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Modelling seed germination and seedling survival of Eucalyptus delegatensis R.T.Baker to facilitate optimal reafforestation.

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Battaglia, M (1993) Modelling seed germination and seedling survival of Eucalyptus delegatensis R.T.Baker to facilitate optimal reafforestation. PhD thesis, University of Tasmania.

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Abstract

Land managers increasingly are being involved in making quantitative evaluation
of management options. Forests, however, are complex biological systems and
predictions require the synthesis of many processes. Traditional approaches to
evaluating options have been to replicate experiments in time and space. Not all
questions are amenable to such approaches, and even where they are, inferences
may be of only limited application. In an increasingly complex decision making
context, land managers will require access to more sophisticated techniques. This
thesis illustrates the collection of basic data, its synthesis into a physiological
model, and its use as a tool to address a typically complex management question -
the time at which Eucalyptus delegatensis R.T.Baker seed should be sown in
forest regeneration operations. The species is widely exploited for commercial
forestry in South-Eastern Australia and its germination physiology is moderately
complex, providing an appropriate test for the usefulness of the modelling
approach.
The germination response of seedlots from five provenances to temperature,
stratification, soil matric potential and interrupted imbibition was examined. The
species was found to have a distinct temperature optimum between 15 and 20°C,
and a minimum temperature for germination of approximately 2°C. Short periods
of exposure to high temperatures did not substantially affect germination
performance. Stratification greatly increased the range of temperatures over
which a high proportion of the seed germinated. Increases in the rate of
germination with stratification are related to accrued thermal time during
stratification. Pre-imbibing seeds at water potentials down to -2 MPa increased
the rate of germination. However, no advantage was found after pre-imibibing at
lesser soil water matric potentials. This increased germination rate was associated
with a shortening of the time to commencement of germination and more
synchronous germination. Germination rate and germination capacity were
impeded by soil matric potentials below -0.01 .MPa, and germination was totally
inhibited by soil matric potentials below -0.5 MPa. Soil matric potential and
temperature interacted in their effects on germination capacity, and seeds
germinating at near optimum temperatures were less sensitive to soil moisture
stress. Seeds survived dehydration within sixty hours of the commencement of
imbibition, but were increasingly affected by dehydration thereafter. The rate of
imbibition was influenced by the ambient temperature and solution water
potential. At modest levels of water stress imbibition was not impeded and the
observed reduction in gennination capacity was probably due to the inhibition of
ii
growth related processes. Differences in germination response were detected
between the seedlots and these could be related to their geographic brigin.
The proportion of variability in seed and germination traits attributable to interand
intra-site components varied between traits examined. The germination rate
of seed was not significantly different between trees within a site, or between
trees from different sites. Variation in seed size and the proportion of dormant
seed in seed samples was mainly affected by site effects. The sensitivity of seed
samples to the water stress levels applied also varied substantially between sites
but additionally the seed from the drier site exhibited a highly significant
between-tree variability. It was concluded that the proportion of variation in seed
and germination characteristics attributable to between-, and to within-site effects,
could be partly related to the scale at which selective forces were presumed to
operate. Nevertheless, a substantial amount of variation in response existed
within the seed collected from the one tree.
The role of age and microtopographical variation in enabling seedlings to
withstand frost and drought was explored in glasshouse studies. The frost
resistance of E. delegatensis was found to vary with seedling age over the first six
months of development. Much of this variation was found to be a result of the
differing sensitivity of leaves originating from different leaf nodes, although older
leaves from the same node may have been more frost resistant than recently
expanded leaves. Newly emergent seedlings appeared to be the most susceptible
stage of the tree's lifecycle to death by frost.
Small scale variation in soil conditions, at the scale of tens of centimetres,
markedly affected the germination and establishment of seeds and seedlings under
moisture limiting conditions. Microsites that afforded protection, and probably
resulted in increased humidity, caused a marked increase in germination number
and rate. The mean survival time was significantly higher on these protected
microsites than on less protected microsites, or on microsites that restricted root
penetration. The importance of this variability in microtopography was strongly
influenced by season and the level of environmental stress, and was diminished as
seedlings aged. Due to the different requirements for seed germination and
seedling growth, a favourable microsite for germination was not necessarily a
favourable site for seedling survival. A comparison of seed and seedling
responses to water stress indicated that for E. delegatensis, at least, selection due
to microsite differences at the time of germination may not affect the
developmental characteristics of the seedlings.
iii
At two geographically close sites that differed significantly in climatic profile,
seed of E. delegatensis and Eucalyptus amygdalina Labill., a species that
frequently replaces E. delegatensis on drier sites, was sown at twelve times of the
year. Regular censuses of seedlings were conducted. The pattern of survival of
over twenty thousand seedlings, comprising one thousand two hundred identified
cohorts was followed. The influences of weather, seed harvesting, site
preparation, time of emergence and time of sowing on emergence, growth and
survival were examined. By modelling temperature and soil moisture it was
found that germination in the field was influenced strongly by ambient
temperature and soil moisture and that the commencement of germination flushes
in spring and autumn were well correlated to threshold values of soil moisture and
air temperature predicted from laboratory studies. A model of seasonal patterns
of seedling mortality was developed and concluded to be highly age dependant.
Although age dependant mortality rate was relatively constant at a given site
between seasons and years, with each season containing its own compliment of
hazards, it was necessary to make allowance for stochastic events, such as severe
frosts and drought, to satisfactorily model survivorship.
A mathematical model of germination was developed for E. delegatensis based on
the physiology of underlying processes. The accuracy of this model in predicting
the time course of germination under conditions of fluctuating temperature and
moisture was examined. This model was used to examine the results from the
field trial. In combination with a mortality function derived from field
observation, this gennination model was used to make recommendations on the
optimum times of sowing for the east coast of Tasmania, to explore the
importance of 'safe sites' for germination, and to investigate the implications of
different seedlot dormancy responses on reproductive success.

Item Type: Thesis (PhD)
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Date Deposited: 04 Feb 2013 04:39
Last Modified: 11 Mar 2016 05:55
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