Concord, Massachusetts, USA has served as an active location for phenological observations since philosopher and naturalist Henry David Thoreau began recording plant and animal occurrence and phenology in 1851. Since that time, numerous naturalists, scientists, and researchers have continued this tradition, creating an invaluable time series of 758 species in a single location. A total of 13,441 phenological records, spanning 118 years, now exist, with observations of many species ongoing. Relative abundance data for an additional 200 plant species is also provided. Thoreau’s published journals and records in Special Collections libraries at the Concord Free Public Library, Harvard University, Peabody Essex Museum, and Morgan Library and Museum provide insight into his methods of routinely walking around Walden Pond, through natural areas, and within the town of Concord, seeking the first leaf or flower on plants, seasonal observations of migratory birds, and fruit maturation times. Several amateur naturalists, and most recently the present research group, have followed this method of regularly searching Concord for the earliest signs of seasonal events, visiting many of the same locations including Walden Pond, the site made famous by Thoreau. While Thoreau’s observations were initially made out of a curious desire to document the natural world, these data have led to dozens to contemporary studies, addressing timely issues such as climate change, conservation, ecology, and invasive species. This time series of data, initiated by Thoreau and continued by others, has resulted in dozens of peer-reviewed publications, a popular science book, and numerous educational and outreach opportunities. These data grow increasingly valuable with time and as new and creative studies are undertaken with Thoreau’s historic records. No copyright restrictions apply to the use of this data set other than citing this publication.

Concord, Massachusetts (42°27′N 71°20’W, 43 m elevation) is approximately 28 km northwest of Boston, in the northeastern United States (Figure 1). Concord is the location of numerous sites of major historic and literary importance (site of an early Revolutionary War battle, now protected as Minuteman National Historical Park; home to formerly enslaved residents and significant authors such as Thoreau, Ralph Waldo Emerson, Louisa May Alcott, and Nathaniel Hawthorne; site for the development of intellectual and political movements such as transcendentalism and abolition), protected natural spaces, and opportunities for recreation. Most notably, Walden Pond, made internationally famous by the writings of Thoreau, is in Concord. These features have made it a destination for intellectuals, naturalists, and environmentalists for over 150 years. Importantly, the draw of Concord and traditions initiated by Henry David Thoreau have resulted in the wealth of phenological data presented here.

There is a diversity of ecosystems and microclimates within the 67.4 km² of Concord. Concord is dominated by temperate forest which today is composed largely of maple (Acer spp.), oak (Quercus spp.), birch (Betula spp.), pine (Pinus spp.) and hickory (Carya spp.) trees. Shade-tolerant shrubs, particularly blueberry (Vaccinium) and huckleberry (Gaylusaccia) species in the Ericaceae, and wildflowers can be found in the understory. Open fields maintained since forest clearing in the 17th and 18th centuries by European settlers provide habitat for sun-loving wildflowers. Opportunistic wildflowers are also found in human-dominated parts of the Concord landscape – in cemeteries, along roadsides and railroad tracks, on the edges of parking lots, and near buildings. Local, state, and federal agencies maintain environments, such as Great Meadows National Wildlife Refuge and Minute Man National Historical Park, that have become dependable habitats for resident and migratory bird species. Concord is at the confluence of three major rivers, Concord, Assabet, and Sudbury, and has numerous springs, streams, and ponds. Notably, Walden Pond State Reservation, the historic site of Thoreau’s cabin and foundation for much of the present work, is located on the southeastern corner of Concord. Much of Concord’s landscape is also protected by land trusts and nonprofit educational institutions, including the Estabrook Woods, a tract of 490 ha owned by Harvard University and the Middlesex School. This abundance of protected land in natural conditions contributes to Concord being well-suited to long-term observations of a wide variety of organisms that occur in a diversity of habitats.

Several environmental characteristics of Concord are of particular relevance to plant and animal phenology and the long-term data set presented here. This region is within the warm summer subtype of the humid continental climate (Dfb) in the Köppen-Geiger climate classification (Beck et al. 2018), with year-round precipitation, winter months that are often below freezing, and summers that are warm and humid. Transitions between seasons are times of temperature variability, with, for example, frost or snow events that may occur after spring thaw and the start of the growing season. An advantage of working in Concord is that it is only 43 km to the Blue Hill Observatory, which is the oldest continuously operating weather observatory in the United States, with weather records going back to the 1830s (Magee et al. 2014). This region of southern New England is also noteworthy for having an increasing trend of warming temperatures that is approximately twice as fast as the average rate of warming for the United States as a whole (Miller-Rushing and Primack 2008; Heberling et al. 2019).

Plant phenology in temperate regions of the northern hemisphere is generally negatively correlated with spring temperature, leading to earlier leafing (Polgar and Primack 2011, Everill et al. 2014, Polgar et al. 2014b, 2014a), flowering (Primack et al. 2004, Miller-Rushing and Primack 2008, Ellwood et al. 2013) fruiting (Gallinat et al. 2015, 2018) and spore release in warmer years (Ellwood et al. 2011). The impacts of climate change on bird migration are much weaker than they are for plants, and are less dependent on local temperatures at arrival sites (Miller-Rushing et al. 2008, Ellwood et al. 2010, 2015). Long-term records, such as those initiated by Thoreau and presented here, have been critical in advancing our understanding of the effects of climate change on migration timing.

