Present-day Climates Geography: Present, Future and Past Plants (Part 1): ‘Greening of the Land’...

Present-day Climates

Geography: Present, Future and Past

Plants (Part 1):‘Greening of the Land’ - Origins, Diversification

Plants (Part 2):‘Icehouse World’ - Carboniferous and Permian‘Hothouse World’ - Jurassic and Cretaceous

PLANT DIVERSITY CLASSALLISTER REES

BIOMES AS ORIGINALLY MAPPED BY WALTER & LIETH, 1967

Elevation >1000m

GlacialArcticCold TemperateMid Latitude DesertCool TemperateWarm TemperateWinterwetSubtropical DesertTropical SummerwetTropical Everwet

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PLANTS (PART 1)

THE GREENING OF THE LAND

Late Silurian (ca 420 Ma) onwards

Do I really want to do this?

Without land plants, there would have been no food for animals and therefore no terrestrial animal life (that includes us – just try and picture yourself in a world without plants….)

Without vegetation, the surface sediments produced by rock weathering would have been highly mobile

River flow would have been highly variable, and flash floods common, producing braided rivers

Wind and water would have transported the loose sediments to the oceans in the absence of rooted vegetation or soils

Vegetation helped bind the sediments together, so rivers flowed more gradually (‘meandered’) to the oceans

Vegetation also produced soils, promoting even more plant growth and expansion across previously barren landscapes

THREE PROBLEMS FACED BY PLANTS COLONIZING LAND:

1. The fight against gravity

2. Bodily fluids

3. Reproduction

1. THE FIGHT AGAINST GRAVITY

Rigid support structures needed, using energy + resources

2. BODILY FLUIDS

Fluid distribution (vascular) system needed to transport water + minerals from the soil

Also need a barrier to prevent water loss (the cuticle) - but this also limits capacity to store water directly from the atmosphere

So, also need roots (these also provide anchorage!)

3. REPRODUCTION

Need to disperse offspring away from the parent plant to prevent overcrowding

Also need to protect the offspring from drying out

Without land plants, the oxygen content of the atmosphere would be much lower than it is today….

(The reverse reaction is used by animals)

…. so much lower that we wouldn’t have enough oxygen to breathe! We all talk about ‘air’, but what we really need is oxygen.

Late Silurian (ca 420 Ma)

Small plants (ca 1cm tall) with simple pore openings in the surface barrier

These allow gas exchange but no regulation of water loss

(Spongeophyton)

Some plants grew a little taller (but still only up to ca 10cm) and had ‘pouches’ at the tips of their stems, containing spores which could be released and spread by wind

(Sporogenites, ca 2cm tall) (Cooksonia, ca 7cm tall)

Taller plants (ca 50-100cm) with increased spore dispersal abilities

Early Devonian (ca 410 Ma)

BUT at a greater cost of construction, requiring more photosynthesis

(Sawdonia, ca 90cm tall)

(Psilophyton, ca 50cm tall)

Although still small, pretty soon plants were developing:

1) greater structural strength

2) a plumbing system to move fluids up (water) and down (sugars produced by photosynthesis) the plant body

3) small leaves to increase gas exchange with the atmosphere

(Aglaeophyton, ca 50cm tall)

(Asteroxylon, ca 30cm tall) (Pertica, ca 1m tall)

(~ small leaf)

(~ surface ‘roots’)

By this time, large trees with woody stems, branches and leaves had developed

They’d also evolved to solve the leaf gas exchange/water loss problem, by developing stomata

They became so efficient that they contributed greatly to reducing atmospheric CO2 levels

Late Devonian (ca 370 Ma)

(Archaeopteris, ca 30m tall)

stoma

guard cells

When guard cells have a lot of water in them, their curvature increases, so a hole opens up between them (the stoma)

With only little water in them, they are straighter and the stoma closes

(false-color SEM of leaf surface of the tobacco plant, Nicotiana tabacum)

However, this increased efficiency created some new problems….

Because plants had contributed so much to decreasing atmospheric CO2, they were no longer just bathed in it, so had to evolve by increasing the amount of stomata

BUT….

This led to greater water loss, so more efficient root and stem systems were needed to supply more water to the leaves

Single conducting (vascular) strand (Early Devonian)

Multiple strands (Carboniferous)

(Rhynia) (Psaronius)

(wat

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ansp

ort c

ells

)

Development of roots

small simple stems

stomata + small ‘stem leaves’

main stem + branches

wood + bark (small trees)

large trees + large leaves

CO2 Level

Time (Ma)

Carbon cycle (1): no vegetation

Carbon cycle (2): vegetation

Estimates of CO2 through time, relative to present day levels

A (very) brief summary of major innovations….

Estimates of plant species diversity through time

(ferns)

(seed plants - conifers, cycads, etc)

(flowering plants)

PLANTS (PART 2)

ICEHOUSE AND HOTHOUSE WORLDS

Carboniferous through Cretaceous

‘ICEHOUSE WORLD’

Glossopteris

Alfred Wegener

Continental Drift

Locations of certain fossil plants and animals on present-day, widely separated continents would form definite patterns, if the continents are rejoined.

‘HOTHOUSE WORLD’

Late JurassicVolgian (150 Ma)

JURASSIC – AN ANTARCTIC CASE STUDY

CRETACEOUS - A SIBERIAN CASE STUDY

Late CretaceousMaastrichtian (70 Ma)

Growth rings in fossil wood

toothed (‘serrate’) margin leaves

smooth (‘entire’) margin leaves

Mid Cretaceous (ca 95 Ma) position of North Alaska - at ca 80N

Cretaceous coal in North Alaska represents ca 1/3 of total US reserves of all ages combined!

Just like Antarctica, these northern regions of Alaska and Siberia had diverse thriving forests and much warmer climates than today

Reconstruction of northern Alaskan forests in the mid-Cretaceous

I’ve shown examples of vegetation history spanning ca. 400 million years. The key point is that plants have had time to adapt and evolve to changing circumstances (e.g., geography, climate), but have also played an active role in determining climate. There were winners and losers, survival of the fittest, etc. But all of those changes have occurred gradually.

I leave you with this question: how do you think the human race is affecting our planet and what sort of future do you envisage? Use your knowledge and imagination here: 50 years or 50 million years? It’s up to you. Feel free to come and discuss (GS 334, rees@geo.arizona.edu).

Present-day Climates

Geography: Present, Future and Past

Plants (Part 1):‘Greening of the Land’ - Origins, Diversification

Plants (Part 2):‘Icehouse World’ - Carboniferous and Permian‘Hothouse World’ - Jurassic and Cretaceous

PLANT DIVERSITY CLASS TOPICS: