Non-invasive imaging of drought-induced cavitation in plants

The increased frequency of extreme events, associated with climate change, can lead to loss of biodiversity and forest dieback. Intense drought has disastrous effects on plants growth and physiology and maintaining hydraulic conductivity during time of water stress is necessary for plants survival. However, during drought, the blockage of the hydraulic pathway by air-seeding results in the loss of conductivity by embolism formation. Therefore, it is essential to be able to accurately measure the conductivity, leading to a better prediction of species vulnerability through vulnerability curves (VCs). The measure of vulnerability thresholds is necessary in order to evaluate the causes of forests dieback. Moreover, understanding the mechanisms behind drought-recovery and embolism repair can contribute to the advance of models used to predict the impact of drought on vegetation dynamics. This PhD was designed to test the accuracy and applicability of new visual techniques and aim to provide alternatives to invasive methods for measure of VCs. While invasive techniques measure conductivity on cut samples, visual techniques allow for measurements on intact samples. However, the question of the accuracy of visual methods are still discussed on the ground that they do not provide a direct measurement of conductivity (PLC) but instead use a proxy through the measurements of loss of vessels (PLV). In conclusion, my PhD research addresses the use of visual techniques as an accurate alternative for invasive methods and aims to provide further knowledge concerning mechanisms that regulates drought-induced embolism and recovery, under controlled conditions as well as in field-based studies. The results of this research suggest that (1) visual techniques may be used with all xylem anatomy, but need precise implementation in order to avoid erroneous results, (2) embolism repair via refilling is not a common for E. saligna and severe drought may be responsible for lagged-mortality also observed in the field and that (3) hydraulic failure driven by drought-induced embolism was one of the factors that contributed to the massive dieback of A. marina in northern Australia. Overall, drought-induced embolism may lead to forest mortality and lagged-mortality in many ecosystems, and the understanding of species dependant embolism recovery responses is necessary to determine species resistance and resilience to extreme events and climate change

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 to -11.93 MPa in stems and -3.13 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% to 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

The role of hydraulic failure in a massive mangrove die-off event

Between late 2015 and early 2016, more than 7,000 ha of mangrove forest died along the coastline of the Gulf of Carpentaria, in northern Australia. This massive die-off was preceded by a strong 2015/2016 El Niño event, resulting in lower precipitation, a drop in sea level and higher than average temperatures in northern Australia. In this study, we investigated the role of hydraulic failure in the mortality and recovery of the dominant species, Avicennia marina, 2 years after the mortality event. We measured predawn water potential (Ψpd) and percent loss of stem hydraulic conductivity (PLC) in surviving individuals across a gradient of impact. We also assessed the vulnerability to drought-induced embolism (Ψ50) for the species. Areas with severe canopy dieback had higher native PLC (39%) than minimally impacted areas (6%), suggesting that hydraulic recovery was ongoing. The high resistance of A. marina to water-stress-induced embolism (Ψ50 = −9.6 MPa), indicates that severe water stress (Ψpd < −10 MPa) would have been required to cause mortality in this species. Our data indicate that the natural gradient of water-stress enhanced the impact of El Niño, leading to hydraulic failure and mortality in A. marina growing on severely impacted (SI) zones. It is likely that lowered sea levels and less frequent inundation by seawater, combined with lower inputs of fresh water, high evaporative demand and high temperatures, led to the development of hyper-salinity and extreme water stress during the 2015/16 summer

Visual and hydraulic techniques produce similar estimates of cavitation resistance in woody species

Hydraulic failure of the plant vascular system is a principal cause of forest die-off under drought. Accurate quantification of this process is essential to our understanding of the physiological mechanisms underpinning plant mortality. Imaging techniques increasingly are applied to estimate xylem cavitation resistance. These techniques allow forin situmeasurement of embolism formation in real time, although the benefits and trade-offs associated with different techniques have not been evaluated in detail. Here we compare two imaging methods, microcomputed tomography (microCT) and optical vulnerability (OV), to standard hydraulic methods for measurement of cavitation resistance in seven woody species representing a diversity of major phylogenetic and xylem anatomical groups. Across the seven species, there was strong agreement between cavitation resistance values (P-50) estimated from visualization techniques (microCT and OV) and between visual techniques and hydraulic techniques. The results indicate that visual techniques provide accurate estimates of cavitation resistance and the degree to which xylem hydraulic function is impacted by embolism. Results are discussed in the context of trade-offs associated with each technique and possible causes of discrepancy between estimates of cavitation resistance provided by visual and hydraulic techniques

Canopy dieback and recovery in Australian native forests following extreme drought

In 2019, south-eastern Australia experienced its driest and hottest year on record, resulting in massive canopy dieback events in eucalypt dominated forests. A subsequent period of high precipitation in 2020 provided a rare opportunity to quantify the impacts of extreme drought and consequent recovery. We quantified canopy health and hydraulic impairment (native percent loss of hydraulic conductivity, PLC) of 18 native tree species growing at 15 sites that were heavily impacted by the drought both during and 8-10 months after the drought. Most species exhibited high PLC during drought (PLC:65.1 +/- 3.3%), with no clear patterns across sites or species. Heavily impaired trees (PLC > 70%) showed extensive canopy browning. In the post-drought period, most surviving trees exhibited hydraulic recovery (PLC:26.1 +/- 5.1%), although PLC remained high in some trees (50-70%). Regained hydraulic function (PLC < 50%) corresponded to decreased canopy browning indicating improved tree health. Similar drought (37.1 +/- 4.2%) and post-drought (35.1 +/- 4.4%) percentages of basal area with dead canopy suggested that trees with severely compromised canopies immediately after drought were not able to recover. This dataset provides insights into the impacts of severe natural drought on the health of mature trees, where hydraulic failure is a major contributor in canopy dieback and tree mortality during extreme drought events

Nutrients from salmon parents alter selection pressures on their offspring

Organisms can modify their surrounding environment, but whether these changes are large enough to feed back and alter their evolutionary trajectories is not well understood, particularly in wild populations. Here we show that nutrient pulses from decomposing Atlantic salmon (Salmo salar) parents alter selection pressures on their offspring with important consequences for their phenotypic and genetic diversity. We found a strong survival advantage to larger eggs and faster juvenile metabolic rates in streams lacking carcasses but not in streams containing this parental nutrient input. Differences in selection intensities led to significant phenotypic divergence in these two traits among stream types. Stronger selection in streams with low parental nutrient input also decreased the number of surviving families compared to streams with high parental nutrient levels. Observed effects of parent-derived nutrients on selection pressures provide experimental evidence for key components of eco-evolutionary feedbacks in wild populations

Hydraulic failure and tree size linked with canopy die-back in eucalypt forest during extreme drought

Globally, drought is one of the most important factors affecting the composition, structure and function of forests (Reichstein et al., 2013). In 2019, Australia experienced its driest and hottest year on record, with many parts of the country undergoing their third consecutive year of drought (Bureau of Meteorology, 2020). These unprecedented conditions resulted in the occurrence of massive canopy die-back events, i.e. foliar death (Atlas of Living Australia, 2020), and record-breaking wildfires across forests and woodlands in eastern Australia (Boer et al., 2020; Nolan et al., 2020a). Extreme drought events such as these are becoming more frequent as increasing temperatures and evaporative demand lead to longer and more intense droughts under climate change (Trenberth et al., 2014; Dai & Zhao, 2017; Miralles et al., 2019). The increased incidence of drought has raised concerns that climate change may lead to broad scale tree mortality in many regions, as the pace of climate change outstrips the capacity of trees to acclimate or adapt to greater drought stress (Brodribb et al., 2020)