Aspen Bark ColorationExplaining Exposure and Phellum Pigment in the Cork Layer

Oren RabinowitzVegetation Ecology, CU EBIO 4100

Summer 2014

Introduction

- Populus tremuloides (Michx) is a well studied tree in the Rocky Mountains. Scientists documented many aspects of the aspen’s physiological adaptations to sub-alpine conifer forests (Pearson 1958, Schaedle et.al. 1967,Debyle 1985, Perala 1990)- One aspect of the physiology of the aspen tree that remains understudied is the outward appearance of the bark as either greenish, orange or white.- Various hypotheses have been proposed to explain the color variation in the tree bark.

Malpighiales-Salicaceae

-Populus

Prior hypotheses

Coloration in bark is readily observed but has never been properly described in the scientific literature as it pertains to the tree’s evolution.

- Marr (1947) noticed that early frost damaged white trees more than orange trees.- Cottam (1954) observed that orange bark trees were found at higher elevations than white bark ones.- Mitton (1980) suggested that the bark provided a distinguishing feature between male and female ramets, or small sapling suckers.

Preliminary Research

Microscopy in the Tripp lab and CU Herbarium revealed distinct layering of the cork cambium, subtended by the chlorenchyma and the vascular cambium.

Hypothesis and Prediction

H1 = Physical weathering is a factor in determining bark colorationH0 = Physical weathering has no effect on bark coloration Initial tests on cork tissue - White at maturity- Hydrophobic- Easily removed by physical force- Burns quickly and completely

Experimental Design

- The Spot Test: performed on nine total trees from three different stands. Six spots drawn with a pen on the surface at breast height on a randomly selected tree. Microscale physical weathering simulation was used on test spots and color differences were recorded using a color scale. - Initial Color and Density: density readings from canopy mirror and initial color from color scale were measured and recorded

Sample Areas

A= Elk Meadow, Mountain Research Station, CO.

B= Mud Lake Open Space, Nederland, CO.

C= Hesse Trailhead, Eldora, CO.

Kruskal-Wallis H Test

Our Kruskal summary on the ranked data indicates that not all of our group means are the same, and this effect is due to the treatment on color change.

Density Correlation Test

Slight correlation between density and color. This is most likely due to a sampling error of small size. Insignificant p-value is recorded, but we hypothesize that an increase in sample size would give better workable data.

Results

Our statistical analysis reveals that the relationship between treatment type and coloration differed significantly for the spot test. Our initial box plot supports the Kruskal summary (p=.04).Dense stands are protected from weathering and are expected to have whiter bark on appearance. Larger sample will most likely increase our statistical power in this correlation (p=.19).

Discussion and Findings

Fire is the most effective form of physical weathering that reduces accumulation of white bark tissue on cork layer. This phenomenon reveals the orange, immature, non-weatherable bark cells beneath. Water and wind are less effective. Marr and Cottam’s previous hypothesis also support this finding.

There may be a relationship between the density of an aspen stand and initial color, highlighting the effect of dense stand resistance to weathering.

Based on our findings, we can reject the null hypothesis (p=.04) and say that physical weathering has an effect on aspen bark color, by removing layers of mature bark. We fail to reject the null hypothesis that density has an effect on initial color(p=.19), but we assume that there may be a relationship.

Further Research

- The specific color of a tree could be more easily quantified using a field tool, or a universal color index for field biologists. Unfortunately these tools have not been invented yet.-Deeply orange/green stands could be reflective of past fire history of an area, but similar tests on coloration and burn history would need to be conducted.- Aspens may contribute significantly to wildfire fuel load from continual shedding of flammable bark tissue.

Additional Questions

How long does an aspen take to replenish a white layer of outer bark tissue after a burn?Does sunlight, time or both bleach the initially orange bark cells?Does the thickness of the cork cambium affect observable tree color?At what age does smooth, photosynthetic bark become furrowed, inactive bark?Does a lack of weathering hinder photosynthetic capabilities of the chlorenchyma?Is chlorenchyma found in other high elevation deciduous species (Salix)?

Thanks!Tim Kittel, Erin Tripp, Summer Sugg, Amy Concilio, and this guy

CitationsCottam, W.P (1954). Prevernal Leafing of Aspen in Utah Mountains. J. of the Arnold Arboretum. Vol. 35 #3 p 239 - 250.

Debyle, N.V., Winokur, R.P. (1985). Aspen : Ecology and Management in the Western United States. USDA Forest Service General Technical Report RM-119.

Marr, John W (1948). Frost Injury to Aspen in Colorado. Ecological Society of America Proceedings. Vol. 28 #4 p 60.

Mitton, Jeffry B. and Michael C. Grant (1980). Observations on the Ecology and Evolution of Quaking Aspen, Populus Tremuloides, in the Colorado Front Range. American Journal of Botany. Vol. 67, #2, pp 202 -209.

Pearson, L.C. and D.B. Lawrence (1958). Photosynthesis in Aspen Bark. American Journal of Botany. Vol 45, #5 pp 383 - 387.

Perala, Donald A.(1990), Populus tremuloides Michx. Quaking Aspen. Aspen Bibliography. Paper 2878, pp 555 - 569

Schaedle, M. P. Iannaccone, and K.C. Foote (1967). Hill Reaction Capacity of Isolated Quaking Aspen Bark Chloroplasts. Forest Science, Vol. 14, #2 pp. 222-223

Yau, Chi. (2004).Kruskal Wallis Test. R Tutorial. Web.(http://www.r-tutor.com/elementary-statistics/non-parametric-methods/kruskal-wallis-test)