Figure 1. The Nosehill forest.
Monday, November 28, 2011
About the Forest, by Felix and Jose
Figure 1. The Nosehill forest.
Do plants tend to grow around already existing trees or away from them? We went to Calgary’s Nose Hill Park on September 22, 2011 to observe and record a set of data to help analyze this question. A 10m x 1m plot of land was set up at 1198m elevation; 51° latitude, 6.799min N; 114° longitude, 7.771 min W; 260° clockwise N. From here, we observed the number of shrubs and forbs (plants that do not have woody bodies and are not grasses) growing within a 50 cm radius of a tree versus those that were not. We found that in total, 37 small shrubs and forbs, excluding grass, where growing within that radius while only 19 grew in areas outside of that range. The total amount of area close to trees was approximately 30% of the total plotted area. Some of the plants were identified to be dandelions, clover, shrub dogwood, snowberry, and wild roses. The tree species was aspen poplar.
Figure 2. Image of snowberry.
What we can draw from this is that shrubs and forbs tend to grow close to trees. Now why is that? There are several possible biological explanations as to why this phenomenon occurs.
In many environments, sunlight is an extremely sought after commodity by plants. This is because plants use the energy gained from solar radiation to facilitate the process of photosynthesis. Through photosynthesis, plants are able to phosphorylate ADP (adenine diphosphate) into ATP (adenine triphosphate) which is used as the main source of fuel for cellular processes. They also create glucose and oxygen in the process which is necessary for human survival. Small plants tend to flourish in areas without large trees because large trees can obstruct sunlight. Grassland biomes, however, are a special case.
The biome that this scientific inquiry was conducted in was a grassland / deciduous forests biome in Alberta. One of the most definitive attributes of this type of is their lack of accessible water. In most cases, plants pursue more sunlight as it provides more energy for photosynthesis thus allowing the plant to flourish. Too much sunlight, however, can result in the accelerated rate of evaporation and can lead to dehydration among plants resulting in their deaths. Calgary, according to Statistics Canada receives an average of approximately 2405 hours of sunlight per year making it the sunniest city in North America. This is why much of Calgary is naturally grassland; grasses consume very little water.
In order for plants like small shrubs and forbs to grow, they have to find water. Large trees provide shade from the sun which slows down evaporation and giving more time for water to be absorbed into the ground. Running water is slowed when it comes in contact with the trees and their roots allowing for more soakage. The accumulation of more water allows the soil to sustain more plants. In this case, water becomes a more valuable commodity than sunlight so plants will actually grow near large trees.
Figure 3. Image of aspen poplar tree canopy.
Notice the amount of area the leaves cover and the sunlight they must block.
Plants rely on seeds for reproduction. To be dependent on the wind scattering them would be very risky; many plants have developed an evolutionary feature to account for that. They place their seeds in parts of the plant that are commonly consumed by animals – fruits. As animals eat them, they ingest the seeds that are not digested and later excreted out. This helps plants spread around areas that are massive distances away. This technique is dependent where the animal goes. Animals require shelter; in grassland like Nose Hill, there are few areas with ample shelter besides some dispersed areas with trees. Animals will naturally congregate and live in these areas and defecate in them. These results in a higher concentration of seeds in areas close to trees resulting in more plant growth.
There are several biological principles, however, that argue the opposite. For example: competition for limited resources like phosphorous and nitrogen compounds that are necessary for the process of photosynthesis and sustaining a plant’s life. Trees consume many times more resources than shrubs do. This creates competition. Plants growing near trees would receive less and therefore be hindered in growth and may die. Therefore, shrubs and forbs should grow away from trees. Grasslands and deciduous forests, however, are known to have rich soils. This is because there is large amount of biomass that decomposes thus adding nutrients into the soil. Biomes like rainforests have poor soil because much of the decaying material is consumed by insects and other animals thus preventing the absorption of nutrients into the soil; grasslands and deciduous forests have almost none. There are also very few animals that consume large quantities of living biomass which allows for more plant growth and later decay. Because the soil is rich in nutrients, there is not much competition for them and therefore it does have a significant effect.
Another possible explanation involves oxygen levels and cellular respiration. The trees with their leaves high above the rest of the shrubs absorb most of the sunlight and therefore have more energy and are able to perform more photosynthesis. One of the products of photosynthesis is oxygen. Oxygen is used in the process of cellular respiration – a process that produces ATP thus fuelling vital cellular processes. Some oxygen produced from photosynthesis is used in cellular respiration while some is exported out as excess. The higher rate of photosynthesis means more oxygen being produced and more being let out as waste filling up the surrounding air with more oxygen. The small shrubs on the ground do not absorb much sunlight and therefore do not photosynthesize as much. This means that sometimes they do not get enough oxygen for cellular respiration. The excess oxygen from the trees, however, can be used to supplement this shortage. This creates a form of commensalistic symbiosis between the small shrubs and trees.
Nose Hill is one of Calgary’s most well preserved natural environments. In many environments, plant species tend to be the most competitive amongst each other. Pilosella Aurantiaca, for example, relentlessly absorbs all resources in the land and reproduces very quickly thus consuming and forcing out native plants. If we are to look at the behaviour of flora in our area of study, however, we can see quite the opposite: a general mutualism or commensalism between different varieties of plants. This is very interesting and must come with an explanation. A possible explanation is the lack for ruthless competition. Dispersed areas with trees and shrubs in grasslands like the one we have studied Nose Hill are in a rare environmental situation; they have abundant resources due to rich soils and very few other competitive species are present. Plants in this situation form a community where they do not need to compete amongst other members. They can, instead, form a mutual alliance with the purpose of protecting their fertile land from invasive foreign species that would deplete the resources and threaten the survival of the community.
From the data collected at our study in Nose Hill Parks and various biological reasoning, we can derive many explanations as to why some plants behave the way they do. We can see how a plant’s biome affects its behaviour and can lead to forms of symbiosis and a community of plants.
Freeman. S. (2008). Biological Science. San Francisco: Pearson Benjamin Cummings
Campbell. N. (1996). Biology: Fourth Edition. San Francisco: The Benjamin/ Cummings Publishing Company
Lehninger, A. L., Cox, M. M., & Nelson, D. L. (2005). Lehninger principles of biochemistry (4th ed.). New York: W.H. Freeman