Propagation of plant species is a form of asexual reproduction, this occurs when a single organism reproduces itself. In relatively simple uni-cellular organism, such as an amoeba, the cell divides in a process called mitosis (or replicative nuclear division), in which an exact copy of the parent cell is passed on through future generations. The terms “asexual” means reproduction without gametes, for plants it means that there is no fertilisation and therefore development through germination, in short it means reproduction and propagation without passing through the seed component of the overall life cycle of the plant.
The result is that the progeny are exactly identical to each other and to the parent organism, they are clones. There are advantages to this reproductive strategy. For instance a large number of individual plants can be produced from a single parent plant, meaning that a particular habitat can be colonised quickly. The daughter plants have exactly the same characteristics as the parent plant so they are already adapted to the particular habitat. It is also the optimum strategy for maintaining the best characteristics of that species, thus enabling it to flourish in the habitat. The obvious disadvantage is that the species is very vulnerable to rapid environmental change, such as those occurring as a result of climate change, or disease.
1: Generic Vegetative propagation
One example is the common blackberry plant, when a shoot reaches the soil from the parent plant. It becomes what is termed a stolon. From this structure roots are produced, which in time grow into a new plant, which then separates from the parent plant. Asexual reproduction is common among prokaryotic organisms, protocists, fungi and plants. It is exceptionally rare, at least naturally in animals. In terms of conservation of tree species the most common method of vegetative propagation is to use cuttings from a parent plant, a shoot from the parent tree is planted in a tree nursery. Intertwined with cuttings shoots can be layered, where a shoot is grown attached to the parent tree before it is separated. Genetic diversity and therefore adaptability can be enhanced by techniques of budding or rafting, where plants from two different (but similar) species are grown together. Cutting then, is a type of selective breeding in which shoots from a parent plant are grown separately so that they grow into completely new individual plants. How understanding the life cycle of a given tree species can ensure its survival is outlined in the example below.
A subspecies of the common olive tree known as the Laperrines olive has evolved over millions of years, certainly since the early Pleistocene epoch, (which ended some 12000 years ago), to withstand the harsh environmental conditions of the Sahara desert and parts of the Sahel. The plant grows in a narrow range which runs from the Saharan mountains, to Sothern Algeria to the North of the Sudan. It grows only at altitudes of between 1400 and 2800m. Clearly, this is a harsh arid and inhospitable environment in which to survive the laperrines olive has had to adapt. A key component of this success has been a reproductive strategy based on vegetative asexual reproduction. The point here is that plants that reproduce in this way do not vary their genetic characteristics, as would occur in fertilisation or sexual reproduction. The Laperrines olive plant reproduces in such a way that its genetic characteristics are preserved and not altered.
The plant is highly valued as local source of olive oil, it is also a fuel source, its leaves are used as feed for animals and as a natural source of medicine. It has been discovered that the Laperrines olive could be used as a genetic resource to improve the drought resistant properties of its Mediterranean cousin. In other words hybridization and selective breeding, all good so far, unfortunately the plant may become extinct as a consequence of climate change and almost non-existent genetic variation over extended periods of time. In other words to maintain its ecological niche, the plant has sacrificed genetic diversity and so cannot adapt to rapidly changing environmental conditions. Conservation efforts are centered on both habitat protection and crossbreeding the Laperrines olive with other species, which safeguard the species itself as well as the genes that have enabled it to survive. If successful a whole new range of drought resistant and genetically diverse plants could be cultivated. The problem is finding the best gene exchange pathways between plants that have been separated by evolution and geography for hundreds of thousands of years.
A project to save a tree species called Torreya Taxifolia shows how this could be achieved. The species is almost extinct; only 1000 individual trees are known to exist. A team from the Atlanta Botanical Gardens is distributing cuttings to a variety of conservation groups across the US. The species was decimated by fungal disease; the problem is that although it is treatable the species requires a cool moist climate and to germinate its seeds must pass through the digestive tract of consumer animals. The hope is that trees with the most beneficial characteristics can be cloned and these then used to repopulate its habitat.
2: Cloning schematic
The management, conservation and diversity of the tree species contained in all forests is a crucial driver of planetary biodiversity. Today approximately 4 billion hectares of the land surface of the Earth is forest, about 55% of this amount is managed or has a varied utility; in other words it is no longer virgin forest. Different tree species have different environmental benefits, so understanding how a given species may be useful should be a cornerstone of forest management policy. In other words some trees may be better at carbon storage, others may provide more food for consumer organisms and still others may have faster growth rates. It is therefore intuitive that mixed forests are more productive and diverse than mono-cultured plantation forests. In an epoch were biodiversity is under increasing pressure to provide humanity with the resources it needs to survive and that biodiversity is great danger of total collapse, it seems sensible to pursue policies which find a sustainable balance between integrity and functioning and the production of resources.
