Light And Patterns Of Seed Germination And Seedling Recruitment In The Eastern Of Qinghai-Tibetan Plateau

Regeneration from seed is a key of community procedure and species diversity maintenance. Predicting community construction from seed——the start point of plant life circle-is the ambitious goal of ecology researchers.Here, we tried to explore the upscale effects of seed traits (seed size and germination) combined with environment factor (light availability) on community level. Two field experiments were conducted. (1) the germination time courses expressed in the field by 695 Angiosperm species from the eastern Qinghai-Tibet Plateau were monitored across three light availability treatments (high light treatment:100% unfiltered natural light; intermediate light treatment: 36% natural light; low light treatment: 2.9% natural light). (2) first-year seedling survival was monitored from germination for 305 angiosperms species of the eastern Tibetan Plateau grasslands, across a light gradient (4% Natural Light, 36% NL, 67% NL, and 100%NL).Specifical questions which we addressed and corresponding main resuls and conclutions as follow.1) Diverse germination behavior in eastern Tibetan Plateau grasslandSpecies within a plant community can exhibit diverse germination behavior, and environmental conditions can alter germination behavior and its diversity within plant communities. Here, we trid to understand the diverse germination behavior in eastern Tibetan Plateau grassland and whether germination behavior change with light availability changing.We found pronounced variation in germination behavior, ranging from very rapid germination and high germination proportions to slow, asynchronous germination at very low levels, with several intermediate categories. Light availability altered the frequencies of germination behavior categories, and tended to canalize germination behavior into the extreme categories of rapid and non-dormant germination or delayed germination and high levels of dormancy. The diversity of germination behavior within a single plant community may be maintained in part by the correlated selection acting on germination and other life-history traits, with seed mass or iteroparity modifying selection on germination. Germination exhibited strong phylogenetic structuring, such that related taxa exhibited similar germination behavior and similar germination responses to light. The observation that closely related taxa within a plant community exhibit similar germination behavior and plasticity suggests that germination can influence the patterns of phylogenetic relatedness among interacting species and within plant communities more generally.2) Light availability, seed mass and temporal niche differentiationGermination timing is a significant determinate of plant performance and success.Within a community,species may germinate at different times so as to mitigate competition and to take advantage of different aspects of the seasonal environment(temporal niche differentiation). How species partition temporal niche space by germination time will depend on the nature of competitive interactions between species. How abiotic and biotic competitive factors combined influence germination timing, however,remains poorly understood. We illustrated a hypothesis of the combined effects of biotic and abiotic competitive factors on germination timing and the subsequent upscale effects on community assembly.We found a significant negative correlation between GT and seed mass in both light treatments. Additionally, small seeds (early germinating seeds) tended to germinate later and large seeds (late germinating seeds) tended to germinate earlier under low light vs. high light conditionsWe found that low light availability can reduce temporal niche differentiation by increasing the overlap in germination time between small and large seeds. In turn, the reduced temporal niche differentiation may strengthen deterministic factors such as competition in the process of community assembly. Given the phylogenetic signal in germination time, shift in germination timing in response to light availability, and seed mass, these dynamics might subsequently influence the phylogenetic structure of a community.3) Light availability, germination timing and life cycleGermination timing is a key transition of life history. It not only links subsequent life-history traits, such as plant height and flower timing, but also provides a link to the previous generation through the influence of the maternal environment.Environment factors may mediate these key links, and consequences of this process may influence species regeneration and dispersal. However, little is known about how environment factors mediate these key links.We found that, in general, there were significant correlations of germination timing with water and light in maternal habitats in high light but not in low light,while there were significant correlations of germination timing with plant height,onset of flowering and elevation in low light but not in high light, i.e. there are light-dependent associations of germination timing with subsequent life-history traits and maternal habitats. In sum,light-dependent associations of germination timing with subsequent life-history traits and maternal habitats may influence the probability of plant species life-cycle completion and influence distribution and dispersal of plant species in natural plant communities.4) Light availability, seed size and offspring survivalTheoretical models predict a positive relationship between seed size and subsequent offspring survival. Yet, this relationship is uncertain under different environment. Furthermore,we know little about the experimental evidence of this relationship in relation to seedling emergence time,specific leaf area, biomass allocation or biomass accumulationWe found the positive size-survival relationships just under lower light treatments. This relationship, however, is the resultant of the indirect nature, via biomass accumulation, of the effect of seed size on seedling survival. The direct effect of seed size on seedling survival is negative. Seed size significantly related biomass accumulation, and biomass accumulation significantly related seedling survival. Both relationships were becoming stronger with light availability decreases. To large extent,light gradient change seedling recruitment, and, specifically, it not only influenced seedling emergence time,specific leaf are, biomass allocation (root: shoot biomass ratio) and accumulation, and seedling survival, but also changed relationships among these traits. In sum, larger seed provides survival advantages through positively influencing biomass accumulation just under lower light treatments. Our results also suggest large seeds can bring a given risk to their large seedlings. Light influences many aspects of seedling recruitment.