Kenneth R. Knoerr

Seasonal patterns of leaf water relations in four co-occurring forest tree species: Parameters from pressure-volume curves

SummaryLeaf water relationships were studied in four widespread forest tree species (Ilex opaca Ait., Cornus florida L., Acer rubrum L., and Liriodendron tulipifera L.). The individuals studied all occurred on the same site and were selected to represent a range of growth forms and water relationships in some of the principal tree species of the region. The water relations of the species were analyzed using the concept of the water potential-water content relationship. The pressure-volume method was used to measure this relationship using leaf material sampled from naturally occurring plants in the field. Water potential components (turgor, osmotic, and matric) were obtained by analysis of the pressure-volume curves.Initial osmotic potentials (the value of the osmotic component at full turgidity) were highest (least negative) at the start of the growing season. They decreased (becoming progressively more negative) as the season progressed through a drought period. Following a period of precipitation at the end of the drought period, initial osmotic potentials increased toward the values measured earlier in the season.Seasonal osmotic adjustments were sufficient in all species to allow maintenance of leaf turgor through the season, with one exception: Acer appeared to undergo some midday turgor loss during the height of the July drought period.In addition to environmental influences, tissue stage of development played a role; young Ilex leaves had higher early season initial osmotic potentials than overwintering leaves from the same tree.The seasonal pattern of initial osmotic potential in Liriodendron and the observed pattern of leaf mortality suggested a possible role of osmotic potentials in the resistance of those leaves to drought conditions. The fraction of total leaf water which is available to affect osmotic potentials, called the osmotic water fraction in this study, was greatest in young tissue early in the season and declined as the season progressed.The results of this study showed that the water potential-water content relationship represents a dynamic mechanism by which plant internal water relations may vary in response to a changing external water-availability regime. The measured water relationships confirmed the relative positions of the species along a water-availability gradient, with Cornus at the wettest end and Ilex at the driest end of the gradient. Acer and Liriodendron were intermediate in their water relations. The spread of these species along a water-availability gradient on the same site suggested that coexistence is partially based on differential water use patterns.

Components of water potential estimated from xylem pressure measurements in five tree species

SummaryPressure volume curves were measured with a pressure bomb in leaves collected in the field from Ilex opaca, Acer rubrum, Liquidambar styraciflua, Liriodendron tulipifera and Cornus florida. Water potential components were calculated from the curves. The species differed in the relationships measured. In all species the trends from summer to fall were toward lower (more negative) osmotic potentials, lower matric potentials more rapid loss of turgor with increasing leaf water deficit, and the occurrence of incipient plasmolysis at lower values of leaf water deficit. Initial osmotic potentials ranged from-14.8 to-19.8 bars, similar to values reported in the literature for other mesophytic plants. These values, however, were much higher than those reported for halophytes and xerophytes. The fraction of leaf water which contributes to the osmotic potential ranged from 0.74 to 0.98 in this study. Values reported for other mesophytes and for halophytes and xerophytes all fall well within this range. Patterns of component water potentials are discussed in relation to potential growth rates and water flow in the total plant system.

Solar radiation variability on the floor of a pine plantation

Abstract Measurements of global radiation were made above and below the canopy of a pine plantation during eight consecutive cloudless days. Several analyses were to be made of the variation in time and space in global radiation reaching the forest floor. One analysis represented the measurements as deviations from the mean global radiation at the floor. This approach revealed that periodic samples would be required from at least three pyranometers in this stand to satisfactorily estimate means over short periods of up to a few hours. A statistical analysis of daily radiation totals showed that significant differences were still present among individual sampling points. However, no significant differences were found among measurements of daily totals on four sample plots, each containing five sample points. The analyses suggest some guidelines for sampling global radiation beneath a forest canopy. The measurements also demonstrated the scattering effects of the canopy. The diffuse component increased from 15% of the global radiation above the canopy to 46% of the portion transmitted to the floor. The mean transmission was 17% during the 8-day experiment.

