Regional patterns of tree population age structures in northern Patagonia: climatic and disturbance influences

Variations in regional patterns of age structures of the conifer Austrocedrus chilensis near the forest/steppe ecotone of northern Patagonia, Argentina were investigated in relation to climatic variation and changes in disturbance regimes. Climatic variation was derived from the instrumental record beginning in c. 1910 and was also inferred from variations in ring widths of Austrocedrus over the past approximately 200 years. Variations in regional patterns of fire were derived from prior studies based on dendrochronological evidence of fire. Most trees that established since c. 1960 could be dated to an annual resolution. Almost no Austrocedrus establishment has occurred since the warm-dry period began in c. 1980. In contrast, many trees established during the cool-wet conditions between 1963 and 1979 with peaks in numbers of surviving trees corresponding to particularly cool-wet conditions in 1964-66 and 1973-75. Throughout the twentieth century, relatively few Austrocedrus survive from periods of extreme and persistent (> 5 years) drought. In contrast, short droughts (1-2 years) are not reflected in age structure patterns. Periods of reduced radial growth reflect spring-summer moisture deficits and coincide with approximately decade-long periods of fewer surviving trees. Prior to c. 1900, relationships of age structure to variation in radial growth are less clear due to the confounding influences of increasingly inaccurate age determinations of older trees, and the cumulative effects of past changes in disturbance regimes. Variations in both climate and disturbance regimes influence the regional pattern of age structures of Austrocedrus. For example, near the ecotone the frequency of surface fires has declined drastically during the present century as has the severity of browsing by livestock during the past approximately 40 years. The reduction in these disturbances was probably a necessary condition for the increase in Austrocedrus establishment during the 1963-79 cool-wet interval.