Canopy Air Research Articles

Environmental controls on net ecosystem‐level carbon exchange and productivity in a Central American tropical wet forest

AbstractDifficulty in balancing the global carbon budget has lead to increased attention on tropical forests, which have been estimated to account for up to one third of global gross primary production. Whether tropical forests are sources, sinks, or neutral with respect to their carbon balance with the atmosphere remains unclear. To address this issue, estimates of net ecosystem exchange of carbon (NEE) were made for 3 years (1998–2000) using the eddy‐covariance technique in a tropical wet forest in Costa Rica. Measurements were made from a 42 m tower centred in an old‐growth forest. Under unstable conditions, the measurement height was at least twice the estimated zeroplane height from the ground. The canopy at the site is extremely rough; under unstable conditions the median aerodynamic roughness length ranged from 2.4 to 3.6 m. No relationship between NEE and friction velocity (u*) was found using all of the 30‐min averages. However, there was a linear relationship between the nighttime NEE and averaged u* (R2 = 0.98). The diurnal pattern of flux was similar to that found in other tropical forests, with mean daytime NEE ca. − 18 μmol CO2 m−2 s−1 and mean nighttime NEE 4.6 μmol CO2 m−2 s−1. However, because ∼ 80% of the nighttime data in this forest were collected during low u* conditions (< 0.2 m s−1), nighttime NEE was likely underestimated. Using an alternative analysis, mean nighttime NEE increased to 7.05 μmol CO2 m−2 s−1. There were interannual differences in NEE, but seasonal differences were not apparent. Irradiance accounted for ∼ 51% of the variation in the daytime fluxes, with temperature and vapour pressure deficit together accounting for another ∼ 20%. Light compensation points ranged from 100 to 207 μmol PPFD m−2 s−1. No was relationship was found between 30‐min nighttime NEE and tower‐top air temperature. A weak relationship was found between hourly nighttime NEE and canopy air temperature using data averaged hourly over the entire sampling period (Q10 = 1.79, R2 = 0.17). The contribution of below‐sensor storage was fairly constant from day to day. Our data indicate that this forest was a slight carbon source in 1998 (0.05 to −1.33 t C ha−1 yr−1), a moderate sink in 1999 (−1.53 to −3.14 t C ha−1 yr−1), and a strong sink in 2000 (−5.97 to −7.92 t C ha−1 yr−1). This trend is interpreted as relating to the dissipation of warm‐phase El Niño effects over the course of this study.

Cooling Fan-ventilated Greenhouses: a Modelling Study

A model for fan-ventilated greenhouse cooling is presented in which the primary heat transfer surfaces (cover/structure, canopy and floor) are represented as three parallel planes. Validation of the model was accomplished using data collected over 14 days. Agreement was good, with canopy temperatures over-predicted by only 0·1%, air temperatures in the canopy under-predicted by 0·5%, humidity of the canopy air under-predicted by 1·6% and transpiration rates under-predicted by 1·4%. Simulation runs suggest that when evaporative pad cooling is not used, little advantage is derived from increasing airflow rates beyond about 0·05 m 3 m −2 s −1. When evaporative pad cooling is used, however, both air and canopy temperatures decline with increasing airflow rates up to 0·13 m 3 m −2 s −1, the highest level considered. Increasing canopy size is predicted to be more influential in reducing air temperatures when evaporative pad cooling is used than when it is not, but its effect on canopy temperature is expected to be approximately the same whether or not evaporative pad cooling is used. With no evaporative pad cooling, the evapotranspiration coefficient (i.e., the ratio of energy used for transpiration to incoming solar energy) is predicted to range from 1·75 for an outside temperature of 36·8°C and an outside humidity ratios of 3·3 g kg −1 to 0·8 for an outside humidity ratio of 29·9 g kg −1 at the same temperature. With evaporative pad cooling, the coefficient is predicted to range from 0·6 to 0·8 at the same outside temperature and the same range of outside humidity ratios.

