Sunday, March 31, 2013

Sycamores, Trees of Wonder

If you were to decide to learn only one tree in your life, I would recommend learning the sycamore (though I don't recommend learning only one tree).
A stately sycamore between our outdoor classroom and playground.

A friend once told me that sycamores are easy to recognize because they are the only tree that wears pants.  A sycamore's upper limbs are white and appear to be uncovered, and its lower trunk is covered with brown, patchy bark - the sycamore's pants.  Sycamore bark consists of three layers: the outer is brown, the middle is greenish and the inner bark is white.  Sometimes you can see all three layers together in a camouflage-like pattern.  The outer two layers peel off of the upper limbs, leaving bone-like white branches that look spectacular against a blue sky.  Scientists aren't sure why the upper bark peels off, but some people think it falls off to prevent vines from being able to grow up into the treetops. 

Sycamores tend to grow near water, and since their white branches are visible from a distance, they are useful for finding water if you are ever lost in the forest.  Early explorers used them to find water sources across North America, but they used sycamores for lots of other things too.  Sycamores are very fast-growing, so they produce a lot of wood.  Though sycamore wood is twisty, it is extremely strong and light, and Native Americans and settlers both used it to make just about everything you can make out of wood.  Before North American forests were logged, most forests contained trees that were a lot older, therefore they were bigger than trees we have now.  Old sycamores tend to be shockingly enormous compared to other trees, and they are often hollow (here's a medium-sized one, and possibly the world's largest), so sometimes they were used by people as a shelter or to corral animals.
A drift of sycamore seeds and a few pieces of fallen sycamore bark.
The sycamore tree at our school is making a mess right now.  While more northern parts of the country still have snowdrifts, here in Middle Tennessee, we get drifts of sycamore seeds.  Sycamores hold their seed balls (technically fruits) up on their branches all winter, but now the seed balls are dispersing their seeds.  Every ball contains hundreds of wind-dispersed seeds that each have a few feathery hairs to catch the wind.  Some of the seed balls break apart while they are on the tree, and the seeds are dispersed from high in the sky.  Many seed balls fall onto the ground as well.  Mostly the seed balls break apart when they fall off the tree.  The seeds are only loosely-held together, so the seed balls usually smash to smithereens on impact with the ground.  If you're lucky, you may find a whole seed ball, which is exceedingly enjoyable to break apart for yourself.
A sycamore seed ball with a few seeds falling out.
 If you lightly crush the seed ball, you can see how the seeds fit together so tightly. 
A lightly-crushed sycamore seed ball.
Once you have completely crushed the seed ball, look for the hard structure inside.  That structure is the base of where the seeds are produced, and it looks like the strangest type of seed or fruit you've ever seen, but it's neither seed nor fruit, just stem.  Many naturalists have been confused when trying to identify these structures when they find them without the surrounding furry sycamore seeds.
A fully-crushed seed ball.
Soon our sycamore will look very different.  It will be covered with giant plate-sized sycamore leaves, but you will still be able to recognize it by its beautiful white branches.

Wednesday, March 20, 2013

Mosses, Lichens and Succession

Life has a way of taking over here on Earth.  Any surface without living things on it will eventually have life growing on it if you wait around long enough.  A new sidewalk of poured cement will have plants growing through cracks after twenty years.  New roofs will eventually become soft and covered with mold, moss and even plants.  Fresh lava from a volcanic eruption will cool, harden and in several decades be covered by a forest (Neat example here).  The invasion and growth of life on nonliving surfaces is called succession, and it's happening right here in our classroom.

Green and grey lichens growing on rock.
The nonliving surfaces we have at the classroom are mostly the rocks.  The big boulders and the flat rocks around the pond are too recently dug from the ground to have life on them yet, but they probably will by the time you graduate from high school.  The rocks with the waterfall behind the pond and the rocks that make up the wall at the back of the classroom have been exposed at the Earth's surface for long enough to have some neat life growing on them. 

