Changes in urban plant phenology in the Pacific Northwest from 1959 to 2016: anthropogenic warming and natural oscillation.

In the Pacific Northwest of the USA, winter and spring temperature vary with the Pacific Decadal Oscillation, making effects of anthropogenic warming difficult to detect. We sought to detect community-level signals of anthropogenic change in a legacy plant phenology dataset. We analyzed both incomplete data from 1959 to 2016 on spring phenology of 115 species and more complete 1996-2016 data on spring and fall events for 607 plant species. We used ordination of the long-term dataset to identify two major axes of community-level change in phenology among years, with the first being a trend toward earlier spring phenology in more recent years. In contrast, for the short-term dataset, variation in spring phenology was mostly PDO-driven and did not reveal a strong trend over time. At both time scales, a second axis of phenological variation reflected summer and fall events, especially earlier appearance of fall color in recent years. In univariate analysis, more than 80% of individual species' leaf out dates and first flower dates occurred earlier over time, for an average advance across all species of 2.5 days per decade from 1959 to 2016. While most events did not advance in the period 1996-2016, fall color advanced by 10.6 days per decade, suggesting that intensification of summer drought has continued regardless of the PDO cycle. While estimates of slope over time depended strongly on the time window chosen for the analysis, estimates of slope versus temperature were consistently negative regardless of time window, averaging 5-7 days per 1 °C for spring events.