Sammendrag
The ericoid shrub Vaccinium myrtillus is one of several deciduous boreal plants that respond to larval
defoliation by compensatory production of a new set of leaves within the same growing season soon after
defoliation. This new set is termed as ‘secondary leaves’. The physiological performance and longevity of
secondary leaves is poorly understood. Following a multi-year larval outbreak in boreal Norway, we
therefore monitored the fate of the secondary leaves from 2014 to 2016. We observed that secondary
leaves were still green upon onset of snow season and that the same leaves were still attached and green
when snow melted in spring. During the early growing season of 2015, the overwintering leaves were
consumed by moth larvae, but the secondary leaves produced in mid-July 2015 overwintered and were
not consumed by larvae the next growing season. Our monitoring shows that most of these leaves
remained attached until late August, while the contents of chlorophyll and nitrogen remained high until
July. The flavonoid (anthocyanin) content of overwintering leaves increased shortly after snowmelt, and
this coincided with a short-lasting colour change from green to violet-bronze. This was probably a
protective response to the abrupt increase in solar radiation. Overall, secondary, overwintering leaves
were free of snow and physiologically active for ca. 6.5 months, which is 2-3 months more than the
longevity of primary leaves in the study area. We conclude that wintergreen leaves probably rendered an
important carbon sequestration contribution in the critical first phase of the growing season until new side
branches with new leaves were developed. Hence, the novel results reported here on the facultative
wintergreen growth habit of V. myrtillus appears to be a successful strategy.
Bilberry, Compensatory growth, Defoliation, Facultative wintergreenness, Geometrid
moths, Larval outbreaks, Leaf longevity, Phenotypic plasticity
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