Plants Leafing Out Earlier in Spring
Plants across nearly all of the Northern Hemisphere are responding to global warming by bursting their leaf buds 1.2 days earlier on average each decade. The trend, measured for 1955 to 2002, is most pronounced for central Europe and certain places throughout North America. In some areas spring arrival is moving ahead three days each decade.
Vegetation is reacting to increasingly warmer temperatures, particularly in winter and spring. For instance, average annual temperatures have climbed 0.3 °C or more each decade since 1960 in Europe, East Asia and Alaska.
Climatic warming is not happening everywhere, though. Across North America, temperature trends are highly variable and eastern parts of the continent have even become cooler. Temperatures have not warmed at all in Central Asia and plants there are also not getting an earlier start on spring.
The warming trend also shows up as a decline in the number of freezing days below 2.2 °C each winter. This has happened most dramatically in East Asia, but also significantly in central North America and Europe. Winters are mostly ending sooner, with the date of last freezing occurring an average of 1.5 days earlier each decade.
Changes to the timing of last frost and leaf growth can end up damaging plants. When plants start growing further in advance of the last date of freezing, they risk heavier frost damage.
In East Asia and western Europe, the gap between bud burst and the last hard frost has narrowed over the decades, lowering the chance of frost damage. Meanwhile in northern and eastern Europe and in some parts of North America, plants have lengthened the period between bud burst and final freezing, putting them in danger of suffering severe damage.
Mark D. Schwartz, Rein Ahas and Anto Aasa. 2006. Onset of spring starting earlier across the Northern Hemisphere. Global Change Biology. 12(2): 343-351.