Transpiration rates and canopy conductance of Pinus radiata growing with different pasture understories in agroforestry systems

摘要

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 under- story, 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 all 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 tile 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.