Abstract
Aim The purpose of this study was to examine the influence of moisture and fire on historical ponderosa pine ( Pinus ponderosa Dougl. ex Laws.) age structure patterns. Location We used a natural experiment created over time by the unique desert island geography of southern Arizona. Methods We sampled tree establishment dates in two sites on Rincon Peak and another site in the Santa Catalina Mountains. The influence of regional moisture patterns was investigated by examining synchrony among tree age peaks and periods of favourable moisture. The influence of fire was investigated by examining the relationship between local fire histories and age peaks. Results Tree age peaks (recruitment events) were synchronized with periods of reduced fire frequencies, suggesting that periods of reduced fire activity allowed time for young trees to develop fire‐resistant characteristics. For example, two age peaks (1670s–1680s and 1770s–1780s) in the Rincon Peak‐north site coincided with a period of reduced fire frequency. These age peaks were absent in the Rincon Peak‐south site where fires were more frequent during that time. All three sites had age peaks in the early 19th century, coinciding with regional climate variability (i.e. El Niño Southern Oscillation patterns) that reduced fire activity. Main conclusions Historically prior to fire suppression, the survivorship, and thus cohort establishment, of ponderosa pine was ultimately determined by local fire history patterns. The importance of fire as a tree establishment regulator highlights the need to use fire at appropriate intervals not just to restore but to maintain healthy forests.