Miaojun Ma

Seasonal dynamics in alpine meadow seed banksalong an altitudinal gradient on the Tibetan Plateau

We studied seasonal dynamics of seed banks along an altitudinal gradient in three alpine meadows on the Tibetan Plateau, as well as seed size distribution relative to depth. Seed bank density and species richness decreased significantly with altitude increase in both April and July. The highest elevations showed highest seed bank depletion. Although species composition of the vegetation changed along the altitudinal gradient, seed bank composition was almost unvaried from April to July. This indicated that seed bank density has an obvious seasonal change but composition does not. We found a low degree of similarity between the species composition of vegetation and of the seed bank along the whole gradient, and this similarity decreased with altitude increase. These suggest that the role of the seed bank decreased gradually with altitude increase. The hypothesis that a species seed bank strategy is an inherited trait was not confirmed because for some species seed bank strategy changed with altitude. We found that persistent seed banks were the most frequent strategy at all three altitudes. Species with persistent seeds tended to have smaller seeds than those with transient seeds only in highest altitude, while the other two altitudes did not show difference. There was no trend in seed size distribution with altitudes and soil depths.

Effect of Diurnal Fluctuating versus Constant Temperatures on Germination of 445 Species from the Eastern Tibet Plateau

Germination response to fluctuating temperatures is a mechanism by which seeds detect gaps in vegetation canopies and depth of burial in soil, and it is very important for plants. Thus, studies on the effect of fluctuating temperature on germination at the community level are valuable for understanding community structure and biodiversity maintenance. We determined the effects of two alternating temperatures (5/25°C and 10/20°C) and one constant temperature (15°C) on seed germination of 445 species in a grassland community on the eastern Tibet Plateau. Seed mass was determined for each species, and data on habitat, type of life cycle, altitudinal distribution and functional group (graminoids or forbs) were obtained from the literature. Taking all species into account, alternating temperatures increased germination percentages regardless of amplitude. Overall, species growing in disturbed ground showed a significant germination response to temperature fluctuation, but those living in Alpine/subalpine meadow, forest margin /scrub, marshland and dry sunny slope habitats did not. Species distributed only at high elevations (>2000m) did not show a significant germination response to temperature fluctuation, whereas those occurring at both high and low elevations had a significant positive response. Germination of annuals/biennials was significantly promoted by 5/25°C, but not by 10/20°C, whereas germination of perennials was significantly promoted by both 5/25°C and 10/20°C. Small-seeded species were more likely than large-seeded species to respond positively to fluctuating temperatures. Germination of forbs had a positive response to temperature fluctuation, but germination of graminoids did not. Regeneration ability by seeds for about 36% of the species studied in the grassland can be increased by temperature fluctuation. The differential response among species to alternating vs. constant temperatures helps maintain community structure and biodiversity. A positive germination response to temperature fluctuation can partly explain why there are more forbs in degraded meadows.

Soil seed bank dynamics in alpine wetland succession on the Tibetan Plateau

The primary goal was to address several questions with regard to how soil seed banks change in a successional series. How does the composition of the viable seed bank change, and how does the relationship of the soil seed bank and vegetation change with succession? Can the seed bank be regarded as a potential as a source of seeds for wetland restoration? We collected soil seed bank samples and sampled the vegetation in four different successional stages and used the NMDS (nonmetric multidimensional scaling) to evaluate the relationship of species composition between the seed banks and vegetation. The difference of seed density and species richness in different habitats and soil depths also was compared. Viable seeds of half (37) the species in the early-successional stage were found in all the successional stages. Similarity between seed bank and vegetation increased with succession. Both seed density and species richness in the seed bank increased with successional age and decreased with soil depth. The majority of species from the early-successional stage produced long-lived seeds. Seed density and species richness increased with succession, mainly as a result of increasing seed production, and hypotheses predicting decreasing density of buried seeds and species richness were not confirmed. Seed banks play a minor role in contributing to the regeneration of vegetation, and managers cannot rely on soil-stored seed banks for restoration of wetlands.

