Biomedical subjects

E David Ford

Publications and source records attributed to E David Ford.

7 recordsLinked to original sources

PubMed2006

Simulation of effects of wood microstructure on water transport.

A tracheid-level model was used to quantify the effects of differences in wood microstructure between coastal and interior Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii and var. glauca) wood on larger scale properties like hydraulic conductivity. The model showed that tracheid length, the ease of flow through a bordered pit and effective tracheid diameter can all limit maximum hydraulic conductivity. Among the model parameters tested, increasing bordered pit conductivity and tracheid length resulted in the greatest increase in maximum conductivity in both the inland and coastal ecotypes. A sensitivity analysis of the uncertainty between parameters governing flow through the bordered pit and air-seeding potential showed that, although decreased pit flow resistance increased maximum hydraulic conductivity, increased cavitation led to lower conductivity over time. The benefits of increasing the number of bordered pits depended on the intensity of the meteorological driving function: in drier environmental conditions, wood with fewer pits was more conductive over time than wood with more pits. Switching the bordered pit characteristics between coastal and interior wood indicated that the conductivity time course of coastal and interior wood was primarily governed by differences in the number of bordered pits and not differences in tracheid dimensions. The rate at which tracheids refilled had little effect on the conductivity time course of either coastal or interior wood during the first two summers when the wood was highly saturated, but had a marked influence in subsequent years once the cavitation profile stabilized. Our work highlights the need for more empirical work on bordered pits to determine whether variation in their number and properties is related to changing environmental conditions. In addition, a detailed simulation model of a bordered pit is needed to understand how variation in pit properties affects the relationship between ease of flow through a bordered pit and its potential for facilitating air-seeding.

Biological Transport↗

PubMed2006

Statistical inference using the g or K point pattern spatial statistics.

Spatial point pattern analysis provides a statistical method to compare an observed spatial pattern against a hypothesized spatial process model. The G statistic, which considers the distribution of nearest neighbor distances, and the K statistic, which evaluates the distribution of all neighbor distances, are commonly used in such analyses. One method of employing these statistics involves building a simulation envelope from the result of many simulated patterns of the hypothesized model. Specifically, a simulation envelope is created by calculating, at every distance, the minimum and maximum results computed across the simulated patterns. A statistical test is performed by evaluating where the results from an observed pattern fall with respect to the simulation envelope. However, this method, which differs from P. Diggle's suggested approach, is invalid for inference because it violates the assumptions of Monte Carlo methods and results in incorrect type I error rate performance. Similarly, using the simulation envelope to estimate the range of distances over which an observed pattern deviates from the hypothesized model is also suspect. The technical details of why the simulation envelope provides incorrect type I error rate performance are described. A valid test is then proposed, and details about how the number of simulated patterns impacts the statistical significance are explained. Finally, an example of using the proposed test within an exploratory data analysis framework is provided.

Data Interpretation, Statistical↗

PubMed2003

Branch and foliage morphological plasticity in old-growth Thuja plicata.

At the Wind River Canopy Crane Facility in southeastern Washington State, USA, we examined phenotypic variation between upper- and lower-canopy branches of old-growth Thuja plicata J. Donn ex D. Don (western red cedar). Lower-canopy branches were longer, sprouted fewer daughter branches per unit stem length and were more horizontal than upper-canopy branches. Thuja plicata holds its foliage in fronds, and these had less projected area per unit mass, measured by specific frond area, and less overlap, measured by silhouette to projected area ratio (SPARmax), in the lower canopy than in the upper canopy. The value of SPARmax, used as an indicator of sun and shade foliage in needle-bearing species, did not differ greatly between upper- and lower-canopy branches. We suggest that branching patterns, as well as frond structure, are important components of morphological plasticity in T. plicata. Our results imply that branches of old-growth T. plicata trees have a guerilla growth pattern, responding to changes in solar irradiance in a localized manner.

Light↗

PubMed2002

Modeling tree water flow as an unsaturated flow through a porous medium.

The electric circuit analogy has had a profound influence on how tree physiologists measure, model and think about tree water flow. For example, previous models that attempt to account for changes in saturation use the electric circuit analogy to define capacitance as the change in saturation per change in pressure. Given that capacitance is constant, this relationship implies that subjecting a block of wood to a pressure of -2.5 MPa for 2 min results in the same change in saturation as subjecting the same block to the same pressure for 2 days. Given the definition of capacitance, it is unclear how the electric circuit analogy could be used to predict changes in saturation separately from changes in pressure. The inadequacies in the electric circuit analogy discussed in this paper necessitate a new theory of tree water flow that recognizes the sapwood as being a porous medium and explicitly deals with the full implications of the unsaturated flow occurring in the sapwood. The theory proposed in this paper combines the Cohesion theory with a mathematical theory of multiphase flow through porous media. Based on this theory, both saturated and unsaturated tree water flow models are presented. Previous partial differential equation models of tree water flow based on the electric circuit analogy are shown to be mathematically equivalent to the model of saturated porous flow. The unsaturated model of tree water flow explicitly models the pressure profile and the rates of change in saturation and specific interfacial area (a measure of how the water in the unsaturated sapwood is partitioned between mobile and immobile components). The unsaturated model highlights the differences between saturated and unsaturated flow and the need to measure the variables governing tree water flow at higher spatial and temporal resolutions.

