Water Gum Research Articles

Shoot Flammability Patterns in Native and Exotic Street Tree Species at the Wildland–Urban Interface of Eastern Australia

Street trees provide ecosystem services such as heat mitigation, improved community well-being, and biodiversity conservation. At the wildland–urban interface (WUI), high-flammability street trees also provide a conflicting ecosystem disservice, heightening risks of wildfire spread into urban areas. We addressed this service–disservice conflict by assessing shoot flammability patterns in 10 street tree species, to identify low-flammability species that can potentially mitigate wildfire risks at the WUI. We found significant differences among species in flammability attributes including time-to-flame (TTF), flame duration (FD), number of flaming events (nF), and flame temperature (FT), and identified low-flammability species for each attribute. Overall, species’ rankings from least to most flammable differed considerably across the four attributes. For example, native water gum (Tristaniopsis laurina) had the slowest TTF, but had the longest FD. Among nine shoot traits, we found that high leafing intensity was the most frequent trait correlated with flammability. In particular, high leafing intensity was significantly related to fast TTF and high FT. Lack of coordination among flammability attributes suggests that, in general, selection of low-flammability street tree species should consider how each flammability attribute differentially contributes to wildfire spread risk. Nonetheless, native Tuckeroo (Cupaniopsis anacardioides) emerged as a potential candidate for further exploration as a low-flammability street tree as it had comparatively long TTF, short FD, and low nF. We found no consistent evidence that exotic species were less flammable than native species, and suggest that native trees be the focus of further research to identify low-flammability street trees.

Tristaenone A: A New Anti-Inflammatory Compound Isolated from the Australian Indigenous Plant Tristaniopsis laurina.

Inspired by ethnopharmacological knowledge, we conducted a bioassay-guided fractionation of the leaves of Tristaniopsis laurina which led to the discovery of a new anti-inflammatory compound, tristaenone A (1). The structure was elucidated by detailed spectroscopic data analysis, and the absolute configuration was established using X-ray crystallography analysis. Tristaenone A (1) suppressed LPS and IFN-γ-induced NO, TNF-α and IL-6 production in RAW 264.7 cells with IC50 values of 37.58 ± 2.45 μM, 80.6 ± 5.82 μM and 125.65 ± 0.34 μM, respectively. It also inhibited NF-κB nuclear translocation by 52.93 ± 14.14% at a concentration of 31.85 μM.

Selecting tree species with high transpiration and drought avoidance to optimise runoff reduction in passive irrigation systems

Rainfall in cities can generate large volumes of stormwater runoff which degrades receiving waterways. Irrigating trees with runoff (passive irrigation) has the potential to increase transpiration and contribute to stormwater management by reducing runoff received by downstream waterways, but the stochastic nature of rainfall may expose trees with high transpiration to drought stress. We hypothesized that for success in passive irrigation systems, tree species should exhibit i) high maximum transpiration rates under well-watered conditions, ii) drought avoidance between rainfall events, and iii) high recovery of transpiration with rainfall following a drought. We assessed 13 commonly planted urban tree species in Melbourne, Australia against three metrics representing these behaviours (crop factor, hydroscape area, and transpiration recovery, respectively) in a glasshouse experiment. To aid species selection, we also investigated the relationships between these three metrics and commonly measured plant traits, including leaf turgor loss point, wood density, and sapwood to leaf area ratio (Huber value). Only one species (Tristaniopsis laurina) exhibited a combination of high crop factor (>1.1 mm mm−1 d−1) indicating high transpiration, small hydroscape area (<3 MPa2) indicating drought avoidance, and high transpiration recovery (>85%) following water deficit. Hence, of the species measured, it had the greatest potential to reduce runoff from passive irrigation systems while avoiding drought stress. Nevertheless, several other species showed moderate transpiration, hydroscape areas and transpiration recovery, indicating a balanced strategy likely suitable for passive irrigation systems. Huber values were negatively related to crop factor and transpiration recovery and may therefore be a useful tool to aid species selection. We propose that selecting tree species with high transpiration rates that can avoid drought and recover well could greatly reduce stormwater runoff, while supporting broader environmental benefits such as urban cooling in cities.

