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1954-2016
2016
Reproduction of threatened, long lived semi arid
Acacia within highly fragmented stands in far
western NSW
Cairo N. Forrest
University of Wollongong
Unless otherwise indicated, the views expressed in this thesis are those of the author and do
not necessarily represent the views of the University of Wollongong.
Recommended Citation
Forrest, Cairo N., Reproduction of threatened, long lived semi arid Acacia within highly fragmented stands in far western NSW,
Doctor of Philosophy thesis, School of Biological Sciences, University of Wollongong, 2016. htps://ro.uow.edu.au/theses/4961
Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library:
Reproduction of threatened, long lived
semi arid Acacia within highly
fragmented stands in far western NSW.
A thesis submitted in fulfilment of the requirements for the award of the
degree
DOCTOR OF PHILOSOPHY
from the
UNIVERSITY OF WOLLONGONG
by
Cairo N. Forrest B. Sc. (Hons)
Department of Biological Sciences
2016
Certification
I, Cairo N. Forrest, declare that this thesis, submitted in fulfilment of the requirements for the
award of Doctor of Philosophy, in the School of Biological Sciences, University of Wollongong,
is wholly my own work unless otherwise referenced or acknowledged. The document has not
been submitted for qualifications at any other academic institution.
Cairo Forrest
22 March 2016
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Abstract
The contraction and aging of stands of overstory tree species in rangelands is well
documented worldwide and largely reflects anthropogenic pressures such as agricultural land
clearing and increased grazing leading to increased mortality and reduced
recruitment. Without recruitment, stands that largely comprise old and senescent plants may
soon go locally extinct. Complicating assessments of plant population health however, is the
diversity of dynamics in populations of plant species; what constitutes reproductive failure and
population contraction in one species can simply represent a natural cycle in another. In far
western New South Wales (NSW), several Acacia species subject to intense grazing by
domestic and feral herbivores display prolonged recruitment failure. Surveys over the past two
decades have also failed to detect fruit set suggesting they are trending to extinction.
Hypotheses explaining the failure of these fragmented populations to reproduce sexually have
included restrictions to mating systems, insufficient genetic diversity, prolonged drought
period, and the widely supported claim that extant plants are senescent. In stark contrast,
some shorter lived co-occurring Acacia species such as A. ligulata and A. victoriae are thriving
and reproducing regularly under the same conditions. It is not understood why this difference
exists. Reproductive effort has not yet been monitored outside a prolonged period of drought,
demographic surveys to date have only been qualitative, and little is known about their mating
systems. Without this information it is impossible to know which of the competing hypotheses
explain their decline, or to recommend conservation strategies for the future. Here I use a
multidisciplinary and comparative approach combining surveys, genetic analysis and manual
pollination and growth experiments to gain this information.
Initially, I conducted the first formal surveys of the condition of populations of the
threatened Acacia species (A. melvillei, A. loderi, and A. carneorum), one potentially
threatened species (A. homalophylla) and the thriving co-occurring A. ligulata, across
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2
approximately 430,000 km of the semi arid region of western NSW. Specifically I estimated
the age of plants within stands as well as their health via five demographic, three
environmental and four plant health measures within 47, 26, 30, 10 and 20 separate stands of
each species respectively. I took advantage of a rare La Niña rain event across the region to
assess and quantify the reproductive (sexual) capacity of these stands under conditions
where reproduction could not be limited by lack of water. I attempt to explain variation in
seed set between stands as measured as 1) the presence of any mature fruit on plants and 2)
the percentage of plant's canopies covered in mature fruit. I used microsatellite markers to
look for differences in levels of genetic diversity as allelic and genotypic richness among stands
of A. loderi and A. carneorum that did and did not set fruit in response to the La Niña rain
event. The viability, fitness and capacity for these species to contribute to a long lived soil
stored seed bank was assessed using 875 seed set after the La Niña rain event from 92 stands
by performing controlled germination and growth experiments and sowing seed in the field.
2
Region-wide surveys of seedling recruitment in 133 stands across 336,000 km were also
conducted, as well as long term growth and survival surveys of these seedlings in the field, to
1) assess the vigour of the seed currently being produced, 2) quantify the natural recruitment
response within stands across the region, 3) assess the health and long term survival prospects
of seedlings and 4) determine what local environmental conditions appear to be key drivers of
recruitment success and failure. Finally I combined pollinator observations, pollen tube
analysis, manual pollination experiments, and paternity analysis techniques, to attempt to
characterize the state of current mating systems operating in several highly fragmented A.
carneorum and A. ligulata stands within Kinchega National Park in western NSW, during this
same period of high water availability.
I found that 100, 69, and 100% of A. melvillei, A. loderi and A. carneorum stands
surveyed across the region respectively were populated predominantly by large mature plants
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only with the majority of A. loderi and A. carneorum plants within these stands displaying
reduced canopy cover indicative of senescence. In contrast, all A. homalophylla and A. ligulata
stands were comprised of plants of a range of sizes with A. homalophylla stands displaying
evidence of substantial recent recruitment through suckering and A. ligulata stands displaying
a more even spread among morphological classes and a high proportion of putative recruits.
