The response of intra-annual stem circumference increase of young European beech provenances to 2012-2014 weather variability

The increasing frequency and severity of extreme weather events, especially droughts, arising from on-going climate changes negatively affect productivity and stability of forest ecosystems. Understanding species responses and suitable ecotypes that are able of adapting to new environmental conditions is increasingly important. The objective of this study was to quantify the relationships between the inter-annual stem circumference increase (SCI) of five European beech (Fagus sylvatica L.) provenances and weather variability during 2012-2014 in a provenance trial located in central Slovakia. SCIs were extracted on daily and weekly scale from hourly data of circumference changes. To detect the main environmental factors influencing SCI seasonal dynamics, intra-seasonal moving correlation functions were calculated. All five provenances responded synchronously to weather conditions, with high correlations among them during the growing season on both daily and weekly scale. The photoperiod exhibited a synchronizing effect on the seasonal peak of SCI as a sign of tree adaptation to long-term seasonal variations in climate. Temperature was the most significant factor influencing SCI dynamics at the beginning of the season. During the summer months, a precipitation deficit, heat waves and the consequently decreased soil water potential significantly affected the SCI of young beech trees, despite the fact that the provenance plot was situated in an area of optimum beech growth. Not only the severity and duration were important but also the timing of drought within a season. Within all seasons, the lowest SCI values were recorded for the provenance from the lowest altitude and the most oceanic climate (northern Germany). A comparison of daily and weekly SCI with first derivatives of growth functions indicated that SCIs were closely related to theoretical incremental processes, especially on a weekly scale. In young beech trees, SCI seemed to represent an appropriate proxy for studying intra-seasonal incremental processes. A newly designed SASB (self adjusting sharp beginning) function fit these processes better than the Gompertz function.