Xiusheng Yang

Vertical distributions of canopy foliage and biologically active radiation in a defoliated/refoliated hardwood forest

Abstract Vertical profiles of foliage area and solar irradiance in the ultraviolet-B (UVB, 280–320 nm), photosynthetically active (PAR, 400–700 nm), and total spectral regions were measured simultaneously in a partially refoliated mixed oak forest, previously defoliated by gypsy moth, using canopy analyzers and broadband radiation sensors mounted on an auto-levelling platform of a mobile, up-down lifting tower. Measurements were taken at ten locations in the stand; at nine vertical positions in each location. Temporal variations also were evaluated in a second experiment with the same protocol at a fixed location. Downward cumulative leaf area index was fit to the Weibull cumulative distribution function. Good agreements were found between the data and their Weibull representations, with nonlinear R 2 value averaged at 0.98 for the ten fittings of the spatial samples and greater than 0.99 for the means. Both the scale and shape parameters of the Weibull cumulative distribution function were significantly correlated and decreased with the canopy leaf area index. As an indicator of the internal consistency of the canopy analyzers, the temporal variation of the leaf area measurements at the fixed location was about 10% for solar zenith angle in the range of 30–45°. The irradiance of UVB, PAR and total solar radiation within the canopy were all found to attenuate with downward cumulative leaf area index, and their vertical distribution could be reasonably well described by Beers law of attenuation. The attenuation rate was greatest for UVB, smallest for total spectral region and intermediate for PAR. Extinction coefficients were 0.86, 0.79 and 0.64 for UVB, PAR and total solar energy, respectively. Ratios of UVB to PAR, UVB to total spectral region and PAR to total spectral region also were shown to decrease with cumulative leaf area index. The use of long-term change in the flux ratio of UVB to PAR to monitor the forest adaptation to, and damage level from, increased exposure to UVB was recommended.

Spatial and temporal variations of atmospheric deposition in interior and Coastal Connecticut

Abstract Both wet and dry deposition fluxes of sulfur, nitrogen, and phosphorus were estimated at three monitoring stations in Connecticut, one inland near the Connecticut-Massachusetts border and two along the Connecticut coastline of Long Island Sound. Three years of data, from 1991 through 1993, were analyzed and are reported in this paper. The total annual deposition ranged from 1.27 to 1.58 gm −2 for sulfur, 0.8 to 0.85 gm −2 for nitrogen, and 3.81 to 4.69 mgm −2 for phosphorus. More than 90% of the phosphorus and about two-thirds of the sulfur and nitrogen were deposited in wet form. The estimated wet deposition quantities were all higher in summer, indicating a higher scavenging efficiency of rainfalls. The dry deposition quantities were higher for particles but lower for gases during the summer seasons. While there were no significant differences in wet deposition among the three monitoring stations, spatial gradients in atmospheric concentration and dry deposition, especially for sulphur, were detected with higher values to the southwest industrial area. Although more sites might be needed to pinpoint the sources of the pollutants, one station is sufficient for a reliable estimation of the overall deposition. The strong episodic nature of the deposition quantities, however, demands more frequent than denser sampling in the area for long-term monitoring. Also, since the weekly deposition quantities were all strongly positively skewed, more descriptive statistics in addition to the mean values are recommended in data reports.

Effect of ground cover, rain intensity and strawberry plants on splash of simulated raindrops

