In this issue of Nature Climate Change and in an online collection we feature research and opinion content focusing on the various aspects of changing tree phenology, and its broad implications. We chose to time the collection for our April issue, which falls closest to the boreal start of spring at the equinox, 20 March. Yet at the time of writing, although the spring equinox has barely passed, trees in parts of the north have already erupted into blossom or bright leaf, and early flowering bulbs have already faded to darker shades of green. Spring has come early — an observation that holds not just for 2022, but is part of a long-term trend of advancement, driven by climate change.

Credit: Oleg Elkov / Alamy Stock Photo

Plants and animals time important life events to optimize their survival and reproductive success. For (deciduous) trees, this includes releasing leaves and flowers at times to best coincide with resources such as light and pollinators, and letting leaves colour and fall before frosty winters set in. Because tree phenology is influenced by temperature, it is not surprising that climate change has led to phenological shifts: the recent assessment report from the IPCC (https://www.ipcc.ch/report/ar6/wg2/) states with very high confidence that climate change has driven early spring, and notes phenological shifts as a major effect of climate change on plants and animals alike.

Studies have consistently indicated that spring advancement and (to a lesser extent) delayed autumns will lead to longer growing seasons in temperate regions, with implications for primary productivity, carbon capture and further future warming. Nature Climate Change has published an array of papers that investigate the mechanisms behind altered tree phenology, and project productivity changes. In addition to this angle, phenological shifts of diverse organisms are in and of themselves a climate change issue, and we maintain a strong interest in research on the mechanisms and impacts of these shifts, particularly within the context of complex ecosystems.

Tree phenology is not just measurable, it is also highly visible, and as such has long captured the eye (and pen) of observers. Climate change science can suffer from a lack of long-term data, but fascination with the start of spring has helped buck this trend for spring phenology observations. In a Correspondence in this issue, Vitasse and colleagues display the world’s five longest time series of spring plant blooming and leaf-out, including records of cherry tree (Prunus jamasakura) blossoms from Kyoto, Japan that stretch back to 812 CE. While showing recent phenological shifts, the authors also demonstrate how — in capturing public interest — tree phenology can be a vital tool for teaching and engagement with climate change science. In line with this, the Correspondence mentions Jean Combes, a citizen scientist who has collected spring leaf-out times of four common UK tree species for nearly 75 years. In our Feature, we discuss how Jean’s love of nature translated to a long-term phenology record, and how the collective work of citizen scientists across the United Kingdom (and the world) has resulted in a growing and invaluable phenology dataset.

The close link between tree phenology and citizens is continued in a Correspondence by Zhou, which spotlights phenological shifts in urban environments. Zhou urges consideration of the diverse human services linked to phenological events. Tree springtime blooms, for example, may be cultural signifiers of spring, tourist attractants, or early stages of tree fruit production, but can also have implications for human health in the form of allergenic pollen output. The Correspondence highlights the complexity of urban environments, the lessons we can learn from this, and the considerations necessary under continued climate change and increased urbanization.

Changes in phenology must be considered not only in the context of human impacts, but should also be integrated into conservation action. In their Comment, Ettinger and colleagues describe three areas in which including phenology can provide opportunities to optimize conservation. Among other points, the authors highlight the role for phenological plasticity in maximizing species persistence under continued change, and the need to consider conservation in the joint context of species that are shifting in space and in time.

Phenological shifts are one of the most visible impacts of climate change, and the links between shifts and anthropogenic climate change have now been recognized for over 20 years. Nonetheless, research continues to unravel complexities in the mechanistic causes and impacts of shifts, and the future implications. While our collection focuses on tree phenology, there is much to be learned still of other primary producers, and other organisms across trophic levels, both individually and in the context of complex ecosystems.