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COMPARING SEEDLING RECRUITMENT BETWEEN EASTERN HEMLOCK(Tsuga Canadensis) AND OTHER TREE SPECIES BY EXAMINING THE

EFFECTS OF SLOPE AND SUNLIGHT PENETRATION ON THERECRUITMENT

Samuel Ayers1, Esther Fadairo1, Elizabeth Soto2, Kathryn Sweeny1

1Wells College 2Biology Major, Wells College

Key Words: eastern hemlock, slope, sunlight level, seedling, low recruitment,germination, shade, understory populations, effects, t-test p-value

INTRODUCTION

Tsuga canadensis, commonlyknown as Eastern hemlock, is aconfiner tree species locatedin the Eastern United States.It has flat needles with whitestripes on its underside.Eastern hemlock is known forgrowing tall and old as well asbeing one of the most shade-tolerant tree species, beingable to survive with as littleas five percent sunlight.(Godman and Lancaster, 1990)Its ability to survive in lowlight is most likely connectedwith its needles creating adeep canopy that can shed outlight. (Hadley and Schedlbauer2002) Eastern hemlock is foundon a wide range of areas suchas ridge tops, and it is alsofound on northern positions ofits range with north and eastfacing slopes. (Farjon, 1990)

An interesting fact hasbeen noted for Eastern

hemlock’s understorypopulations compared to othertree species. For some unknownreason, seedlings have adifficult time germinatingunder hemlock trees, resultingin a low density of little tono seedlings under hemlocks.(Rogers, 1978) In thisexperiment we will examine theeffects of slope and sunlightpenetration on Eastern Hemlockto help us predict the lowseedling recruitment underhemlock trees. While hemlocksare well adapted to grow onsteep slopes, seedlings have adifficult time germinatingunder higher sloped conditions,therefore, steep slopes inhibitnew growth. Additionally, sincehemlock needles create a canopythat blocks out light,understory seedling populationsare reduced. We expect slopeand light levels to be directlycorrelated in their effects on

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the low amount of seedlingsunder hemlocks because northfacing slopes receive lesslight, which seedlings need togerminate.

MATERIALS AND METHODS

Study Site

A team of ecologists and Iwent out to the Aurora forestjust behind the Macmillanbuilding at Wells College torecord slope and light detailsfor various hemlock and non-hemlock trees. To get a betterestimate of the tree populationwe decided to sample not onlythe trees right behind thebuilding but also trees thatwere further in the forest.

Sampling

On October 7, 2013 we wentout to the forest and markedtrees accordingly by slope andwhether the tree was hemlock ornon-hemlock. Using aclinometer, we established aflat area as an area with aslope less than 15 degrees anda sloped area as an area with aslope greater than 15 degrees.Knowing this, we marked twentytrees total: Five hemlocks onflat grounds, five non-hemlockson flat grounds, five hemlockson sloped grounds, and fivenon-hemlocks on sloped grounds.

After marking a tree, weused a 1x1 meter quadrat tocount the number of seedlings,any woody plant, under eachtree. To keep consistency, thequadrat was always placed onthe south side of the tree. Torecord the light level thateach tree was receiving, weused a PAR ceptometer at thetrunks of the trees, again, onthe south side for consistency.

One of the group members,Sam Ayers, returned to the sametrees for six more days spreadbetween two weeks. He recordedlight levels again for eachtree under different weatherconditions, such as sunny andcloudy, to get a better view ofthe overall light level foreach tree. He also took thesunlight level in the parkinglot as a control reading withno trees, though this readingwas later not used. To makelight levels uniform, themeasurements were converted topercentages (used in Figure 1)

Additionally, a t-test wasconducted for both slope dataand light data using Excel.Analysis ToolPak was added toExcel and under [Tools], [DataAnalysis] we chose the T-Test:Paired Two Sample for Means.When the t-test results weregiven, we looked at the p-value(one tail). P-values below 0.05

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are considered statisticallysignificant, while P-valuesgreater than or equal to 0.05

are considered statisticallyinsignificant.

RESULTS

Data for the twenty trees and seven days was recorded in a chart,and then created into appropriate graphs for interpretation.Patterns and trends in the data can be seen.

FIG. 1. Relationship between light penetration and number ofseedlings based on slope and tree species. The graph aboveshows two patterns. One pattern is that hemlocks tend to get alow amount of light penetration and a small amount ofunderstory seedlings while other tree species tend to get ahigher percentage of light penetration and more seedlings.Another pattern is that flat areas, or small slopes, generallyreceived more light and had more seedlings under trees whilesloped areas received less light and had fewer seedlings. Wealso found our t-test p-value for sunlight penetration to be0.116, making our results statistically insignificant.

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0 5 10 15 20 25 30 35 40 45 500

2

4

6

8

10

12

R² = 0.122275665796819

R² = 0.240694675227205

HemlockLinear (Hemlock)Other Tree Sp.Linear (Other Tree Sp.)

Slope (degrees)

# of

Seedl

ings

Und

er T

ree

FIG. 2. Relationship between slope and number of seedlings basedtree species. The graph above shows a pattern that the higher thedegrees in slope, the lower the number of seedlings under bothhemlock and other tree species. The linear lines for both hemlockand other tree species data make it easier to see a trend betweenthe data, though the R2 values for both sets of data are small.Between the two, the other tree species’ data has a more obvioustrend than the hemlock data. We also found our t-test p-value forslope to be 0.118, making our results statisticallyinsignificant.

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DISCUSSION

According to our findings,our hypothesis is correct. Bothslope and light penetrationlevels affect the overallnumber of seedlings, and forboth cases the lower the lightpenetration and the steeper theslope, the lower the number ofseedlings under the trees.Also, light levels decreased atabout an equal rate for bothhemlocks and non-hemlocks whenslope conditions changed,meaning our data shows clearpatterns and trends.

