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

The picture shows a coltsfoot in flower. Click to enlarge
Nowadays, coltsfoot flowers earlier in the year than even in the middle of the previous century.
Source: Photograph: © kraichgaufoto / stock.adobe.com

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents

 

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 67 years. While winter has become distinctly shorter, early spring has become istinctly 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.

The graph shows a phenological clock. Three periods 1951 to 1980, 1981 to 2010 and 1988 to 2017 are plotted concentrically. The changes in the following ten leading phases represented by wild plants are shown.
BD-I-1: Phenological changes in wild plant species

The graph shows a phenological clock. Three periods 1951 to 1980, 1981 to 2010 and 1988 to 2017 are plotted concentrically. Shown are the changes in the following ten leading phases represented by wild plants; in the following, the numerical values of the three periods are read respectively: English oak (onset of leaf fall) winter: 143, 135 and 133 days, coltsfoot (beginning of flowering) for early spring: 14, 14 and 13 days, wood anemone (beginning of flowering) for first spring: 31, 31 and 31 days, English oak (beginning of leaf unfolding) for full spring: 30, 28 and 28 days, black elder (beginning of flowering) for early summer: 20, 22 and 23 days, large-leaved lime (beginning of flowering) for midsummer: 49, 43 and 44 days, mountain ash (development of first ripe fruits) for late summer: 21, 24 and 23 days, black elderberry (development of first ripe fruits) for early autumn: 29, 39 and 43 days, weeping birch (start of leaf discolouration) for full autumn: 20, 22 and 22 days, copper beech (start of leaf fall) for late autumn: 7,7 and 7 days.

Source: DWD (phenological observation network)
 

Temporal development of wild plant species undergoes seasonal shifts

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 or common alder. For this development, it is longer-term weather patterns preceding flowering rather than individual warm or cold days which are crucial. If temperatures remain high e.g. 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 D 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 agricultural actions.

The interpretation of shifts in seasonal cycles can only produce reliable results if this is done on the basis of 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 1951 to 1980 and the comparative period 1981 to 2010 with the starting points of the period from 1988 to 2017, 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 1951 to 1980 whereas the seasons of autumn, late autumn and winter started later. This means that especially the early autumn in the mean of the years 1988 to 2017 was approximately 14 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 three periods in question remained unchanged amounting to approximately 90 days whereas the beginning and end of summer in the period 1988 to 2017 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 1988 to 2017 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 development between organisms. This affects established interactions for example between plants and their pollinators or interactions in predator-prey relationships. This effect impacts on the structure and functions of ecosystems and can put animal species and plant species at risk. For example, in the Netherlands it was proven that in pied flycatcher populations the number of individuals declined owing to the temporal decoupling of the time when nestlings are reared from the time when there was an optimal supply of their food source.35 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 1951 to 1980 amounted to just 222 days, it was extended on average by 8 days to 230 days in the period 1981 to 2010, and in the period 1988 to 2017 by an average of 10 days to 232 days. 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 studies in Germany of the effects of an extended vegetation period on biodiversity.

35 -  Both C., Bouwhuis S., Lessells C.M., Visser M.E. 2006: Climate change and population declines in a longdistance migratory bird. Nature 441: 81–83.
DOI: 10.1038/nature04539

 

Interfaces

LW-I-1: Agrophenological phase shift

LW-R-1: Adaptation of management rhythms

WW-I-6: Start of the spring algal bloom in standing waters - case study

 

Objectives

Buffering and minimising the impacts of climate change on biodiversity in Germany (e.g. shifts in vegetation zones, changes in bird migration behaviour, threats to psychrophilic species) (NBS, ch. B 3.2)