Linda M. Broadhurst

Seed supply for broadscale restoration: maximizing evolutionary potential.

Restoring degraded land to combat environmental degradation requires the collection of vast quantities of germplasm (seed). Sourcing this material raises questions related to provenance selection, seed quality and harvest sustainability. Restoration guidelines strongly recommend using local sources to maximize local adaptation and prevent outbreeding depression, but in highly modified landscapes this restricts collection to small remnants where limited, poor quality seed is available, and where harvesting impacts may be high. We review three principles guiding the sourcing of restoration germplasm: (i) the appropriateness of using ‘local’ seed, (ii) sample sizes and population characteristics required to capture sufficient genetic diversity to establish self‐sustaining populations and (iii) the impact of over‐harvesting source populations. We review these topics by examining current collection guidelines and the evidence supporting these, then we consider if the guidelines can be improved and the consequences of not doing so. We find that the emphasis on local seed sourcing will, in many cases, lead to poor restoration outcomes, particularly at broad geographic scales. We suggest that seed sourcing should concentrate less on local collection and more on capturing high quality and genetically diverse seed to maximize the adaptive potential of restoration efforts to current and future environmental change.

Genetic characterization of root-nodule bacteria associated with Acacia salicina and A. stenophylla (Mimosaceae) across south-eastern Australia.

Symbiotic relationships between legumes and nitrogen-fixing soil micro-organisms are of ecological importance in plant communities worldwide. For example, nutrient-poor Australian soils are often dominated by shrubby legumes (e.g. species of Acacia). However, relatively few studies have quantified patterns of diversity, host-specificity and effectiveness of these ecologically important plant-microbe interactions. In this study, 16S rRNA gene sequence and PCR-RFLP analyses were used to examine bacterial strains isolated from the root nodules of two widespread south-eastern Australian legumes, Acacia salicina and Acacia stenophylla, across nearly 60 sites. The results showed that there was extensive genetic diversity in microbial populations, including a broad range of novel genomic species. While previous studies have suggested that most native Australian legumes nodulate primarily with species of the genus Bradyrhizobium, our results indicate significant associations with members of other root-nodule-forming bacterial genera, including Rhizobium, Ensifer, Mesorhizobium, Burkholderia, Phyllobacterium and Devosia. Genetic analyses also revealed a diverse suite of non-nodulating bacterial endophytes, only a subset of which have been previously recorded. Although the ecological roles of these endosymbionts are not well understood, they may play both direct and indirect roles in promoting plant growth, nodulation and disease suppression.

Assessing limitations on population growth in two critically endangered Acacia taxa

Abstract Flowering phenology, soil seedbank and the impact of fire, weeds and grazing were investigated in two rare Acacia taxa restricted to the agricultural district north of Perth Western Australia. Acacia aprica is known from six extant populations and one extinct population. Five of the extant populations are restricted to linear road reserves with the sixth population located in a small native vegetation remnant. Acacia cochlocarpa ssp . cochlocarpa is known from a single population also situated on a road verge. Size class structure, levels of canopy death and an absence of juveniles indicated that all populations are in decline. Flowering intensity and success varied between populations and years in A. aprica and between years in A. cochlocarpa ssp. cochlocarpa. Seed bank analysis indicated that seeds were patchily distributed while experimental fires demonstrated that such events could break seed dormancy and promote germination. Both taxa have similar germination physiologies and showed increased germination after seeds were exposed to heat. Following emergence, however, competition with annual weeds had a negative impact on seedling growth and survival in both taxa. While vertebrate grazing had some influence, weeds were the major inhibitory influence on recruitment. Reduced fire frequencies since fragmentation may be responsible for population decline in both taxa but other site specific factors such as weeds and grazing may affect the establishment of seedlings following fire.

Microbial phylotype composition and diversity predicts plant productivity and plant-soil feedbacks.

