Effects of cluster fragmentation on root growth
At the beginning of this study, C2 had more than 2-fold root biomass than C1, but at the end of study, C2 showed 1.26-fold, 1.34-fold, 1.44-fold, 1.42-fold and 1.79-fold higher total root length, root surface area, root volume, root tips and root biomass than that of C1. This result might indicate that the root growth rate of S. breviflora was higher for the small sub-cluster than the large one. Moreover, the effects of nitrogen, phosphorus and water addition on root morphological traits and root biomass under the treatment of C1 and C2 were different. During the further analysis, within the treatment of C1 we found that the total root length, root surface area and root biomass were significantly lower under high–level N addition than that under low–level N addition, whereas there were no changes within the treatment of C2 (P ≤ 0.05, Table S1). These results also can be explained by the ideas of Hodge (2004), the proliferation of root systems was determined by the content of extra and intra nutrients of plants together. For example, compared with the control,Poa pratensis had a shorter root length, lower root length density and smaller root biomass in a N–rich patch, where they can capture 13% of its total N content (Hodge et al. 1998), but in another study where N patch represented a larger proportion of its total N (18%), they had a higher root length density and higher root biomass (Hodge et al. 1999). In our study, compared with C2, C1 did not need more nitrogen to support their individual growth, which may be caused by high-N addition exceeding the specific thresholds of nitrogen to induce root growth for C1. So, N was a major limiting factor to the root growth of small sub - cluster.