Henry David Thoreau was an acute observer of the plants of Concord, Massachusetts and Walden Pond. He carefully recorded the abundance and phylogeny of plants in the area. Phylogeny refers to the timing of seasonal activities such as flowering. Thoreau’s data provides information about species abundance and flowering time of the plants of Concord 150 years ago.
Since Thoreau’s time, several botanists have revisited the natural areas around Concord, creating a unique sequence of observations of change in vegetation over time. Charles Willis and colleagues at Harvard and Boston Universities have analyzed Thoreau’s data and those of botanists since his time and analyzed the abundance of the same species in Thoreau’s old neighborhood over time. Concord is unusual in still having large areas of land undeveloped or protected since Thoreau’s time.
Willis and colleagues analyzed data for 473 plant species and included information on changes in species abundance, habitat, and sensitivity of flowering to temperature. They also constructed a phylogeny (relationship diagram) for all the species.
The study shows that climate change has had a profound influence on the plants of Thoreau’s woods. Of the plants that existed in Concord in Thoreau’s time, 27% have disappeared, and 36% have decreased in abundance so much that they are likely to disappear soon. Some of the decline is due to forest succession, some to development, but much of the change appears to be due to climate change. The mean annual temperature has increased by 2.4 degrees C in the last 100 years. Plant species are flowering an average of 7 days earlier than in Thoreau’s time.
The species that have disappeared or declined are more likely to be closely related than one would predict from random chance. The authors explain that this is probably due to the similarity in flowering response to temperature among closely related species.
Some plant species can track seasonal temperature change. These species will flower earlier when temperatures are warmer. Other species are not as good at tracking temperature, continuing to flower at the same time every year regardless of temperature. The good trackers increased in abundance or held their own, while the poor trackers were more likely to decline in abundance. And the poor temperature trackers tended to be related to one another. This means that poor temperature tracking seems to be a trait of certain plant families, but not of others.
The plants most likely to decline were asters, bladderworts, buttercups, dogwoods, lilies, louseworts, mints, orchids, saxifrages and violets.
This study is interesting because of the connection to Thoreau. It is important because it shows that the risk of extinction due to climate change is shared among closely related plants. Species whose flowering times do not change with seasonal temperature changes are more likely to become extinct (globally or locally) than species that do shift their flowering times. And, the species that don’t change flowering times are likely to be closely related.
Dogwoods (Cornus spp.) are of particular concern because they are declining for other reasons as well. Dogwoods are suffering since the introduction of the serious disease dogwood anthracnose, caused by the newly identified fungus Discula destructiva, dogwood populations may be uniquely vulnerable to extinction caused by a combination of climate change and disease.
Links:
- Phylogenetic patterns of species loss in Thoreau’s woods are driven by climate change – news report, PNAS
- C.G. Willis, B Ruhfel, R.B. Primack, A.J. Miller-Rushing, C.C. Davis. 2008 Phylogenetic patterns of species loss in Thoreau’s woods are driven by climate change. PNAS 105: 17029-17032. DOI: 10.1073/pnas. 080656105
