hence, our results showed that a seeding rate of 600–700 seeds m−2, which resulted in uniculm growth habit, is advantageous in terms of grain yield at high latitudes due to higher biomass accumulation and subsequently greater interception of PAR.

Evapotranspiration of beech stands and transpiration of beech leaves subject to atmospheric CO(2) enrichment.

Beech trees (Fagus sylvatica L.) show reduced stomatal conductance and increased leaf area index in response to increased atmospheric CO(2) concentration. To determine whether the reduction in stomatal conductance results in lower stand evapotranspiration, we compared transpiration on a leaf-area basis and stand evapotranspiration on a ground-area basis in young European beech trees growing in greenhouses at ambient (360 +/- 34 micro mol mol(-1)) and elevated (698 +/- 10 micro mol mol(-1)) CO(2) concentrations. Trees were grown in homogenized natural soil at constant soil water supply for two growing seasons. At light saturation, leaf transpiration rates were, on average, 18% lower in the elevated CO(2) treatment than in the ambient CO(2) treatment. Mean transpiration coefficients (transpiration/net CO(2) uptake) of leaves were 179 and 110 in the ambient and elevated CO(2) treatments, respectively, indicating improved water use efficiency in trees in the elevated CO(2) treatment. Total leaf conductance was decreased by 32% at light saturation. The elevated CO(2) treatment resulted in a 14% reduction in stand evapotranspiration. In both CO(2) treatments, evapotranspiration increased linearly at a rate of 0.2 kg H(2)O m(-2) day(-1) for each 1 degrees C rise in air temperature between 14 and 25 degrees C. We conclude that, under Central European conditions, water losses from deciduous forest stands will be reduced by a doubling of tropospheric CO(2) concentration.

Interception of photosynthetic photon flux density by woody elements in a hinoki (Chamaecyparis obtusa [Sieb. et Zucc.] Endl.) stand

AbstractA hinoki stand was divided into the following two layers: one was a leafed upper layer consisting of leaves and woody elements, such as stems, living branches and dead branches; the other was a leafless layer consisted of woody elements, such as stems and dead branches. Photosynthetic photon flux density (PPFD) penetrating through the stand was measured in relation to the silhouette area of leaves and woody elements. The silhouette area, on a ground area basis, was 11.2 ha ha−1 for leaves (leaf area index), 0.26 ha ha−1 for stems, 0.40 ha ha−1 for living branches and 0.69 ha ha−1 for dead branches, yielding a total wood silhouette area of 1.35 ha ha−1. The apparent extinction coefficient, K, was computed to be 0.420 ha−1 ha for the leafed layer, while the extinction coefficient of woody elements, Kc, was computed to be 1.01 ha−1 ha for the leafless layer. The cumulative wood silhouette area density from the top of the canopy down to a given depth increased with an increase in the corresponding leaf area density within the leafed layer. The extinction coefficient of leaves, KF, was estimated to be 0.367 ha−1 ha. Of the PPFD extinguished within the canopy, the fraction (KF/K) due to leaves alone was evaluated to be 87.4%.

Dynamics of Soybean Growth in Variable Planting Patterns

AbstractReported soybean [Glycine max (L.) Merr.] yield responses to variable plant populations have been inconsistent. The varied responses are tied to the capacity of the plant stand to attain a minimal level of dry matter prior to reproductive growth. This study examined soybean growth expressed as functions of both plant and ground area to identify associations with yielding ability. Treatments consisted of high, medium, and low populations (approximately 21, 13, and 3 plants per square meter) at 0.96‐m row widths and a medium population (approximately 13 plants m−2) at 0.43‐m row widths for 2 yr. The values of total dry matter (TDM), stem DM, leaf DM, leaf area, height, and main stem node number were determined on both a whole plant and ground area basis. In addition, relative growth rate (RGR), relative leaf area growth rate (RLAGR) and net assimilation rate (NAR) were examined. Treatments with the greatest interplant space had the largest RGR, RLAGR, and NAR early in growth, thus resulting in compensation for reduced plant population. Decreases in plant DM at 91 DAP in response to increasing plant population were greater in 1988 than in 1989, and were related to poorer growing conditions in the latter year. The related TDM values were larger in 1988, with all treatments exhibiting a TDM above 500 g m−2 by early podfill (R5). In contrast, the TDM at 91 DAP of the wide row, low and medium population treatments was 300 and 426 g m−2, respectively in 1989. Seed yield differences were found in 1989 but not 1988. Differences among years and treatments concerning growth rates, plant DM, TDM, and seed yield emphasize the impact of limiting environmental factors on plant compensation in response to increased space.

Effects of daytime carbon dioxide concentration on dark respiration in rice

ABSTRACTRising atmospheric carbon dioxide concentration ([CO2]) has generated considerable interest in the response of agricultural crops to [CO2]. The objectives of this study were to determine the effects of a wide range of daytime [CO2] on dark respiration of rice (Oryza sativa L. cv. IR‐30). Rice plants were grown season‐long in naturally sunlit plant growth chambers in subambient (160 and 250), ambient (330), or super‐ambient (500, 660 and 900 μmol CO2 mol−1 air) [CO2] treatments. Canopy dark respiration, expressed on a ground area basis (Rd) increased with increasing [CO2] treatment from 160 to 500 μmol mol−1 treatments and was very similar among the superambient treatments. The trends in Rd over time and in response to increasing daytime [CO2] treatment were associated with and similar to trends previously described for photosynthesis. Specific respiration rate (Rdw) decreased with time during the growing season and was higher in the subambient than the ambient and superambient [CO2] treatments. This greater Rdw in the subambient [CO2] treatments was attributed to a higher specific maintenance respiration rate and was associated with higher plant tissue nitrogen concentration.

Deuterium tracing for the estimation of transpiration from trees Part 3. Measurements of transpiration from Eucalyptus plantation, India

Measurements of transpiration from individual trees of Eucalyptus from plantations at four different sites in Karnataka, Southern India, are presented. These show large (as much as tenfold) differences in the transpiration between premonsoon and postmonsoon periods, a reflection of the effects of soil-moisture stress in the premonsoon periods. For trees with diameters at breast height (DBH) less than 10 cm the transpiration rate of individual trees is proportional to the square of the DBH. For trees which are not experiencing soil-water stress the daily transpiration rate of individual trees, q, is well represented by the relation: q = (6.6 ± 0.3) g (m 3 day −1 where g (m 2) is the tree basal area. On a unit ground area basis the transpiration rate, expressed as a depth per day is given by the relation: E t = (0.66 ± 0.03) G (mm day −1 where g(m 2ha −1) is the total basal area per hectare. For all the sites studied, although there is evidence for the ‘mining’ of soil water as roots penetrate deeper depths in the soil each year, there is no evidence for direct abstraction from the water table.