Effects of climate change on pome fruit phenology and precipitation

Meteorological data more than 50 years (1958-2010) from the University of Bonn, Klein-Altendorf research centre's own weather station, including air and soil temperature as well as precipitation, were analysed for climate change effects. Phenological data over 50 years on hand-written filing cards were calendar dates of full bloom, harvest and leaf drop, as well as late frost and consequent yield loss, for a range of apple and pear cultivars, using only bearing fruit trees at the time. This implied i) the use of several generations of fruit trees/orchards and ii) use of original pome varieties, which existed as bearing trees since 1958, and were replanted after the orchard had been grubbed; the error rate in the 18,000 air temperature data was 0.77% and corrected where possible. Analysis and correlation of the meteorological data from the last 50 years showed two distinct climate phases in Klein-Altendorf: an early 30 year period (1958-1987) with a temperature of -0.4°C below the long-term, 50-year average of 9.4°C; followed by a ca. 20 year period of a +0.6°C temperature rise (1988 to date). A comparison of the phenological data of phase II (1988 to date) with phase I (1958-1987) showed 4 days earlier full bloom, but only 2 days earlier harvest without change in leaf drop date. This resulted in a 2 days longer fruit development - despite the warming - for cultivar 'Cox's Orange Pippin' at Klein-Altendorf. This may be explained by temperatures exceeding the optimum for photosynthesis of pome fruit and leaves. A European comparison showed Klein-Altendorf to have the oldest combined phenology and weather data and the latest apple flowering (F1 of cultivar 'Golden Delicious'), except for Gembloux in Belgium, where phenology records started in 1985. There was no change in the amount of annual precipitation of 594 mm in Klein-Altendorf over the 50 years of observation and records, but the relative distribution changed slightly from ca. 50 mm less precipitation in the summer during the fruit growing period to spring, with possible drought stress in summer (July) resulting in e.g. smaller fruit size.