Estimation of Momentum and Heat Fluxes Using Dissipation and Flux-Variance Methods in the Unstable Surface Layer

Dissipation and flux-variance methods, derived from the turbulent kinetic energy and temperature variance budget equations in conjunction with Monin-Obukov similarity theory, were used to estimate surface fluxes of momentum and sensible heat. To examine the performance of these two methods, direct eddy correlation measurements were carried out above a nonuniform grass-covered forest clearing in Durham, North Carolina. The dissipation method sensible heat flux predictions were in good agreement with eddy correlation measurements. Also, the flux-variance method reproduced the measured sensible heat flux well following an adjustment to the similarity constant. However, the momentum flux (or friction velocity) estimated by the dissipation and flux-variance methods were both inferior to those for sensible heat flux. The data from this experiment indicated that the above two methods are sensitive to the dimensionless wind shear (ϕm) and temperature standard deviation (ϕθ) functions. On the basis of dimensional analysis and the temperature variance budget equation a new dissipation approach for estimating sensible heat flux was derived. The similarity constant for this new approach was shown to be around 1.6 for uniform surfaces and from the data of this experiment.

Estimating solar radiation on mountain slopes

Abstract The amount of solar irradiation on a mountain slope is an important parameter for describing the climatology of a sloping site, but measurements of such radiation are not easily obtained. Daily totals of solar irradiation can be estimated by using published tables of potential solar irradiation if the effects of atmospheric transmissivity and cloud cover are included. Daily totals of solar irradiation (K↓s) were estimated as: K↓s = k↓(Sps/Sp) where Sps and Sp are the table values of potential solar irradiation for a slope and for a horizontal surface and K↓ is the daily total of global solar radiation measured on a horizontal surface at a site near enough to have the same cloud cover as the mountain slope. A test of this method shows that valid estimates of solar radiation input were obtained for two opposite-facing slopes.

Comparison of surrogate surface techniques for estimation of sulfate dry deposition

Abstract The dry deposition rates of sulfate particles to artificial surfaces within and above a mature hardwood forest were measured over an annual range of synoptic weather conditions. Artificial, or ‘surrogate’, surfaces representing both rough and smooth textural types included deposition buckets, petri dishes, filter paper, Teflon configurations and polycarbonate membranes. Ambient concentrations of sulfate and sulfur dioxide were also monitored. The artificial surfaces were evaluated on the basis of the magnitude of the sulfate dry deposition rates and measurement precision. Correlations between techniques and the magnitude of the deposition velocities identified technique similarities. Ambient concentrations of the sulfur oxides and the deposition rates were not well correlated. For diverse reasons, many of the techniques were found to have limited reliability. The petri dish, bucket inside and filter plate surfaces were found to represent the most precise devices for the estimation of dry deposition to smooth, complex and rough artificial surfaces, respectively. Seasonal averages for samplers exposed at all heights were 11.2, 27.7 and 71.2μg SO 4 2− m −2 h −1 , yielding mean deposition velocities to surfaces exposed within the forest canopy of 0.03,0.11 and 0.14 cm s −1 and an annual estimate of the potential dry deposition to a foliated hardwood forest of 4.0, 11.5 and 21.0 kg SO 4 2− ha −1 for the petri dish, bucket inside and filter plate surfaces, respectively. The indirect ratio between deposition rates and velocities results from varying concentrations of ambient sulfate between sampling periods. The accuracy of the filter plate data is suspect due to a significant correlation with sulfur dioxide concentrations. Sulfur concentration and deposition rate gradients indicate the forest is providing a net sink for sulfur pollutants during periods with foliage. The wide range of dry deposition rates estimated from the variety of deposition surfaces emphasizes the uncertainty of the artificial surface measurement techniques. In spite of these limitations, surrogate surfaces provide an estimate of sulfate flux rates not currently obtainable from natural surfaces.

