Broad-leaves

145 - 66 mya Cretaceous

Paleosols Flowering Plants Mycorrhiza Animals

Broad-leaves

Could the plant part we call 'leaf’ help explain the abominable mystery? We tend to overlook its possible role compared with the exotic flowers. Yet these new green leaves provide a food source for flying insects much more profound than some nectar. They also dropped to the ground where they were decomposed - a process many of us now take for granted - as the prime function of soil.

There are so many insects making a meal out of these leaves these days, from caterpillars to whitefly. Angiosperms have become most of the crops we grow today, and many because of their leaves. There are great tomes written about crop - i.e angiosperm - pests and diseases. And some of us are fortunate enough to have been able to complete a degree, on the subject - when they had such degrees - in Integrated Pest Management. Most of what we consider pests and diseases would have arisen in this period, as these new eco-niches rich in nutrients appeared. But some would have got lost along the way.

Angiosperm leaves play a much bigger part in the cycle of life than previous thin pine needles and fern fronds. Their big evolutionary jump was that they gained many more veins during the Cretaceous
“The veins that irrigate leaves during photosynthesis are demonstrated to be strikingly more abundant in flowering plants than in any other vascular plant lineage (Boyce et al 2009). This would have provided them with more water for photosynthesis, and allowed them to grow more rapidly. These high assimilation rates are likely to have been unique throughout evolutionary history and the transpiration-driven recycling of water would have altered the surrounding environment significantly.

Deciduous

Another aspect of the possible role of veined-leaves would be the appearance of ‘deciduousness’. Deciduousness (meaning ‘to fall off’) refers to trees which lose their leaves in winter or the dry season. When the leaves that deciduous trees, shrubs, and vines sport in summer fall in autumn, this is called ‘The Fall’ in North America. 'Deciduous' come from the same Latin root as ‘decadent , meaning "fall."

The leaves of many trees in many parts of the world fall (hence the American term for Autumn) off the trees annually. They are said to be deciduous. Deciduous woodlands are absent in regions with extreme climates, whether cold, as in Polar regions, dry, as in deserts or consistently warm and wet like tropical rainforests. They are found in Eastern North America, Eastern Asia and throughtout Europe except Southern Spain and Greece.

What is staggering for those of us, who live in the temperate Northern Hemisphere, is when we find that trees do not do this in the Southern Hemisphere.
“The northern and southern temperate forests are very dissimilar in composition, for the genera differ in each area and most of them belong to divergent families. The fossil record suggest that the morphologic differences between the plants that contribute to these forests are the result of their independent evolution on opposite sides of the tropical rainforest belt during Cretaceous and later times….However the factors that may account for the presence of a dominantly deciduous habit for woody angiosperms in the temperate forests of one region as opposed to typically evergreen in the other, are obscure, as is the evolution of the deciduous habit itself in northern temperate hardwood forests" (Axelrod 1966)

When autumn leaves..

As the days shorten leaves lose their chlorophyll (giving plants their green colour and used to catch sunlight), thus exposing other pigments. These include carotenes (yellow) and anthocyanins (reds and pinks) which create those fabulous autumnal colours. Plants then produce special cells to enable the leaves to fall leaving the plant to heal the cut and provide protection in winter. It is what many of us of feel the soil is for – composting the leaves. But this was quite late on in the life of soil, a mere 150 mya. But what a change it was to bring.

The leaves of deciduous trees turn vibrant hues of red, yellow, and orange, as the days get shorter. Oaks tend to retain their carotene pigments till leaf fall whereas hazel has lost their pigments by fall time. Many soil bacteria are pigmented. Most of the pigmented bacteria in the top layer of the soil are photoprotected from exposure to huge amounts of UVA radiation on a daily basis by carotenoids. (Kumar et al 2015) The leaves swirl to the ground. Many annual plants die and wither. This finally completes ‘Law of Return’ where plants use up soil nutrients during the growing season, and give them back when the season is over.

