Ecology Field Trip
The purpose of this field experiment was to determine the composition of the oak forest on and around Hubbard Hill, determining tree type frequency and size through a random sampling process. This is important for ecological purposes in determining the overall needs and structure of the oak forest ecosystem, at least insofar as different tree species are concerned, and for tracking levels of forest growth, loss, or change through repeated measurements/sampled conducted over time. A secondary purpose of this field exercise more directly aimed at the educational objectives was an exposure to the methods and skills necessary for conducting an accurate and effective sample, giving students needed experience.
Methods
Two different sampling methods were utilized in the actual conducting of the field experiment, and the results later compared and combined in analysis. Both sampling methods began with the use of a random number chart to determine transect lines and compass directions, and to determine points along these transect lines for observations/sampling. A drag line was extended along the transect line, once determined, to aid in consistency and the ease of conducting the sample n an accurate manner. Trees included in the measurement were those greater than ten centimeters (in practice, nine-and-a-half centimeters, to account for measurement discrepancies) in diameter at breast height, with breast height defined as 137 centimeters. Measurements/estimates for split-stemmed trees were taken nearer the ground, below the split.
For the plot sample method, a square plot ten meters by ten meters was created around each plot, extending ten meters from the pre-determined observation point along the length of the transect line, and five meters in both directions perpendicular to the transect line. All trees within this square of appropriate size were recorded by species and size. The second sampling method originated with the same randomly predetermined point on the transect line that was used to mark the location of the plot, however this point was used as the center of four quadrants defined by the drag line and an imagined perpendicular line, and the tree nearest to the center point in each quadrant that met the size criteria was recorded. This led to some degree of overlap in the sampling, as there was always the potential for the trees in the forward quadrants to have also been included in the plot, and indeed this was frequently the case.
Results
The first table demonstrates the results of the plot sampling method, which examined all trees of diameter greater than 9.5 centimeters when measured at breast height within each of the randomly identified ten-meter square plots. A total of three hundred and thirty two trees were included in the observations made via this method, and a full third of these trees were white pines -- by far the most common tree in the forest according to the sample, and by a wide enough margin to make this a comfortable statement to apply to the forest as a whole. Yellow birches and red oaks were almost equally represented, the point of having statistically equal populations and frequencies in the forest, with significant red maple populations as well. Other varieties of oak, birch, maple, and more occurred with far lower levels of frequency.
Tree Type
Frequency
Avg. Siz (DBH)
White Pine
42.1
Yellow Birch
43
16.8
Red Oak
42
45
Red Maple
33
31.4
White Oak
18
33.6
Sugar Maple
17
27.1
Beech
12
18.5
American Elm
7
18
Black Birch
7
13.4
Hemlock
6
16
Hornbeam
6
19
White Ash
6
26.7
Sweet Birch
5
22.2
Paper Birch
4
19.5
American Birch
3
10.7
Red Pine
3
48
Silver Maple
2
20
White Birch
2
14.5
Cherry
1
20
Mountain Maple
1
37
Pin Cherry
1
13
Pitch Pine
1
41
Yellow Maple
1
55
Total:
26.5
The second table shows the results of the point-quarter sampling method, which yielded comparable results that nonetheless differ in significant ways (discussed at greater length below). Only one-hundred and forty-four trees were included in this sample, which is easily predicted given four trees per randomly selected observation point and a total of thirty-six observation points; white pines were still the majority population, though they comprised less than a quarter of the trees observed. Other general assessments of relative populations and the increased observation of certain species over others held basically the same in these observations, though again proportions differed. There were also fewer tree species identified altogether in the point-quarter sampling method compared to the plot sampling.
Tree Type
Frequency
Avg. Siz (DBH)
White Pine
33
40.7
Red Oak
23
43.6
Yellow Birch
18
26.7
Red Maple
16
30.8
Beech
10
19.3
Black Birch
9
16.6
Sugar Maple
9
28.2
White Oak
9
44.7
White Ash
7
27.1
Paper Birch
4
11.5
Sweet Birch
2
21.0
American Elm
1
33.0
Mountain Maple
1
37.0
Silver Maple
1
26.0
White Birch
1
12.0
Total:
27.9
Discussion
The results of these two sampling methods have a variety of implications both for the ecological understanding of this forested area and for the practices of sampling and data collection that were utilized and explored in this field exercise. First, it is possible that the term "oak forest" is something of a misnomer here, as not only do pine trees represent a much more substantial portion of the tree population but also because compared to the whole, oak trees do not make up a majority or even an especially sizable minority of the forest's trees. This is not just an issue of nomenclature, but also could lead to different perspectives on fundamental ecological issues regarding the mix and balance of species -- both flora as well as fauna -- likely to be found and potentially threatened in this particular ecosystem.
The differences in the two sampling methods are perhaps of even greater importance, given the more direct nature of the results that can be observed in this area. With a population less than half that of the plot sampling method, it is of little surprise that the point-quarter method is less accurate, but the degree of difference is perhaps a bit surprising. Entirely different proportions of trees were found using the two methods, and in the second method there were several specific species that went entirely unobserved despite being fairly well represented in the plot sampling method. These differences could be significant ecologically -- almost certainly are significant, in fact -- as they represent more unique niches in terms of resources attainment and provision for other species of plant and animal life in the forest. There is also a fairly substantial risk of rather complex bias in the point-quart method of sampling, as the method of sampling necessarily chooses trees that are in a fairly close proximal relationship, even more so than with the plot-sampling method (which does contain some of the same bias). With only four trees selected for observation each point and each tree being the closest tree of substantial size to that point, if there was any reason that two species of tree or two specimens of the same species of tree would or would not bas as prone to grow in proximity to each other, then this sampling method is going to demonstrate this relationship rather than an accurate rendering of the forest as a whole.
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