Why Don't Fish Need Mittens?

Brrrr!  It's been cold the last few nights!  Air temperatures dropped into the mid 20's, which is way lower than freezing.  I bundled up in many layers to survive being outside for about an hour last evening.  I felt a little bad for the fish in our outdoor classroom - they are stuck in cold water without any hats or mittens or even hot cocoa to warm them up.

A mosquitofish alive and well after several nights of freezing temperatures.

Humans are like tropical animals in terms of their thermal comfort zone.  We are comfortable living in temperatures in the 60's to 90's on the Fahrenheit scale.  We have created many devices to keep ourselves at a comfortable temperature: clothes, buildings, heat, air conditioning, insulation and ice cubes all help us maintain comfortable body temperatures whether we are in the tropics or in the Arctic.  Animals can be classified as endotherms or ectotherms, and we are of the endotherm variety.  Endotherms use some of the energy in the food they eat to keep their bodies warm.  Even though the temperature of the air inside our buildings is usually around 72 degrees, our bodies stay at 98.6 degrees.  Mammals, birds and even some fish like tuna can keep their body temperature warm using energy from food.

Mosquitofish are happy as clams in a much broader range of temperatures than we can stand.  They can live in the very warm water of shallow sunny pools in the summer, and they can survive a fairly cold winter too.  Mosquitofish are ectotherms, like most fish, amphibians, reptiles, insects and mollusks.  They don't keep their body temperature warm - they let it cool off when the environment cools off.  And as the temperature drops, they simply slow down.  Their bodies move more slowly, they eat less food, and they stay more hidden.  Many ectotherms hibernate, essentially sleeping in a cold state until the weather becomes warm enough to move around again.  If you watch our mosquitofish, you will notice that they are much slower on cold days than warm days. 

Mosquitofish can't survive if the pond freezes all they way through.  Fortunately for them, water temperature usually doesn't get as low as air temperature, so the pond is going to be warmer than the air temperature, and it won't usually freeze.  Also, ponds freeze at their surface, then the ice acts as an insulator, keeping the lower layer of the pond from freezing.  So even if you see ice on our pond this winter, it is likely that the mosquitofish will be swimming slowly in the water under the surface. 

Do mosquitofish feel cold?  I don't know.  I suppose you would have to put a mosquitofish in a fish tank with a cold area and a warm area and see where it chooses to spend its time!

Here are some other ways you can see organisms responding to the temperature at the outdoor classroom this week:

It's easy to see which plants survive freezing right now.  I'll write about this more in the deep winter, but it's probably easier to see now before the dead plants blow away and decompose.  The dead leaves in the picture below didn't survive freezing.  Either their seeds will survive the winter or their roots will survive in the ground, but it will not grow again until the spring.  The plant on the left is just fine with freezing temperatures, and it will stay growing, though very slowly, through the winter.  there are lots of winter-growing plants in our classroom.

The fern on the left survived freezing, the plant on the right did not.

The honey bees are still drinking at our pond on warm days!  They must have a fairly warm location for their hive.  Bees do some temperature regulation of their hives by eating food then shaking their wings really hard inside the hive to generate heat.  Our bodies do a similar thing - they shiver to generate heat.  Bees also flap their wings to fan the hive if it gets too hot.  Even though insects are ectotherms, bees have some endotherm ability.  Neat!

Honey bees are still drinking from our pond on warmer days despite the freezing nights.

Winter, Spring, Summer, Abscission

It's happening everywhere right now!  Plants are chopping off their own organs, and they are piling up in yards all over town!  How come no one is worried about this epidemic of leaf death??!!  Well, it happens every year, so I'm pretty sure the plants are going to recover.  Still, why on earth would plants get rid of their most important organs?  That's what we'll address in today's post.

Closeup of leaf abscission zone on sourwood.

In the picture above, you can see the color difference between the pale pink of a leaf petiole (technical term for a leaf stem), and the bright red of a sourwood twig.  The line between those two differently-colored plant parts is called the abscission (ab-SIZH-uhn) zone. 

