Oil mallees: physiology and growth under variable water availability in low rainfall agroforestry systems

[Truncated abstract] Radical changes in vegetation cover due to land clearing for agriculture in the Western Australian wheatbelt have resulted in environmental degradation issues ranging from soil erosion to water-logging and the spread of secondary salinity. The reintroduction of deep-rooted perennial crops or other plantings has been proposed as one of a number of possible remedies, but to date, only limited research has been done on how these perennials will perform when planted into this environment. This thesis is concerned with the effect of water availability on the growth and water-use patterns of mallee eucalypts integrated into annual farming systems. Three experimental chapters focus on Eucalyptus kochii subsp. borealis (C. Gardner) D. Nicolle and are concerned with the effect of the presence of, and depth to a fresh water table on trees planted in a range of landscape positions. A fourth experimental chapter compares the patterns of water use of three species of mallee eucalypts – E. kochii subsp. borealis, E. polybractea R.T. Baker and E. loxophleba subsp. lissophloia L.A.S. Johnson & K.D. Hill – at a single site. Measurements focused on sap flow in stems, lateral roots and tap roots but included soil water content monitoring, biomass measurements, development of allometric relationships, leaf water relations, irrigation with labeled water and xylem cell anatomy. The main aim of the study was to quantify sap flow in stems and roots to investigate how patterns of water use were affected by water availability, and to assess the importance of hydraulic redistribution. The implications of these patterns of water use are discussed in terms of field scale water balance and the suitability of mallee species for planting into a drying environment.