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(1D) SYMPOSIUM: Field-based manipulative experiments for understanding environmental change (part 1)

Tracks
Track 4
Monday, November 25, 2019
11:00 - 13:00
Chancellor 5

Speaker

A/Prof. Mark Hovenden
Associate Professor
University of Tasmania

Warming an extremely carbon-dense, high altitude ecosystem stimulates a massive increase in ecosystem carbon emissions

11:00 - 11:15

ESA abstract

Evidence of warming-induced increases in CO₂ efflux from soils has led to suggestions that the response of ecosystem respiration will trigger a positive land C–climate feedback cycle, ultimately warming the earth further. Currently, there is little consensus about the mechanisms driving the warming-induced respiration response, and importantly the literature is underrepresented for regions that are large C sinks. Here, we present results from a five-year warming experiment in the C-rich soils of a high-altitude, native Tasmanian grassy sedgeland, and analyse whether alterations of plant community composition or differences in microbial respiratory potential could contribute to any changes in ecosystem respiration. In situ, 3 degrees of warming increased ecosystem respiration by an average of 28% and this effect was consistent over time and across plant community composition treatments. In contrast, warming had no impact on microbial respiratory potential in incubation experiments. Therefore, the ecosystem respiration response to warming was not due to either microbial function or plant community effects, and is more likely due to increases in belowground autotrophic respiration, and the supply of labile substrate through root exudates. CO₂ efflux from this ecosystem with C-dense soils increased by more than a quarter in response to modest warming, suggesting C inputs need to increase by at least this amount if soil C stocks are to be maintained.

Professor Glenda Wardle
Professor of Ecology and Evolution
The University of Sydney

Too much or too little: plant responses to nutrient and drought manipulations in arid grasslands.

11:15 - 11:30

ESA abstract

Grasslands feed the world. However, increasing pressure from extended droughts, warmer temperatures, over-fertilizing and altered fire regimes are degrading ecosystem function in the rangeland systems that support human food production and biodiversity. A key gap in knowledge is how the native plant species in the extensive hummock grasslands of Australia will tolerate or decline under these multiplicative stressors.

To address this gap we conducted a series of long-term manipulative field experiments using standardized protocols to ensure that we can scale our findings up regionally, and globally. Nutrient Network measures the response of plant biodiversity and productivity in grasslands to additional nutrients (NPK + micronutrients) and exclusion of herbivory via fenced plots. We report here on changes in plant biomass and composition from 2013-2019, including an addition to the design by comparing responses in burnt and unburnt sites. Similarly we report on our Drought-Net experiments, to compare biodiversity responses from 2016-2019 to extreme drought in burnt vs unburnt vegetation. Treatments were imposed by fixed shelters, with 85% reduction in rainfall based on the 1% extreme of the 100-year record. We find that plant response is highly dependent on growing season rainfall, recovery of plant cover was delayed in burnt sites, and highest in the NPK+fence treatment. Higher nutrient levels tended to favour plants that increase after overgrazing.

These findings provide mechanistic explanations for the extent of plant resilience to environmental change and to inform near-term forecasts of ecosystem responses to guide management responses in both the agricultural and environmental sectors.

Professor Sally Power
Professor
Western Sydney University

Pastures and climate extremes (PACE): Understanding pasture responses to elevated temperature and drought

11:30 - 11:45

ESA abstract

Climate change is driving ever more extreme temperatures, alongside dramatic shifts in the size, frequency and seasonality of rainfall events. Sustained periods of drought and warming are likely to result in major shifts in the performance, species suitability and sustainability of managed grasslands across Australia, with important economic implications for the pasture-dependent industries. Our Pastures and Climate Extremes (PACE) project exposes key pasture grass and legume species to a winter+spring drought (60% reduction in rainfall) and a 3oC increase in temperature, under field conditions, to investigate species differences in resistance and resilience under climate extremes.

In the first year of the project we found large species differences in aboveground productivity responses to drought, with spring biomass reductions ranging from 45% (Themeda australis) up to 77% (Lolium perenne) or 79% (Digitaria eriantha). Productivity declines were accompanied by species-specific reductions in tiller density, loss of physiological function and altered water use strategies, although most species recovered rapidly once normal watering resumed. Warming had an almost uniformly negative impact on productivity - even during the cooler months - that appears to be associated with lower soil water contents in warmed plots. Drought and warming were both associated with changes in plant nutrition, including reduced crude protein content, increased forage dry matter content and changes in both digestibility and plant macronutrient concentrations. Species’ carbon allocation, hydraulic and nutritional strategies are being investigated to determine the mechanisms underpinning differences in sensitivity and inform recommendations for sustainable pasture management under future, more extreme climates.

