A response from Jeff Severinghaus on why the trees don't make good thermometers after 1950 – "I did indeed feel at the time that Mike Mann had not given me a straight answer. "

Plants also have respiration and also release CO2 in addition to taking it up.
But it is all overblown anyway. If you have a fully mature forest that is no longer putting on net biomas, it is a net zero for carbon sequestration. “Old growth” forests provide no net reduction in atmospheric CO2. For every young tree that is growing putting on carbon, one is decaying releasing carbon. For every new branch sprouted, one is broken off from wind. At some point at full maturity, the forest stops adding net carbon biomass.
The best way to reduce CO2 is to plant fast growing trees, cut them down for paper, sequester the paper in a landfill. Paper recycling actually increases the rate of atmospheric CO2 increase by reducing the rate of tree planting for paper.
The second best way to reduce CO2 growth is through “sustainable” logging and forest management techniques that allow selective logging of the oldest trees in a mixed age forest and forest floor fuel management by regular controlled burning of the underbrush to prevent fuel load buildup resulting in extremely destructive, very hot fires.

Never seen serious study comparing daily minimums and maximums and attributing night warming to GHE. Of course, night warming is also prone to UHI.

I have seen experimental results only on conifer species. In those species, elevated CO2 does two things: 1) it accelerates growth and 2) it GREATLY increases the number of viable seeds produced. The growth increase is 2 to 4 times depending on the species. Seed production can be as much as 10x increased. That actually makes sense in other contexts. Gymnosperm species were dominant when CO2 levels were highest. Angiosperm species take over as CO2 levels drop. They (the broad leaf species) also tend to conserve their CO2. In the fall they drop their leaves. In the following year, these decompose releasing CO2 that the forest takes up again. The conifer species drop needles but these tend to decompose only very slowly often taking many years due to the resins they contain acting as a preservative. Those resins tend to act to poison the ground to make it more difficult for other species to invade. But as CO2 levels drop, the conifers produce fewer seeds. CO2 doesn’t seem to have quite as dramatic impact on seed production of broad leaf species.
One thing I might look for if atmospheric CO2 is increasing is to look at areas such as Northern California where you might have a margin of Douglass fir meeting a mixed oak/madrone forest. I would expect to see the fir trees starting to expand their range and start moving into the areas that had been mostly oak/madrone. And in fact I do see just such an occurrence though it might just be local to that specific area. I have a friend with about 500 acres in Mendocino County, California. On this property, over the past 5 years, the Douglass fir seedlings have begin encroaching into the oak and madrone area. My friend regards the firs as weeds and they seem to be a bit vexatious in their spread into the hardwood tree area. It seems to be almost a reverse forest succession with a conifer species starting to crowd out hardwoods.
I would also be interested in any changes recently in the populations of Araucaria globally (“Monkey Puzzle” trees, Norfolk Island pine, Bunya-bunya, etc.) These trees are a VERY old line which evolved when CO2 levels were much higher than today. I wonder if they are doing better now that CO2 levels are higher. Are wild Norfolk Island pines suddenly increasing in number?

John says
The other thing is that CO2 is a fertilizer. The Idsos and many other researchers have shown time and again that plants grow more quickly with more CO2. Tropical trees and temperate trees add more carbon lately, in our now higher CO2 environment, we’ve seen in a number of studies. So if the trees add more carbon, why would tree rings show less growth?
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Trees photosynthesise during the day and respire both day and night. So tree growth is the net effect of photosynthesis (CO2 absorption) and respiration (CO2 expiration). Respiration increases with temperature. Photosynthesis is affected by temperature and the amount of CO2 in the air. So if nights are warm respiration may partially cancel out the increased daytime photosynthesis.

Well there may be some link somewhere between some tree species and local temperatures, but when we look at 150+ years of temperature records in northern Russia, the tree ring records really do not correlate at all, whereas the thermometer records correlate well with each other.
Treering records should not be smoothed. The correspondences between years (the “lacework details” of the thermometer records are just as important as the longer-term correspondences, to determine usefulness as proxies.
See my work on all this.

CO2 can only cause indirect warming at night. Surface land and ocean temps cool at night via radiation and at different rates. In dry atmospheric conditions, that cooling can be quite impressive. Cooling is tempered by cloud cover (IE an abundance of water vapor). If diurnal temps have increased in this case, the AGW hypothesis would be of course that night time air must be anomalously humid to cause this warming, and that this additional cloud producing humidity must be caused by the portion of CO2 that is AGW CO2.
I would ask the doctor for observational based (not modeled) references before this can be accepted. There are very powerful natural long and short term cloud producing mechanisms that readily temper night time cooling. That we can place this tempered cooling related to these trees on the back of such a tiny increase in CO2 is quite a stretch.
There is an old saying in medical circles: You must rule out the first encountered pathology. And the first encountered pathology must be short and long term natural weather and climate variation.

