Many species of herbs (including grasses) have some of their seeds dispersed by the large grazing mammals that consume the seeds along with foliage. This is an interaction that has probably been oc...

Dispersal of Small Seeds by Big Herbivores: Foliage is the Fruit

Hydrochory, or the passive dispersal of organisms by water, is an important means of propagule transport, especially for plants. During recent years, knowledge about hydrochory and its ecological consequences has increased considerably and a substantial body of literature has been produced. Here, we review this literature and define the state of the art of the discipline. A substantial proportion of species growing in or near water have propagules (fruits, seeds or vegetative units) able to disperse by water, either floating, submerged in flowing water, or with the help of floating vessels. Hydrochory can enable plants to colonize sites out of reach with other dispersal vectors, but the timing of dispersal and mechanisms of establishment are important for successful establishment. At the population level, hydrochory may increase the effective size and longevity of populations, and control their spatial configuration. Hydrochory is also an important source of species colonizing recruitment-limited riparian and wetland communities, contributing to maintenance of community species richness. Dispersal by water may even influence community composition in different landscape elements, resulting in landscape-level patterns. Genetically, hydrochory may reduce spatial aggregation of genetically related individuals, lead to high gene flow among populations, and increase genetic diversity in populations receiving many propagules. Humans have impacted hydrochory in many ways. For example, dams affect hydrochory by reducing peak flows and hence dispersal capacity, altering the timing of dispersal, and by presenting physical barriers to dispersal, with consequences for riverine plant communities. Hydrochory has been inferred to be an important vector for the spread of many invasive species, but there is also the potential for enhancing ecosystem restoration by improving or restoring water dispersal pathways. Climate change may alter the role of hydrochory by modifying the hydrology of water-bodies as well as conditions for propagule release and plant colonization.

The role of hydrochory in structuring riparian and wetland vegetation

In the process of plant invasion rivers may act as dispersal agents, supporting downstream movement of diaspores by water (e.g. Stani-forth & Cavers, 1976; van der Pijl, 1982; Schneider & Sharitz, 1988; Skoglund, 1989). Moreover, periodic disturbances resulting from destructive flooding generally destroy or damage a large part of riparian vegetation (Ellenberg, 1988) thus creating openings that provide suitable habitats and favourable nutrient conditions for seedling establishment and subsequent colonization (Walker, Zasada & Chapin, 1986). Consequently, riparian habitats into which an alien species was successfully introduced and naturalized may serve as foci for subsequent spread into the adjacent landscape (Pysek, 1991, 1993).

Plant invasions and the role of riparian habitats: a comparison of four species alien to central Europe.

The importance of hydrochory, or seed dispersal by water, to the regeneration of Taxodium distichum (bald cypress) and Nyssa aquatica (water tupelo) was examined in a forested floodplain of the Savannah River in South Carolina. Seedfall and dispersal by water were quantified for 2 yr using floating seed traps. Water depth, surface velocity, and flow direction were monitored over the same period. Seedfall for bald cypress and water tupelo occurred primarily from early fall throughout the winter, when water levels were rising in the swamp. Extended buoyancy periods for newly released seeds and fruits (bald cypress: 42 + 37 d; water tupelo: 85 ? 36 d) prolonged dispersal for both species. Flowing water transported experimentally released seeds long distances, but in a uniform direction, and concentrated them nonrandomly against logs, trees, knees, and other emergent substrates. An examination of the soil seed banks in five microsite types supported the results of the seed transport study. Lowest woody seed densities occurred in the open areas, and highest seed densities occurred in sediments adjacent to emergent substrates such as logs. Additionally, elevated water levels of 1-2 m caused by short-term, high-discharge floods scoured seeds of Nyssa sylvatica var. biflora, Quercus spp., Liquidambar styraciflua, Pinus taeda, and other species from adjacent bot- tomland hardwood communities and transported them into the bald cypress-water tupelo forest. We conclude that elevated water levels, which occur during late fall when seeds are released, influence the distribution of bald cypress and water tupelo seeds and their avail- ability for recruitment. Short-term, deeper floods may also be important for seed transport among bottomland hardwood communities that are spatially separated or differ in species composition.

Hydrochory and Regeneration in A Bald Cypress‐Water Tupelo Swamp Forest

1. The landscapes of large floodplain rivers are characterised by heterogeneous environments related to the interplay of flood flows, sediment transport and vegetation dynamics.

2. The large rivers of Europe, and probably most rivers throughout the forest biomes, were characterised by islands but over the period of major human interference, many have become dominated by incision and narrowing so that they are now characterised by single-thread and relatively simple channel forms.

3. Vegetation plays an active role in developing heterogeneous channel forms through (a) biotic processes such as seed dispersal, vegetative regeneration and succession and (b) abiotic effects such as increasing flow resistance inducing sedimentation, and decreasing bank erodibility.

4. In particular, accumulations of living driftwood (cf. dead driftwood accumulations and dispersed seedlings) accelerate sedimentation and island development.

5. River reaches with vegetated islands have a high habitat diversity.

6. The natural influences of flood disturbance, wood accumulation, vegetation growth, island development and tree die-off, cause island-dominated reaches to undergo cycles of island growth and decay that are related to cycles of aquatic habitat diversification and simplification.

Island‐dominated landscapes of large floodplain rivers, a European perspective

The annual smartweeds Polygonum lapathifolium L., P. pensylvanicum L., and P. persicaria L. grow on alluvial river bars during the summer and autumn when their substrate is free of flood waters and...

P. B. Cavers

Papers

An experimental study of water dispersal in Polygonum spp.