Fine root phenology differs among subtropical evergreen broadleaved forests with increasing tree diversities

Fine root phenology represents ecological strategy of plants in utilizing soil resources and plays an essential role in carbon and nutrient cycling. Here, we aimed to understand how fine root phenology could change in subtropical broadleaved forests with increasing tree species diversity. Three 37-40 years old Castanopsis carlesii forests with low to high tree species diversity were selected in southern China: a plantation forest (PF), a secondary forest through the assisted regeneration (AR), and a secondary forest through natural regeneration (NR), with a nearby natural C. carlesii forest as a control. Fine root length production in these forests was measured monthly during January 2013 to December 2014 using minirhizotron, and fine root production phenology and its correlations with environmental factors were compared among these forests. No significant difference in fine root length production was found among these forests; however, there was distinct fine root phenology. The timing of fine root growth initiation and growth peak was quite stable across the two years in PF, while they were more flexible in AR and NR which were higher in tree species diversity. However, the timing of fine root growth cessation was quite fixed for all the forests. The duration and inter-annual stability of fine root growth both decreased with increasing tree species diversity. Moreover, the correlations of monthly fine root production with monthly air and soil temperature both increased with increasing tree species diversity. It is concluded that the timing of fine root growth in higher tree species diversity stand seems more flexible and more synchronous with seasonal and annual fluctuations of soil resources caused by weather variability, which helps our understanding of the relationships between tree diversity and ecosystem carbon and nutrient cycling.