We take deciduousness for granted, and many of us believe that the main function of soil is to break down those leaves that fall to earth. Most of us take this granted. How did it give a positive advantage for plants – especially trees- to lose their leaves in winter/dryness? Perhaps the vein leaves are sensitive to cold and dry, but by falling off they leave the tree in a good position to withstand cold dry nights, and the fallen leaves may well protect their roots from freezing soil.

Deciduous hardwoods

Deciduous hardwoods first appeared early in this period in the lower middle latitudes in the Northern Hemisphere, associated with broadleaved evergreens in areas of temperate climate. The deciduousness may have been an adaptation to moderate drought in the cooler part of the year, but proved useful when living in higher latitudes with more intemperate climes, more moist, dry or cold which have marked periodicity. Although they evolved marginal to the tropical zone of the Southern hemisphere southward migration may not have been encouraged by rainforest like conditions on smaller land masses. Evergreen flourished there and still do.

We can find deciduous trees nearly 400 million years back, but deciduous hardwoods first appeared at lower middle latitudes in the Northern Hemisphere from about 145mya to 100mya This difference is put down to independent evolution either side of the tropic later in the period – as the continents diverged.

If the ways that this could happen had already been there for 250 m years, why did it take till this period to spread?? Conifers can withstand cold and dryness without dropping their leaves, yet spruce has developed the deciduous habit. Deciduous trees must have been put at an advantage as they were to increase dramatically over next few million years – and have stayed to this day. Their leaves are clearly a part, but perhaps the deciduousness protected the most productive part, the leaves, from more extreme conditions.

Deciduous Savannah

In the Southern hemisphere, evergreen savannah species have been selected for leaf traits that avoid excessive light and heat stress during the dry season, extending the longevity of their leaves at the cost of water use efficiency. High temperatures can impair cellular enzymes. Deciduous savannah species may accommodate this damage because they can lose their leaves every dry season. Evergreen species may allow for evaporative cooling to pre-empt heat damage. Evergreen species orientated their leaf blades or phyllodes with steep angles to the horizontal, while deciduous species orientate their blades horizontally.

Deciduousness may be evolved in separate times and ways. Evolutionists like to trace everything back to a common ancestor, in order to make a ‘clade’. But it is hard to see how this can be done with a process like this. Deciduousness provides one of the most obvious examples of convergent evolution - the process whereby distantly related organisms independently evolve similar traits to adapt to similar necessities. This went on over many millions of years in different sources.

Higher Processing

“Deciduous species exhibited general higher processing rates than evergreen species In deciduous species, processing is faster during the spring–summer than in the autumn–winter period, which may be attributed to the greater nutritional value and less consistency of the leaves during this season. Within evergreen species, pine had a significantly faster processing rate in autumn, attributed to greater physical fragmentation of the needles" (Lopez et al 2001)

These flowering plants dieback each year and their foliage fall back to ground. To take over as they have, this must be a great advantage. How has that come about? Soil had been able to deal with dead trees for some time, but how did this regular dieback create new environments for soils? The young delicious deciduous leaves will bring food for many sorts of creatures kept warmer by them in winter.

Less lignin

Deciduous tree species are often associated with greater macrofaunal (worms) activity than coniferous forests (Nickels & Prescott 2021), which mix surface organic material with mineral soil. Deciduous leaves have lower lignin content 10-24%, compared with substrates high in lignin > 25% such as pine needles.

Some organisms and creatures will fall to earth with the leaves – as that can give us an idea how soils would have adapted further to these new circumstances. The creatures must have adapted to this sudden change in their environment. This meant the recycling of nutrients would be more extensive and continuous - and thereby contributing to an ‘abominable’ increase in the number of 'higher plants' - whatever Darwin meant.

