Boundary Layer Climates.

The turbulent exchanges of CO2 and water vapour between an aggrading deciduous forest in the north-eastern United States (Harvard Forest) and the atmosphere were measured from 1990 to 1994 using the eddy covariance technique. We present a detailed description of the methods used and a rigorous evaluation of the precision and accuracy of these measurements. We partition the sources of error into three categories: (1) uniform systematic errors are constant and independent of measurement conditions (2) selective systematic errors result when the accuracy of the exchange measurement varies as a function of the physical environment, and (3) sampling uncertainty results when summing an incomplete data set to calculate long-term exchange. Analysis of the surface energy budget indicates a uniform systematic error in the turbulent exchange measurements of -20 to 0%. A comparison of nocturnal eddy flux with chamber measurements indicates a selective systematic underestimation during calm (friction velocity < 0.17 m S-l) nocturnal periods. We describe an approach to correct for this error. The integrated carbon sequestration in 1994 was 2.1 t C ha-l y-l with a 90% confidence interval due to sampling uncertainty of :!:0.3 t C ha-l y-l determined by Monte Carlo simulation. Sampling uncertainty may be reduced by estimating the flux as a function of the physical environment during periods when direct observations are unavailable, and by minimizing the length of intervals without flux data. These analyses lead us to place an overall uncertainty on the annual carbon sequestration in 1994 of --0.3 to +0.8 t C ha-l y-l.

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Measurements of carbon sequestration by long‐term eddy covariance: methods and a critical evaluation of accuracy

The predominant emphasis on harmful effects of environmental stresses on growth of woody plants has obscured some very beneficial effects of such stresses. Slowly increasing stresses may induce physiological adjustment that protects plants from the growth inhibition and/or injury that follow when environmental stresses are abruptly imposed. In addition, short exposures of woody plants to extreme environmental conditions at critical times in their development often improve growth. Furthermore, maintaining harvested seedlings and plant products at very low temperatures extends their longevity. Drought tolerance: Seedlings previously exposed to water stress often undergo less inhibition of growth and other processes following transplanting than do seedlings not previously exposed to such stress. Controlled wetting and drying cycles often promote early budset, dormancy, and drought tolerance. In many species increased drought tolerance following such cycles is associated with osmotic adjustment that ...

Acclimation and adaptive responses of woody plants to environmental stresses

Leaf dry mass per unit area (LMA) is a product of leaf thickness (T) and of density (D). Greater T is associated with greater foliar photosynthetic rates per unit area because of accumulation of photosynthetic compounds; greater D results in decreased foliage photosynthetic potentials per unit dry mass because of lower concentrations of assimilative leaf compounds and decreases in intercellular transfer conductance to CO2. To understand the considerable variation in T and D at the global scale, literature data were analyzed for 558 broad-leaved and 39 needle-leaved shrubs and trees from 182 geographical locations distributed over all major earth biomes with woody vegetation. Site climatic data were interpolated from long-term world climatologies (monthly precipitation, surface temperature) or modeled using the Canadian Climate Center Model (monthly global solar radiation). Influences of total annual precipitation (WT), precipitation of the driest month (Wmin), monthly mean precipitation of the three dries...

Global-scale climatic controls of leaf dry mass per area, density, and thickness in trees and shrubs

Publisher Summary This chapter provides an overview of land surface processes and climate-surface albedos, and attempts to bridge the gap between disciplinary studies of atmospheric versus surface climate. The chapter describes physical processes involving the land surface that are of concern to scientists interested in modeling the global climate system. It looks at the physics of land surface albedos by presenting a conceptual basis for understanding the basic concepts. Physical reasoning and simple mathematical models are used to summarize how these factors determine surface albedos. Some of the aspects of surface energy balance considerations that are important for global climate models are summarized. Factors that are important for determining the albedos and energy balance of land surfaces are also outlined. The mathematical theory of plant canopy albedos is presented, and a summary of parameters determining plant canopy albedo is provided. Concepts related to snow albedos, soil albedos, and surface albedos for climate models are discussed. Modeling temperature of a transpiring canopy, and modeling energy exchange over a snow surface are also described.

Land Surface Processes and Climate—Surface Albedos and Energy Balance

Leaf 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.

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

Pressure 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.

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

Solar radiation variability on the floor of a pine plantation

Measures of leaf tissue elasticity were determined by analyzing the turgor pressure-water content relation developed from pressure-volume experiments, in four naturally occurring forest tree species. The relation between the bulk tissue elastic modulus and tissue turgor pressure varied by species and varied with state of leaf maturation through the growing season. Although the values of the elastic modulus reported here agree in magnitude with values reported in general for higher plant tissue, the functional dependency of the apparent elasticity on tissue water status was more complex in this study than can be accounted for by current models.

Seasonal variation of leaf tissue elasticity in four forest tree species

A model of the energy exchange between the atmosphere and a vegetated surface has been developed and used to investigate the sources of energy available for evaporation of precipitation intercepted by a forest canopy. Simulations with this model have demonstrated that a forest canopy wetted by rainfall will partition more of the absorbed radiant energy into latent heat exchange than an unwetted canopy in the same environment. This energy diversion creates a decrease in sensible heat transfer from the canopy to the atmosphere and a smaller decrease in a long-wave radiation emitted by the canopy.

Kenneth R. Knoerr

Papers

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

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

Solar radiation variability on the floor of a pine plantation

Seasonal variation of leaf tissue elasticity in four forest tree species

The evaporation of intercepted rainfall from a forest stand: An analysis by simulation