Spring phenology of temperate forest trees has advanced substantially over the last decades due to climate warming, but this advancement is slowing down despite continuous temperature rise. The decline in spring advancement is often attributed to winter warming, which could reduce chilling and thus delay dormancy release. However, mechanistic evidence of a phenological response to warmer winter temperatures is missing. We aimed to understand the contrasting effects of warming on plants leaf phenology and to disentangle temperature effects during different seasons.

With a series of monthly experimental warming by ca. 2.4 °C from late summer until spring, we quantified phenological responses of forest tree to warming for each month separately, using seedlings of four common European tree species. To reveal the underlying mechanism, we tracked the development of dormancy depth under ambient conditions as well as directly after each experimental warming. In addition, we quantified the temperature response of leaf senescence.

As expected, warmer spring temperatures led to earlier leaf-out. The advancing effect of warming started already in January and increased towards the time of flushing, reaching 2.5 days/°C. Most interestingly, however, warming in October had the opposite effect and delayed spring phenology by 2.4 days/°C on average; despite six months between the warming and the flushing. The switch between the delaying and advancing effect occurred already in December. We conclude that not warmer winters but rather the shortening of winter, i.e. warming in autumn, is a major reason for the decline in spring phenology.

Any information on the experiment, it´s background and detailed methods, can be found in the manuscript:

Beil, I., Kreyling, J., Meyer, C., Lemcke, N., & Malyshev, A. V. (2021). Late to bed, late to rise—Warmer autumn temperatures delay spring phenology by delaying dormancy. Global Change Biology, 27, 5806–5817. https://doi. org/10.1111/gcb.15858

The following datasets are provided:

Autumn leaf senescence
We assed autumn leaf senescence before, during, and after a warming manipulation in September and October, respectively. For quantifying chlorophyll degradation, we used an SPAD 502 Plus Chlorophyll Meter. The values are unitless “SPAD values”.  In order to avoid selection bias, we always choose the greenest leaves. Since the Chlorophyll Meter does not give meaningful values for brown leaves, we set the value to 0 for brown or fallen leaves.

Dormancy depth ambient
To track the pattern of dormancy induction and development, we assessed bud dormancy depth on seedlings that were purely grown in the common garden, without any warming manipulation. Every week, from August 2018 onward until spring flushing, we transferred three seedlings of each species from the common garden into a warm greenhouse (“forcing greenhouse”) with optimal growing conditions (21.5°C ± 2.3°C SD and 16 h light). The time, or better the growing degree days (GDD), that it took each seedling to unfold the first leaves was used as a measure of its dormancy depth.
GDDs were calculated as:
                                               GDD = _t0^t₁T_mean - T_base                                   

where t₀ is the starting day at which the seedling was transferred to the warm, t₁ is the day at which leaf unfolding was observed, T_mean is the daily mean temperature, and T_base is a constant set to 5°C.

Dormancy depth after each temperature manipulation
Directly after each warming manipulation, we assessed dormancy depth in the same way as for the “ambient” seedlings.

Spring leaf flushing in response to warming
We observed spring phenology three times per week and recorded the day, at which first leaf was unfolded, defined by the petiole being visible.

Temperature and climate data ambient
Temperature, precipitation and rel. humidity at the common garden are given. The seedlings were placed here most of the time, unless they were in the greenhouse for temperature manipulation. The data were recorded in 2 m height, ca. 8 m away from the seedlings.

Temperatures during warming manipulation
Temperature in forcing greenhouse
We worked with three different greenhouses. The first two where used for warming manipulation from autumn till spring, with temperatures being slightly above outside in the control compartment) and temperatures being ca. 2.4°C above the control in the warming compartment, respectively. The third greenhouse was used as a forcing chamber to assess dormancy depth. It was set to optimal growing conditions with ca. 21.5°C and 16 h light.