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>> Has anybody considered that? <<
Yes. But when the AGW crowd is cherry-picking trees for temperature spikes, you won't get THEM to consider it.
Great news Larry, 'Tree World'.
Wonderful to see real science in operation. The WGAW's will just have to face the fact that Nature has been at the game of climate adaptation for a few billion years or so. If us mere humans think that we can hoodwink or second guess it, we are on a 'hiding for nothing'. Trees have been at the game for long enough to get it right, 'they' are forced to bend to the eternal Laws of Physics.
What is illuminating about this study of photosynthesis in trees, is the equilibrium temperature which is maintained by the Process: it confirms my own theories which started to take shape in the early 60's. I was, at the time, engaged in a study of methane digesters for biogas production (on a chicken farm), very smelly! The key to the economics of the operation was to determine the energy input required during the cold season, and the cooling energy output during the warm season, as anaerobic bacterial activity must be held between 35 and 39 C, --- 37 C being the optimum. As this is identical to the the body core temperature maintained by humans, and we are just a mobile stomach with anaerobes in a digestive gut, I considered that I had learned something important about how, and why, nature optimises chemical processes at temperatures to suit the prevailing environment.
Now cycle 20 years on to the early 80's. To further my combustion research, I was then studying plant physiology in order to understand the conversion of Carbon Dioxide to Carbo-Hydrates and Hydro-Carbons at very low temp. and atmos. pressure. (At the same time I was making cross comparisons with the well known Steam Re-forming process, i.e. Hydrogenation of Syn-Gas (CO) and CO2, which operates at very high temp. and pressure). In plants, the Exothermic side of the process is initiated by the magic of photosynthesis, but the Endothermic (hydrogenation) side is controlled by a plant's access to water. The optimum temperature of the process then, is determined not by photosynthesis, but by the bio-chemistry of plant life, which works best at around 21 C, (very close to the ideal for our external surfaces)?
Thus I can confirm but from a very different direction, that Trees, like all our plant life, are no more dependent on the atmospheric temperature ambient for their growth rate than we humans are. They, like us, are wholly dependent on Water, both for life's essential chemical processes and its thermal equilibrium, (heat exchange via evaporative cooling). --- Consequently, from a scientific point of view, 'Tree Rings' as a proxy for climate temperature have as much relevance as 'Tea Leaf' patterns in a tea cup!
Trust the above helps to dispel the AGW myths, but really, if you want to know what controls the growth rate of plants? --- Just ask any farmer.