When Trees Don’t Know They’re Dead.

Last week a neighbor of mine called me up to ask how likely it was that their 4 year old (or so) crab apple tree was dead.  Sometime over the course of the winter cute fuzzy bunnies had decided that the tree’s bark was tasty and decided to eat it.  Naturally they ate it all the way around the circumference of the tree with the exception of a strip about an inch wide.  At this point you’re probably asking yourself why the neighbors suspected the tree might be alive.  The reason they were calling me was that the tree was leafing out–  so they figured that maybe the tree would make it — that maybe, just maybe, it wasn’t as bad as it looked.  My answer — Sorry, the tree is dead, it just doesn’t know it yet.  As a rule of thumb you can have up to a third of the circumference of a young tree girdled and the tree has a decent chance of growing out of it.  More than that and, though the tree might live for a few years, you’re dealing with so much damage to the vascular tissue that you’re just putting off the inevitable by not cutting it down.  A tree with as much damage as my neighbors tree had was just going through the motions.

When bark is eaten what is destroyed is the phloem — the tissue which carries the carbohydrates made by the leaves down the plant’s stem.  The cambium — which creates new phloem and xylem — is also destroyed.  But the xylem — the innermost tissue which transports water and nutrients up the stem — is left largely intact.  So girdled trees will flush out in the spring (using resources provided by the xylem), perhaps even two springs, but ultimately the tree will succumb.

But there is an up-side!  Girdled trees will be under a lot of stress.  Stressed trees tend to flower heavily — so enjoy the show first, then cut down the tree.

Planting trouble: multiple trees in one hole

[I enjoyed Jeff’s Valentine story so much that I thought I’d stick to the theme of togetherness…for better or worse.]

A week or so ago a reader asked about the practice of planting three or four fruit trees in the same hole.  Having not heard of this before, I checked on the web and found many “how to” pages geared to home gardeners who either want a longer harvest of a particular fruit (early to late) or a mixture of different species.  Doesn’t it sound just great, especially for smaller urban yards?

One of these sites has these written instructions: “Plant each grouping of 3 or 4 trees in one hole at least 12 to 15 inches apart.”

Now, I’m sorry, but this is just asking for trouble down the road.  Readers of this blog know that root systems extend far past the drip line, and that roots from different trees are going to compete with one another.  You’ll end up with three unhappy trees, all jostling for space and resources, just like kids in the back seat during those long car rides.

But wait! you might say.  There’s research on high density tree planting, and it’s been shown to increase fruit yield on a per acre basis!

Yes, in fact there is a lot of planting density research on many different species of fruit trees.  What’s considered by researchers to be “high density” varies, but it rarely exceeds 2698 trees/acre (6666/ha for our international readers).  Optimal and sustainable levels of high density planting are also variable, as they depend on not only species but rootstock and the crown architecture; 1214/acre (3000/ha) might be a mid-range number.  This can be converted to a per-tree requirement of 36 sq. ft. or a 6’x6’ planting area.

How does this compare to the 12-15” recommendation given earlier?  If we’re generous and use the 15” recommendation, this translates to 6.25 sq. ft. per tree or 6970 trees/acre.  The 12” recommendation would lead to a whopping equivalent of 10,890 trees/acre.  (And no, it doesn’t matter if you’re using dwarfing rootstock or not; most of the higher densities in the literature are for dwarfing rootstocks.)

You don’t have to be a math whiz to see that these densities are totally out of line with reality.  Sure, you can probably keep overcrowded trees alive with lots of water and fertilizer, but they’ll be under enough chronic stress so that pests and disease might take hold, and fruit production will likely be poor.  And it’s about as far from a sustainable practice as you can get.

Good to the last drop

As part of our discussion of the relative merits of fall planting, Linda mentioned an article in Arboriculture and Urban Forestry that suggests that frequent, light irrigation might be better for landscape trees then the usual recommendation of infrequent soakings.  While I will withhold final judgment until I see the article (I did a scan of the last two year’s table of contents for A&UF but missed the article in question), here’s my rational for following the standard recommendation.


