GROWTH OF EUCALYPTUS-MARGINATA (JARRAH) SEEDLINGS IN MEDITERRANEAN-CLIMATE FOREST IN SOUTH-WEST AUSTRALIA IN RESPONSE TO OVERSTORY, SITE AND FERTILIZER APPLICATION

Shoot growth, leaf growth and net rate of leaf photosynthesis were mea sured on Eucalyptus marginata Donn ex Smith (jarrah) seedlings in medi terranean-climate forest of south-west Western Australia to determine the effects of overstorey, type of site and application of fertiliser. Soil temperature, soil water deficit, leaf water potential, rainfall and photosynthetically active radiation (PAR) were monitored. Seedling s on sites on which the overstorey had been removed experienced signif icantly higher soil temperatures for 9 months of the year (of up to 6 degrees C), smaller soil and leaf water deficits during the dry season (minimum predawn leaf water potentials of - 0.5 MPa compared with - 1 .6 MPa), and greater PAR (maximum values of 1900 mu mol m(-2) s(-1) co mpared with 900 mu mol m(-2) s(-1)) than seedlings on sites with an ov erstorey. Shoot dry weight was eight times greater, leaf area was more than 20 times greater and photosynthesis was two to 10 times greater on sites with the overstorey removed. Application of fertiliser increa sed shoot dry weight by a factor of two. For sites with the overstorey removed, seedlings on a high quality site achieved six times the shoo t dry weight of those on a bauxite pit and three times the shoot dry w eight of those on a low quality site. Leaf growth commenced in October when soil temperature at 40 cm reached about 17 degrees C. Leaf growt h decreased during January when water deficits were severe, increased again in March and April after rainfall, but decreased in May when soi l temperature fell below 15 degrees C. Leaf growth and photosynthesis both decreased over the dry season as water deficits developed. Seedli ngs on the site with the overstorey removed had higher rates of photos ynthesis and greater photosynthetic capacity than those on the site wi th the overstorey retained. We conclude that the major effect of overs torey removal was to reduce some of the limitations that water deficit s impose on growth. Secondary effects of overstorey removal also favou ring seedling growth were increases in soil temperature and PAR.