Abstract A study was conducted to characterize the general features of splash droplets produced from impacting raindrops in an effort to better understand rain splash dispersal of fungal plant pathogens. A rain generator was used to produce simulated raindrops, with a distribution of drop sizes which was very similar to natural rain, that impacted on different ground covers or strawberry plants through a 100-cm 2 opening of a rain shelter. Three ground covers, i.e. soil, straw, and plastic mulch, were tested with simulated rains of 15 and 30 mm h −1 intensity. Effect of plant density was also evaluated by placing strawberry plants of different leaf area indices in the exposed area of the rain shelter. Water sensitive papers were used to collect splash droplets in order to determine droplet size distribution, number and mass flux densities over distance from the raindrop impact area, and also mass reflection. The distribution of splash droplet size was found to be highly and positively skewed and was well fitted by the Weibull distribution function. The median size of splash droplets was less than 0.26 mm in diameter, and the Weibull parameters were largely independent of rain intensity. Both flux densities of splash droplet number and mass declined with distance from the window edge, and were represented by an exponential gradient model. The steepness of the gradients was found to increase with surface roughness (i.e. steepest for straw, shallowest for plastic), and decrease with rain intensity. Mass ratio of the splash droplets to the incident raindrops was in the range of 0.29 to 0.38 without the presence of strawberry plants, and was affected by ground cover, rain intensity, and their interaction. With raindrop impactions on plants, mass reflection was greatly reduced to 0.02–0.09. Results were consistent with, and help explain, the previously demonstrated relationship between surface topography and splash dispersal of fungal spores.

Estimation of Nutrient Atmospheric Deposition to Long Island Sound

Anthropogenic nutrient sources (especially nitrogen and phosphorus) released into the Long Island Sound (LIS) causes excessive phytoplankton growth resulting in hypoxic conditions. Atmospheric deposition (both wet and dry deposition) has significant effect on aquatic and terrestrial ecosystems. Two dry deposition monitoring sites were established along the north shore of LIS in February 1991. Wet and dry deposition samples were collected since then. A dry deposition velocity model, based on the characteristics of the water surface, was used to estimate the loading as well as the seasonal variability (dynamics) of nutrients in atmospheric deposition to LIS. The average nitrogen flux from each site was 6.64 kg (as-N) ha-yr-1. The total atmospheric nitrogen loading was estimated to be 2240 metric tons yr-1 which correcponds to 2.5% of the estimated total nitrogen loading to the Sound from all sources. The average phosphorus flux was 37.44 g (as-P) ha-yr-1. The total atmospheric phosphorus loading to the Sound was estimated to be 12.62 metric tons yr-1. The results show that wet deposition was the predominant source of atmospheric contribution to the Sound.

Radiative properties of hardwood leaves to ultraviolet irradiation

Spectral reflectance and transmittance of leaves to ultraviolet irradiation were determined under laboratory conditions for seven species of hardwood trees, namely red oak (Quercus rubra, L), black oak (Q. velutina, Lamarch), white oak (Q. alba, L.), sugar maple (Acer saccharum), Norway maple (A. plantanoides), hickory (Carya tomemtosa), sweetgum (Liquidambar styraciflua), and black oak litter. The experimental system consisted of a solar simulator, an integrating sphere, and a spectroradiometer. Experiments were repeated three to five times for both adaxial and abaxial surfaces of fresh leaves chosen at randomly. The spectral distributions and simple averages of the radiative properties in the wavelength ranges of ultraviolet-B (UV-B, 280–320 nm) and ultraviolet-A (UV-A, 320–400 nm) were determined. The spectral distributions of reflectance were similar between adaxial and abaxial surfaces, although the magnitude varied among tree species. Leaf reflectance was very low for the ultraviolet spectrum in general and varied among species and between adaxial and abaxial surfaces. It was generally higher over the UV-A waveband compared to UV-B, and higher on the abaxial than adaxial surface. The broadband reflectance in the UV-A range (over all species) was 5.0 and 3.9% for abaxial and adaxial surface, respectively, compared to 3.5 and 2.8% in UV-B. The transmittance through leaves was extremely small in the UV-B (<0.1%) and nearly zero in the UV-A spectral range. Consequently, the absorptance of ultraviolet radiation by leaves, as determined from the measured reflectance and transmittance, was quite high, being more than 90% for all the combinations of species and wavebands examined. The reported results are useful for studies requiring spectral radiative properties of the examined leaves with respect to ultraviolet irradiation.

