Dispersal characteristics of the invasive plant Mimulus guttatus
Concern as how environmental change may impact on non-native plant invasions has largely focused on the direct effect of climate. By contrast, the potential effects of changes in the seasonality and volume of precipitation on non-native plant spread has received far less attention. Unpredictable flood pulses may increase the likelihood of plant invasion by altering the competitive balance between native and non-native species, redistributing nutrients, facilitating the colonization of non-native species through increased disturbance and/or enhancing the dispersal of propagules by water.
The riparian invasive species Mimulus guttatus (Scrophulariaceae), originating from North America has been used as a model to address the likelihood that flood pulses might facilitate the spread of non-native plants. This perennial riparian herb was introduced in the United Kingdom around 1812 and established in the wild by 1824. The plant is found in wet places, it is competitive, has a high seed production, a short germination period, an effective long-and short-distance dispersal mechanisms and a rapid growth. M. guttatus reproduces both vegetatively by fragments, and by seeds.
The study focussed on the dispersal characteristics of M. guttatus, and especially the roles of vegetative fragmentation with increasing water velocities, subsequent fragment survival, regeneration and colonization, as well as the buoyancy, survival and germination success of seeds.
The vegetative regeneration of M. guttatus fragments was high: after only 1 week, 60% of the fragments produced new shoots, and at 6 weeks, 96% of fragments had new shoots. Moreover, fragments of 1 and 2 nodes produced significantly more shoots per node than fragments of 4 or 5 nodes, or whole plants. After 1 week, 36% of fragments formed roots at the nodes, and 99% after 6 weeks. The proportional fragmentation was significantly greater at higher water velocity: 74% of stems were broken at 1.48 m s-1 compared with 17% at 0.88 m s-1. Fragments length, stem diameter and the number of nodes per fragment all increased significantly with increased velocity. At the lower velocities, it was principally the most recent plant growth that fragmented, whereas at higher velocities, stem breakage also occurred further down the plant.
M. guttatus seeds are small (0.02 mg, 0.5 mm wide x 1 mm long). Individual plants release on average 7000 seeds, however, seeds have a short buoyancy period so the timing and magnitude of high-flow events is crucial in determining potential dispersal distances. Seeds germinated readily both in water and on sand with an average 33% germination rate within 9 days.
M. guttatus exhibited similar, if not higher (95% versus 75%), regeneration growth and survival rates compared with the very invasive Fallopia japonica. The mean fragmentation was significantly greater at higher velocities. Nevertheless, M. guttatus possesses a shallow root system (1-2 cm depth), and where it colonizes fine sediments such as silt, plants can be easily uprooted under high flow conditions. In real situations, high-flow events will affect different stages of M. guttatus colonization, establishment and spread. If high-flow events occur at too high a frequency, populations may not re-establish before the next high-flow events, preventing growth and long-term survival.
Sources
Truscott AM, Soulsby C, Palmer SCF, Newell L, Hulme PE (2006) The dispersal characteristics of the invasive plant Mimulus guttatus and the ecological significance of increased occurrence of high-flow events. Journal of Ecology, 94 1080-1091.