Climate-driven conifer mortality in Siberia

Aim An 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. Location Central Siberia. Time period 1950-2018. Major taxa studied Pinus sibirica Du Tour and Abies sibirica Ledeb. Methods We used geospatial analysis of satellite-derived (MODIS, Landsat) data, topography (elevation, slope steepness and exposure) and climatic variables [precipitation, thermal degree days (TDD = n-ary sumation (t > 0 degrees 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). Results Siberian 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 conclusion Siberian 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.

Automated summary

The mortality rates of Siberian pine and fir have significantly increased in recent decades due to elevated air temperatures, acute droughts, and insect attacks. Mortality mainly occurs at low elevations, while areas with sufficient moisture see an increase in tree growth increment and forest area.