BD-I-1: Phenological changes in wild plant species | Umweltbundesamt

BD-I-1: Phenological changes in wild plant species

On average, the beginning of phenological spring, summer and autumn has advanced in the course of the past 71 years. While winter has become distinctly shorter, early autumn has become distinctly longer. These changes reflect the adaptability of plants to the changed climate. On the other hand, they can also have consequential effects on biodiversity, potentially leading to a situation which ultimately puts animal and plant species at risk.

Spring, summer and autumn begin earlier year by year

In our climes, the seasonal development of plants is primarily influenced by climate- and weather-related temperature patterns. A warm winter, for example, leads to very early flowering of trees such as hazel (Corylus avellana) or common alder (Alnus glutinosa). For this development, it is not individual warm or cold days which are crucial; in fact, the crucial parameter is longer-term weather patterns that precede flowering. If temperatures remain high, for instance during several consecutive weeks in winter, a sum total of warmth accumulates thus accelerating a plant’s development.

Changes in natural seasonal rhythms and associated temporal shifts in the development of plants have been studied and documented for years by means of so-called phenological observations. These nationwide studies involve the beginning of certain periodically recurring biological phenomena such as leaf and bud formation, flowering, maturity of fruit or leaf fall. The phenological observation network operated by DWD includes the observation of a broad spectrum of wild plants whose specific development phases mark the beginning of phenological seasons. Wild plants are particularly suitable for the observation of phenological shifts, as their responses are free from the influence of human manipulation in breeding processes or from cultivation activities (cf. Indicators LW-I-1, and LW-R-1).

The interpretation of shifts in seasonal cycles can only produce reliable results if this is done over extended sequences. This is why phenological data as well as climate-related data are averaged over periods of 30 years. Using the so-called phenological clock to compare the mean starting points of phenological seasons for the reference period of 1951 to 1980 and the comparative period of 1981 to 2010 with the starting points of the period from 1992 to 2021, the following pattern emerges: Regarding the phenological seasons from pre-spring via early summer until early autumn, the two periods after 1981 started earlier than in the reference period of 1951 to 1980 whereas the seasons of autumn, late autumn and winter started later. Pre-spring is the only time when the statistical difference is not significant. This means that especially the early autumn in the mean of the years 1992 to 2021 was approximately 17 days longer than in the reference period 1951 to 1980 whereas the winter season was approximately ten days shorter compared to the winter seasons between 1951 and 1980. This comparison also demonstrates that the summer mean of the threeperiods in question remained almost unchanged amounting to approximately 90 days whereas the beginning and end of summer in the period 1992 to 2021 was on average approximately twelve days earlier than in the reference period 1951 to 1980. An analysis of the starting dates of phenological seasons in the period 1992 to 2021 compared to the reference period 1951 to 1980 reveals statistically significant – and in most cases highly significant – differences between the two periods for all seasons.

On one hand, shifts of phenological seasons reflect the adaptability of plants and animals to changed climatic conditions. On the other, changes in development cycles caused by climate change also indicate consequential impacts on biodiversity. Phenological shifts can, in some cases, decouple the synchronicity of developments between organisms. This affects established interactions, for example between plants and their pollinators or interactions in prey-predator relationships. This effect impacts in turn on the structure and functions of ecosystems and can ultimately put animal and plant species at risk. For example, in the Netherlands it was proven that in pied flycatcher (Ficedula hypoleuca) populations the number of individuals declined owing to the decoupling of the time when nestlings are reared from the time when there was an optimal supply of their food source.¹²⁷ Pied flycatchers are long-distance migrants which spend winters in Africa; hence they are unable to respond adequately to the changed cycles in the development of their food organisms.

In Germany, there have been no wide-ranging studies or systematic observations of the consequences of such changes in relationships between plants and animals caused by phenological shifts. This is why at this point in time it is only possible to say that further shifts in phenological phases are to be expected.

The same applies to temporal extensions observed in respect of phenological vegetation periods. Those periods are equivalent to the sum of the days of phenological spring, summer and autumn. While the mean vegetation period in the years of 1951 to 1980 amounted to just 222 days, it was extended on average by 8 days to 230 days in the period of 1981 to 2010, and in the period of 1992 to 2021 by an average of 10 days to 232 days. In this context, it is important to note that the duration varies considerably from year to year. For example, an extension of the vegetation period can result in higher productivity of ecosystems which, in turn, can affect the relationships between various species. So far, there have been no systematic nationwide studies in Germany regarding the effects of an extended vegetation period on biodiversity.

^{127 - Both C., Bouwhuis S., Lessells C.M., Visser M.E. 2006: Climate change and population declines in a long-distancemigratory bird. Nature 441: 81-83. doi: 101.038/nature04539.}