[Effects of tree species identity and diversity on young tree growth in a south subtropical plantation]

To select the tree species assembly model for improving the productivity in south subtropical plantations, we carried out an experiment following a random block design with eight native tree species across a richness gradient of 1, 2, 4, and 6 species. The effects of tree species diversity and species mixing with different functional identities on the young tree growth were investigated in the 5th year of the experiment. The results showed that tree growth was not positively correlated with tree species richness. The growth of fast-growing tree species (Pinus massoniana and Mytilaria laosensis) in the monoculture was 2.5-4.5 times of the valuable broadleaved tree species (Castanopsis hystrix and Erythrophleum fordii) monoculture. Tree growth was significantly increased by 51.5%-132.8% in the conifer and broadleaved tree species mixing plantations and in the fast-growing and nitrogen fixation tree species mixing plantations, when two tree species or four tree species were mixed. There was no significant difference in tree growth among different tree species mixed types, when six tree species were mixed. The contents of soil nitrogen, phosphorus and organic matter were the main factors affecting tree growth. The results indicated that young tree growth could be improved through the selecting conifer and broadleaved tree species mixing, fast-growing and nitrogen fixation tree species mixing in south subtropical plantations.

Structure and function of soil bacterial communities in the monoculture and mixed plantation of Pinus massoniana and Castanopsis hystrix in southern subtropical China

Establishing monoculture of native broadleaved tree species and mixed coniferous broadleaved plantations is the tendency for forest management in subtropical China. The variations of structure and function of soil bacterial community in monoculture and mixed tree plantations are still not clear. We examined soil bacterial community structure and function under different soil layers (0-20, 20-40 and 40-60 cm) in three planted forests, including broadleaved Castanopsis hystrix, coniferous Pinus massoniana and their mixed plantation, in south subtropical China, using 16S rRNA gene high-throughput sequencing and PICRUSt prediction. The results showed that soil bacterial community structure of mixed plantation and P. massoniana plantation were similar but being significant different from that in C. hystrix plantation. The diversity, biological pathways metabolic function, and nitrogen cycling function of soil bacterial community in C. hystrix plantation were lower than those in P. massoniana plantation and mixed plantation. Soil total nitrogen, nitrate nitrogen and C/N were the main factors driving the variations of soil bacterial community structure and function among different forest types. Our results suggested that the mixed plantation of C. hystrix and P. massoniana is better than C. hystrix plantation in this area in terms of soil bacterial community structure and function.

[Effects of retention density on growth, biomass, and economic benefit of Cunninghamia lanceolata plantation]

We examined the effects of retention density on plant DBH (diameter at breast height), height, volume growth, stand biomass, and stand economic benefit of Cunninghamia lanceolata plantation by Pingxiang, Guangxi Province. Four treatments of different retention density were set up a 14-year middle-aged Cunninghamia lanceolata plantation, 500, 750 and 1000 trees·hm^-2, with stand without thinning as the control (1500 trees·hm^-2). Results showed that DBH (20.55 cm), increment in height (15.70 m), and large-diameter timber volume (18.31 m³·hm^-2) of the C. lanceolata plantation were the highest in 500 trees·hm^-2 treatment. The volume of living trees was the highest in the control (199.63 m³·hm^-2), which was significantly higher than that in 500 and 750 trees·hm^-2. The biomass of arbor layer and ecosystem, as well as the economic benefit differed significantly across the treatments, with arborous biomass (90.72 t·hm^-2), ecosystem biomass (94.97 t·hm^-2), and economic benefit (1.184×10⁵ yuan·hm^-2) of 1000 trees·hm^-2 treatment being significantly higher than others. Reducing stand retention density increased the DBH, plant height, timber diameter, proportion of large diameter timber, average volume and biomass of single timber, but it did not enhance the volume of living trees. The retention density of 1000 trees·hm^-2 was the optimum for middle-aged C. lanceolata plantation. Compared with the control, it significantly increased the total stand volume, arbor biomass, ecosystem biomass, and economic benefit by 2.3%, 5.7%, 4.7%, and 5.8%, respectively.

Differential effects of conifer and broadleaf litter inputs on soil organic carbon chemical composition through altered soil microbial community composition

A strategic selection of tree species will shift the type and quality of litter input, and subsequently magnitude and composition of the soil organic carbon (SOC) through soil microbial community. We conducted a manipulative experiment in randomized block design with leaf litter inputs of four native subtropical tree species in a Pinus massoniana plantation in southern China and found that the chemical composition of SOC did not differ significantly among treatments until after 28 months of the experiment. Contrasting leaf litter inputs had significant impacts on the amounts of total microbial, Gram-positive bacterial, and actinomycic PLFAs, but not on the amounts of total bacterial, Gram-negative bacterial, and fungal PLFAs. There were significant differences in alkyl/O-alkyl C in soils among the leaf litter input treatments, but no apparent differences in the proportions of chemical compositions (alkyl, O-alkyl, aromatic, and carbonyl C) in SOC. Soil alkyl/O-alkyl C was significantly related to the amounts of total microbial, and Gram-positive bacterial PLFAs, but not to the chemical compositions of leaf litter. Our findings suggest that changes in forest leaf litter inputs could result in changes in chemical stability of SOC through the altered microbial community composition.

[Characteristics of carbon storage and its allocation in Erythrophleum fordii plantations with different ages]

Carbon storage and its allocation of 7-, 29- and 32-year-old Erythrophleum fordii plantation ecosystems in Guangxi were studied on the basis of biomass survey. The results showed that the carbon contents in different organs of E. fordii, ranging from 509.0 to 572.4 g x kg(-1), were in the order of stem > branch > root > bark > leaf. No significant differences in carbon content were observed among the shrub, herb and litter layers of the E. fordii plantations with different ages. Carbon content in the soil layer (0-100 cm) decreased with increasing the soil depth, but increased with increasing the stand age. The carbon storage of the arbor layer was 21.8, 100.0 and 121.6 t x hm(-2) for 7-, 29- and 32-year-old stands, respectively, and the order of carbon storage allocation in different organs was same as the order of carbon content. The 7-, 29- and 32-year-old E. fordii plantation ecosystems stored carbon at 132.6, 220.2 and 242.6 t x hm(-2), respectively. The arbor layer and soil layer were the main carbon pools, accounting for more than 97% of carbon storage in the ecosystem. Carbon storage allocation increased in arbor layer but decreased in soil layer with increasing the stand age. The influence of stand age on carbon storage allocation in shrub, herb and litter layers did not show a obvious regular pattern.