The influence that climate has on natural and managed systems and the rhythm that it imposes (Helm et al. 2013; Visser et al. 2010) on the timing of plants and animals’ key life stages—such as flowering, fruiting, and migration—has been noted since ancient times (Aitken 1974; Woodward and McTaggart 2019). These life stages, in turn, drive a wide range of processes at the community and population levels, at local and regional spatial scales.

Phenology is the study of these rhythms and processes (Lieth 1974). Phenological studies have featured in the International Journal of Biometeorology from the late 1950s—the decade in which it was first published (Donnelly and Yu 2017; Sheridan and Allen 2017). However, from the late 1990s, phenological studies were a significant theme (Donnelly and Yu 2017; Sheridan and Allen 2017), as evidence for and concerns about changes to the global climate system grew.

The papers in this supplement consider climate change and are based on the international conference “Phenology 2018: One planet, two hemispheres, many regions” which was held in Melbourne, Australia. This was the fifth international meeting focusing on phenology (Donnelly et al. 2011; Schwartz and Donnelly 2014; Menzel 2000; Chmielewski 2015), supported by the International Society of Biometeorology.

As with each of the conferences, the integrative approach within phenology (Schwartz 2013) is highlighted. At this conference, there were 100 delegates from 30 countries representing many different disciplines (e.g., ecologists, foresters, geographers, health scientists, horticulturists, modelers, meteorologists, statisticians). The topics addressed were across the following themes: aerobiology, agricultural phenology, phenological methods, phenology and citizen science, phenology and conservation biology, remote sensing, traditional ecological knowledge, tropical phenology, and urban phenology.

Although each of the themes are relevant to plants and animals, the conference was clearly plant and vegetation focused (Fig. 1). The final selected papers reflect this.

Fig. 1
figure 1

Word cloud based on Phenology 2018 abstracts

Full size image

Recognizing that studies on shrubs are limited, despite the role that they play in ecosystem function (e.g., carbon storage, habitat, soil stability), Donnelly and Yu (2021) undertook a review to identify research and knowledge gaps in the phenology of shrubs in deciduous, temperate forests. Their findings reinforce the lack of studies, with only 32 papers meeting their criteria. All the studies occurred in the Northern Hemisphere, with 21 being from the USA. The majority (> 70%) used data that had been collected by direct observation and involved the study of invasive species, with one species Amur honeysuckle (Lonicera maackii) featuring prominently.

The authors advocate for a greater geographical range for monitoring of shrub and tree phenology together—given their interrelationship and combined contribution to forest ecosystem functioning. In addition, they make six recommendations for future research, including quantifying the length of time between leaf out and fall for shrubs and trees and the implications that change in tree leafing has for shrubs.

Moving from the understory to the canopy, Denéchère et al. (2021) note that individuals and their traits contribute to populations, but as with shrub phenology, they observe that within-population variability is not well studied. Hence, they examine the within-population variability (defined by standard variation) of leaf budburst and senescence of nine species variously distributed across four countries (England, France, Germany, and Romania). Species was a contributing factor to variation in both budburst and senescence. They found that warmer spring temperatures reduced the within-population variability in budburst, as development was faster. Conversely, later occurrence of senescence and warmer temperature resulted in greater variation. The authors determined that a sample size of 28 for budburst and 23 for leaf senescence was required to have a precision of 3 and 7 days, respectively.

Based on standard phenological observations and measurements of individuals taken weekly, Langvall and Löfvenius (2021) present a method to quantify the average rates of development and interannual variability for leaves from dormant bud to final size and fruits from flowers to ripe fruits. The method is illustrated using data gathered on three tree species and two shrubs across an altitudinal gradient from forest sites in Sweden, during the period 2006–2017. The method is most applicable to phenological phases which are correlated with air temperature and have a long season of vegetative growth.

Wang et al. (2021) also examine the development time of budburst, but of flowers as well as temperature sensitivity in an experimental setting. Twigs of the three trees and two shrubs were planted in growth chambers and subjected to temperatures between 5 and 30 °C. Their results reinforce that the relationship between phenophases and temperature is often non-linear (Keatley and Hudson 2010; Sparks et al. 2000) and highlight that temperature sensitivity was reduced with increasing temperature.

Highlighting the value of long-term observational data, Xu et al. (2021) investigate the chilling and forcing requirements for leaf unfolding of four tree species across temperate and sub-tropical regions in China, via partial least square regression and chilling models (dynamic and positive Utah models) and forcing temperatures (growing degree hour). They found differences between regions in the length of the chilling period: the temperate regions being longer, as chilling commenced earlier and finished later than the subtropics. This also resulted in higher chilling requirements. Although there was no difference between the regions in the length of forcing periods, plants in the temperate region required lower forcing temperatures. The authors also discuss the limits of model parameterization of the two chilling models they used and highlight that further experimental work would be of benefit in understanding local adaption of the different species.