“60 Years on”—Effects of Climatic Change on Tree Phenology—A Case Study Using Pome Fruit

Abstract

Phenological records of apple and pear trees, including flowering, harvest and leaf drop, and concomitant weather records at Klein-Altendorf (50° N) near Bonn, Germany were correlated using two approaches: (a) linear curve fitting and (b) comparison of mean values of the first 30 years (1956–1988) versus the recent 30 years of climate change (1989–2017). The annual air temperature increased by 1.7 °C from 8.6 °C in 1958 to 10.3 °C in 2017 over the last 60 years and similarly in the vegetation period (1 April–30 October) from 13.7 °C in 1958 to 15.2 °C in 2017 by 1.5 °C. The combination of stronger increase in winter temperatures (by +1.2 °C) than in the summer (air +1.0 °C) with advanced bud break and −0.3 °C lower minimum temperatures in April during flowering resulted a continued risk of a late frost, as experienced in 2017. The strongest climate change effect, i.e., 11–14 days advanced flowering (in apple and pear) highly correlated (R2 −0.7) with the March/April temperature. Fruit ontogeny was 4 days shorter in cv. ‘Lucas’ pear, but 5 days longer in cv. ‘Cox’ and 10 days longer in cv. ‘Boskoop’, but remained unchanged in cv. ‘Golden Delicious’, irrespective of early or late ripening variety and contradictory climate effects, fruit matured 4–12 days earlier indicating its sole dependency on variety. Climate data and (earlier) harvest date closely correlated (R2: 0.6–0.7). The lowest correlation was between canopy duration (bloom to leaf fall), which was consistently extended by 6–10 days and the leaf drop stage beginning 2–4 days earlier. The correlations indicated that the Meckenheim fruit growing region is strongly affected by climate change and the comparison between two equally-balanced 30-year phases gave more realistic results than linear curve fitting.