Oak trees are one of the most common in world; they are definitely one of my favourites and have been as long as I can remember. I have always enjoyed climbing them throughout my career as an arborist. I have found looking into the life cycle of the Oak tree from seed to sapling very interesting and rewarding.
The journey I want to describe starts with the production of acorns on an Oak tree. There are many factors involved in the production of acorns. Contained in an acorn is all the information needed to grow a whole new tree and the stored energy to support a sapling until the small tree has become self sufficient and makes its own energy from photosynthesis and absorb its own nutrients from the ground.
In order to make acorns the Oak has to first have its delicate flowers pollinated. This means getting through early spring without any hard frosts damaging them. Once the flowers are pollinated the tree must produce the acorns. The creation of these seeds requires a large amount of energy to produce due to their large store of energy in the form of cotyledons inside the acorn. Then the actual acorns need a certain amount of heat and light to mature. This is possibly why generally Oaks do not produce acorns for decades after they start growing as acorns are quite an investment.
When oaks produce around 5 million seeds in their lifetime, if they’re lucky only grow half a dozen fully mature offspring. The actual success rate of each seed is very low; this is because of the number of factors against them.
In its life an Oak can produce millions of seeds and a birch can produce hundreds of millions in a comparatively short time, this shows that the Oak does invest more energy per seed than the Birch for example. It isn’t the largest seed in the world. One of the largest is the coconut which has on average 4 seeds per Kilogram. Acorns are on average 290 per Kilogram and Birch seeds have an average of 5.9 million per Kilogram. (Thomas 2011)
Seed size varies greatly in acorns; the larger seeds have the advantage of being a greater food source for the sapling in its early stages, however the larger the seed the more energy it takes to make the seed and the harder it is to disperse further from the parent.
The larger the seed the further down the first root is able to go to secure a reliable source of water, nutrients and footing in the ground. Also the more energy that the sapling has to utilise above ground, the higher the stem would be able to grow. This would give the young tree an advantage over surrounding vegetation growing. The higher the young plant, the more light the plant would be able to get.
In 2014 Łukasz Łuczaj et al found from the 2500 acorns that he collected across Poland that on average in Quercus robur acorns were 6.9% water, They ranged between 2.26–3.66 cm high and 1.16–1.73cm wide.
Nataša P. Nikoliã et al also studied the variable characteristics of the acorn, they collected acorns from 17 different Quercus robur genotypes. They measured the microscopic differences between the acorns look at seed longevity and viability.
Once acorns have ripened in the tree, they fall from the tree in autumn time.
A large quantity of seeds can accumulate underneath the surface of any area of plants or trees. Many seeds will lay in wait for the opportunity of the correct amount of light etc to grow and become successful mature specimens, Acorns however don’t normally accumulate, they will grow almost straight away. Here they face a new set of challenges and factors against them. Some of these factors also help with the dispersal of the seeds from the parent plant.
Acorns are a good source of food for insects, small herbivorous mammals and larger herbivorous mammals. Acorns in years gone past have been a food source for humans when other sources of food have been scarce.
Small herbivorous animals will collect acorns as well as other seeds as a food store for winter time. Łukasz Łuczaj et al, Looked at tannin levels in acorns from Quercus robur and other Quercus species. Acorns were used as a food source by humans in foraging times, also more recently in times of food shortages, acorns were used as an addition to flour to make bread, as a substitute for coffee amongst other things. The problem with eating Acorns is the levels of Tannins in them. In high concentrations they can cause kidney or liver failure. In lesser concentrations they can cause digestive problems, the tannins make the taste quite bitter. All of the things are a deterrent for the seed being eaten and not being able to germinate.
Christian Smit and Mart Verwijmeren (2010) looked into the survival and establishment of Acorns through to saplings; they found that the success rate of saplings seemed to be higher if they had been buried on the outskirts of thorny shrubs. In the wild small herbivorous mammals such as mice and squirrels preferably store their winter food supply next to or around shrubs, there are different theories for this. One is that mice store acorns next to undergrowth to give them shade from predators whilst storing their food supply, (Kollmann 1995).Another theory is that the animals use the shrubby plants as memory aids so they know where to come back to when they need their food.
Smit and Verwijmeren hypothesised that mice were an important method of seed dispersal and that thorny shrubs gave protection to saplings from large grazing herbivores.
Mellanby found 5000 oak saplings per hectare that had been dispersed by animals in 1668
Acorns are more likely to germinate if buried, firstly because buried seeds have better conditions for germination also they are less accessible to other herbivores as a food supply.
Jared P. Haas et al (2005) studied the habits of mammalian predators on acorns, they looked at the feeding habits of mice, squirrels and deer.
Maria Birkedal et al (2009) looked at the impact of rodents feeding on freshly planted seeds, the research was carried out for forestry purposes to look at more successful ways of planting broadleaf seeds with a reduced failure rate. They found that beech nuts seemed to be the favourites but the acorns were also dug up and eaten. They found that seeds sown in mixed conifer forests had a greater success rate, they tried installing perches for birds of prey, and this did not seem to make a difference.
