Leaf out times using herbarium specimens

Posted by Richard B. Primack

Scientists are interested in knowing how a warming climate will affect the leafing out times of trees, and if leafing out times are earlier now than in the past due to a warming climate. If trees are leafing out earlier now, that could extend the growing season, and forests could be absorbing more carbon dioxide and producing more wood. However, until recently there was no good way to determine when tree species leafed out in the past.

Figure 1.Two specimens of Populus grandidentata accessed online from the G.S. Torrey Herbarium at the University of Connecticut (EEB, 2004). Young leaves and flowers are visible on the left specimen (collected 10 May 1936), but only mature leaves on the right specimen (collected 8 June 1932).

We have found that herbarium specimens of most New England tree species can be used to determine past leaf out times. These herbarium specimens are flattened dried twigs, often collected when the plants are in full flower. These specimens each have labels that show the date and location of collection. About 15% of the specimens also have young, unfolding leaves on them, and so can be used to determine past leaf out dates.

Figure 2. Leaf-out dates are significantly correlated with annual variation in April temperatures, with early leaf-out in warmer years (slope = -2.70; P < 0.01). (Figure 4 in Am. J. Bot article)

Using over 1500 New England herbarium specimens with young leaves, we were able to determine that trees in New England leaf out earlier in warm years than cold years, that trees are leafing out earlier now than in the past, and that trees leaf out earlier in warmer more southern locations in New England than in more northern and colder locations. This method promises to be useful to climate change biologists and ecosystem ecologists when applied to other areas and over larger regions.

Figure 3. Mean leaf-out dates for each town. (A) Towns with at least two data points. Major geographical features are labeled. Towns with only one or no data points are shown in white. (B) Towns with at least five data points; towns that are notably early or late are labeled by name. Towns with four or fewer data points are shown in white. (Figure 5 in Am. J. Bot. article)

To see the full article, published in American Journal of Botany, click here.