Monday, December 24, 2007

How trees changed the world

Finding: A nearly completly preserved tree dating 385 million years ago from the Gilboa Forests was found. It came from the first forests on Earth.

The Gilboa tree dates from the middle of the Devonian period (416 to 359 million years ago), a time of explosive evolutionary action among land plants. During this period trees evolved from small, primitive forms that would have barely brushed your ankle into genuine trees up to 30 metres tall. And with the evolution of trees, they and all the other plants - hitherto confined to marshy environments - went on to conquer the surface of the planet.

The First Forests
These first forests changed the face of the Earth. Early land plants had already started leaking oxygen into the atmosphere, creating soils and providing food and shelter for animals, and the evolution of trees upped the pace of change. They weathered rocks, made soils deeper and richer, created complex habitats and changed the climate beyond recognition. By the end of the Devonian, an ecologically modern world had appeared. Discoveries such as the Gilboa tree are bringing stunning new insights into how the foresting of the world came about.

Plants in the Ordovician Period
Plants first colonised land in the Ordovician period, around 465 million years ago (see Chart). By the early Devonian they had developed many of the features of modern plants, including a protective waxy cuticle, vascular plumbing for transporting water and nutrients, and pores called stomata to draw in carbon dioxide. But there were many differences from modern plants too. Seeds had yet to evolve - early land plants instead reproduced by way of spores. Wood, large leaves and deep roots were unknown. Few plants were taller than a few centimetres.

The Rhynie ecosystem
The best place to catch a glimpse of this primitive terrestrial forest is in the hills around the village of Rhynie in Aberdeenshire, UK. Here the finely crystallized quartz of the Rhynie chert preserves in extraordinary detail an entire ecosystem that was engulfed and petrified by silica-rich waters from a volcanic spring 410 million years ago. The fossil plants still stand upright, and even their cells remain visible. Tiny creatures such as insects, centipedes, mites, harvestmen and spider-like trigonotarbids are preserved in great detail. Some still cling to the stems on which they lived and died.

Originally, Rhynie wouldn't have been recognisable as a modern environment. Every plant would have been on a small scale - knee height and lower. However, by the late Devonian, plants are on a scale that humans are used to. You can walk around a woodland and identify a tree canopy layer, a shrub layer and a herbaceous layer. Now the whole environment looks more modern.

The Rhynie landscape was not entirely devoid of large living things, however. Dotted here and there were featureless columns standing up to 6 metres high and a metre wide at the base. When first described in 1857, from fossils found in Quebec, they were identified as conifer trunks and named Prototaxites. However, studies into the structure of their "wood" soon revealed that they were not trees. Various alternatives were proposed, including giant algae, lichen and fungi, but the identity of Prototaxites remained uncertain.

Last year a research team University of Chicago decided to settle matters. They measured the ratio of different carbon isotopes in Prototaxites fossils to reveal whether they were making a living from photosynthesis or by eating rotting matter as fungi do today. The results clearly showed that Prototaxites was a fungus. It was both tougher and stronger

And there was more. The isotopes also revealed that some Prototaxites were probably living off microbial crusts composed of bacteria, algae and lichens. These crusts still exist today but are confined to places where vascular plants can't grow, such as deserts. What Prototaxites shows is that there were large patches of the early Devonian landscape with no vascular plants. This means that plants didn't conquer the land as quickly and completely as scientists once assumed.

The Rise of the Forests
With large patches of land still free of vascular plants, clearly an upgrade of their plumbing and support systems was needed for the terrestrial conquest to continue. This was achieved through "secondary growth", the evolution of tougher and stronger tissues to carry water and nutrients up longer stems. Once these were in place, plants were able to grow much bigger. It was these advances that allowed the trees of the Gilboa forest to reach their full height of 8 meters or more. The foresting of the Earth had begun.

The appearance of trees changed the rules of the game, with evolutionary scramble for height, for light to power photosynthesis, and for prime positions to disperse spores. Once it became possible to be a tree, then the race is on for size and dominance. If you look at modern ecosystems, the trees that are dominant hog the light space of the environment. The goal is to be at the top and collect the most light.

