Friday puzzle answer(s)

Wow!  What a lot of great brainstorming over the weekend!  I would venture to say that The Garden Professors have the smartest students in the world.

On to the answer…or answers.  First, the phenomenon.  It’s called paraheliotropism – literally, a movement to protect (the leaves) from the sun (yes, Trena, it is a tropism!). This is the opposite of another phenomenon called heliotropism, or solar tracking.  Sunflowers famously do this, as do a number of arctic species that collect solar warmth for the benefit of their pollinators.  (An aside:  if you have never watched David Attenborough’s The Private Life of Plants you must add it to your Netflix queue.  Right now.)   

But our saxifrage (thanks, Holly! I’m such a taxonomy imbecile) is reducing solar exposure by positioning its leaves in parallel to the sun’s rays.  This is a reversible movement and helps reduce photooxidative stress, leaf temperature, and water loss.  It’s an important strategy as the newly emerging leaves are actively expanding.  If turgor is reduced by high temperature or water loss, so is the final size of the leaf. 

Finally, these rapidly expanding leaves have relatively thin cuticles (if they were thicker the leaves wouldn’t be able to expand as well).  The cuticle gives further protection to the leaf from water loss due to heat, drought, wind, or even late season freezing events (thanks for that addition, John!).  The cuticle will mature after the leaf has reached its full size.

So, as Foy suggested, this is a way for leaves to "harden off" and reach full size before exposing themselves to the sun.  Aren’t plants cool?

And you are all such great participants!  Group hug!  Now, back to work.

Friday puzzle

Spring is coming…and soon herbaceous perennials will poking their leaves up through the mulch:

Obviously as leaves first emerge they’ll be vertically oriented – but these ones have remained vertical days after emerging.  Eventually they’ll become horizontal.  But today’s question is – what’s the advantage in remaining vertical?  And what’s this phenomenon called?

Answer on Monday – have a nice weekend!

Odds ‘n Ends

Some odds and ends today that I either #1 was asked to post or #2 couldn’t resist posting.  First for the picture that I was asked to post.

This, as far as we can tell (we being myself, my technician, and our grounds department), is the American elm tree that was being planted in that picture from 1909 which I posted on January 21.  Dutch elm disease was devastating here in the mid 1900s as it was everywhere, but this region of the world was lucky and there were a number of escapes — and resistant trees (that’s an ongoing project of mine — working with DED resistant elms — I’ll probably post more about it this spring).  Anyway, the tree is a little smaller than I would normally expect for an elm of this age, but the proximity of the road and sidewalk could easily have stunted its growth.

Now for the stuff that I can’t resist posting — mostly having to do with Bert’s post on January 25.  Chad (my technician — if you follow the blog you’ll remember him, 6’4″ — etc.) was showing me a book titled Shade-Trees in Towns and Cites by William Solotaroff published in 1911 and it had this great shot of filling a tree cavity.  So here it is:

The book also had a great shot of what they did to a trees canopy before they planted:

This type of pruning isn’t necessary at all.  When trees are planted they adapt to the amount of roots which they have by producing fewer, or smaller leaves.

Update:

Here is a photo of the leaves of a freeman maple which was severely rootpruned right before planting and, below it, the leaves of a similar maple whose roots were left pretty much intact (both plants were container grown).

As you can see, trees have their own methods of dealing with root loss — no need for us to come in and clip their tops off.  Now, two years later, both of these trees (and all of the others in the research plot) look pretty much identical.

Mistaken Identity, or The Truth?

In the wake of The Garden Professors’ sudden notoriety (see Linda’s Jan. 26 post), my department head sent out a very kind e-mail announcement to our faculty, staff, and grad students. 

However, he referred to us as the "Hort Professors" blog, sans hyperlink. 

A curious staff member (the lovely and talented Pris Sears) searched that title, resulting in the following:

Hort Professors, Hot Professors…kind of the same thing. Thanks, Google! 

“Being wrong” counterpoint

I haven’t finished with the water droplets story yet – but I just had to add some more evidence to the tree planting discussion from last week.

Consider this series of photos below.  This is a street tree in Kennewick, WA (in the southeastern part of the state, where summers can be intensely hot and dry).  Every spring, this tree leafs out just fine – and every summer the leaves suffer marginal and tip scorch.  This is a classic symptom of chronic drought:

As an amenity, the tree fails.  Even though the landscape is well-watered, as shown by the healthy turf in the next photo, the canopy is sparse and dry.

An excavation of the roots explains why:  the tree was planted too deeply and has developed a secondary set of  roots:

Note how sparse these roots are – which is typical of many adventitious root systems.  While the roots are adequate for water uptake during the cool spring weather, the hot dry summers suck away more water from the leaves than this puny root system can absorb, even when well-watered.

My point:  sure, trees might survive being planted too deeply.  But thrive?  Not in this case – and this is a well-managed landscape!  With less care this tree would have died long ago.  The only solution here would be to replace this tree – correctly.

