Tree leaves are unfurling earlier with increasing temperatures, but this tendency is slowing down. That was the message of an article that came out recently in the high-profile science journal Nature called “Declining global warming effects on the phenology of spring leaf unfolding.”
The authors of this article used a European phenology database called PEP725 that has highest coverage over Austria, Switzerland, and Germany. (And some of the data in the PEP725 database comes from citizen scientists!) They used 9 common trees that occurred at 100 or more sites each and looked at tree leaf unfurling time in relation to yearly temperatures, going back to 1980. What they found was that leaves have tended to unfurl earlier as time has gone on. This itself isn’t surprising. Many studies have found that leaves come out earlier as temperatures get warmer. What they also found, though, was that for very warm springs, the trees weren’t quite as early as you would expect based on mildly warm springs. So for the first couple degrees Celsius warmer, trees unfurl their leaves several days earlier. But the number of days earlier is less than double if you double the number of degrees above average for a particular spring. In other words, trees’ sensitivity to increasing warming slows down.
This seems reasonable, though it hasn’t been shown with mature trees across a broad area like this. What still isn’t really understood, though, is why. We know, for example, that trees need a certain amount of light to photosynthesize. And they need a certain amount of warmth to get all the enzymes and metabolic processes moving. Many also need a “chilling period” over the winter to trigger a switch from losing leaves in the fall to putting out new leaves in the spring. But which of these reasons is in play as climate gets warmer is in debate. If the amount of light matters a lot, then the effects of warming will be limited. If warmer winters mean less of a “chilling period”, that might actually counter the effects of warmer springs. But if not, then warmer springs may trigger earlier and earlier leaf unfurling.
Also contested is what matters to trees when we refer to “warmer” temperatures. Is it the average temperature over the spring? Is it the highest high temperatures or the lowest low temperatures that matter? We know, of course, that a frost event (a lowest low) that happens after leaves start to unfurl can damage and kill leaves. So a tree has to balance the effects of putting out leaves earlier — and risking a frost — versus putting them out later and missing out on possible growth. And it has to do this repeatedly even though the temperatures change from year to year. With all this complexity, perhaps it’s no wonder that it’s still a bit of a mystery exactly when leaves will come out in the spring.
Y.H. Fu et. al. “Declining global warming effects on the phenology of spring leaf unfolding.” Nature, vol. 526, pp. 104–107 (October 1, 2015). doi:10.1038/nature15402