Why Is Seed Dispersal Important for Forest Restoration?

Think plants can’t move? You’re only half right. Plants move through the dispersal of their seeds by water, wind, gravity, ballistics, or animals. Dispersal allows plant offspring to colonize areas with less competition and more resources. Without dispersal, plant offspring are less likely to survive, making it crucial for plants to reproduce and to sustain forest diversity.

In addition to its role in maintaining healthy forests, seed dispersal is needed for forest restoration. Current global forest restoration goals are ambitious. The Bonn Challenge, an agreement signed by 61 countries, aims to restore forests on 1.3 million square miles, or 2% of Earth’s surface, by 2030. 

Some of this reforestation will involve directly planting trees, or active restoration. Given the scope of global restoration goals, however, this approach is too time-consuming and costly. Instead, passive restoration, where natural ecological processes drive forest regeneration, will be critical for large-scale forest restoration. Thus, understanding how seed dispersal works and identifying threats to this mechanism are needed to preserve this critical ecosystem process for forest regrowth.

Seed Dispersal Methods

The many forms of seed dispersal are an evolutionary marvel. Plants produce seeds specially adapted for dispersal by one of five forces: wind, water, gravity, ballistics, or animals. 

Wind

Wind dispersed seeds have special adaptations like hairs, wings, or inflated structures that help them be carried by a breeze more easily. Dandelions, for example, have white bristles that create drag and cause them to float on updrafts of wind.

Many wind-dispersed species make small and lightweight seeds. Orchids, one of the largest plant families, produce “dust seeds,” which are the smallest seeds among flowering plants.

Generally, wind dispersed seeds only travel short distances. Rare extreme weather events, however, can transport wind-dispersed seeds farther. These long-distance dispersal events are uncommon, but can have major ecological consequences, like allowing a species to colonize a novel environment.

Water

Water seed dispersal has evolved in many species that live beside or partially in water. These species have fruit and seed adaptations, including fruits that can float and hard, durable seed coats that allow them to be carried by rivers or ocean currents. One well-known example is the fruits of the coconut palm, which can be at sea for up to 110 days and still germinate.

Water dispersed seeds can contribute to the restoration of riparian forests—areas next to rivers and streams. However, researchers note that it is important to maintain connectivity between waterways to allow for continued dispersal.

Paradoxically, water can even play an important role in the dispersal of some desert plants. When a rare, heavy rain occurs, seeds are transported by water where they then have the opportunity to quickly germinate when conditions are good.

Gravity 

Gravity dispersal is often the first step of a multistep dispersal process, where the second disperser is an animal. In gravity dispersed species, fruits become heavier as they ripen and as the seeds mature. Once fully ripe, the fruits and their enclosed seeds fall to the ground. There, ground-dwelling animals can consume ripe fruits, then pass the seeds through their guts. 

Seeds dispersed by gravity generally do not have any structural adaptations for dispersal. For instance, fruiting trees, like domestic apple trees, rely on gravity seed dispersal. The seeds are encased in fruits that grow as the seeds mature and fall to the ground when they are ready for dispersal. 

Ballistic

Ballistic seed dispersal occurs when seeds are shot like projectiles from the parent plant. Plants have evolved several mechanisms that make this possible. For example, jewelweed seed pods curl inward when they open, which can project seeds over 16 feet away from the parent plant. In the case of Chinese witch-hazel, drying fruits create pressure until the fruit splits and releases the enclosed seed at speeds up to 27.5 mph.

Ballistic dispersal is often found in “weedy” plants—species that grow quickly, but do not live for long. They are able to quickly become established, grow, and reproduce before they are outcompeted by other species.

Animals

Animal seed dispersal may be the most complex dispersal strategy because of the extraordinary number of plant-animal relationships involved. Animals may transport seeds internally or externally. While animal seed dispersal can be found in forests around the world, it is especially important in tropical forests where an estimated 70% of tree species rely on animals for dispersal.

In external dispersal, seeds fasten to an animal’s body with bristles, hooks, or a glue-like substance. Externally dispersed seeds tend to make excellent invasive species because they can easily attach to humans and become established in new environments.

Animals also disperse seeds internally—plants offer seed dispersers fruit and in return the seed is either spit out by the disperser or defecated after passing through its gut. Gut passage makes some seeds more likely to germinate because the seed coat is weakened in places. The size of fruits affects which animals can disperse them, with bigger animals more able to process bigger fruits. So, when animals like elephants and primates are poached, the dispersal of large-fruited and large-seeded species is at risk. 

Animal dispersers range in size from towering savanna elephants to dung beetles and ants. Each type of animal seed disperser plays a unique role in the ecosystem, with differences in how many seeds they consume and how far they move seeds. For example, spider monkeys native to Latin America have diets that are mostly fruit, so they can move lots of large seeds over long distances. Smaller fruit-eating birds may only be able to eat small fruits, but are less dependent on an intact forest for their movement, which may make them better at facilitating forest restoration. 

Both fruits and seeds have evolved adaptations that facilitate this plant-animal mutualism. For example, fruits attract animal dispersers with color and odor, offering a nutritious incentive to disperse the enclosed seeds. Some seeds require some damage to their seed coats to germinate, so passing through an animal’s gut actually makes them more viable.

Why Is Seed Dispersal Important? 

Seed dispersal is a necessary component of healthy forests. When plants do not have dispersers and fall under their parent trees, they are much less likely to survive. Researchers believe this increased mortality is caused by species-specific pathogens that are most effective when seeds of the same species are close together. Undispersed seeds also face more competition from the parent plant and their sibling for critical resources including sunlight, water, and space. 

Many experiments that examine the importance of seed dispersal focus on species spread by animals. Without it, animal dispersed species become less abundant and some trees are more likely to go extinct.

Threats to Seed Dispersal and Their Consequences

Human changes to the environment are altering seed dispersal processes and may be jeopardizing the future of this essential ecosystem service. Hunting, logging, habitat loss, and climate change all pose major threats to seed dispersal, especially animal dispersal.

Ecosystems respond differently to changes in seed dispersal, but one of the most troubling trends is a loss of plant diversity in forests. Changes in seed dispersal may also affect the resources available for species that depend on those plants for survival, thus causing ecological cascades throughout an ecosystem.

Because so many tropical species depend on animals for seed dispersal, the negative effects of humans on seed dispersal may be most prominent in tropical forests. A synthesis of 35 studies found that hunting and logging reduced the distances that seeds were moved and also caused a shift toward the dispersal of small seeds.

In most cases, it is too soon to fully understand the long-term effects of human disturbances on seed dispersal. But given what is known about its importance for healthy ecosystems, we likely do not want to wait until that happens to see the consequences.