The evolution of tree care

I’m one of those people who can’t resist things that are free.  When you pass a yard that has a ‘FREE’ sign on a rusted-out lawn mower or an old piece of exercise equipment and wonder, “Who on earth would take that home?” Uh… that would be me.  Actually, I blame my dad.  The Old Man’s garage was crammed full with outboard motors missing pull-cords, mismatched lawn furniture, and all manner of secondhand tools in varying states of disrepair. So my defect is genetic.  Only my hyper-organized wife prevents my garage from suffering the same fate as my dad’s.  But, like a blind pig finds an acorn, sometimes a scavenger finds something interesting.  Recently a retiring professor in our building was cleaning out his office and put a stack of old books on a table by his door.  And there was the magic word: FREE.  I sifted through the stack and found my treasure; a 1948 edition of Maintenance of Shade and Ornamental Trees by Dr. P.P. Pirone.  Granted, I have ready access to several modern tree care texts plus an ocean of information on the International Society of Arboriculture (ISA) website, but after thumbing through a few pages I knew this one was a keeper.  The main reason the Pirone book interested me was to see how tree care has changed over the years and how things have stayed the same.


What’s changed?  The most obvious example of how tree care has changed in 60-plus years is that an entire chapter of the 1948 text is devoted to cavity filling.  For those not familiar with the process; cavity filling, as the name implies, is the practice of filling hollow areas in trees with concrete, just as a dentist fills a tooth cavity with amalgam or composite materials.  Filling cavities in trees was once a relatively common practice.  Today we recognize that filling does not stop decay and there is little value in the practice (and it certainly complicates removals).  If anything, we would replace this chapter with ‘hazard tree assessment’, seeking to determine if the tree has enough solid wood to be structurally sound.


Filling a tree cavity.


The finished job

What hasn’t changed?  I was surprised to see a fairly lengthy discussion of problems related to girdling roots from 60 years ago.  This actually causes me to eat a little crow.  I have sometimes questioned whether the current ‘epidemic’ of girdling roots is actually related to the fact that arborists and urban foresters are spending more time looking for them and have better tools (specifically air spades) for finding them.  Of course, Pirone’s text also suggests that girdling roots were an issue before the advent of evil nursery production systems such as container growing.  The discussion of treating girdling roots points out another change in practices.  In 1948 the arborist was advised to carefully dress the wound with paint or tar.  Today we generally advise against treating wounds except for situations which risk certain exposures such as if oak trees must be pruned when they are at risk for infection from oak wilt.


Girdling root removed and wound dressed.

So there you go, another book to clutter my shelf and another example of one man’s trash turning into another man’s treasure.  I think Dad would have been proud.

Friday Physiology Fun Followup

Astute readers pointed out several morphological adaptations found in drought-tolerant turf weeds:  fleshy taproots, reflective leaf surfaces, etc.  What we can’t see is what many of these plants do physiologically – and that’s photosynthesize using a biochemical pathway that temperate turfgrasses don’t possess. 

This pathway, called C4 photosynthesis, contains some extra preliminary steps not found in plants using traditional (C3) photosynthesis.  The downside:  it takes more solar energy for the plant to photosynthesize.  The upside:  these extra steps allow the plant to "fix" carbon (transforming it from gas to solid) faster, especially when it’s sunny, warm, and droughty.  Practically speaking, this means that C4 plants do not have to keep their stomata open as long and they conserve water more efficiently than C3 plants.

So in the summer – when it’s hot, sunny and dry – the C4 plants in your lawn are operating under optimal conditions, while the C3 grasses go dormant.  The tables turn when the seasons do:  cool, moist conditions favor traditional photosynthesis, and the C4 plants are overtaken by the turfgrasses.

Cool, huh?

Friday physiology fun

It’s still cold and wintery, so let’s imagine ourselves in a happy place…warm, sunny, dry…with dead lawns.

As the photo shows, the turfgrass is dead; this happens every summer during the Pacific Northwest’s droughty summers.  Yet many of the weedy species are obviously thriving.  Why?

Remember, this is a physiology quiz.  You can discount herbicides, fertilizers, etc.  This is a cool (no pun intended) adaptation that many species native to dry, subtropical to temperate environments possess.  And there are serious implications for water use related directly to this adaptation, or lack thereof.

Let’s see lots of brainstorming on this – no points deducted for trying!  (And if you are a true ecophysiology geek, let other people try first before posting the answer.)

Making a difference?

I promise I’ll post a Friday puzzle later.  But I just had to let you know that if you Google “water droplets burn leaves” (without quotes) you’ll get 436,000 hits as of this morning.  Number 3 on the list?  Our blog!

I think I’ll discuss this paper one more time on Wednesday – there are several other serious issues that I think are worth mentioning.

Have a good weekend!