The stable carbon isotope composition of the terrestrial biosphere: Modeling at scales from the leaf to the globe

Global data sets of the stable carbon isotope composition of plant leaves, of CO2 in canopy air, and of CO2 in the background atmosphere were compiled and compared to results of a global vegetation model (BIOME4) that simulated, at these three scales, the magnitude, direction, and timing of fluxes of CO2 and 13C between the biosphere and the atmosphere. Carbon isotope data on leaves were classified into 12 Plant Functional Types (PFTs), and measurements from canopy flasks were assigned to 16 biomes, for direct comparison to model results. BIOME4 simulated the observed leaf δ13C values to within 1 standard deviation of the measured mean for most PFTs. Modeled δ13C for C3 grasses, tundra shrubs, and herbaceous plants of cold climates deviated only slightly more from measurements, perhaps as a result of the wide geographic range and a limited set of measurements of these PFTs. Modeled ecosystem isotopic discrimination against 13C (Δe) averaged 18.6 globally when simulating potential natural vegetation and 18.1 when an agricultural crop mask was superimposed. The difference was mainly due to the influence of C4 agriculture in areas that are naturally dominated by C3 vegetation. Model results show a gradient in Δe among C3‐dominated biomes as a result of stomatal responses to aridity; this model result is supported by canopy air measurements. At the troposphere scale, BIOME4 was coupled to a matrix representation of an atmospheric tracer transport model to simulate seasonally varying concentrations of CO2 and 13C at remote Northern Hemisphere measuring stations. Ocean CO2 and 13C flux fields were included, using the HAMOCC3 ocean biogeochemistry model [Six and Maier‐Reimer, 1996]. Model results and observations show similar seasonal cycles, and the model reproduces the inferred latitudinal trend toward smaller isotopic discrimination by the biosphere at lower latitudes. These results indicate that biologically mediated variations in 13C discrimination by terrestrial ecosystems may be significant for atmospheric inverse modeling of carbon sources and sinks, and that such variations can be simulated using a process‐based model.

Simulation of air temperature inside the canopy by the LAPS surface scheme

A land–air parameterization scheme (LAPS) describes water vapour, heat and momentum transfer between the land surface and the atmosphere. The scheme is designed as a software package, which can be run as part of an atmospheric, hydrological or ecological model, or as a stand-alone model that operates with seven prognostic variables and 16 morphological and physiological input parameters. Such large number of parameters provides reliable simulation of diurnal courses of meteorological elements inside the crop. In this paper, for simulating the diurnal course of air temperature inside a sunflower field, the LAPS scheme has been used. The results obtained are compared with the results of micrometeorological measurements of a sunflower experimental field of the Oil Crops Department at Rimski Šančevi (Yugoslavia) using data sets from 18 July to 24 July, 1998. The LAPS scheme was compared with two other surface schemes using canopy air temperature data derived from measured air temperature above a maize field at an experimental site in De Sinderhoeve (The Netherlands) for 18 August, 8 September, and 4 October 1988.

Mulching, farmyard manure and exogenous application of osmo-protectants confer terminal heat tolerance in wheat (Tritcum aestivum)

A field experiment was conducted at Ludhiana, Punjab (India) with combinations of 2 sowing dates [timely sowing (10 November) and late sowing (15 December)] and 6 heat-stress-management treatments includes recommended dose of fertilizer (RDF: 150 : 27.3 : 24.8 N, P and K kg/ha), RDF + FYM (10 t/ha farmyard manure), RDF + mulch (rice straw mulch @ 4 t/ha), RDF + FYM + mulch, RDF + FYM + mulch + ZnSO4 (0.5%) and RDF + FYM + mulch + KCl (0.2%) in randomized block design with 3 replications during 201516 and 201617. The timely sown wheat showed higher emergence count, canopy air temperature difference, normalized difference vegetative index, days to earing/maturity, yield attributes, grain yield and water-use efficiency than the late-sown one. The grain yield recorded in RDF + FYM + mulch + KCl was the highest, which was statistically similar to RDF + 10 t FYM, RDF + Mulch, RDF + FYM + mulch and RDF + FYM + mulch + ZnSO4. The grain yield and water-use efficiency recorded in RDF + FYM + mulch + KCl, RDF + FYM + mulch + ZnSO4 and RDF + FYM + mulch was at par under late sown condition. Under late-sown conditions, RDF + FYM + Mulch + KCl can be used to ameliorate the adverse effect of terminal heat stress on wheat and enhance the grain yield and water use efficiency.