First, the context.  In discussing landscape tree irrigation I am talking about watering trees during establishment, typically during the first year after planting and maybe the second if the tree is lucky.  The goal of watering in this case is ensuring survival.  The questions are whether deep soakings are more likely to encourage deeper rooting where water availability is less variable than near the surface after irrigation ceases and whether infrequent watering increases drought tolerance over more frequent irrigation.


Roots follow resources
As my Woody Plant Phys students quickly learn, we avoid the teleological ‘roots seek out water’; nevertheless, roots do proliferate where resources are available.  A couple of illustrations.  As a Tree Physiology Project Leader with International Paper I supervised a 25 acre hardwood fertigation trial.   Trees were watered daily via drip irrigation system with emitters spaced every 3’ down a row.  As part of the study we did periodic root harvests.  My technicians quickly learned it was an easy job: just look for the drip emitters – every three feet there was a mop of roots right next to the drippers.  The notion of roots following resources is also widely reported in the ecology literature on tree utilization of ‘patchy resources’ (e.g. Gloser et al. 2008 Tree Phys 28:37-44 ).  Other factors being equal deeper watering should result in deeper rooting.


Trees habituate to frequent irrigation
Another short rotation forestry example.  In eastern Washington and Oregon forestry companies Potlatch and Boise Cascade operated intensively managed ‘fiber farms’ which grew 70’ tall, 7” diameter hybrid poplars on a 7 year rotation.  To maintain these growth rates, trees were irrigated daily.  But there was a downside: If one day’s irrigation was missed the leaders to the trees would start to wilt.  Three days without water would result in leaf drop. The daily irrigation was great for growth but it turned the trees into physiological wusses.


Periodic water stress improves drought tolerance and survival
A common adaptation for trees to tolerate drought is osmotic adjustment, which is an active accumulation of solutes that enables plant cells to maintain turgor pressure during dehydration.  Plants that have acclimated to stress via osmotic adjustments and other physiological adjustments are able to survive better during prolonged drought than plants that have not been pre-conditioned.  For example ponderosa pine seedlings that had been subjected to brief drought events survived a terminal dry-down two weeks longer than seedlings that had been watered 3 times a week before the final dry-down (Cregg 1994 Tree Phys. 14:883-898.


What would it take to change my mind?
Obviously some of my examples here are anecdotal (though there’s plenty of hard data on osmotic adjustment and other drought conditioning effects on trees).  To recommend frequent (2 or 3 times a week), shallow irrigation I would need to see: a well designed and executed experiment that compared frequent irrigation to periodic (once every 7-10 days) applying the same amount of water weekly (0.5 to 1” per week) for the first year and then documented improved survival of the trees after irrigation had been discontinued.  I’m not saying it’s not possible but it goes against my personal observations with irrigated trees in a variety of settings and relevant data with which I’m familiar.

Surviving the desert with beauty and efficiency

I’m away this week for an out-of-state seminar and a little annual leave.   Some of my favorite places to visit this time of year are the high deserts of California.  Today we hiked to Horse Thief Creek, a relatively easy trail in the Santa Rosa Wilderness.  It’s the perfect time of year to see the high desert in bloom, especially with last winter’s substantial rainfall.

In graduate school I became interested in environmental stress physiology, and I still am entranced by the plant kingdom’s ability to overcome nearly every environmental extreme on earth.  Desert ecosystems are particularly harsh, as rainfall is limited to a short period of time, often in the winter or spring.  While many perennials are able to tolerate the subsequent dry season, annual species cannot.  In essence, they escape drought stress altogether by existing only in seed form for most of the year.  Seeds contain relatively little water anyway, and are so protected against environmental extremes that they can remain viable for decades or even centuries.

But back to our desert.  After the rainy season, seeds of annual plants go into overdrive, germinating, growing, setting seed, and dying back all the span of a few weeks.  Thus, the lucky hiker can find an abundance of tiny, brilliant desert annuals when seasons and vacation schedules coincide.

