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Timing of spring phenological phases of bird cherry and silver birch under climate change in 21st century in Latvia
Abstract and Figures
Phenology is a study of timing of event taking place in nature. In a functional ecosystem phenological phases for mutually dependent are synchronized. In a rapidly changing climate there is a risk of decoupling, that is spring development of interdependent could species proceed at different peace. We are investigating the likely timing of spring phenological phases during the last and upcoming centuries for two tree species common in northern and central Europe: bird cherry (Prunus padus known also as Padus racemosa or Padus avium) and silver birch (Betula pendula). The study is based on long term voluntary-citizens-scientist-phenological observations since 1927. A thermal-time phenological model is used to describe the timing of the spring phenological phases of bird cherry and silver birch. The phenological model was parametrized using the observations from national meteorological stations. The shifts of the timing of phenological phases due to climate change are analysed using simulated meteorological data from bias-corrected ENSEMBLES data set. It is found that the phases onset, modelled with ENSEMBLES data, in comparison to observations at some stations are biased by few days earlier, but the trends for modelled and observed phase timing are overlapping. During the last 50 years the average timing for flowering of the bird cherry have sifted by a week earlier, while the leaf unfolding, and flowering of the silver birch has shifted even more. Modelling results suggest that the trend will continue in future as well. The warm winters in future with sudden could spells in early spring rises risks that the prematurely unfolded leaves may suffer from frost damage.
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Recent boreal winters have exhibited a large-scale seesaw temperature pattern characterized by an unusually warm Arctic and cold continents. Whether there is any physical link between Arctic variability and Northern Hemisphere (NH) extreme weather is an active area of research. Using a recently developed index of severe winter weather, we show that the occurrence of severe winter weather in the United States is significantly related to anomalies in pan-Arctic geopotential heights and temperatures. As the Arctic transitions from a relatively cold state to a warmer one, the frequency of severe winter weather in mid-latitudes increases through the transition. However, this relationship is strongest in the eastern US and mixed to even opposite along the western US. We also show that during mid-winter to late-winter of recent decades, when the Arctic warming trend is greatest and extends into the upper troposphere and lower stratosphere, severe winter weather-including both cold spells and heavy snows-became more frequent in the eastern United States.
A historical phenological record and meteorological data of the period 1960-2009 are used to analyse the ability of seven phenological models to predict leaf unfolding and beginning of flowering for two tree species-silver birch Betula pendula and bird cherry Padus racemosa-in Latvia. Model stability is estimated performing multiple model fitting runs using half of the data for model training and the other half for evaluation. Correlation coefficient, mean absolute error and mean squared error are used to evaluate model performance. UniChill (a model using sigmoidal development rate and temperature relationship and taking into account the necessity for dormancy release) and DDcos (a simple degree-day model considering the diurnal temperature fluctuations) are found to be the best models for describing the considered spring phases. A strong collinearity between base temperature and required heat sum is found for several model fitting runs of the simple degree-day based models. Large variation of the model parameters between different model fitting runs in case of more complex models indicates similar collinearity and over-parameterization of these models. It is suggested that model performance can be improved by incorporating the resolved daily temperature fluctuations of the DDcos model into the framework of the more complex models (e.g. UniChill). The average base temperature, as found by DDcos model, for B. pendula leaf unfolding is 5.6 °C and for the start of the flowering 6.7 °C; for P. racemosa, the respective base temperatures are 3.2 °C and 3.4 °C.
To investigate the impact of recent climatic changes on plant development, this study used phenological data of the volunteer networks in Latvia and Lithuania from the 1971-2000 period. The phenological calendar method was applied. Phenological seasons were described using data on 6 phenological phases at 10 stations. The growing season was described using birch Betula pendula as an example. Correlation analysis, linear regression and non-parametric Mann-Kendall trend tests were applied to establish the relationship between phenological phases and meteorological factors (temperature, precipitation) and the North Atlantic Oscillation (NAO). The results indicate a statistically significant trend toward earlier onset of spring and summer phases. The study found that, unlike the majority of trends observed in Europe, on average the onset of phenological autumn in Latvia and Lithuania also started earlier. The observed trends in spring correlated well with temperatures in the preceding months and with the NAO. A strong correlation (in 12 cases out of 20) was found between spring phenophases and precipitation in February.
Winters and early springs are predicted to become warmer in temperate climates under continued global warming, which in turn is expected to promote earlier plant development. By contrast, there is no consensus about the changes in the occurrence and severity of late spring frosts. If the frequency and severity of late spring frosts remain unchanged in the future or change less than spring phenology of plants does, vulnerable plant organs (dehardened buds, young leaves, flowers or young fruits) may be more exposed to frost damage. Here we analyzed long-term temperature data from the period 1975–2016 in 50 locations in Switzerland and used different phenological models calibrated with long-term series of the flowering and leaf-out timing of two fruit trees (apple and cherry) and two forest trees (Norway spruce and European beech) to test whether the risk of frost damage has increased during this period. Overall, despite the substantial increase in temperature during the study period, the risk of frost damage was not reduced because spring phenology has advanced at a faster rate than the date of the last spring frost. In contrast, we found that the risk of frost exposure and subsequent potential damage has increased for all four species at the vast majority of stations located at elevations higher than 800 m while remaining unchanged at lower elevations. The different trends between lower and higher elevations are due to the date of the last spring frost moving less at higher altitudes than at lower altitudes, combined with stronger phenological shifts at higher elevations. This latter trend likely results from a stronger warming during late compared to earlier spring and from the increasing role of other limiting factors at lower elevations (chilling and photoperiod). Our results suggest that frost risk needs to be considered carefully when promoting the introduction of new varieties of fruit trees or exotic forest tree species adapted to warmer and drier climates or when considering new plantations at higher elevations.
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