Root-shoot allometry and drought response of dominant and co-dominant European beech (Fagus sylvatica [L.]) and Norway spruce (Picea abies [L.] Karst.) in monospecific versus mixed-species stands in Bavaria, Germany

Abstract Climate change and associated drought stress significantly impact tree root systems and canopies, essential for water and nutrient provision. We examined the root-shoot allometry of dominant European beech ( Fagus sylvatica [L.]) and Norway spruce ( Picea abies [L.] Karst.) in monospecific and mixed-species stands across five triplets along a climatic and geological gradient from North to South Bavaria, focusing on their drought responses. We used Lloret indices of resistance, resilience, and recovery to assess the effects of the exceptionally hot drought year of 2003 on root and shoot growth. Data were categorized by stand type: beech pure (BP), beech mixed (BM), spruce pure (SP), and spruce mixed (SM). Results showed that the allometric factor decreased in the order SP (− 2.986) > SM (− 3.960) > BP (− 7.434) > BM (− 8.628), and the allometric exponent in the order BM (3.159) > BP (2.795) > SM (1.914) > SP (1.657), with significant differences between species. For European beech, the allometric factor was significantly affected by stand density and site index, and the allometric exponent by tree species, mixed stand type, and site index. European beech exhibited higher resistance and resilience in shoot growth compared to Norway spruce, with no significant influence from stand type. Root growth patterns were similar, with European beech being more resilient and resistant than Norway spruce, although to a lesser extent. European beech also recovered faster than Norway spruce, with significant differences between their respective monospecific stands. We concluded that root growth for both species was enhanced compared to shoot growth, improving anchoring and the supply of water and nutrients to the aboveground tree organs. European beech experienced less drought stress than Norway spruce, attributed to differences in their physiology and root system characteristics.