Nearly 400 Million Years of Forests

[lpetrich]

Chris Scotese PALEOMAP Home Page - for illustrating Earth History and Climate History reconstructions.

I decided to look at the coal deposits, evidence of forests.

While the average temperature of the Earth has varied a sizable amount, it's mostly variation in high-latitude temperatures. In the Late Cretaceous, one of the warmest periods, the polar regions had a cool-temperate climate, while nowadays, they are freezing cold.

Our planet has had forests since the mid-Devonian, about 380 million years ago. Both tropical forests and temperate-zone forests.

Surprisingly complex community discovered in the mid-Devonian fossil forest at Gilboa | Nature - "The origin of trees by the mid-Devonian epoch (398–385 million years ago) signals a major change in terrestrial ecosystems with potential long-term consequences including increased weathering, drop in atmospheric CO2, modified climate, changes in sedimentation patterns and mass extinction."

The earliest known tree is a fernlike one called  Wattieza that lived in the mid-Devonian. It grew to 8 m / 26 ft tall.

The earliest known land plant is  Cooksonia, starting at about 430 million years ago, in the Wenlock series in the Silurian. Cooksonia plants were mosslike, growing, only a few cm tall. By the early Devonian, about 400 million years ago, plants grew up to around 1 m tall.

 

[lpetrich]

The first trees were ferns, horsetails, and lycopods - rather primitive plants. Primitive enough to need wet ground for reproduction.

How that happens I must explain in more detail. A fundamental aspect of eukaryote biology is alternation of generations.

Haploid phase: X
Cell fusion: X + X -> XX
Diploid phase: XX
Meiosis: XX -> X + X
Haploid phase again

Each phase may be able to do mitosis -- "normal" asexual cell division:
Haploid: X -> XX -> X + X
Diploid: XX -> XXXX -> XX + XX

Meiosis works much like diploid mitosis, but with an extra division stage:
Meiosis: XX -> XXXX -> XX + XX -> X + X + X + X

Organisms can have sexual reproduction only in the haploid phase (many fungi), only in the diploid phase (many animals), and in both phases (plants).


Plants practice alternation of generations, between a multicellular haploid phase, the gametophyte (it makes gametes, sex cells), and a multicellular diploid phase, the sporophyte (it makes spores).

In mosses, the gametophyte is the most prominent part, with the sporophyte growing on top. In ferns, horsetails, and lycophytes, the gametophyte is a small blob living on the ground. In these plants, the egg cells stay inside the female gametophytes and the male gametophytes release sperm cells which swim to the egg cells. That is why these plants need moist ground to reproduce.

Once a sperm meets an egg, they make a sporophyte. In ferns, horsetails, and lycopods, the sporophyte is the most prominent part of the plant - it's what one sees unless one looks closely at the ground.

Sporophytes make spores, and when they reach suitable ground, they make gametophytes, completing the cycle.

 

[lpetrich]

I now turn to seed plants.

• Pteridosperms or "seed ferns", named from their fernlike leaves, Late Devonian, around 350 Mya
• Cycads, which look like palm trees, Early Permian 280 Mya, or possible late Carboniferous, 300-325 Mya
• Ginkgo, around the P-Tr mass extinction, around 250 Mya
• Conifers, Late Carboniferous, around 300 Mya
• Gnetophytes, Permian, around 280 Mya
• Angiosperms ("encased seed") or flowering plants, the Late Jurassic, around 130 Mya

Non-angiosperm seed plants are gymnosperms ("naked seed").

Glossopteris was a seed fern that lived in the Permian, and that went extinct in the P-Tr mass extinction. Some of them grew as much as 30 meters tall.

Cycads have palm-like or fern-like leaves that are something like seed-fern leaves.

Conifers are mostly trees, and they became much more common after the P-Tr event. They were the main kind of tree for most of the Mesozoic, and they are still common in some places today. Some present-day families, like Araucariaceae and Cupressaceae, are known from the Mesozoic.

Gnetophytes include trees in genus Gnetum, some tropical evergreen trees.

 

[lpetrich]

The early history of angiosperms is very obscure, but they started becoming prominent in the Late Jurassic, and angiosperm trees displaced conifers over the Cretaceous.

Most angiosperm trees are broadleaf ones: eudicots (Fagales, Rosales, etc.) and magnoliids. There are some monocot trees, like palm trees and bamboo. I've searched for details on their fossil records, with only limited success. But some of these trees do indeed go back to the Mesozoic -- Late Cretaceous and earlier.