Organisms that can colonize bare rock are called pioneer species.  Lichens are usually the first pioneer species, and they look like color splotches on the surface of rocks - white, green, grey, yellow or even orange.  Lichens are actually two organisms for the price of one: a fungus and an alga living together.  The fungus and alga form a mutualism - an interaction where both organisms benefit.  If you remember from the beginning of the year, algae grow in our pond - algae can only live where they don't dry out.  In lichens, they live surrounded by cells of fungus so they can live outside of a pond.  In return for this good protection, the algae provide the fungus with food from doing photosynthesis.  Together, the organisms that form lichens make acids that slowly dissolve the rock on which they grow, which makes tiny crevices in the rocks.
White, green and grey lichens plus dark green mosses growing on a rock.
Once lichens have been growing on rocks for a while, mosses are able to survive there too.  Mosses are plants that don't have flowers or stems or roots - just tiny green leaf-like structures and microscopic hair-like structures.  Mosses send their hair-like structures into the crevices the lichens made in order to anchor themselves on the rock.  Then the mosses grow bigger.  They die back during harsh weather and grow more in good weather.  As they die back, their dead parts decompose in place, and they turn into a tiny bit of soil.  After several years, mosses build up enough soil underneath themselves that other plants can move in.  Mosses can also start to grow in cracks and pockets in rocks.

Just as mosses build habitat for small flowering plants, the flowering plants provide habitat and food for more creatures.  Flowering plants have roots that hold the soil in place, and they also add to the soil as they die back each winter and decompose.  Mosses and plants can host tiny insects, adding to the variety of life growing on a formerly bare rock.  As the years go on, the soil builds and builds and larger plants, shrubs and eventually trees can grow on what was once bare ground.  Eventually a mature forest might be found where once there was bare rock, and succession has been a success.
A rather large moss behind the waterfall.
Next time you are near an older neighborhood or a vacant lot in Nashville, see if you can recognize succession.  Old houses have mossy roofs.  Ancient stone walls are covered in plants with trees growing through them and lizards living between the stones and roots.  Old parking lots or yards are infiltrated with weeds and dotted with butterflies drinking from the weeds' flowers.  You can see the results of succession at the River Campus too.  Most of what is now the wetland used to be an open farm field with only grass - only 15 years ago!  Now it has grown into a young forested wetland with lots of plants and small trees.  Life certainly does take over!

Thursday, March 7, 2013

Turdus migratorius, American Robin

The robins are here!  For such a pretty and lovable bird, they have a very unflattering scientific name: Turdus migratorius.  The migratorius part isn't so bad, and robins are indeed migratory birds.  But why the name Turdus?  Scientific names are in Latin, and 'turdus' means 'thrush' in Latin, which is a bad deal for the robin.  A thrush is a type of bird that is usually small and plump and searches for food on the ground.  Bluebirds and wood thrushes are other common Middle Tennessee thrushes.
A male robin.
Robins are grayish with rusty undersides.  The males' heads are darker than their backs, and their underbellies are usually brighter than females'.  Last week a male and female were getting to know each other at our outdoor classroom. 
A female robin.
 In Nashville, this time of year robins are just finishing up their winter migration.  Robins group together near the end of winter into massive flocks (did you notice them a couple weeks ago?), then they migrate to follow food sources and warm weather.  They may not migrate straight north like most other migratory birds do - they just go wherever life is good for robins, which explains why we have some robins here year-round.  At the end of the winter migration, birds form pairs and begin to find a home range to nest in.  They may come back to the same place as last year, but they may not.
A female robin between the two Japanese quinces.
Robins are generalists in both nesting and in feeding.  They build nests in a variety of habitats, from landscaped yards to meadows to forests.  They eat a variety of insects, worms and fruits, depending on what is available to them to eat.  In the spring and summer, they tend to eat more insects, and in the fall and winter they eat more fruit and berries.  This time of year, you are likely to see the cliché of a robin with a worm dangling from its beak.  Robins can usually find their food and shelter requirements around where humans live, so they tend to be very familiar to us.
A female foraging for ground insects or worms and a male sitting on the fence.
If the male and female I saw at our outdoor classroom become a pair, they will have 2-3 broods of baby robins this spring and summer.  The female will soon start building a cup-shaped nest of three layers: twigs first then mud then grass on the inside.  She will probably lay 3 light blue oblong eggs in the nest a couple of days after it is finished.  The female sits on the nest for 12-14 days, getting up every once in a while to turn the eggs or go get food.  The male might bring her some food or might not.  The female's belly has a patch of skin with extra blood vessels that keep the eggs warm as she sits on them.  If the eggs get too cold, they will die.  When the eggs hatch, the female tosses the egg shells out of the nest and broods (sits) for another 3-4 days.  After that, the hatchlings are able to keep themselves warm enough without being sat upon constantly.