Tomorrow it’s the Salton Sea.  Not sure what we’ll see in terms of plant life, but we’re hoping to catch some of the migratory waterfowl on their journey north.





Better Red than Dead!!!

David, one of our newer readers, asked why his red-stemmed roses seem to be more cold hardy than the green-stemmed cultivars.  So today’s blog will be dedicated to a brief discussion of why it’s better to be red than dead.

The brilliant red, blue, and purple colors seen in flowers and fruits are due to anthocyanins (and the closely related betacyanins).  These water-soluble, non-photosynthetic pigments are also commonly found in stems, leaves and other vegetative tissues.  In 1999 I wrote a review article exploring the reasons that leaves and stems might turn red.  A few years later I wrote another review, more specifically looking at how anthocyanins might influence plant water relations.  (This last phrase is plant physiology-geek jargon, and I have to admit that the class I took on this topic during my PhD work was the hardest, and probably most hated, of all the classes I took.  And now it’s turned out to be one of the most valuable.  Go  figure.)

While you hard-core types can read the review articles that I’ve hot-linked above, what I’ll try to do is summarize my hypothesis for why leaves (and stems) turn red.  Some leaves are red when young, then turn green when older.  Green, deciduous leaves turn red before they fall off in the autumn.  And some plants are genetically programmed to have red leaves all their lives.

The environment can also influence leaf reddening.  Drought, nutrient deficiency or toxicity, salts, heavy metals in soils, cold temperatures, low soil oxygen, whew!  All of these environmental factors have been attributed to temporary reddening.  What do these factors have in common?

It turns out that all of these environmental stresses directly or indirectly affect the ability of plants to take up and/or retain water. Because anthocyanins are water-soluble, they effectively dilute the concentration of water in the plant.  Look at it this way: any limited area will only hold so many water molecules.  A test tube of pure water has the maximum number of water molecules possible.  A test tube of water plus sugar (or salt, or anthocyanins for that matter) will have fewer water molecules, because the other substances take up space, too.  So effectively, anthocyanins reduced the apparent concentration of water in plant tissues.

Why is this important?  Well, anthocyanins in leaves helps reduce water loss, because the concentration of water in the leaves is reduced and evaporation slows down.  They also could serve as antifreeze compounds, allowing red leaves (and stems, David!) to be more cold hardy.  And if anthocyanins aren’t amazing enough already, they also (1) bind and transport sugars during fall leaf color change, (2) protect tissues against high levels of solar radiation, and (3) are natural antioxidants.  (That’s why you’re supposed to eat red fruits!)

I could go on and on, but I hope this might help explain why David’s red stemmed roses might be more cold hardy than the green variety. (And my thanks to my daughter Charlotte for allowing me to use her photos here.)

Are natives the answer?

Last week Jeff kicked off a lively discussion about invasive plants.  Let me state up front that no one on this blog is promoting invasive plants.  But the issues surrounding invasive plants are extremely complex and have profound implications for many groups with whom we work in landscape horticulture and urban and community forestry.  It is essential in these discussions that we separate fact from hyperbole.  In some quarters, lines have been blurred and people fail to make key distinctions and lump exotic, alien, or non-native species together with invasives.  According to the Federal Executive Order on Invasive species “Invasive species” means an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.  All invasives are alien but only a small fraction of alien species are invasive (all humans are mammals but not all mammals are humans).  Nevertheless, there is a temptation to ‘hedge all bets’ and promote only native species for horticultural planting since native plants, by definition, cannot be invasive.  In addition, there is a ‘feel good’ aura that surrounds native plants – if they’re native they must be good – that clouds some of the logic in the argument.

