| Fine root of trees is an important unit with absorbing ability and quick turnover functions. They are important in the acquisition of water and essential nutrients in whole plant, make a significant contribution to energy flowing and carbon cycling at the ecosystem level. Therefore, the fine roots attracted more and more attentions of plant physiological ecologists. The root system of trees is a multi-level branching structure. The traditional standards to define fine roots by diameter size (<0.5mm,<1mm, or<2mm) are not uniform. The roots in the same diameter size include a number of branching orders, which have different physiological and ecological function heterogeneity. So far, there is no standard to define the fine roots currently. Therefore, it is necessary to reasearch the functional heterogeneity from the perspective of root branching orders, to distinct the different functional modules in root systems of trees.In this study, we studied root architecture across the first six root branch orders of 21 typical tree species from subtropical forest in Hubei Province. Our objectives were to determine how root morphology (e.g., diameter, length, specific root length), anatomical character(epidermis, cortex and stele), TNC(non-structural carbon) and total-N concentration changes across root branch orders and how such variations of fine roots differ between three types of tree species(ancient gymnosperm, ancient angiosperm, evolved angiosperm). The results are summarized as follows:(1) The diameter, length and SRL among 21 different species are of great heterogenerty. The average value of diameter (0.04-0.74 mm) and length (1.27-17.23 mm) of first root order presented minimum, the SRL of first order were maximum (28.28-1300.2 m-g-1). Respectively, the average value of diameter (3.97-9.84 mm) and length (144.98-987.10 mm) of sixth order achieved maximum, the SRL of sixth order were minimum (0.10-2.76 m-g-1). With ascending root orders, the mean fine root diameter and root length increased (P<0.05), while the specific root length (SRL) decreased significantly (P＜0.05). Gerenally, the average diameter of ancient species was greater than the evolved species, but the difference of the average root length was not significantly.(2) The diameter of stele increased significantly with the ascending orders, the percentage of stele in total diameter also increased. Differently, the cortex thickness's increasing trend with the ascending orders is not significant, the percentage of cortex in total diameter decreased. The thickening of fine roots was mainly due to the growth and thickening of the stele. Moreover, the cortex of the fourth and higher root branch orders in all species came off or became incomplete. The results showed that the first three orders were the module with absorptive function.(3) The TNC concentration increased with ascending orders. The TNC of first order fine root was lowest (8.85-147.37 g-kg'1), while the sixth order fine root was highest (103.17-219.96 g-kg"1). The total-N concentration decreased significantly with ascending orders. The total N of first order fine root was highest (8.98-23.41 g-kg"1), while the sixth order fine root was lowest (2.46-11.99 g-kg"1). The variation of TNC concentration and total-N concentration also indicated that the lower-level roots were to absorb nutrients and the higher-level were to store substance.In conclusion, owing to the variation of morphology, anatomical architecture, TNC and total-N concentration across root branching orders, it stated that the inflection point of key indicators in most species appeared at the third orders. Therefore, the first three orders of 21 typical subtropical forest tree species were defined as fine roots, which have lower diameter, length and TNC content, higher SRL and total-N content. The total-N concentration appears to be a key factor in understanding the C cost and C turnover. Therefore, the function of the first three orders was majoring in absorption, and the coarse roots were majoring in storage and transportation. These results confirmed the theory that the first three orders were with shedding masks, died and turnovered easily from the perspective of anatomical structure and chemical content. Besides four ancient tree species which are Liriodendron chinense, Magnolia grandiflora, Michelia maudiae, Cedrus deodara, the diameter of the first three orders are |
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