Canopy temperature and water stress quantificaiton in rainfed pigeonpea ( Cajanus cajan (L.) Millsp.)

Field studies were conducted at Anand (22°35′N, 72°55′E), India, to investigate utility of remotely sensed canopy temperature to assess crop water status and to quantify terminal water stress in rainfed pigeonpea ( Cajanus cajan (L.) Millsp.) during the southwest monsoon seasons of 1993–1994 and 1994–1995, using different planting dates. Canopy–air temperature differential were measured by thermal IR thermometer. Differences in temporal variation in canopy–air temperature differential clearly described the differential response of pigeonpea to soil water availability. The canopy–air temperature differential ( T c− T a) was significantly ( P≤0.05) related to soil moisture, soil water content in the root zone and evapotranspiration. Significantly negative relationship was also noticed for yield attributes with both T c− T a and stress degree days (SDD) during reproductive ontogeny. Total water use during reproductive phase in pigeonpea decreased with increasing SDD. A close relationship between seed yield per plant and SDD in flowering stage clearly showed the potential utility of canopy temperature to describe water stress effects in rainfed pigeonpea.

A crop water stress index for tall fescue ( Festuca arundinacea Schreb.) irrigation decision-making — a traditional method

A high irradiance plant growth chamber was used to study crop water stress indices (CWSI) and baselines with increasing soil water deficit for Tall Fescue ( Festuca arundinacea Schreb.). Canopy temperatures for turf plugs were continuously measured with infrared thermometers, along with plant water use, measured with electronic mini-lysimeters. Net radiation, canopy and air temperatures, and vapor pressure deficit (VPD) levels were recorded and analyzed statistically. The canopy–air temperature differential ( T c− T a) increased with a decrease in soil moisture content. T c− T a increased as net radiation became greater, independent of soil water deficit. Canopy temperature of well-watered plants decreased at rate of 2.4°C for each 1 kPa reduction in air vapor pressure deficit for all net radiation levels. For each 100 Wm −2 increase in net radiation, canopy temperature of well-watered plants increased at a rate of 0.6°C and was well correlated (well-watered baseline) with VPD. Increases in canopy temperature coupled with a decrease in transpiration rate were hallmark signs of water stress progression. However, ( T c− T a) and VPD baseline relationships correlated poorly for moderate-stress and severe stress conditions regardless of net radiation levels. Thus, even with the increased precision and replications of a controlled environment study, lower limit crop water stress baselines were quite variable.

Tree Fruit Reflective Film Improves Red Skin Coloration and Advances Maturity in Peach

Replicated trials were conducted during the summers of 1998 and 1999 at commercial orchards in South Carolina to determine the influence of ground application of a metalized, high density polyethylene reflective film on fruit red skin color and maturity of peach (Prunus persica) cultivars that historically have poor red coloration. At each site there were two experimental treatments: 1) control and 2) reflective film (film). Film was applied 2 to 4 weeks before anticipated first harvest date by laying a 150-cm (5-ft) wide strip of plastic on either side of the tree row in the middles. Treatment areas at a given farm ranged from 0.25 to 0.5 ha (0.5 to 1.0 acre) in size and each treatment was replicated four times at each site. At harvest, two 50-fruit samples were picked from each plot per treatment. All fruit were sized and visually sorted for color (1 = 0% to 25%, 2 = 26% to 50%, 3 = 51% to 75%, and 4 = 76% to 100% red surface, respectively). A 10-fruit subsample was selected following color sorting and evaluated for firmness and soluble solids concentration (SSC). All cultivars tested (`CVN1', `Loring', `Bounty', `Summer Gold', `Sunprince', `Cresthaven' and `Encore') experienced significant increases in percent red surface when film was used in 1998 and 1999. This color improvement ranged from 16% to 44% (mean = 28%). On average, fruit from film were 4.2 N (0.9 lb force) softer and had 0.3% higher SSC than control fruit. Growers harvested more fruit earlier and in fewer harvests for film. Fruit size was not affected by film. Reflected solar radiation from film was not different in quality than incident sunlight. Film resulted in an increase in canopy air temperature and a reduction in canopy relative humidity during daylight hours.

