Seed dispersal

For anyone who walks through the forest, the most obvious living things all around will be plants. Less obvious, however, is how they ended up where they are. While animals can generally spread further afield quite easily, plants are less mobile, and have to use other means to allow their seeds to disperse.

Ensuring that their own kind thrives into the future is high on the list of priorities for plants. It is also important that their offspring disperse; being too crowded together can end up in too much competition for food, water and other essentials, and the species, as a whole, suffers. Colonising new sites also reduces the risk of a plant having all its proverbial eggs in one basket. If all members of a species are crowded into one area they are more at the mercy of chance. Fire or disease could easily seal the fate of a species in an isolated patch of forest; if they are more widely dispersed, this risk is reduced enormously.

However, plants are faced with a range of strategic problems. Imagine you are a plant setting seed: how will your seeds travel any distance and end up in suitable ground? How do you ensure your offspring have enough energy reserves to give them a good start in life? On top of all this there is the problem of making sure that a sufficient number of your seeds manage to germinate, having run the gauntlet of seed predators. In response to these kinds of problems, plants (and fungi) have evolved some ingenious ways of using their environment to help them disperse and germinate, as we will see with some of the many examples of plants in the Caledonian Forest.

Blowing in the wind

There are many plant species in the Caledonian Forest that disperse through the air - an effective strategy, considering there’s no shortage of wind in the Highlands! Scots pine (Pinus sylvestris) seeds are wind-dispersed, typically travelling up to 100 metres from their parent tree, although in snowy landscapes, they can travel several kilometres, blown across the frozen surface of the ground. Birch (Betula spp.) also produces huge numbers of lightweight seeds. These come complete with two tiny wings that help them float on the air. Birch is a pioneer species, ie one that colonises a bare site rapidly and in large numbers, although it is relatively short-lived. It was among the first tree species to arrive on the bleak tundra after the last Ice Age.

The seeds of plants such as rosebay willowherb (Chamerion angustifolium) and willows (Salix spp.) use downy hairs as a flight mechanism. Even more sophisticated are the tiny parachutes used by members of the daisy family, such as thistles (Cirsium spp). This can ensure they travel much further, drifting on the breeze.

Orchids, such as creeping ladies-tresses (Goodyera repens) invest in their seeds being almost powder-like – well suited to air travel – but because the seeds are so fine they do not have enough energy reserves to allow them to germinate. Like all orchids, they rely on mycorrhizal fungi to start them off. In this mutually-beneficial relationship, the fungus provides the seedling with nutrients which it could not access on its own. In return the fungus can get sugars that the plant photosynthesises as it starts to grow.

Fungi and some plants such as ferns and mosses produce tiny lightweight spores that also carry very easily on the breeze. Puffballs, including Lycoperdon pyriforme, release their tiny spores when rain, or an animal’s foot, puffs the spores through the exit hole in the top of the fruiting body. The puffballs therefore rely on a combination of dispersal agents, which can include animals, water and wind. (Scientific names often reflect some of the characteristics of the species, and are not always lofty as they may appear: Lycoperdon translates as ‘wolf fart’, which is an alternative common name for this fungus!)

Water

While water is not used as commonly as wind to spread seeds, it plays an important role in dispersing those of alder (Alnus glutinosa). The seeds can be dispersed by the wind, but their ‘wings’ also contain pockets of air, that enable them to float on the water and root further downstream. Alder is typically a riverside tree, and so this strategy helps continue its dispersal along watercourses, ensuring they land in appropriate spots. It is rare to see trees in rows in a natural forest, but with alder one can sometimes find lines of seedlings resulting from seeds deposited along the water’s edge during a flood. Seeds of some plants, such as marsh thistle (Cirsium palustre) and alder germinate on the water itself. By the time they are washed on to land, they are ready to root and grow.

‘Hitch-hiking’ on animals and birds

Dispersal by animals is fascinating, as the places the plants end up in is closely tied to the lifestyle and movements of the animals involved. Some plants have barbs or similar mechanisms on their seeds. Twinflower (Linnaea borealis) rarely sets seed, but when it does the seeds have small hooks that adhere to animal fur and bird feathers, allowing them to be carried to new sites. Humans also play a part in this process: we have carried seeds far beyond their normal range on clothes and shoes. Some seeds are dispersed and pushed into the ground by the hooves of large mammals. The loss of herbivores such as the wild ox (Bos primigenius), has deprived the forest of an important dispersal agent.

Birds are among the most mobile creatures, and there are many plants that have evolved to hitch a ride with them. As well as sticking to the outside, plants can tempt birds to swallow their seeds, and so carry them in their gut to other areas. Attracting the birds is crucial, and what better way to do it than with energy-rich berries? With bright berries such as those of rowan (Sorbus aucuparia) the colour acts as an advertisement for birds, who have good colour vision. This is a true mutualistic relationship: both parties gain something from the bargain.

The second problem is how to protect the seed itself. After all, a bird’s digestive system is a forbidding place to be, with grinding gizzard (an extension of the gut packed with stones), and powerful acids. The stone that you spit out after eating a cherry is in fact hard casing, protecting the seed within. Wild cherry or gean (Prunus avium) and bird cherry (Prunus padus) have co-evolved so closely with birds that the seeds need this harsh treatment to break their dormancy.

It is quite common to see clusters of young rowan growing beneath a Scots pine, or even a single seedling within the pine’s branches. We can then deduce that a bird (typically a member of the thrush family) at some point sat in the branches of the tree, and passed the seeds out in its droppings.

Heavier seeds, such as nuts and acorns, have large energy reserves, enabling them to spend time getting firmly established, and giving them a better chance of surviving to maturity. However, all that weight causes problems with moving around, and oak seeds are very reluctant to germinate in the shadow of their parents.