Both parents feed the hatchlings after they escape their eggshells.  For the first few days, the menu is regurgitated food the parents already ate.  And if that isn't gross enough, after that the parents bring soft-bodied insects and worms to feed the poor little birds.  The hatchlings beg and peep like mad for their food, so they must like it.  Begging is an important skill for robin hatchlings, because the most aggressive peeper with the longest neck and widest-open beak will get the most food and is most likely to survive to adulthood. 

At two weeks old, the robins usually fledge (leave the nest).  They still don't fly well or know how to find food, so the parents hop around them on the ground alerting them to danger and bringing them insects and berries.  At first, the mom feeds the fledglings, but then when they are starting to become independent, the dad will feed them, and the mom will go and build a new nest for the next brood.  The fledgeling stage is very dangerous for the birds because the young ones can't fly yet.  They might become the food that a mother hawk brings home to her hatchlings, or they might fall prey to a cat.

There are many types of birds at our outdoor classroom right now.  Next time you go, try to count the different types of birds you see.  Last time I was there, I saw 3 kinds.  Pay special attention to the robins, and see if you can figure out what they are doing when you see them (feeding? gathering nest materials? searching for a good nest site? fighting off other birds? or are they watching you?).   

Friday, March 1, 2013

Hey Buds!

Today felt like winter, but if you read nature instead of reading the thermometer, spring is already here.  Tree buds are among the first things to reveal that winter is over, and many of the buds at our outdoor classroom are already saying spring.  Twigs are the ends of tree or shrub branches, and the caps at the end of twigs are called buds.  Buds are the most exciting things about twigs (and trust me, there are a lot of exciting things about twigs).
Buckeye twig with three buds.  The end bud is starting to open.
Generally, ends of plant parts are very important, because these parts contain the only plant structures that can make new growth.  Parts capable of plant growth are called meristems.  Look at the end of a tree’s twig and you will see a structure called a bud.  Each bud contains a meristem covered with tiny leaf-like things called bud scales.  Scales are the protectors that keep the meristem inside from dying in the freezing cold of winter.  This time of year, the meristem starts to grow, and it pushes the scales aside.  As the meristem grows, it starts to produce either new leaves, stems and twigs or new flowers.   If you’re curious, cut off a swollen bud, slice it in half from top to bottom, and look at the cut surface with a magnifying glass.  You’ll see sliced immature leaves or flower petals.
Large buckeye bud just starting to open.  Notice the bud pushing the bud scales aside.
Plants grow very differently than people do.  People get longer and wider in every area of their bodies as they grow from child to adult.  So your arm in first grade will be both shorter and thinner in all sections, upper and lower arm, hand and fingers, than your arm in the twelfth grade.  Most plants only grow longer at their tips. (Can you guess what plants don’t grow from their tips?)*  Imagine if your body only grew longer at the ends of your toes and fingers – your adult body would look VERY different.  Plants’ stems (including tree trunks) and roots can grow wider at any point, which is why it takes more people to hug around old trees than young trees, but they only grow longer at the tips.  People often think that if they were to carve something into a tree and come back in 20 years, the carving would be very high off the ground.  This is false, since trees only get taller at the ends of their branches.  [Just a reminder, please don’t carve things into trees – tree bark is the plant organ that carries food between the roots and the leaves, and damaging the bark can kill a tree.]

Back to the buds.  Look at several trees and shrubs at the outdoor classroom.  Right now many trees have some buds just starting to open and other buds in their closed-up winter stage.  It’s a great time to compare winter buds and spring buds on the same plant.  Notice the different shapes of bud scales – pointy or rounded, separate or overlapping, striped or not, green or brown.  You might also notice how some twigs become very colorful just as their buds begin to expand.
Rhododendron flower bud
My favorite thing about buds is that bud scales leave scars on the twigs when they fall off.  Bud scale scars look like tiny clustered rings around a twig.  If you look at a twig starting at the tip and move back along the twig, you will encounter several tiny rings in the same place running around the twig like tight bracelets.  Those rings are scars from where last year’s bud scales were, and everything from there to the tip is last year’s new growth!  Look further down the twig toward the main branch and you may find additional years’ of bud scale scars.  You can tell the age of a twig by counting back bands of bud scale scars from the tips to the trunk.
Rhododendron flower bud opening.
*Grass and other grass-like plants grow from meristems that are near the ground.  That’s why we can mow the grass and cut off all the leaf tips but the grass keeps growing and needs to be cut again soon.