Modeling Energy Fluxes from Sparse Canopies and Understorys

Land surfaces are an assemblage of component surface types, for instance overstory vegetation species, understory vegetation species, and bare soil. Often two or more surface types absorb a significant fraction of the available energy to the land surface as a whole. In these cases the interaction of fluxes from the component surfaces may be important to the total land surface energy balance. We compare three models of land surface energy balance: a Penman‐Monteith model; a model with two component surfaces that don't interact (patch model); and a model with interacting component surfaces (Shuttleworth‐Wallace model). Data from six published studies are used to investigate which models best represent a particular land surface taking account of water supply to the component surfaces and overstory canopy architecture. Flux interaction between component surfaces was only found to be important when there was a large difference between the surface resistances (i.e., water availability to the surfaces). Also, all three models were found to estimate the same land surface energy fluxes (to within 50 W m−2) when both surface resistances were >300 s m−1. The ratio of (aerodynamic resistance between the canopy air space and the reference height) to (mean component surface boundary layer resistance) was useful for indicating the level of interaction between component surfaces.

Canopy Microenvironments and Applied Bacteria Population Dynamics in Shaded Tall Fescue

Shading of the turfgrass canopy is assumed to support increased microbial activity and therefore more potential disease problems. The goal of this research was to determine how shading affects microenvironmental conditions, and how these conditions affect phylloplane colonization by applied bacterial strains. Shade cloth was suspended over tall fescue (Festuca arundinacea Schreb.) turf in field plots and environmental parameters were measured in shaded and full sun areas. Shading increased leaf wetness duration and relative humidity (RH), but decreased the average and range of canopy air and foliage temperatures. Three species of bacteria (Bacillus megaterium, Stenotrophomonas maltophilia, and Enterobacter cloacae) were applied to all plots. Populations of B. megaterium were the lowest of the three species in both study years. Data showed that measurable differences in microclimate occur as a result of shading and that bacterial colonization was generally favored by shading. The lack of consistency between the two assay methods and among bacterial species regarding the effects of shading suggests that differences in response to microclimate conditions may exist among bacterial species or strains. Results indicate that shading should be considered when screening biological control agents for disease management.

Vertical Profiles of CO2 Concentration and δ^13C Values in a Subalpine Forest of Taiwan

We investigated the vertical gradients in CO2 concentration and stable carbon isotope ratio (δ^13Cco2) of the canopy air within a coniferous-hardwood, Chamaecyparis and Rhododendron dominated, subalpine forest in Taiwan. The stable carbon isotope ratio of vascular plants and a epiphytic bryophyte species (Bazzania fauriana) (δ^13)C(subscript leaf)) from different heights within the forest were also analyzed. Results revealed that CO2 and δ^13Cco(subscript 2) gradients did exist within the forest, with higher CO2 concentrations and more negative δ^13Cco(subscript 2) values in air sampled from the lower canopy. The average vertical gradients in CO2 and δ^13Cco(subscript 2) value of the CO2 of the 12 sampling dates were 28.5±6.1ppm and 1.3±0.3‰, respectively. Seasonal patterns of the relationship between l/[CO2] and the corresponding δ^13C were pronounced, with steady decreases in the slopes and increases in the intercepts found from January to August. A decreasing δ^13C(subscript leaf) with decreasing height was also measured in a bryophyte (ranging from-27.4 to -29.2%o), canopy and understory leaves (ranging from -28.6 to -33.5%o). It was estimated that photosynthetic physiology affected by microclimates within the forest contributed approximately 2.8% of variation of the vertical gradient of vascular plant δ^13C values.

