The phenology of an organism is comprised of numerous phases. In the example we present here of a common eastern tree species, Acer rubrum (red maple), its spring phenology begins with flower development, followed by fruit development, and lastly leaf development. Within each of these categories, the phenology can be examined on a finer scale, e.g., flower buds visible, budbreak of flowers, mature flowers with identifiable male and female parts, flowers senescing, etc. Much of the research done to date has investigated phenology using a small number of phenophases, e.g., first flower or first leaf out, or has investigated finer-scale phenology for a limited number of years or locations. Here we examine fine-scale phenology of Acer rubrum across its range in the eastern United States and southeastern Canada for over 100 years. The only known source of these data is herbarium specimens. We examined thousands of herbarium specimens in person and online, and coded their state of reproduction on a scale of 0-9 where stage 0 is no flowerbuds visible, stages 1-8 follow flower and fruit development, and stage 9 where seeds have dispersed. We also did this for leaf phenology on a scale of 0-4 where 0 is no leaves and 4 is fully mature leaves. We related these phenological stages to abiotic variables including elevation and latitude.
We found that the timing of flowering and fruiting varied widely over Acer rubrum's geographic range. Latitude and elevation were significant predictors of reproductive phenophases (p<0.001, R2=0.15 for latitude and p=0.02, R2=0.04 for elevation). These same factors however, are not significant predictors of leaf out (p=0.042, R2=0.00 for latitude and p=0.77, R2=0.00 for elevation). Thus, large-scale latitudinal trends differed between reproductive and leafing phenophases, with the timing between flowering and fruiting of Acer rubrum more condensed in the north and lengthened in the south. In Massachusetts, for example, flowering has generally occurred by April 27, with fruiting 40 days later, and leafing 5 days after fruiting. In north Florida, on the other hand, there are generally 55 days between flowering and fruiting, then leafing 17 days after fruiting. Preliminary analyses suggest that categorizing phenological stages into more fine-scale categories can provide more information on phenological stages, but at the cost of greater observer error. Our results suggest that for commonly collected species, herbarium specimens can provide valuable information on phenology at broad spatial scales and long temporal scales.