Breakdown

The leaves that were not eaten dropped to ground, where they provided a new food source. The dead leaves provided a new habitat and safety for many organisms and soil processes. Conifers tend to reduce process of decomposition. The angiosperm leaves created a complete new ecosystem. The dead brown leaves are almost as important as the living green leaves. For the first time in plant evolution, the plants are dropping their leaves regularly, providing soil with material to decompose and recycle nutrients continually.

Mineralisation

Mineralisation is the decomposition (i.e., oxidation) of the chemical compounds in organic matter, Nutrients, like nitrogen and phosphorus, in those compounds are released in soluble inorganic forms that may be available to plants by increasing the bioavailability of the nutrients. Many agronomists spend their lives talking about ‘mineralisation’ and its importance in the decomposing organic compounds, to release nitrogen and phosphates - the N&P of NPK fertilisers

Microbial breakdown

Much microbial breakdown occurs in the anoxic conditions of animal guts, particularly worms and mites and we call this humification

Humus

The result of worms and mites chewing and pooing leads to more glueing. The breakdown products are mainly humus and humic substances, the latter helping glue detritus and debris togther.

Decay

The soil process we now take for granted, leaf decay, developed dramatically during this period. There were already the necessary movers - the earthworms with their mucous, and the munchers’ - mites with their bacteria. But now they could diversify.

“The breakdown of broad leaved compared with conifer litter was measured for 2-5 years in forests of each species to determine (i) if broadleaf litter decomposed faster than needle litter, (ii) if litter decomposed faster in broadleaf or mixed wood forests than in coniferous forests, and (iii) if mixing with broadleaf hastened decomposition of needle litter. The broadleaf litters decomposed faster than needles during the first year but, thereafter, decomposed more slowly, so differences were small after 3 years" (Prescott et al 2011) So deciduous produce more litter and faster turn over in early stages, which would have been a clear winner.

Breakdown of broad leaves would take less energy than that previously used for lignin breakdown, as there is little lignin in the leaves. So less energy, more food. That seems quite an advantage. The cover of leaves will also provide an advantage as it will protect soil from winter elements – higher rainfall and frosts, both likely to break up topsoil. Leaves will keep temperatures higher and protect soil from rain impacts. The more you think about it, the more we realise what a change it must have made to soils, when those leaves started to land. The return of nutrients, protect soils, and keeping the soil warmer.

Have you ever wondered how leaves get decomposed at the time of the year when temperatures are falling? We know that biological life gets faster when it warms up – about a doubling every 10C. Yet deciduous trees usually shed those leaves when things are cooling down.

This happens in all areas outside the tropics. Just when you need them most these organisms and creatures must be slowing down.

Litter layer

This leaf ‘litter layer’ differs from other soil layers and is a distinct layer with its own properties. The importance of this layer over time is not just a matter of decomposition but also the new structure it provides. The leafy litter layer is the layer of mainly dead plant organic material present on top of the mineral soil surface and composed of debris in different stages of decomposition.” The leaf litter probably holds dead plant matter turning over every few years. That sounds obvious, but don’t forget that the rest of the soil is based more on structures around mineral particles. This leaf litter layer is totally organic and distinct, with little mixing.

While it is nearer to compost than litter in the previous millions of years, the creatures from the previous versions would have been laying in wait The leaf litter layer, provide habitat to a diverse community of fauna that play an important role, not only in litter decomposition, but also recycling nutrients and building soil structures. Existing creatures would include earthworms, woodlice, mites, and slugs, that live in and on the soil surface shred plant materials into smaller and smaller pieces, increasing the surface area on which soil bacteria and fungi can prey. See Litter Louts for more

Nutrients

The saprophytic fungi that had been around for a couple of hundred million years, would have had a field day and “send out” filamentous threads, called hyphae, that operate much like plant roots. These hyphae release acids and enzymes necessary to break down dead plant material. This makes nutrients available to plants to sustain their own growth. You may have seen this whitish “mould” under leaves. It’s quite hard-working, and adds a lot to soil. As the litterfall is consumed by the decomposer food web, water and inorganic nutrients (i.e. nitrogen and phosphorus) are released into the soil, from whence they came where they can be taken up again by plants to foster new growth. “Plant litter is the main source of organic phosphate (P) and acts as a stubble return to drive the P cycle" (Jiang et al 2013).

