O Christmas Tree!

What would Christmas be without trees tangled up in tinsel and fairy lights? A range of different trees are grown across the world destined to become bauble-laden Christmas trees, from Douglas firs to Norwegian spruces to Virginia pines. All of these trees belong to the Pine family (Pinacaea) of conifer trees but how long have these festive flora lived on our planet and how did they evolve?

christmas_tree_farm_ia

Christmas Tree farm in Iowa

Our Christmas tree story begins almost 500 million years ago, with the invasion of plants onto land. This is one of the most important events in the evolutionary history of life – before the evolution of land plants, the Earth’s surface would have been a harsh, arid landscape of barren rock. Pioneer plant invaders changed global climates, allowed soils to develop and therefore permitted other lifeforms to evolve and eventually occupy almost every corner of the continents.

The earliest evidence we have for land colonisation comes from fossilised microscopic spores of liverwort plants of Ordovician age (approximately 470 million years old). These primitive plants lack stems or roots and are thought to have evolved from freshwater green algae.

Lepiododendon fossil – a type of lycopsid plant from the Carboniferous period showing characteristic leaf scars

Lepiododendon fossil – a type of lycopsid plant from the Carboniferous period showing characteristic leaf scars

We don’t find any macrofossil plants until the Silurian period, around 425 million years ago. Here, plants began to diversify into different groups such as the lycopods – plants with long thin leaves which leave characteristic scars on the stems. These groups (as well as some early arachnids!) are exceptionally preserved in the early Devonian Rhynie chert formations of Scotland (approx. 410 million years old). These early Devonian plants would not have reached any taller than waist height as they lacked a robust system for water transport.

Reconstruction of Archaeopteris - one of the first trees to evolve. Not quite the Christmas tree we know and love!

Reconstruction of Archaeopteris – one of the first trees to evolve. Not quite the Christmas tree we know and love!

The first group of plants to attain heights of several meters – like our Christmas conifers – were the ferns of the middle Devonian (approx. 360 million years ago); through the evolution of woody tissues to transport water. By the late Devonian, the first tree-like plants and distant ancestors to the gymnosperms (the seed-bearing group to which our festive friends belong) such as Archaeorpteris had also developed woody tissue and could grow to a lofty height of 30 meters.

Our Christmas conifers reproduce via cone-shaped seeds rather than spores, as in ferns today. Before the evolution of the seed, spore-bearing plants such as the lycopods could only inhabit moist, swampy environments as they required moisture to transport their spores through the soil. The evolution of seeds in the late Devonian enabled plants to colonise non-wet land for the first time and enabled trees to form vast forests over the continental landmasses. These early seed plants (known as seed ferns) ranged from trees to small shrubs with woody stems, but still with fern-like foliage.

Our yuletide companions belong to a group of plants called the gymnosperms (literally meaning ‘naked-seed’). They have seeds that develop on the surface of leaves which are often modified to form cones, as seen for example in pines, spruces and firs. These gymnosperms (including the cycads and ginkgos) evolved after the seed ferns in the late Carboniferous period (approx. 319 million years ago) and they began to replace the typical swampy, coal-forming, lycopsid dominant rainforests.

Skipping on 20 million years, we move from the warm swampy Carboniferous and into the cold and dry Permian period and here at last we see the first glimpse of a conifer! Cooler climate adapted conifers, with their drought-resistant needle-shaped leaves, evolved and radiated across the globe throughout the Permian. They became the most dominant land plants throughout most of the Mesozoic and would have provided the main food source for hungry herbivorous dinosaurs.

 Conifer forests of the Mesozoic providing tasty snacks for hungry herbivores.

Conifer forests of the Mesozoic providing tasty snacks for hungry herbivores.

Some primitive conifers like the monkey puzzle tree have remained unchanged since the Triassic era, but our Christmas pines evolved later on in the Cretaceous period. The earliest known pine tree is a 140 million year old twig fossil preserved in charcoal in a Canadian quarry. The fossil, discovered this year in March, shows evidence of ducts which would have carried sticky flammable resins.

Fossil conifer cone of Jurassic age

Fossil conifer cone of Jurassic age

This find suggests that fires shaped the evolution of our festive pine trees. Temperatures and oxygen levels were much higher in the Cretaceous than at present, meaning that wildfires were abundant. The fires were beneficial to the pines and, aided by their flammable resins, would have removed competitors from the forest floor giving new pinecones space to germinate and grow.

So there we have it – our wintery pines actually evolved in a time of prolific wildfires over 140 million years ago, alongside Tyrannosaurids and Iguanodons. As you hang your festive baubles this season and hoover up the supposedly ‘non-drop’ needles, why not have a think about the conifers’ long evolutionary journey?

By Amy Ball, reposted from Geological Society of London blog.

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