Climate change is shading out forest wildflowers in Thoreau’s Concord

Caitlin McDonough MacKenzie and Richard B. Primack

A man can never say of any landscape that he has exhausted it. 
—Thoreau in his Journal. 1850.

Many spring wildflowers in New England flower and leaf out in April before the trees start leafing out. This period of a few weeks of full sunlight is crucial for the wildflowers — the high light environment provides the energy for growth, flowering, and fruit production. In the past, our lab has shown that spring flowering and leaf out for wildflowers and trees are occurring earlier now than in Thoreau’s time.

Birdfoot violet leafs out and flowers early in the spring

In a recent paper, we’ve compared the rate of change in tree leaf out (days/°C) to the rate of change in wildflower flowering (days/°C). To our surprise, we found that trees are more responsive to a warming climate than wildflowers, with trees leafing out 2 weeks earlier now than in the 1850s when Henry David Thoreau was recording phenology, whereas wildflowers have only shifted by one week earlier.  The result is that wildflowers now have one week less of full sunlight in the spring before the tree produce their leaves.

Black oak trees are leafing out earlier now than in the past

We asked our colleague Mason Heberling how this might affect the energy budget of these spring wildflowers. Mason’s models show that the spring energy budgets for many spring wildflowers, such as Jack-in-the-pulpit and wild ginger, have already declined from Thoreau’s time until now by as much as 26%, with as much as 48% decline in energy budget projected by the end of this century.

Richard monitoring the flowering time of marsh marigolds

This loss of energy reserves means that many wildflower species may not have enough energy to mature their fruits and produce flower buds for next year. Further, many of these species will have a reduced chance of growing and surviving in these habitats in coming decades.


Read the full paper in Ecology Letters.

Heberling, J.M., McDonough MacKenzie, C., Fridley, J.D., Kalisz, S. & Primack, R.B. 2019. Phenological mismatch with trees reduces wildflower carbon budgets. Ecology Letters doi: 10.111/ele.13224 (WEB LINK HERE)