Fresh leaf scar where the abscission zone dissolved and the leaf fell off.

This time of year, the layers of abscission zones are changing.  One layer is hardening and filling up with a corky substance called suberin.  Suberin is waterproof and heals what would otherwise be a wound where the leaf falls off.  The leaf scar in the picture above is dry and not losing sap because suberin has sealed the wound.  The second layer in the abscission zone is made of thin-walled, weak cells that self-dissolve when the plant is ready to shed its leaves.  Abscission zones are usually quite noticeable this time of year on any plant that is in the process of losing its leaves.  Take a look at the next two pictures and find the abscission zones.

Sourwood leaves and petioles (stems) about to undergo abscission.

The abscission zone is at the base of the leaf petiole where it attaches to the twig.

It is extremely unusual for living organisms to shed any part of themselves except for the production of offspring.  Some lizards have tails that fall off to distract predators, and many plants lose their leaves in the fall - but I can't think of other examples of falling-off body parts.  Of course, most organisms constantly rebuild their outer-coverings and some organisms can replace body parts that are bitten off, but voluntary amputation is strange, indeed! 

The loss of body parts comes at a huge cost.  Plants work all summer to catch enough sunlight to grow more leaves and get bigger, and leaf abscission every fall would seem to waste that energy.   But as with the lizards that lose their tails, there are also benefits.  Lizards' bodies escape to live another day and regrow another tail.  Plants benefit from shedding leaves by not having to maintain those leaves during the winter.  Leaves are tender tissues that would become disfigured and die when frozen.  Try putting some lettuce leaves in the freezer over night and then take them out to thaw.  You will notice they turn to mush when they return to room temperature.  In order for plants' leaves to survive winter, they would have to be tough, like holly, magnolia or spruce leaves, which take much more energy to produce.  Plants with leaves that survive freezing grow more slowly than ones that shed their leaves.

Dogwood with remnants of chlorophyll along veins and lots of anthocycanins (red pigment).

Plants have many ways to minimize the costs of losing their leaves.  They move all available nutrients out of their leaves and down into their roots to save the food for the next growing season.  Leaves fall near the plant that grew them and decompose, releasing their nutrients into the soil and further increasing the amount of nutrients recovered by the plant.  In this way, deciduous plants grow their own mulch.  Some plants, like walnut trees, even deposit compounds in their leaves that suppress the growth of competitor plants as the leaves decompose throughout the winter and spring. 

Rainbow of fall colors.

As leaves senesce (slow down and die) in the fall, they turn the variety of amazing colors we are so familiar with.  Plants' normal color is green, due to the most important compound in the world: chlorophyll.  Chlorophyll is the substance in plants that allows them to absorb sunlight and use the energy from sun to make food, a process called photosynthesis.  In the fall, chlorophyll breaks down, revealing other colorful substances plants use for photosynthesis: xanthophyll (ZAN-tho-fill), a yellow pigment, and carotene (CARE-oh-teen), an orange pigment.  As temperatures drop, some plants make anthocyanin (AN-tho-SIGH-uh-nin), a red pigment that helps the plants store sugars for winter.  Some plants reveal tanins (TAN-ins) in their leaves in the fall.  Tannins are brown in color and are thought to be waste molecules produced by plants.  They have a bitter flavor, though some tannins are pleasant, including the ones found in tea leaves.

Leaf scar on a buckeye showing scars where the leaf veins were sealed off with suberin.

So leaf abscission is a trade-off that works in parts of the world with four seasons.  Plants in the tropics and plants in colder regions keep their leaves.  Tropical plants don't have to deal with cold, so they don't shed their leaves unless there is a yearly dry season.  Plants nearer the poles of the planet don't have a long-enough growing season to start from scratch every year, so they have to grow slowly and produce evergreen leaves and needles.  We lucked out, and we get to see the beautiful fall colors that accompany leaf abscission.

