Trends and temperature response in the phenology of crops in Germany

The phenology of 78 agricultural and horticultural events from a national survey in Germany spanning the years 1951-2004 is examined. The majority of events are significantly earlier now than 53 years ago, with a mean advance of 1.1-1.3 days per decade. The mean trends for 'true phases', such as emergence and flowering, of annual and perennial crops are not significantly different, although more trends (78% vs. 46%) are significant for annual crops. We attempt to remove the influence of technological advance or altered farming practices on phenology by detrending the respective time series by linear regression of date (day number) on year. Subsequently, we estimate responses to mean monthly and seasonal temperature by correlation and regression in two ways; with and without removing the year trend first. Nearly all (97%) correlation coefficients are negative, suggesting earlier events in warmer years. Between 82% and 94% of the coefficients with seasonal spring and summer temperatures are significant. The conservative estimate (detrended) of mean temperature response against mean March-May temperature (-3.73 days degrees C-1) is significantly less than the full estimate (-4.31 days degrees C-1), the 'true' size of phenological temperature response may lie in between. Perennial crops exhibited a significantly higher temperature response to mean spring temperature than the annual crops.