As with all forms of modern agronomics, forestry is almost exclusively based on monoculture, principally conifers in the UK and species such as pine or spruce in Scandinavia. The principal reason for this is economics; such systems are considered, rational, profitable and easier to manage. Recent research from Sweden suggests that this not the case and that a mixed forest is not only more productive but additionally has a positive impact on forest ecosystems. In essence the more tree species present the more growth, carbon storage, fruit and berry production, food for consumer organisms, dead wood, (thus more decomposing organisms and therefore more nutrient rich soil) and biodiversity. The reason is obvious, different trees have in terms of ecology different roles and functions, such that no one tree can provide all the inputs necessary for an ecosystem to thrive. The implication is that conservation is improved due to a greater number of species, which in turn provides overall a greater rate of flux of ecological throughputs. The research from Sweden is augmented by similar research findings from Central European, Canadian and Mediterranean forests. Taken holistically, forest ecologists infer a direct and positive relationship between diversity and the combinations of different tree species and a healthy forest ecosystem.
When we think of deforestation we should be rightly concerned over the consequences Amazonian or African rainforest destruction. However, it is important to remember that such destruction has been occurring for thousands of years much closer to home. For example, The Caledonian forest has been severely depleted by human activity, today it is less than 1% of its original size, and similar statistics can be applied all over the UK. As a consequence the remaining habitat has become fragmented; different parts of the forest are no longer connected, which further compromises there ecological viability. In addition the forest remnants are less diverse in terms of both habitat and tree species. They cannot sustain damage caused by disease, fire or storms, which under natural circumstances are beneficial in the long run because these impacts promote diversity in the forest as a whole. The level of destruction is such that once common species such as juniper, holly, aspen and rowan have been selectively bred out, which in turn has affected the diversity of consumer organisms which depend on them. Aspen which is found naturally across the world from the Arctic Circle to China is particularly rare in Scotland. The species does not generally produce seed, so the species is propagated from root cuttings, using the techniques outlined above.
3: The Former and Current Range Of The Caledonian Forest
The Scottish charity treesforlife has since 1989 been working to restore the diversity and extent of the Caledonian forest. Since 1991 the organisation has been developing techniques of aspen root propagation and now grows up, to 3000 trees every year. In early 2013 the charity received a grant of £7,443 enabling the organisation to expand its operations in this area. The charity has set an ambitious goal of planting a million very rare but native Caledonian tree species over the next five years. Such initiatives need to be seen in a global and historical context, no-one can say what any forest would be like today if human beings had not exploited them. It is however fair to say that the Caledonian forest (and other forests) would be much more expansive and populated by more diverse organisms (many of which are long since extinct in the UK), had the forest been managed with sustainability as the goal as opposed to greed and exploitation. The following example outlines what happens when this is the case
In 1973 a US backed military coup in Chile, deposed the democratically elected government lead by Salvador Allende and replaced it with a fascist dictatorship lead by Augusto Pinochet. The regime was responsible for the deaths, torture and imprisonment of tens of thousands of people. Aside from this terror, the regime was also responsible for a series of appalling environmental crimes. For instance the clearance of thousands of square kilometres of virgin, boreal trees effectively destroyed the Cordillera forest. This act still stands as arguably the single greatest and most rapid loss of such forest to date documented on the planet. The forest was replaced with faster growing non-indigenous tree species, with the result that once common tree tress such as the ruil and Chilean plum yew are now almost extinct.
In early 2013 a team from the Royal Botanic gardens in Edinburgh, discovered a tiny band of ruil trees whilst on an expedition to salvage seeds from other threatened tree species. According to the team this discovery is in conservation terms priceless. They state with authority that the young generation Chile believes the pine plantations which replaced the forest are in fact natural. The discovery of Ruil and other tree species means that propagation techniques can hopefully be used to repair some of the damage and restore some of the original forest.
In the long term propagation by cloning will become increasingly important both for natural and managed systems. Vegetative propagation means that more beneficial traits can be selected and although such techniques do not yield the diversity that sexual reproduction by seed fertilisation produces, the big advantage is that propagation allows the rapid selection of beneficial traits. As such propagation has a major role to play in the conservation and the reforestation of regions such as the Caledonian forest. For managed systems it is possible to improve their diversity and ecological viability whilst not sacrificing their productivity by re-introducing different tree species to the forest system. At a time where habitat destruction has put a fifth of all plant species in danger of extinction the urgency required cannot be understated, of course the real answer is to not destroy the habitat in the first place.