Busy Fall Ants

Edward O. Wilson is one of my scientist heroes, and he has studied ants for most of the 83 years of his life.  As a child, he loved to go outside and observe ants for hours because they exhibit such a variety of behaviors.  E. O. Wilson eventually became the world's leading myrmecologist (ant expert), as well as an expert on ecology, animal behavior and conservation biology.  Thanks to him, I know some really amazing things about ant behavior, and I always think of him when I observe ants.

A foraging ant.

The ants at our outdoor classroom are busy, busy, busy this time of year.  Frost is coming soon, and the ants are foraging for their last bits of food to help get them through the cold weather coming our way.  If you stop and observe the rocks around the pond, you will start to see some patterns in the ants' behaviors as the ants bustle around in a mad rush to get ready for winter.  Below are some patterns in ant behavior that I observed.

An ant and her shadow.

The ant above was exploring to find food, also known as foraging.  Any ant exploring on its own in a zig-zag or random fashion is most likely foraging for food.  When the ant finds food, it will pick up the food and bring its food back to the ant's nest to share with the other ants in its colony.  If there is more food than it can carry, the ant will do something incredible.  It will leave a scent trail on its return to the nest to signal to its nest mates to go and get the rest of the food!  How amazing that these tiny creatures can communicate such complex information to each other.

Ants following a scent trail.

Ants are social insects that live in colonies.  The ants in a row in the picture above are interacting as a social group by following a common scent trail.  Either they are all going to get food or they are moving their colony.  Ants usually maintain a nest in a space in or near the ground.  The nest stores their food and eggs.  The ants in the picture above are all sisters!  I know this because all worker ants are female and are sisters.  The sisters work together to keep the colony alive and take care of their mom, the queen.  The queen stays in the ants nest and lays eggs.  If you look closely at a line of ants, you might be able to see if they are carrying bits of food or eggs.  If they are carrying ant eggs, they are moving the colony.  There is a colony of ants outside my back door that moves its nest every time it rains: from under the flower pot in dry weather to under a loose brick when it's rainy.  They never seem to get tired of carrying eggs around.

Ants deciding if they are friends or enemies.

If you observe a line of ants, you will probably notice ants are going in both directions, like in the picture above.  That means the ants run into each other.  Every time an ant runs into another ant, it needs to determine if the other ant is a friend or an enemy.  Enemy ants must be run off the territory or killed and eaten, and friendly ants must be allowed to pass.  Ants don't recognize each others' faces; rather, they smell each other with their antennae.  It takes just a flash for the ants to touch antennae, recognize each other, then head on their way. 

Ants are extremely important creatures on Earth.  They live in the soil and on trees and other plants, and they help recycle nutrients in ecosystems.  Ants build soil, eat pest organisms, and provide food for other insects and for birds.  Some plants are pollinated by ants, and some seeds are dispersed by them too.  E. O. Wilson has estimated that ants account for about the same amount of mass on Earth as humans do! I wonder which has had a greater impact on our planet.  I know humans have built cities and houses and reshaped the ecosystems, but ants have built the soil that all other terrestrial ecosystems are built on.

How to Read Bark Scars in Sourwood Trees

Sourwood trees are among the first to turn colors in the Fall.

The sourwood trees in our outdoor classroom are the first to put on their fall colors for the season.  Sourwoods are wonderful, smallish trees with beautiful foliage and interesting bark.  They are named for the sour taste of their leaves, which you can experience if you touch a bit of torn leaf to your tongue.  The leaves contain oxalic acid, which tastes pleasantly sour (all acids taste sour).  Tasting the leaf is not harmful, but the leaves are not considered edible and shouldn't be eaten.

The small orange-leaved tree in the picture is one of our sourwoods.

We have two sourwood trees.  Above you can see the location of one sourwood - it's the orange-leaved small tree in the center of the picture.  See if you can find the second sourwood tree when you visit the classroom.

Lenticels in young bark of the sourwood tree.

