The yield of many crops is limited by the amount of nitrate ions available in the soil. Nitrate gets into the soil naturally when nitrogen gas from the air is fixed by bacteria or by lightning, or when organic substances such as dead plants or animal droppings are decayed by bacteria. In order to increase production, farmers add nitrogen-containing fertilisers to the soil.
At first glance, it might seem very easy for plants to obtain their nitrogen. However, atmospheric nitrogen is unavailable to plants or animals. Plants usually obtain the nitrogen they need by absorbing nitrate ions or ammonium ions through their roots. However some plants obtain much of their nitrogen by forming a symbiotic relationship with nitrogen fixing bacteria. Vaccinium may be able to do this, which would explain why it is a pioneer plant in the burnt area. Nitrogen fixation is the conversion of nitrogen gas into ammonia. This is carried out by nitrogen fixing bacteria such as Rhizobium, Azotobacter and Frankia.
There are also far more areas in the unburnt area where there is no vaccinium at all then there are in the burnt area. This would suggest the vaccinium being a pioneer plant, which as the calluna grows more and competes the vaccinium loses out more.
On the graph for the unburnt moorland, there is often a peak in the amount of vaccinium, on the same quadrat as there is a trough in the number of calluna. This means that where the two species have been growing for many years and no burning has taken place, the vaccinium grows better when there is no calluna present. Whereas on the graph for the burnt moorland, when there is a peak of calluna, there often also is a peak of vaccinium, which means that when the area has been burnt and the species have recently grown back, the two plants grow better when they are both present. This may be the calluna being helped by the vaccinium or the vaccinium being helped by the calluna.
In the unburnt area there are far more plants of all kinds. The calluna and the vaccinium are both more prominent in the unburnt area than they are in the burnt area. However in the burnt area the vaccinium plants are not outnumbered by the calluna plants by as high a percentage. If we went to the moorland closer to the time when the burning had taken place, there would have been more vaccinium than calluna, or possible no calluna at all. This is because some plants such as vaccinium are better adapted for living in conditions where the ground is in poor condition and there are fewer nutrients. The vaccinium was a pioneer plant and began primary succession, which enabled the calluna plant to grow there. The calluna plant is better adapted to grow in good conditions and will then overtake the vaccinium as the conditions get better and the amount of calluna therefore increases, the vaccinium may not decrease in number but they can’t keep up with the calluna’s numbers
Competition
Communities consist of individuals living together. Competition occurs whenever two or more individuals have to share resources in short supply. Competition between members of two different species, like the competition between the Calluna plant and the Vaccinium plant, is called interspecific competition. The calluna plant has small pine leaves so it reduces transpiration. There is a smaller surface area so less transpiration occurs. The vaccinia plant has larger leaves so has a larger surface area, therefore more photosynthesis can occur.
Succession
Species distribution does not necessarily remain constant over long periods of time. After the moorland is burnt the plants will have to begin growing there again. The first species to colonise bare ground like this are called pioneer plants. They are able to survive in very difficult conditions. Some plants are able to fix nitrogen, which helps them to grow even where there is almost no nitrate in the soil. The presence of the pioneer plants gradually changes the environmental conditions: they provide shelter for other seeds to germinate, or for insects to hide. Dead leaves that fall from them provide humus that becomes part of the soil, improving its water-holding capacities and nutrient content. So, over many years, the environmental conditions become suitable for a wider range of plants to live. The number of species increases, and some of the pioneer plants will disappear because they are not good at competing with the newcomers. This gradual, directional change in a community over time is called succession. In this case, where the starting point was burnt with no living things present, it is known as primary succession.
Nitrogen Cycle
The yield of many crops is limited by the amount of nitrate ions available in the soil. Nitrate gets into the soil naturally when nitrogen gas from the air is fixed by bacteria or by lightning, or when organic substances such as dead plants or animal droppings are decayed by bacteria. In order to increase production, farmers add nitrogen-containing fertilisers to the soil.
At first glance, it might seem very easy for plants to obtain their nitrogen. However, atmospheric nitrogen is unavailable to plants or animals. Plants usually obtain the nitrogen they need by absorbing nitrate ions or ammonium ions through their roots. However some plants obtain much of their nitrogen by forming.
Unburned
Observed Value
Expected Value
Burnt
Observed Value
Expected Value
Chi Squared
χ² = ∑ (O-E)² / E
Unburned
χ² = ∑ (35-36)² / 36 + (4-3)² / 3 + (11-10)² / 10 + (4-1)² / 1 = 9.5
Burnt
χ² = ∑ (39-35)² / 35 + (1-5)² / 5 + (5-9)² / 9 + (5-1)² / 1 = 21.4
Critical Value
Critical value = 3.84 (at the 5% accuracy level)
The value calculated (i.e. 21.4) is greater than the critical value (i.e. 3.84) then there is a significant difference between the observed and expected value.
As the observed value is greater than the expected value it is possible to say that vaccinium and calluna are associated i.e. they grow together more often than could be expected by chance.
The null hypotheses is rejected.