For the first time, I documented landscape wide sexual reproductive success in the form
of generally high levels of fruiting / seed set in 83, 80, and 81% of A. melvillei, A. homalophylla
and A. loderi stands surveyed respectively, in at least one of the two consecutive years
following the La Niña rain. While every A. ligulata stand surveyed produced seed, only 13% of
all A. carneorum stands surveyed in the region set seed and only in relatively small amounts. I
also found that the seemingly oldest plants of all five Acacia species within these stands were
equivalently fecund as plants of seemingly younger age. In contrast however, only four of the
30 A. carneorum stands were found to set any fruit, with fecundity levels being extremely low
in comparison to the other four species.
Great intra-specific variance in demographic characteristics of stands of all five species
was uncovered including stand size, density and proximity to other stands. However, all were
likely to have populations greatly reduced by clearing combined with subsequent recruitment
failure. 64% to 100% of A. melvillei, A. homalophylla, A. loderi and A. carneorum stands
surveyed across the region now consist of fewer than 200 plants while 47, 89, 73, and 90% of
these same stands are now isolated from the closest neighbouring stand by at least 4 km.
Nevertheless, I found great similarities between stands of all four threatened species in the
apparent condition of plants within them, the level of flowering effort and the amount of
pollen deposited on stigmas by their pollinators. Furthermore, I found that differences in the
five structural, three environmental, four plant health and two climatic variables, often
associated with reproductive success or failure in others species, were not good indicators in
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these species. For all four threatened species, the most fragmented stands with senescent
plants were found to be comparably fecund as those in the largest, densest stands populated
by apparently vigorous mature plants. The lack of genetic diversity found in all five of the 26 A.
loderi stands surveyed that failed to set fruit (all 5 were monoclonal), despite most stands
containing much genetic diversity however, suggests a genetic component to maternal sterility
in these species.
I found high levels of seed viability in unparasitized seed ranging from 68% (SE±0.1) to
77% (SE±0.0) which was comparable with A. ligulata 46% (SE±0.1). A substantial proportion of
the seedlings grown from the A. melvillei, A. homalophylla, A. loderi and A. carneorum seed
survived to two years of age under coastal conditions, ranging from 50% (SE±5.0) to 54%
(SE±5.1) , which was also comparable with A. ligulata (44 % (SE±1.8)). Highly variable
recruitment was found within stands and even among plants within stands of A. melvillei, A.
homalophylla, A. loderi and A. ligulata, ranging from as high as 369 seedlings per plant to zero
seedlings. In contrast, no sexual recruitment was found in any of the A. carneorum stands
surveyed irrespective of whether they were observed to set fruit or not. Understory vegetation
was found to be important in protecting seedlings from grazing, with A. melvillei, A. loderi and
A. ligulata seedlings located outside the canopy of nurse plants being grazed more often than
seedlings located under a ‘nurse plant’.
A diverse pollinator assemblage of 17 and 23 native insects was found successfully
depositing viable pollen onto the majority of A. carneorum (37.3% (SE±0.3)) and A. ligulata
flowers (55.1% (SE±0.1)) in all stands surveyed, irrespective of their reproductive histories. In
contrast however, I found that while many of the same native insects visited both A.
carneorum and A. ligulata, the pollination system of A. ligulata was now dominated by
European honeybees (Apis mellifera), which tended to move pollen within plants and between
local plants more so than the native pollinators. Paternity analysis of fruit collected from seed
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produced in a fruiting A. carneorum stand revealed approximately one third of fruit in both
stands to be selfed and the other two thirds to be outcrossed pollen from the only
neighbouring stand 1 km away. This indicates that while these plants are self compatible, a
mate choice mechanism preferentially selects for outcrossed pollen, given the largely localized
movements the insect pollinators were found to be making. Naturally produced A. ligulata
seed grew on average 18% and 26% slower than seed produced through manual outcrossing in
the first and second years the experiment was run respectively, irrespective of the source of
the outcrossed pollen applied.
My findings provide the first rigorous support for the claim that stands of these
threatened species are at risk due to senescence and prolonged recruitment
failure. Moreover, my findings suggest that while cyclical large scale rain events allow plants
to initiate sexual reproduction in A. melvillei, A. homalophylla and A. loderi, persistent
reproductive failure within all A. carneorum stands and a few A melvillei and A. loderi stands,
implies that for some, despite appearing equally healthy and receiving ample viable pollen,
reproduction is connected to some other cryptic deficiency, or is limited by their natural
reproductive strategy. For A. carneorum at least, this hypothesis is further supported by my
own and previous carbon dating results which found that the clonal structure of stands likely
pre-dates the worst effects of anthropogenic disturbance. For species that clearly rely on
sexual reproduction to maintain populations and genetic diversity, degeneration of ground
conditions suitable for sexual recruitment is likely to explain why current recruitment rates in
many stands at least, are grossly inadequate. This combined with the unnaturally intense
grazing regime in the region, means that long term mortality rates of these new recruits is
likely to be unacceptably high.
Whilst A. ligulata seems to be recruiting far better than the threatened species, the
finding that honeybees dominate pollination of many populations is cause for concern given
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