Sourwood bark is wonderful - it has so many visible features that give clues to what the tree is doing and what it has gone through during the tree's life.  A lot of people think tree bark is a dead part of the tree, but the opposite is true: tree bark is a living, important tree tissue that changes as trees grow.  Bark is mostly responsible for moving sugars (a tree's food, made from photosynthesis) between the leaves and roots.  Bark is filled with phloem tubes for transporting the sugar.  The above picture of a young twig contains tiny spots called lenticels.  Lenticels are tiny holes in the bark to allow air to get into and out of the inner tissues of the twig.  Compare the above twig to the one below.

Sourwood twig with cicada damage.

The twig in the picture above is about the same age as the one in the previous picture, but something looks wrong!  This giant gash in the bark is the healed wound cut into the bark by one of last year's cicadas.  Cicadas cut into young bark and lay their eggs in the gash where the developing offspring can feed on tree sap.  The living bark responds by slowly growing a scar to heal the wound and seal off the wood, which is what you see above.  

Older twig with young bark splitting as the twig grows larger.

Bark naturally stretches and tears and re-heals to allow tree twigs and trunks to increase in girth.  The twig above shows the first tears in young bark as the twig is getting thicker through the years.  The stretch marks get bigger as the tree gets bigger, and large branches and trunks might have deep furrows in the bark.

Scar from where a branch broke off the tree.

When branches fall off or are broken off, the bark around the broken area swells up and heals over the scar.  The scar above looks like a pretty big scar, so I suspect the branch that used to grow here was torn off unevenly.  Notice the larger tears in the normal bark above and below the branch scar.

Large gash in bark that is healing over - possibly damage from planting the tree.

Here is an even bigger scar from some major damage to the trunk.  Something cut into the bark of this tree.  Perhaps it was damaged as it was being transported or planted here.  Such an injury can weaken or kill a tree, because it can let diseases into the tree, just like a wound in our skin can become infected.  I wish more people realized this so they wouldn't carve their initials into trees' bark.  Nevertheless, this tree appears to be healing from its damage.  You can see exposed wood through the gash in this bark.  If the wound to this tree were to have cut through the bark all the way around the tree, the tree would have died, since the bark would be unable to move sugars up and down the tree.  Plant managers who need to kill trees use this technique - it's called girdling a tree.

Check out the bark on our sourwood trees and look for lenticels, branch scars, normal tears in the bark, and possible injuries to the bark.  Then take a look at other types of trees and see if you can read the scars in the bark.  Can you tell where branches used to be?  Can you see how the bark split at the tree got bigger?

Click to zoom in and see how the leaf veins connect.

When you check out the bark on our sourwoods, be sure to look at the leaves too.  Sourwood leaf veins are large, and it's easy to see the network of how the veins connect.  Also be sure to look for the remnants of flowers, now turning into fruits, at the ends of some of the branches. 

Little Green Farm Workers

With the greens of the farm plants fading, two bright green animals brought themselves to our attention on the farm this week.  Both of them are welcome creatures that help us keep the numbers of pest insects low. 

The first green creature of the week, a rough green snake, was hanging out in the barn.  I found him in a basket I was about to use for eggplants.  We quickly caught him and put him in this jar (with air holes), so that the farm owner's son could see him too.  These little green snakes are a rare sight on the farm, and they hold a special place in the hearts of this farm family.  We marveled and the bright emerald luminescent color of this sleek, friendly snake.

Rough Green Snake with grass- we released him quickly!

Rough green snakes don't get large.  They can grow to almost 3 feet long, but they always stay skinny.  They are great climbers and spend their time hunting insects and spiders in any kind of vegetation from grass to trees, but they prefer to be higher up rather than on the ground.  Green snakes are well camouflaged for their preferred habitat, and I've probably seen dozens of them without realizing it.  They coil up in branches to sleep at night, and in the cooler weather, they seek refuge under logs or other debris.  This may be why our green snake ventured into the barn.  He probably thought he found a good place to overwinter.

