We measured tree transpiration and canopy conductance in Pinus radiata D. Don at two low rainfall sites of differing soil fertility in Canterbury, New Zealand. At the more fertile Lincoln site, we also assessed the effects of two common pasture grasses on tree transpiration and canopy conductance. At the less fertile Eyrewell Forest site, the effect of no understory, and the effects of irrigation in combination with mixtures of grass or legume species were determined. Tree xylem sap flux (F(d)') was measured by the heat pulse method. Total canopy conductance to diffusion of water vapor (G(t)) was calculated by inverting a simplified Penman-Monteith model. The different treatment effects were modeled by the simple decaying exponential relationship G(t) = G(tmax)e((-bD)), where D = air saturation deficit. At the Lincoln site, trees with an understory of cocksfoot had lower F(d)' and G(tmax) than trees with an understory of ryegrass, although the sensitivity of G(t) to increasing D (i.e., the value of b) did not differ between treatments. At the Eyrewell site, irrigation only increased F(d)' in the absence of an understory, whereas the presence of understory vegetation, or lack of irrigation, or both, significantly reduced G(tmax) and increased b. We conclude that the selection of understory species is critical in designing successful agroforestry systems for low rainfall areas.

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Surface soils developed in Sicily Island loess (thermoluminescence age 75 000–95 000 yr B.P.) and Peoria loess (thermoluminescence and 14C age 9000–22 000 yr B.P.) were sampled to study a chronosequence in the lower Mississippi River valley. A toposequence composed of an Udalf and an Aqualf was sampled in each of the loesses to evaluate relationships between topography and soil formation. Morphological, physical, chemical, and mineralogical properties were examined to determine the influence of time and topography on soil formation. Soils developed in the older Sicily Island loess had thicker argillic horizons and sola, a greater degree of horizonation, and greater maximum and total sola clay contents than younger Peoria loess‐derived pedons. Chemically, soils developed in the older loess had less exchangeable Ca, more exchangeable Al, lower mean effective cation exchange capacity (ECEC) per unit clay for the whole argillic horizon, lower base saturations, and were more acidic than their younger counterparts. Kaolinite and vermiculite were more abundant in the older Sicily Island loess‐derived soils while smectite and mica percentages were higher in Peoria loess‐derived pedons. Topography influenced soil formation through differences in soil drainage conditions. Soils occupying better‐drained sites had thicker sola and argillic horizons, thinner total surface (A + E) horizons, redder hues, and were more acidic with less exchangeable Ca, lower base saturations, and lower mean ECEC per unit clay for the whole argillic horizon than soils forming under poorly drained conditions. Kaolinite and mica were more abundant in the better‐drained soils while smectite was in greater quantities at the poorly drained sites.