Image analysis techniques for studying phenology and annual production of fine roots in forests: a review

Fine root production (FRP) is an important process in carbon and nutrient cycling in forest ecosystems. Here, we review methodologies for estimating FRP nondestructively, such as minirhizotron and root scanner methods. These nondestructive methods can track individual roots, enabling detailed observation of temporal changes in fine root growth. They can also include the growth of fine roots with a short lifespan in FRP estimates, which is overlooked in destructive methods. Recent studies have also proposed automatic image analysis techniques, which would drastically increase the amount of information available in a less laborious manner than conventional manual analyses. However, these nondestructive methods need to convert two-dimensional image data to three-dimensional mass data for evaluating the contribution of FRP to forest productivity. In this review, we show several calculation procedures for such data conversion and discuss issues to be improved in future studies. In addition, we identified 38 reported values of annual FRP across Japan, 8 of which were obtained using nondestructive methods (77–726 g m⁻² year⁻¹). The FRP estimates by the nondestructive methods were similar to those by sequential coring and tended to be greater than those by the net sheet and ingrowth core methods. The annual FRP generally correlated with the examined soil depth and fine root biomass (FRB) in the same stands, which might be partly due to smaller FRP by the net sheet and ingrowth core methods with smaller sampling depth. Further improvement of the nondestructive methods would contribute to better estimating of FRP in future studies.