Global data sets provide strong evidence of convergence for leaf structure with leaf longevity such that species having thick leaves, low specific leaf area, low mass-based nitrogen concentrations, and low photosynthetic rates typically exhibit long leaf life span. Leaf longevity and corresponding leaf structure have also been widely linked to plant potential growth rate, plant competition, and nutrient cycling. We hypothesized that selection forces leading to variation in leaf longevity and leaf structure have acted simultaneously and in similar directions on the longevity and structure of the finest root orders. Our four-year study investigated the links between root and leaf life span and root and leaf structure among 11 north-temperate tree species in a common garden in central Poland. Study species included the hardwoods Acer pseudoplatanus L., Acer platanoides L., Fagus sylvatica L., Quercus robur L., and Tilia cordata Mill.; and the conifers Abies alba Mill., Larix decidua Mill., Picea abies (L.) Karst., Pinus nigra Arnold, Pinus sylvestris L., and Pseudotsuga menziesii (Mirbel) Franco. Leaf life span, estimated by phenological observations and needle cohort measurements, ranged from 0.5 to 8 yr among species. Median fine-root life span, estimated using minirhizotron images of individual roots, ranged from 0.5 to 2.5 yr among species. Root life span was not correlated with leaf life span, but specific root length was significantly correlated with specific leaf area. Root nitrogen : carbon ratio was negatively correlated with root longevity, which corroborates previous research that has suggested a trade-off between organ life span and higher organ N concentrations. Specific traits such as thickened outer tangential walls of the exodermis were better predictors of long-lived roots than tissue density or specific root length, which have been correlated with life span in previous studies. Although theories linking organ structure and function suggest that similar root and leaf traits should be linked to life span and that root and leaf life span should be positively correlated, our results suggest that tissue structure and longevity aboveground (leaves) can contrast markedly with that belowground (roots).

Summary 1.Root life span regulates the quantity and quality of root-derived organic matter transferred to the soil organic matter pool. However, poor understanding of the rates and controls of root life span has hindered the prediction of carbon (C) flow and nutrient cycling dynamics at the ecosystem scale. 2. We examined the effects of root branch order, root diameter, mycorrhizal colonization, season of birth, depth in the soil, nitrogen (N) fertilization and foliage removal on root life span in a longleaf pine ( Pinus palustris Mill.) forest from 2001 to 2004 using minirhizotron and soil monolith sampling. 3 . Among all factors, root branch order had the strongest and most consistent effect on life span, with higher order roots having a 46% longer life span than roots one order lower. 4 . Within first order roots, mycorrhizal colonization significantly increased root life span by > 45% in 2 of 3 years. 5 . Roots born in winter and spring generally lived longer than roots born in summer and autumn. Root life span was positively correlated with depth in the soil and root diameter, but the correlations were weaker than with order, year and season. Neither N fertilization nor foliage removal had a significant impact on root life span. 6 . When biomass mortality and associated N flux were estimated based on order-specific mean life span, N concentration and ecosystem-scale biomass estimates, first order roots constituted approximately 50% of the total biomass mortality and > 60% of the N flux for the first three root orders combined. 7. Synthesis. Our results show that (i) root branch order was the strongest predictor of life span among all covariates and can effectively partition the distal longleaf pine root systems into three or more populations with different turnover rates; (ii) only a fraction of fine roots turns over annually, whereas models of C cycles assume an annual turnover for the entire fine root system. We conclude that an order-based approach holds greater promise than the traditional diameter class approach for evaluating the role of different fine root populations in C flow and nutrient cycling.

Summary• A wide variety of transparent materials are currently used for minirhizotron tubes. We tested the null hypothesis that minirhizotron composition does not influence root morphology and dynamics.• Minirhizotron data were compared for glass, acrylic and butyrate tubes in apple ( Malus domestica ) and acrylic and butyrate tubes in a study with six forest tree species.• Root phenology and morphology were generally similar among tubes. Apple root production was greatest against glass; these roots became pigmented later and lived longer than roots near acrylic or butyrate. Roots generally became pigmented faster next to butyrate than next to acrylic. Root survivorship was shorter near butyrate tubes in three of the four hardwood species; however, survivorship was shorter near acrylic tubes for the three conifer species. Comparison of minirhizotron standing crop data with root standing crop from cores showed that the acrylic data matched more closely than the butyrate data.• This study reveals that the transparent material used often has little effect on root production but can substantially influence root survivorship in some plants.

Long-term minirhizotron observations of absorptive fine roots provide insights into seasonal patterns of belowground root production and carbon dynamics. Our objective was to compare root dynamics over time across mature individuals of 11 temperate trees species: five evergreen and six deciduous. We analyzed the timing and growth on 1st-and 2nd-order roots in minirhizotron images down to a vertical depth of 35 cm, as well as monthly and total annual length production. Production patterns were related to total annual precipitation of the actual and previous year of root production over 6 years. The main or largest peak of annual fine-root production occurred between June and September for almost all species and years. In most years, when peaks occurred, the timing of peak root production was synchronized across all species. A linear mixed model revealed significant differences in monthly fine-root length production across species in certain years (species x year, P < 0.0001), which was strongly influenced by three tree species. Total annual root production was much higher in 2000–2002, when there was above-average rainfall in the previous year, compared with production in 2005–2007, which followed years of lower-than-average rainfall (2003–2006). Compared to the wetter period all species experienced a decline of at least 75% in annual production in the drier years. Total annual root length production was more strongly associated with previous year’s (P < 0.001) compared with the actual year’s precipitation (P = 0.003). Remarkably similar timing of monthly absorptive fine-root growth can occur across multiple species of diverse phylogeny and leaf habit in a given year, suggesting a strong influence of extrinsic factors on absorptive fine-root growth. The influence of previous year precipitation on annual absorptive fine-root growth underscores the importance of legacy effects in biological responses and suggests that a growth response of temperate trees to extreme precipitation or drought events can be exacerbated across years.