Age structure and recruiment of blue oak (Quercus douglasii) at Deer Creek Hills Preserve, Sacramento County, CA

Abstract

To understand past recruitment and the effect of climate on blue oak (Quercus douglasii) regeneration, we examined age structure associated with canopy cover changes in a blue oak population over the last 50 years at Deer Creek Hills Preserve in Sacramento County, California. Historical (1953) and current (2004) aerial photos were compared to identify sites experiencing increased and decreased canopy cover over the intervening time period. To examine whether mean tree age corresponds with canopy cover change, over 100 trees in each of the canopy increase and canopy decrease sites were sampled for height and diameter at breast height (DBH). To asses mean tree age and age structure for the preserve, 400 randomly selected trees were measured for height and DBH. To determine the establishment date of trees, 100 trees (including random, increase, and decrease) between 9 and 49 cm DBH were cored. A regression of tree age (by ring counts, not age adjusted for coring height) versus DBH was generated to identify the age of un-cored trees. Analysis of 244 trees throughout the preserve suggests that mean tree age in canopy decrease sites ( =67±2) is significantly greater than in randomly selected sites ( = 49±.7). However, there was no significant difference between mean tree age of canopy increase ( = 49±.4) and random sites ( = 49±.7). Analysis of recruitment patterns for 110 randomly selected trees produced a bell-shaped curve, suggesting a past pulse of recruitment. These results suggest the potential for blue oak replacement and indicate that past climate may play a role in influencing long-term blue oak recruitment patterns.