Department of Geography, The University of Iowa, Iowa City, IA 52242. Age data from each of the three study sites are shown in Fig. 5a , 5b , and 5c .
Trees can invade further upslope only by positive feedback.
With continued warming, this same threshold is reached for higher elevations. Finally, warming is sufficient for the establishment and survival of scattered, individual trees. References Holtmeier, F.-K. 1982. Noble, I.R. 1993. A model of the responses of ecotones to climate change.