Posted by Gus on 12/5/2008, 10:57 pm, in reply to "Re: Results of recent record freezes" Link: Eucalyptologics: Info resources on eucalypt cultivation worldwide
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Hello Treeman and friends 
First, thanks a lot for recording the data, and for sharing your experience
There would be many comments to make, but there is little time lately, so I will focus on E. nitens because we have had the opportunity to study quite well its reaction to several "nice" (because of how it happened) frost events in our trials here.
The absolute minimum was some 17ºF, and the sampled population was some 20,000 trees, some 3 year old and planted in a frost hollow around here.
First comment, even if the temperature was not yet in the range of the mentioned ones as the limit for the species, frost damage soon appeared. This is normal, because not every plant is equal to the one growing besides it. There is genetic variability. And even in homogeneous sites as this one, there is variability for some site parameters too.
So we classified the sampled trees in 4 different levels of damage, according to the % of live crown that was damaged. And now I comment on the two extremes:
1) The cold hardy extreme: +80% of the trees had nil damage, or it happened on less than 25% of live crown. +95% of these recovered totally and have put near 2 metres additional growth as average this year.
2) The non cold hardy extreme: <3% of the trees had 100% live crown damage, most of them the less vigorous, so shorter, so more prone to radiational freeze damage (the damaged tissue is "too close to the freezing soil"). Of these, roughly a 50% died and a 50% of them survived and later recovered, but after the loss of most of the aerial part (so they are sprouting from the trunk, near the base, which ruins them as practical trees for timber crop).
So, a couple useful comments, some of which I have made before:
1) Cold hardiness is different for fragile seedlings at nursery or planted out than for larger planted trees, as Howard has well mentioned for his experiences in France. For at least three reasons: a) plant tissue for seedlings is softer, loaded of water and less sturdy than for saplings, and more in chilly winds; b) plants at nursery (or small seedlings planted too late in the season) are raised near the soil or over the soil, which means there is higher risk of root damage either by substrate getting frozen or by the whole plant being in the most immediate layer of air near the soil, which is the one radiating very low temperatures in radiational frost events; and c) because the first can be buried by snow and the later rarely do (which has implications, either for insulation protecting from chillier winds or frost, or for worse damage due to wet snow water freezing while melting).
2) Even for a cold hardy species (whatever works better in each area), there will be variability for frost damage. And it will appear in some plants even when temperatures do not yet reach the "absolute minimum" quoted by literature based in other experiences. For the case we described of E. nitens, if the absolute minimum of 17ºC reached by the studied trees was even lower, so approaching the "theoretical limit", then the % of damaged trees would increase. Even for larger trees. Upon a temperature, say, in which a critical number of trees was damaged (in the most extreme case, 19,999 damaged and 1 undamaged), it could yet mean a +80% of survival later in the year. But if a significant number of these damaged and recovered trees have no use later for timber (or whatever desired use), then, regardless of where the true "limits" are, that temperature is already a "No no". So we could easily say "it is true this species can take a temperature of N degrees and survive", and be truthful. But we should also know that "this species is better grown where temperatures dont go lower than this Y range if you want to make a viable crop of it". And it would be truthful also.
3) Experiences at nursery stage like the one of Treeman are useful from many perspectives. One of the most obvious is that for a given plant lot at nursery in as homogeneous conditions as possible, even if heavy damage appears and kills many plants, if any survives unscathed or almost unscathed, then the "natural culling" yields candidate specimens with higher frost tolerance. And those are valuable plants from the point of view of tree domestication and breeding. But they should be tested later in the field.
4) Experiences at nursery are a good indicator of cold hardiness for eucalypts in a given area. But we must remember it is just an indicator. A fully equiped nursery designed for it and a wise scheduling could make those same toasted plant lots survive in your location. And maybe, some of the not so good performers in your conditions now could "pass the test" at field planting stage, providing suitable conditions during the first winter outdoors. The disadvantage would be that no culling took place, so there is no selection effort in the earliest stage (nursery), and this must take place later (field). Which in any of the cases mean plant losses, either for the nurseryman, or for the tree planters.
5) There is no need to grow 20,000 planted trees to find out if they are hardy or not. That would be nonsense. It can be done with fewer. But it is important to remember that a minimum statistical sample is needed, if some conclusion of general use is to be produced from the experience. Results from testing 1 (or 2) plants in a given location are still useful and valuable, and the effort of every grower is to be praised. But it is not enough to give really sound conclusions. It is however, another good initial indicator.
All this said, I am sorry for not providing you with pictures of this all at the moment, but I cannot do that until the study is finished and the results published. Just know they exist!
And to end, just saying that any experience done by a grower that helps the grower to refine the methods for plant production, or to sort out which types of them are better for their local conditions, is positive. So well done to all of you trying these plants. Remember to see the glass half full instead of half empty.
Gus
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