Assessing the influence of shrubs and their interspaces on enhancing infiltration in an arid Australian shrubland.

Abstract

In assessments of rangeland condition, the fraction of the soil surface lying beneath the canopy of a plant has been widely adopted as a measure of soil protection afforded by the plant, and of the size of the area of altered micro-environment associated with it. In this local environment, various factors produce more permeable soils that allow faster uptake of rainwater than occurs through soils in the open interspaces between plants, or in degraded rangeland where plant canopy cover has been diminished. However, the nature and location of the transition from 'shrub' to 'interspace' hydrologic behaviour has remained illdefined, and the appropriateness of this two-phase subdivision of shrublands remains to be established. This paper reports on a study designed to begin this process by assessing the size of the 'zone of influence' exerted by bluebush shrubs (Maireana spp.) on water infiltration rates in their vicinity, using a test site at Fowlers Gap, north of Broken Hill, NSW. Using radial transects of infiltrometer data extending from close to the stem into the open interspace, it is shown that infiltration rates do not change abruptly at the canopy margin. Rather, enhanced water uptake persists for some distance into the interspace. According to present results, the plant canopy only covers the central one-third of this larger zone of influence, and so provides an inadequate measure of it. Measures of the persistence in time and lateral extent of the zone of influence may provide more meaningful and more sensitive measures of rangeland condition in the arid and semi-arid shrublands.