Fine root vitality decline results in reduced branch formation in mature beech stands after drought

Abstract

Climate change is deteriorating the health, growth and thus functioning and services of forests through ongoing droughts and an increase in the severity of these effects is predicted. Although fine roots can serve as an early indicator of drought stress, we still lack a basic understanding of how tree fine root vitality is reflected in aboveground tree morphology. Our study uses multitemporal fine root data obtained by sequential coring, as well as high-resolution tree crown data obtained by terrestrial laser scanning in mature European beech (Fagus sylvatica) stands. First, we assessed which fine root vitality traits are affected by drought, and second, evaluated the relationship between fine root vitality and crown vitality including trees within three different radial distances (5, 8 or 10 m) from the fine root sampling point. We found that fine root necromass, fine root biomass to necromass ratio and turnover were correlated with drought. Mainly fine roots in the upper 10 cm of soil affected the crown vitality of beech trees within a 5-m radius of the fine root sampling point, highlighting the tree-centred horizontal distribution of fine roots and shallow rooting pattern of beech. A decreased fine root vitality in the upper soil in the previous growing seasons led to decreased crown vitality. Including fine root inventories in forest monitoring protocols could substantially improve our understanding of the status and trends of forests and enable forest management to deal more effectively with the growing pressure on forests.