Coupled Atmosphere–Biophysics–Hydrology Models for Environmental Modeling

The formulation and implementation of LEAF-2, the Land Ecosystem‐Atmosphere Feedback model, which comprises the representation of land‐surface processes in the Regional Atmospheric Modeling System (RAMS), is described. LEAF-2 is a prognostic model for the temperature and water content of soil, snow cover, vegetation, and canopy air, and includes turbulent and radiative exchanges between these components and with the atmosphere. Subdivision of a RAMS surface grid cell into multiple areas of distinct land-use types is allowed, with each subgrid area, or patch, containing its own LEAF-2 model, and each patch interacts with the overlying atmospheric column with a weight proportional to its fractional area in the grid cell. A description is also given of TOPMODEL, a land hydrology model that represents surface and subsurface downslope lateral transport of groundwater. Details of the incorporation of a modified form of TOPMODEL into LEAF-2 are presented. Sensitivity tests of the coupled system are presented that demonstrate the potential importance of the patch representation and of lateral water transport in idealized model simulations. Independent studies that have applied LEAF-2 and verified its performance against observational data are cited. Linkage of RAMS and TOPMODEL through LEAF-2 creates a modeling system that can be used to explore the coupled atmosphere‐biophysical‐ hydrologic response to altered climate forcing at local watershed and regional basin scales.

Turbulence Statistics Above And Within Two Amazon Rain Forest Canopies

The turbulence structure in two Amazon rain forestswas characterised for a range of above-canopystability conditions, and the results compared withprevious studies in other forest canopies and recenttheory for the generation of turbulent eddies justabove forest canopies. Three-dimensional wind speedand temperature fluctuation data were collectedsimultaneously at up to five levels inside and abovetwo canopies of 30–40 m tall forests, during threeseparate periods. We analysed hourly statistics, jointprobability distributions, length scales, spatialcorrelations and coherence, as well as power spectraof vertical and horizontal wind speed. The daytime results show a sharp attenuation ofturbulence in the top third of the canopies, resultingin very little movement, and almost Gaussianprobability distributions of wind speeds, in the lowercanopy. This contrasts with strongly skewed andkurtotic distributions in the upper canopy. At night,attenuation was even stronger and skewness vanishedeven in the upper canopy. Power spectral peaks in thelower canopy are shifted to lower frequencies relativeto the upper canopy, and spatial correlations andcoherences were low throughout the canopy. Integrallength scales of vertical wind speed at the top of thecanopy were small, about 0.15 h compared to avalue of 0.28 h expected from the shear lengthscale at the canopy top, based on the hypothesis that theupper canopy air behaves as a plane mixing layer. Allthis suggests that, although exchange is not totallyinhibited, tropical rain forest canopies differ from other forests in that rapid, coherentdownward sweeps do not penetrate into the lowercanopy, and that length scales are suppressed. This isassociated with a persistent inversion of stability inthat region compared to above-canopy conditions. Theinversion is likely to be maintained by strong heatabsorption in the leaves concentrated near thecanopy top, with the generally weak turbulence beingunable to destroy the temperature gradients over thelarge canopy depth.

A Remote Sensing Study of the NDVI– T s Relationship and the Transpiration from Sparse Vegetation in the Sahel Based on High-Resolution Satellite Data

This article proposes a new approach for estimation of the transpiration rate in sparse canopies. The method relies on a combination of techniques; some of which having a successful background of solid experimental and theoretical justification, while others having only recently been introduced as promising tools for the extraction of environmental information from satellite data. The transpiration rate (λ E v ) is calculated by applying an energy balance approach to the vegetation component of the land surface: λ E v=R n v−H v , where R n v is the net radiation absorbed by the vegetation, and H v is the sensible heat flux between the leaves and the air within the canopy. R n v is calculated through the use of remote sensing and standard meteorological data by combining a conventional method for estimation of the land surface net radiation with a ground-calibrated function of NDVI (normalized differential vegetation index). H v is assessed as a linear function of the temperature difference between vegetation ( T v ) and the mean canopy air stream ( T 0). Because the surface temperature ( T s ) recorded by satellite contains combined information of both soil and vegetation, T v is evaluated on the basis of the linear NDVI– T s relationship for individual surface types. T 0 is assessed utilizing recent evidence that ( T s−T 0) is linearly related to the difference in surface temperature and air temperature above the canopy ( T s−T a ), with the slope coefficient depending only on canopy structure. The method is tested using remote sensing data ranging from ground-based, airborne, and satellite recordings. The modeled transpiration rates compared well to measurements of sapflow data and latent heat fluxes recorded for a wide range of surface types (agricultural crops, natural vegetation, forest vegetation).

