Abstract:

Aims Fine root longevity plays a key role in the processes of root production and mortality and is important to precisely estimate carbon and nutrient cycling in terrestrial ecosystems. However, few studies have simultaneously examined the influence of multiple factors on fine root longevity. Our objectives were to examine: 1) seasonal patterns of fine root length production and mortality in plantation-grown Pinus sylvestris var. mongolica, 2) effects of fine root diameter, order, season of fine root birth, soil depth and mycorrhizal colonization on fine root longevity, and 3) relative strength of the influences of these factors on fine root survivorship.

Methods Minirhizotrons were used to estimate fine root length production, mortality and longevity. In July 2007, three 20 m × 30 m plots were established in a P. sylvestris plantation, and six minirhizotron tubes (90 cm length, 5.5 cm inside diameter) were installed randomly throughout the plot. Video images were collected at approximately four-week intervals during the growing season from 5 May 2008 to 22 October 2009. Video images were digitized via RooTracker software (NC, USA). Kaplan-Meier method in survival analysis was used to estimate mean longevity, median longevity and cumulative survival rate. The effects on root longevity by single factors, including root diameter, order, season of root birth, soil depth and mycorrhizal colonization, were tested by Log-rank test. The relative strength of the influences of these factors on fine root survivorship was examined by Cox proportional hazards regression analysis.

Important findings Fine root length production and mortality exhibited strong seasonal patterns, in which production peaked in spring and summer and mortality peaked in late summer, autumn and winter. Mean and median fine root longevity were (322 ± 10) d and (310 ± 15) d, respectively. Log-rank test indicated that fine-root longevity was significantly affected by root diameter, order, season of birth, soil depth and mycorrhizal colonization. Cox proportional hazards regression suggested that mycorrhizal colonization, root diameter and soil depth significantly influenced fine root survivorship, controlling for other factors (covariates). The risk of fine root mortality decreased 99% with a 1 mm diameter increase, and fine root longevity increased 5% with a 1 cm increase in soil depth. However, the risk of mortality of mycorrhizal roots was 175% higher than for non-mycorrhizal roots. Root order and season of root birth had no significant effects on fine root longevity. Our findings highlight that fine root longevity is influenced by endogenous and exogenous factors simultaneously, and multi-covariate analysis is an efficient way to reveal variation of fine root longevity.

Key words: Cox proportional hazards regression, fine root longevity, fine root production, fine root mortality, minirhizotron, Pinus sylvestris var. mongolica