From: rim@csadfa.cs.adfa.oz.au (Bob McKay)
Subject: Re: List of cold climate gums
Organization: Australian Defence Force Academy, Canberra, Australia
Date: Thu, 19 Nov 1992 07:01:01 GMT

arkdr@uniwa.uwa.edu.au (Dave Rindos) writes:

>In this discusssion, I think  we are acting as if the minimum recorded
>temperature on a thermometer were the critical factor in cold hardiness.
>It strikes me that this leaves out a much more important factor: the 
>duration of the cold temperature itself.  

This is certainly correct, and I agree we probably haven't been careful
enough to distinguish the two, but I don't actually think it changes the
conclusions all that much.

>In much of the the US (continental climate and all that), the winters
>are not only cold, but consistently cold.  Day and night temperatures
>stay low with little fluctuation.  Taking Canberra, in contrast, the day
>temperatures -- even in winter -- tend to be fairly high.  Hence, the
>minimum recorded temperature is at that low level for only a relatively
>short period of time.  Hence, plant tissues (say in the trunk of  a
>tree) will NOT have a chance to go as low as air temperatures.  In most
>of the US, in strong contrast, the minima are maintained for a long
>enough time to effect the plants the same way  a much lower temperature
>for a shorter period of time would.  Or at least this seems probable. 

Again, this is undoubtedly correct. Since it refers specifically to 
Canberra, US readers are probably going to treat it as a refutation of my
earlier comments, so I better explain how I think it affects them.
Where I thought the guide was over optimistic, Dave's argument strengthens
the point: if a species can't take that temperature as an overnight minimum,
it's not going to like it for weeks on end. So it really comes down to the
two species - pauciflora and stellulata - where I thought the guide was
underestimating their cold tolerance. In both cases, I'm talking about their
behaviour in their wild habitat: high (for Oz) mountain areas, with snow
cover for some months of the year, and with little more Winter diurnal 
temperature variation than you get in the continental States (probably more
Winter sun, but that's an issue I've already covered).

>It would seem to me that using PLANT SPECIES as indicators of climate is
>much more reasonable than using thermometers.  For example, is a
>particular gum as cold hardy as a Lemon, or an Orange, or a Mango?  For
>a gum to grown in the colder parts of the US it would have to be *much*
>hardier than a Lemon.  Indeed, I find the average ozzie astonished that
>citrus can be grown in only very very limited regions in the US (e.g.
>you can't even grow lemons in most of the south-eastern states, save the
>more southern parts of Florida!); Lemons do just fine even in "cool"
>places such as Melbourne.

A very nice point, and in general, plant species are going to be a better
indicator. For this particular issue, though, I can't use it - there are only
a few people living in the areas we're talking about, and I don't
know what they can or can't grow there - I doubt if there's enough folk 
knowledge in those areas that anyone knows, actually. Even if I did know, I
couldn't be sure if it was limited by cold or something else (eg there are
wild apples in the areas that pauciflora and stellulata grow, but they seem to
peter out before those two species; should I conclude that they are cold hardier
than apples? I doubt it, I suspect some other factor, perhaps Summer 
dessication, is limiting the apples higher up). On the other hand, to take 
Dave's point, Canberra is right on the limit of the ability to grow citrus: 
there is one particularly cold-tolerant variety of lemon (Meyer) that is 
reasonably reliable (if you cover it for the first few Winters). You can't 
grow other citrus in normal circumstances. But then again, citrus are 
fairly thin barked, and so are perhaps more susceptible to short cold events 
than are gums - it's all too complicated!!!!!
-- 
Bob McKay		   	Phone:	+61 6 268 8169	    fax: +61 6 268 8581
Dept. Computer Science		Internet:	rim@csadfa.cs.adfa.oz.au
ADFA, Northcott Dve 		UUCP:...!uunet!munnari!csadfa.cs.adfa.oz.au!rim
Campbell ACT 2600 AUSTRALIA	ARPA:	rim%csadfa.cs.adfa.oz.au@uunet.uu.net
Newsgroups: rec.gardens