In 1789, British naturalist Robert Marsham published a chart in the Philosophical Transactions of the Royal Society called “Indications of spring.” In this chart he lists the first day of the year on which he noticed particular events at his home in Norfolk — the blooming of certain flowers, the emergence of leaves on various species of trees, the first croaks of frogs and toads, the appearance of a butterfly, and the appearance, nest-building, and hatching of various birds.What is notable about this chart is that it spans more than half a century, from 1736 to 1788. (From a modern perspective, it is also notable in that the chart was published without comment, explanation, or context, other than the location where the observations were made. It is just a stand-alone chart. Today, it would be impossible to publish such a stand-alone chart in a scientific journal.) Marsham’s descendents would continue his spring phenology observations into the twentieth century, making this record one of the longest ones available and of high scientific value.
Recently, scientists in the UK used the Marsham phenology records to understand more about how climate affects spring leaf-out dates in trees. Earlier this year they published their results in an article in the scientific journal Global Change Biology called “Predicting a change in the order of spring phenology in temperate forests.” In the article, they describe how they took 13 tree species and modeled how various aspects of climate affect the date on which leaves come out on the trees. As part of their analysis, they also did a future projection of their model, where they looked at what might happen to the leafing dates of trees with climate warming.
This figure from their paper shows the thirteen trees species and when the leaves come out on the trees. Here’s how to read these graphs (called “violin plots”): across the bottom of each of the three graphs is time — number of days since the beginning of the year, which is often called the “Day of Year” or “Julian Day”. Each diamond represents a species, with the middle black bar showing the middle (median) date for when that tree’s leaves first came out. So, for example, for the very top diamond, which is red, we learn that Ash leaves came out, on average, around the 120th day of the year — April 30.
But, of course, Ash leaves didn’t come out exactly on April 30 every year — there was some variation from year to year. That’s what the rest of the diamond shows. For the top Ash diamond, we see that Ash leaves came out between about days 85 (March 26) and 140 (May 20). In general, wider diamonds show greater variation and skinnier diamonds show less variation. The colors of the diamonds just correspond to the middle date, with earlier dates being blue and later dates being red.
The three graphs show different time periods. The top graph (“Historic”) shows the results directly from the Marsham records. The middle graph shows the modeled future projections for the near future (“2010-2039”) and the bottom graph shows the projections for the middle of the century (“2040-2069”).
Because different tree species respond to climate differently, you can see that we expect many species to leaf-out earlier. But some will likely not change too much in their leafing times and a few may even leaf out later. This last group seems counter-intuitive, but the timing of tree leaf-out is complicated and often involves some amount of winter chilling. With warmer winters, that chilling may take longer to happen or may not happen at all.
I enjoy reading about early scientific efforts, in part because some great pieces of research such as the Marsham records are mixed in with other reports that we might find bemusing today. Around the same time he was recording spring indicators, Robert Marsham was also investigating the usefulness of washing a tree’s trunk to increase its health and growth: