|Fig. 1. Swedish Columnar Aspen (Populus tremula 'Erecta'). Back entrance of General Services Building, University of Alberta.|
But before I continue on what puzzles me in this picture, let me quickly introduce you to this species. These trees are known as Swedish Columnar Aspen or scientifically Populus tremula 'Erecta'. Populus tremula is Latin and stands for European aspen. The term in single quotes indicates that these individuals belong to a cultivar named 'Erecta' given its columnar statue. 'Cultivar' is a blending of the words 'cultivated' and 'variety' and represents a product of selective breeding to highlight certain features such as the columnar growth form. Once the desired traits are present, they are preserved by consecutively propagating these individuals asexually, for example using cuttings from stems or roots. Asexual propagation results in genetically identical individuals, which are also known as clones.
Due to their columnar statue they make a great choice for small and narrow patches along walls or other space-limited places in urban areas. The cultivar 'Erecta' consists of only male clones to avoid the massive amounts of downy fluff produced by the females each year during summer. The fluff contains the seeds and ensures seed dispersal by wind. It is often heard that people react "allergic" to the fluff, which, however, is incorrect since true allergic reactions are due to the male pollen shed in early spring.
Coming back to the photo now, what's odd in this picture? How come that there seems to be a gradient in autumn leaf colouration, i.e. trees to the left are much greener than trees to the right. It also seems that there is a gradient in size although this could be an effect of the slight slope these trees are planted on. My first guess was that the smaller trees to the right are exposed to much more wind and colder temperatures, while the trees to the left are somewhat sheltered from this effect and experiencing their own little microclimate. Furthermore, the big spruce tree to the right might dominate the world below the ground by claiming the majority of the available water for itself. All this could result in an earlier initiation of leaf colouration, or leaf senescence, and less growth for the aspen trees closest to the corner of the building. What are your thoughts? Feel free to comment below!
But why do trees shed their leaves anyway? The answer is to escape unfavourable growing conditions. Such conditions can be experienced within the growing season itself (drought, nutrient deficiency, etc.), or represent the well-known pattern of annual recurring leaf coloration and leaf abscission in autumn to avoid frost damage in winter.
|Fig. 3. Mid-winter run in Edmonton. Author very frost hardy.|
But how do trees exit dormancy so that they can produce new leaves in spring? This process is primarily driven by temperature. After trees experienced a required amount of chilling in the winter, which is needed to release dormancy, they start "counting" the number of days above a certain temperature threshold until a needed heat sum is acquired, which then will initiate bud break. Required heat sums are species-specific and under strong genetic control, i.e. trees that were moved across latitudes will still require the same amount of heat sum to which they were adapted to irrespective of their present location. This can have dramatic consequences if a species breaks bud too early in an environment which experiences late spring frosts. We can determine the required heat sum for each species by adding the average daily temperature for days above a defined temperature threshold (e.g. 0°C) starting from a defined point in time (e.g. January 1st) and observing when bud break actually occurs on the studied tree – et voilà! Once the conditions are right (i.e. warm and moist), bud break and the emergence of leaves can progress very fast (see images below).
|Fig. 4a. Timing of bud break at a hybrid poplar plantation by Boyle, Alberta, Canada. May 13, 2011.|
|Fig. 4b. Timing of bud break at a hybrid poplar plantation by Boyle, Alberta, Canada. May 15, 2011.|
|Fig. 4c. Timing of bud break at a hybrid poplar plantation by Boyle, Alberta, Canada. May 17, 2011.|
Images: All pictures taken by Stefan Schreiber.
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