Abstract
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.
Highlights
Germination is one of the most critical stages in the life cycle of plants [1,2], and it is the main source of variation in the regeneration niche [3], which is one of the core mechanisms maintaining biodiversity within a plant community
Alternating temperatures had a significant effect on mean germination percentage of the 445 species, which was significantly higher at the two alternating temperature regimes than it was at the one constant temperature (Figure 1)
Temperature fluctuation improved germination at the community level, and the germination response to temperature fluctuation was affected by seed mass, life cycle type, habitat, altitudinal distribution and functional group (Figure 3)
Summary
Germination is one of the most critical stages in the life cycle of plants [1,2], and it is the main source of variation in the regeneration niche [3], which is one of the core mechanisms maintaining biodiversity within a plant community. Information on seed germination is required for understanding the distribution and life cycle traits of plant species and how they coexist in the same community. Germination is controlled by many environmental factors such as light, temperature and soil moisture [4,5,6]. These environmental factors can affect germination directly or indirectly. Temperature can regulate germination directly by activating enzymatic reactions occurring in the process of germination and by preventing or promoting the synthesis of hormones that affect the status of seed dormancy [5,7]. If seeds require the low temperatures of winter for dormancy break, they usually become nondormant during winter and can germinate the following spring or early summer. Maximum, optimum and fluctuating temperature requirements for seed germination can partly explain the germination niche of a species and its habitat requirements and distribution
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