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.

Maples in Early Spring

If you live in the temperate eastern United States, and you only know one kind of tree, it's probably going to be a maple.  Everyone knows maples.  People either recognize and love maples' unique, pointy leaves, enjoy maple syrup, admire bright fall maple trees, or played with maple helicopter seeds as kids.  Few people know what maples are up to this time of year, though.

Even without leaves, maples are very busy this time of year.  Look at the ends of the branches on this maple tree below:  there are lumps all along the branches.

Swollen maple buds ready to pop.  Early March.

Those lumps are flower buds.  Many maples flower and fruit before they leaf out.  Here is a closeup of maple flowers:

Mid March, maple flowers.

Maple flowers are pollination generalists.  Some are pollinated by insects and bees, some are wind pollinated, and some are self-pollinated.  From the tree's perspective, it pays to be flexible with pollination strategies if you bloom very early in the growing season, because it's difficult to insure that insects will be out when you're ready to bloom.  Insects are really the best pollinators.  They are great at pollinating over long distances with small amounts of pollen, but they require warmer temperatures to do their work.  Wind pollinates cheaply - you don't have to feed it nectar or a portion of your pollen to get it to carry your pollen to another flower.  But wind isn't very specific in direction, so you usually need to make a lot of pollen if you are using wind (more on this next time!).  Self pollinating is convenient, but let's face it, you don't get much genetic variety if you make kids using only your own genes. 

Either way, lots of pollination has happened, because the maples in Chicago are LOADED with maple fruit.  Notice I called these helicopter things seeds earlier in the post, and now I'm calling them fruit.  I didn't want to alarm you earlier, but here's how this works:  fruits are plant parts that hold seeds.  An apple fruit has seeds in it, and so does a cucumber, and so does a maple fruit.  The maple fruit consists of a wing and a case around the actual seed.  Open up the swollen end of the fruit, and you will find a sticky seed (and you can stick the fruit on your nose or fingers like we did when we were kids).

Maple fruits (samaras) in late March.

Maple fruits are winged, and they are adapted to being carried far away from their parent tree by the wind.  They do indeed work like helicopters - their wing catches the wind and spins them along to hopefully sunnier ground than the ground just under their parent tree (maples are indeed shade trees).  There are many types of fruits out there: berries, capsules, hesperidia, drupes, pepoes, etc.  Fruits with wings are called samaras.  Both maples and ash trees have samaras to carry their seeds away.

New (red!) maple leaves, plus some maple samaras, late March.

Above you can see some new leaves just starting to grow on this maple. I had to look hard to find maple leaves on this type of maple tree - they mostly have only fruit right now.  Below you can see two pictures of early leaf growth on a Japanese maple.  Japanese maples seem to usually leaf out before they set fruit.

Japanese maple leaf buds opened and showing the new expanding leaves, late March.

Slightly older Japanese maple leaves, late March.

Hail, Mary!

What you see here is not only my first picture posted to this blog but also the largest hail I have ever seen.  It fell on my house, and it is officially called, "golf ball-sized hail".  The coin in the picture is a quarter.  The house is fine.  The car has a little cellulite now.  The garden is licking a few wounds, but really OK.

This rather charismatic weather event got me thinking more about weather on the farm.  A co-worker on the farm remarked that she would not like do farming as her primary profession because she didn't want her job to be at the mercy of the weather.  While all are livelihoods are linked to weather at some level (try doing any work in a tornado), she clearly has a point.  Great weather means healthy, productive crops, which in turn means income.  Drought, hail, flood, tornado and hurricane are the dreaded words that mean an entire season's income could be down the drain (or evaporated, as the case may be).  Farmers can buy crop insurance for these unpredictable events, but it is only a safety net, not a satisfying season's income.

Fortune has smiled on the farm this spring, with ample rain, early warmth to start the crops, later cool to work in and no hail or tornadoes.  The crops are growing as fast as the weeds.  The hail that fell on my house thirty miles away would have been disastrous for the farm.  It would have destroyed all the greens (chard, kale and lettuce) and would have seriously stunted the ornamental flowers and tomatoes.  The leaves would have been shot through with holes, like those on the leaves of my zucchini plant at home.  The tomatoes would have been dented badly, with bruises that rot in a day or two.

Hail is a very poorly understood weather phenomenon, but it has a lot in common with tornadoes, which are well-known here in Nashville.  Conditions that create tornadoes are also likely to create hail - the clouds pile high (cumulonimbus clouds), often with thunder and lightning, ominous winds, massive rain and a greenish tint to the sky.

Hail and tornadoes tend to occur on the leading edge of a thunderhead as it enters the area.  A cumulonimbus cloud indicates a tall pile of cool to cold air over an area of warmer air.  Think about it - it's summer and you're standing outside in the heat.  Next a thundercloud comes up and does it's thing.  After the storm, the air that has moved in with the storm is cooler.  A big mixing of air has occurred, usually squeezing the water out of the air as some form of precipitation.  When the temperature difference is larger, for example when it's hot on the ground and freezing in the cloud, the air mixes very fast (tornado), and the precipitation freezes (hail).  If it's cold on at ground level, for example in winter, hail doesn't form because the water in the air doesn't form liquid water droplets that can freeze into solid balls, instead water molecules in the air collide and grow onto crystals of water, forming snow.  We will all probably become more familiar with this process as the Earth warms, since climate change is increasing the severity of storms. 

The strangest thing about precipitation of any form is that it doesn't just automatically fall out of the air when it forms.  We all know that water or ice are denser than air, so they fall to the earth when we drop them.  But they don't fall when there are updrafts blowing them higher up in the clouds or when they are so small they are pulled more strongly by the air molecules than by gravity.  A cumulonimbus cloud is tall because it has updrafts.  If it has updrafts and downdrafts and is very, very cold, a piece of hail can form and grow on the updraft, fall and melt a little, blow back up and grow, etc. until it grows to the size of a dime, a golf ball, or even a grapefruit.  Next time it hails golf balls near you, observe the hail balls and you will probably see layers from the hail freezing differently as it rises and falls in the clouds.  You will probably also notice that the hail is irregular in shape due to multiple hail pieces freezing together somewhere above your head.

Today is the first day of summer.  Precipitation will be less predictable for  the next few months.  I know I'll be paying a lot more attention to the weather than I normally do - you can't help it when you grow plants outside.  Even without checking the rain gauge, you notice the dampness of the soil, the color of the ground, the turbidity of the leaves and stems and the dustiness of the air.  With a little luck, it'll be a good crop.