Conclusions
The results indicate that only a small proportion of the variation in stand-level FRB can be explained by environmental or tree stand variables, whereas there seems to be a higher correlation between the tree-level FRB and the mean basal area of the forest stand. The clear relationships between the ≤1 mm and ≤2 mm diameter classes and between the ≤2 mm and ≤5 mm enabled us to standardize the FRB data to the ≤2 mm diameter class. The analysis indicated that the sampling depth in most reports did not cover the whole rooting depth. If this is not taken into account, biased underestimates might be obtained for the FRB in some biomes. A more consistent determination of fine root parameters and reporting of stand and environmental characteristics in the future would facilitate the use of the results of the fine root studies for gaining a better understanding of the C cycle in forest ecosystems.