Morphology and longevity of different-order fine roots in poplar (Populus × euramericana) plantations with contrasting forest productivities

Abstract

Fine roots play important roles in the allocation of forest net primary productivity. Here, to provide insights into the intrinsic and extrinsic factors affecting the root growth, we examined the morphology and longevity of different-order fine roots in two poplar (Populus × euramericana (Dode) Guinier) plantations with contrasting forest productivities. The results indicated that the biomass of fine roots decreased significantly as the root orders increased (P < 0.05). In the plantation with relatively high productivity, the specific root length of the first- and second-order roots was significantly greater than that of the third- to fifth-order roots (P < 0.05). Observations of minirhizotrons showed that roots of the third-order and higher lived longer than the first- and second-order roots. Compared with those in the plantation with relatively low productivity, the cumulative survival rate of the first-order roots was significantly higher in the plantation with relatively high productivity, while that of the third-order or higher roots was lower. The life-span of the first-order roots correlated negatively with soil available inorganic nitrogen, while that of the second-order roots correlated positively with the soil phenolic acid content. A Cox proportional hazards analysis revealed that soil conditions, root orders, season of root birth, and soil depth significantly affected fine-root longevity. These findings suggest that roots of different orders contribute unequally to the poplar plantation productivity.