Oak versus conifer: competition, climate and drought effects in oak woodlands of northern California

Abstract

Oregon white oak (Quercus garryana Douglas ex Hook.) is experiencing increasing competition from Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) across its range at the same time as climate models are predicting increasing climate variability, including drought. Across a broad range of sites and crowding conditions, we assessed stand and age structure of oaks and conifers in northern California. We also evaluated the effects of tree characteristics, competition, and climate on Oregon white oak and Douglas-fir tree growth (2002-2016) and drought response (2013-2015). Across the 10 sites, the majority (>80%) of oak stems dated between 1850 and 1910, while the majority of Douglas-fir (73%) established after 1970. Oregon white oak growth had positive relationships with winter and summer precipitation, spring mean maximum temperature, and September maximum temperature; and negative relationships with oak crowding, Douglas-fir crowding, and summer temperature. Douglas-fir growth had a positive relationship with late spring precipitation and negative relationships with Douglas-fir crowding and summer mean maximum temperature. Oregon white oak growth was more resistant to prolonged drought than Douglas-fir. However, oak resistance to drought was also negatively related to Douglas-fir crowding and positively related to oak crowding. These results suggest that Oregon white oak may be better suited to a future climate than Douglas-fir, but Douglas-fir encroachment may threaten the ability of Oregon white oak to resist future drought. Management of Douglas-fir encroachment and maintenance of variable oak density is expected to optimize the vigor and drought resistance of Oregon white oak and best preserve ecosystem function under climate change.