Saturday, March 29, 2014

More than one way to grow a tree

eventually, redwood seeds really do reach the sky
Think small and picture yourself as a tree seed, but one with big dreams of growing up to reach the sky and the stars. But there are other trees around you, so how will you get up there?

You can be a hare or a tortoise, so to speak.

Some trees have genes that enable them to start growing quickly and grow tall very fast.

They also have genes that allow them to thrive in areas with lots of light and even shade out other trees. These trees live fast and die young.

They are called pioneer species because they are usually the first plants to establish themselves in areas where forest has been cleared, burned, or otherwise opened up, such as in abandoned agricultural areas where native vegetation is growing back.


pioneer species race to
colonize new clearings
Pioneer trees thrive in open areas with lots of light and can even shade out other trees. You can distinguish pioneer tree species by several visible characteristics:

– big wide hat to protect from the sun leaves to capture maximum light;
tough durable blue jeans soft wood, since their emphasis is on growth, not durability;
big beard small seeds that are dispersed by wind (rather than animals) to reach open areas where animals might not for fast dispersal.

Their fast, furious lifestyle makes pioneer species the top dogs of a newly regenerating forest, but eventually they will be replaced by slower-growing, longer-lived species.

The quaking aspen of North America is the epitome of a pioneer species – it is an early colonizer of areas cleared by fire or other disturbance, grows in many soil types, including bare ground, and can be found from the Arctic Circle to Mexico.

Quaking aspen has a thin bark that makes it vulnerable to fire damage; yet fire, ironically, and the sunny open environment it leaves behind are important for the germination of the quaking aspen seeds.

sun-loving aspens are pioneer species: the conifer come in later
photo: Jesse Varner, Wikimedia Commons

Like other pioneer species, quaking aspen needs that sunny environment for its seeds to germinate and loses out once other tree species have caught up (though this may take over 100 years and more than one generation of this short-lived species). In the case of the quaking aspen, various fir, spruce, and pine species eventually shade out and become the dominant trees in western North America.

Similar cycles happen in the tropics. Those of you who have visited Central or South America might recognize the Cecropia - a genus of trees that is the big star of second-growth forest &ndash: one that has been cleared and is regrowing all at once. Like the quaking aspen, Cecropia trees pop up and thrive in direct sunlight, mature early, have fruit (reproduce) at a relatively young age, and live 70-100 years. These are the hares of the tree world.

Fast-growing, big-leaved Cecropia trees on either side of a trail
- bonus points if you can find the monkey jumping across!
those leaves get really big --
this is a single (dead) leaf with many lobes

Old timers

OK, so what about the tortoises of the tree world?

shade-tolerant trees can get huge
The growth genes of some other trees (called shade-tolerant species) are designed not for fast growth, but to allow the tree to maintain itself as a sapling, growing slowly in the dark, low-energy understory of a forest until enough light is available to speed up the process.

The seedlings of shade-tolerant trees stay alive in this shady, low-energy environment, and they grow quickly once an opening occurs in the canopy (when, say, an old tree dies and falls and creates a small gap), but they can’t compete with the pioneers in the high heat and dryness of a large open area, such as a clearcut.

Shade-tolerant trees hang in there by using what little light reaches down to the forest floor, including keeping their leaves for longer times than do the pioneer tree species and using light at the red end of the spectrum, which reaches the understory more than light at shorter wavelengths.

They still need sunlight, but they can photosynthesize with the minimal light that reaches the forest floor, so they can survive long enough to eventually outcompete those fast-growing pioneer species, once the conditions are right.

the base of a Dipteryx tree in Peru: it grows slowly
but steadily to become one of the giants of the rainforest
Given all that waiting, shade-tolerant trees tend to live for hundreds of years and grow very large. Particularly in the tropics, they usually also develop the hard wood that people value for its strength, durability, and beauty, for flooring, furniture, utensils, veneer, and even baseball bats.

In Central and South America, underneath the shade of those fast-growing Cecropia trees wait the seedlings and saplings of Dipteryx trees.

These future giants grow slowly in the shade until the light reaches them through a gap in the canopy created by a fallen tree. They then take advantage of the extra solar energy to shoot up into the canopy, where they will stay and continue to harden and widen for the next few hundred years.

Niche diversity

These different types of trees demonstrate that there is often more than one strategy to evolutionary success (i.e. high biological fitness) if one looks at an ecosystem over time, whereas at any given time, a particular strategy might seem best.

left out to pasture: an isolated Dipteryx tree in a Costa Rican
pasture with a researcher checking a natural cavity
in the trunk that was housing the nest of a great green macaw.
These birds nest in the natural holes of these extremely hard trees,
and until recently, farmers left them because they broke their saws.
(yes, it's me)

It might seem that the fast-growing tree wins here, but fast growth upwards means little growth outwards and less dense wood (so, generally not what you want for furniture).

These trees (such as those of the quaking aspen or the Cecropia) typically have short lives, whereas some of the huge emergent trees (such as the Dipteryx or other valuable woods, such as rosewood, ebony, or mahogany) that may have to wait for a treefall gap and take longer to reach their full height may live for many hundreds of years and thus have many seasons to reproduce.

Dipteryx seeds are so hard, only certain animals can open them (hint: not us!)

The two growth strategies of course work side by side in the forest. In fact, pioneer trees like Cecropia typically create the shady conditions that allow shade-tolerant trees to grow well.

And when a large Dipteryx or other huge emergent tree falls (boom!) and creates a big gap in the canopy, guess who gets another chance to grow and reproduce? The pioneer tree... and the cycle continues.

pioneers and old-timers co-exist:
here in Thailand, tall Dipterocarp trees in the back likely
grew up in the shade of some long-gone pioneer trees,
like the large-leaved Macaranga in the foreground
(sorry, lousy photo)

Thus, one strategy may look advantageous at any given time or in a certain condition (an open, hot, dry area created by a fire or treefall vs closed, dark, moist intact rain forest), but each of these trees has its ecological niche.

And each of these niches has proven to be successful – they are still with us today!

even surrounded by cloud forest, this huge old shade-tolerant
tree dominates the Monteverde, Costa Rica skyline

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