Seeds differ in how long they take to germinate; this is mostly due to not having the right site or environment to be successfully established. Seeds have been known to be dormant for 1200 years and still be germinated and grown as plants. Lots of seeds are kept by seed banks and nurseries in a dry cold environment; this increases the longevity of the viable seed. The three different types of dormancy are Morphological dormancy – where the seed has to develop further before being ready to germinate, physiological dormancy where the seed’s chemical structure needs to be modified for germination and then physical dormancy where the something physically happens to the seed before it can germinate.
The role of the seed coat is controlling dormancy and germination.
Some species of tree are very sensitive to the type of light needed for germination; the seed will only start the growing process if the right intensity is given. This will prevent the sapling from growing directly underneath a tree as this would mean a hard fight for survival.
Germination of an acorn occurs under the soil which is called hypogeal germination, this type the cotyledons remain under ground, the plant has to still function from the food source in the seed as the plant cannot photosynthesise until the leaves are formed, the cotyledons are essential for sustaining growth up to this point. A type of bird called a Jay is known for dispersing acorns as they collect acorns and bury them in the ground to be eaten at a later date. The Jay will remember where they have left the acorn and when they return to collect the acorn they pull the acorn from the ground leaving the root, stem and leaves, the plant continues to grow.
When the seed has reached is suitable site, the seed will need the correct combination of conditions to germinate, the three factors involved are light temperature and moisture. Species vary greatly in the different levels of each factor, once these factors are at the right levels the next process will occur. How fast the seedlings grow also varies due to different factors, depending on the size of seed and food stores within the seed, also the environment that it is growing. At this stage there could be competition from other plants in the surrounding area, for light.
Acorns come off of the tree at about 45% moisture, but will cease to germinate if their moisture levels drop below 25%, If soaked in water for more than 48 hours this process can be reversed.
Xiaogai Hou et al (2010) studied the survival rate of acorns after the cotyledon of the seed had been damaged or partially removed, this happens in the wild from insects and small mammals eating the seeds food reserves to survive. Amazingly all the acorns in his experiment sprouted and grew a stem with leaves, however they did find a significant difference in the height and number of leaves compared with other acorns that had different parts of the cotyledons removed. They concluded that the most important part of the seed was the top. Damage at the top of the acorn was more detrimental to the seedlings survival and success than damage to the lower half and therefore that insect infestation could be responsible for the decline in Oak seedling survival in infested areas.
Anna M. Jensen et al (2012) carried out research looking at the relationship between oak saplings and other vegetation such as shrubs, herbacious vegetation and the effects on the total biomass accumulated by the oak saplings with these other plants present. All plants compete with their neighbouring plants; Anna Jensen et al found that shrubs indirectly helped the oak saplings by suppressing the herbaceous plants at the start of the saplings life. The saplings that were grown in amongst the herbaceous vegetation did invest in their root system, compared to the saplings grown with no competition or competition with shrubs only. During the third year of growth the shrub did seem to hinder the growth of the oak saplings, however as this study was only over three years the longer known effect of these nursery plants was not known.
When seedlings are developing their first roots they will either put a long taproot down into the ground or lots of fibrous roots, the advantages of having a large taproot is that it gives the sapling a great advantage for finding a reliable source of water and nutrients, however the down side is that these plants are very difficult to transplant as the long taproot is rarely completely removed from the ground. A Blue Oak acorn was planted in central California, the tap root was found to be around 1 metre long. (Thomas 2011).
Biotic factors that affect saplings are: Pathogens, Fungal/Bacterial, animals grazing, competition with other plants for water and nutrients
There are insects and Mammals that live off of trees throughout their life cycle, they are an important part of the ecosystem and usually feed or a particular species or avoid a particular species. This can be a problem for the tree at any stage but each species of tree has its own adaption’s, physical and chemical as a defence against attack as the tree is unable to move away from the predator.
An example of a parasite living in Oak trees is the gall wasp, the larvae of this insect feeds on the seed when it is still on the parent tree and developing, this will cause the acorn to abnormally form into what’s commonly called an Oak Apple.
Bastien Castagneyrol et al (2012) carried out a controlled field experiment on oak saplings. They were looking at the genetic diversity of Quercus Robur saplings and the success for insects feeding outside the plants and inside these plants. Their conclusion was that a greater diversity of a plant does not reduce the attack from these types of herbivores, and that instead the type of herbivore grazing just became more of a general browser rather than one that feeds on one particular plant.
Abiotic factors that affect saplings are Weather, soil, avalanches, fires, floods and volcanoes .
Tropisms are a plants reaction to an outside influence upon the plant, such as geotropism (growth on a gradient) phototropism (growth towards a light source) this reaction causes certain types of growth mostly which is visible externally and can paint a picture of the life the tree has had up to that point.
I think you will agree that it is miraculous that any oak tree survives with the number of difficulties it has to overcome in its formative years.
References
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Nataša P. Nikoliã, Ljiljana S. Merkulov, Borivoj Ð. Krstiã, Slobodanka P. Pajeviã, Milan K. Boriše, Saša S. Orlovi.. (2010). VARIABILITY OF ACORN ANATOMICAL CHARACTERISTICS IN QUERCUS ROBUR L. GENOTYPES. Proc. Nat. Sci, Matica Srpska Novi Sad. 118, 47—58.
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Xiaogai Hou, Xianfeng Yi, Yueqin Yang, Wenjing Liu. (10 February 2010). Acorn germination and seedling survival of Q. variabilis: effects of cotyledon excision. effects of cotyledon excision . 711, 1-7.