Above- and below-ground biomass relationships of Leucaena leucocephala (Lam.) de Wit in different plant stands

The above and below-ground biomass (AGB and BGB) relationship is often used to assess the impact of biotic and abiotic effects on the growth and development of individual plants. The AGB and BGB relationship of the same tree species in different habitats can change significantly because of environmental stress. To investigate how the tree size, the biomass allocation and BGB/AGB ratio of Leucaena leucocephala (Lam.) de Wit varied according to spacing and mixed plant patterns in a valley-type savanna of southwest China, we examined the growth of L. leucocephala, and sampled 23 individuals for biomass measurement in each of four treatments (close/wide spacing of Leucaena leucocephala monocultures, mixed plantation of Leucaena leucocephala and Eucalyptus camaldulensis, and mixed plantation of Leucaena leucocephala and Eucalyptus citriodora), and then determined the regression relationships between AGB and BGB of L. leucocephala in different plant stands. Our results indicated that mixed planting significantly reduced all growth metrics for the tree sizes of L. leucocephala and increased the value of BGB/AGB. Changing plant spacing in monocultures had a significant impact on AGB and TB (Total Biomass) of L. leucocephala, but it had no significant effect on the other metrics. Within mixed plant schemes, L. leucocephala significantly reduced the biomass allocation to leaves and small roots and increased the allocation to coarse root biomass. There were no significant differences in tree size and biomass allocation of L. leucocephala between different spacing regimes in monocultures or between different mixtures in mixed plant stands. The correlation between BGB and AGB of L. leucocephala in all plant stands was consistent with the model of allometric growth, and AGB can be used to accurately estimate BGB. Interestingly, the correlations were not exactly the same. BGB and AGB in monoculture showed isometric growth, and their values in mixed plant stands showed allometric growth. BGB also increased faster than AGB. The findings indicated that L. leucocephala allocated more biomass to the root system when it was planted with Eucalyptus.