Advancing frost dates have reduced frost risk among most North American angiosperms since 1980.

In recent decades, the final frost dates of winter have advanced throughout North America, and many angiosperm taxa have simultaneously advanced their flowering times as the climate has warmed. Phenological advancement may reduce plant fitness, as flowering prior to the final frost date of the winter/spring transition may damage flower buds or open flowers, limiting fruit and seed production. The risk of floral exposure to frost in the recent past and in the future, however, also depends on whether the last day of winter frost is advancing more rapidly, or less rapidly, than the date of onset of flowering in response to climate warming. This study presents the first continental-scale assessment of recent changes in frost risk to floral tissues, using digital records of 475,694 herbarium specimens representing 1,653 angiosperm species collected across North America from 1920 to 2015. For most species, among sites from which they have been collected, dates of last frost have advanced much more rapidly than flowering dates. As a result, frost risk has declined in 66% of sampled species. Moreover, exotic species consistently exhibit lower frost risk than native species, primarily because the former occupy warmer habitats where the annual frost-free period begins earlier. While reducing the probability of exposure to frost has clear benefits for the survival of flower buds and flowers, such phenological advancement may disrupt other ecological processes across North America, including pollination, herbivory, and disease transmission.

Contrasting resistance and resilience to extreme drought and late spring frost in five major European tree species.

Extreme climate events (ECEs) such as severe droughts, heat waves, and late spring frosts are rare but exert a paramount role in shaping tree species distributions. The frequency of such ECEs is expected to increase with climate warming, threatening the sustainability of temperate forests. Here, we analyzed 2,844 tree-ring width series of five dominant European tree species from 104 Swiss sites ranging from 400 to 2,200 m a.s.l. for the period 1930-2016. We found that (a) the broadleaved oak and beech are sensitive to late frosts that strongly reduce current year growth; however, tree growth is highly resilient and fully recovers within 2 years; (b) radial growth of the conifers larch and spruce is strongly and enduringly reduced by spring droughts-these species are the least resistant and resilient to droughts; (c) oak, silver fir, and to a lower extent beech, show higher resistance and resilience to spring droughts and seem therefore better adapted to the future climate. Our results allow a robust comparison of the tree growth responses to drought and spring frost across large climatic gradients and provide striking evidence that the growth of some of the most abundant and economically important European tree species will be increasingly limited by climate warming. These results could serve for supporting species selection to maintain the sustainability of forest ecosystem services under the expected increase in ECEs.

Changing risk of spring frost damage in grapevines due to climate change? A case study in the Swiss Rhone Valley.

Late spring frost is a severe risk during early plant development. It may cause important economic damage to grapevine production. In a warming climate, late frost risk either could decline due to the reduction in frost days and an advancement of the last day of frost or increase due to a more pronounced shift forward of the start of the active growing period of the plants. These possibilities were analyzed in a case study for two locations in the lower Swiss Rhone Valley (Sion, Aigle) where viticulture is an important part of agriculture. Twelve phenology models were calibrated for the developmental stage BBCH09 (bud burst) using measured or reconstructed temperature data for two vineyards in Changins (1958 to 2012) and Leytron (1977 to 2014) together with observed phenological data. The day of year (DOY) for BBCH09 was then modelled for the years 1951 to 2050 using the best performing phenology model in combination with ten downscaled and bias-corrected climate scenarios. A 100-day period starting with BBCH09 was defined, during which daily mean and minimum temperatures were used to calculate three frost risk indices in each year. These indices were compared between the periods 1961-1990 (reference) and 2021-2050 (climate change scenario). Based on the average of the ensemble of climate model chains, BBCH09 advanced by 9 (range 7-11) (Aigle) and 7 (range 5-8) (Sion) days between the two time periods, similar to the shift in the last day of frost. The separate results of the different model chains suggest that, in the near future, late spring frost risk may increase or decrease, depending on location and climate change projections. While for the reference, the risk is larger at the warmer site (Sion) compared to that at the cooler site (Aigle), for the period 2021-2050, small shifts in both phenology and occurrence of frost (i.e., days with daily minimum temperature below 0 °C) lead to a small decrease in frost risk at the warmer but an increase at the cooler site. However, considerable uncertainties remain that are mostly related to climate model chains. Consequently, shifts in frost risk remain uncertain for the time period considered and the two study locations.

Unchanged risk of frost exposure for subalpine and alpine plants after snowmelt in Switzerland despite climate warming.

The length of the snow-free season is a key factor regulating plant phenology and shaping plant community composition in cold regions. While global warming has significantly advanced the time of snowmelt and the growth period at all elevations in the Swiss Alps, it remains unclear if it has altered the likelihood of frost risk for alpine plants. Here, we analyzed the influence of the snowmelt timing on the risk of frost exposure for subalpine and alpine plants shortly after snowmelt, i.e., during their most vulnerable period to frost at the beginning of their growth period. Furthermore, we tested whether recent climate warming has changed the risk of exposure of plants to frost after snowmelt. We analyzed snow and air temperature data in the Swiss Alps using six weather stations covering the period 1970-2016 and 77 weather stations covering the period 1998-2016, spanning elevations from 1418 to 2950 m asl. When analyzed across all years within each station, our results showed strong negative relationships between the time of snowmelt and the frequency and intensity of frost during the most vulnerable period to frost for subalpine and alpine plants, indicating a higher frost risk damage for plants during years with earlier snowmelt. However, over the last 46 years, the time of snowmelt and the last spring frost date have advanced at similar rates, so that the frequency and intensity of frost during the vulnerable period for plants remained unchanged.

