Peter K. Ades

Mechanical stability of trees under dynamic loads

Tree stability in windstorms and tree failure are important issues in urban areas where there can be risks of damage to people and property and in forests where wind damage causes economic loss. Current methods of managing trees, including pruning and assessment of mechanical strength, are mainly based on visual assessment or the experience of people such as trained arborists. Only limited data are available to assess tree strength and stability in winds, and most recent methods have used a static approach to estimate loads. Recent research on the measurement of dynamic wind loads and the effect on tree stability is giving a better understanding of how different trees cope with winds. Dynamic loads have been measured on trees with different canopy shapes and branch structures including a palm (Washingtonia robusta), a slender Italian cypress (Cupressus sempervirens) and trees with many branches and broad canopies including hoop pine (Araucaria cunninghamii) and two species of eucalypt (Eucalyptus grandis, E. teretecornus). Results indicate that sway is not a harmonic, but is very complex due to the dynamic interaction of branches. A new dynamic model of a tree is described, incorporating the dynamic structural properties of the trunk and branches. The branch mass contributes a dynamic damping, termed mass damping, which acts to reduce dangerous harmonic sway motion of the trunk and so minimizes loads and increases the mechanical stability of the tree. The results from 12 months of monitoring sway motion and wind loading forces are presented and discussed.

Stochastic structure and individual-tree growth models

The majority of past and current individual-tree growth modelling methodologies have failed to characterise and incorporate structured stochastic components. Rather, they have relied on deterministic predictions or have added an unstructured random component to predictions. In particular, spatial stochastic structure has been neglected, despite being present in most applications of individual-tree growth models. Spatial stochastic structure (also called spatial dependence or spatial autocorrelation) eventuates when spatial influences such as competition and micro-site effects are not fully captured in models. Temporal stochastic structure (also called temporal dependence or temporal autocorrelation) eventuates when a sequence of measurements is taken on an individual-tree over time, and variables explaining temporal variation in these measurements are not included in the model. Nested stochastic structure eventuates when measurements are combined across sampling units and differences among the sampling units are not fully captured in the model. This review examines spatial, temporal, and nested stochastic structure and instances where each has been characterised in the forest biometry and statistical literature. Methodologies for incorporating stochastic structure in growth model estimation and prediction are described. Benefits from incorporation of stochastic structure include valid statistical inference, improved estimation efficiency, and more realistic and theoretically sound predictions. It is proposed in this review that individual-tree modelling methodologies need to characterise and include structured stochasticity. Possibilities for future research are discussed.

Neutral DNA markers fail to detect genetic divergence in an ecologically important trait

The development of strategies for in situ, ex situ conservation and reforestation of the monkey puzzle tree (Araucaria araucana), a vulnerable tree endemic to southern South America, has led to an interest in the level and distribution of the genetic diversity of the species. Neutral DNA markers (RAPDs) and quantitative genetic techniques were used to characterise genetic heterogeneity within and among populations from throughout the natural range of the species. Both the level and pattern of genetic variation estimated using the different techniques were essentially uncorrelated. An important discrepancy was found with the neutral markers failing to detect an important quantitative genetic divergence across the Andean Range relating to drought tolerance. This study clearly demonstrates the potential problems associated with making recommendations for conserving the genetic resource of threatened species based solely on neutral marker studies. Alternative approaches are discussed, including a stronger focus on ecologically important traits and the potential use of surrogate measures of genetic variability.

Prospecting for sources of resistance to ascochyta blight in wild Cicer species

A collection of 114 Cicer accessions including 95 wild Cicer accessions from four different species (C. bijugum K. H. Rech., C. echinospermum P. H. Davis, C. pinnatifidum Jaub. & Sp. and C. reticulatum Ladiz.) was screened in glasshouse trials for resistance to ascochyta blight (caused by Ascochyta rabiei (Pass.) Lab.). Resistance was identified in accessions from all four wild Cicer species. There was variation for resistance within accessions of C. echinospermum, C. pinnatifidum and C. reticulatum, with only C. bijugum containing accessions in which all plants were completely resistant. Resistant accessions from C. echinospermum and C. reticulatum may offer accessible sources of resistance because fully fertile hybrids with cultivated chickpea can be readily obtained.

