Stand structure variability and dynamics of Quercus garryana and Quercus kelloggii woodlands across northwestern California

Background/Question/Methods

Oak woodlands on the northern coast of California are unique to the region and support a high level of biodiversity, but their integrity and extent have been substantially altered. Past studies hypothesize that 19^th century Euro-American settlement altered historical disturbance regimes (i.e., through the suppression of fire and increases in grazing) leading to denser oak ecosystems. To assess change over the last century in northwestern California, we evaluated tree age and stand structure across a gradient of conifer overstory dominance within and across four Quercus garryana and Quercus kelloggii mixed oak woodland stands. We hypothesized that: 1) across all stands, oak establishment pre-dates conifer establishment, 2) at the plot scale (0.1 ha), both oak and conifer established during distinct cohorts, and 3) at the stand scale, conifer establishment was continual during the 20^thcentury, in contrast to multiple cohorts of oak establishment. To evaluate these hypotheses, we collected 180 tree cores and stand structure data from 9 plots (0.1 ha) per stand (707 cores total).  Patterns of tree establishment and differences in tree ages between species were tested using ANOVA and t-tests to evaluate the temporal dynamics of northwestern California oak woodlands.

Results/Conclusions

Across all stands, we found that oak trees established 50 to 150 years earlier than conifer trees.  The majority of oak establishment occurred during the mid to late 1800’s with no tree establishment after 1950.  Oak establishment occurred in distinct cohorts at plot scales, but varied between single to multiple cohorts at stand scales depending on the site. Across all stands, the majority of conifer establishment occurred during the mid-20^th century and exhibited a relatively continuous establishment pattern. These results suggest that the oak stands sampled in this study possibly established after disturbance in the mid to late 19^th century, and that the scale of cohort-initiating disturbance varied across sites. These results support findings elsewhere and suggest that 20^th century establishment of conifer into nearby oak stands has significantly altered stand structure towards a conifer-dominated landscape.