Marked differences in standing biomass, length density, anatomy and physiological activity between white and brown roots in Fraxinus mandshurica Rupr. plantation

Abstract

Root browning is closely related to physiological function, but the dynamic and underlying mechanism are poorly understood. Our objective was to explore the interrelationship between root browning, in the context of root branch order, and standing biomass, root length density, and soil resources availability at the ecosystem level. We measured the standing biomass and length density of white, brown and black roots of the first three branch orders in Fraxinus mandshurica Rupr.. Root anatomy, morphology, physiological activity, as well as soil total available nitrogen, moisture, and temperature were also determined. White roots had longer specific root length, higher physiological activity, and intact cortex compared with brown roots. Grand means of biomass and length density in white roots were 5.6-fold and 4.4-fold higher than those in brown roots, respectively. Sampling time, soil depth, and branch order generally had significant effects on the biomass and length density of both white and brown roots. In addition, sampling time consistently had significant effects on the brown to white root ratio, whereas the ratio was only moderately correlated with soil temperature. Root browning has pronounced impacts on tree root resource acquisition and belowground carbon allocation in F. mandshurica at both individual root and ecosystem levels.