Idempotent Generative Network

We propose a new approach for generative modeling based on training a neural network to be idempotent. An idempotent operator is one that can be applied sequentially without changing the result beyond the initial application, namely $f(f(z))=f(z)$. The proposed model $f$ is trained to map a source distribution (e.g, Gaussian noise) to a target distribution (e.g. realistic images) using the following objectives: (1) Instances from the target distribution should map to themselves, namely $f(x)=x$. We define the target manifold as the set of all instances that $f$ maps to themselves. (2) Instances that form the source distribution should map onto the defined target manifold. This is achieved by optimizing the idempotence term, $f(f(z))=f(z)$ which encourages the range of $f(z)$ to be on the target manifold. Under ideal assumptions such a process provably converges to the target distribution. This strategy results in a model capable of generating an output in one step, maintaining a consistent latent space, while also allowing sequential applications for refinement. Additionally, we find that by processing inputs from both target and source distributions, the model adeptly projects corrupted or modified data back to the target manifold. This work is a first step towards a ``global projector'' that enables projecting any input into a target data distribution

Performance of GEDI space-borne LiDAR for quantifying structural variation in the temperate forests of South-Eastern Australia

Monitoring forest structural properties is critical for a range of applications because structure is key to understanding and quantifying forest biophysical functioning, including stand dynamics, evapotranspiration, habitat, and recovery from disturbances. Monitoring of forest structural properties at desirable frequencies and cost globally is enabled by space-borne LiDAR missions such as the global ecosystem dynamics investigation (GEDI) mission. This study assessed the accuracy of GEDI estimates for canopy height, total plant area index (PAI), and vertical profile of plant area volume density (PAVD) and elevation over a gradient of canopy height and terrain slope, compared to estimates derived from airborne laser scanning (ALS) across two forest age-classes in the Central Highlands region of south-eastern Australia. ALS was used as a reference dataset for validation of GEDI (Version 2) dataset. Canopy height and total PAI analyses were carried out at the landscape level to understand the influence of beam-type, height of the canopy, and terrain slope. An assessment of GEDI’s terrain elevation accuracy was also carried out at the landscape level. The PAVD profile evaluation was carried out using footprints grouped into two forest age-classes, based on the areas of mountain ash (Eucalyptus regnans) forest burnt in the Central Highlands during the 1939 and 2009 wildfires. The results indicate that although GEDI is found to significantly under-estimate the total PAI and slightly over-estimate the canopy height, the GEDI estimates of canopy height and the vertical PAVD profile (above 25 m) show a good level of accuracy. Both beam-types had comparable accuracies, with increasing slope having a slightly detrimental effect on accuracy. The elevation accuracy of GEDI found the RMSE to be 10.58 m and bias to be 1.28 m, with an R2 of 1.00. The results showed GEDI is suitable for canopy densities and height in complex forests of south-eastern Australia

Adaptation to Delayed Force Perturbations in Reaching Movements

Adaptation to deterministic force perturbations during reaching movements was extensively studied in the last few decades. Here, we use this methodology to explore the ability of the brain to adapt to a delayed velocity-dependent force field. Two groups of subjects preformed a standard reaching experiment under a velocity dependent force field. The force was either immediately proportional to the current velocity (Control) or lagged it by 50 ms (Test). The results demonstrate clear adaptation to the delayed force perturbations. Deviations from a straight line during catch trials were shifted in time compared to post-adaptation to a non-delayed velocity dependent field (Control), indicating expectation to the delayed force field. Adaptation to force fields is considered to be a process in which the motor system predicts the forces to be expected based on the state that a limb will assume in response to motor commands. This study demonstrates for the first time that the temporal window of this prediction needs not to be fixed. This is relevant to the ability of the adaptive mechanisms to compensate for variability in the transmission of information across the sensory-motor system

The role of fire in the coevolution of vegetation, soil and landscapes in south eastern Australia