Photosynthetic temperature response of the Antarctic vascular plants Colobanthus quitensis and Deschampsia antarctica

The photosynthetic temperature response of the Antarctic vascular plants Colobanthus quitensis and Deschampsia antarctica was examined by measuring whole‐canopy CO2 gas exchange and chlorophyll (Chl) a fluorescence of plants growing near Palmer Station along the Antarctic Peninsula. Both species had negligible midday net photosynthetic rates (Pn) on warm, usually sunny, days (canopy air temperature [Tc]> 20°C), but had relatively high Pn on cool days (Tc<10°C). Laboratory measurements of light and temperature responses of Pn showed that high temperature, not visible irradiance, was responsible for depressions in Pn on warm sunny days. The optimal leaf temperatures (Tl) for Pn in C. quitensis and D. antarctica were 14 and 10°C, respectively. Both species had substantial positive Pn at 0°C Tl, which were 28 (C. quitensis) and 32% (D. antarctica) of their maximal Pn, and we estimate that their low‐temperature compensation points occurred at −2°C Tl (C. quitensis) and −3°C (D. antarctica). Because of the strong warming trend along the peninsula over recent decades and predictions that this will continue, we were particularly interested in the mechanisms responsible for their negligible rates of Pn on warm days and their unusually low high‐temperature compensation points (i.e., 26°C in C. quitensis and 22°C in D. antarctica). Low Pn at supraoptimal temperature (25°C) appeared to be largely due to high rates of temperature‐enhanced respiration. However, there was also evidence for direct impairment of the photosynthetic apparatus at supraoptimal temperature, based on Chl fluorescence and Pn/intercellular CO2 concentration (ci) response curve analyses. The breakpoint or critical temperature (Tcr) of minimal fluorescence (Fo) was ≈42°C in both species, which was well above the temperatures where reductions in Pn were evident, indicating that thylakoid membranes were structurally intact at supraoptimal temperatures for Pn. The optimal Tl for photochemical quenching (qp) and the quantum yield of photosystem II (PSII) electron transfer (φPSII) were 9 and 7°C in C. quitensis and D. antarctica, respectively. Supraoptimal temperatures resulted in lower qp and greater non‐photochemical quenching (qNP), but had little effect on Fo, maximal fluorescence (Fm) or the ratio of variable to maximal fluorescence (Fv/Fm) in both species. In addition, carboxylation efficiencies or initial slopes of their Pn/ci response were lower at supraoptimal temperatures, suggesting reduced activity of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco). Although continued warming along the peninsula will increase the frequency of supraoptimal temperatures, Tc at our field site averaged 4.3°C and was below the temperature optima for Pn in these species for the majority of diurnal periods (86%) during the growing season, suggesting that continued warming will usually improve their rates of Pn.

531 Relating the Accumulation of Heat Units to Changes in the Phenology and Frost Hardiness of Cranberry during Spring

Little is known about the growth and development of the cranberry plant (Vaccinium macrocarpon Ait.) in response to air and soil temperatures in the spring. During this period, marked changes in cranberry bud hardiness are known to occur (from –20 to 0 °C), with the greatest changes occuring before bud elongation. The ability to predict changes in bud phenology and hardiness in relation to thermal time would be useful to growers in making frost management decisions. To establish a working growth model, canopy air and soil temperatures were continuously recorded in 1996, 1997, and 1998 in a cranberry bed (cv. Stevens) in central Wisconsin. In spring, samples of uprights were randomly collected from several locations within the bed and sorted according to a nine stage bud classification from tight bud to bloom. Controlled freezing tests were performed on uprights from the most advanced stages present that constituted 10% or more of a sample on a given date. Heat units were calculated from hourly canopy air temperatures. Despite the varied weather conditions over the3 years, a distinct relationship existed between the accumulation of heat units and the advancement of the crop. Spring 1998 was very early and resulted in the accumulation of more heat units before initial and advanced bud swell was observed compared to the other 2 years. Initial evaluation suggests that soil temperatures between 5 to 10 °C and photoperiod may play a role in modulating the effect of air temperatures. Further refinement of this model and the predictive value for frost hardiness changes will be discussed.