Polybrominated diphenyl ethers and polybrominated biphenyls in Australian sewage sludge

This paper presents a brief review of the international scientific literature of polybrominated diphenyl ethers (PBDEs) and polybrominated biphenyls (PBBs) in sewage sludge and a survey of these compounds in sewage sludge from 16 Australian wastewater treatment plants (WWTPs). The SigmaPBDE mean concentration in the Australian study was 1137microgkg(-1) dry weight (d.w.) (s.d. 1116) and ranged between 5 and 4 230microgkg(-1)d.w. The urban mean of 1308microgkg(-1) (s.d. 1320) and the rural mean of 911microgkg(-1) (s.d. 831) are not statistically different and are similar to levels in European sludges. Principal components analysis was performed on the data set and revealed that 76% of the data variation could be explained by two components that corresponded to overall concentration of the pentaBDE and the decaBDE commercial formulations. An analysis of variance was performed comparing PBDEs levels at three WWTPs over the years 2005 and 2006, finding differences between treatment plants (BDE-47) but no significant difference in PBDE levels in the years 2005 and 2006. Low levels of BB-153 were detected in all samples of this survey (n=16); mean 0.6microgkg(-1)d.w. (s.d. 0.5). This compound has rarely been reported in any other study of sewage sludges undertaken outside Australia. This work highlights the need for a risk assessment of PBDEs in sewage sludge when used for land application, taking into account typical levels found in Australian sludges and soils.

Genetic diversity of isolates of the Leptosphaeria maculans species complex from Australia, Europe and North America using amplified fragment length polymorphism analysis

Amplified Fragment Length Polymorphism (AFLP) analysis has been used to analyse 100 Australian, European and North American isolates of Leptosphaeria maculans. All isolates had distinct AFLP profiles. They could be classified into five types, which had very few AFLP bands in common and corresponded to classifications made previously on the basis of ability to cause stem cankers on Brassica napus (A group), or inability to do so (B group), and on host range. Four isolates had AFLP profiles completely dissimilar to these groups and to each other. Genetic diversity and geographic differentiation were analysed separately within AFLP types 1 and 2. UPGMA analysis of the 66 AFLP type 1 (A group) isolates using 50 polymorphic bands did not provide evidence for clustering according to geographic origin. Non-metric multidimensional scaling (NMDS) analysis suggested that the Australian and European populations were separate adjacent clusters, while the North American population partially overlapped both the others. This geographic differentiation was supported by Wrights fixation index (Fst) analysis. Three measures of genetic variability between isolates within regions (effective number of alleles, gene diversity, and Shannon index) showed that the North American A group isolates were less diverse than those from Australia and Europe. The 21 AFLP type 2 (B group; NA1 sub-group) isolates did not cluster based on geographic region, which was confirmed by NMDS and Fst analysis. There was a similar degree of genetic diversity within A group and the NA1 sub-group of B group isolates. Unlike other techniques, AFLP analysis can readily discriminate between group B isolates of the L. maculans complex that were previously difficult to classify and also provides individual fingerprints for isolates. Isolates of the A group and of the NA1 sub-group of B group could be also distinguished readily by electrophoretic karyotyping, as the latter isolates had more bands smaller than 1.4 Mb than the A group isolates.

Analysis of a diverse global Pisum sp. collection and comparison to a Chinese local P. sativum collection with microsatellite markers.

Twenty-one informative microsatellite loci were used to assess and compare the genetic diversity among Pisum genotypes sourced from within and outside China. The Chinese germplasm comprised 1243 P. sativum genotypes from 28 provinces and this was compared to 774 P. sativum genotypes that represented a globally diverse germplasm collection, as well as 103 genotypes from related Pisum species. The Chinese P. sativum germplasm was found to contain genotypes genetically distinct from the global gene pool sourced outside China. The Chinese spring type genotypes were separate from the global gene pool and from the other main Chinese gene pool of winter types. The distinct Chinese spring gene pool comprised genotypes from Inner Mongolia and Sha’anxi provinces, with those from Sha’anxi showing the greatest diversity. The other main gene pool within China included both spring types from other northern provinces and winter types from central and southern China, plus some accessions from Inner Mongolia and Sha’anxi. A core collection of Chinese landraces chosen to represent molecular diversity was compared both to the wider Chinese collection and to a geographically diverse core collection of Chinese landraces. The average gene diversity and allelic richness per locus of both the micro-satellite based core and the wider collection were similar, and greater than the geographically diverse core. The genetic diversity of P. sativum within China appears to be quite different to that detected in the global gene pool, including the presence of several rare alleles, and may be a useful source of allelic variation for both major gene and quantitative traits.

