Shrub-ring responses to climate of the common hawthorn (Crataegus monogyna)

Summary Land-use changes are fueling the transformation of formerly grazed European landscapes. Abandoned grasslands and woodlands are undergoing rapid processes of shrub encroachment (“shrubification”), but data on the patterns and paces of these dynamics are lacking. To fill this research gap, the common hawthorn ( Crataegus monogyna ), a widely distributed thorny shrub species dominating succession in many formerly grazed woodlands, was studied. To quantify age and growth rates, dendrochronological analyses were conducted on six hawthorn populations exposed to varying climate conditions in northern Spain. To test whether growth and aboveground phenology were coupled, data on leaf phenology were obtained for two study sites and related it to radial-growth indices. Lastly, growth indices were correlated with climate (temperature, precipitation) and hydrological (river flow) variables. A positive correlation between growth indices and the leaf flush date was found in one site. Warmer spring conditions lead to earlier leaf flushing, whereas leaf fall was delayed by warm autumn conditions. Growth was enhanced by low winter and summer temperatures, and by wet winter-to-spring conditions. However, a high river flow in spring reduced growth in a floodplain forest site. In the case of hawthorn, encroachment in dry sites could be slowed down by warmer spring and summer temperatures, which constrain its radial growth, whereas wetter spring conditions could accelerate it. The pace of ongoing “shrubification” across formerly grazed regions could be modulated by aridification.