Severinghaus says that warmer nights mean that trees respire more CO2, hence have less carbon to add to tree rings? That’s very interesting, but I’m not so sure he’s right. It has less to do with CO2 and more to do with the way plants function at night under different day/night temperature conditions.
I’m not a botanist, but I am an avid gardener. Ambient CO2 levels naturally go UP at night because the plants aren’t taking it in. But respiration doesn’t depend on sunlight and occurs not just at night but during the day, too. They not only produce CO2 but release energy and consume O2. On balance, though, the plants take in far more CO2 than they respire.
Now, many species of flowering plants (I know, we’re talking about trees, but I can’t imagine the processes would be any different) grow well ONLY when temperatures during the part of the diurnal cycle that normally comes at night are lower than temperatures during the day. Yes, plants and trees are diurnal and certain functions and processes are more efficient at certain times. For example, cell division takes places just before dawn.
In other words, some species only do well when there is a big enough difference between daytime and nighttime temperatures. If the nighttime minimum temperatures are too warm, it affects plant growth by not exerting enough pressure on the root systems, which affects calcium distribution (and other compounds) to the leaves, buds, and flowers by slowing it down. Too-warm nighttime temperatures also rapidly speeds up respiration, which causes the plant to burn through the assimilates (sugars) it created during the day via photosynthesis. (Cooler temps at night also promotes sugar transport.) When that happens, plant growth is severely limited.
So I think Severinghaus is half-right. He’s in the right church, but the wrong pew. CO2 level has nothing whatever to do with it.

My current best guess is that the higher CO2 since then has caused greater
warming at night (which is corroborated by minimum temperature trends,
since minimum temperatures usually occur at night).
I thought the increased minimums have mostly occurred in Winter. In cities.
Trees respire more at higher temperature, so they lose carbon when nights are warmer
than average. So their ring width has not increased as much as it would
have if the warming had been uniformly distributed over the diurnal cycle.
I’ve found info indicating that roots grow at night at a rate greater than the average, according to an official summary of this question I’d have to go find again. This should help a tree grow rings. And do the rings grow at night using the same respiratory mechanism, given energy availability? Otherwise, it appears that no one knows enough about roots in general.
If he’s talking about the increase in respiratory energy lost at night with warmer temps, then another question is, even if it is even significantly “lost”, can it be effectively compensated for, if needed, during the day by photosynthesis. Trees seem to be very specialized and adaptable at the same time.
Too many variables, especially with wild trees, where no one knows what has gone on both below and above ground throughout each of their histories. That’s why they’re called “wild”.

Tilo Reber says:
November 29, 2011 at 7:52 am
As many have pointed out, including Tilo, there has been no empirical research that can tell us why tree ring width declined. What The Team have offered and offers is nothing but hunches. Until there is empirical research on at least one variety of tree that can tell us about the changes in growth rates for tree rings and the environmental changes that cause them, tree ring data cannot be used as a proxy for temperature. This applies to all tree ring data extending into the past. That tree ring data has not been supported through empirical research.
The point should be extended to all paleo data. Paleo data is used today only because it has been used in the past. But none of it, present or past, has been subjected to serious empirical research that would provide some support for the data and, more important, some scientific basis for distinguishing good paleo data from bad. By contrast, archeologists can use radiocarbon dating to assign dates to historic and pre-historic artifacts with reasonable confidence because there is a well-confirmed science that explains techniques of radiocarbon dating. There is no such thing for tree rings as proxies for temperature. For that reason, all claims based on tree ring proxies must be treated as hunches and not genuine science.
This message came to The Team more than a decade ago from prominent scientists, such as Professor Daly, who warned them that they must adopt scientific method in the practice of paleoclimatology. I am sad to say that I have come to believe that the members of The Team truly do not understand the message. They truly do not understand that their claims about tree rings as proxies for temperature are not empirical claims at all. These studies must be rejected because there is no empirical support for them. All articles by The Team that rely on tree ring width data, past and present articles, must be withdrawn.

“Gail Combs says:
November 29, 2011 at 10:07 am”
Dittos. http://gustofhotair.blogspot.com/2009/04/analysis-of-australian-temperature-part_16.html
I got to Lowe’s site last night while looking for night time minimums. But it’s only Australia, dontcha know, so it doesn’t “fit”.