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"Taiga"
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Molecule generation using transformers and policy gradient reinforcement learning
Generating novel valid molecules is often a difficult task, because the vast chemical space relies on the intuition of experienced chemists. In recent years, deep learning models have helped accelerate this process. These advanced models can also help identify suitable molecules for disease treatment. In this paper, we propose Taiga, a transformer-based architecture for the generation of molecules with desired properties. Using a two-stage approach, we first treat the problem as a language modeling task of predicting the next token, using SMILES strings. Then, we use reinforcement learning to optimize molecular properties such as QED. This approach allows our model to learn the underlying rules of chemistry and more easily optimize for molecules with desired properties. Our evaluation of Taiga, which was performed with multiple datasets and tasks, shows that Taiga is comparable to, or even outperforms, state-of-the-art baselines for molecule optimization, with improvements in the QED ranging from 2 to over 20 percent. The improvement was demonstrated both on datasets containing lead molecules and random molecules. We also show that with its two stages, Taiga is capable of generating molecules with higher biological property scores than the same model without reinforcement learning.
Journal Article
The taiga : discover this forested biome
\"Discusses the plants and animals of the taiga biome, including their roles in the food chain\"-- Provided by publisher.
Book
Soil organic layer combustion in boreal black spruce and jack pine stands of the Northwest Territories, Canada
Increased fire frequency, extent and severity are expected to strongly affect the structure and function of boreal forest ecosystems. In this study, we examined 213 plots in boreal forests dominated by black spruce (Picea mariana) or jack pine (Pinus banksiana) of the Northwest Territories, Canada, after an unprecedentedly large area burned in 2014. Large fire size is associated with high fire intensity and severity, which would manifest as areas with deep burning of the soil organic layer (SOL). Our primary objectives were to estimate burn depth in these fires and then to characterise landscapes vulnerable to deep burning throughout this region. Here we quantify burn depth in black spruce stands using the position of adventitious roots within the soil column, and in jack pine stands using measurements of burned and unburned SOL depths. Using these estimates, we then evaluate how burn depth and the proportion of SOL combusted varies among forest type, ecozone, plot-level moisture and stand density. Our results suggest that most of the SOL was combusted in jack pine stands regardless of plot moisture class, but that black spruce forests experience complete combustion of the SOL only in dry and moderately well-drained landscape positions. The models and calibrations we present in this study should allow future research to more accurately estimate burn depth in Canadian boreal forests.
Journal Article
Heavily hunted wolves have higher stress and reproductive steroids than wolves with lower hunting pressure
Summary Human‐caused harassment and mortality (e.g. hunting) affects many aspects of wildlife population dynamics and social structure. Little is known, however, about the social and physiological effects of hunting, which might provide valuable insights into the mechanisms by which wildlife respond to human‐caused mortality. To investigate physiological consequences of hunting, we measured stress and reproductive hormones in hair, which reflect endocrine activity during hair growth. Applying this novel approach, we compared steroid hormone levels in hair of wolves (Canis lupus) living in Canada's tundra–taiga (n = 103) that experience heavy rates of hunting with those in the northern boreal forest (n = 45) where hunting pressure is substantially lower. The hair samples revealed that progesterone was higher in tundra–taiga wolves, possibly reflecting increased reproductive effort and social disruption in response to human‐related mortality. Tundra–taiga wolves also had higher testosterone and cortisol levels, which may reflect social instability. To control for habitat differences, we also measured cortisol in an out‐group of boreal forest wolves (n = 30) that were killed as part of a control programme. Cortisol was higher in the boreal out‐group than in our study population from the northern boreal forest. Overall, our findings support the social and physiological consequences of human‐caused mortality. Long‐term implications of altered physiological responses should be considered in management and conservations strategies. Lay Summary
Journal Article
Taiga
Introduces animals and plants that typically live in taigas, which are forests in very cold areas.
Book
Wildfire Dynamics along a North-Central Siberian Latitudinal Transect Assessed Using Landsat Imagery
The history of wildfires along a latitudinal transect from forest–tundra to middle taiga in North-Central Siberia was reconstructed for the period from 1985 to 2020 using Landsat imagery. The transect passed through four key regions (75 × 75 km2) with different climate and landscape conditions that allowed us to evaluate regional wildfire dynamics as well as estimate differences in post-fire forest recovery. The Level-2A Landsat data (TM, ETM+, and OLI) were used to derive: (i) burned area (BA) locations, (ii) timing of wildfire occurrence (date, month, or season), (iii) fire severity, and (iv) trends in post-fire vegetation recovery. We used pre-selected and pre-processed scenes suitable for BA mapping taken within four consecutive time intervals covering the entire period of data analysis (1985–2020). Pre- and post-fire dynamics of forest vegetation were described using spectral indices, i.e., NBR and NDVI. We found that during the last three decades, the maximum BA occurred in the southernmost Vanavara region where ≈58% of the area burned. Total BA gradually decreased to the northwest with a minimum in the Igarka region (≈1%). Nearly half of these BAs appeared between summer 2013 and autumn 2020 due to higher frequency of hot and dry weather. The most severe wildfires were detected in the most northeastern Tura region. Analysis of NDVI and NBR dynamics showed that the mean period of post-fire vegetation recovery ranged between 20 and 25 years. The time of vegetation recovery at BAs with repeat wildfires and high severity was significantly longer.
Journal Article
Sufficient conditions for rapid range expansion of a boreal conifer
Unprecedented modern rates of warming are expected to advance boreal forest into Arctic tundra
1
, thereby reducing albedo
2
–
4
, altering carbon cycling
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and further changing climate
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–
4
, yet the patterns and processes of this biome shift remain unclear
5
. Climate warming, required for previous boreal advances
6
–
17
, is not sufficient by itself for modern range expansion of conifers forming forest–tundra ecotones
5
,
12
–
15
,
17
–
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. No high-latitude population of conifers, the dominant North American Arctic treeline taxon, has previously been documented
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advancing at rates following the last glacial maximum (LGM)
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–
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. Here we describe a population of white spruce (
Picea glauca
) advancing at post-LGM rates
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across an Arctic basin distant from established treelines and provide evidence of mechanisms sustaining the advance. The population doubles each decade, with exponential radial growth in the main stems of individual trees correlating positively with July air temperature. Lateral branches in adults and terminal leaders in large juveniles grow almost twice as fast as those at established treelines. We conclude that surpassing temperature thresholds
1
,
6
–
17
, together with winter winds facilitating long-distance dispersal, deeper snowpack and increased soil nutrient availability promoting recruitment and growth, provides sufficient conditions for boreal forest advance. These observations enable forecast modelling with important insights into the environmental conditions converting tundra into forest.
A boreal conifer is advancing northwards into Arctic tundra, with this treeline advance facilitated by climate warming together with winter winds, deeper snow and increased soil nutrient availability.
Journal Article