The relationship between ecological variation and microbial genetic composition is critical to understanding microbial influence on community and ecosystem function. In glasshouse trials using nine native legume species and 40 rhizobial strains, we find that bacterial rRNA phylotype accounts for 68% of amoung isolate variability in symbiotic effectiveness and 79% of host specificity in growth response. We also find that rhizobial phylotype diversity and composition of soils collected from a geographical breadth of sites explains the growth responses of two acacia species. Positive soil microbial feedback between the two acacia hosts was largely driven by changes in diversity of rhizobia. Greater rhizobial diversity accumulated in association with the less responsive host species, Acacia salicina, and negatively affected the growth of the more responsive Acacia stenophylla. Together, this work demonstrates correspondence of phylotype with microbial function, and demonstrates that the dynamics of rhizobia on host species can feed back on plant population performance.

Symbiotic Effectiveness of Rhizobial Mutualists Varies in Interactions with Native Australian Legume Genera

Background and Objectives Interactions between plants and beneficial soil organisms (e.g. rhizobial bacteria, mycorrhizal fungi) are models for investigating the ecological impacts of such associations in plant communities, and the evolution and maintenance of variation in mutualisms (e.g. host specificity and the level of benefits provided). With relatively few exceptions, variation in symbiotic effectiveness across wild host species is largely unexplored. Methods We evaluated these associations using representatives of several legume genera which commonly co-occur in natural ecosystems in south-eastern Australia and an extensive set of rhizobial strains isolated from these hosts. These strains had been previously assigned to specific phylotypes on the basis of molecular analyses. In the first of two inoculation experiments, the growth responses of each host species was evaluated with rhizobial strains isolated from that species. The second experiment assessed performance across genera and the extent of host specificity using a subset of these strains. Results While host growth responses to their own (sympatric) isolates varied considerably, rhizobial phylotype was a significant predictor of symbiotic performance, indicating that bacterial species designations on the basis of molecular markers have ecological importance. Hosts responded in qualitatively different ways to sympatric and allopatric strains of rhizobia, ranging from species with a clear preference for their own strains, to those that were broad generalists, through to species that grew significantly better with allopatric strains. Conclusion Theory has focused on trade-offs between the provision of benefits and symbiont competitive ability that might explain the persistence of less beneficial strains. However, differences in performance among co-occurring host species could also drive such patterns. Our results thus highlight the likely importance of plant community structure in maintaining variation in symbiotic effectiveness.

The influence of sampling strategies and spatial variation on the detected soil bacterial communities under three different land‐use types

To determine the influence of pooling strategies on detected soil bacterial communities, we sampled 45 soil cores each from a eucalypt woodland, a sown pasture and a revegetated site in an Australian landscape. We assessed the spatial variation within each land-use plot, including the influence of sampling distance, soil chemical characteristics and, where appropriate, proximity to trees on the soil bacterial community, by generating terminal restriction fragment length polymorphism profiles of the bacterial 16S rRNA genes. The soil bacterial community under the revegetated site was more similar to the original woodland than the pasture, and this result was found regardless of the soil- or the DNA-pooling strategy used. Analyzing as few as eight cores per plot was sufficient to detect significant differences between the bacterial communities under the different plots to be distinguished. Soil pH was found to be most strongly associated with soil bacterial community composition within the plots and there was no association found with proximity to trees. This study has investigated sampling strategies for further research into the transitions of soil microbial communities with land-use change across broader temporal and spatial scales.

Seeing the wood and the trees—predicting the future for fragmented plant populations in Australian landscapes

Australian landscapes face significant environmental challenges in the coming decade. The fragmentation of vegetation following broadscale land-clearing has rapidly altered critical genetic and demographic processes within and among the remnants that still reside in these landscapes. These perturbations threaten the long-term persistence of many species. Although considerable research has been directed towards the management of rare species, little is understood about how common and widespread species respond to these new challenges. In this paper we review the current state of knowledge regarding species biology for some key Australian taxonomic groups to develop broad predictions about the major threats to species persistence, particularly for some of the most common and widespread floral components of fragmented Australian landscapes. The main focus is on associations between reproductive strategy, vulnerability to demographic and genetic threats, and implications for fecundity.

