Population differentiation and phenotypic plasticity in temperature response of bud burst in Frangula alnus provenances of different latitude

Abstract

Flushing in spring marks an important adaptive process in a tree’s growth cycle. We studied bud burst in three provenances of a common small tree Frangula alnus Mill., originating from Italy, Belgium and Sweden. We observed timing of bud burst and leaf senescence in a common garden, and timing and duration of bud burst in greenhouse conditions (cuttings) with different temperature regimes, all located in Belgium. The early bud burst response of the southern European provenance together with the later leaf senescence compared to the local provenance indicated population differentiation that may, at least partly, be driven by local adaptation to a longer growing season. The duration of the process was longer in the cold greenhouse compared to the local provenance, whereas it responded similar as the local provenance in the warm greenhouse, suggesting adaptation to warmer conditions. Unexpectedly, the northern European provenance expressed a plastic reaction to warmer temperatures in the field trial and in the greenhouse conditions flushing in both cases earlier than the local provenance. The duration of the process in the warm greenhouse compared to the local provenance was shorter, whereas in the cold greenhouse it was similar. This result again suggested that not only the onset, but also the duration of the bud burst process was a plastic reaction to the warmer conditions compared to its site of origin. Together, our results suggest two mechanisms driving bud burst in F. alnus provenances, a co-gradient and a counter-gradient variation depending on the latitude of origin.