This study investigated the synchronous responses of vegetation to extreme temperatures and/or precipitation in the middle to high latitudes of Asia using semi-monthly observations of the leaf area index (LAI) from 1982 to 2016. The extreme states of vegetation and climate were specified using standard anomalies of the annual cycle removed variables. The results show that the area with the maximum/minimum LAI increased or decreased in correspondence with global warming. The LAI reached its maximum mostly in spring and autumn, and its minimum in summer. Generally, extreme cold and/or wet conditions inhibited forest and crop growth in the area south of 60°N, particularly from October to November. In contrast, extremely hot and/or dry conditions promoted forest growth, particularly in the central and northern parts of Siberia from August to September. However, in the arid areas of Central Asia and the Mongolian Highlands which are covered mainly by sparse vegetation and grasses, low temperature extremes and/or strong precipitation promoted vegetation growth, while high temperature extremes and/or low precipitation had adverse effects on vegetation growth. The compound extreme climates of hot-and-dry and cold-and-wet were more frequent than the cold-and-dry and hot-and-wet climates. The overall positive response of vegetation was superior to that of the negative response. The results of this study suggest a continuous increase in vegetation density and coverage over the boreal region in the future if the warming trend persists. The consequent climate feedback at regional and global scales should be given more attention.