Seasonal shoot and needle growth of loblolly pine responds to thinning, fertilization, and crown position

The impacts of thinning, fertilization and crown position on seasonal growth of current-year shoots and foliage were studied in a 13-year-old loblolly pine ( Pinus taeda L.) plantation in the sixth post-treatment year (1994). Length of new flushes, and their needle length, leaf area, and oven-dry weight were measured in the upper and lower crown from March through November. Total shoot length was the cumulative length of all flushes on a given shoot and total leaf area per shoot was the sum of leaf areas of the flushes. By the end of June, first-flush foliage reached 70% of the November needle length (14.3 cm) and 65% of the final leaf area (15.0 cm 2). Cumulative shoot length of first- and second-flush shoots achieved 95% of the annual length (30.3 cm), whereas total leaf area per shoot was 55% of the final value (75.3 dm 2). Fertilization consistently stimulated fascicle needle length, dry weight, and leaf area in the upper crown. Mean leaf area of upper-crown shoots was increased by 64% six years after fertilization. A significant thinning effect was found to decrease mean leaf area per shoot in the crown. For most of the growing season, the thinned-fertilized trees produced substantially more leaf area per shoot throughout the crown than the thinned-nonfertilized trees. These thinned-fertilized trees also had greater needle length and dry weight, longer first flush shoots, and more leaf area per flush than trees in the thinned-nonfertilized plots. Needle length and leaf area of first flush shoots between April and July were linearly related to previous-month canopy air temperature ( T a). Total shoot length strongly depended on vertical light gradient (PPFD) within the canopy, whereas shoot leaf area was a function of both PPFD and T a. Thus, trees produced larger and heavier fascicles, more and longer flush shoots, and more leaf area per shoot in the upper crown than the lower crown. We conclude that thinning, fertilization, and crown position regulate annual leaf area production of current-year shoots largely by affecting the expansion of first flush shoots and their foliage during the first half of the growing season.

Early- and mid-season temperature effects on the growth and composition of satsuma mandarins

SummaryThe growth and composition of satsuma mandarin fruit from trees which had been covered by a tunnel house during either stage one (the first ten weeks after anthesis), stage two (10±20 weeks after anthesis) or throughout fruit growth, were compared with those grown under ambient conditions. During each stage temperature was identified as the key factor influencing fruit development and changes to fruit were generally maintained when trees were returned to ambient conditions. Raising maximum air temperatures in the canopy of trees by 2.4.K during stage one increased both fruit growth rates and accumulation of sugars throughout their development. Titratable acidity levels were also increased and quinate levels decreased when trees were warmed during stage one, but these differences did not persist until harvest. Fruit grown under elevated temperatures during stage one had larger pedicels and dorsal vascular bundles, which suggests that temperatures during early fruit development determine the fruits' subsequent ability to import and unload carbohydrate. Raising canopy air temperatures during the second stage of fruit growth lowered tritratable acidity, total acids, citrate and malate levels compared with those in control fruit, and these lower levels were maintained until harvest. Fruit from trees grown in a tunnel house all season and those from control trees were supplied with photosynthate synthesized from 14CO2 or injected with 14C sucrose during either stage one or stage two of fruit development. During stage one, warmed fruit incorporated less radioactivity into structural components and quinate, and more into other soluble components in juice sacs. This suggests that elevated temperatures advanced early fruit development and shortened the duration of the cell division phase. During stage two, less radioactivity was incorporated into acids and more into sugars in warmed fruit.

