Mediterranean climate effects. II. Conifer growth phenology across a Sierra Nevada ecotone

Growth and xylem water potential of the lower elevation conifers Pinus jeffreyi and Abies concolor and the higher elevation Pinus monticola and Abies magnificawere monitored in their montane Mediterranean habitat of the southernmost Sierra Nevada mountains of California. Measurements were made across the ecotone between the midmontane and upper montane forests and through light and heavy snowfall years. Radial stem growth, averaging ;1.5 mm/yr, started 2 wk after snow melt, providing that maximum air temperatures had reached 218C, and ended when predawn water potentials fell rapidly at the onset of the summer drought. Leader growth started on or after a fixed date, providing that minimum air temperatures were above 248C for Pinus species or 12.58C for Abies species. The cue for leader growth was inferred to be photoperiodic. Leader growth ended when either a determinate internode length of ; 1m m was reached or predawn water potentials fell rapidly. Abies magnificagrew more rapidly than the low-elevation species, but had a shorter growth period; its annual leader growth, as a consequence, was only 35 mm/yr vs. 50 mm/yr for the low-elevation species. Needle growth was similarly determinate in the absence of early drought. This growth phenology contributes to determining species distribution across the ecotone. The montane Mediterranean climate of the Sierra Nevada range of California is characterized by heavy winter precipitation—rain and snow in the midmontane forest and snow in the upper montane forest—and an extended period of summer drought (Oosting and Billings, 1943; Axelrod, 1976; Major, 1990). The ecotone between the midmontane forest and upper montane forest is marked by a major discontinuity in the distribution of conifer and other species (Mellmann-Brown and Barbour, 1995; Barbour and Minnich, 2000). In the southernmost Sierra Nevada this ecotone occurs at elevations between ;2400 and 2700 m. The overall objective of the work reported in this and in a companion paper in this issue (Royce and Barbour, 2001) was increased understanding of environmental factors and their interactions affecting conifer species distribution across this ecotone. The midmontane (mixed conifer) forest of the southernmost Sierra Nevada is dominated by yellow pine (Pinus ponderosa Laws or Pinus jeffreyi Grev. & Balf.), Sierra white fir (Abies concolor Gordon & Glend. var. lowiana (Gordon) Murray), sugar pine (Pinus lambertiana Douglas), incense cedar (Calocedrus decurrens (Torry) Florin), and black oak (Quercus kelloggii Newb.). The upper montane (red fir) forest is dominated by Shasta red fir (Abies magnificaMurray, var. shastensis Lemmon) and western white pine (Pinus monticola Douglas). Trees found within the ecotone include P. jeffreyi, P. monticola, A. concolor, and A. magnifica(Barbour and