Hat sich die Zeitspanne von der Saat bis zur Reife für Sommerkulturen in Mitteldeutschland infolge von Temperaturerhöhung verkürzt?

Abstract

Pickert, M., Wolter, M., Döring, J., Borg, H.: Did the time span between sowing and maturity of summer crops in central Germany become shorter due to increasing temperatures? - Hercynia N. F. 48 (2015): 5 – 20. At present anthropogenic climate change is a much discussed topic. In Germany the mean annual temperature is said to have increased by 1°C since 1901, the decade from 1990 to 1999 is considered to be the warmest in the 20th century, and the temperatures in the years since were mostly above the long term mean. Our study examines if this temperature pattern is reflected in a shortening of the time span between sowing and maturity (ZSE) of summer crops in Central Germany. Observed ZSE cannot be used for this purpose, because the growth conditions in the individual years were rarely ideal so that the ZSE were not just affected by temperature, but also by other factors. Furthermore, over the years the varieties planted and their management changed, which affects the ZSE, too. Hence, the growing degree day method was used to calculate the ZSE from 1901 to 2012 (with a look back to 1851). In this method daily temperatures above a plant species specific base temperature and below an upper temperature limit, which a plant is exposed to during its growth, are summed up. From sowing to maturity a given species always requires the same temperature sum (TSE), if no other factors limit its growth. The ZSE results from the number of days passed until the TSE is reached. At warm temperatures it is reached faster than at cool ones so that the ZSE is less. Calculations were carried out for summer crops with a TSE-requirement of 900, 1300 and 1700 °C d and for two sowing dates (March 15th and April 15th) using temperature data for Halle (Saale). It was assumed that all other growth conditions were ideal and that the only variable affecting plant growth was temperature. The results for the six scenarios differ in the length of the ZSE, but the variation between 1901 and 2012 is very similar. There are two periods (1943 to 1953 and 1998 to 2012) when the ZSE were mostly quite short and rarely exceeded the mean for 1901 to 2012. In the first period the ZSE were on the whole shorter than in the second, but the second period is longer and probably not over yet. Looking back to 1851 there is an additional period between 1857 and 1880 when the ZSE were mostly below the long term mean. This is the longest such period to date. The ZSE were partly as short as those between 1998 and 2012, but longer on average. Note that before 1851 only mean monthly temperatures were available, which do not allow as precise a calculation of the ZSE as daily means do. There are also two periods (1881 or 1901, respectively, to 1942 and 1954 to 1997) when the ZSE were mostly above the mean of the total observation period. In the second period the ZSE were generally longer than in the first. Since 1901 the ZSE did not become shorter, which means that crop development was not accelerated by higher temperatures. The claimed temperature rise since 1901 is not recognizable in the ZSE. Since 1998 the ZSE were often rather short, which matches the observation that temperatures since 1998 were mostly above the long term mean. However, between 1943 and 1953 the ZSE were similarly short and at times even shorter, which means that these were the warmest years, at least during the period of crop growth. The years since 1998 exhibit rather short, though not the shortest ZSE for a rather long, though not the longest period. So far, periods with short ZSE were always followed by periods with above average ZSE. Key words: climate change, growing degree days, growth period, temperature sum