Nathan Turk, Tom Ryan, Jose Lopez
September 29, 2005
FOR 332
Lab Report: Red Pine Demography
Sunlight Intensity on Forest Floors of Contrasting Stands
Abstract
Measuring the intensity of sunlight per unit area upon the forest floor is a possible means
of evaluating light conditions in terms of a standard determined by a light reading taken
in an open area. Using a light meter, we measured light intensity in two different red pine
plantations at Heiberg Forest, near Tully, NY. We expected to find greater variation in
readings taken in plots in the second site than in the first. The experiment corroborated
this, although time constraints precluded readings at all of Site 1’s plots. This means
future approaches can be revised. Sunlight intensity not only correlates with relative light
conditions but also has ties with species composition, diameter growth and live crown
ratios. It also poses prospects for future management endeavors.
Introduction
Many variables affect intensity of light on forests’ floors. Sometimes sources of that
variability can be determined through light intensity readings at many plots within a
stand. Plots with high crown density and basal area, for example, might experience more
interception of sunlight through their canopies (Spurr and Barnes, 1980). Sites that are
less dense or have experienced gaps in their canopies due to disturbance, mortality or
thinning might experience greater sunlight intensity at the forest floor. Ultimately the
higher the light intensity at the forest floor, the closer its conditions are to experiencing
full sunlight, such as open plots do. Open plots, however, are still subject to absorption
and scattering from water vapor, dust and permanent atmospheric gases (Grasovsky,
1929), which is why it's important to compare measurements to those from open areas
near the plots, versus universal values. In our experiment we used a light meter to
measure the intensity of light in terms of microEinsteins, or instantaneous photon flux
density, per square meter at ground level. We expected to find greater variability in light
intensity readings within the second stand than in the first.
Methods
The study was conducted in compartments 27 and 24B at Heiberg Experimental Forest,
in Tully, NY. Each compartment had 9 plots, each about 50 feet from each other. We
took measurements from plots 1, 3, 5, 6, 7 and 9 at compartment 27 (Site 1) and plots 1
through 9 at compartment 24B (Site 2). Before taking light meter readings from each
compartment, we measured the intensity of sunlight in open areas not shaded by trees.
The full-sunlight reading was established as the reference point by which we could
quantify, rather than simply observe, how shaded forest conditions were. We also
checked whether cloud coverage obstructed sunlight prior to taking any measurements. If
clouds were present, we delayed the reading until they passed. Otherwise we took the
sunlight readings with the light meter at a distance of one pace south from the center of
each plot site, using the afternoon sun as a westerly reference point. We then recorded
each reading.
Results
Comparative Analysis of Amount of Sunlight
at Ground Level
Fig. 1
Amount of Sunlight
50
40
30
Site 1
20
Site 2
10
0
1
2
3
4
5
6
7
8
9
Flag Number
Although time restraints precluded the measurement of light intensity at every plot in Site
1, results show a generally consistent measure of five or less microEinsteins of light
intensity per square meter in plots 1, 3, 5, 6, 7 and 9. The distribution of light intensities
in Site 2 was more widespread, from approximately 2 microEinsteins at plot 2 to close to
45 at plot 9, which was the highest of either site’s intensities.
Discussions
Though canopy cover was not measured during the red pine demography lab, a link
between over story vigor and amount of sunlight reaching the forest floor is suggested
with Team Gray Van’s measurements of crown ratio, or crown height as compared to
total tree height, among the stands’ Pinus resinosa trees. Stand 1’s red pines had two of
the highest crown ratios in plots one (26%) and seven (29%), where we found the lowest
sunlight intensity readings (fig. 1). In Stand 2, both the highest crown ratio percentage
(47%) and one of the lowest sunlight intensity readings--2 microEinsteins-- coincided in
plot two, while crown ratios as small as 11% in plot four coexisted with sunlight intensity
on par with Site 1’s generally low readings. Such comparisons indicate that vigorous
crowns among the stands’ dominant trees may stymie light from reaching the forest floor
in high intensities.
Woods and DeSantis’ findings show the dominance of Pinus resinosa in terms of both
height and quantity. Considering red pines’ shade intolerance, the trees’ success at Stand
1’s less light-intense places such as plot one (fig. 1) suggests the pre-forest conditions
might have consisted of far more light upon perhaps a drier soil, on which the species
thrives (Hardin, Leopold and White, 2001). Sunny, open conditions might have allowed
their rise to dominance in Site 1’s DBH distribution, after which their successful lateral
and vertical growth decreased the intensity of sunlight on the forest floor, leading up to
the relatively low-intensity conditions we measured. Site 2, however, shows comparative
diameter growth among moderately shade-tolerant Pinus strobus, while the site’s largest
DBH class (16 to 18 inches) is occupied by Fraxinus Americana and aspens, which are
both generally shade-intolerant. The greater range of diameters among Stand 2’s species
compared to those of Stand 1 might reflect the wider range of light intensities measured,
since white pine, for example, could succeed with partial light (Stand 2) better than it
might in Stand 1’s relative shade. The variation in tolerances among the stand’s trees
might also be a reflection of an erratic canopy cover, a possibility that our wide array of
measured intensities supported. Species’ proven success in particular conditions of
sunlight intensity could help future operations aiming to accommodate understory
regeneration, as well as efforts toward reforestation or a forestation of formerly clear,
open sites.
Determining the species composition and diameter class distribution of plots three
through nine in Site 1 and Site 2 may have painted a clearer picture of how light intensity
affects diameter, as well as how patterns of shade tolerance versus intolerance unfold in
differing light intensity conditions. Additionally, readings at Site 1’s plots two, four and
eight may have indicated more clearly where the strongest and weakest intensities of light
occurred, and might have further corroborated the inverse relationship we observed
between live crown ratio and sunlight intensity.
References
Grasovsky, Amihud. “Some Aspects of Light in the Forest.” Yale University School of
Forestry Bulletin 23 (1929): 9-28.
Hardin James W., Donald J. Leopold, Fred M. White. 2001. Textbook of Dendrology.
McGraw-Hill Higher Education. Boston: 214.
Spurr, Stephen H., Burton V. Barnes. 1980. Forest Ecology. John Wiley & Sons, Inc.
New York: pp. 119-120.
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