Investigation and Analysis of Understory Vegetation in Casuarina equisetifolia Forests in Northern Hainan Province

Understory vegetation in the Casuarina equisetifolia forests at different ages was sampled and analyzed fi rstly, and then the structural features, biomass, and distribution characteristics of carbon stock of the understory vegetation, as well as the quantity and thickness of the understory litter were studied. The results show that the understory vegetation in the C. equisetifolia forests is mainly composed of herbaceous plants, and there are nearly no shrubs in the study area. Moreover, the quantity of herbaceous plants rises obviously with the increase of forest age. There is a big difference between the woodlands at different ages in total biomass of herbaceous plants, and the maximum of total biomass can be found in 15-year-old woodland, reaching 1.68 t/hm^2; the biomass of overground parts of herbaceous plants is higher than that of underground parts. There is a big difference between the woodlands at different ages in thickness of the understory litter, and the thickness of understory litter in 10-yearold woodland is the highest, reaching 6.67 cm. The maximum quantity of the understory litter has reached 14.69 t/hm^2, and the thickness of the half decomposed litter is larger than that of the undercomposed litter in most woodlands.

Ecological niche modeling of the main forest-forming species in the Caucasus

Background:Ecological niche modeling of the main forest-forming species within the same geographic range contributes significantly to understanding the coexistence of species and the regularities of formation of their current spatial distribution.The main abiotic and biotic environmental variables,as well as species dispersal capability,affecting the spatial distribution of the main forest-forming species in the Caucasus,have not been sufficiently studied.Methods:We conducted studies within the physiographic boundaries of the Caucasus,including the Russian Federation,Georgia,Armenia,and Azerbaijan.Our studies focused on ecological niche modeling of pure fir,spruce,pine,beech,hornbeam,and birch forests through species distribution modeling and the concept of BAM(Biotic-Abiotic-Movement)diagram.We selected 648 geographic records of pure forests occurrence.ENVIREM and SoilGrids databases,statistical tools in R,Maxent were used to assess the influence of abiotic,biotic,and movement factors on the spatial distribution of the forest-forming species.Results:Geographic expression of fundamental ecological niches of the main forest-forming species depended mainly on topographic conditions and water regime.Competitor influence reduced the potential ranges of the studied species by 1.2–1.7 times to the geographic expression of their realized niches and led to differences in the distribution of species with similar requirements for abiotic conditions.Movement factor significantly limited the areas suitable for pure forests(by 1.2–1.8 times compared with geographic expression of realized ecological niches),except for birch forests.Conclusion:Distribution maps,constructed by abiotic,biotic,and movement factors,were the models of the occupied distributional area of the forest-forming species in the Caucasus.Biotic and movement factors should be considered in modeling studies of forest ecosystems if models are to have biological meaning and reality.