© 2018 Dr. Assaf InbarFire is an important process in the earth system, with biological, ecological, hydrological and geomorphological consequences varying from negligible to severe. The short-term effect of fire on earth system processes had been studied in detail, however, its role in the coevolution of soil and vegetation within the critical zone has never been addressed. In South Eastern (SE) Australia, local studies have shown that post fire runoff and erosion rates increase with aridity (the ratio between potential evapotranspiration and precipitation). The systematic variation in forest type, fire frequency and post fire response make SE Australian uplands an excellent natural laboratory to study the role of fire in coevolution of the critical zone. The aim of this study was to explore the role of fire in coevolution and to identify the key mechanisms, processes and feedbacks involved. Observations in which drier forests burn more frequently and yield more post-fire runoff and erosion, were used to hypothesize that in SE Australian uplands, fire has a critical role in the coevolution of the critical zone, and that its contribution increases systematically with aridity. Three different methods were used to address the presented aim. The first method focused on the long-term fingerprints of coevolution, soil depth and landform. By considering the observed climate-related differences in forest type, fire frequency and erosion rates, I hypothesised that soil depth and hillslope gradient are north-south asymmetric, and that the magnitude of that asymmetry varies systematically with climate. I addressed these hypotheses by analysing data from soil depth measurements and topographic analysis of digital elevation models. Results showed that soil depth decreased non-linearly with aridity, and that south facing hillslopes were on average steeper and their soils deeper than those facing north. Indices of asymmetry in soil depth (SAI) and hillslope gradient (HAI) expressed a humped-type relationship with aridity, with a peak close to the water-energy limit boundary, pointing to the key role of climate and possibly fire in controlling differential hillslope-scale coevolution across pedomorphic and geomorphic timescales. In the second method, I used a new numerical model in order to: (i) test the hypothesis that fire related processes and feedbacks are critical to explain observed patterns and magnitude of differences in system states across the landscape, and that their effect increases with aridity; and (if the hypothesis was supported), (ii) evaluate the role of fire related mechanisms in the coevolution process. The model was formulated and parameterised to express processes typical to SE Australian systems, and was evaluated with literature and empirical data. Simulations with stochastic fire controlled by soil moisture deficit replicated the observed pattern and magnitude of difference in system states. The net effect of fire on soil depth increased non-linearly with aridity when results from these simulations were compared to those without fire (i.e., coevolution only controlled by climate). Analysis of simulations designed to isolate the key processes affected by fire indicated that model outputs are sensitive to fire frequency and the effect of individual fires on infiltration capacity (Ic), and less so to the effect of fire on forest cover. Using model simulations, a fire-related eco-hydro-geomorphic feedback was identified in which a long-term increase in post-fire erosion might contribute to more frequent fires and more erosion. The aim of the third approach was to evaluate and quantify, using intensive field measurements, the way in which contemporary vegetation and soil depth affect the partitioning of rainfall and solar radiation, and to estimate the implications of this on processes in the coevolution of the critical zone. Sub-canopy microclimate (and open reference sites) was measured at sites across an aridity gradient, and the effect of partitioning of rainfall and solar radiation on coevolution was addressed by analysing soil moisture and temperature data, which are central to several processes in coevolution: productivity, flammability and weathering. Results showed that throughfall decrease and net shortwave radiation under the canopy increase with aridity due to the lower rainfall and higher canopy openness (respectively). On wet (dry) sites, the closed (open) canopy and the deep (shallow) soils partition water and energy in a way that resulted in wetter (drier) soils throughout the year, pointing to lower (higher) flammability and higher (lower) productivity. Mean annual soil water stores decreased non-linearly with aridity, and were more than 5 times higher on wet sites, despite annual rainfall only differing by a factor of ~2. Soil weathering is affected by soil moisture, and the results indicates that the differences between the system states may be amplified by weathering rate differences. The results points to a coevolutionary feedback between weathering, productivity, erosion and fire, which is controlled by the partitioning of water and energy across the vegetation and soil. Overall, results show that fire can play a significant role in the coevolution of soil, vegetation and landscapes in SE Australia. This work is the first to show the importance of fire related eco-hydro-geomorphic feedbacks in coevolution, controlled by soil moisture. Fire was found to operate within feedback loops between its effects on system properties and consequential changes in fire frequency. Two feedback loops were identified: between fire frequency and erosion, and between soil development and fire frequency. By its effect on infiltration capacity and the corresponding reduction in soil depth in direr forests, fire was found to exaggerate the effect of climate on coevolution, and helps to explain the extreme differences in observed system states across an aridity gradient

