Do Thoreau’s observations and herbarium records see the same changes in fruiting?

One of the simplest but most valuable scientific observations you can make is watching the seasons turn. Phenology is the study of seasonal change. It could be the first budding or blossom on a tree, or it could be hearing the first call of a cuckoo. These signs can reveal long-term changes in the ecology of a location. Fruiting could be a valuable tool in studying phenology, but fruits are not so well studied compared to leaves and flowers. A study by Tara Miller of Boston University, and colleagues in the US and Germany, seeks to help change this. Herbarium records would seem to be the answer. Yet, while they are a record, are they accurate? Miller and colleagues have looked for an independent record to compare with herbaria. They have found it in an overlooked manuscript by American naturalist and philosopher Henry David Thoreau.

Thoreau is best known for his work Walden, published in 1854. However, for their work Miller and colleagues turned to Wild Fruits. Wild Fruits was found on the author’s death in 1862. It wasn’t a final draft, and the process of editing the text took a while. A scholarly edition appeared in 2001. The text has both detailed descriptions and dating for fruits. It means Thoreau has provided field observations of fruiting phenology for New England. This evidence could help botanists understand the phenology of herbarium samples, say Miller and colleagues.

“Herbarium collections are based on methods different from those used to make field observations,” write Miller and colleagues, “for example, collectors are not always gathering specimens with the intention to capture first fruiting dates or other phenophases, whereas field observations are often conducted to do just that. Herbarium specimens are collected more frequently in spring and summer, so they may fail to accurately capture the end of fruiting seasons (Daru et al., 2017). In addition, herbaria contain a range of specimens that can be used to infer different metrics: earliest specimen to be collected in the season among all of the specimens, mean date of collection among all of the specimens, and last specimen collected among all of the specimens, instead of start, peak and end of fruiting over the course of a season at a location. It is not clear whether metrics using herbarium specimens are comparable with field observation metrics, as they do not directly capture the same phenophases. There are few historical datasets on wild fruits available, so it is important for researchers to know how to work with these differences and synthesize or choose between historical datasets.”

To find out how herbarium specimens compared to field observations, the team focused on 67 fleshy-fruited plant species. From Thoreau, they looked for first, peak and last observed fruiting. From the herbaria, they looked for earliest, mean and latest specimen, as well as fruiting durations.

The results were interesting. The earliest herbarium specimens tended to be around 28 days earlier than Thoreau’s observation. The latest herbarium specimens lagged by 18 days. But the peak fruiting dates noted by Thoreau were the same as the mean specimen dates in the herbaria.

“First date and earliest specimen, peak date and mean specimen, and last date and latest specimen are highly correlated between Thoreau’s observations and the herbarium specimens. These relationships indicate that there is a very consistent order of fruiting across species, and that this pattern is robust to differences in collection method and phenophase metric. Both historical datasets appear to be capturing a real biological trend in fruiting patterns: the sequence of fruiting for these 67 species is relatively consistent across New England,” write the authors.

One of the surprises in the results was how strong the correlations were between the two records. The team expected that the mean and peak dates would be the most consistent matches. However, it was the last fruiting dates and last specimen dates that most strongly correlated. 

“It is possible that the stronger correlation between last dates and latest specimens in our study reflects a consistent order in which fruits are consumed by wildlife,” argue Miller and colleagues. “Last dates may combine information on both fruiting order and frugivore feeding preferences. For example, fruits with higher antioxidant contents, like arrowwood (Viburnum dentatum), may reliably be eaten first, whereas less desirable fruits, like winterberries (Ilex verticillata), may consistently be left until later in the season (Bolser et al., 2013), which could be reflected in later field observations and specimen collections.”

The authors note that the differences between the two data sets mean that field observations and herbaria cannot be used as a simple substitute for each other. Yet, with care, botanists can use them to understand how the fruiting season is changing around a location.

“This study demonstrates that two different historical datasets – Thoreau’s observations and herbarium specimens – capture very similar information about the order in which woody and herbaceous species fruit in New England. The high correlations of metrics within datasets confirm findings from previous studies and help validate these historical datasets. The high correlations between the metrics of fruiting in Thoreau’s observations and the herbarium specimens suggest that we may successfully use and compare different methodologies for studying the order in which species fruit,” conclude the authors.