Well, it’s still winter out there.  It was 18^o F at dawn in Pittsburgh but by Monday it will be back to 41^o.

These yo-yo temperatures can wreak havoc on roads and bridges and our landslide-prone hillsides.  If the temperature drops fast and far enough it even hurts living things.  At super low temperatures the trees explode.

I had never heard of this phenomenon until a conversation in Maine last fall when I asked Ann Sweet, who runs the Harbourside Inn, how cold it gets in winter at Acadia National Park.  Ann said the ocean keeps the island warmer than interior Maine but every once in a while it gets so cold that the trees explode.

Wow! And why?

Tree sap contains water and water expands when it freezes.  The expansion increases pressure under the bark and in extreme cases causes the bark to explode.  This doesn’t happen all the time because trees draw down sap into their roots in autumn, leaving room under the bark for expansion.  If they didn’t do this they wouldn’t live through the winter.

The danger for cold-explosion comes when the trees haven’t had time to draw down their sap or when the temperature falls extremely low.  Both occurred in north-central Washington in December 1968 when temperatures fell to -47^oF.  The fruit trees in Wally and Shirley Loudon’s orchard exploded.

Native Americans were well aware of this phenomenon.  According to Wikipedia, the Sioux and Cree called the first full moon of January “The moon of cold-exploding trees.”

When the moon was full on January 9, Pittsburgh’s average temperature was 10 degrees above normal.  I don’t think we’re in any danger of exploding trees.

(photo of tree exploded by lightning in Central Park, New York by David Shankbone.  Click on the image to see the original on Wikimedia Commons)

p.s. It is much more common for trees to explode when hit by lightning.

It’s winter.  Mostly.  The plants are dormant but there’s no snow to brighten the ground.  With temperatures in the 40′s and overcast skies the landscape is brown in Pittsburgh.

But what is this?  A spot of green on this last day of 2011.  What plants are braving the cold in Schenley Park?

Two alien invasives:

Goutweed …

… and Garlic mustard.

Perhaps it’s not wintry enough for them.

(photos by Kate St. John)

The Holly and The Ivy is one of my favorite Christmas tunes. This week the lyrics inspired me to learn more about holly.

The holly and the ivy, when they are both full grown,

Of all the trees that are in the wood, the holly bears the crown.

Like mistletoe, holly is a European Christmas tradition that easily transferred to North America.  In Europe they decorate with Ilex aquifolium. When the colonists arrived in Virginia they found an abundance of the very similar Ilex opaca.

Since ancient times holly has been associated with the winter solstice because it remains green all year.  Each evergreen leaf with its sharp spiky edges remains on the tree for two to three years and only falls in the spring when a new bud forces it off the branch.

The holly bears a blossom as white as lily flower…

In Esther Allen’s photo you can see the holly’s white flowers which bloom in May and are pollinated by bees, wasps, ants and moths.  Hollies are dioecious — some trees are male, others are female.  Only the female trees bear fruit.

The holly bears a berry as red as any blood…

Holly berries stay on the trees in winter and are often eaten by birds.  Esther’s photo shows that uneaten berries can persist into spring alongside the flowers.

The holly bears a prickle as sharp as any thorn…

The leaf edges are sharp.  No doubt about it.  I tested one with my finger.  Ow!

The holly bears a bark as bitter as any gall…

This, I did not test.  I’ll take their word for it.

.

The holly and the ivy, when they are both full grown,

Of all the trees that are in the wood, the holly bears the crown.

…a crown of thorns, greenery and red berries.  Very beautiful.

.

(photo by Esther Allen, remembered fondly this Christmas)

The Christmas tradition of kissing under the mistletoe came to North America with European colonists. Back home they used Viscum album for this purpose.  Here they found a very similar native plant, Phoradendron serotinum, that worked just as well.

Growing up in Pittsburgh I always thought of mistletoe as an exotic import that could only be bought in the store.  I never saw it in the wild because we’re just north of its range but it’s quite common in the south, noticeable as balls of greenery in bare trees at this time of year.