The Gilboa Forest
So what was the Gilboa forest like? Its trees, known as cladoxylopsids, looked a bit like tree ferns, although they are not related. Their trunks were long and slender, with a bulbous base and shallow roots. They did not have leaves, instead sporting a goblet-shaped crown of branches and thread-like branchlets. These were probably green to carry out photosynthesis. In other words it must have looked like a strange, giant bottle brush.

Gilboa, 385 million years ago, was a warm, wet flood plain 10 degrees south of the equator. Since the cladoxylopsids lacked leaves, the forest would have been airier and brighter than any modern forest. There was a diverse understorey that included club mosses and ferns, but the only animal inhabitants were arthropods, including insects, centipedes and mites. It would also have been wet underfoot: as with today's ferns and mosses, cladoxylopsid spores could only be fertilised on wet surfaces, so the trees were confined to flood plains.

World Wide Representation
Cladoxylopsids achieved worldwide success: fossils of their crowns (which until the Gilboa tree were thought to be complete plants) have been found in Europe, China and the Americas. However, in many respects they were primitive, and lurking in their shadows was a group of plants that would soon put them in the shade.

Other Plant Developments
These were the archaeopterids, relatives of the conifers. At the time of the Gilboa forest, archaeopterids were no bigger than shrubs, but their lineage soon made some key evolutionary advances, including wood, deep roots and large leaves. By 370 million years ago, a fully fledged tree, Archaeopteris, had emerged from the archaeopterid ranks.

With the trunk of a conifer and fern-like leaves, Archaeopteris reached 30 metres - as tall as a mature oak - and dominated late Devonian forests all over the world. Its wooden trunk permitted it to grow much taller than the cladoxylopsids. There is a stronger material around, per mass, than wood says a leading researcher in the chemical make-up of fossil plants. Wood also led to the formation of the first complex soils. Soil humus is, by and large, lignin, the polymer that makes wood tough.

The Decline of CO2 Levels and the Rise of Deep Roots
Archaeopteris was also the first plant to evolve deep roots. Roots eat away at rocks, burrowing into and dissolving them with acids in pursuit of nutrients. Over an immense period of time the weathered material gets washed into the oceans, where it combines with dissolved CO2 to form sediments that are eventually subducted into the Earth's interior by tectonic activity. This process removed huge amounts of CO2 from the oceans and atmosphere, with profound consequences for the climate. Between the beginning and end of the Devonian, levels of the gas plummeted by up to 95 per cent. Greenhouse conditions vanished, to be replaced by an ice age that at its peak 300 million years ago saw glaciers approaching the tropics.

Deep Roots and Large Leaves
But oddly it was the climatic upheaval brought about by roots that appears to have driven the next great innovation - large leaves. These first appeared 390 million years ago, but only became widespread with Archaeopteris 15 million years later. The stripping of CO2 from the Devonian atmosphere helped to remove an obstacle that had been inhibiting the evolution of large leaves.

Although large, flat leaves are very efficient at capturing sunlight for photosynthesis, they are difficult to keep cool. To prevent overheating, leaves need to release water vapour through their pore-like stomata - the plant equivalent of sweating. The problem is the number of stomata is regulated by a genetic switch that responds to CO2 levels in the atmosphere: the more CO2, the lower the stomatal density. If that genetic switch was already in place in the Devonian, high CO2 levels would have prevented plants from evolving large leaves. If large leaves had appeared then they would have cooked.

Only with falling levels of CO2, and improvements in roots and vascular systems to supply cooling water, could plants evolve the high stomatal densities that make large leaves viable. Studies on fossil leaves have so far supported this idea: earlier leaves were smaller and had far fewer pores than later ones, and only after CO2 levels fell did large leaves become abundant.

