DOI: 10.5061/dryad.866t1g1s3
title: Late to bed, late to rise—Warmer autumn temperatures delay spring phenology by delaying dormancy

description: 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.
description: 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. <i>Global Change Biology</i>,
 27, 5806–5817. https://doi. org/10.1111/gcb.15858
description: The following datasets are
 provided: <b>Autumn leaf
 senescence</b><br> 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.
 <b>Dormancy depth ambient</b><br>
 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.<br> GDDs were calculated as:<br>
                                                <i>GDD</i> =
 <sub><i>t</i><sub>0</sub></sub><sup><i>t</i><sub>1</sub></sup><i>T</i><sub><i>mean</i> </sub>- <i>T</i><sub><i>base</i></sub>                                    where t<sub>0</sub> is the starting day at which the seedling was transferred to the warm, t<sub>1</sub> is the day at which leaf unfolding was observed, T<sub>mean</sub> is the daily mean temperature, and T<sub>base</sub> is a constant set to 5°C. <b>Dormancy depth after each temperature manipulation</b><br> Directly after each warming manipulation, we assessed dormancy depth in the same way as for the “ambient” seedlings. <b>Spring leaf flushing in response to warming</b><br> We observed spring phenology three times per week and recorded the day, at which first leaf was unfolded, defined by the petiole being visible. <b>Temperature and climate data ambient </b><br> 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. <b>Temperatures during warming manipulation<br> Temperature in forcing greenhouse</b><br> 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.