Effects of insect herbivory on the performance of Larix sibirica in a forest-steppe ecotone

The potential of insects to cause temporary spatial shifts of the forest-steppe borderline was investigated in a case study in the northern Mongolian mountain taiga, where Larix sibirica forests border on montane meadow steppe. Insect herbivores of L. sibirica in northern Mongolia include gypsy moth (Lymantria dispar) and grasshoppers, which defoliate trees. Grasshoppers have (like mice) an additional detrimental effect by decorticating stems of tree seedlings. The hypothesis was tested that insect herbivores cause spatial shifts of the forest-steppe borderline by, first, increasing the mortality of mature trees and, secondly, inhibiting rejuvenation.The first hypothesis was tested by investigating a L. sibirica-meadow steppe ecotone, which was heavily defoliated by gypsy moth in early summer 2005. Defoliation was more severe at the forest edge than in the forest interior. Though only 10% of the larch needles at the forest edge endured the gypsy moth invasion without feeding damage, trees were not sustainably affected, as trees were fully foliated in the subsequent year. This suggests that single gypsy moth invasions, which are frequent in Mongolia's forest-steppe ecotone, do not necessarily result in permanent damage of L. sibirica and, with it, not necessarily lead to local shifts of the treeline, though entire forest edges are often completely defoliated.The second hypothesis was tested by planting 2-year-old seedlings of L. sibirica along the treeline towards the meadow steppe and in the interior of the adjacent light taiga forest. Seedling mortality within 3 months was significantly higher at the forest edge (87%) than in the forest interior (40%). Seedlings at the forest edge died either due to insect and small mammal herbivory (65%) or due to drought (25%). Herbivore damage in the seedlings included defoliation by gypsy moth and grasshoppers as well as decortication by grasshoppers and mice. The high feeding pressure for seedlings at the forest edge suggests that insects and mice inhibit or at least retard forest regeneration at the treeline and can thereby lead to temporary spatial shifts of the treeline towards the steppe, after trees have died, e.g., due to fire or logging.