Effectivess of granular polyacrylamide to reduce soil erosion during consecutive rainstorms in a calcic regosol exposed to different fire conditions

Fire severity varies widely among and within wildfires. The objective of this work was to test the effectiveness of granular polyacrylamide (PAM) to reduce erosion in a Calcic Regosol exposed to different fire conditions. Three treatments were selected representing disturbances that coexist after a wildfire: unburned, low-moderate severity direct fire, and prolonged heating under moderate temperature [heated (HT)]. Granular PAM was spread on the surface of disturbed samples at two rates: (i) 0 (control) and (ii) 50 kg ha−1. Additional application rates of 25 and 100 kg ha−1 were tested in HT. Three 80-mm rainstorms were applied with an intensity of 47 mm h−1, separated by drying periods. PAM reduced soil loss in all storms in unburned and direct fire, although runoff increased during the first storm due to increased runoff viscosity. In the highly stable HT, soil loss was reduced only with an application rate of 100 kg ha−1 and after a drying period. In many cases, granular PAM could be effective to reduce post-fire erosion. In soils with high structural stability, a PAM dose should be selected to find a right balance between its stabilising effect on soil structure and its effect increasing runoff during the first storm

The role of fire in the coevolution of soils and temperate forests

Climate drives the coevolution of vegetation and the soil that supports it. Wildfire dramatically affects many key eco‐hydro‐geomorphic processes, but its potential role in coevolution of soil‐forest systems has been largely overlooked. The steep landscapes of southeastern Australia provide an excellent natural laboratory to study the role of fire in the coevolution of soil and forests, as they are characterized by temperate forest types, fire frequencies, and soil depths that vary systematically with aridity. The aims of this study were (i) to test the hypothesis that in Southeastern Australia, fire‐related processes are critical to explain the variations in coevolved soil‐forest system states across an aridity gradient and (ii) to identify the key processes and (iii) feedbacks involved. To achieve these aims, we developed a numerical model that simulates the coevolution of soil‐forest systems which employ eco‐hydro‐geomorphic processes that are typical of the flammable forests of southeastern Australia. A stepwise model evaluation, using measurements and published data, confirms the robustness of the model to simulate eco‐hydro‐geomorphic processes across the aridity gradient. Simulations that included fire replicated patterns of observed soil depth and forest cover across an aridity gradient, supporting our hypothesis. The contribution of fire to coevolution increased in magnitude with aridity, mainly due to the higher fire frequency and lower post‐fire infiltration capacity, increasing the rates of fire‐related surface runoff and erosion. Our results show that critical feedbacks between soil depth, vegetation, and fire frequency dictate the trajectory and pace of the coevolution of flammable temperate forests and soils

