Tree growth near treeline: Abrupt or gradual reduction with altitude?

Natural climatic treelines are relatively discrete boundaries in the landsc ape established at a certain elevation within an otherwise continuous gradi ent of environmental change. By studying tree rings along elevational trans ects at and below the upper treeline in the European Alps, we (1) determine whether radial stem growth declines abruptly or gradually, and (2) test cl imatic influences on trees near treeline by investigating transects for cli matically different historical periods. While tree height decreases gradual ly toward the treeline, there is no such general trend for radial tree grow th. We found rather abrupt changes which imply threshold effects of tempera ture which moved upslope in a wave-like manner as temperatures increased ov er the past 150 yr. Currently radial tree growth at treeline in the Alps is the same magnitude as at several hundred meters below current treeline. Ov er short intervals, tree-ring width is more dependent on interannual climat ic variability than on altitudinal distance to treeline, We conclude that ( 1) the elevational response of toe-rings includes a threshold component (a minimal seasonal temperature) and that (2) radial growth is more strongly c orrelated with year to year variation in climate than with treeline elevati on as such. Our data indicate that the current treeline position reflects i nfluences of past climates and nor the current climate.