Amanda Gallinat, Colby College, USA, and Theresa Crimmins, USA National Phenology Network, discuss their article: Combined volunteer and ecological network observations show broad-scale temperature-sensitivity patterns for deciduous plant flowering and leaf-out times across the Eastern USA
Around the globe, the timing of seasonal activity in plants and animals – termed phenology – is shifting substantially in response to warming temperatures. These changes have far-reaching ecological and economic consequences. However, understanding how changes in the timing of seasonality is affecting the hundreds of thousands of plant and animal species across large regions, such as the entirety of the U.S., is very challenging.
Our best option for compiling observations on lots of species across the country is to engage as many people as possible in documenting what they see happening. Through the Nature’s Notebook platform, the USA National Phenology Network enlists both volunteers and professionals in tracking the seasonal activity of plants and animals. The program is now in its 17th year of data collection and more than 42 million records of plant and animal phenology have been contributed from across the country, primarily by volunteer observers.
While these observations are our most promising resource for understanding how species are responding to rapidly changing conditions, some have wondered whether volunteer-contributed records are of sufficient quality to be used in assessments of these changes. To answer this question, we compared observations contributed to Nature’s Notebook by volunteers across the country to observations of the same species collected by professionally trained scientists at National Ecological Observatory Network (NEON) sites.
We found that volunteers’ reports yielded very similar patterns to those recorded by NEON scientists. These findings boosted our confidence that we and other scientists could use the combined dataset from both approaches in large-scale evaluations of phenological change.
The majority of studies documenting phenological changes have evaluated observations collected at a single site or a handful of sites, and they frequently only evaluate a small number of species. In this study, we tested whether several patterns that have been established in previous studies hold when scaled up to over 100 species, across the Eastern USA. The primary response variable we evaluated across species was their sensitivity to temperature, which indicates how many days plants advance (or delay) the timing of their activity – leaf-out or flowering – in response to warming.
Previous studies have indicated that both introduced species and species active earlier in the season are often more sensitive to temperature changes. We assessed differences in species’ sensitivity to temperature between native and introduced species as well as their relative timing of leaf-out and flowering within the spring season. We also evaluated differences in sensitivity among trees, shrubs, and forbs. This is an open question in the literature: previous studies have reported conflicting findings regarding which functional groups are most responsive to temperature, with differing implications for competition among plants for spring light. Finally, we explored whether species vary in their sensitivity to temperature across their ranges. Few previous studies have been able to address this question, as it necessitates observations collected consistently across large regions.
Our findings largely confirmed patterns that have been established in smaller-scale studies. Introduced species tend to show greater sensitivity to temperature than native plants, shifting the timing of both leaf-out and flowering to a greater degree than native species. While this feature gives them a competitive advantage in the spring, enabling earlier growth and access to nutrients and sunlight, it can also put them at greater risk of damaging frost events.
Among functional groups, we found that shrubs and forbs generally showed greater sensitivity to temperature than trees, suggesting that they may be able to maintain windows of carbon capture prior to shifting canopy closure under climate change. And consistent with many previous studies, we found that species active earlier in the season exhibited greater temperature sensitivity than later-season species.
Finally, our observations, collected across a large geographic region, showed that plants in the southern reaches of species’ ranges showed stronger sensitivity to temperature than those at northern latitudes. This finding, which is consistent with experimental work, suggests that individuals in more northern, colder latitudes, are more conservative with their springtime activity, only initiating leaf-out or flowering once they have been exposed to a particular amount of chill in the winter or days have lengthened sufficiently. These requirements protect plants from exposing sensitive tissues too early in the spring season.
This study underscores the value of large-scale data to unite and verify more broadly the trends previously observed only in smaller-scale studies. Importantly, our comparison of observations contributed by volunteers and professionals demonstrates the suitability of volunteer-contributed data in scientific evaluations.
Nature’s Notebook participants are building a multi-taxa, large-scale phenology dataset that is unprecedented for the U.S. and critically important for furthering our understanding of how species and ecosystems are responding to rapidly changing conditions. As these and NEON-collected observations continue to grow, so do the opportunities to study broad-scale phenological trends.