Later in the week, we were working in the greenhouse, and we found two gigantic praying mantises, both the brightest of green.  One of the mantises was half brown and the other was all green.  I wrote a little about mantids earlier, but here's some more information about them.

In Tennessee, our most noticeable mantids come in three color variations: green, brown, and green plus brown.  The green ones are European mantids, the brown are Carolina mantids, and the green plus brown are Chinese mantids.  Only the Carolina ones are native, and the other two were introduced to the US to help control garden pests.  These introduced species do not appear to be particularly invasive, though they can reduce numbers of helpful organisms like wolf spiders.  People generally regard them as welcome workers in farms and gardens.   There are several other mantis species that are illegal to import because they pose a threat to native ecosystems.  They can probably reproduce very quickly and overeat beneficial insects.

European mantis, about 6" long.

Female mantises are disconcertingly large this time of year.  They grow big from hunting all season, and their abdomens are filled with eggs.  Now they are laying their egg cases on vegetation.  The egg cases look like brown trilobytes - they are oblong with ridges and about 1-2" long.  They are eggs encased in a foamy mass that hardens after it is laid.  The eggs overwinter to hatch in the spring, releasing hundreds of tiny, springy green mantises into the area.  The tiny, thin mantises need to disperse fast, because their siblings pose a significant predation threat.

Surprises of Fall

Fall came so quickly this year.  Today is the first day of fall, but we have had fall weather for the past several weeks.  Spending entire days every week interacting with the earth and plants has made me notice the season's changes much more acutely this year.  Here are the things I've noticed most as the weather has changed:

• The bees and wasps are already gone.  The flowers are still going strong, but that cloud of buzzing has disappeared.  There are still a few slow bumblebees here and there. 
• The spiders are out in force.  Many blooms have their own resident flower spider, and there are lots of webs strung up between the plants.  We had the biggest garden spider I've ever seen in the hoop house. 
  

  

  Garden Spider Source
  

• It feels strange to eat cherry tomatoes when it's cool and cloudy.  The tomatoes taste the same, it's just not as heavenly to pop them in my mouth when I walk by the tomatoes.  Now I want to nibble the turnip leaves.
• The smell of tomatoes rotting in the field is almost intoxicating.  It's difficult to describe why this is so wonderful, but there's a toasty, dusty, cheesy, roasted tomato smell all around the tomato rows from the unusable tomatoes that makes my head spin.  Rotting squash smell great in the field too.  Don't try this at home - it doesn't work without sunshine and dirt.
• The crops are all different now.  Instead of tomatoes, squash and melons, we have turnips (the best vegetable), chard and beets.  It happened so fast.
• The weeds have slowed down a lot, thank goodness.  Even though I can see the scattered crab grass seeds everywhere, and I know what's ahead for next summer, the pressure's backed off a bit for now.
• It's easy to get a LOT done now that it's not 100 degrees.  In the extreme heat, work slows down due to the body's physiological constraints.  These crisp, cool days mean that I can work fast and easily, and everything feels good.
• I only go fill my water bottle once or twice a day now, instead of four or five times.
• Ironically, the work is starting to taper off even as our capacity to do it increases.  Since fewer crops grow during the winter, a lot of the fields are lying fallow, and we have planted cover crops.  Here is the melon field, all disked in and planted with a mixture of vetch, radish and rye for the winter:

  

  This field is done for the year.
  

• The farm is looking more neat and tidy.  With things growing more slowly, there is time to organize and clean up.  June and July felt like a race to keep up with the creeping jungle of crops and weeds, and now it feels like we are getting ahead.

I only have one or two more days to work on the farm, then I'm moving to the heart of Chicago for a while.  I expect the contrast to be a little jarring.  I'll be reporting on what biological phenomena I observe in the city.  In the mean time, I'm savoring the last few hours of fresh air, big skies and working on the earth here in Middle Tennessee.