The Rise of Seeds
Archaeopteris had one more innovation to offer. It evolved a method of reproduction that partially freed trees from the flood plains that had confined the cladoxylopsids. Male and female cladoxylopsid spores were the same size, and fertilisation could only occur on wet ground where nutrients were readily available to nourish the embryo. In contrast, female Archaeopteris spores were larger than male ones and stored a food supply for the embryo. From this beginning, seeds evolved.

Seed plants - the grasses, flowers, shrubs and trees that are everywhere today - are thought to have descended from Archaeopteris and its relatives. With the evolution of seeds, plants could now spread to all sorts of places that had previously been out of bounds. Seeding freed plants from a reproductive necessity on water. Seeds allow trees to occupy and colonise drier environments.

Where Do Trees Colonise?
One of the most difficult places for trees to colonise was the uplands. In 2003, scientists discovered the oldest-known fossils of mountain plants, in Blanche Brook, a remote river in Newfoundland. Hundreds of giant trees lying in the a remote part of the river bed. The trees turned out to be 305 million years old, from the late Carboniferous. Known as cordaitaleans and standing up to 50 metres tall, they were seed plants related to conifers and looked a bit like monkey puzzle trees.

What about the Carboniferous Sporing vs Seeds?
Elsewhere in the Carboniferous, however, sporing plants still held sway. In the swampy lowland rainforests, Archaeopteris and the cladoxylopsids were overshadowed by giant club moss and horsetail trees that towered above the seed plants jostling for space below. In modern forests the opposite is true, with seed-bearing trees dominating the sporing ferns, mosses and horsetails of the understorey. It's like the world was turned upside down. Spores prevailed over seeds for the last time.

Organic Carbon and Coal
These late Carboniferous lowlands are noted for the sheer fecundity of their tree and plant fossils. Much of Europe, Asia and North America were near the equator and were blanketed by huge tracts of rainforest. Tectonic forces meant that the basins where the rainforests grew were slowly subsiding. This process, coupled with regular flooding, led to colossal amounts of organic carbon being buried. Much of the world's coal reserves formed in this 20-million-year period.

Then around 300 million years ago, a catastrophic earthquake caused one coal forest in what is now Illinois to slump below sea level, where it was rapidly buried. Low-oxygen conditions preserved a 1000-hectare expanse of this forest floor in near-pristine condition. The forest floor now forms the ceiling of the Riola and Vermilion Grove coal mines in Illinois.

The Ancient Rainforest
Scientists conducted the largest ever study of this ancient forest. The coal that had been mined out used to be the soil of the ancient rainforest, so as you walked around looking up you could see roots hanging down just above your head, and you could see giant fallen trees complete with their roots, trunk and crown.

You can see the roots of an ancient rainforest hanging down just above your head.
Spectacular 40-metre club mosses and shorter tree ferns monopolised the Illinois forest. There is really nothing today that looks anything like the giant club mosses. They probably grew much closer together than do trees in a modern rainforest because they don't have large canopies, so you don't have a lot of shadow cast by the trees. The forest would also have been greener than its modern counterpart, as the abundant club mosses had green scale-like leaf cushions all over their trunks and branches.

Swampworld and Tectonic Forces
Swampworld was not to last. As tectonic forces dragged the world's landmasses together to form the supercontinent Pangaea, the climate changed. Dryer, harsher conditions set in, and by the end of the Carboniferous the coal swamps had disappeared from most of the world. The mighty sporing trees could no longer find the water they needed to reproduce, and seed plants gained the upper hand. The reign of modern plants had truly begun.

1 comment:

Debbie said...

Absolutely fascinating, and in its own way, poetic. The relationship today between man and the trees that created the environment that allowed man to develop is really sacred, if that is the right word. To see mankind now decimating today's rain forests is tragic. There is a quotation from the Baha'i Writings which states: "O moving form of dust! I desire communion with thee, but thou wouldst put no trust in Me. The sword of thy rebellion hath felled the tree of thy hope.. . While there is yet time, return, and lose not thy chance."
Thank you for this illuminating article.

Deborah Eckert, Antigua, West Indies