Professor Alfredo Huete
Professor
University of Technology Sydney

Phenocam-observed pasture responses to variable rainfall: experiences from the DRI-GRASS drought experiment

11:45 - 12:00

ESA abstract

Changes in rainfall regime and the occurrence of extreme events such as drought are major threats to Australia’s grazing economy, with high uncertainties associated with future climate impacts on pasture productivity. Here, we used time-lapse digital cameras (phenocams) to quantify the response of mixed grass species to variable rainfall treatments through the DRI-GRASS experiment at the Hawkesbury Institute for the Environment (HIE) in western Sydney. Phenocam images were processed to the green chromatic coordinate (GCC) values to track changes in vegetative phenology. The integral of the GCC was used as an estimate of aboveground net primary productivity. GCC accurately tracked grassland dynamic responses to the rainfall treatments. Increasing quantities of rainfall resulted in significantly higher productivity throughout the year compared to ambient, whereas decreasing rainfall quantity reduced productivity. The intensification treatments increased grassland productivity during cooler months, but was equivalent to ambient rainfall during summer months. Summer drought unexpectedly drove higher GCC during non-drought periods, which was attributed to exotic forb invasions following the drought disturbance. We suggest further research into intensification of rainfall regimes to investigate whether our results are replicable across multiple years, and to determine if observed greater productivity during cooler periods is due to ecological (i.e. species composition) or physiological (i.e. increased biomass) means. Our results from the drought treatment stress the importance of using field observations to validate remote sensing and also highlight the increasing threat of exotic species invasions where increased drought is predicted.

Ms Kirsty Milner
University of Technology Sydney

Time is of the essence: shifting heat stress thresholds can happen quickly and markedly

12:00 - 12:15

ESA abstract

Predictions about plant species persistence under a warming climate scenario often are based on a thermal tolerance threshold measured once for the species in question. Where thresholds are measured more than once, it is generally at different times of year, reflecting acclimation to changing temperatures through the seasons. Little is known about whether thresholds can shift at much shorter time scales, such as days or even hours, but such insight is critical for valid assessments of species vulnerability to environmental change. In addition, measured heat tolerance thresholds rarely account for other environmental factors, such as water stress, which may in turn influence heat stress, particularly in hot, dry regions such as deserts. In a manipulative water access experiment under field conditions, we investigated the effects of ambient high summer temperatures, on a desert-dwelling Australian native shrub, Myoporum montanum, measured in situ in arid South Australia. Using chlorophyll fluorometry, we determined photosynthetic thermal tolerance thresholds, T₅₀, for leaves at pre-dawn and mid-afternoon every three days over a 12-day experimental period. For both morning- and afternoon-measured plants, we found that thermal thresholds were significantly higher for water-stressed than well-watered plants. Further, irrespective of water status, thermal thresholds significantly increased from morning to afternoon, representing a shift of up to 2 ⁰C during a nine-hour period. Our findings have clear implications for forecasting and comparing species vulnerability to temperature extremes, and highlight the importance of environmental and temporal context in framing experimental research on plant thermal tolerance.

Andy Leigh
UTS Sydney

High heat tolerance does not always mean lower thermal vulnerability: why water status matters

12:15 - 12:30

ESA abstract

With increasing frequency and intensity of extreme temperature events, assessments of organisms’ thermal vulnerability are becoming a common application of physiological heat tolerance measurements. For plants, thermal vulnerability assessments of a species compare measured physiological heat tolerance to habitat air temperature to obtain a thermal safety margin. However, a fundamental consideration frequently not accounted for is that leaf temperature rarely equals air temperature. This is partly because a plants access to water can decouple leaf temperature from air temperature, depending on the extent to which leaves transpire and remain cool. The magnitude to which calculating thermal safety margins with air, rather than leaf temperatures, leads to inaccurate conclusions about thermal vulnerability has not yet been considered.

We water-stressed Myoporum montanum desert shrubs in situ in arid South Australia over 12 days during extreme high temperatures in summer and measured leaf and air temperatures, leaf water potential and photosynthetic heat tolerance. Water-stressed plants experienced high leaf temperatures, high heat tolerances and consequently narrow safety margins (high vulnerability). Well-watered plants avoided high leaf temperatures, had lower heat tolerances and wide safety margins (low vulnerability). Using local and regional maximum air temperatures to calculate safety margins, flipped the thermal vulnerability results between water treatments, so that water stressed plants had wider safety margins and lower vulnerability than well-watered plants. Our findings demonstrate the importance of accounting for plant water status and recent leaf temperatures rather than air temperature when interpreting heat tolerance measurements and making assessments of thermal vulnerability under climate change.