Because of the strong link between local temperature and plant phenology, the first individuals of a plant species to exhibit a given phenophase are likely to be found in the warmest microclimates. Like Thoreau, we sought flowering plants in warm microclimates. During the 2003 field season, we spent considerable effort finding where plants occurred and where they first flowered and leafed out in Concord; when we located plants of a particular species in this initial season, they were often already in flower or leaf. Thus, the data from that season were not useful for comparison with earlier records, and are omitted here (Miller-Rushing and Primack 2008).

Sampling design

Thoreau flowering, leaf out, fruiting, and bird arrival—From 1851-1858, Henry David Thoreau took walks around Concord and recorded his observations in journals (Figure 2). His primary motivation for doing this was to gain a better understanding of the plants and animals around him, as well as to record the cyclical phases of development and movement for these organisms. He later compiled these observations into tables for particular years. Thoreau’s botanical tables are now housed in the Morgan Library in New York and his bird observations are at the Ernst Mayr Library, Museum of Comparative Zoology (MCZ), Harvard University Cambridge, Massachusetts. While Thoreau’s methods, such as the amount of time spent on his field excursions or his exact walking routes, are unknown, his dated journal entries and descriptions provide insight into the fact that he sought the earliest leaves, flowers, and fruits and often found them in the warmest locations. Thoreau’s flowering, leafing and bird arrival records are available as single dates of observation in given years. Fruiting phenology data, however, are given as representative dates, in which a single value is provided as an estimated date of “first”, “peak”, or “last” fruiting phenophase based on observations made from 1850-1860 (Thoreau 2001).

Hosmer flowering—From 1878-1902, Alfred Hosmer, an amateur botanist who was inspired by Thoreau, continued flowering observations in a way comparable to those of Thoreau, and also recorded estimates of species relative abundance in Concord and the surrounding area. Hosmer also recorded spore release dates of fern and gymnosperm species that are included in this data set. Hosmer’s data are in the form of an unpublished book housed in Special Collections at the Concord Free Public Library.

Eaton flowering—Richard Eaton was an accomplished botanist who conducted fieldwork in Concord during the mid-twentieth century. Eaton published a flora of Concord (Eaton 1974) and recorded estimates of species relative abundance.

Primack and Miller-Rushing flowering—Primack and Miller-Rushing transcribed relevant entries of Thoreau’s plant phenology tables into digital spreadsheets. From there, Primack and Miller-Rushing added comparable records of flowering times from Alfred Hosmer and species relative abundance from Hosmer and Richard Eaton (Miller-Rushing and Primack 2008, Primack et al. 2009). In 2003, Primack and Miller-Rushing began searching Concord for the earliest flowers (Primack and Miller-Rushing 2012). The data from this pilot year are not included here as species were often first located well after they had started to flower (Miller-Rushing and Primack 2008). From 2004 to the present, Primack, along with undergraduate and graduate students and various colleagues, visited Concord two to three days a week to search for and record flowering times of the same species of plants described in Thoreau’s journals. Plants were recorded as flowering when the first open flowers of the species were found anywhere in Concord.

Primack and Polgar leaf out—In 2009, Primack along with Caroline Polgar and others started to also record leaf-out dates of common woody trees and shrubs that were observed by Thoreau, following a similar method to the flowering field work (Polgar et al. 2014a). Species were recorded as leafing out when a plant could be found anywhere in Concord in which the leaves were unfolded on at least three branches on one tree and their adult shape was discernable, even if the leaf was still small and unexpanded.

Relative abundance—All phenological and abundance observations were of plants in situ, in habitats that ranged from protected conservation areas to relatively disturbed in-town areas such as train stations and roadsides. Plants growing in cultivation, especially from current and abandoned ornamental gardens, were not included, except in the few cases where Thoreau recorded the leafing out time of woody nonnative plants, such as lilacs and apples. Data reflect the phenology and relative abundance at the scale of the Town of Concord, though Hosmer included observations made in the surrounding vicinity as well. Hosmer and Eaton used many terms to describe species relative abundance but did not define them quantitatively. Primack and Miller-Rushing defined the following categories: very common (found throughout the area), common (occurring in >3 localities), frequent (occurring in 3 localities), infrequent (occurring in 2 localities), rare (occur in 1 locality), and very rare (10 or less individuals in a single locality. For all observers, abundance data represent the result of several years of observations, rather than annual assessments of abundance, which can be influenced by species that may be present but not detectable in each year (Chen et al. 2013). In the case of Primack and Miller-Rushing, observations on abundance were based on five years of fieldwork (2003-2007) with some later updates. See Willis et al. (2008) and Primack et al. (2009) for more details on how we treated abundance categories in our own analyses. Readers might want to take note of two articles by Angelo (2014 a, b) and our replies (Primack and Miller-Rushing 2014, Primack et al. 2014) on the effectiveness and rigor of our methods. Also, as this article was in the final stages of review, Ray Angelo (2021) made available on-line the flowering times of Concord plants, and their relative abundances based on observations and flowering times based on herbarium specimens (https://archive.org/details/calendar-flowering-times-concord-massachusetts/page/n11/mode/2up). Detailed information the distribution and past collection of species is provided by Angelo (2020).

