Stem Morphology for Quaking Aspen (Populus tremuloides) and Limber Pine (Pinus flexilis) By Alan Rosacker Winter Ecology – Spring 2005 Mountain Research Station – University of Colorado, Boulder

My Question I wanted to look at specific winter adaptations for trees in the high montane environment The two species we will be looking at are Quaking Aspen, also known as Trembling Aspen, and Limber Pine

Overview What are the adaptations that Quaking Aspen and Limber Pine have developed to survive the winter environment? Quaking Aspen –How Does Photosynthetic Bark Help Quaking Aspen? Limber Pine –How Do Highly Flexible Stems Help Limber Pine?

Quaking Aspen ( Populus tremuloides ) The Mechanism For Corticular Photosynthesis: Photosynthetic Bark---what does that mean? –The bark is made up of Photosynthetic organelles Chloroenchymes –Chlorophyll-containing tissues within the stems –Refixation-These chloroenchymes use the limited amount of carbon dioxide given off by the tree during respiration –Some numbers: »The bark of young Aspen contains up to 42% of the total tree chlorophyll- (Kharouk, 1995) »10-15% of total photosynthesis is corticular in mid summer, but larger during times where leaf contributions are limited-winter (Aschen, 2001)

The Refixation of Carbon Dioxide for Aspen Bark and Leaves under Varying Light Conditions (Aschen, 2001) The levels of carbon dioxide given off by the leaf are much higher, but the stem is able to refix more carbon dioxide as the light energy increases The open and closed circles represent leaves and twigs that were exposed to 100% and 20% sunlight respectively

The Refixation of Carbon Dioxide for Aspen Bark at Different Ages (Aschen, 2001) PFD (umol photons m^-2 s^-1) Less Than 1 Year Old1 Year Old As the tree develops, the bark layers become thicker and less light energy can penetrate into the chloroenchymes

Populus Tremuloides The Importance of Corticular Photosynthesis: So we understand the Mechanism, but why is this important? –Important during stressful (winter) times-sugars can be made for the tree –During non-stressful times- adds to overall photosynthesis –Get a jump start when spring comes- much like many evergreen trees –Recycling of internal Carbon Dioxide –Creating a CO2 O2 ratio that may help in defense against phyto-pathogenic fungi (Jensen, 1969)

Overview What are the adaptations that Trembling Aspen and Limber Pine have to survive the winter environment Quaking Aspen –How Does Photosynthetic Bark Help Trembling Aspen? Limber Pine –How Do Highly Flexible Stems Help Limber Pine?

Limber Pine (Pinus flexilis) The Mechanism of flexibility: A study of the Goldenrod Tree (Bosea yervamora) – Wide bands of Conjunctive Tissue Composed of thin-walled collenchyma cells (Carlquist, 2003) The collenchyma cells of plants usually have thickened walls which stiffen the leaves and stems (Freeman, 2002)

Collenchyma Cell Structure The structure of collenchyma cells in stiff stems and branches

Limber Pine (Pinus flexilis) The Importance of Flexibility: Branches bend under the burden of snow –Thin Limber Pine branches can be bent completely back on themselves without strain or cracking – (Wier, 1998) Branches flex under high winds

Summary Adaptations for Winter –Quaking Aspen Mechanism –Chloroenchymes –Refixation of carbon dioxide Function –Jump start on photosynthesis –Limber Pine Mechanism –Wide bands of thin-walled Collenchyma cells Function –Flexibility under heavy snow

Reference: Aschan, G; Wittman, C and Pfanz, H. (2001): Age-Dependent Bark Photosynthesis of aspen twigs.-Trees 15: Aschan, G; Wittman, C and Pfanz, H. (2002): Ecology and ecophysiology of tree stems: corticular and wood photosythesis. Naturwissenschaft 89: Aschan, G; Wittman, C and Pfanz, H. (2001): Leaf and twig photosynthesis of young beech and aspen tress grown under different light regime. Basic Applied Ecology 2; Stuart Wier. Accessed 2/26/ Stuart Wier. Accessed 2/26/2005 Carlquist, Sherwin. (2003): Wood and stem anatomy of woody Amaranthaceae: ecology, systematics and the problems of defining rays in dicotyledons. Botanical Journal of the Linnean Society 143: Accessed 03/05/2005http:// Freeman, Scott. (2002): Biological Science. Prentice Hall Jensen, KF. (1969). Measuring oxygen and carbon dioxide in read oak trees. US Forest Service research note NE-74.