Non-invasive imaging reveals convergence in root and stem vulnerability to cavitation across five tree species

Root vulnerability to cavitation is challenging to measure and under-represented in current datasets. This gap limits the precision of models used to predict plant responses to drought because roots comprise the critical interface between plant and soil. In this study, we measured vulnerability to drought-induced cavitation in woody roots and stems of five tree species (Acacia aneura, Cedrus deodara, Eucalyptus crebra, Eucalytus saligna, and Quercus palustris) with a wide range of xylem anatomies. X-ray microtomography was used to visualize the accumulation of xylem embolism in stems and roots of intact plants that were naturally dehydrated to varying levels of water stress. Vulnerability to cavitation, defined as the water potential causing a 50% loss of hydraulic function (P50), varied broadly among the species (–4.51 MPa to –11.93 MPa in stems and –3.13 MPa to –9.64 MPa in roots). The P50 of roots and stems was significantly related across species, with species that had more vulnerable stems also having more vulnerable roots. While there was strong convergence in root and stem vulnerability to cavitation, the P50 of roots was significantly higher than the P50 of stems in three species. However, the difference in root and stem vulnerability for these species was small; between 1% and 31% of stem P50. Thus, while some differences existed between organs, roots were not dramatically more vulnerable to embolism than stems, and the differences observed were less than those reported in previous studies. Further study is required to evaluate the vulnerability across root orders and to extend these conclusions to a greater number of species and xylem functional types.

Changes in forest structure over 60 years: tree densities continue to increase in the Pilliga forests, New South Wales, Australia

Studies of long-term vegetation changes are critical for enhancing our understanding of successional dynamics in natural ecosystems. By comparing forest inventory data from the 1940s against field data from 2005, we document changes in stand structure over 60 years in forests co-dominated by Callitris glaucophylla J.Thompson & L.Johnson, Allocasuarina luehmannii (R.Baker) L.Johnson and Eucalyptus crebra F.Muell., in central Pilliga, New South Wales (NSW), Australia. Sampling was stratified across two forest types and across a 1951 wildfire boundary, to assess the effects of initial stand structure and early disturbance on stand dynamics. Stems in the size range tallied in the 1940s (>8.9 cm DBH for Callitris and >11.4 cm for Allocasuarina and Eucalyptus) of each genus increased about three-fold in density and about four-fold in basal area over 60 years, with similar trends in both forest types and fire zones. On average, there were 3638 stems ha–1 in 2010, of which 86% were small Allocasuarina and Callitris (<11.4-cm and <8.9-cm diameter at breast height, DBH, respectively). These results illustrate a continuation of forest encroachment that was initially documented in the late 1800s. However, increases in Allocasuarina have received little attention compared with Callitris recruitment. In the absence of disturbance, ongoing increases in stand stocking may be expected.

Essential oil variation in Eucalyptus crebra, E. melanophloia (Myrtaceae) and their hybrids

The leaf essential oils obtained from Eucalyptus melanophloia F.Muell., E. crebra F.Muell. and their hybrids from four widely separated sites were investigated. The major components that were observed, of the predominantly monoterpenoid oils, were α-pinene (1–58%), 1,8-cineole (0.2–60%) and p-cymene (0.2–21%). Oils tended to be unique to each locality investigated and although the parental species were discrete on their profiles, the oils of the hybrids appeared not to differ from those of their putative parents when commonly used methods of analysis (e.g. principal components analysis, PCA) were applied. Leaf oils are a complex mixture of components and elucidating differences among large datasets such as these can be difficult by traditional methods of analysis. Linear discriminant analysis (LDA) was applied to the oils data and it was found that with a forward stepwise LDA with 13 or 16 components of 70, it was possible to distinguish between the parents and their hybrids at a particular site or where the parents and hybrids originated.

Seasonal responses of xylem sap velocity to VPD and solar radiation during drought in a stand of native trees in temperate Australia

Xylem sap velocity of two dominant tree species, Eucalyptus crebra F. Muell. and Callitris glaucophylla J. Thompson & L.A.S. Johnson, in a native remnant forest of eastern Australia was measured in winter and summer during a prolonged (> 12 months) and extensive drought. The influence of vapour pressure deficit (VPD) and solar radiation levels on the velocity of sap was determined. Pronounced hysteresis in sap velocity was observed in both species as a function of VPD and solar radiation. However, the rotation of the hysteresis curve was clockwise for the response of sap velocity to VPD but anti-clockwise in the response of sap velocity to radiation levels. A possible reason for this difference is discussed.The degree of hysteresis (area bounded by the curve) was larger for the VPD response than the response to solar radiation and also varied with season. A simple linear model was able to predict sap velocity from knowledge of VPD and solar radiation in winter and summer. The consistent presence of hysteresis in the response to sap velocity to VPD and solar radiation suggests that large temporal and spatial models of vegetation water use may require some provision for the different responses of sap velocity, and hence water use, to VPD and solar radiation, between morning and afternoon and between seasons.

Allometric relationships and community biomass estimates for some dominant eucalypts in Central Queensland woodlands

Allometric equations are presented relating stem circumference to branch, leaf, trunk, bark, total above-ground and lignotuber biomass for Eucalyptus crebra F.Muell. (woodland trees), E. melanophloia Sol. Ex Gaerth. (both woodland and regrowth community trees) and E. populnea F.Muell. (woodland trees). There were no significant differences (P > 0.05) between the slopes of individual lognormal regression lines plotting stem circumference against total above-ground biomass for E. crebra, E. melanophloia and E. populnea. Root-to-shoot ratios and leaf area indices were also determined for the stands contributing to each regression. The regressions were then applied to measured eucalypt stems in the associated plant community to give estimates of each stand’s component (eucalypt tree fraction only) biomass per hectare. These eucalypt regressions were next applied to measured stems of each species on a total of 33 woodland sites in which these eucalypts individually contributed > 75% of total site basal area. Above-ground biomass/basal area relationships averaged 6.74 0.29 t m–2 basal area for 11 E. crebra sites, 5.11 0.28 t m–2 for 12 E. melanophloia sites and 5.81 0.11 t m–2 for 10 E. populnea sites. The mean relationship for all sites was 5.86 0.18 t m–2 basal area. The allometric relationships presented at both individual tree and stand levels, along with calculated biomass : basal area relationships, enable ready estimates to be made of above-ground biomass (carbon stocks) in woodlands dominated by these eucalypts in Queensland, assuming individual stem circumferences or community basal areas are known. However, to document changes in carbon stocks (e.g. for Greenhouse Gas Inventory or Carbon Offset trading purposes), more attention needs to be placed on monitoring fluxes in the independent variables (predictors) of these allometric equations.