Regional genetic differentiation in Western Australian sandalwood (Santalum spicatum) as revealed by nuclear RFLP analysis

Abstract Western Australian sandalwood, Santalum spicatum, is widespread in the semi-arid and arid regions of Western Australia, and there is some morphological variation suggestive of two ecotypes. The level and structuring of genetic diversity within the species was investigated using anonymous nuclear RFLP loci. Santalum spicatum showed moderate levels of genetic diversity compared to other Australian tree species. The northern populations in the arid region showed greater levels of diversity and less population differentiation than the southern populations in the semi-arid region due to differences in the distribution of rare alleles. Equilibrium between drift and gene flow in the northern populations indicated that they have been established for a long period of time with stable conditions conducive to gene flow. In contrast, the southern populations showed a relationship between drift and gene flow indicative of a pattern of fragmentation and isolation where drift has greater effect than gene flow. The different patterns of diversity suggest that the ecotypes in the two regions have been subject to differences in the relative influences of drift and gene flow during their evolutionary history.

Genetic diversity within and divergence between rare and geographically widespread taxa of the Acacia acuminata Benth. (Mimosaceae) complex.

The Acacia acuminata complex is a group of woody shrubs and small trees comprised of three formally described taxa (A. oldfieldii, A. acuminata ssp. acuminata and ssp. burkittii) and two informal taxa (‘small seed’ and ‘narrow phyllode’) with contrasting geographical distributions within the south-west of Western Australia. In addition, a series of variant populations of possible hybrid origin exist. Population genetic structure was investigated in 25 populations representing the taxa and variants using 16 polymorphic allozyme loci. All taxa and variants exhibited relatively high levels of genetic variation compared with other woody angiosperms with similar geographic distributions. Levels of genetic diversity in the widely distributed ssp. burkittii and ‘narrow phyllode’ taxon were considerably higher (He, 0.311 and 0.319, respectively) than expected for widespread woody shrubs. In contrast, the rare and highly restricted A. oldfieldii exhibited significantly lower levels of genetic diversity (He, 0.173) compared with the other taxa in the complex, but higher than other rare woody shrubs. Although morphologically close, associations based on genetic distance showed A. oldfieldii was highly divergent from the other taxa (D = 0.85) while including variant populations confused systematic alliances. The unusual placement of some of these populations and high degree of population differentiation (GST, 12.7%) supports the suggestion that these may represent a series of hybridisation events between the various taxa.

Non-additive effects of pollen limitation and self-incompatibility reduce plant reproductive success and population viability

BACKGROUND AND AIMS Mating system is a primary determinant of the ecological and evolutionary dynamics of wild plant populations. Pollen limitation and loss of self-incompatibility genotypes can both act independently to reduce seed set and these effects are commonly observed in fragmented landscapes. This study used a simulation modelling approach to assess the interacting effects of these two processes on plant reproductive performance and population viability for a range of pollination likelihood, self-incompatibility systems and S-allele richness conditions. METHODS A spatially explicit, individual-based, genetic and demographic simulation model parameterized to represent a generic self-incompatible, short-lived perennial herb was used to conduct simulation experiments in which pollination probability, self-incompatibility type (gametophytic and sporophytic) and S-allele richness were systematically varied in combination to assess their independent and interacting effects on the demographic response variables of mate availability, seed set, population size and population persistence. KEY RESULTS Joint effects of reduced pollination probability and low S-allele richness were greater than independent effects for all demographic response variables except population persistence under high pollinator service (>50 %). At intermediate values of 15-25 % pollination probability, non-linear interactions with S-allele richness generated significant reductions in population performance beyond those expected by the simple additive effect of each independently. This was due to the impacts of reduced effective population size on the ability of populations to retain S alleles and maintain mate availability. Across a limited set of pollination and S-allele conditions (P = 0·15 and S = 20) populations with gametophytic SI showed reduced S-allele erosion relative to those with sporophytic SI, but this had limited effects on individual fecundity and translated into only modest increases in population persistence. CONCLUSIONS Interactions between pollen limitation and loss of S alleles have the potential to significantly reduce the viability of populations of a few hundred plants. Population decline may occur more rapidly than expected when pollination probabilities drop below 25 % and S alleles are fewer than 20 due to non-additive interactions. These are likely to be common conditions experienced by plants in small populations in fragmented landscapes and are also those under which differences in response between gameptophytic and sporophtyic systems are observed.