Managing the risk of extreme climate events in Australian major wheat production systems.

Extreme climate events (ECEs) such as drought, frost risk and heat stress cause significant economic losses in Australia. The risk posed by ECEs in the wheat production systems of Australia could be better managed through the identification of safe flowering (SFW) and optimal time of sowing (TOS) windows. To address this issue, three locations (Narrabri, Roseworthy and Merredin), three cultivars (Suntop and Gregory for Narrabri, Mace for both Roseworthy and Merredin) and 20 TOS at 1-week intervals between 1 April and 12 August for the period from 1957 to 2007 were evaluated using the Agricultural Production System sIMulator (APSIM)-Wheat model. Simulation results show that (1) the average frequency of frost events decreased with TOS from 8 to 0 days (d) across the four cases (the combination of locations and cultivars), (2) the average frequency of heat stress events increased with TOS across all cases from 0 to 10 d, (3) soil moisture stress (SMS) increased with earlier TOS before reaching a plateau and then slightly decreasing for Suntop and Gregory at Narrabri and Mace at Roseworthy while SMS increased with TOS for Mace at Merredin from 0.1 to 0.8, (4) Mace at Merredin had the earliest and widest SFW (216-260) while Mace at Roseworthy had latest SFW (257-280), (5) frost risk and heat stress determine SFW at wetter sites (i.e. Narrabri and Roseworthy) while frost risk and SMS determine SFW at drier site (i.e. Merredin) and (6) the optimal TOS (window) to maximise wheat yield are 6-20 May, 13-27 May and 15 April at Narrabri, Roseworthy and Merredin, respectively. These findings provide important and specific information for wheat growers about the management of ECE risk on farm. Furthermore, the coupling of the APSIM crop models with state-of-the-art seasonal and intra-seasonal climate forecast information provides an important tool for improved management of the risk of ECEs in economically important cropping industries in the foreseeable future.

Limited alpine climatic warming and modeled phenology advancement for three alpine species in the Northeast United States.

UNLABELLED: • PREMISE OF THE STUDY: Most alpine plants in the Northeast United States are perennial and flower early in the growing season, extending their limited growing season. Concurrently, they risk the loss of reproductive efforts to late frosts. Quantifying long-term trends in northeastern alpine flower phenology and late-spring/early-summer frost risk is limited by a dearth of phenology and climate data, except for Mount Washington, New Hampshire (1916 m a.s.l.).• METHODS: Logistic phenology models for three northeastern US alpine species (Diapensia lapponica, Carex bigelowii and Vaccinium vitis-idaea) were developed from 4 yr (2008-2011) of phenology and air temperature measurements from 12 plots proximate to Mount Washington's long-term summit meteorological station. Plot-level air temperature, the logistic phenology models, and Mount Washington's climate data were used to hindcast model yearly (1935-2011) floral phenology and frost damage risk for the focal species.• KEY RESULTS: Day of year and air growing degree-days with threshold temperatures of -4°C (D. lapponica and C. bigelowii) and -2°C (V. vitis-idaea) best predicted flowering. Modeled historic flowering dates trended significantly earlier but the 77-yr change was small (1.2-2.1 d) and did not significantly increase early-flowering risk from late-spring/early-summer frost damage.• CONCLUSIONS: Modeled trends in phenological advancement and sensitivity for three northeastern alpine species are less pronounced compared with lower elevations in the region, and this small shift in flower timing did not increase risk of frost damage. Potential reasons for limited earlier phenological advancement at higher elevations include a slower warming trend and increased cloud exposure with elevation and/or inadequate chilling requirements.

Agroclimatic Metrics for the Main Stone Fruit Producing Areas in Spain in Current and Future Climate Change Scenarios: Implications From an Adaptive Point of View.

Stone fruit production has enormous economic importance in Spain. Cultivation locations for these fruit species (i.e., peach, apricot, plum, and sweet cherry) cover wide and climatically diverse geographical areas within the country. Climate change is already producing an increase in average temperatures with special intensity in certain areas like the Mediterranean ones. These changes lead to a decrease in the accumulated chill, which can have a profound impact on the phenology of Prunus species like stone fruits due to, e.g., difficulties to cover the chilling requirements to break endodormancy, the occurrence of late frost events, or abnormal early high temperatures. All these factors can severely affect fruit production and quality and therefore provoke very negative consequences from the socio-economic point of view in the incumbent regions. Thus, characterization of current cultivation areas in terms of agroclimatic variables (e.g., chill and heat accumulation and probabilities of frost and early abnormal heat events), based on data from 270 weather stations for the past 20 years, is carried out in this work to produce an informative picture of the current situation. Besides, future climatic projections from different global climate models (data retrieved from the Meteorological State Agency of Spain-AEMET) up to 2065 for two Representative Concentration Pathway scenarios (i.e., RCP4.5 and RCP8.5) are also analyzed. Using the current situation as a baseline and considering the future scenarios, information on the current and future adaptive suitability of the different species/cultivars to the different growing areas can be inferred. This information could be the basis of a decision support tool to help the different stakeholders to take optimal decisions regarding current and future stone fruit or other temperate species cultivation in Spain.