Bird arrivals—Thoreau also faithfully recorded the first arrivals of migratory birds each spring and sought favorable locations for these observations. William Brewster, founder of the American Ornithologists’ Union and the Nuttall Ornithological Club, and curator of birds at the MCZ, recorded observations of migratory birds for the years 1886 and 1900-1919 and donated these to MCZ’s library. Ludlow Griscom followed in Brewster’s footsteps in many ways – he was president of the American Ornithologists’ Union and the Nuttall Ornithological Club and was also curator of birds at the MCZ. He authored The Birds of Concord (Griscom 1949) and Birds of Massachusetts (Griscom and Snyder 1955) and his observations of migratory birds for the years 1930, 1931, and 1933-1954 are included here. Griscom’s records are housed at the Peabody Essex Museum in Salem, Massachusetts. Rosita Corey, a schoolteacher in Concord and avid birder, provided us with her observations of migratory birds that span two distinct periods, 1956-1973 and 1988-2007. These observers produced numerous arrival records for 22 species, totaling 1,106 data points. There are also observations available for fewer years for some species, which are not included here.

Ice out—In addition to phenology observations, Thoreau also recorded the date of ice-out of Walden Pond (1845-1854, discontinuous). The timing of ice-out is a further metric of warming temperatures in Concord, and the impact of climate change on organisms and the environment. Ice-out is not explicitly defined in Thoreau’s records, but a common definition, and the one used in contemporary observations, is when ice covers less than ten percent of the pond and does not return later in the winter or early spring. Our dataset includes ice-out records for 1995-2014 from the citizen science project, Journey North (journeynorth.org).

Phenology definitions—Most past observers (e.g., Thoreau, Hosmer) did not explicitly define the phenophases they observed, but we were able to infer their definitions based on material in their journals, notes, and data. Based on the historical materials, we defined the phenophases as below. See referenced literature for more information.

• Leaf out: the date that at least one fully unfolded leaf on at least three branches of an individual plant is observed (Polgar et al 2014)
• First flower: the date that a single open flower with petals extended such that it can be pollinated is observed
• First fruit: the date that ripe fruit is first observed (Miller et al 2021)
• Peak fruit: the date or estimated date of ‘prime’ fruiting (2 of 31 Peak fruit dates are estimated; see Miller et al 2021 Table 1). While this likely refers to the time when the majority of plants have ripe fruits or the majority of fruits on a plant are ripe, Thoreau did not record a precise definition for this phenophase.
• Last fruit: the date or estimated date that the last ripe fruit is observed (26 of 29 Last fruit dates are estimated; see Miller et al 2021 Table 1)
• Spore release: the date that the first spore capsule, or numerous capsules, are observed to be open
• First arrival: the date when the first individual of a migratory bird species is observed (Ellwood et al. 2010)
• Ice out: the date when ice covers <10% of Walden Pond, and does not return later in the season (Primack 2014; journeynorth.org)

Dates of observations— Dates of observations were recorded as the day of year such that during most years January 1 = 1, January 2 = 2, March 1 = 60, … and December 31 = 365. During leap years the calendar has 366 days and therefore February 29 = 60, March 1 = 61, etc. Thoreau observed fruiting phenology from 1850-1860 and compiled them in aggregate for that time period. We therefore present records of fruiting phenology during this time as decadal, as opposed to annual, to reflect his aggregated observations (Miller et al. 2021).

Other data—Our research based on these records has also benefited from the inclusion of other types of data sources such as photographs, herbarium specimens, and observations from gardeners and birdwatchers, (e.g., Primack et al. 2004, Primack and Miller-Rushing 2012, Everill et al. 2014, Willis et al. 2017, Gallinat et al. 2018, Miller et al. 2021, Miller-Rushing et al. 2021), though we do not include those data in this dataset. Most notably, the Concord Public Library has a large collection of dated photographs taken by the landscape photographer Henry W. Gleason (1855-1937), many of which are of wildflowers. Working with a combination of data types has helped to fill in spatial, taxonomic, and temporal gaps.

Total records—Since Thoreau’s first observations in the 1850s, a total of 13,441 phenological records were documented in Concord and are presented here. These data include phenology and/or abundance data for 958 species. Additional information about the observations can be found in Thoreau’s and Hosmer’s notebooks, Eaton’s book and our publications.

Taxonomy and systematics

Plant names have changed considerably since Thoreau’s time, and have continued to change in the past several decades. In our data set, we have used the names given by Eaton (1974), as these are widely known and used in our region. Users of our data set may wish to consult other data sources for updated names.