Ecology of Eucalyptus victrix in grassland in the floodplain of the Fortescue River.

Abstract

The WA coolibah tree, Eucalyptus victrix L. Johnson & K. Hill forms an unique and pristine woodland in the Fortescue Valley, in the Pilbara district of Western Australia. Until recently, no research had been done on E. victrix ecology and concern had been expressed by pastoral managers and others about the condition of the woodland occupied by this species. This research was an attempt to understand the ecology of coolibah using a multi-disciplinary approach.A population demographic study of stands in E. victrix woodland reveals that the present tree populations occupy larger areas than saplings and seedlings. Soil moisture and warm summer temperatures are believed to be stimulating factors for seedling recruitment of E. victrix. Different size classes (height/diameter) reflect different recruitment events and these reflect past availability of seeds and adequate soil moisture in that particular area of the woodland. Occasional grasshopper outbreaks and drought cause canopy shrinkage. Presence of hollow boles, restrict dendrochronological examination of tree ages.An important population study was of a post-seedling cohort at Roy Hill, where height distribution reflected a typical single recruitment event. In subsequent years (1995 - 1998) height measurement showed several peaks, suggesting that seedlings were now growing at different rates. Uniform and steady height growth was observed on saplings found at the edges of gilgai. During May 1998 several saplings flowered, it was assumed that E. victrix attained its reproductive maturity at mean height of 2.50 m and with a stem diameter of 50 mm.Seedling recruitment and subsequent growth mainly depend on heavy rainfall flooding events. Seedling recruitment occurs only from current seed rain. Seed longevity reduces after 54 days of burial at 50 mm depth. Mortality (4 - 6 leaf stage) of newly recruited seedlings during subsequent dry months is very high. Furthermore, allelopathic effects (root competition from established grass and insect herbivory) are additional causes of seedling mortality in the years of recruitment.Seedlings recruited at a burnt (disturbed) site, grew faster compared with undisturbed sites with less mortality. This suggested that fire created a suitable condition by reducing root competition, increasing soil nutrients and also creating a gap which providing sufficient solar radiation for seedlings to establish and develop into a healthy population. It is suggested that once newly recruited seedlings overcome the first summer, mortality rates during subsequent years are drastically reduced.Long-term waterlogging of E. victrix seedlings significantly increases seedling stem diameter. Large numbers of adventitious roots are developed and lenticels proliferate on the submerged portion of the stem. Flooding reduces each photosynthesis, transpiration and stomatal conductance. Flooding does not increase shoot fresh or dry weight on 4-, 8- or 17- week old seedlings. Leaf emergence may be stimulated on flooded seedlings compared with unflooded seedlings. While root dry weight is greater in 17-week old flooded plants than 13 - week seedlings, this difference is not significant. It is suggested that maintenance of a high root/shoot ratio is a drought adaptation. Furthermore, comparative study of flood tolerance in semi-arid eucalypt species suggests that those species intolerant of flooding seldom express morphological adaptations and fail to recover from physiological damage.The annual grass Setaria dielsii occurs under the canopy of mature E.victrix trees of the coolibah woodland. This species has probably displaced more palatable perennial grasses. Soil moisture under trees is slightly higher and soil temperatures are less extreme than away from the canopy. Growth of S. dielsii appears to be more associated with soil nutrient status. Levels of total N, Mg, K, and of electrical conductivity (EC) under trees are significantly higher than those away from the tree. Levels of Ca, Na, Fe, and organic carbon do not differ. The under story sub-shrub Malvastrum americanum is an important competitor with S. dielsii. Light availability may determine relative biomass contributions of the two species.The effect of the density of grass species, growing space and time of establishment on E. victrix seedlings (inter-specific competition), and the effect of density and growing space within E. victrix seedling populations (intra-specific competition) were studied under controlled conditions. Results indicate resources necessary for growth of individual E. victrix seedlings were more limiting under increase density of neighbouring grass species than under intra-specific competition. In particular photosynthetic area was drastically reduced in mixed culture.Lack of ground cover beneath the canopy of isolated E. victrix trees was ascribed to toxic or phenolic substances present in leaf, bark and leaf litter of E. victrix. Lactuca sativa seed germination subjected to extracts and leachate demonstrate that the fresh leaf of E. victrix possesses toxic substances which cause deleterious effects on both germination and radicle development. Similarly, increasing concentrations of leaf and bark leachate show reduced germination percentage of L. sativa seeds. Shoot and root biomass of grass and eucalyptus treated with leaf leachate were reduced. E. victrix leaf leachate significantly reduced shoot and root biomass of its own seedlings. High Performance Liquid Chromatogram (HPLC) analysis separated 11 and 8 possible toxic substances from leaf and bark extract respectively.