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Factors affecting the establishment of jarrah (Eucalyptus marginata) from seed in the northern jarrah forest of Western Australia

Stoneman, Geoff (1992) Factors affecting the establishment of jarrah (Eucalyptus marginata) from seed in the northern jarrah forest of Western Australia. PhD thesis, Murdoch University.

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Field experiments were established on seven sites in the northern jarrah forest of south-west Western Australia to test the effect of (i) overstorey, site, seedbed, seeding date and seed harvesting by vertebrates and invertebrates on emergence, (ii) overstorey, site. seedbed, application of fertiliser and grazing on mortality, and (iii) overstorey, site and application of fertiliser on growth of Eucalyptus marginata seedlings. Rainfall, soil temperature, soil water deficits. leaf water potential and photosynthetically active radiation (PAR) were monitored over two years.

Seed harvesting by small vertebrates substantially reduced emergence, whereas invertebrates only slightly reduced emergence. Ninety per cent of seed was removed within one day where it was on the soil surface and in the open, whereas seed removal was insignificant when the seed was covered by soil. Vertebrates harvested seed more effectively when the understorey and litter were removed and the seed was more visible on the soil surface. Seed harvesting by small vertebrates had less impact on a site where there had been bauxite mining possibly because the surrounding area was being revegetated with large numbers of seed and the area afforded poor cover for vertebrates. Emergence was less where the overstorey was removed compared to where it was retained and on the bauxite pit compared to the forest sites. There was no difference in emergence between the low and high quality forest sites and so the emergence phase could not explain the variation in abundance of E. marginata trees on the different quality sites.

Seedlings on sites with the overstorey removed experienced significantly higher soil temperatures for nine months of the year (of up to 6°C)/ smaller soil and leaf water deficits during the dry season (minimum predawn leaf water potentials of -0.5 MPa compared to -1.6 MPa), and higher intensities of PAR (maximum values of 1900 [umol m-2 s- compared to 900 1 /jmol m-2 s-1 compared to seedlings on sites with the overstorey retained.

Mortality of E. marglnata seedlings was greater on sites where the overstorey was retained compared to where the overstorey was removed, on low quality sites than high quality sites and on undisturbed and lightly disturbed seedbeds compared to heavily disturbed seedbeds. Neither fertilising nor grazing by vertebrates had a statistically significant effect on mortality. Most of the mortality occurred as water deficits developed in late spring and summer and the major cause of mortality was attributed to water deficits. Some mortality occurred prior to this and was probably caused by pathogenic fungi. Mortality of E. marginata seedlings in the northern jarrah forest of Western Australia can be minimised by reducing the density of the overstorey and by seeding into a seedbed which has had litter and ground cover removed and the soil disturbed.

Shoot dry weight was eight times greater, leaf area was more than 20 times and photosynthesis was two to 10 times greater on sites with the overstorey removed. Fertilising increased shoot dry weight by a factor of two. For sites with the overstorey removed, seedlings on the high quality site achieved six times the shoot dry weight of those on the bauxite pit and three times the shoot dry weight of those on the low quality site. Shoot dry weight was strongly related to leaf area index, independent of the effects of overstorey, site quality and fertiliser. Leaf growth commenced in spring when soil temperature at 40 cm reached 17°C, it was then primarily limited by water deficits until late autumn when soil temperature fell below 15°C, at which point leaf growth had nearly stopped. Leaf growth and photosynthesis both decreased as water deficits developed over the dry Seedlings season. on the site with the overstorey removed had higher rates of photosynthesis and greater photosynthetic capacity than those on the site with the overstorey retained.

To understand the effects of soil temperature, shade by an overstorey and water deficits on seedling growth, a series of controlled environment studies was undertaken where each factor alone was studied.

The effect of soil temperature on the growth of E. marginata seedlings was studied in the first glasshouse experiment. Soil temperature had a significant effect on the growth of all plant parts except the cotyledons. Total plant growth and shoot growth were maximal at a soil temperature of 30°C, but root growth had a slightly lower optimum such that the root:shoot ratio was highest at 20°C. Roots grown at 15°C had about 30% less length per unit of DW than those grown at 20 - 35°C.

The effect of shade on plant growth, dry weight partitioning and photosynthetic response to light of E. marginata seedlings was studied in the second glasshouse experiment. Plant dry weight and that of all plant parts declined in response to shade, as did the root:shoot ratio. Plant leaf area was less for plants grown under no shade than for plants grown under shade. Specific leaf area increased with shade. Seedlings grown under no shade had a higher light saturated rate of photosynthesis, a higher light compensation point and a higher light saturation level than seedlings grown under 70% shade. E. marginata seedlings responded to shade in a very similar way to other eucalypts which naturally regenerate below an overstorey. Plant dry weight and leaf dry weight were strongly related, independent of shade, whereas the relationship between plant dry weight and plant leaf area was dependent on the level of shade. Therefore, leaf dry weight may be a better predictor of biomass production in forest stands where shade is likely to be a significant factor affecting growth.

Leaf growth, net rate of leaf photosynthesis and tissue water relations of shoots of E. marginata seedlings was studied over the course of a soil drying and rewatering cycle in the third glasshouse experiment to determine the effect of water deficits on these parameters. The relationship between midday photosynthesis and predawn leaf water potential showed that photosynthesis of E. marginata seedlings is very sensitive to water deficits. Midday photosynthesis declined to 40% of that of well hydrated seedlings at a predawn leaf water potential of -1 MPa and reached zero at -2.2 MPa. Photosynthesis recovered rapidly following rewatering and was nearly completely recovered by the second day after rewatering. The rate of leaf growth decreased linearly with predawn leaf water potential to reach zero at -1.5 MPa. Leaf growth did not recover completely within the first three days after rewatering. All tissue water relations parameters, except the bulk modulus of elasticity, changed significantly as the soil dried and recovered completely by 3 days after rewatering. The changes in osmotic pressure at full turgor of 0.4 MPa indicates considerable ability by E. marginata seedlings to adjust osmotically. However, there was little osmotic adjustment until predawn leaf water potential fell below -1.5 MPa so it would not have conferred any advantage to the growth of the seedlings, but may aid in survival. The sensitivity of photosynthesis and relative water content to water deficits shows that E. marginata behaves as a mesophyte despite the fact that it grows in a drought-prone environment.

The glasshouse experiments dealing with the effects of soil temperature and shade showed that both the higher soil temperatures and the higher light intensities on sites with the overstorey removed could account for significant increases in growth of E. marginata seedlings, but that these effects were a relatively small component of the growth response due to overstorey removal. The glasshouse experiment on the effects of water deficits showed that rates of leaf growth and photosynthesis are very sensitive to water deficits and therefore the greater water deficits that seedlings experienced on sites with the overstorey retained will cause large reductions in the growth of E. marginata seedlings.

In conclusion, secondary effects of overstorey removal are increases in soil temperature and light which promote greater growth of E. marginata seedlings. The major effect of overstorey removal is to reduce some of the limits that water deficits impose on growth of E. marginata seedlings in the northern jarrah forest of south west Western Australia.

The major factors affecting establishment of E. marginata seedlings in the forest were seed harvesting by small vertebrates which greatly reduced emergence, and water deficits which increased mortality and reduced growth of E. marginata seedlings.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Dell, Bernard and Turner, Neil
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