Age structure and trends in annual stem increment of Larix sibirica in two neighboring Mongolian forest–steppe regions differing in land use history

Abstract

Selective logging affects climate sensitivity of Larix sibirica in the Mongolian forest–steppe mediated by changes in age and stand structure. Climate warming increasingly limits the productivity of boreal forests via increased drought stress, especially at the southern fringe of the biome. The southernmost boreal forests are exposed to more intensive human disturbance than most forests at more northern latitudes. We asked the question of how forest use through logging and moderate forest grazing interferes with the climate response of the annual radial stem increment. We conducted a case study in Larix sibirica stands of the Mongolian forest–steppe involving two neighboring forest regions (20 km distance) differing in logging and grazing intensity. One site was subjected to heavy logging until 25 years ago and low intensity of livestock grazing; another site was exposed to moderate selective logging and higher, but still moderate livestock numbers. While the differences in grazing had no detectable effect, former heavy logging led to younger and more even-aged forest stands. Forests at both sites showed recent increases in missing-ring frequency, which probably indicated increased drought vulnerability. Climate-response analysis indicated that heavy logging 25 years ago was associated with high sensitivity of stemwood formation to high summer (especially June) temperatures. These findings suggest that: (1) recent logging under the conditions of climate warming has increased the sensitivity of tree growth to temperature in these southern boreal forests; (2) high replication at the stand level is needed to avoid bias in dendrochronological analyses in regions exposed to spatially heterogeneous logging intensities.