Models, Biological↗

PubMed2002

Parameterizing a model of Douglas fir water flow using a tracheid-level model.

The theory of tree water flow proposed in Aumann & Ford (submitted) is assessed by numerically solving the model developed from this theory under a variety of functional parameterizations. The unknown functions in this nonlinear partial differential equation model are determined using a tracheid-level model of water flow in a block of Douglas fir tracheids. The processes of flow, cavitation, pit aspiration/deaspiration, flow through the cell wall and ray exudation in a block of approximately 79 000 tracheids are modeled. Output from the tracheid model facilitates determination of the hydraulic conductivities in the sapwood as a function of saturation and interfacial area between liquid and gaseous phases of water, the function governing the rate of change in saturation, and the function governing the rate of change in interfacial area. The models show complementary things. The tracheid model shows that capacitance, or the change in saturation per change in pressure, is not constant. When all refilling is stopped, it takes over 180 days for the hydraulic conductivity in the vertical direction to reach 1/4 of its maximal value, showing the robustness of the transpiration stream for conducting water. The shape of the functions determined with the tracheid model change with different tracheid-level assumptions. When these functions are used in the differential equation model, it is shown that cell-wall conductivity plays an important part in the lag in flow observed in many conifers. The flow velocities and rates of change in saturation predicted by the differential equation model agree with those measured in Douglas fir. Both models support the theory of tree water flow presented in Aumann & Ford (submitted) and undermine the theory that water flow in trees is analogous to the flow of current in electric circuits.

Cell Wall↗

PubMed2002

Light interacts with auxin during leaf elongation and leaf angle development in young corn seedlings.

Modern corn ( Zea mays L.) varieties have been selected for their ability to maintain productivity in dense plantings. We have tested the possibility that the physiological consequence of the selection of the modern hybrid, 3394, for increased crop yield includes changes in responsiveness to auxin and light. Etiolated seedlings in the modern line are shorter than in an older hybrid, 307, since they produce shorter coleoptile, mesocotyl, and leaves (blade as well as sheath). Etiolated 3394 seedlings, as well as isolated mesocotyl and sheath segments, were less responsive to auxin and an inhibitor of polar auxin transport, N-1-naphthylphthalamic acid (NPA). Reduced response of 3394 to auxin was associated with less reduction of elongation growth by light (white, red, far-red, blue) than in 307, whereas the activity of polar auxin transport (PAT) and its reduction by red or far-red light was similar in both genotypes. NPA reduced PAT in etiolated 3394 seedlings much less than in 307. A characteristic feature of 3394 plants is more erect leaves. In both hybrids, light (white, red, blue) increases leaf declination from the vertical, whereas NPA reduces leaf declination in 307, but not in 3394. Our results support findings that auxin and PAT are involved in elongation growth of corn seedlings, and we show that light interacts with auxin or PAT in regulation of leaf declination. We hypothesize that, relative to 307, more erect leaves in the modern hybrid may be primarily a consequence of a reduced amount of auxin receptor(s) and reduced responsiveness to light in etiolated 3394 plants. The more erect leaves in 3394 may contribute to the tolerance of the modern corn hybrid to dense planting.

Adaptation, Physiological↗

PubMed2002

Variation in specific needle area of old-growth Douglas-fir in relation to needle age, within-crown position and epicormic shoot production.

Variation in specific needle area (SNA; cm2 projected fresh needle area g-1 oven-dried needle weight) was investigated in relation to needle age, within-crown position and epicormic shoot production in 450-year-old Douglas-fir (Pseudotsuga menziesii Mirb. (Franco) var. menziesii) trees. Specific needle area decreased with increasing needle age. The magnitude and rate of change in SNA with needle age were greatest for lower-crown branches, and decreased toward the middle- and upper-crown branches. For all branches, there was no difference between regular and epicormic shoots in the relationship between SNA and needle age. Specific needle area decreased with increasing distance from branch base, and this relationship was significant for the majority of needle age classes of the upper- and middle-crown branches. In the lowercrown branches, SNA did not vary with distance from branch base for the majority of needle age classes. For all branches, there was no difference between regular and epicormic shoots in the relationship between SNA and distance from branch base for the majority of needle age classes. These results indicate that renewal of foliage by epicormic shoot production maintains needle quality. Branch SNA increased linearly with decreasing height in the crown at a mean rate of 0.951 +/- 0.110 cm2 g-1 per vertical meter. Total needle area of branches was estimated from total needle dry weight taking into account within-branch variation in SNA. Analyses of allometric relationships between branch size and foliage amount (needle area and needle dry weight) showed that branch length was a better predictor of foliage amount than branch diameter for old Douglas-fir trees. Total needle dry weight and needle area of the sample trees, estimated from branch length and branch height and taking into account vertical within-crown variation in branch SNA, ranged from 42.4 to 154.2 kg and from 246.2 to 816.0 m2 per tree, respectively.

Plant Leaves↗