A river basin spatial model to quantitively advance understanding of riverine tree response dynamics to water availability and hydrological management.

Ecological condition continues to decline in arid and semi-arid river basins globally due to hydrological over-abstraction combined with changing climatic conditions. Whilst provision of water for the environment has been a primary approach to alleviate ecological decline, how to accurately monitor changes in riverine trees at fine spatial and temporal scales, remains a substantial challenge. This is further complicated by constantly changing water availability across expansive river basins with varying climatic zones. Within, we combine rare, fine-scale, high frequency temporal in-situ field collected data with machine learning and remote sensing, to provide a robust model that enables broadscale monitoring of physiological tree water stress response to environmental changes via actual evapotranspiration (ET). Physiological variation of Eucalyptus camaldulensis (River Red Gum) and E. largiflorens (Black Box) trees across 10 study locations in the southern Murray-Darling Basin, Australia, was captured instantaneously using sap flow sensors, substantially reducing tree response lags encountered by monitoring visual canopy changes. Actual ET measurement of both species was used to bias correct a national spatial ET product where a Random Forest model was trained using continuous timeseries of in-situ data of up to four years. Precise monthly AMLETT (Australia-wide Machine Learning ET for Trees) ET outputs in 30 m pixel resolution from 2012 to 2021, were derived by incorporating additional remote sensing layers such as soil moisture, land surface temperature, radiation and EVI and NDVI in the Random Forest model. Landsat and Sentinal-2 correlation results between in-situ ET and AMLETT ET returned R2 of 0.94 (RMSE 6.63 mm period-1) and 0.92 (RMSE 6.89 mm period-1), respectively. In comparison, correlation between in-situ ET and a national ET product returned R2 of 0.44 (RMSE 34.08 mm period-1) highlighting the need for bias correction to generate accurate absolute ET values. The AMLETT method presented here, enhances environmental management in river basins worldwide. Such robust broadscale monitoring can inform water accounting and importantly, assist decisions on where to prioritize water for the environment to restore and protect key ecological assets and preserve floodplain and riparian ecological function.

Invasive species in the Anthropocene: Help or hindrance?

Under predicted climate change scenarios many parts of the world will be hotter. Higher temperature extremes present significant physiological challenges to ectothermic freshwater species that cannot regulate body temperature. Willows (Salix spp.) are highly invasive deciduous northern hemisphere shrubs and trees that have colonised riparian zones of southern hemisphere streams. Non-native willows are criticised for their high consumption of water and their capacity to form dense monostands along the margins and within waterways that limit light to streams in summer, alter the timing and quality of allochthonous inputs and modify ecosystem function. As such, governments invest heavily in the removal of willows from streams in order to preserve ecosystem integrity. Although detrimental effects of non-native willows are well documented, little attention has been focussed on consideration of potential ecosystem services that non-native willow infestation may provide under predicted climate warming. Here, we use a case study to illustrate that shading by non-native willows can provide thermal refugia for temperature sensitive endemic taxa and we provide a holistic approach to non-native willow removal that may provide benefits to aquatic species amid changing climate. We present a simple decision matrix for prioritising willow removal activities that may be applied to other invasive species and we discuss traditional views of invasive species management and river restoration and their relevance in a rapidly warming world. The concepts we discuss are of immediate relevance to environmental managers challenged with maintaining and restoring ecosystems that are rapidly changing in structure and function in response to climate warming.