Mistletoe is a hemiparasitic plant that grows on tree branches, sending its roots into the bark.  It is partly (hemi) parasitic because it produces its own food through photosynthesis but needs its host for water and minerals.  It doesn’t normally kill its host.  That would be suicidal.  It needs its host to survive.

Like many plants mistletoe relies on birds to spread its seeds but there’s a twist.  Birds like to eat mistletoe berries and they don’t digest the seeds but the seed must survive the bird’s gut and stick to a tree branch long enough to germinate.  To do this the seeds are coated in a very sticky substance.  Birds either avoid swallowing the seeds and wipe their sticky bills on a branch, or they swallow the seed and fly elsewhere to “deposit” it.  Favorite perching trees receive more deposits. That’s why you see a lot of mistletoe in some trees but not others.

Some birds are mistletoe specialists.  In the desert West, phainopeplas get their water and nutrition from desert mistletoe for much of the year.  A phainopepla can eat up to 1,100 mistletoe berries per day.

Though mistletoe’s parasitic nature gave it a bad name, recent studies have shown that animals, birds and habitat are more diverse where mistletoe thrives.  It apparently holds the ecosystem together … like a kiss joins a couple.

Have you hung a sprig of mistletoe this year?

(photo of mistletoe in Delaware by Charlie Hickey)

Grandidieri’s baobab (Adansonia grandidieri) grows only in the western part of Madagascar, an island off the coast of Africa.   As is typical for baobabs it has a very wide trunk compared to its crown.  Of the eight baobab species on earth, six are endemic to Madagascar.

If surrounded by a forest this tree would not stand out.  At 80 feet tall it’s about the height of a red oak and would blend in from a distance.  But its trunk is 10 feet wide, three times the diameter of a red oak.  This is one fat tree!

The IUCN says that Grandidieri’s baobab probably occurred in dry deciduous forests close to water but is now found in degraded agricultural land.  Sadly this is typical of Madagascar where deforestation is a huge problem.

That’s why this baobab is endangered … and more unusual than it ought to be.

(photo by Bernard Gagnon on Wikimedia Commons.  Click on the photo to see the original.)

As I’ve snapped photographs of bark for my winter tree identification series, I’ve had no trouble finding clean, lichen-free trees in Schenley Park.  It turns out the lack of lichens is bad news for our air quality.

Lichens are two organisms that operate as one, a symbiotic partnership of a fungus with a green or blue-green algae (sometimes all three).  The algae’s photosynthesis feeds the fungus.  The fungus gathers and retains water and nutrients and protects the algae.

This amazing combination allows lichens to thrive in some of the harshest habitats on earth but they’re sensitive to air pollution.  The ones that grow on trees are epiphytes, totally dependent on the surrounding air and precipitation for their nutrition.  Ultimately their tissues absorb elements in concentrations that mimic what’s in the air.

We’ve known for a long time that there’s a correlation between the absence of lichens and poor air quality.  Back in 1866, the Finnish botanist William Nylander showed that lichens were present in the Luxembourg Gardens that had disappeared from the polluted sections of Paris, France.  Sadly, air pollution increased in Paris and within 30 years the Luxembourg Gardens’ lichens had disappeared as well.

Lichens are used in air quality studies today because they are widespread, accurate indicators and far less expensive than man-made monitors.  You don’t have to be an expert to participate.  In the 1960′s schoolchildren in Great Britain gathered data in a nationwide lichen-based air quality study that produced the “Mucky Air” map.  Here’s a list of a few more recent lichen studies:

• A lichen air quality study conducted in Pittsburgh by Matthew Opdyke
• Air pollution-related lichen monitoring by the National Park Service
• The OPAL Air Survey currently underway in the U.K.
• USDA Forest Service air quality lichen survey in Oregon and Washington
• Utah lichen study showing that lichen tissues mimic air quality concentrations

Even if you can’t identify lichens you can make a rough guess of the local air quality by the types of lichens you see.  Basically, “the further it stands out from the tree, the cleaner the air.”  Crusty lichens (crustose) are the hardiest because they have the least surface area, leafy (foliose) lichens are in the middle, shrubby (fruticose) lichens are the most sensitive.  Hypogymnia physodes, a foliose lichen pictured above, is often used as an indicator species because it’s widely distributed and it “stands up.”  I’ve seen lichens like this in Maine but not in Pittsburgh.

