Detecting flowering time response to climate change in Denmark

Richard B. Primack

 

“Let us spend one day as deliberately as Nature, and not be thrown off the tracks by every nutshell and mosquito’s wing that falls on the rails.” Henry David Thoreau in Walden.

 

Lots of new phenology data is becoming available from digitized herbarium specimens, citizen science programs, and photographs uploaded to the iNaturalist app. But how do these data sets compare in terms of detecting the effects of temperature and climate change on flowering times?  

 

A new study compares three separate phenology datasets from Denmark: i) herbarium specimen data spanning 145 years, ii) one year of flowering data collected from a citizen science program, and, iii) data derived from a year of photographs uploaded to iNaturalist. Each dataset includes flowering dates of three common plant species: Allium ursinum (ramsons), Aesculus hippocastanum (horse chestnut), and Sambucus nigra (black elderberry).

 

Photo:  Ransoms, horse chestnut, and black elderberry.

 

The iNaturalist dataset provided the most extensive geographic coverage across Denmark and the largest sample size, and recorded peak flowering in a way comparable to herbarium specimens. The directed citizen science dataset recorded much earlier flowering dates because the program objective was to report first flowering, and so was less comparable to the other two data sets. 

 

Figure 1: Distribution of data points across Denmark for: A) the herbarium dataset; B) citizen science dataset; and C) iNaturalist dataset. The herbarium data shows the widest range of temperatures. The iNaturalist data set has the most observations.

Herbarium data demonstrated the strongest effect of spring temperature on flowering in Denmark, possibly because it included year-to-year variation in temperature, while the other datasets only included one year of spatial variation in temperature across Denmark. Combining herbarium data with iNaturalist data provides an even more effective method for detecting climatic effects on phenology. Phenology observations from citizen science programs and iNaturalist will increase in value for climate change research once additional years are available.

 

This article is published as: 

Iwanycki Ahlstrand N, Primack RB, Tøttrup AP. 2022. A comparison of herbarium and citizen science phenology datasets for detecting response of flowering time to climate change in Denmark. International Journal of Biometeorology https://doi.org/10.1007/s00484-022-02238-w

Forests changing slowly and quickly in Webster Woods

Richard B. Primack

 

“The higher the mountain on which you stand, the less change in the prospect from year to year, from age to age. Above a certain height, there is no change.” Henry David Thoreau in Familiar Letters. 

 

Some parts of the Webster Woods in Newton, Massachusetts, have not noticeable altered in my lifetime, and others have changed very quickly.

 

The history of the woods begins with the retreat of glaciers from Massachusetts about 16,000 years ago. We can still see evidence of the glaciers in the giant boulders they left behind in the woods. 

Boulder on Gooch’s Cave left behind by the glaciers.

For the last 400 years, hardy souls used the land for woodlots and pasture, as evidenced by the rock walls running through the woods These walls were likely constructed in colonial times and had wooden rail fences running along the top. 

 

Giant rocks forming a wall.

Other areas of the woods were extensively altered for water use. The canal emerging from Hammond Pond supplies water to the flooded meadow just south of the train tracks, which might have been managed for hay and cranberries in the past. 

Flooded field is linked to Hammond Pond by a canal. 

On the ridge and plateau above Hammond Pond and the plateau just to the west of Bare Pond, oak, black birch, and red maple trees are only about 20 to 30 feet tall with twisted, often broken trunks. The short, bonsai-like stature of these trees is likely due to thin, nutrient-poor soils. 

 

Stunted pitch pine tree on a ridge above Bare Pond. 

Big winds were responsible for toppling many of the oaks that fell. The Great New England Hurricane of 1938, with gusts of wind up to 186 miles per hour, knocked down many trees and broke the tops off others. Two years ago, a windstorm broke many trees (most weakened by disease). 

 

Oak tree broken in a windstorm. 

Other parts of the woods are being transformed more quickly. Just east of the Parkway, white pine saplings are rapidly growing up and forming thickets in the former deer park. Within a few decades, this field will become a pine forest. 

 

The ridge above Hammond Pond is also changing rapidly. The giant hemlock trees are sick and dying due to the introduced hemlock woolly adelgid insect, which infests their twigs, leaving behind dead branches. 

 

About four years ago, fire killed many small and medium-sized trees and shrubs, leaving the area more open. The roots of some trees, especially oaks and sassafras, survived and are producing new sprouts. 

 

Sassafras tree sprouting after a fire.

Tree diseases have also altered the Webster Woods. Chestnut trees were once common in the woods, but 90 years ago a fungus infected their stems and killed them, creating opportunities for young birch and oak trees to grow into the forest canopy. Fungal diseases are weakening beech, oak, and birch trees. These diseases reflect the declining health of our forests.

 

Diseased black birch tree with many fungal bodies. 

 

Now that the City of Newton has acquired the remaining block of undeveloped forest from Boston College, the future of the Websters Woods seems more secure. However, the people of Newton still need to monitor the woods and take action in response to ongoing and future threats, such as fires, tree diseases, and the invasion of non-native native. 

 

This is a condensed version of an article published in the Newton Conservators Newsletter:

 

https://newtonconservators.org/wp-content/uploads/2022/02/Newsletter-2022-Spring-Forests-Changing-Webster-Woods.pdf