Role of the Soil Seed Bank during Succession in a Subalpine Meadow on the Tibetan Plateau

Abstract We studied the seed bank of abandoned fields in four successional stages (habitat A: 1 year; habitat B: 10 years; habitat C: 20 years; and habitat D: mature meadow) in eastern Tibetan Plateau. The seed density in seed banks decreased with successional age, but species richness and diversity increased, and the highest species diversity and species richness appeared in habitat C. Similarity between the seed bank and vegetation decreased gradually with succession in the whole. The vegetation is more similar to the seed bank in the shallow layer than to the seed bank in the deeper soil, which shows that the vegetation contributes less to the seed bank as soil depth increases. The seed bank was mainly composed of first successional species during the whole successional range. Species of the later successional stages contributed little to the seed bank. Most of species from early successional stage produced longer-lived seeds, which stayed viable in the soil for a long period (more than 20 years). Hypotheses about changes in seed bank during succession, predicting decreasing species richness and seed diversity, were not confirmed. The hypothesis that density of buried seeds declined during succession was confirmed. We conclude that seed bank plays an important role on vegetation in the early succession stage. In the later succession stages, seed banks role becomes weaker and weaker, and which likely relies on vegetative reproduction and dispersal.

Seasonal Dynamics of the Plant Community and Soil Seed Bank along a Successional Gradient in a Subalpine Meadow on the Tibetan Plateau

Background Knowledge about how change the importance of soil seed bank and relationship between seed mass and abundance during vegetation succession is crucial for understanding vegetation dynamics. Many studies have been conducted, but their ecological mechanisms of community assembly are not fully understood. Methodology We examined the seasonal dynamics of the vegetation and soil seed bank as well as seed size distribution along a successional gradient. We also explored the potential role of the soil seed bank in plant community regeneration, the relationship between seed mass and species abundance, and the relative importance of deterministic and stochastic processes along a successional gradient. Principal Findings Species richness of seed bank increased (shallow layer and the total) and seed density decreased (each layer and the total) significantly with succession. Species richness and seed density differed significantly between different seasons and among soil depths. Seed mass showed a significant negative relationship with relative abundance in the earliest successional stage, but the relationships were not significant in later stages. Seed mass showed no relationship with relative abundance in the whole successional series in seed bank. Results were similar for both July 2005 and April 2006. Conclusions The seed mass and abundance relationship was determined by a complex interaction between small and larger seeded species and environmental factors. Both stochastic processes and deterministic processes were important determinants of the structure of the earliest stage. The importance of seed bank decreased with succession. The restoration of abandoned farmed and grazed meadows to the species-rich subalpine meadow in Tibetan Plateau can be successfully achieved from the soil seed bank. However, at least 20 years are required to fully restore an abandoned agricultural meadow to a natural mature subalpine meadow.

Effects of Water Level on Three Wetlands Soil Seed Banks on the Tibetan Plateau

Background Although the effect of water level on germination in soil seed banks has been documented in many ecosystems, the mechanism is not fully understood, and to date no empirical studies on this subject exist. Further, no work has been done on the effect of water level on seed banks of drying and saline-alkaline wetlands in alpine areas on the Tibetan Plateau. Methodology We examined the effects of water level (0 cm, 5 cm and 10 cm) on seed germination and seedling establishment from soil seed banks at 0–5 cm and 5–10 cm depths in typical, drying, and saline-alkaline wetlands. We also explore the potential role of soil seed bank in restoration of drying and saline-alkaline wetlands. Principal Findings Species richness decreased with increase in water level, but there almost no change in seed density. A huge difference exists in species composition of the seed bank among different water levels in all three wetlands, especially between 0 cm and 5 cm and 0 cm and 10 cm. Similarity of species composition between seed bank and plant community was higher in 0 cm water level in drying wetland than in the other two wetlands. The similarity was much higher in 0 cm water level than in 5 cm and 10 cm water levels in all three wetlands. Species composition of the alpine wetland plant community changed significantly after drying and salinization, however, species composition of the seed bank was unchanged regardless of the environment change. Conclusions/Significance Water level greatly affects seed bank recruitment and plant community establishment. Further, different water levels in restored habitats are likely to determine its species composition of the plant community. The seed bank is important in restoration of degraded wetlands. Successful restoration of drying and salinization wetlands could depend on the seed bank.