Differences in the Germination Characteristics of Heracleum mantegazzianum and H sosnowskyi

To compare the germination of both species, seeds of H. mantegazzianum (Moravcovâ et al., 2006 and Chapter 5, this volume) and H. sosnowskyi were germinated at different temperature regimes. Seeds of both species were first stratified for 2 months at temperatures of 4-6°C to break the dormancy and then germinated at different temperatures: 2, 6, 10/5, 20/5, 15/10, 25/10 and 22°C (see Fig. 10.5).

The majority (71-94%) of the seeds of H. sosnowskyi germinated almost regardless of the temperature regime. The lowest germination percentages were recorded at 22°C (Fig. 10.5). However, 10% germinated during the stratification process, before setting the temperature for germination. These results suggest that the majority of the seeds of H. sosnowkyi break dormancy at almost the same time, following a stratification period as short as or less than 2 months, and immediately germinate, independently of the temperature. This accords with the early spring massive germination of H. sosnowskyi seeds, observed in the field in Lithuania as well as in the common garden burial experiment conducted in the Czech Republic. This experiment also showed that the seeds of H. sosnowskyi only germinated before March.

This is very different from H. mantegazzianum, where dormancy is broken gradually and germination can extend over several years (3 years minimally; cf. Figs 5.6 and 5.7). A study of the germination requirements, using the same design as described here for H. sosnowskyi, provided different

Fig. 10.5. Percentage of H. sosnowskyi seeds (mean ± se) that germinated when subjected to various temperature regimes and under two germination periods. The seeds were cold-stratified for 2 months at 4-6°C prior to the experiment. Seven temperature regimes were used: 2, 6, 10/5, 20/5, 15/10, 25/10 and 22°C (with alternating day/night temperatures; the day and night each lasted 12 hours). Percentage of seeds that germinated was ascertained after 2 and 6 months. Differences between times within the temperature regime are shown above bars (t-test for paired comparisons).

Fig. 10.5. Percentage of H. sosnowskyi seeds (mean ± se) that germinated when subjected to various temperature regimes and under two germination periods. The seeds were cold-stratified for 2 months at 4-6°C prior to the experiment. Seven temperature regimes were used: 2, 6, 10/5, 20/5, 15/10, 25/10 and 22°C (with alternating day/night temperatures; the day and night each lasted 12 hours). Percentage of seeds that germinated was ascertained after 2 and 6 months. Differences between times within the temperature regime are shown above bars (t-test for paired comparisons).

results for H. mantegazzianum. The stratification period for seeds of H. man-tegazzianum needed for breaking dormancy lasts at least 2 months. The highest percentage and fastest germination occurred at alternating temperatures of 20/5°C, with slower but comparably high percentages of germination at 6°C, following stratification at the same temperature. In H. mantegazzianum, all germination at the higher temperatures occurred in the first 2 months and then stopped. However, germination at low temperatures of 2°C and 6°C continued (Moravcova et al., 2006 and Chapter 5, this volume). This indicates that the higher temperatures represent a constraint on the breaking of dormancy in H. mantegazzianum and seeds remain dormant until the next cold period.

The above results can be interpreted with regard to the distribution limits of both species in Europe. The European distribution of these two Heracleum species is distinct, with H. sosnowskyi confined to the northern and eastern parts of the continent and H. mantegazzianum having the centre of the invaded distribution range in central and western Europe (see Jahodova et al., Chapter 1, this volume). An attempt to elucidate climatic factors that might have played a role in shaping this distribution was made using H. mantegazzianum as a model species. Pysek et al. (1998) show that the distribution of this species in the Czech Republic is significantly affected by the temperature in January; the number of reported localities decreases with increasing temperature, but the distribution is also determined by human population density, which reflects possibilities for dispersal.

Although the transient seed bank and immediate germination not extending over a period longer than 1 year may be a disadvantage in terms of the long-term population dynamics (Pyke, 1990; Van Clef and Stiles, 2001), the germination characteristics of H. sosnowskyi indicate that invasion by this species is unlikely to be limited by temperature. The current distribution of H. sosnowskyi seems to be driven mainly by human activities and the history of introductions, namely the massive planting in eastern Europe, than by ecological constraints.

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