Conifer chill requirements

[Xenie]

Long time listener, first time caller.

I have been studying quite a bit about the effects of a warming climate on boreal forests. The researchers seem to be suggesting that a number of timer tree species will have spring bud burst issues due to warming winters. The cold induced dormant period will be affected by warming winters. This got me wondering about my collection. This is my first winter with a number of conifers, so the spring will be my first time experiencing bud burst for many of these. I live in San Francisco, where winter lows average about 50 degrees F. I have read that various amounts of “chill hours” at ~ 40 degrees F are required for many conifers to properly bud burst in spring (400+ Hours in most cases). The climate change researchers suggest that bud production and spring burst is negatively affected when the required chilling hours are not achieved. Yet, we have spruces and Doug Firs etc. as street trees (not super plentiful, but quite a few) and arboretum specimens. So, I guess long story short — does anyone have experience growing cold hardy conifers in temperate coastal winters?

 

[Xenie]

Long time listener, first time caller.

I have been studying quite a bit about the effects of a warming climate on boreal forests. The researchers seem to be suggesting that a number of timer tree species will have spring bud burst issues due to warming winters. The cold induced dormant period will be affected by warming winters. This got me wondering about my collection. This is my first winter with a number of conifers, so the spring will be my first time experiencing bud burst for many of these. I live in San Francisco, where winter lows average about 50 degrees F. I have read that various amounts of “chill hours” at ~ 40 degrees F are required for many conifers to properly bud burst in spring (400+ Hours in most cases). The climate change researchers suggest that bud production and spring burst is negatively affected when the required chilling hours are not achieved. Yet, we have spruces and Doug Firs etc. as street trees (not super plentiful, but quite a few) and arboretum specimens. So, I guess long story short — does anyone have experience growing cold hardy conifers in temperate coastal winters?

Timber trees, I mean to say...

 

[Gustavo Martins]

Same here. No chilling hours yet lots of deciduous species. My thinking is that in a natural setting these trees may have a disadvantage and are outcompeted by better adapted species. However, in urban settings and in our pots competition among tree species is likely reduced or absent, which allows them to grow. They may be short-lived too compared to their native environment. Anyways, this is just me trying to find a justification for their presence in places they should not naturally occur.

 

[Wires_Guy_wires]

Chill hours are just part of the dormancy trigger. Some plants use multiple systems to 'remember' the time of the year.
Change in spectrum, decreased daylight hours, lower light levels, higher humidity/water levels all play a part in this. Plants on shorelines wouldn't have gotten old if they continuously miss their ideal spring burst window.

The old trees don't adapt a lot. They might rely on the single-measurement system. But 50 years from now, everything that is still alive will have (had) adaptations and will use multiple point checks instead of single. It's evolution at its finest.

My best guess is that local arboterums and local nurseries would only sell the types of trees that can survive in those situations. Otherwise they'd be out of business by the next year (or double their business because they get to sell everything twice, but that style of running nurseries doesn't last long).

 

[WNC Bonsai]

Recent reserach found that of 173 tree species studied only 35% relied on spring photoperiod as a trigger to break dromancy. Interestingly these were all lower latitude species. The speies from higher latitudes mainly relied on tempratures as the trigger. If you consider this it is logical. In northern areas by the time the day length is long enough to support growth the temps have also warmed. However in lower temperate zone areas due to fluctuating tempertures the trees need to remain dormant until there is adequate daylight and ignore short periods of above normal temps. So having the dual temp-photoperiod is important. This really is dire for northern trees since if they rely only on temp triggers to break dromany they are likely to start coming out too early and get whacked back by late freezes.

 

[WNC Bonsai]

Back to the OP question. There are several chill models in use. The basic one counts all houars <45F equally. A slightly more complex one counts only hours between 32-45F. The Utah and Richardson models ae similar and have several ranges of temps and count either 0.5 or 1.0 hr for each hour in that range:
<34=0 hrs
34-36=0.5 hr
37-48=1.0 hr
49-54=0.5 hr
55-60=0 hr
61-65=-0.5 hr
>65=-1.0 hr

So there is some hope for trees in warmer areas depending on which model best describes the tree species chill physiology. Unfortunately most of the published research is on commerical species like Doig fir and there is a lot on fruit trees. Most of what is known about other species is anecdotal, so we are in a learning phase. Isn’t it great to be living through a global experiment! As the old Chinese curse says “may you live in interesting times”.

 

[Leo in N E Illinois]

@Xenie
You are in the San Francisco Bay area, there are several bonsai clubs, a few commercial vendors and a few serious, well trained bonsai instructors in your area. Hopefully one of the SOB's will post his contact information. That is ''Students of Boon'' (SOB) and Boon is a Japan trained educator in the Bay area. Peter Tea is east, by an hour or two drive east of Sacramento. But there are a number of experienced bonsai growers who can tell you more about which conifers do well in the many microclimates around the San Francisco Bay area.

