I always find discussions about this misleading, because I look at my trees and see plenty growing while we've been at 95-100 for weeks (34-37c really). Perhaps some is because instead of saying photosynthesis drops off like you did, people say grows stops in high temps, which isn't correct.
Some awfully good questions that are difficult to answer simply, because of the interconnectedness of different variables we balance when keeping a bonsai alive… and at high temperatures.
I’ll try my best, but don’t promise this explanation will be entirely free of miscues. If so, my apologies in advance and I look forward to corrections with clear examples.
Starting with a few basics. (Please bear with me.)
Bonsai aren‘t kept like wild trees. We grow a tree in artificial conditions.
We use artificial conditions to attain our goals and hopefully keep our bonsai healthy at the same time. Watering and fertilizing trees frequently are both artificial conditions, necessary for our trees which are kept in small pots… and our design goals.
During normal conditions this is easy to do. But there are times a careful balancing act is done between attaining our goals for our trees and their health. Styling/pruning events are the usual examples of this.
It is at temperature extremes where hobbyists walk a more difficult tightrope where our trees health can easily go out of balance.
This is due to the tree’s reaction to the extreme conditions. Normally, in extreme heat
Photosynthesis slows down as temperatures increase. A tree is essentially a biochemical engine. When the engine gets hot it doesn’t work well, proteins get misshapen, water/gas exchange is altered, and the organism focuses on ramping up transpiration to cool the tree ‘factory’ itself.….
The tree is shifting more and more of the available water towards the increased transpiration needs to just keep the xylem flow intact so water can evaporate in the needle/leaf cells to keep cool. Here’s where our watering bonsai helps keep the trees living. The tree is essentially acting like a misting station. Water flowing up the pipes (xylem) from its source (available water in the media)… to evaporate out the needle/leaf stomata, taking heat with it and producing a cooling effect.
With the biochemical engine slowing down, not as many nutrients are needed. It’s in a lifesaving mode. Tossing fertilizing into the mix is essentially creating an osmotic imbalance between the media fluid and the cells of the roots. The more water that is pulled out of the media into the roots the greater the imbalance becomes. The more fertilizer added… makes the situation worse.
Note … neglecting the effect of evaporation in the media increasing the fertilizer imbalance and not watering enough just to simplify the explanation.
In other words the fertilizer salts are creating a condition of less percentage water molecules in the solution outside the cells compared to inside the root cells.
As the imbalance becomes too great a couple things happen. The roots cells give up water to the media. The root cells start to desiccate… and the pressure to draw in the water in the xylem cooling system continues to try to pull water up the tree.
So damage to the roots. Worst case air pockets form in the xylem creating cavitation.
In a normal condition eventually things get too hot and the biochemical engine slows to a halt.
Around this time the needle/leaf stomata close due to the heat and/or drought stress normally induced at high temperatures.… a lifesaving measure to prevent cavitation from occurring in the xylem.
I hope this helps answer your questions and wish you all a happy Birthday (America’s that is)
cheers
DSD sends