Jays (Garrulus glandarius) eat acorns (Quercus spp.) and can carry them (up to five per flight) for several kilometres. These birds store the acorns by burying them, to eat at a later time. They often have a preference for burying them near a tree or shrub, at the edge of the forest. In one autumn they can bury as many as 4,600 acorns, and can remember the locations of them all! Harsh weather and hungry predators, however, will take their share of jays, and this enables some seeds to germinate. Jays play an important part in dispersing oak woodlands, and both oaks and jays are present in Glen Moriston, in the south of our target area.

Hazelnuts are often spread to new areas by red squirrels (Sciurus vulgaris) and mice, that may store them for later use. Again, while some seeds will be eaten, others aren't reclaimed and manage to grow.

Remarkably, even ants can help to disperse the seeds of plants. Wood anemone (Anemone nemorosa) and cow-wheat (Melampyrum spp.) seeds have a small parcel of fatty tissue - called an elaiosome - attached to them. Ants take the seeds back to their nest so they can feed the elaiosome to their larvae, and so the seed is transported to a new location.

Other methods of dispersal

Some plants don’t invest much energy in complex mechanisms for dispersal. Bluebells or wild hyacinths (Hyacinthoides non-scripta) are one example of a plant that simply drops its seeds directly to the ground. However, the result is that such plants will tend to spread and colonise new areas very slowly indeed.

There are plants that can disperse their seeds under their own power. Wood cranesbill (Geranium sylvaticum) has seed pods that explode when ripe, throwing the seeds away from the parent plant. Common dog violet (Viola riviniana) uses this strategy, after which the seeds are sometimes spread further still by ants.

Some plants set seed very rarely, but can ensure their ongoing survival by generating clones of themselves. Aspen (Populus tremula), is one example. In Scotland, it rarely sets seed, but readily sends out suckers (known as ‘ramets’) from its roots. The young trees are genetically identical to the parent. The disadvantage here is that they miss out on the genetic variation and robustness that can result from sexual reproduction. The advantage is that they can survive in one area for hundreds, if not thousands of years.

Dispersal in time

Other plants avoid competition by delaying their germination until a more suitable time. Heather (Calluna vulgaris) produces numerous seeds which fall to the ground below. Some of these become dormant, forming a seed bank in the peat. If a fire sweeps through the area, the plants above ground may be killed, but the heat then warms the seeds to a temperature suitable for germination, and they can grow, free from competition.

Safety in numbers

As if transportation isn’t enough, plants have to ensure that at least some of their seeds avoid the many hungry mouths that inhabit the forest. One strategy to reduce the risk of seeds being eaten is utilised by hazel, which invests in giving its nut a protective shell that requires effort to get into.

Many plants produce large numbers of seed to improve the chances at least some seeds surviving. Forest trees that have a lot of seed predators, such as Scots pine and oak, have ‘mast years’, in which they produce a huge glut of seeds every few years. Seed predators, such as jays or squirrels, have their fill, and while many seeds may end up in unsuitable ground, at least some are almost guaranteed to germinate. This does take a heavy toll on the tree’s resources however, and it becomes virtually sterile the following year in order to recover.

These cycles have a powerful knock-on effect in the forest ecosystem. The abundant food allows seed predators - herbivores such as rodents - to raise more young than usual, and there is a population explosion. This in turn can lead to an increase in weasels (Mustela nivalis) or other predators. However, in the barren period that follows, the expanded rodent population may crash once again.

Seed dispersal and ecological restoration

Understanding how wild plants disperse is important in restoring the Caledonian Forest, or any terrestrial ecosystem. Ideally we use natural regeneration to restore a healthy forest, but the further from the seed source, the less likely it is that seeds will colonise an area. The effective colonisation distance varies with the species of plant. For example, because trees such as Scots pine have seeds that travel relatively short distances, they are less likely to colonise more remote, denuded areas, even when they would be able to grow well there. Rowan berries, on the other hand, can be carried long distances in the gut of birds, and it is not uncommon to find rowan seedlings growing from the cracks in a rocky perch in an otherwise treeless landscape. This knowledge helps us to prioritise what we plant - we rarely plant rowan, instead focussing on those trees that need most assistance.

The same principles apply to smaller plants. Trees for Life’s Woodland Ground Flora Project aims to identify those species of wild flower that are under-represented in more remote areas. For example, knowing that bluebells would be unable to cross highly Finally, understanding seed dispersal enables us to grow new trees in the nursery. For instance, the seeds of wild cherry have to pass through a bird’s digestive system for them to germinate. We know therefore that we have to use other methods to break the dormancy of the seeds, so that they will germinate in the nursery.

The plants in the Caledonian Forest are well equipped to disperse and regenerate, given the right conditions, although they are often hindered by forest fragmentation and overgrazing. In restoring forest ecosystems, humans are now consciously playing a positive role in dispersing wild plants.

Dan Puplett

 

Sources & further reading

  • Begon M., Harper J.L., and Townsend C.R, (1996). Ecology (3rd ed.) Blackwell Science: Oxford.
  • Detheridge, A. (2006). Ground flora regeneration in replanted Caledonian woodland in Glen Affric. Unpublished MSc Thesis. Imperial College, London.
  • Fitter, A. (1987). New Generation Guide to the Wild Flowers of Britain and Northern Europe. Collins: Glasgow.
  • Grime, J.P. (1979). Plant Strategies and Vegetation Processes. John Wiley & Sons: Chichester.
  • Tudge, C. (2006). The Secret Life of Trees: How They Live and Why They Matter. Penguin: London.

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