Soil Protection

Increasingly we are recognising the importance of this surface leaf litter layer in protecting and building the soil underneath. The emerging regenerative farming movement emphasises the importance of ‘cover crops’- crops which cover the ground in winter so that rains and frosts do not batter them. “Cover cropping strengthens nutrient cycling in agricultural systems under different conditions, increasing crop phosphate nutrition and yield, as this practice could enhance soil microbial communities. (Hallama et al 2018)

According to the European Food Safety Agency “Not protecting the natural processes and organisms in litter will fail the objectives of the EU regarding biodiversity, soil erosion, organic matter decline, etc. We are increasingly recognising the role of leaf litter in increasing soil biodiversity

N:C Ratio

For the previous 300 million years there has been decomposition of plant material, but this period presents a progression, as there is more green stuff, in the shape of leaves and annual plants falling to the ground. The surface now much more like compost mulch as we know it. When composting we are told that we should have about half green stuff and about half brown stuff. The green provides nitrogen for nutrient while the brown provides carbon compounds for structure.

We are finding that the Carbon : Nitrogen (C:N) ratio points to what may be going on, and provide a way of balancing the two main properties of leaf litter - the breakdown and the protection..

Crop ratios

The ratio of the mass of carbon-to-nitrogen in soils can tell us a lot. Residues differ as to their C:N ratio.
Mature alfalfa hay has nearly the perfect ratio with a 25:1 C:N ratio, so microbes consume it quickly and leave little excess C or N.
Wheat straw has a C:N ratio of 80:1 while hairy vetch has a C:N ratio of 11:1. The lower the ratio the more rapidly nitrogen will be released into the soil for immediate plant uptake.

The quicker microbes consume residue, the less time the soil is covered. That lessens the time residue protects against soil-shattering raindrops, or shades soils from moisture-sucking sunshine and scorching winds. Clearly this also protects all those creatures running round. Eating seeds and stuff in the compost mulch. This is litter is as we now know it.

A ration of 20-30:1 is a good balance between the two. A moisture content around 50% and C:N ratio around 30:1, both within the recommended ranges, make composting go well

24:1

Bacteria and fungi, seem to purr in soils with a C:N ratio of 24:1. (24 parts carbon to 1 part nitrogen). Around 16 parts of carbon are used for energy and eight parts for maintenance. Under these optimum conditions, soil microbes spur release the release of nutrients like N, phosphorous and zinc to crops, provide organic breakdown while also providing a protective cover to the soil. The 24:1 ration provides that ideal balance.

The major distinction made between deciduous (above) and coniferous litter is that the higher the C/N or lignin/N ratios and the higher the polyphenol (lignin) content, the lower the abundance and activity of soil organisms. (Korboulewsky et al 2021). The C/N or lignin/N tests do not always agree in predicting litter decay (Taylor et al 1989) We have seen throughout this history that lignin is hard to decompose, so the new delicious deciduous leaves would have proved to be easier to digest.

Straw

With a high C:N ration like 80:1, a microbe does a lot of digesting but it takes some chomping. The deficit of N means microbes have to find additional N to balance out the excess straw carbon. This could come from excessive N in the soil, but it may create an N deficit. This is called immobilization and could continue until some microbes die and release N contained in their bodies, which is called mineralisation.

The delicious deciduousness must have made great changes to soil, so much so that it is possibly the last great missing piece in the 3-D jigsaw that builds the soil architecture. These luscious leaves enrich the familiar functions of soil, so this was perhaps the time in geological history which completed the soil as we know it today.

Flowers

Root nodules
Fungi

Animals

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