Some examples:

Natives are more stress tolerant and better adapted than exotics.
Really.  If native plants are always better adapted, why do we have invasives?  Shouldn’t the “better adapted” natives out-compete them? Stress tolerance and adaption are a function of natural selection pressures of the environment in which a species or population evolves.  The world is full of stressful environments and, therefore, lots of stress tolerant species.  There is no a priori reason, for example, to believe that a native species needs less water than an exotic.  The ability to withstand drought depends on the particular species in question.  I’ve done a lot of research on stress physiology of Scots pine – few, if any, native species here in Michigan can match it for drought and cold hardiness.  Moreover, as Jeff pointed out, most of our urban and suburban environments no longer reflect native conditions.  Urban heat islands can result in temperatures 10-20 deg. F warmer than the native countryside.  In our research on heat island effects in downtown Lincoln, NE we logged temperatures in tree canopies in excess of 125 deg. F.  These temperatures were coupled that with the usual urban conditions of impervious surfaces and compacted soils – what tree species is native to that ecosystem?

Native restoration?  This nurse-log ecosystem is typical of forests in western Oregon & Washington.  Trying to keep it alive in downtown Portland requires constant mist irrigation..

Native plants are more pest resistant than exotics.  This would be true if native pests were all we had to contend with.  But the exotic pest train has already left the station.  Emerald ash borer, Dutch elm disease, white pine blister rust, chestnut blight, Asian long horned beetle, and sirex wood wasp are here and here to stay.  And their friends are coming.  The continued expansion of global trade will almost undoubtedly mean that exotic pests, for which native trees have not evolved resistance, will become more, not less, of a problem in the future.   Relying exclusively on native trees means more, not fewer, catastrophic tree failures.  Heavy planting of green and white ash, which are both native in Michigan, has resulted in the loss of 30% or more of the urban tree canopy to EAB in some Michigan communities.

Natives increase diversity  This presupposes that exotic species do not or cannot fill niches occupied by natives.  Exotics can certainly add structural diversity and age class diversity to an urban and community forest.  I would also argue that they add to species biodiversity as well.  If we consider an urban community such as Lansing or Detroit, there are maybe six or seven native tree species that we could expect to have reasonable longevity as street trees.  If we expand our choices to include non-natives we can expand the list to twenty or so.  Not a huge number to be sure, but still a better hedge against catastrophic urban tree loss that the ‘native only’ policy.

Where to go from here?  We cannot ignore that fact the invasive plants are a huge economic and environmental issue.  Presently we do not have models that will accurately predict which exotics will become invasive and which ones won’t.  Trees that are demonstrated to be invasive in a given environment need to be dropped from planting programs.  Except for the desert Southwest and parts of the Plains, every region of the country has great native trees that can. and should, be an integral part of their urban and community forests.  While it’s tempting to play it safe and promote natives only, this policy has significant shortcomings.  Urban and community forests provide enormous economic, environmental, and societal benefits.  In order for our urban forests to provide these functions over the long term we need as broad an array of trees species as possible, including appropriate exotics.

Friday puzzle solved!

Great discussion over the weekend, with some very astute observations.  If you looked at the brown needles under the tree in Friday’s picture, you may have noticed that some of them weren’t needles:

Not only was this tree planted too deeply, as several of you pointed out, but the burlap and twine were left intact.  It appears the nylon twine has already started to girdle the trunk, based on the trunk swelling just above where the twine is wrapped.

I’ve ranted about this practice already, so I’ll just sigh and move on to the first question – what directly caused the needle drop from the lower part of the tree?  It’s a young tree facing west so the lower half gets plenty of sunlight.  And though needle drop is normal with all conifers, the upper portion of the tree does not show the same drop with its interior needles.  My guess is that ethylene gas is responsible.

Plant roots under stress often release ethylene, a natural plant growth regulator more commonly associated with fruit ripening.  It also induces leaf drop, so as it percolates out of the soil it affects the lower leaves, but dissipates before it reaches leaves higher in the crown.  It’s a common phenomenon with over-watered house plants.

Thanks to all of you who participated in the diagnosis discussion – this is more fun than my 20 years of college teaching!