The difference in Ecomorphology of the Outer and Inner leaves from a Quercus robur (Pedunculate Oak tree).

Khalil Betz-Heinemann – DI308

This study is to confirm whether the outer leaves’ morphology is adapted to have an overall larger sinus area to allow more light to reach the leaves on the inner canopy of the tree. The hypothesis is that outer leaves on an Oak tree will have larger sinuses than the inner leaves on the same Oak tree. This will be tested by comparing the sinus area between outer and inner leaves to see whether the outer leaves have significantly larger sinuses for light to pass through.

The Oak tree is a photoautotroph, which means it uses sunlight as an energy source to convert carbon dioxide and water into oxygen and glucose. The sunlight is trapped by chlorophyll in the chloroplasts of the Oak tree’s leaves. This whole process is called photosynthesis. Due to some leaves on a tree being on the outer surface of its canopy, they will receive the largest share of the sunlight reaching the tree, because they will block some of the sunlight from reaching the leaves underneath or below them. However Oak trees have gaps in their leaves called sinuses. These sinuses mean that a leaf can capture sunlight while some sunlight can pass through these sinus gaps to the leaves below (Horn 1971). Research has shown that sinus’s in Oak leaves are deeper in outer leaves than inner leaves suggesting that the larger sinus is an adaptation to allow light to pass through (Osborn & Taylor 1990). Oak trees’ adaptability is important to study as they are the most common deciduous tree in the U.K (Figure 1). They provide a variety of habitats and are part of many other species’ life cycles especially those that make up the symbiotic Lichen (Figure 2). It is therefore increasingly important to understand how the Oak tree has adapted to become an important part of the U.K. ecosystem so that it can be maintained appropriately. This study is to see whether an Oak tree’s leaf morphology is adapted to allow the maximum amount of photosynthesis to take place.

Figure 1 (Forestry Statistics 1999). Figure 2 (Agate 2002).

Figure 2.

Method

An Oak tree with a fairly uniform foliage circumference with no neighbouring trees or buildings and of mature age was selected, so as to remove the possibility of non-uniform exposure to sunlight. A sample size of ten leaves exposed fully to the sunlight was collected and ten leaves from the inner shaded foliage of the tree (see Figure 2). All the leaves were collected at random from the same cross section of the tree so that they would all have been in a similar position relative to the sun. Additionally no leaves with visible signs of insect or parasite activity were selected.

The surface area of each leaf was worked out by placing each on a grid and calculating the number of 2mm^2 squares each covered (Figure 3). The gaps or sinus area for each leaf was defined by drawing a ruled line between the tips of each lobe. The surface area between the edges of the leaf and the ruled lines was then also calculated using the afore-mentioned grid method, to give the sinus area.

Figure 3.

Results

Having collected the data, the sinus area of each leaf was divided by the corresponding leaf area. This then gave a ratio for each leaf’s sinus area in comparison to its leaf area, where the leaf area is 1 and the sinus area is a ratio to this, as displayed in the ‘ratio of sinus to leaf area’ column in the Tables 1 & 2. When an average of leaf area to sinus area for Outer Leaves is compared to that of Inner Leaves we can see that Outer leaves have an average ratio of 1:0.12 and for the Inner leaves it is 1:0.09. Therefore the data supports the hypothesis that sinus area in outer leaves is larger because 0.12 (Outer Leaf sinus area) is larger than 0.09 (Inner Leaf sinus area).

Results Table 1.

Outer Leaves Sinus area Leaf area Ratio of Sinus to Leaf Area

114.5

971.5

0.12

132

1413

0.09

130

1074

0.12

119

738

0.16

110

1043

0.11

211

1637

0.13

112.5

957.5

0.12

87.5

879

0.10

108.5

968

0.11

85

670.5

0.13

average

0.12

variance

0.00036

Results Table 2.

Inner Leaves Sinus area Leaf area Ratio of Sinus to Leaf Area

49

736

0.07

76

958

0.08

108

842

0.13

130

1294

0.10

66

683

0.10

140

1060

0.13

93

1227

0.08

49

604

0.08

44

688

0.06

43

607

0.07

average

0.09

variance

0.00060


However to test whether the two data sets used are actually significantly different enough statistically, a one-tailed t-test was performed. To use the correct t-test an f-test was first done to see whether the variances of each data set are significantly different.