Functional adaptations and trait plasticity of urban trees along a climatic gradient

In urban environments, long-term tree survival and performance requires physiological tolerance or phenotypic plasticity in plant functional traits. Knowledge of these traits can inform the likely persistence of urban forests under future, more severe climates. We assessed the plasticity of morphological and physiological traits of tree species planted along an urban climatic gradient in the Greater Sydney region during a severe, multi-year drought in eastern Australia. We selected four sites along a ∼55 km east-west transect, ranging from the cool/wet coast to the warm/dry inland. We assessed five tree species (four natives, one exotic) with different predicted climatic vulnerability based on climate-origins, estimating functional traits indicative of drought tolerance: carbon isotope composition (δ13C), Huber value (HV), specific leaf area (SLA), wood density (WD), and leaf turgor loss point (πtlp). Broadly, trees planted in warm/dry sites had more negative πtlp, higher WD, δ13C and HV, and lower SLA than cool/wet sites, indicating phenotypic plasticity to drought. The leaf-level traits πtlp, δ13C and SLA were more strongly correlated with temperature and precipitation, compared to HV and WD. Species differed in the extent of their trait shifts along the transect, with greater plasticity evident in the exotic Celtis australis and the more temperate cool-climate Tristaniopsis laurina, compared to the more tropical, warm-climate Cupaniopsis anacardioides, which showed limited plasticity and lower drought tolerance. Our findings reveal adaptive capacity of urban trees to climate via plasticity in drought tolerance traits, which can direct species selection to improve urban forests resistance to climate change.

Genera of diaporthalean coelomycetes associated with leaf spots of tree hosts.

Four different genera of diaporthalean coelomycetous fungi associated with leaf spots of tree hosts are morphologically treated and phylogenetically compared based on the DNA sequence data of the large subunit nuclear ribosomal DNA gene (LSU) and the internal transcribed spacers and 5.8S rRNA gene of the nrDNA operon. These include two new Australian genera, namely Auratiopycnidiella, proposed for a leaf spotting fungus occurring on Tristaniopsis laurina in New South Wales, and Disculoides, proposed for two species occurring on leaf spots of Eucalyptus leaves in Victoria. Two new species are described in Aurantiosacculus, a hitherto monotypic genus associated with leaf spots of Eucalyptus in Australia, namely A. acutatus on E. viminalis, and A. eucalyptorum on E. globulus, both occurring in Tasmania. Lastly, an epitype specimen is designated for Erythrogloeum hymenaeae, the type species of the genus Erythrogloeum, and causal agent of a prominent leaf spot disease on Hymenaea courbaril in South America. All four genera are shown to be allied to Diaporthales, although only Aurantiosacculus (Cryphonectriaceae) could be resolved to family level, the rest being incertae sedis.

Differences in resprouting ability are not related to seed size or seedling growth in four riparian woody species

Summary Resprouting is a key plant attribute facilitating persistence in disturbance‐prone environments. Resprouting ability in seedlings may depend on both developmental ontogeny and seed size. However, the relationships between these factors are not well explored, especially for woody species with comparatively small seeds and epigeal germination. We investigated resprouting capacity in seedlings from four subtropical, riparian, Myrtaceous tree species, Melaleuca leucadendra, Asteromyrtus symphyocarpa, Eucalyptus camaldulensis var. obtusa and Tristaniopsis laurina, displaying these characteristics. We recorded resprouting in response to simulated disturbance as a function of seed mass and developmental age (5–150 days post‐emergence) and examined the acquisition of resprouting ability in relation to growth and biomass allocation patterns. Patterns of resprouting were distinct among species, but the acquisition of resprouting ability was not determined by seed mass. The ‘small’ seeded M. leucadendra and the ‘intermediate’ seeded E. camaldulensis showed unexpectedly high shoot resprouting vigour from cotyledon stage (70% resprouting at 5 days post‐emergence), as well as greatest ongoing allocation to root mass and lateral root development. In contrast, in A. symphyocarpa (another species with ‘intermediate’ seed mass) and T. laurina (a ‘large’ seeded species) resprouting rates during early development were much lower (&lt; 10%), although there was a trend towards increasing resprouting ability with age in A. symphyocarpa (&gt; 150 days). Resprouting capacity was also independent of seedling size and relative growth rate. Our results indicate that the size‐dependency of resprouting capacity varies considerably among these species. This suggests physiological and morphological species traits other than those directly related to reserve size or relative growth rate may convey survivorship in river environments. Our findings show that resprouting capacity was not related to seed size and seedling growth patterns in these four species. This is different to evidence from comparative studies undertaken in fire‐prone and other temperate environments. A broader survey of seedling resprouting ability including more species is required to determine the generality of our findings in riparian species.