Abstract:

Fine root is a critical determinant of root turnover and therefore carbon (C) and nutrient flow in terrestrial ecosystems. Despite the ecological importance of fine root longevity and the tremendous research efforts devoted to it,the understanding of fine root longevity and turnover is still rudimentary. This article reviews some of the most important factors that control fine root longevity,including carbon allocation,fine root structure,soil nitrogen (N) and water availability,soil temperature,and soil biota in forest ecosystems,with the purpose of providing a brief summary of recent advances in fine root longevity research and to point out the gaps of understanding and directions for future root longevity research in China.The most important function of fine roots is resource acquisition from the soil. To perform this function,C fixed in leaves must be used to build the fine root biomass and to supply energy needed for root growth,resource uptake,and maintenance of the fine roots. Consequently,C availability to roots may exert strong control over root longevity,and the interactions between C source (leaves) and C sink (roots) has been postulated as a mechanism through which root longevity may be explained. However,due to the lack of experimental evidence,the mechanisms by which C is allocated to roots and how C availability in roots controls root longevity remains poorly understood. Detailed studies on C allocation and utilization in processes such as root growth and root respiration (in growth and maintenance) are needed.In addition to physiological controls of root longevity (e.g.,C availability),structural characteristics of root systems also have a strong influence over root longevity. Recent studies showed that both root diameter and branching order were important regulators having shorter lifespans and turn over more rapidly than larger diameter,higher order roots. These findings are likely to contribute greatly to a more accurate quantification and prediction of C and nutrient flow via root turnover. The efficiency model,which suggests that the mortality of fine roots may occur when the C costs of plant roots exceed the benefits (e.g.,nutrient acquisition) they provide,also has been invoked to explain root mortality and longevity. This model necessitates that root longevity is closely related to resource availability in the soil. Some evidence suggests that roots in resource rich sites live longer,while others suggest the opposite. Much of the controversy may result from different methods used in estimating fine root longevity and turnover,and differences in C sink-source relationships among different species,stand development,as well as the length of the experiments in which the influence of resource levels on root longevity are tested. Climatic factors also influence root longevity. Fine root longevity appears to be the longest in cold environments with marked seasonal variations in seasonal environments. However,most recent evidence suggests that root longevity may be more dependent on root structure than on root environment,with first order roots having similar longevity regardless of differences in species and the root environment.Finally,ecosystem-scale processes,such as environmental stresses and pathogen and herbivory pressure,may also influence root longevity. Drought and high temperatures may shorten root longevity. However,because soil moisture,temperature,leaf C fixation,and soil resource availability are tightly linked,independent tests of moisture and temperature influences on root longevity are difficult to conduct. Evaluating the regulation of herbivores and pathogens on root longevity is also difficult,in part because little quantitative information is available on the populations of root pathogens and herbivores in different ecosystems and the responses of root systems to different levels of pathogen and herbivory pressure. In sum,root longevity is a critical but difficult research subject. There are many opportunities as well as challenges for future root longevity research in China,of which we list only a few: 1) C allocation and C utilization in fine roots may be a primary mechanism controlling root longevity,but,to understand this mechanism fully,the methods for accurately estimating C allocation and utilization in roots of different structural and functional characteristics must be developed first;2) Root longevity is clearly related to soil resource availability (e.g. N and moisture) and environmental factors (e.g.,soil temperature and moisture),and further studies on the impact of these factors on the longevity of individual roots and root systems as a whole may prove fruitful. Studies in this area should be large-scale and long-term to encompass large environmental variability frequently observed in natural ecosystems and to provide relevant information for a better understanding of the belowground

Key words: Forests, Fine root longevity, Fine root turnover, Root carbon and nitrogen, Root structure