Chemical reactivity descriptors are computed by the use of three alternative approaches derived within the framework of density functional theory. These schemes consider the computation of orbital Fukui indices, where all valence orbitals are taken into account; the Fukui indices of each atom in the molecule from the atomic resolved hardness matrix; and the atom in molecule softnesses, expressed in mixed LCAO representation of second quantization as functions of Mayer atomic valences. The hardness matrix is constructed from the Kohn-Sham orbitals by the use of the fractional occupation number concept and Janak's extension of density functional theory. The site reactivity of molecules involved in radical attack reactions of some substituted olefins and isocyanide addition to dipolarophiles is rationalized in terms of the orbital and atomic resolved reactivity indices. The reactivity descriptors of thiophene, furane, and pyrrole are also reported and discussed. In addition, the nucleophilic attack on the allyl coordinated to the electronically asymmetric [Pd(phosphine)-(imine)] fragment was considered.

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Fire history within the northern larch forests of Central Siberia was studied (65+°N). Fires within this area are predominantly caused by lightning strikes rather than human activity. Mean fire return intervals (FRI) were found to be 112 ± 49 years (based on fire scars) and 106 ± 36 years (based on fire scars and tree natality dates). FRI were increased with latitude increase, and observed to be about 80 years at 64°N, about 200 years near the Arctic Circle, and about 300 years nearby the northern range limit of larch stands (~71°+N). Northward FRI increase correlated with incoming solar radiation (r = − 0.95). Post Little Ice Age (LIA) warming (after 1850) caused approximately a doubling of fire events (in comparison with a similar period during LIA). The data obtained support a hypothesis of climate-induced fire frequency increase.

AimAn increase in conifer mortality has been observed widely across the boreal forest biome. We investigate the causes of this mortality, in addition to the geospatial and temporal dynamics of mortality, in Siberian pine and fir stands.LocationCentral Siberia.Time period1950–2018.Major taxa studiedPinus sibirica Du Tour and Abies sibirica Ledeb.MethodsWe used geospatial analysis of satellite‐derived (MODIS, Landsat) data, topography (elevation, slope steepness and exposure) and climatic variables [precipitation, thermal degree days (TDD = ∑(t > 0 °C), standardized precipitation evapotranspiration index (SPEI) and root zone moisture content (RZM)], together with in situ data. Dendrochronology was applied for analysis of the radial growth increment (GI).ResultsSiberian pine and fir mortality has increased greatly in recent decades. The mortality of forest stands and trees was dependent on the TDD, RZM and SPEI. Mortality occurred mainly within the southern part of the species ranges and decreased northward, correlated with latitudinal gradients of TDD and SPEI. Mortality was observed mostly at elevations < 1,000 m and decreased with increasing elevation, whereas the area of forests and GI of trees increased with elevation. Forest mortality was preceded by the changes in tree GI. Since the onset of climate warming, GI increased until a breakpoint in the mid‐1980s. Further temperature increase caused a reduction in GI owing to moisture stress and division of the tree population into “decliners” and “survivors”. Mortality was caused by the combined impact of moisture stress and bark beetle attacks.Main conclusionSiberian pine and fir mortality was preceded by a reduction in the GI of trees caused by elevated air temperatures, acute droughts and subsequent insect attacks. Forest mortality was observed mostly at low elevations, whereas within the areas with sufficient moisture availability (at elevations c. < 1,000 m) the tree GI and forest area increased. With the projected increase in drought, Siberian pine and fir trees are predicted to retreat from their southern low‐elevation ranges.