Temperature effects on root exudation in mature beech (Fagus sylvatica L.) forests along an elevational gradient

Aims: Root exudation may have a large impact on soil biological activity and nutrient cycling. Recent advances in in situ-measurement techniques have enabled deeper insights into the impact of tree root exudation on rhizosphere processes, but the abiotic and biotic controls of exudation rate remain poorly understood. We explored the temperature dependence of root exudation in mature beech (Fagus sylvatica L.) trees. Methods: We measured fine root exudation in seven beech forests along an elevational gradient (310–800 m a.s.l.) and related carbon (C)-flux rates to mean daily temperature, actual precipitation, mean summer temperature (MST) and precipitation (MAP), soil moisture (SWC), and stand structure. Results: Average mass-specific exudation (averaged over all sampling dates) ranged from 12.2 µg C g ⁻¹ h ⁻¹ to 21.6 µg C g ⁻¹ h ⁻¹ with lowest rates measured at highest elevations and peak rates at mid-elevation (490 m). Regression analyses showed a highly significant positive effect of site-specific daily air and soil temperature on exudation rates (p < 0.01) with an average increase by 2 µg C g ⁻¹ h ⁻¹ per 1 °C-temperature increase, while the relation to mean summer or annual temperature and mean temperature of the measuring year was less tight. Exudation decreased with increases in mean annual precipitation and soil moisture, but increased with increasing stem density. Conclusions: The root exudation rate of beech trees roughly triples between 10 °C and 20 °C mean daily temperature, evidencing a large temperature influence on root-borne C flux to the soil.