Results Table 3.

f-test  

0.455

t-test  

0.004

As the result of the f-test is 0.455 (Table 3) which is larger than 0.05, this shows that the variance distributions of the two data sets do overlap significantly as the overlap is more than 5%. This type of variance distribution is tested using a ‘type 3’ t-test. The result of which is 0.004 (Table 3), which shows that the two data sets are significantly different as the similarity is less than 5%.

Overall the data collected supports the hypothesis and is statistically viable. However the methodology of this investigation may have biased the results towards this conclusion. The samples we have taken have all been from one individual Oak tree thus providing no possibility for comparative analysis with the rest of the population (Sudman 1976). It is therefore highly possible that the single tree chosen could have been abnormal in comparison to the population average. Using a sample size of ten inner leaves and ten outer leaves is also very limited because one abnormal leaf will already influence the data by magnifying any random error/anomaly by 10% (Larget 2003). Additionally no distinctive observations were made as to how much sunlight actually reaches inner leaves in comparison to outer leaves, meaning that the distinction in sinus areas found is not necessarily an adaptation for an increased amount of sunlight to reach the inner leaves.

The actual samples taken from the individual tree were picked without using a random selection method, but by merely plucking the most available leaves at random intervals. This means the leaves taken could be biased towards the collector’s height and judgement of what constitutes a good leaf for sampling. This collector’s judgement was also used to decide what inner and outer leaves were, without a specific rule of distinction used. In comparison the method for measuring the sinus area and leaf area was consistent leaving no room for differential measuring of each leaf, which would make the data irrelevant.

Comparisons to other studies conducted on leaf shape have shown sinus areas are more indented in leaves from the outer canopy of many tree species including the Oak (Westmoreland 1989). However this is not necessarily just due to sunlight but can be influenced by factors such as wind, humidity, elevation and location (Givnish 1987). It has also been proposed that leaves separated into segments by larger sinuses have better gaseous exchange across their surface (Saupe 2009), suggesting that the differentiation in sinus area in comparison to leaf area may not only be an adaptation for efficient sunlight absorption. Studies related directly to sinus area versus sunlight absorption efficiency do suggest that the results of this investigation are in agreement with a general scientific consensus; selective pressure mediated by phylogenetic constraints is responsible for outer leaves on some tree species including Oak, having larger sinus areas than the inner leaves (Givnish 1987). While efficient photosynthesis is seen as responsible for being a commanding selective pressure in the formation of this alternate geometry between leaves on the same tree (Chabot & Jurik 1979).

In conclusion this investigation is lacking statistical weight and methodological viability as described and requires further research into all the related components involved in Oak tree leaf morphology. However as this investigation does not contradict separate in-depth studies of a similar nature and is statistically correct within itself, it can be said to further support the science that Outer leaves on an Oak tree do have larger sinus areas and that this is mainly due to a selective adaptation for allowing light to pass through to the Inner leaves.

Bibliography

Horn, H. S. (1971). The Adaptive Geometry of Trees. New Jersey: Princeton University Press.

Osborn, J. M. (1990). Morphological and ultra structural studies of plant cuticular membranes. I. Sun and shade leaves of Quercus velutina (Fagaceae). Botanical Gazette 151: 465-476.

Forestry Statistics (2008). Woodland Areas and Planting – Main Tree Species in GB. UK: National Inventory of Woodland and Trees.

Agate, E. (2002). Woodlands. UK: BCTV.

Sudman, S. (1976). Applied Sampling. New York: Academic Press.

Larget, B. (2003). ‘Probability’ Lecture. Madison: University of Wisconsin, Department of Statistics.

Westmoreland, D (1989). Leaf morphology & light microenvironments: A field exercise.  American Biology Teacher 51: 303 – 306.  

Givnish, T. J. (1987). Comparative studies of leaf form:  Assessing the relative roles of selective pressures and phylogenetic constraints. New Phytologist 106 (Supple): 131 – 160.

Saupe, S. G. (2009). ‘Plant Physiology (Biology 327)’ Lecture. Minnesota: St. John’s University, Biology Department.

Chabot, B. F. & Jurik, T. W. (1979). Influence of instantaneous and integrated light-flux density on leaf anatomy and photosynthesis. American Journal of Botany 66: 940-945.

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