Tree stems 2:106 yr old (i.e., established before significant European influence in this area) in a 7.3-ha old-growth ponderosa pine (Pinus ponderosa) forest in northern Arizona were aged ~nd mapped. Age ~tructu~e analysis showed that successful establishment of ponderosa pine was mfrequent. The penods Without successful establishment could be quite long, as suggested by four consecutive decades in which only two surviving trees were established.. The st~~s w~re strongly aggregated, as measured with nearest neighbor analysis, and groups were visu~lly dtstmct m the field. Most of the stems occurred in groups of three or more, with group size ra.n~ng from 3 to 44 st.ems and area occupied by a group ranging from 0.02 to 0.29 ha. Ages of stems Wlthm groups were vanable, the most homogeneous group having a range of 33 yr and the least having a range of 268 yr. . The data are not consistent with the commonly held view that southwestern ponderosa pine occurs m e~en-aged groups and ~hat each gr~up became est_ablished following the demise of the group previOusly occupymg the site. Instead, It seems more likely that seedlings became established when ~me or two trees within the group died, the additional fuel surrounding the dead trees causing an mtensely burned spot in the otherwise low-intensity fires that were frequent in the area. The hot spot would create a potenti~l seedbed for pine by eliminating, at least temporarily, the competing grasses on that small area. Thts decreased competition, in conjunction with adequate seed production and favorable moisture conditions in the spring and early summer, may well have been critical for ponderosa pine establishment. The relative infrequency of all these events occurring in the necessary sequence could explain the erratic age structure data from this area.

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The contribution of changes in meristem behavior to age-related decline in forest productivity is poorly understood. We studied age-related trends in needle morphology and gas exchange in a population of red spruce (Picea rubens Sarg.) growing in a multi-cohort stand where trees ranged from first-year germinants to trees over 150 years old, as well as in grafted scions from these trees. In the field study, age-related trends in foliar morphology were determined in six cohorts ranging in age from 2 to 120 years, and differences in gas exchange characteristics were compared between 60- and 120-year age classes. In a common-rootstock study, scions from trees representing 20-, 60-, and 120-year cohorts were grafted onto juvenile rootstock and maintained for three growing seasons, after which morphological and physiological foliar attributes were evaluated. The field study revealed significant age-related trends in foliar morphology, including decreasing specific leaf area, and increasing needle width, projected area, and width/length ratio. Similar trends were apparent in foliage from the grafted scions. Both in situ foliage and shoots of grafted scions from the oldest cohort showed significantly lower photosynthetic rates than their counterparts from younger trees; however, differences in stomatal conductance and internal CO(2) concentrations were not significant. These results suggest that: (1) foliage of red spruce exhibits age-related trends in both morphology and physiology; (2) age-related decreases in photosynthetic rates contribute to declining productivity in old red spruce; (3) declines in photosynthetic rates result from nonstomatal limitations; and (4) age-related changes in morphology and physiology are inherent in meristems and persist for at least 3 years in scions grafted to juvenile rootstock.