Seed Dispersal And Seedling Regeneration Patterns In The Jiaohe Broadleaf-conifer Forests, Jilin Province

The regeneration of seed plants may be divided into three distinct stages:seed rain, seed dispersal and seedling establishment. These stages have been regarded as very important in determining the future composition and structure of a forest community. Understanding the relationship between seed dispersal and seedling regeneration, and the relative importance of dispersal limitations, negative density dependence and niche processes in seedling regeneration, contributes to understanding the formation, maintenance, and evolution mechanism of biodiversity and provides a theoretical basis for biodiversity conservation in a forest ecosystem.We established a total of 415 seed-seedling census stations in three large permanent field plots. The plots belong to three successional stages, a half-mature forest (HF,21.84 ha), a mature forest (MF,42 ha), and an old-growth forest (OGF,30 ha) in the conifer and broad-leaved mixed forests. Based on a 3-year observation period, we analyzed the seed rain and seedling composition patterns over time and space. The effect of negative density dependence and niche processes on seedling survival was verified at local scales. In addition, the role of seed dispersal, negative density dependence and niche processing in maintaining the biodiversity was evaluated. The main results of the study are as follows:1. Seed rain exhibits a seasonal dynamic in all three forest plots and at different successional stages, sometimes showing a double-peak pattern. The seed yield of most species showed obvious annual and spatial variation, the spatial variation being greater than the annual variation. However, the annual and spatial variations in the seed yield did not differ among the different successional stages.2. Multiple linear regression was used to test the relationship between seedling abundance (for five major species in the three forest types) and two variables:a) the sum of the basal areas of conspecific adults within 20 m of a seed trap and b) canopy openness. The results show that: (1) The species composition of seedlings slightly varied among different successional stages, but was similar with that of adult trees in the plots. The rank of importance values for seedling species among different years and successional stages varied slightly. (2) The number of seedlings and the number of seedling species was greatest in the 5-20 cm height classes, and decreased with the increasing height class. The number of seedlings decreased faster than that of seedling species. This result indicates that compared with interspecific competition, intraspecific competition was the main driver that led to seedling death, which also confirmed the role of the negative density dependence (NDD) in affecting seedling regeneration. (3) The amount of seedlings for the five major species (Fraxinus mandshurica、Acer mono、Acer tegmentosum-Acer mandshuricum-Tilia amurensis) was significantly positively related to the sum of the basal area of conspecific adults, which indicated that the quantity and distribution of seedlings were affected by dispersal limitation as well as niche processes. Our research thus confirmed the role of negative density dependence, dispersal limitation and niche processes on seedling composition.3. We analyzed the quantitative distribution of woody seedlings and the spatial relationship between seedlings and adult trees for four major seedling species:F. mandshurica, A. pictum subsp. mono, Abies holophylla and A. mandshuricum. A Syrjala test was used to analyse interannual differences in the spatial pattern of the number of seedling individuals and the number of seedling species at station (1 trap and 4 quadrats) level. Our results show that:(1) There are obvious differences in the quantitative distribution of woody seedlings between different communities and years. Seedling density shows great spatial variability at station level. However, the spatial hereogeneity of seedling richness was low which indicates that the spatial hereogeneity of the seedling distribution had an important impact on seedling density. (2) The spatial pattern of the number of seedling individuals and the number of seedling species varied among different years at station level which partly demonstrates the influence of seed production, dispersal mechanism and community composition on the spatial distribution of seedlings. (3) Judging from the spatial relationship between seedlings and adult trees, the four major seedling species showed similar distribution patterns compared with those of the adult trees. This result in turn indicates that the distribution pattern of adult trees affects the distribution pattern of the seedlings.4. This investigation has revealed severe seed and establishment limitations in the study area. The seed limitation was greater in the early successional stage than in the middle and late stages. Recruitment limitation in the early and middle stage was more effected by the lack of seed yield and dispersal limitation, while in the late stage of succession, it was effected mainly by dispersal limitation. There was no significant negative relationship between seed and establishment limitation, which indicates that our result does not support the hypothesis of a colonization-competition tradeoff which promotes species coexistence. However, the dispersal limitation was positively related to the establishment limitation, which indirectly demonstrates the role of negative density dependence in promoting species coexistence. In addition, we did not find that seed and establishment limitation was regulated by seed size. However, seedling establishment was affected by biotic factors.5. We analyzed the effect of biotic and abiotic neighborhoods and evaluated the variation among species regarding the effect of biotic neighborhoods on seedling survival. We found convincing evidence for the existence of NDD in the temperate forest ecosystems. The biotic and abiotic variables affecting seedling survival change with successional stage, seedling size, and age. The strength of NDD for the smaller (<20 cm in height) and younger seedlings (1-2 years) as well as all seedlings combined varies significantly among species. We found no evidence that a community compensatory trend (CCT) existed in our study area. The results of this study demonstrate that the relative importance of NDD and habitat niche partitioning in driving seedling survival varies with seedling size and age and that the biotic and abiotic factors affecting seedling survival change with successional stage.