Simultaneous determinations of the sensible and latent heat transfer coefficients for tree leaves

The heat and mass transfer coefficients for exchange across the fluid dynamic boundary layer over tree leaves were simultaneously determined in a controlled environment chamber. The mass transfer coefficients were calculated from measured values of evaporation, air specific humidity and a value of leaf specific humidity at leaf temperature. The heat transfer coefficients were calculated from measured values of air temperature, leaf temperature and an estimate of the sensible heat flux density calculated as the measured net radiation at the leaf surfaces minus the latent heat flux density. The experiments described in this paper indicate that the equations based on laminar boundary-layer theory can give reasonable estimates of the transfer coefficients of real tree leaves for the velocities most commonly experienced in plant canopies, if they are adjusted by a constant multiplier greater than one. Calculations of local mass transfer coefficients based on temperature measurements at three locations at different distances from the leading edge of the leaves, indicate that the deviation from theory is probably the result of transition to turbulent boundary-layer flow at some distance from the leading edge.

Microscale pressure fluctuations in a mature deciduous forest

Static pressure fluctuations in the microscale range were measured in a mature deciduous forest. Pressure measurements were taken at the ground and above the canopy, and mean profile data of windspeed were collected from above the canopy to near the forest floor. Time series, spectra, and cross-correlations were calculated under different canopy conditions, and relationships between surface pressure fluctuations and mean windspeeds were determined. High-frequency pressure fluctuations that occur over aerodynamically smoother surfaces do not occur at the forest floor. These surface fluctuations are advected by the wind above the canopy, not that within the trunk space. The shapes of the pressure spectra are affected by changes in windspeed. Comparisons of spectra above and below the canopy also show some effect of the canopy itself on the shape of the pressure spectra.

The radiation budget of a forest canopy

SummaryModels were developed to separate the radiation-transfer processes of plant canopies from those of the underlying soil surface. The effect of the canopy was then examined by applying the model to springtime measurements of the fluxes of short- and longwave radiation entering and leaving a pine plantation. The model showed that 10 percent of the shortwave radiation that was absorbed within the canopy and 20 percent of that leaving the upper canopy boundary originated as reflection from the forest floor. Longwave radiation emitted upward by the canopy during the daylight period exceeded that emitted downward by about 6 percent. The model confirmed the importance of the longwave component to net radiation of the canopy.ZusammenfassungEs wurden Modelle entwickelt, die es gestatten, die Strahlungsprozesse der Baumkronen von denen der unterliegenden Erdoberfläche zu trennen. Der Effekt der Baumkronen wurde durch Anwendung des Modells auf Messungen der kurz- und der langwelligen Strahlung im Frühling überprüft, welche in einem Kiefernbestand herrscht. Die Modellrechnungen zeigen, daß 10% der in den Baumkronen absorbierten kurzwelligen Strahlung und 20% der langwelligen Strahlung, die aus der Obergrenze der Baumkronen austritt, von der am Waldboden reflektierten Strahlung herrühren. Die langwellige Strahlung, welche während der Tagesstunden von den Baumkronen nach oben hin ausgesendet wird, übertrifft die nach unten ausgesendete Strahlung um ungefähr 6%. Das Modell bestätigt die Bedeutung der langwelligen Strahlungskomponente für die Strahlungsbilanz der Baumkronen.

Leaf emergence and flow-through effects on mean windspeed profiles and microscale pressure fluctuations in a deciduous forest

Abstract Mean windspeed profiles and microscale pressure fluctuations at the surface were measured before and after leaf emergence in a deciduous forest. Effects of the leaf emergence were evident in the windspeed profiles, as well as effects of flow-through from an open border. However, no effects of the leaf emergence could be seen on the relationship of pressure fluctuations at the surface to mean windspeed above the canopy, unless the direction of the flow was from the open border to the site of measurements.