Carbon isotope dynamics in Abies amabilis stands in the Cascades

Carbon isotope ratios ( delta 13C) of canopy air and carbon isotope discrimination at the ecosystem level were studied in three montane Pacific silver fir (Abies amabilis (Dougl. ex Loud.) Dougl. ex J. Forbes) stands, an old-growth and two younger stands. Spatial and temporal variations of canopy CO2 concentrations ([CO2]) and their delta 13C were strongly related to stand structure. Within the old-growth stand, both daytime canopy [CO2] and canopy delta 13C stayed close to those of the troposphere, either indicating low overall photosynthetic rates or high turbulent mixing. Pronounced periods of photosynthetic drawdown below baseline [CO2] accompanied by more enriched canopy delta 13C were observed for the two younger and denser stands. Canopy [CO2] profiles seemed closely related to changes in soil conditions. Soil respiration rates were positively related to soil temperature, but negatively to soil moisture. delta 13C of soil-respired CO2 stayed relatively constant at -24.55 ± 0.20%oduring the growing season. Significant relationships existed between canopy delta 13C and 1/[CO2] in all three stands. Using the intercepts of these regressions, we calculated an average delta 13C for ecosystem respiration of -26.4 ± 0.1%o. Ecosystem carbon isotope discrimination ( DELTA e), an integrating measure for carbon exchange between the troposphere and the entire ecosystem, stayed relatively constant through time. DELTA e showed no significant stand structure effect (leaf area index, density) and averaged 18.9%o for the old-growth and 19.2 ± 0.2%ofor the two younger stands.

Maize canopies under two soil water regimes: IV. Validity of Bowen ratio–energy balance technique for measuring water vapor and carbon dioxide fluxes at 5-min intervals

The feasibility of determining at 5-min intervals the fluxes of water vapor and CO 2 using a Bowen ratio/energy balance/CO 2 gradient (BREB+) apparatus was studied. Details of the BREB+ apparatus are given. The results obtained over maize canopies at the short 5-min signal averaging time are examined in several aspects. One aspect is the fact that on days when net radiation ( R n) changed rapidly because of cloud movement, latent heat flux ( λE) fluctuated nearly in unison at 5-min intervals with the fluctuations in independently measured R n, Bowen ratio, and canopy temperature. The direction of the fluctuations is consistent with the principles of energy balance and gradient transport. Hence, the results support the use of the short signal averaging time. Another aspect is the coincidental fluctuations in the measured differential in water vapor (Δ q) and in carbon dioxide (ΔCO 2) at two heights above the canopy on clear days when the trend in radiation was steady, which appeared to be the result of measurement noise at first glance. It turned out that Δ q and ΔCO 2 fluctuated largely in unison, indicating the likelihood of real accumulation of water vapor and depletion of CO 2 near the top of the canopy at times of low turbulence, and the opposite effects of reduced accumulation and depletion at times of high turbulence. The calculated values of eddy transfer coefficient for water vapor, K w, is consistent with this interpretation. In spite of the fluctuations in Δ q, the time trend in λ E was steady on clear days. In contrast, the time trend in the 5-min canopy CO 2 flux ( A cf) often exhibited substantial short term peaks and dips. Parts of these fluctuations in A cf may be attributed to the measurement noise in ΔCO 2 and the inherent larger errors in calculating A cf. The error contributed by the changes in CO 2 and water vapor stored within the canopy air volume from one measurement interval to the next were evaluated. The limited results show that the errors due to storage were largely minor and were similar in relative size for λ E and for A cf. The 5-min mean data were compared with the 30-min mean data to evaluate whether the shorter averaging time is sufficient to make horizontal divergence of energy components insignificant. The comparison indicates negligible mean horizontal movements in λE and sensible heat H and demonstrates the fulfillment of the assumption of one-dimensional transport underlying the BREB+ technique. On days of variable clouds, it is shown that the 30-min mean values distorted or obliterated the expected responses of the canopy to sudden changes in radiation. Some limitations and other inaccuracies of the BREB+ technique are discussed. Overall, the results are strongly supportive of the validity of the BREB+ technique in general, and the use of the novel averaging time of 5 min for λ E and H over field crops in particular. The necessary averaging time for A cf is less certain and further study is desirable.