Intra- and interannual variability in diameter increment of Fagus sylvatica L. and Picea abies L. Karst. in relation to weather variables

We examined the effect of weather variables on radial growth of Norway spruce and European beech at the intra- and interannual level. We used database of regular growth measurements at 9 forest sites distributed along an altitudinal and spatial gradient within the Slovakia and Czech Republic. In the period of 2010–2017, we analysed data from 213 dendrometers with manual reading in monthly or biweekly interval. The mean daily and annual diameter increments were analysed in relation to air temperatures and precipitation observed during the respective growing seasons. The general intra-annual diameter increment was modelled using a log-normal function. Results of modelling suggest that precipitation was a better predicting factor of the increment in spruce, while air temperature enhanced predictions of increments in beech. The highest, eight-year-cumulative increment of spruce (31.1 mm) and beech (22.8 mm) was found in the mixed mountainous forest at Poľana site, where both species occur in their growth optimum. The interspecific comparison of radial growth at this site revealed earlier culmination of increment in spruce compared to beech. The growth-limiting weather conditions for spruce occurred especially during the dry season 2015, while in beech sites the slight decrease of annual increment was observed in 2016. In the lowest altitudes of studied forest sites (beech 350 m a.s.l., spruce 440 m a.s.l.) the radial growth was reduced due to high summer temperatures. In the context of further predicted increase of air temperatures, these altitudinal limits for tree growth should be considered in the future forest management in Central Europe.