Mechanisms of Dispersal

Fruits of H. mantegazzianum are elliptical, winged and dispersed mostly by wind, water and human activities. The majority of ripe fruits fall close to mother plants. For plants 2-m high, 60-90% of fruits fall within a radius of 4 m from the mother plant (Nielsen et al., 2005). Clegg and Grace (1974) and Ochsmann (1996) argue that dispersal by wind could be important only over short distances. There is no direct evidence of dispersal by animals, but it can be supposed that adherence to animal skin could only play a role in short distance dispersal. Since at the landscape scale, long-distance dispersal and random events can play a crucial role in the dynamics of plant species, buoyancy can potentially affect the distance the species can reach. Clegg and Grace (1974) and Dawe and White (1979) report an ability to float up to 3 days for H. mantegazzianum, but L. Moravcova (unpublished results) found that 6-month-old fruits sink within 8 h. Such time, nevertheless, is likely to be sufficient for spreading a long distance, especially by fast-flowing streams. Other important dispersal vectors are humans, who spread fruit of H. mantegazzianum stuck to car tyres along roads, move them to new

Fig. 5.7. The proportion of viable dormant seeds in the soil is rather low after the first year and rapidly decreases further. Survival over 3 years of seeds buried at ten localities in the Czech Republic in November 2002 is shown. Seeds were taken from the soil in October of the following 3 years and tested for viability by tetrazolium. Numbers are means of five replicates. Based on data from Moravcova et al. (2006).

Fig. 5.7. The proportion of viable dormant seeds in the soil is rather low after the first year and rapidly decreases further. Survival over 3 years of seeds buried at ten localities in the Czech Republic in November 2002 is shown. Seeds were taken from the soil in October of the following 3 years and tested for viability by tetrazolium. Numbers are means of five replicates. Based on data from Moravcova et al. (2006).

locations with soil transport, or deliberately transport decorative umbels with dry fruit (Tiley et al., 1996). Given that long-distance dispersal is important to the success of possible invasion (Pysek and Hulme, 2005), dispersal by water and humans seem to be the most significant factors in this respect.

If suitable sites are available, high rate of spreading is realized at both local and regional scales. At the scale of the Czech Republic the number of localities doubled each 14 years during the exponential phase of invasion (see Pysek et al., Chapter 3, this volume). Mullerova et al. (2005) report an average rate of spread of about 10 m/year, and increase in the area invaded by more than 1200 m2 each year in the Slavkovsky les region, Czech Republic.

To illustrate the spreading from local populations to wider surroundings, aerial photographs can be explored (Mullerova et al., 2005). Diaspore output of H. mantegazzianum populations can be calculated and evaluated by using additional data from experiments running in the sites analysed (Krinke et al., 2005; Perglova et al., Chapter 4, this volume; Pergl et al., Chapter 6, this volume). Density of flowering plants as recorded from aerial photographs varied around 1.76 plants per m2 at an average site (Mullerova et al., 2005) and this value corresponds reasonably well to that recorded in permanent plots in the field (J. Pergl et al., unpublished data). For the site harbouring the largest population of H. mantegazzianum (see Perglova et al., Chapter 4, this volume, for the size of populations in individual sites), 14,164 flowering plants

Fig. 5.8. Embryo in (A) a freshly harvested, (B) 2-months stratified and (C) 5-months stratified seed of H. mantegazzianum. Seeds were stratified in the soil. Photo: L. Moravcová.

Fig. 5.8. Embryo in (A) a freshly harvested, (B) 2-months stratified and (C) 5-months stratified seed of H. mantegazzianum. Seeds were stratified in the soil. Photo: L. Moravcová.

were estimated to be present from aerial photographs (Fig. 3.4). Given the mean fecundity of 20,500 fruits per plant in the study area (see Perglová et al., Chapter 4, this volume), the total fruit number per 60-ha area (size of the research plot used by Müllerová et al., 2005) is over 290,000,000, representing an annual input of 484 fruits/m2/year. Relating the total fruit set to the mean area actually infested at a site (31,946 m2) gives 9089 fruits/m2/year. These values can be compared with the number of seeds in seed banks, estimated in permanent plots: the mean value from autumn 2002, after the fruits were shed, was 3650 seeds/m2 (Krinke et al., 2005). Bearing in mind that the values derived from aerial photographs are rough estimates, they provide some idea of how large a proportion of fruits are spread outside the actual stands. The value of 484 seeds/m2/year is a theoretical one since seeds are not dispersed evenly across the whole site. On the other hand, the amount produced by monitored populations (9089) greatly exceeds the value recorded in the field (3650); this difference indicates that a large proportion of fruits are spread into surroundings, making further population growth possible.

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