Figure 3.

(A) Forest reduction between 1954 and 1963, estimated from GIS analyses. Forest reduction was greatest at lower elevations. (B) Field observations of the persistence (⋅) and mortality (⧫) of ponderosa pines, determined from remains of dead trees as a function of elevation and topographic position. Topographic position indices ranged from small values for exposed ridgetops to large values for sheltered valley bottoms (22, 26). Across most topographic positions, mortality was greatest at lower elevations. (C) Mean changes in stem diameter during calendar year 1996 for ponderosa pine along an elevation/moisture gradient (10 trees were measured at each of three sites: 2,010 m elevation (bold line) with 41 cm/year precipitation; 2,320 m (dashed line) with 51 cm/year; 2,780 m (thin line) with 89 cm/year). Annual stem diameter increment (tree growth) was greater at the mesic, high-elevation site than at the xeric, low-elevation site. In addition, at the end of a dry winter/spring period in 1995–1996, stem diameter actually decreased because of water stress at the low-elevation site between April and mid-June, normally the time of most rapid growth.