LW-I-1: Agrophenological phase shifts | Umweltbundesamt

LW-I-1: Agrophenological phase shifts

Changes in seasonal weather patterns result in new challenges facing agricultural businesses in terms of scheduling their management operations. Owing to higher accumulations of warmth in spring, both apple trees and winter rapeseed  flower earlier and earlier.

Spring crops starting earlier year by year

Hardly any other form of land use is as tied to natural seasonal rhythms as agricultural land use. People who work in agriculture always need to adapt the management of their specific crop to the annually changing weather patterns prevailing at any particular time. Changes in weather patterns can impact on crops in both positive and negative ways. On one hand, greater accumulations of warmth in the presence of adequate availability of water further the growth of certain types of crop. On the other, excessively high temperatures or drought can entail losses in yield or quality, when for instance a cereal crop ripens too early.

Climate change influences seasonal weather patterns which in turn impact the seasonal development processes of agricultural crops. Spring is expected to bring rising temperatures, summers are likely to become drier and hotter while winters have already become milder and wetter. Such circumstances make it impossible to infer any simplified conclusions regarding volume, quality or stability of yields, because the impacts of climate change can vary subject to the crop cultivated and to crop
rotation. Above all, there are regional differences to be taken into account.

Changes in natural seasonal rhythms and associated temporal shifts in the development of plants have been studied and documented for years by means of phenological observations. These nationwide studies involve the onset of certain biological phenomena recurring periodically such as leaf and bud formation, flowering, maturity of fruit or leaf fall. The phenological observation network operated by DWD extends to wild plants, agricultural crops and management operations, thus providing indications regarding impacts on agricultural management, because changes in seasonal weather patterns confront agriculture with new challenges. Farming businesses have to adapt their choices of crop and crop variety, crop rotation and the scheduling of management operations according to new circumstances arising.

In Germany the onset of spring is marked by the flowering of apple trees and winter rapeseed. Both flowering periods are independent of the influence of preceding agricultural operations such as the date of sowing. Apples belong to the category of permanent crops, while winter rapeseed is sown the previous year, no later than September. The flowering period is therefore linked directly to climatic factors, especially accumulations of warmth occurring in the first few months of the new year.

Depending on the progress of weather patterns, the start of flowering in respect of apple trees and winter rapeseed tends to fluctuate from year to year, and in some cases such fluctuations are considerable. The differences from year to year can be as much as up to three weeks. However, looking back over a little more than the past fifty years, both crop types show a significant trend towards an earlier onset of flowering. Compared to the 1970s, winter rapeseed started to flower 18 days earlier – as indicated by the mean calculated for the past ten years – while apple trees started to flower roughly 15 days earlier.

As far as fruit-growing is concerned, early flowering can increase the risk of damage from late frost. In cases where warm spring temperatures have led to flowering and fructification being well advanced, only to be hit by late frost, this is possible to cause frost damage leading to substantial yield losses and possibly even to the loss of the entire harvest. In 2016, winter returned with a vengeance in late April, with snow falling all the way from Scandinavia to the Alpine region. Even though that year does not stand out in terms of the phenological time series for apple blossom, the resulting damage was immense, owing to the very late occurrence of frost. Likewise, in 2017 (the subsequent year), the late onset of cold weather – with numerous frosty nights – descended on large parts of Europe between 17th April and 10th May. Owing to the previous warm weather pattern which had prevailed from March until the beginning of April, the vegetation was already comparatively far ahead in that spring. As far as fruit-growing is concerned, the blossom had already partly gone in that year, whereas there were already young fruit which are even more vulnerable to frost than the blossom. In respect of fruit growing on trees, the extent of damage reached devastating proportions. 2020 was another year in which late frosts caused yield losses as a result of a frosty May; whilst in 2021 several successive frosty April nights did the damage.

In many locations the orchardists are already responding to this development by increasingly applying targeted anti-frost irrigation, in other words, spraying plants with very small water droplets. While the water freezes, crystallisation heat is released, which protects both leaves and blooms from frost damage. Nevertheless, frost protection irrigation requires massive quantities of water amounting to 30,000 per hectare and hour. This factor naturally limits the application of frost protection irrigation, not least for sustainability reasons. A more sustainable solution might be the planting of less frost-sensitive varieties in locations vulnerable to frost.

The situation is quite different, however, with regard to winter rapeseed. In this case, early flowering can entail benefits for the management of pest organisms and crop rotation. The infestation of winter rapeseed with rape beetle is apt to increase owing to rising winter and spring temperatures (cf. Indicator LW-I-4). The use of early-flowering varieties can provide a tool for the prevention of this infestation in the vulnerable stage of budding. This is indeed one of the reasons why agricultural businesses increasingly give preference to early-flowering winter rapeseed. However, phenological observation is unable to take a specific variety into account. This circumstance is the reason why the effects of using different varieties are also reflected in the flowering periods observed within the framework of phenological analysis. Hence, the relevant indicator fulfils two purposes at the same time: it is both impact and response indicator. In fact, it clearly illustrates the combined effects described.