Dynamics of shade tolerance, biomass, and individual growth of five understory plant species in Eucalyptus urophylla plantations

In order to investigate the ecophysiological characteristics and growth adaptability of different understory plants in Eucalyptus urophylla plantations, five understory species with high economic and medicinal values, including Ilex asprella, Gardenia jasminoides, Melastoma candidum, Rhodomyrtus tomentosa, and Litsea cubeba, were selected to measure leaf characteristics, chlorophyll content,and photosynthetic light response curves. In addition, the dynamics of biomass and individual number were analyzed based on field sampling survey data in 2006 and 2011. Comparative analysis and cluster analysis revealed that: (1) the leaf width, thickness, and area of M. candidum were the largest, whereas those of I. asprella were the smallest. Overall, the net photosynthetic rate of the selected understory plants increased with increasing light intensity. However, the net photosynthetic rate of I. asprella and G. jasminoides increased more rapidly than that of M. candidum and L. cubeba, and reached the light saturation point more quickly. R. tomentosa, M. candidum, and L. cubeba had higher light saturation points than that of I. asprella and G. jasminoides. The selected understory plants could be classified into two groups in terms of shade-tolerance ability, based on hierarchical cluster analysis. I. asprella and G. jasminoides were assigned to the strong shade-tolerant group, whereas M. candidum, L. cubeba, and R. tomentosa were assigned to the low shade-tolerant group. (2) Biomass of the five selected understory species increased with stand age,but the range of increment varied. The greatest increase was observed in M. candidum (74 fold), followed by R. tomentosa (18 fold), and the least was observed in L. cubeba (1.3 fold). The increase in biomass of M. candidum, R. tomentosa, and L. cubeba was mainly caused by the increase in individual height and basal diameter,whereas the increase in biomass of I. asprella and G. jasminoides was mainly attributed to the increase in individual number, height, and basal diameter. (3) I. asprella, a shade-tolerant understory species with high economic value,dominated the shrub community with increasing stand age of the Eucalyptus plantations, mainly owing to the increase in density, individual height, and basal diameter. G. jasminoides showed lower dominance over the shrub community owing to lesser increase in individual height and basal diameter than those of I. asprella. The density of L. cubeba, R. tomentosa, and M. candidum sho