Climate-induced shifts in leaf unfolding and frost risk of European trees and shrubs

Abstract

Climate warming has advanced leaf unfolding of trees and shrubs, thus extending the growing period but potentially exposing plants to increased frost risk. The relative shifts in the timing of leaf unfolding vs. the timing and intensity of frost events determine whether frost risk changes under climate warming. Here we test whether the frost risk for unfolding leaves of 13 European tree and shrub species has changed over more than 60 years using dynamic state-space models and phenological observations from 264 sites located between 200 and 1900 m a.s.l. across Switzerland. Trees and shrubs currently feature sufficient safety margins regarding frost risk, which increase from early- to late-leafing species and tend to decrease with increasing elevation. Particularly after 1970 to 1990 and at higher elevations, leaf unfolding has advanced across all species. While the time between the last critical frost and leaf unfolding has shifted from predominantly positive trends in the late 1950s and 1960s to a trend reversal since the 2000s, the minimum temperature during leaf unfolding has mostly increased since the 1980s. These dynamic shifts in leaf unfolding and frost risk demonstrate species- and site-specific responses of trees and shrubs to climate cooling and warming.

Figure 1

Frost damage on unfolding leaves following late spring frost. Pictures of leaves from (a) sycamore (Acer pseudoplatanus), (b) European beech (Fagus sylvatica) and (c) Norway spruce (Picea abies) were taken on 24 April 2017 (Zugerberg, Switzerland; ca. 47.14°N, 8.53°E; sycamore and beech at 940 m a.s.l., spruce at 800 m a.s.l.). Spring temperatures based on daily average temperatures (T_ave) from 16 March to 5 May 2017 were 1.18 K above the long-term mean (2017: 6.42 °C; 1931 to 2016: 5.24 ± 1.43 °C, mean ± sd) and resulted in unusually early leaf unfolding. The warm spring was followed by two severe frost events (daily minimum temperatures T_min of −6.8 °C during the nights from 19–20 and 20–21 April 2017). Climate data are from the nearby station Einsiedeln (ca. 47.13°N, 8.76°E, 910 m a.s.l.). Photographs by C. Bigler.

Figure 2

Trend analysis of leaf unfolding based on dynamic linear model (DLM) and linear model (LM). Data for beech at the station La Valsainte (see Supplementary Table S1 and Supplementary Fig. S9): (a) observed dates of leaf unfolding, smoothed levels based on DLM, and regression line based on LM, (b) smoothed slopes based on DLM and slope based on LM. The DLM indicates a positive slope in 1960 (0.53 days/year) and a negative slope in 2010 (−0.46 days/year), whereas the LM suggests a constant negative slope (−0.20 days/year).

Figure 3

Distributions of leaf unfolding dates and frost risk (date safety margin and temperature safety margin). Box plots of: (a) leaf unfolding dates, (b) date safety margins (equation 2), and (c) temperature safety margins (equation 3). Boxes include the 25th, 50th and 75th percentiles. To ensure comparability, only observations for the common period (1996 to 2011; 1980 to 1995 for European aspen) at the 34 common stations are shown (number of observations: leaf unfolding dates, n = 5′827; date safety margins, n = 5′484; temperature safety margins, n = 5′827). The species are ordered from early- (blue) to late-leafing species (red) according to median dates of leaf unfolding. See Supplementary Fig. S3 for leaf unfolding and safety margins at all stations from 1951–2011; see Supplementary Figs S1, S2, and S4 for leaf unfolding and safety margins in different elevation bands.

Figure 4

Shifts in leaf unfolding dates and frost risk (date safety margin and temperature safety margin) based on dynamic linear models (DLMs). Shown are percentages of stations assigned to four elevation bands with positive smoothed slopes (positive percentages on y-axis) and negative smoothed slopes (negative percentages on y-axis; see equations 5–8). The species are ordered according to median dates of leaf unfolding (see Fig. 3a). Only series from 1956 (1996 for species with later beginning of observations) to 2011 (2014 for leaf unfolding; 1995 for European aspen) without missing estimates of smoothed slopes are shown (number of series: larch, n = 46–47; horse chestnut, n = 31–33; hazel, n = 39–44; silver birch, n = 74–77; rowan, n = 90–94; sycamore, n = 79; large-leaved linden, n = 70–73; beech, n = 41–42; small-leaved lime, n = 63–64; European aspen, n = 22; sweet chestnut, n = 15; Norway spruce, n = 35–36; black locust, n = 29–30). Non-significant slopes (n.s.) are shown with semi-transparent colors, significant slopes (sig.) with opaque colors. For each species and year, the absolute values of positive and negative percentages across all elevation bands sum up to 100%. See also Supplementary Figs S8, S12 and S13 for observations, smoothed levels and slopes of leaf unfolding and safety margins, Supplementary Fig. S14 for results based on all series, and Supplementary Fig. S7 for results based on linear models (LMs).