A comparison of spray drift predictions to lidar data

A field trial was conducted to monitor aerially applied pesticide movement above an oak forest in a near-neutral planetary boundary layer using a lidar system. A 36.6-m tower, instrumented at six levels with fast response micrometeorological sensors, recorded wind, turbulence, and temperature profiles during the spray runs. The micrometeorology data was used to drive two spray drift models for a comparison of drift estimates. The models used in this study were USDA Forest Service Cramer-Barry-Grim (FSCBG), an industry standard engineering model, and University of Connecticut Spray Transport (UCST), a heavy particle Lagrangian transport model. Over five spray runs, the FSCBG model predicted < 0.0001% mass fraction deposition occurring beyond 200 m. The UCST model predicted between 0.0034% and 0.3076% deposited beyond 200 m for the same. The lidar returns detected material drift extending out 2000 m from the spray line indicating that both models fall short of predicting the extent of long-range drift drift near-neutral conditions.

Effect of Aerial Conditions on Heat and Mass Exchange Between Plants and Air in Greenhouses

Experiments and analysis were conducted to examine the effects of aerodynamic and thermal conditions of greenhouse air on estimation of heat and mass fluxes from a uniform canopy, and the ‘aerodynamic resistance’ formulations in delineating the aerial effects by using similarity numbers or via calibration procedures. The air flow mode in greenhouses was found to be complex, variable, and largely in mixed mode. The differences of air temperature and velocity between above- and within-canopy were so significant that use of different reference values may cause different classifications of the air flow mode. It was shown that the resistance determined using similarity numbers is locally defined, must be calculated from within-canopy aerial parameters, and has no power in explaining the vertical transport processes in the air column. The aerodynamic resistance calibrated from the sensible heat flux and the temperature difference between leaves and above-canopy air was shown to be conceptually different from that determined using similarity numbers, and deficient in describing the exchange processes between plants and the greenhouse air. Differentiation between the aerial conditions within a canopy from those above the canopy was strongly recommended for future studies on greenhouse microclimate and transport processes.

Characteristics of Evaporation from Water-based Bacterial Pesticide Droplets

Measurements were conducted to quantify the effect of selected environmental parameters on the evaporation rate from droplets of several commercial formulations of the biological pesticide Bacillus thuringiensis (Bt). Results confirmed the positive relationships of evaporation rate with air temperature and wind speed, and the negative relationship with humidity, but failed to reveal any significant dependence of evaporation on turbulence intensity. Evaporation rates from Bt formulations were essentially the same as those from water droplets until the droplet crystallized. The crystallized droplet size was shown to be a constant proportion of the initial droplet diameter. The general characteristics of evaporation from water and pesticide solutions can be fairly well described by a theoretical model that takes the effect of temperature, humidity, and wind speed into consideration.

Comments on ‘Thermal and aerodynamic conditions in greenhouses in relation to estimation of heat flux and evapotranspiration’

Abstract This paper provides further analysis on the aerodynamic effect on greenhouse heat transfer and evapotranspiration, following the discussion of Stanghellini (1993, Agricultural and Forest Meteorology , 66: 111–117) and Zhang and Lemeur (1992, Agricultural and Forest Meteorology , 58: 209–228; 1993, Agricultural and Forest Meteorology , 66: 119–120). It is shown that the similarity numbers are locally defined, and must be evaluated using mean air velocity and temperature within the canopy, not above. The fluxes determined using similarity numbers are conceptually different from those measured above the canopy. The aerodynamic resistance determined via calibration through above-canopy measurements is merely a calibration constant, incapable of explaining the causality of interested variables, and incomparable with that obtained using similarity numbers.

Characteristics of winds and their effect on dry deposition at the Connecticut coastline of Long Island Sound

Abstract Atmospheric concentration samples and meteorological data of onshore and offshore wind classes during 1994 were collected and analyzed to study the directional characteristics of winds and dry deposition of SO 4 2− , NO 3 − , SO 2 and HNO 3 at the Connecticut coastline of Long Island Sound. The dry deposition velocities during onshore and offshore winds were approximately equal for all the species. The atmospheric concentrations, however, were higher during onshore than offshore winds throughout the study period for all the measured chemicals. The poor air quality during onshore winds indicated that the industrial complex southwest of the monitoring station was a major source area of the pollutants deposited to the area. Seasonal air quality varied with species, with higher SO 4 2− concentrations and lower SO 2 concentrations in summer for both wind classes. The seasonal variation in total dry deposition depended on the time fractions of wind classes. During the warm season, about two thirds of dry deposition were from onshore winds.