The boreal forest of Interior Alaska is experiencing rapid warming: the number of days in summer during which temperatures remain above freezing has increased by 45% (from 85 to 123 days)in the past 100 years. We evaluated whether earlier springs, warmer summers, and extended falls provide an advantage to non-native plants by tracking the phenology of 39 plant species (27 native and 12 non-native) in three sharply contrasting years. Compared to long-term averages, 2013 had a very late spring, warm summer, and a greatly extended fall, 2014 a moderately early spring, cool summer, and average fall, and 2015 a very early spring, average summer, and short fall.
Native plants had significantly earlier dates of first flower, peak flowering, first leaf and peak leaf production than non-native plants, and flowering was advanced in both groups in the early spring year compared to the late spring, but there was no difference in the size of the shift between native and non-native species. Developmental times (e.g., from peak flower to half of fruits ripe) were shorter for native species than for non-native species, but did not differ between years. Non-native plants produced their last leaf later than non-natives, but this difference was much greater in 2013 (extended fall: 52 days) than in years with an average fall (2014: 8 days). Similarly, senescence of non-natives was delayed by two weeks in 2013 compared to natives, but concurrent in 2014. Non-native species also extended their flowering period beyond that of native species, and more so in the late fall year, but this did not result in extended fruit production. We found no evidence that earlier springs or warmer summer gave an advantage to non-native species. Extended autumns may benefit non-native species more than native species by prolonging leaf production and retention, but are unlikely to increase fruit set.