Plant phenology is driven by local environmental conditions such as meteorology and soil. In temperate and boreal regions, temporal variations in plant phenology are dominated by changes in temperature [1]. For example, budburst and leaf unfolding are controlled by the occurrence of warm spring temperatures [2], typically referred as “forcing” and quantified as the daily accumulated air temperature above a certain threshold over the preseason (i.e. period preceding budburst). On the opposite, leaf senescence is partly governed by the occurrence of cold temperatures in autumn [3].

For these reasons, rising temperatures have globally lengthened the growing season length of vegetation by advancing leaf unfolding in spring and delaying leaf unfolding in autumn over the past decades [4,5]. However, the sensitivity of vegetation to temperature change is complex and heterogeneous, with variations observed among species and regions [6]. In addition, recent evidence suggests that European forests are getting less sensitive to warming [7].

Indeed, tree phenology is not solely driven by temperature but is co-limited by multiple factors. About 35 % of Northern tree species is thought to rely directly on light for leaf unfolding as a safety mechanism against late frost damages [8], while water availability is shown to be a major driver of leaf unfolding in arid and semi-arid places [9], and of leaf senescence in general [3,10]. In addition, several observations highlighted significant effects of soil composition and nutrient availability [11], as well as the effect of tree seasonality on plant phenology. For example, summer temperatures can impact leaf senescence as well as leaf unfolding the following year [12].

Tree phenology is adapted to previous environmental conditions [13] and we still don’t know if it is going to keep pace with global change. The heterogeneity of phenological responses induced by the environmental co-limitation leads to poor understanding and future projections when only temperature is considered. It stresses the need for concomitant observations of both phenology, local meteorology and soil properties if we want to improve our understanding of the underlying processes governing plant phenology.

This graph shows the evolution of canopy greenness (green) and daily mean temperature (°C), incoming radiation (W/m²) and precipitation sum (mm) for 2016. (Example from PhenoCam and CRU-NCEP data, Duke forest, US.) Click on variable name’s in legend to remove/add the corresponding curve. Show references

[1] Schwartz, M. D. & others. Phenology: an integrative environmental science. (Springer, 2003)

[2] Hänninen, H. & Kramer, K. A framework for modelling the annual cycle of trees in boreal and temperate regions. Silva Fenn. 41, 167–205 (2007).

[3] Delpierre, N. et al. Modelling interannual and spatial variability of leaf senescence for three deciduous tree species in France. Agric. For. Meteorol. 149, 938–948 (2009)

[4] Cleland, E. E., Chuine, I., Menzel, A., Mooney, H. A. & Schwartz, M. D. Shifting plant phenology in response to global change. Trends Ecol. Evol. 22, 357–365 (2007)

[5] Peñuelas, J. & Filella, I. Responses to a warming world. Science 294, 793–795 (2001).

[6] Wang, S. et al. Temporal trends and spatial variability of vegetation phenology over the Northern Hemisphere during 1982-2012. PloS One 11, e0157134 (2016)

[7] Fu, Y. H. et al. Declining global warming effects on the phenology of spring leaf unfolding. Nature 526, 104–107 (2015).

[8] Zohner, C. M., Benito, B. M., Svenning, J.-C. & Renner, S. S. Day length unlikely to constrain climate-driven shifts in leaf-out times of northern woody plants. Nat. Clim. Change 6, 1120 (2016)

[9] Peñuelas, J. et al. Complex spatiotemporal phenological shifts as a response to rainfall changes. New Phytol. 161, 837–846 (2004).

[10] Xie, Y., Wang, X. & Silander, J. A. Deciduous forest responses to temperature, precipitation, and drought imply complex climate change impacts. Proc. Natl. Acad. Sci. 112, 13585–13590 (2015).

[11] Classen, A. T. et al. Direct and indirect effects of climate change on soil microbial and soil microbial-plant interactions: What lies ahead? Ecosphere 6, 1–21 (2015).

[12] Fu, Y. S. et al. Variation in leaf flushing date influences autumnal senescence and next year’s flushing date in two temperate tree species. Proc. Natl. Acad. Sci. 201321727 (2014).

[13] Peaucelle et al. Spatial variance of spring phenology in temperate deciduous forests is constrained by background climatic conditions. Nat. Comm. (2019) (Accepcted)