Forest fires go back a long way, well into the Paleozoic. At scholar.google.com I found several hits for paleowildfire.

Fossil charcoal, its recognition and palaeoatmospheric significance - ScienceDirect

As we have more or less continuous fossil evidence of forest trees from the Late Devonian onwards, and a similarly sustained record of fossil charcoal from that time to the present (Cope, 1984), this constraints oxygen levels between 13% and 35% over that period (Rabash and Langford, 1968; Watson et al., 1978).

Fire as an evolutionary pressure shaping plant traits - ScienceDirect

However, fire has been a factor throughout the history of land-plant evolution and is not strictly a Neogene phenomenon. Mesozoic fossils show evidence of fire-adaptive traits and, in some lineages, these might have persisted to the present as fire adaptations.

A 350‐million‐year legacy of fire adaptation among conifers - He - 2016 - Journal of Ecology - Wiley Online Library

Synthesis. Coupled with strong evidence for frequent fire throughout the Permian‐Carboniferous and fossil evidence for other fire‐related traits, we conclude that many early conifers were serotinous in response to intense crown fires, indicating that fire may have had a major impact on the evolution of plant traits as far back as 350 Ma.

Fire-adapted Gondwanan Angiosperm floras evolved in the Cretaceous | BMC Evolutionary Biology | Full Text

Fires have been widespread over the last 250 million years, peaking 60−125 million years ago (Ma), and might therefore have played a key role in the evolution of Angiosperms. ...

Focusing on the widespread 113-million-year-old family Proteaceae, fireproneness among Gondwanan Angiosperm floras can now be traced back almost 90 million years into the fiery Cretaceous. The associated evolution of on-plant (serotiny) and soil seed storage, and later ant dispersal, affirms them as ancient adaptations to fire among flowering plants.

Frontiers | The impact of fire on the Late Paleozoic Earth system | Plant Science

Analyses of bulk petrographic data indicate that during the Late Paleozoic wildfires were more prevalent than at present. We propose that the development of fire systems through this interval was controlled predominantly by the elevated atmospheric oxygen concentration (p(O2)) that mass balance models predict prevailed.

 

[fromderinside]

Three cheers for fire ecology and the overthrow of Rooseveltian forest service active fire fighting policy and of not cutting brush near right-of-ways in the thirties. Turns out fire clearing of small brush under cedars produced very healthy forests even reduced damage by fire to forests. Nothing worse than a fire crowning because of juvenile plant growth.

This was all proved in the thirties by southern farmer-hunter scofflaws with their annual undergrowth burning practices. Helped hunting as well since it encouraged new growth during the right times of the year. As for fire fighting in places where humans don't reside much, say yellowstone, letting fires burn results in very healthy forests twenty years on with greater game and productivity of other stuff.

Now in Oregon and probably other places state services keep dead branches and fire prone woods well away from right of ways where negligent cigarette and shiny glass and can stuff castings from cars are causes of most forest fires. That we don't have major restrictions on spark causing RVs on trails is a sad give away to the guzzling unthinking freedom-for-me classes.

 

[lpetrich]

Yes, a good way to prevent big forest fires is to make lots of small ones.

But that may be a hard sell for many property owners. They won't enjoy having vegetation fires near their homes and other buildings. So a good strategy would be to work out how to harden buildings against fire.

Hardening Your Home – Ready for Wildfire
Preparing Your Home - Fire in California
Building a fire resilient home – Harden Up - Protecting Queensland


Mentions cycads as present-day seed ferns: Cycadophyta

Seed-plant highest-level phylogeny has been hard to resolve. There are six main groups of them: seed ferns (all extinct), cycads, ginkgo, conifers, gnetophytes, and angiosperms. Seed ferns are not a very well-defined group, but I won't get into how they should be split up. Of the present-day ones, leaving out the gnetophytes gives

(angiosperms, acrogynmosperms: (cycads, ginkgo, conifers))

I've seen ((cycads, ginkgo), conifers)

The gnetophytes fall into 3 genera: Gnetum spp., Ephedra spp., and Welwitschia mirabilis, a very weird one. They have phylogeny (Ephedra, (Gnetum, Welwitschia)), but their closest relatives among the other seed plants has been a source of controversy. Here are the hypotheses that I've seen:

• All other present-day seed plants
• Angiosperms - anthophyte hypothesis
• Conifers - gnetifer hypothesis
• Pinaceae - gnepine hypothesis
• Cupressaceae - gnecup hypothesis

Conifers are (Pinaceae, Cupressaceae + other taxa)

Cupressaceae contains cypress trees and the giant redwoods. Pinaceae contains the "Christmas tree" trees: pine, spruce, fir

Why gnetophytes jump around a lot is something that I haven't seen a good hypothesis for.