Critical climate thresholds for fire in wet, temperate forests

Climate models predict more frequent droughts and more severe fire weather. Wildfires in wet forests, while historically uncommon, can have catastrophic impacts on forest values and services. Therefore, it is important to ask whether climate change is increasing the frequency of fires in wet forests. Long-term fire histories and weather records were compared in wet Eucalyptus forests supplying water to Melbourne, Australia, to identify the combination of dryness and fire weather under which stand-replacing fires occur. The effect of climate change on stand replacement frequency was predicted using down-scaled regional climate change projections for RCP4.5 and RCP8.5. An index of surface soil dryness (SSD), which is a proxy for live and dead fuel moisture, and daily maximum vapor pressure deficit (Dmax), which is a proxy for daily fire weather, were used to rank each day of each fire season from 1900/01 to 2019/20, within 898 km2 of water supply catchments. The two indices were compared for days with and without stand-replacing fire activity within the study area. Threshold values for the combination of both indices were identified. Damaging fires occurred on about 10 % of days falling above this threshold. The two worst fires, between them burning 86 % of the wet forest in the study area, occurred on days with moderate rather than severe SSD (within the top 2 % of days but only about half of the maximum SSD) but with the most extreme Dmax (highest and second highest ranked out of 43,920 days). The frequency of long fire seasons (SSD > 65 mm for 30 or more days) increased from 1 in ∼30 years during the 20th century to 1 in 4 years in the past 15 years, due to a doubling in the number of warm dry days (Dmax > 2.0 kPa) per fire season. The frequency of extreme fire weather days (Dmax > 5.5 kPa) was also higher in the past 15 years than for most of the previous century. These observations suggest that fire frequency and severity are likely to increase in these wet forests, potentially threatening a suite of ecosystem services. Based on regional climate change projections, increases in the frequency of stand replacing fire will be driven more by increases in maximum temperature than by reductions in rainfall. Under RCP4.5, stand replacing fire frequency in the study area could increase from 1 in ∼140 years historically to 1 in ∼40 years by 2050 and 1 in ∼22 years by 2090 (1 in ∼6 years under RCP8.5), which would result in widespread loss of these iconic forests, including Eucalyptus regnans, the world's tallest Angiosperm

Long-term hydrological response emerges from forest self-thinning behaviour and tree sapwood allometry

Fires in forested catchments are of great concern to catchment managers due to their potential effect on water yield. Among other factors such as meteorological conditions and topography, dominant vegetation and its regeneration traits can play a key role in controlling the variability in the type and recovery-time of the hydrological response between forested catchments after stand-replacing fires. In temperate South-Eastern Australia, a long-term reduction in streamflow from catchments dominated by regenerating tall-wet Eucalyptus obligate seeder forests was observed, which has substantial implications for Melbourne's water supply. While the unusual hydrological response has been attributed to the higher water-use of the regrowth forests, the dominant underlying mechanism has not yet been identified. Here we show analytically and with a closed-form solution that this streamflow pattern can emerge from forest dynamics, namely the combination of growth and tree mortality as constrained by the self-thinning line (STL) and sapwood allometry of the dominant overstory tree species under non-limiting rainfall regimes. A sensitivity analysis shows that observed variations in the relative streamflow anomaly trend can be explained by parameters controlling: (i) the shape of the STL; (ii) regeneration success; (iii) radial tree growth rate; and (iv) fire severity. We conclude that the observed variation in long-term post-disturbance streamflow behaviour might have resulted from different trajectories of forest dynamics and suggest that to minimize uncertainty in future water-balance predictions, eco-hydrological models for even aged forests include a mechanistic representation of stand demography processes that are constrained by forest inventory data

Climate dictates magnitude of asymmetry in soil depth and hillslope gradient

Hillslope asymmetry is often attributed to differential eco-hydro-geomorphic processes resulting from aspect-related differences in insolation. At midlatitudes, polar facing hillslopes are steeper, wetter, have denser vegetation, and deeper soils than their equatorial facing counterparts. We propose that at regional scales, the magnitude in insolation-driven hillslope asymmetry is sensitive to variations in climate, and investigate the fire-prone landscapes in southeastern Australia to evaluate this hypothesis. Patterns of asymmetry in soil depth and landform were quantified using soil depth measurements and topographic analysis across a contemporary rainfall gradient. Results show that polar facing hillslopes are steeper, and have greater soil depth, than equatorial facing slopes. Furthermore, we show that the magnitude of this asymmetry varies systematically with aridity index, with a maximum at the transition between water and energy limitation, suggesting a possible long-term role of climate in hillslope development