REGIONAL PATTERNS OF TREE POPULATION AGE STRUCTURES IN NORTHERN PATAGONIA - CLIMATIC AND DISTURBANCE INFLUENCES

1 Variations in regional patterns of age structures of the conifer Aus trocerdus chilensis near the forest/steppe ecotone of northern Patagon ia, Argentina were investigated in relation to climatic variation and changes in disturbance regimes, Climatic variation was derived from th e instrumental record beginning in c. 1910 and was also inferred from variations in ring widths of Austrocedrus over the past approximate to 200 years, Variations in regional patterns of fire were derived from prior studies based on dendrochronological evidence of fire. 2 Most tr ees that established since c. 1960 could be dated to an annual resolut ion. Almost no Austrocedrus establishment has occurred since the warm- dry period began in c. 1980, In contrast, many trees established durin g the cool-wet conditions between 1963 and 1979 with peaks in numbers of surviving trees corresponding to particularly cool-wet conditions i n 1964-66 and 1973-75. 3 Throughout the twentieth century, relatively few Austrocedrus survive from periods of extreme and persistent (> 5 y ears) drought. In contrast, short droughts (1-2 years) are not reflect ed in age structure patterns, Periods of reduced radial growth reflect spring-summer moisture deficits and coincide with approximately decad e-long periods of fewer surviving trees. 4 Prior to c. 1900, relations hips of age structure to variation in radial growth are less clear due to the confounding influences of increasingly inaccurate age determin ations of older trees, and the cumulative effects of past changes in d isturbance regimes, 5 Variations in both climate and disturbance regim es influence the regional pattern of age structures of Austrocedrus. F or example, near the ecotone the frequency of surface fires has declin ed drastically during the present century as has the severity of brows ing by livestock during the past approximate to 40 years, The reductio n in these disturbances was probably a necessary condition for the inc rease in Austrocedrus establishment during the 1963-79 cool-wet interv al.