COMPARING VEGETATION CHARACTERISTICS

BETWEEN THE NE AND SW SLOPES AT ARAPAHOE

PASS VALLEY

Caleb Benson, SeniorVegetation Ecology Summer 2013

CU Mountain Research Station

Introduction

• Sub-alpine life zone (3000-3500m)• Arapahoe pass physiogeography• Northeast: Aspect 21 deg. & slope 15.5

deg • Southwest: Aspect 208 deg. & slope

22.2 deg• Comparing vegetation characteristics

between north and south facing slopes.

• South slopes are theoretically drier due to more direct sunlight during the day leading to higher evapotranspiration.

• “Westerlies redistribute snow, creating deep, long-lasting snow fields on leeward (west) slopes while leaving adjacent windward slopes snow free” (Isard).• This should lead to a greater diversity

of herbaceous, low-growing vegetation.

Introduction

Engelmann Spruce Shade tolerant Low establishment rate Longevity Wet environments

Sub-alpine Fir Shade tolerant High recruitment rate Rapid height growth Wet environments

South facing slope bear open scrub vegetation, whereas north facing have evergreen woodlands.

Chamaephytes and Hemicryptophytes

Hypotheses

Hypothesis #1: Overall basal area of tree species (P. engelmannii, A. lasiocarpa, P. tremuloides, P. flexilis ) will be greatest on the northeast slope.

Hypothesis #2: Canopy cover will be greater on the northeast slope.

Hypothesis #3: Species diversity of undergrowth vegetation will be greater on the southwest slope.

Methods

Northeast and Southwest facing slopes Aspect and slope Daubenmire plots

10 per slope Random and continuous Number of Species Percent Cover

Methods

•Spherical Crown Densitometer •Canopy Cover•10 per slope

•Bitterlich Variable Radius Method•Tree Density•15 per slope

Bitterlich Results

Fir Spruce Aspen Limber Pine0

5

10

15

20

25

SW-PlotNE-Plot

• ANOVA• Rows Slopes• Columns

Species

• Fail to reject hypothesis #1 due to very low p-values.

Densitometer Results

• Canopy Cover t-test

• Southwest Slope• Mean = 0.699

(69.9%)• SE = 0.063

• Northeast Slope• Mean = 0.811

(81.1%)• SE = 0.023

• P-value = 0.1221• Reject hypothesis

#2

Daubenmire Results• Undergrowth diversity • T-test• Southwest Slope• Mean = 3.9 species

per plot• SE = 0.79

• Northeast Slope• Mean = 2.6 species

per plot• SE = 0.31

• P-value = 0.1443• Reject hypothesis

#3SW species: 16 total NE species: 10 total

Discussion

Basal area greater on NE slope due to: Shade Longer growing season Fewer disturbances

Canopy cover greater due to greater leaf area on NE slopes from limited sun light.

Species diversity greater on SW slope due to: Less canopy cover Snow blankets causing shorter growing

seasons Recent disturbances

Conclusion

Higher number of Daubenmire plots would provide more significant data. Daubenmire uses 50 evenly spaced(1m) subplots per

plot (50x50m) to obtain an objective evaluation of the undergrowth vegetation (Mueller-Dombois, 121).

Northeast slope more moisture greater tree basal area

Larger basal area greater canopy cover decreased species diversity

Larger basal area increased competition decreased species diversity

Literature CitedAplet, Gregory, and Richard Laven . "Patterns of Community

Dynamics in Colorado Engelmann Spruce-Subalpine Fir Forest." Ecology. 69.2 (1988). Armesto, Juan J., and Josέ A. Martίnez. "Relations between vegetation structure and slope aspect in the mediterranean region of Chile." The Journal of Ecology (1978): 881-889.

Isard, Scott A. "Factors influencing soil moisture and plant community distribution on Niwot Ridge, Front Range,

Colorado, USA." Arctic and Alpine Research (1986): 83-96.

Kutiel, Pua. "LOPE ASPECT EFFECT ON SOIL AND VEGETATION IN A MEDITERRANEAN ECOSYSTEM." Israel Journal of

Botany. 41.4-6 (1992).

Mueller-Dombois, Dieter, and Ellenberg Heinz. Aims and Methods of Vegetation Ecology. Caldwell: The Blackburn Press, 2002. Print.