Dr Brett Howland
Ecologist
ACT Government

Investigation into the role of fire to manage and restore long-unburnt natural temperate grasslands

12:30 - 12:45

ESA abstract

Natural temperate grasslands (NTG) are one of Australia’s most threatened ecological communities with less than 1% of the original extent remaining. These ecosystems evolved alongside fire and grazing by native herbivores, with both of these disturbance regimes considered critical in maintaining native biodiversity.
The Australian Capital Territory has some of the largest, best quality and most connected patches of NTG remaining in south-eastern Australia, with the majority of NTG in the ACT protected within nature reserves. While fire is considered an important disturbance in the evolution of these grasslands, fire has been largely absent across ACT grasslands for the past 50 years. Given the length of absence, the reinstatement of this disturbance regime may negatively affect these ecosystems, especially through impacts on fire sensitive threatened reptiles and fire responsive weeds.
To further explore the role of fire in grassland restoration and management a large-scale long term project was started in 2015 at seven grassland reserves. The aims of this project were:
1. Implement fire at a scale-relevant to management (>10 ha)
2. Understand how fire affects native plant diversity, weeds, and threatened fauna

After four years of data collection we found that fire; increased native plant richness by 69% on average, increased the area of habitat occupied by the endangered grassland earless dragon, and resulted in decline of exotic annual grasses especially when undertaken in spring. However, we also found fire reduced the abundance of some common native grasses and resulted in short-term declines of the vulnerable striped legless lizard.

Ms Merinda Day-Smith
Honours Student
La Trobe University

Is grass just grass? An Investigation of Variation in Flammability of Savanna Grass Species

12:45 - 12:50

ESA abstract

It is well recognised that grasses have a unique response to fire and influence fire behaviour, and yet they are generally treated as a uniform group or layer. But can we be sure all grasses burn the same way? Are they all homogenous in burning behaviours such as consumption speed, heat release and heat distribution? This presentation will share a recent study which looked at variable inherent flammability of different grass species in fire-prone systems. Building on a 15-year, manipulative experiment at the Wildlife Territory Park, Northern Territory, we have constructed a specialised methodology to investigate the variation of inherent flammability of 12 prominent savanna grass species. It was predicted that species will exhibit different flammability strategies that may be tied to evolution pressures and phylogeny. The results, will provide a greater capacity to more accurately map the role of the grass-layer and how individual species can drive fire behaviour, shape communities and further contribute to an opportunity to understand changes in fire intensity in savanna ecosystems. This may well prove imperative to making informed conservation and land management decisions in savanna systems of Australia. The presentation will incorporate a mixed media/video of the burning experiment in action as well as an informative poster.

Ms Susanna Bryceson
PhD candidate
La Trobe University

How deep is your shade? Investigating effects of light on Australian native grasses.

12:50 - 12:55

ESA abstract

Global understanding of savanna dynamics is largely based on northern hemisphere studies. On the face of it, Australian savannas are similar to African, Asian and South American, with landscapes of trees and C4 grasses responding to cycles of frequent fire.
A closer look shows that Australian trees are taller, narrower and angle their leaves downwards unlike their northern hemisphere trees which cast deep shade courtesy of horizontal leaves and more leaf area overall. Think about how little shelter you get from rain or sun when standing under a eucalypt, compared to a pine tree. It’s no surprise to find that eucalypts have the lowest leaf density of all trees, from flowering evergreens trees, to deciduous trees and conifers.
Savanna grasses are known to thrive in open environments and are precluded from shade. All of Australia’s native savanna grasses evolved under these conditions in the northern hemisphere before arriving in Australia around 3.5 million years ago. We investigated whether the peculiarities of eucalypt canopies and their dappled shade may have played a role in enabling these C4 grasses to invade a land of trees and shrubs.
This presentation reports on the results of an experiment which began by comparing light availability under eucalypts to exotic trees and morphed into a 2-year growth trial involving over 30 native C3 and C4 grass species from all over Australia, exposed to 4 different light treatments.
The results and conclusions have implications for interpreting ecological patterns and system dynamics in Australian woodlands and savannas.

Mr Jinyan Yang
PhD
Hawkesbury Institute for the Environment

Predicting grassland responses to rainfall in Australia: combining traits and processes in an ecophysiological model

12:55 - 13:00

ESA abstract

Grasslands are a key element of the Australian terrestrial biosphere, covering in total 70% of land surface. Understanding and predicting dynamics of grassland productivity thus bears great ecological and economic importance. In Australia, the major driver of grassland productivity is rainfall. It is thus crucial to understand how plant physiological processes, particularly carbon uptake and the timing of leaf greening and browning, are affected by soil moisture availability. These physiological responses differ among species, according to their functional traits. For instance, plant photosynthetic responses to rainfall differ between C3 and C4 species, while the timing of greening and browning depends on species’ rooting depth. Consequently, a quantitative understanding of process responses to water availability, and their dependence on species traits, is necessary if we are to predict the dynamics of plant productivity under variable rainfall.

Here, we present a generalisable ecophysiological modelling framework that incorporates physiological processes and traits to predict the dynamics of grassland productivity across Australian landscapes. We first illustrate how responses of plant physiological processes and traits to rainfall can be quantified with observations and then demonstrate how that information can be used to improve model predictions of grassland productivity. Examples of application will be given, by evaluating the model against data from rainfall manipulation experiments. We encourage data inputs across landscapes for a comprehensive evaluation and application of this framework.


Chair

Susanna Venn
Senior Lecturer
Deakin University

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