In this study, four tree species with different life forms (evergreen broad-leaved, coniferous species, deciduous broad-leaved, and coniferous species) were used as the experimental materials. The growth dynamics of fine roots were observed by minirhizotron method. We compared differences in the root lifespan for tree species with different life forms within and between species, and also explored the main factors affecting the lifespan of fine roots. The study would help us to understand carbon and nutrient cycling processes in forest ecosystems. The results showed that: (1) fine root morphological characteristics (branched structure and root diameter) significantly affected the lifespan of fine roots within the species. The fine root lifespan with the lower branching order and the smaller diameter was shorter than that of the higher-order roots and the coarser roots. (2) The root lifespan showed obvious soil layer effect and the seasonal effect, that is, with the increase of soil depth, the cumulative survival rate of fine roots gradually increased and the root lifespan was prolonged; while the lifespan pattern of fine roots born in different seasons was inconsistent among tree species. The lifespan of the fine roots born in spring or summer was longer than that born in autumn and winter. (3) The fine roots lifespan of evergreen trees (Lithocarpus glaber and Cryptomeria fortunei) was longer than that of deciduous trees (Quercus acutissima and Taxodium ascendens). The fine root lifespan of conifers trees was longer than that of broadleaf species. Generally, within the tree species, the fine root lifespan is significantly affected by factors such as fine root diameter, root branching structure, and soil environmental factors (soil layer). However, between tree species, fine root lifespan may be more dependent on the differences in functional traits of trees as a whole, such as tree growth rate, carbohydrate distribution patterns.