Dynamics of understory vegetation biomass in successive rotations of Chinese fir (Cunninghamia lanceolata) plantations

Understory vegetation is an important component of forest ecosystems,and is known to have beneficial effects on soil fertility and nutrient cycling.Understory abundance and structure can be expected to change with forest development and understanding of dynamic properties of understory vegetation communities is of high significance in sustainable management of a forest ecosystem.However,quantitative assessment of understory dynamics in subtropical forests is lacking.The objective of this study was to examine how understory vegetation abundance,species composition and biomass production change with stand development after the second successive rotations of Chinese fir(Cunninghamia lanceolata) plantations were established at the same sites.This study has been carried out in Chinese fir forested small watersheds,located at the Huitong Ecosystem Research Station of the Central South University of Forestry and Technology in Hunan Province,one of the National Field Station for Scientific Observation and Experiment in China.The results showed that understory vegetation species was declined at 14 year after the establishment of the second rotation of Chinese fir plantations when compared with those at 3 year,of which woody plant species and herbaceous plant species decreased 40.0% and 47.1%,respectively.Understory vegetation biomass declined from 29.48 t/hm2 at 3 year to 2.53 t/hm2 at 14 year of the studied Chinese fir plantations,of which biomass of woody plant species and herbaceous plant species declined from 7.07 t/hm2 to 1.25 t/hm2 and from 22.41 t/hm2 to 1.28 t/hm2,respectively.In other words,understory biomass of woody and herbaceous plant species at 14-year old stands decreased by 82.3% and 94.3% compared that at 3-year old stands,respectively.Tree seedlings(6068.97 kg/hm2) dominated understory woody plant species biomass at 3 year of the studied stands,and the next was liana species biomass(736.97 kg/hm2) and shrubs species biomass(259.87 kg/hm2). Tree seedlings,liana and shrubs accounted for 85.9%,10.4% and 3.7% of the total understory woody plant species biomass.In contrast,understory shrubs biomass was the highest(881.87 kg/hm2) at 14 year in the studied stands,and the second was liana biomass(247.07 kg/hm2),and the tree seedlings biomass was the lowest(117.87 kg/hm2). The three group accounted for 70.7%,19.8% and 9.5% of the total understory woody plant biomass.Among understory herbaceous plant biomass at 3 year after the second rotation of Chinese fir stands were planted,biomass of fern(8391.44 kg/hm2) was the highest and accounted for 37.4% of the total herbaceous biomass,and Lysimachia christinae biomass was lowest(36.77 kg/hm2),only accounting for 0.16% of the total biomass.When the stands reached at 14 year old,biomass of Miscanthus sinensis(573.00 kg/hm2) was the highest and Hedyotis chrysotricha(2.93 kg/hm2) was the lowest among the understory herbaceous biomass components.The results demonstrated that understory biomass production and distribution change with stands development in the second successive rotations of Chinese fir plantations.