A possible hypothesis is incomplete lineage sorting. A gene duplication is often undone by one of the pair of genes dropping out, though that can take some time. In the meantime, the descendants of the gene-duplicated organism may split off into several species, and these species may have different gene dropouts.

 

[lpetrich]

The Fantastically Strange Origin of Most Coal on Earth

Huge forests of fernlike trees that could grow as much as 50 meters / 160 feet tall, and that did not decompose very much when they died. Dead tree after dead tree after dead tree piled up on each other, and they became heated and squeezed, driving off their volatile components. That left behind their carbon, and the resulting carbon-containing rocks are coal.

 Carboniferous, The Carboniferous Period at UCMP Berkeley -- some 360 to 300 million years ago.

About 90% of our planet's coal was deposited during then, with coal deposition slowing down due to the evolution of lignin digestion. This enabled microbes to decompose dead trees, thus keeping them from making coal.

The first forests | Accumulating Glitches | Learn Science at Scitable - more technical

 

[lpetrich]

 Fossil record of fire

The fossil record of fire first appears with the establishment of a land-based flora in the Middle Ordovician period, 470 million years ago,[1] permitting the accumulation of oxygen in the atmosphere as never before, as the new hordes of land plants pumped it out as a waste product. When this concentration rose above 13%, it permitted the possibility of wildfire.[2] Wildfire is first recorded in the Late Silurian fossil record, 420 million years ago, by fossils of charcoalified plants.[3][4] Apart from a controversial gap in the Late Devonian, charcoal is present ever since.[4] The level of atmospheric oxygen is closely related to the prevalence of charcoal: clearly oxygen is the key factor in the abundance of wildfire.[5] Fire also became more abundant when grasses radiated and became the dominant component of many ecosystems, around 6 to 7 million years ago;[6] this kindling provided tinder which allowed for the more rapid spread of fire.[5] These widespread fires may have initiated a positive feedback process, whereby they produced a warmer, drier climate more conducive to fire.[5]

The earliest land plants needed conditions moist enough for their sperm cells to swim through the ground to reach their egg cells. Mosses and ferns and similar sorts of plants still have that. If they needed that, then how would they have gotten dry enough to burn? The answer is simple: alternation of wet seasons and dry seasons. The plants grow in wet seasons then dry out in dry seasons.

Seed plants avoid sperm cells swimming through ground by the male reproductive parts making pollen. When a pollen grain arrives at a female reproductive part, it either makes a sperm cell or a pollen tube. Among present-day plants, cycads make sperm cells, and most others make pollen tubes. The sperm cell then swims inside this female reproductive part to an egg cell, or else a pollen tube grows inside this female reproductive part until it reaches an egg cell. Thus fertilizing that egg cell. It then grows into an embryo and that embryo is released in a seed.

This enabled seed plants to live in places are are dry all year round, though not completely dry.

Fires among the low, scrubby, wetland plants of the Silurian can only have been limited in scope. Not until the forests of the Middle Devonian could large-scale wildfires really gain a foothold.[8] Fires really took off in the high-oxygen, high-biomass period of the Carboniferous, where the coal-forming forests frequently burned; the coal that is the fossilised remains of those trees may contain as much as 10-20% charcoal by volume. These represent fires which may have had approximately a 100-year repeat cycle.[8]

At the end of the Permian, oxygen levels plummeted, and fires became less common.[8] In the early Triassic, after the 'mother of all extinctions' at the end of the Permian, there is an enigmatic coal gap, suggesting a very low biomass;[11] this is accompanied by a paucity of charcoal throughout the entire Triassic period.[8]

Fires again become significant in the late Jurassic through the Cretaceous. They are especially useful as charcoalified flowers provide a key piece of evidence for tracking the origin of the angiosperm lineage.[8] Contrary to popular perception, there is no evidence of a global inferno at the end of the Cretaceous, when the dinosaurs became extinct; the record of fire after this point is somewhat sparse until the advent of human intervention around half a million years ago, although this may be biased by a lack of investigations from this period.[8]

So the big forests of the Carboniferous had big forest fires.