[FLPERMACULTURE] Interesting tree tidbit

[Jon Bosak]

| what the link basically says is trees maintain an average internal
| temperature of their leaves of about 70 deg. F regardless of
| climate.

Not exactly (though it's been widely reported this way).  While
the researchers did find that "tree leaves were internally cooler
than surrounding air temperatures in warm climes, and warmer in
cool climes," they did not find that trees kept their internal
temperature at 70 degrees.  What they did find, which is more
subtle and in a way more interesting, is that trees did virtually
all their growing at about 70 degrees.  So while it's true that
most of a leaf comes into being at about 70 degrees, and does most
of its photosynthesis at about 70 degrees, that's not the same as
saying that a leaf surviving in air at 40 degrees has an internal
temperature of 70 degrees.  According to the study, it would be
more like 47 to 49 degrees ("seven to nine degrees F higher than
the cool, ambient air in the Swiss Alps.")

Not to say that this isn't a mighty interesting finding, but it's
not quite what's being reported.  There's a somewhat more careful
summary at

    http://www.sciencedaily.com/releases/2008/06/080611135100.htm

which notes that

    Perhaps the more remarkable finding was that across such a
    large area and across so many types of tree that the leaves
    seemed to be operating at the same temperature [...]

It's very easy to jump from the statement that the leaves are
"operating at the same temperature" to thinking that they are
spending most of their time at the same temperature, but that's
not what the study found at all.  What it found was that 70
degrees was the temperature at which they do most of their
operating -- and that they have a very surprising but nonetheless
limited ability to move in the direction of that temperature when
the air is cooler or warmer than the optimum.

I didn't figure this out myself; someone interviewed on NPR noted
it.

Jon

Micheal Wheeler wrote:
> what the link basically says is trees maintain an average internal 
> temperature of their leaves of about 70 deg. F regardless of climate.  
> 
> http://dsc.discovery.com/news/2008/06/11/tree-leaf-temperature.html
> 
>  >They compared current observed records of humidity and temperature 
> against the isotope ratios in the trees, and found that tree leaves were 
> internally cooler than surrounding air temperatures in warm climes, and 
> warmer in cool climes.
>>Tree leaves keep cool through constant evaporation and reducing sun 
> exposure through leaf angles or reflective qualities. Warmth is gained 
> by decreasing evaporation and increasing the number of leaves per branch.
>  >The fact that part of this adaptation occurs at the level of entire 
> forest canopies, and not just within individual leaves, is one reason 
> direct measurements of tree temperatures have been so hard.
>>Measured across an entire growing season, the forest canopy temperatures 
> were found to be seven to nine degrees F higher than the cool, ambient 
> air in the Swiss Alps.
> 
> -- 
> "True affluence is not needing anything." - Gary Snyder
> 
> 
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