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The 3-PG model (Landsberg and Waring, 1997) was parameterized to predict potential productivity across 170 000 ha of Eucalyptus grandis hybrid plantation distributed in 19 regions in eastern Brazil. The regions were defined on the basis of meteorological measurements made by automatic weather stations. Mean annual increments estimated by the model for a 6-year rotation were compared with available observations made annually in permanent sample plots (PSPs). The goodness of fit between estimated and observed growth was determined by R2=0.92. Comparisons between model estimates and measurements such as basal area and total volume are presented. An empirical model called E-GROW ARCEL was developed and fitted using PSP data from the same region. The model is based on recovering the parameters of the Weibull probability density function by matching their moments to estimated stand level variables. Stand models were fitted for projections of stand basal area, mortality, dominant height, tree height, DBH (diameter at breast height) variance and stem taper. Volume of log types in the DBH distribution can be estimated. Mean annual increment (MAI), one of the outputs of 3-PG, was used to establish a hybrid approach, linking the two models by matching the relationship between MAI and site index from E-GROW ARCEL. Growth curves and yields are generated. The hybrid approach is being established as a basis for decision making and management of fast-growing E. grandis hybrid plantations in eastern Brazil.

Changes in the transpiration rate of wheat in drying soils were followed in experiments in which plants were grown in two small weighable lysimeters in a glasshouse. Hourly measurements of soil water potential (Ψs) were made at three depths in each lysimeter. The water potential of flag leaves was measured with a pressure chamber, and stomatal resistance with a pressure drop porometer. Data on root densities and distribution were also obtained. Transpiration rates fell below estimated potential levels when the average value of Ψs in the root zone was reduced to –1 to –5 bars, depending on soil storage, root distribution and potential transpiration rate. From this point Ψs fell rapidly in the surface layers, more slowly at depth. It was found that accurate calculations of daily water uptake could be made from changes in soil water content. The minimum value of leaf water potential (�1 )attained each day declined progressively through the drying cycle, but there was evidence that stomatal resistance (rs) is not uniquely related to Ψ1; initial stomatal closure occurred at Ψ1, values which decreased from –11 to –25 bars as drying progressed. This adaptive mechanism is related to changes in osmotic potential of the leaves. Whole plant resistances (Rp), derived from leaf water potentials and fluxes through individual stems, increased as stem populations increased. In the high population lysimeter Rp decreased from 300 to 100 bar sec mm-3 as canopy transpiration rates increased from 1.5 to 4.5 x 10-4 mm sec-1. In the low population lysimeter Rp decreased from 70 to 30 bar sec mm-3 as transpiration increased from about 2.2 to 4.5 x 10-4 mm sec-1. The higher resistances appear to confer significant advantages in terms of water conservation and adaptation to drought.

A three-layer electrical analogue model was used to calculate resistance to water movement through the roots of wheat plants growing in small weighing lysimeters. In one experiment the wheat was grown in two soil types; in a second experiment one soil type was used but different root systems were induced by controlling the water table before the start of the experimental period. Resistance calculations were based on hourly measurements of transpiration rate, leaf water potential and water uptake from three soil layers (qi), calculated from measurements of soil water potential at three depths. The number of main roots per stem (required for the model) and root surface area in each layer (Ai) were obtained from measurements of root lengths and diameters in soil cores taken at the end of each experiment. Estimates of the resistance to flow through stems led to estimates of ψ0), the water potential at the stem base, at any stem flow rate. Axial (main) root resistances (Rxi) were calculated from the Poiseuille equation. Values of the resistance to water movement through the roots in layer i were calculated from the set of equations describing uptake from each layer in terms of flow rates, potential gradients and resistances; these values, inserted in the solution for 1/10 from the set of three equations, yielded total root resistances (RT) and estimates of the effective soil moisture potential (^ψs) for the whole profile. (RT) ranged from 63.9 to 627.3 bar sec mm-3 (cf. stem resistance between 24 and 70 bar sec mm-3) and was inversely related to flow rate through the main roots, which indicated a constant potential drop (^ψs – ψ0) of about 10 bars, irrespective of soil type or root system. Radial root resistances, estimated as At(<ψsi – ψ0)/qi, ranged from 4.6 x 104 to 4.2 x 106 bar sec mm-l and were inversely related to qi. Inaccuracies in estimates of Rxi do not affect the results much and the model used is potentially valuable as a framework for field research.

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Mature lucerne on shallow granite sand was irrigated at different intervals, determined by four factors, 0-4, o-6, o-8 and i-o, applied to daily evaporation (E o ) as measured by a USWB Class A pan. Moisture balances were calculated for each treatment, and when the accumulated sum of daily evaporation, multiplied by the appropriate factor, reached 1 • 2 inches, which was about 50 per cent of the available moisture in the top 2 feet of soil, this amount of water was applied to the plots of the treatment concerned. Irrigation on the basis of 0-8 E o gave the best overall results, although the highest yield from a single cut was obtained with the irrigation frequency denned by 1 • o E o . During the rainy season yields from all treatments declined markedly, those from the most frequently irrigated treatment showing the greatest drop. The lucerne under the irrigation regimes denned by 0-4 and 0-6 E o , although subjected to considerable moisture stress in the dry weather, suffered no lasting ill effects and yielded well during the rains. The experimental area was almost certainly subject to advective energy but the results are considered valid for local conditions.