Lichens are especially sensitive to sulfur dioxide (SO2).  So are people.  In Pennsylvania most of our SO2 is produced by coal-burning power plants and coking facilities.   High SO2 causes respiratory distress and triggers asthma so it’s been regulated since the Clean Air Act of 1970.  Lichens have rebounded in many areas of the U.S. since then.

In June 2010 EPA issued tighter 1-hour SO2 standards (75 ppb, measured hourly) to protect public health from high short term exposures ranging from 5 minutes to 24 hours.  Because we’ve been measuring SO2 for so long, we already know that the Pennsylvania counties of Allegheny, Beaver, Indiana and Warren have exceeded the new SO2 standard.  Coal-burning facilities in these counties will have to control their SO2 emissions even further.  As they do, we’ll all breathe a little easier.

And we’ll have more lichens in the future.

(photo in the public domain from Wikimedia Commons. Click on the photo to see the original.)

I discussed epiphytes a couple of days ago because I wanted to use this beautiful photo as a quiz. 

Though this looks like an artistic squiggle it’s actually a close-up of a plant. 

Here are some hints to its identity:

• It’s an epiphyte.
• It’s native to the southeastern U.S. where the climate is warm with high humidity.
• It has tiny inconspicuous flowers.  (As many times as I’ve seen this plant I’ve never noticed any flowers.)
• Its leaves are alternate, thin, heavily scaled and curved.  These are its leaves. 
• The leaves appear to form long chains.
• Big hint: It’s commonly found hanging from southern live oaks and bald cypress trees.

Can you guess what it is?

Leave a comment with your answer.

(photo by Ernest V. More in the public domain on Wikimedia Commons)

That’s what epiphyte means in Greek (epi=upon, phyte=plant) and that’s what an epiphyte is:  a plant upon a plant.

I never thought about this word until I saw some interesting epiphytes in the forest while visiting my family in southeastern Virginia.

True epiphytes are sometimes called air plants because they collect their water and nutrients from rainfall, mist, dust and the surrounding air.  Though they’re held aloft by a host plant they aren’t parasites and never directly harm their host.

As proof, here’s a photo of epiphytes growing on telephone wires in Bolivia.

I’ve seen this in Florida too.  I’m sure it annoys the phone company.

We normally think of epiphytes as tropical plants like the red orchid pictured above, but all kinds of plants-upon-plants grow wherever there’s enough humidity or rainfall and clean air.

Mosses, lichens and ferns are the epiphytes I usually see in Pennsylvania.  They seem almost boring because I’m so used to them.

Do you have interesting epiphytes where you live?

(photos via Wikimedia Commons.  Click on each photo to see the originals)

Jonathan Nadle is fascinated by porcelain berry.  He wondered if his cat would be too.

I think it’s safe to say the cat did not catch Jonathan’s enthusiasm. 

Click on the photo of Larry Cat to see the very mild reaction of one imported species (Felis catus) examining another (Ampelopsis brevipedunculata).

(photos by Jonathan Nadle)

As the sunlight shines through this leaf, each vein is illuminated.

Alocasia sanderiana is beautiful up close and from afar.

Its leaves are arrow-shaped, dark green, and very shiny with prominent pale green veins.  The leaf edges are so amazingly wavy that in English it’s called the kris plant, named for the kalis (or kris) daggers of its homeland.

Here’s what the whole leaf looks like:

Alocasia sanderiana is native to the Philippines but is critically endangered in the wild.  It grows in only two locations, both legally protected, but the protection is not enforced.  It’s existence is threatened by logging and by being collected as a house plant.

Ironically, if the kris plant disappears from the wild, its beauty will save it from extinction because it’s been propagated “in captivity” for many, many years.

(close-up of an Alocasia leaf by Joan Guerin; whole-leaf photo from Wikimedia Commons)