 

[amatbrewer]

Does anyone know if the mentioned research is done on only 'wild' forest trees, or includes trees in controlled environments?
The reason I ask is that this, like many things, is probably only a contributing factor to the trees success/longevity. In the wild the trees tend to endure more hardship and stress than ones in captivity (yards, parks, etc), and trees like Bonsai tend to receive FAR more favorable care (the abuse we inflict on them such as trimming, wiring, re-potting, etc not withstanding).
I would think that these 'domestic' trees may be able to handle much less favorable temperatures (and/or other conditions) then their wild counterparts, hence the success some have had growing trees far outside their normal conditions. e.g. my wife's ficus which has lived indoors, mostly in the dark, and in the crappiest soil possible for 15+ years?!?!

I think we often get misled by a definitive "[X] will kill a tree" only to have someone show an exception. We sometimes miss that "X" is often just an added stress (maybe a really BIG one) that under the right conditions may NOT push the tree to the point it can't recover. The corollary to that is also true, something that should be fine, under the right (wrong?) conditions can kill a tree.

But I could be totally wrong. It would not be the first time...not even the first time today.

 

[Leo in N E Illinois]

Back to the OP question. There are several chill models in use. The basic one counts all houars <45F equally. A slightly more complex one counts only hours between 32-45F. The Utah and Richardson models ae similar and have several ranges of temps and count either 0.5 or 1.0 hr for each hour in that range:
<34=0 hrs
34-36=0.5 hr
37-48=1.0 hr
49-54=0.5 hr
55-60=0 hr
61-65=-0.5 hr
>65=-1.0 hr

So there is some hope for trees in warmer areas depending on which model best describes the tree species chill physiology. Unfortunately most of the published research is on commercial species like Doug fir and there is a lot on fruit trees. Most of what is known about other species is anecdotal, so we are in a learning phase. Isn’t it great to be living through a global experiment! As the old Chinese curse says “may you live in interesting times”.

An additional factor that has not been well explored, but has to some degree been documented, in Wisconsin at least. In Wisconsin, Pinus strobus will release as volatile hydrocarbons nearly 35% of the carbon that was fixed by photosynthesis each day. This and similar processes are responsible for the blue haze that gives the Blue Ridge Mountains their name. A significant percentage of the hydrocarbons are sesquiterpenes, which include a number of growth regulators. It was discovered some species of broadleaf shrubs and trees put no energy into maintaining their own biological clock to keep track of the seasons. It turns out they simply ''read'' the sesquiterpenes in the air to ''know'' what time of year it is. If you count detecting chemicals in the air as a signal, or ''talking'', the point is trees and the forest community ''talks'' to each other, to the extent that it includes serving as the ''clock'' for the whole community. This is why my spring flowering shrubs and orchids in my under lights set up in my basement still seem to flower like clockwork, same time of year every year. This is because air coming in from outside has the volatile hydrocarbons, the sesquiterpenes that signal where we are on the calendar.

Sadly it is difficult to research this so only a handful of species has this been documented, the rest is extrapolation.

But stretching the extrapolation, this effect could play a role in breaking dormancy for trees in the Bay area.

 

[Xenie]

Thanks for the replies! A different acronym came to mind when I saw SOBs

I guess the idea is that cold hardy conifers with unmet chill requirements produced stunted or otherwise irregular growth in anticipation of a brutal summer. It is easier for them to handle harsh weather when they do not have a huge amount of new growth to maintain. Some trees seem to be more clairvoyant than others. There are many christmas tree farms just south of SF in Halfmoon Bay. I spoke with one of the owners and he said that noble firs etc. do fine, but norway spruce take 13 years to reach harvest-able size. I suppose slow growth could be good for bonsai. My main concern is that the trees will be healthy in the long run, which seems to be the case with some of the stately out-of-town conifers seen in the arboretum.

 

[0soyoung]

An additional factor that has not been well explored, but has to some degree been documented, in Wisconsin at least. In Wisconsin, Pinus strobus will release as volatile hydrocarbons nearly 35% of the carbon that was fixed by photosynthesis each day. This and similar processes are responsible for the blue haze that gives the Blue Ridge Mountains their name. A significant percentage of the hydrocarbons are sesquiterpenes, which include a number of growth regulators. It was discovered some species of broadleaf shrubs and trees put no energy into maintaining their own biological clock to keep track of the seasons. It turns out they simply ''read'' the sesquiterpenes in the air to ''know'' what time of year it is. If you count detecting chemicals in the air as a signal, or ''talking'', the point is trees and the forest community ''talks'' to each other, to the extent that it includes serving as the ''clock'' for the whole community. This is why my spring flowering shrubs and orchids in my under lights set up in my basement still seem to flower like clockwork, same time of year every year. This is because air coming in from outside has the volatile hydrocarbons, the sesquiterpenes that signal where we are on the calendar.