In Figure 2, one cannotice that even though thenumber of seedlings decreasedas slope increased, non-hemlocktrees still had a higherunderstory population. Thismakes sense because Easternhemlock needles create a canopythat already sheds light (asseen in Figure 1), and whenplaced on sloped conditions,more extreme microclimates arecreated not only underhemlocks, but also around them.This phenomena shows locallight availability is parallelto the size of the canopy ofsurrounding hemlocks. (Canham,Finzi, Pacala, and Burbank,1994)

Assumptions

In order to interpret thedata, one must assume certainassumptions to hold the data astrue as can be. First, weassumed that the forest growthis natural, so in other words,the Aurora forest was nottampered with in any way, soeverything that we observed wasnatural. No seedlings werepurposely placed in certainareas and no trees purposelygrew in certain areas. Thisassumption can be heldpartially true because eventhough all growth seemednatural, we do not know forsure whether people havechanged any aspect of thetested area.

Seedling recruitment isnot affected by tree age. Sincewe randomly selected treeswithout noting age or any otherspecific characteristic otherthan hemlock/non-hemlockspecies, then we assumed thatany other specific treecharacteristic like age doesnot affect understorypopulations.

We also assumed that allnon-hemlock trees are uniform,again, since we did not takenote of any othercharacteristics other thanhemlock species. We also

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assumed that the surroundingarea around the trees weresimilar to the tested area inthe quadrat and that observingonly the area in the quadratdoes not affect the overallresults, since we only testedsouth facing sides of thetrees. Additionally, we assumedthat no other factors had aneffect on the data, such aschanging weather conditions,machine error, and small humanerrors such as pointing the PARceptometer at a different angleevery test day.

Other Factors and Experiments

We noticed that eventhough Figure 1 and Figure 2proved our first hypothesis tobe true, the results are notcompletely significant.Regardless of the slopeconditions, non-hemlock treesalways received more light thanhemlocks, meaning that althoughslope affects light penetrationwhich affects seedlingrecruitment, these are not thesole reasons for the lowunderstory populations. Thereare most likely other factorsthan come into play that we didnot test in our experiment. Forinstance, since hemlocks thriveon moist and acidic soil, thenmaybe the pH of the soil orsome other soil matter is also

contributing to the smallnumber of seedlings underhemlocks. (Finzi, Canham, andVanBreeman, 1998) A study in1982 suggested that “Easternhemlock is allelopathic (whenan organism produces abiochemical that influenceanother organism’s growth,survival, and reproduction) toits own regeneration” due tolitter extracts under hemlocks.Their findings proved theirhypothesis to be correct forsix day old hemlock seedlings,but showed that litter extractshad no effects on hemlockseedlings older than two weeks.(Ward and McCormick, 1982) Theenvironment is complex, andphenomena such as the Hemlocks’reduced seedling recruitmentcan be a result of acombination of several factors,two of which most likelyinclude slope elevations andlight penetration. Because ofthis, further research isneeded.

A source of error would beour small sample size. We onlytested twenty trees, whilehundreds of trees exist in theAurora forest alone. Anothersource would be that any of ourassumptions was false. Forfuture similar experiments wewill test other factors such assoil conditions along with

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slope and light penetration,and we will use a larger numberof tree variables (both hemlockand non-hemlock). Additionally,we will record the species ofthe non-hemlock trees andobserve whether differing non-hemlock tree species havedifferent effects on understorypopulations when compared tohemlocks and each other.

Overall, our resultsclearly show patterns andtrends that prove that Easternhemlocks’ low seedlingrecruitment is partially due toa direct correlation betweenslope and light conditions.Slope seems to be the dominantfactor in understorypopulations, so any conditionsthat change with slope probablyaffect seedling recruitment aswell.

AKNOLEDGEMENTSWe thank Dr. Jackie Schnurr forallowing us to perform thisexperiment and providing uswith the necessary equipment tomark the trees and takeappropriate readings.

LITERATURE CITED

Canham, C.D., Finzi, A.C., Pacala, S.W., and Burbank,D.H. (1994). Causes and

consequences of resource heterogeneity in forests: interspecific variation inlight transmission by canopy trees. Canadian Journal of Forest Research. Vol: 24, pp. 337-349

Farjon, A. (1990). Pinaceae, drawings and descriptions of the genera Abies, Cedrus, Pseudolarix, Keteleeria, Nothotsuga, Tsuga, Cathaya, Pseudotsuga, Larix and Picea. Konigstein: Keltz Scientific Books; pp. 330

Finzi, A. C., Canham, C. D., and Van Breemen, N. (1998). Canopy tree-soil interactions within temperate forests: specieseffects on pH and cations.Ecological Applications, Vol: 8(2), pp. 447-454.

Godman, R. M., & Lancaster, K. (1990). Tsuga canadensis (L.) Carr. eastern hemlock. Silvics of North America, Vol: 1, 604-612.

Hadley, J.L., Schedlbauer, J.L.(2002). Carbon exchange ofan old-growth eastern

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hemlock (Tsuga canadensis)forest in central New England. Tree Physiology Vol: 22, pp. 1079-1092.

Rogers, R.S. (1978). Forests dominated by hemlock (Tsuga canadensis): distribution as related tosite and postsettlement history. Canadian Journal of Botany, Vol: 56(7): pp.843-854

Ward, H. A., McCormick, L. H. (1982) Notes: Eastern Hemlock Allelopathy. Forest Science, Volume 28, Number 4, pp. 681-686