[1] For the past forty years, a weather station in the Verdance Valley has recorded a steady rise in the valley's average annual temperature. [2] Over the same four decades, botanists monitoring a shrub native to the valley have found that it now produces its first blossoms, on average, nearly three weeks earlier in the year than it did when their records began. [3] Warmer conditions are known to accelerate the development of many temperate plants. [4] A researcher therefore concludes that the temperature increase recorded at the station is the cause of the shrub's advancing bloom, and predicts that further warming will push the blooming date earlier still.
Which of the following, if true, would most seriously weaken the researcher's conclusion?