One tree farts and away they all go, like a herd to slaughter (as they say)?

 

[GGB]

wow this is wild, I didn't know any of this. Makes me glad I have a pinus strobus planted in my yard. ALSO I have been telling folks on this site for years that warmth does play a big factor in breaking dormancy. apparently I was "sometimes" right

 

[Shibui]

There are no Pinus strobus within hundreds of miles of me. My spring flowering shrubs and orchids must just have to take a wild guess about when to flower each year?

 

[0soyoung]

There are no Pinus strobus within hundreds of miles of me. My spring flowering shrubs and orchids must just have to take a wild guess about when to flower each year?

Every kind of tree (and likely most all plants) seems to be able to sense the day length. The growth patterns switch following the summer solstice (shortening days). Tropicals sense the winter solstice which trees lacking leaves would have difficulty doing, it would seem. Likely many evergreens do as well. Conceivably, plants/trees with green bark could sense lengthening days.

Throw all this in a tumbler and shake it up. What comes out is complicated.

 

[Anthony]

All the trees on my island are from zone 9 to 7.
Birds brought the seeds and some adapted.
So well was the adaption that many are logging trees,

The Sageretia t is a zone 7 in the ground climbing shrub [ China ]
Grows well here, in pot or the ground.Just don't touch the roots
after October or so.

Never had to test say zone 4 or 5. Hmm?

Anyhow, the Chinese trees use light to bring on dormancy as
our lows are no where near what they are accustommed to.

Are we going to see a huge range of adaptaton ?
Good Day
Anthony

And oh yes we use the refridgerator for winter - 2 months.
Celtis l

 

[Leo in N E Illinois]

There are no Pinus strobus within hundreds of miles of me. My spring flowering shrubs and orchids must just have to take a wild guess about when to flower each year?

Many of species of trees, not just P. strobus have biological clocks. Other species of trees also gas off, or volitalize sesquiterpenes. The trees with the metabolic biological clocks, often are species that are dominant in the local ecosystem. My guess in Australia, certain gums, Eucalypts, have metabolic biological clocks. The species that take advantage, by not spending metabolic energy on biological clocks, tend to be understory trees, or not the dominant species. At least if I remember the few papers I read on it. It was P. strobus that was used to do the full chemical profile of volatile hydrocarbons, but I wager if you explored the research by the Australian Forestry Service, you might find similar papers. Hit your local university's library website and search.

 

[BrianBay9]

I was just looking into this last week and found a study with the graph below. Basically these people found that there in trees that use chill hours as a cue, the fewer chill hours they get, the more warm hours, "forcing units" on this graph, are required to get bud break. So, too few chill hours delays bud break, but mostly they get there eventually. https://www.fs.fed.us/pnw/sciencef/scifi183.pdf

 

[WNC Bonsai]

I was just looking into this last week and found a study with the graph below. Basically these people found that there in trees that use chill hours as a cue, the fewer chill hours they get, the more warm hours, "forcing units" on this graph, are required to get bud break. So, too few chill hours delays bud break, but mostly they get there eventually. https://www.fs.fed.us/pnw/sciencef/scifi183.pdf

That makes sense as it still allows the trees to remain dormant even when theri chill hour requirement has not been fully met.

 

[Gustavo Martins]

I was just looking into this last week and found a study with the graph below. Basically these people found that there in trees that use chill hours as a cue, the fewer chill hours they get, the more warm hours, "forcing units" on this graph, are required to get bud break. So, too few chill hours delays bud break, but mostly they get there eventually. https://www.fs.fed.us/pnw/sciencef/scifi183.pdf

Interesting. I find that over here, at least some deciduous have a very delayed bud break (May) considering my climate zone. Populus or Tilia come to mind when I think about these. But the again. most deciduous here only loose leaves around mid-to-late January... Other species (Acer palmatum, negundo, trident) are not affected and bud break readily.

 

[Xenie]

I was just looking into this last week and found a study with the graph below. Basically these people found that there in trees that use chill hours as a cue, the fewer chill hours they get, the more warm hours, "forcing units" on this graph, are required to get bud break. So, too few chill hours delays bud break, but mostly they get there eventually. https://www.fs.fed.us/pnw/sciencef/scifi183.pdf

I am experiencing this now. Some of the conifers I purchased from Oregon in January are already bursting, while the ones I purchased in October are dormant as can be.