Glasshouse and field studies on the effects of groundcovers on banana and macadamia growth and water relations

Abstract

Groundcovers can be useful for controlling erosion in macadamia orchards but they can have adverse effects on tree growth and crop yield. The effects of groundcovers versus bare soil on banana and macadamia were compared in a glasshouse study and on macadamia in the field. A glasshouse trial compared the effect of 3 water regimes with unmown or mown 'Arachis pintoi cv'. Amarillo groundcover versus bare soil, on stomatal conductance and growth of banana and macadamia. Mean stomatal conductance was higher overall for macadamia (178 mmol/m².s) compared with banana (90 mmol/m².s) when data were pooled across groundcover and water treatments. Medium and dry substrate moisture treatments significantly reduced the growth and vigour of banana compared with the wet treatments, as indicated by reduced total dry matter, leaf area, number of live leaves at harvest, and total root length, but had no significant effect on macadamia. Groundcover had a more adverse effect on the growth of banana than macadamia compared with bare soil and, likewise, unmown cover had a greater effect on growth of banana than mown cover, while there was no mowing effect on macadamia. In an unirrigated field trial, mown and unmown groundcover and bare soil treatments were compared for their effect on early morning leaf xylem water potential of young and older macadamia trees at 3 sites where groundcover was established at different tree ages. Water potential was generally <0.2 MPa more negative in the groundcover treatment than bare soil in spring and summer under dry seasonal conditions over 2 years, although there was no apparent effect on tree health. The difference in water potential between groundcover and bare soil was highest in young trees planted in established groundcover. The more adverse effect of groundcover on water status of young trees in established cover correlates with a greater reduction in growth compared with older trees reported separately (Firth et al. 2003 Aust. J. Exp. Agric. 43, 419–423). Soil bulk density under groundcover was slightly reduced (0.028 g/cm³) at 0–10 cm depth, and penetrometer resistance also slightly reduced (<0.3 MPa) at 0–12 cm depth, compared with bare soil. Higher penetrometer readings in the traffic zone (often >4 MPa at 30–40 cm in the initial survey) compared with the tree line indicate the potential long-term benefits to be derived from groundcovers.