Warmer temperatures have been associated with an earlier emergence of spring leaves each year. New data, however, suggest that leaf emergence is becoming less sensitive to temperature as global temperatures rise. See Letter p.104
For centuries, people have been fascinated by the timing of the arrival of spring, a season named for the 'springing forth' of the leaves of deciduous trees. It has long been known that spring leaf emergence is strongly linked to temperature1,2 — even in ancient Rome, Pliny the Elder realized that leaf emergence was a much better indicator of weather than were the constellations3. Leaves have emerged earlier over the past century, as spring has become warmer. With global anthropogenic emissions currently exceeding previous worst-case scenarios4, considerable warming is expected in the coming decades. Will future warming lead to even earlier and greener springs? In this issue, Fu et al.5 (page 104) report results suggesting that the relationship between the seasonal timing of leaf emergence — spring phenology —and temperature is changing.
The relationship between spring temperatures and leaf emergence has allowed scientists of the Intergovernmental Panel on Climate Change to use changes in the timing of emergence as a key indicator of the ecological impact of climate change6. Apart from resulting in a greener spring, earlier leaf emergence affects various aspects of ecosystem function, and generates multiple feedbacks to the climate system7. It has thus been built into state-of-the-art Earth-system models, which predict a large advance in the timing of leaf emergence under future climate warming. To test the relationship between leaf emergence and warming, Fu and colleagues examined 33 years of observations of 7 forest species across 1,245 sites in Europe. Surprisingly, they discovered that spring leaf emergence has been getting less sensitive to temperature over time (Fig. 1). Their observation-based results call into question current model projections, and suggest that spring leaves might not emerge as early under future warming as had been previously expected.
Although it is generally accepted that temperature is the dominant driver of spring phenology in temperate deciduous forests, there is considerable uncertainty about the pathways of temperature's influence, with little agreement between models, experiments and observations8,9. The timing of warming matters10, and the response seems to vary by species and perhaps by location or population11. Many other factors could also play a part — primarily day-length (photoperiod) and winter-dormancy requirements, but also humidity and temperature variance. Photoperiod has been shown to have a strong influence on some species, particularly on the Fagus (beech) genus, for which the effect of warm temperatures is limited if the days are too short12.
Many species have also been shown to require a certain amount of chilling in winter before their release from dormancy13. This evolutionary mechanism, which is designed to prevent the costly damage a late frost can inflict on young leaves, ensures that winter has truly passed before leaves emerge. Changes in any of these factors could potentially modify the temperature response of leaf emergence, and explain the observed decline in temperature sensitivity reported by Fu and colleagues.
The authors tested three hypotheses to examine the potential underlying causes of their observations. They assessed the role of photoperiod, but could neither confirm nor rule out its influence. They also found no significant changes in the timing of leaf emergence due to temperature-variance changes, which suggests a limited role for this factor. The third hypothesis tested was that warmer winters had resulted in reductions in winter chilling, which could dampen the response of spring leaf emergence to a warmer spring. Using multiple models, the authors showed that declines in winter chilling could indeed lead to a lower temperature sensitivity, although the change in temperature sensitivity predicted by the models was considerably smaller than that observed.
This long-term trend of a decline in winter chilling, in concert with a decline in the temperature sensitivity of spring leaf emergence, raises questions about the extent to which factors such as chilling requirements are already limiting the response of spring phenology to climate warming. However, the association falls short of a causal attribution, as the authors note, because temperature sensitivity was not observed to be markedly different in years that had more chilling than in years with less chilling. Furthermore, not all deciduous plants have chilling requirements, and many have low requirements that are met even under experimental warming14. For most species, the effect of chilling requirements is poorly understood.
The declining temperature sensitivity reported by Fu and colleagues is intriguing, but its root cause is still uncertain. More research is needed to assess whether other species and locations demonstrate a similar decline in temperature sensitivity — in particular, to examine the many other long-term records around the world, in combination with satellite observations of vegetation, experimental data and theoretical understanding.
Leaves emerge in spring as a result of responses that are hard-wired into the genetic code of trees. This might suggest that the response of phenology to environmental drivers should be highly predictable15, yet we are far from having a predictive science of phenology. Observations such as those presented by Fu et al., which challenge models and contemporary understanding, go a long way towards getting us there.Footnote 1
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