Take a walk in a winter forest and you can’t help but notice beech trees. Silky smooth bark and sand-colored dry leaves stick out like Christmas lights against a dull and gloomy background. While every other leaf drifted to the forest floor months ago, beech leaves hold tight like cat hair on a sweater.
It’s called marcescence – these leaves that just won’t drop – and it’s common in oak and beech (the trees are close relatives). But why keep the leaves? Are these trees just photosynthetic versions of hoarders?
One possible reason may be to protect that bud, the thin tapered structure often described as “cigar-shaped.” Inside the scaly covering are the beginnings of the new year’s growth. Hungry deer can ruin a tree’s plans for spring. But with beech trees, deer tend to get a mouthful of dry leaves whenever aiming for a yummy bud. (1)
What about attacks from smaller enemies? Insects seem to prefer infesting trees with leaves hanging on over winter. R. Karban decided to yank all the leaves off a few dozen small oaks and compare infestation levels of a tree-noshing wasp. (2) His numbers indicate that wasps prefer leaf-hoarding trees three-to-one compared to his denuded ones.
I believe Nature is constantly sending messages of wisdom if we’ll just listen. In this case, perhaps she’s saying “every action has an upside and downside, but with diversity, there’s always hope for a better future.”
- Svendsen, Claus R. 2001. Effects of marcescent leaves on winter browsing by large herbivores in northern temperate deciduous forests. Alces 37(2): 475-482.
- Karban, R. 2007. Deciduous leaf drop reduces insect herbivory. Oecologia. 153: 81-88.
The bane of many a Southerner’s existence is springtime pollen. All that yellow dust swirling on the breeze and coating your car, that’s pine tree sperm.
The male cones of a Loblolly Pine (Pinus taeda) look like a bunch of tiny bananas growing from twig tips. If you’re thinking, “wait, that’s not a cone,” the woody cone we use to hot glue decorative wreaths or smear with peanut butter for DIY bird feeders is the female cone. Its spirals of woody shingles (or bracts) protect the tree’s eggs and, after fertilization, the developing pine embryos inside.
Male cones are much smaller and shorter lived. They release pollen for a couple of weeks each spring. And it’s a LOT of pollen: 3-5 pounds per tree. Why so much? Pines transfer pollen from male to female cones by wind. It’s not a very efficient system. More pollen increases the chance of fertilization.
With Climate Change, pollen’s gonna get worse. Ladeau and Clark (2006) found that pines growing in an elevated CO2 environment produce more pollen cones, and more pollen, at younger ages.
p.s. If you ever wondered what a pine pollen grain looks like, it’s a microscopic Mickey Mouse logo!
Ladeau SL, Clark JS. 2006. Pollen production by Pinus taeda growing in elevated atmospheric CO2. Functional Ecology. 20(3) 541-547.
Every autumn, vibrant leaves float down from the tree tops to stitch a patchwork quilt resting on the forest floor.
Over time, leaves are broken down by fungi, bacteria and other detrivores (organisms that eat dead stuff) like earthworms. The superpower of the earthworm (Lumbricus terrestris) is its ability to compost vegetation and return vital nutrients to the soil.
Charles Darwin was fascinated by earthworms, conducting wonderful experiments to determine how much soil they moved and whether worms preferred to collect leaves from the broad end or pointed end. Worms pull leaves into their burrows (narrow end first) to plug the opening and protect themselves from ‘early birds’.
In much of North America, earthworms were killed off in the last ice age (~10,000 years ago). The worms you see now in Michigan, Maine and Minnesota are all invaders. Sounds great, right? More nutrients in the soil? Unfortunately, the northern hardwood forest ecosystem is adapted to a thicker leaf litter layer and slower release of nutrients. So now, the introduction of the earthworm changes which seeds can germinate (and which trees will continue to survive), nutrient run-off, and which animals live in the new, de-littered forest. (1)
If you’re interested in appreciating the awesome recycling power of the worm, check out a delightful little book by Amy Stewart – “The Earth Moved: On the Remarkable Achievements of Earthworms”.
Want to read more specifics about leaf-earthworm experiments? Natural History Magazine has an entertaining write-up.
- Hale CM et al. 2008. Exotic earthworm effects on hardword forest floor, nutrient availability and native plants a mesochosm study. Oecologia. 155:509-518.
Branches of the Live Oak (Quercus virginiana) loop and twist their way toward openings in the forest canopy. Many branches sag down to the ground before stretching back up again.
These low branches help the oak survive in the hurricane-prone regions of the Southeastern U.S. Short, wide trees resist strong winds better than tall, thin ones. Those curvy branches helped the USS Constitution stay afloat during the War of 1812 too. Live Oak limbs were frequently used in ship building due to their natural bends, strength and density.
Too young to vote in the November 4th elections? You can vote for your favorite leaf color!
Already voted for the politicians? Now vote for the trees!
The winner will be the subject of next week’s post. Have fun!