Model-based analysis of the likelihood of gene introgression from genetically modified crops into wild relatives

The proliferation of genetically modified crops has created a need for methods to predict the likelihood of gene introgression into related species in situ. We present a model of a modified crop and an associated unmodified plant population removed spatially from the modified crop but not completely isolated from it, reflecting standard practices for isolation of field trials. We develop models for two kinds of life histories, broadly based on Brassica and Gossypium, taxa that are targets for genetic modification. We find that current prescriptions for field trials are likely to lead to escape of transgenes into wild populations when outcrossing rates are moderate and hybrids are fertile. The results are sensitive to pollen rain within plausible bounds for model parameters, suggesting buffer widths are an important aspect of the design of field trials. When gene introgression requires the spontaneous development of a polyploid, the likelihood of gene introgression is lower but still appreciable in realistic circumstances. Events that are unlikely over periods of a few years become almost certain within scales of a few decades, emphasising the need for gene risk assessments to be set in specified time frames. The models serve to identify the parts of the system that are poorly known and that are important in determining outcomes, providing a focus for future research. There is a need for research on the consequences of changes in fitness due to the transgenes, competitive interactions between related species, and the broader ecological consequences of changes in agricultural practice resulting from the use of genetically modified crops.

Mycosphaerella leaf disease reduces growth of plantation-grown Eucalyptus globulus

Summary Species of Mycosphaerella cause significant leaf damage and defoliation in young eucalypt plantations in southern Australia and overseas. The effects on growth, however, have not been adequately quantified, although over 310 000 ha of Eucalyptus globulus had been planted in Australia by the end of 2000. In a replicated field experiment (2-y-old E. globulus family trial) exposed to natural infection by species of Mycosphaerella, we periodically sprayed fungicides (benomyl and chlorothalonil) on half of the trees and not the other half. Quantitative assessments of disease (both severity of leaf spots and defoliation) and measurements of tree height and diameter were conducted periodically over the following 17 mo. The fungicides effectively controlled the disease caused by the species of Mycosphaerella present in the trial (M. cryptica and M. nubilosa). Even at low levels of infection by the Mycosphaerella spp. (less than 10% leaf area affected), the unsprayed trees had significantly poorer growth, with height increment reduced by 13% and diameter increment by 4%. Defoliation of unsprayed trees (61 %) was double that of sprayed trees (32%). Severity of leaf spots was positively correlated with defoliation (r = 0.36, P < 0.001), and defoliation was negatively correlated with both height increment (r = -0.37, P < 0.001) and diameter increment (r = -0.29, P < 0.001) — evidence that disease was reducing growth. While spraying controls disease, it is time-consuming and may not be commercially viable. Breeding for resistance to reduce the impact of Mycosphaerella leaf disease may be the most economic control strategy.

Mycosphaerella diseases of juvenile foliage in a eucalypt species and provenance trial in Victoria, Australia

Summary The incidence of mycosphaerella leaf diseases and the severity of seven species of Mycosphaerella were assessed on the juvenile foliage of 13 eucalypt species and 35 provenances in a 2–3 year old trial in south-eastern Australia. Significant variation was observed between the 13 host species, with E. cypellocarpa and E. nitens recording the highest disease incidence and E. elata and E. oreades having no assessable disease. While wide variation was observed between provenances within species, variation was significant only between provenances of E. nitens. Seven species of Mycosphaerella were recorded on diseased leaves collected from the trial. M. cryptica and M. nubilosa were the most damaging, with M. cryptica being the most common and having the widest host range.