Thursday, 16 February 2017

The Hidden Life of Trees by Peter Wohlleben

When a dedicated ecologist/forester manages trees for a lifetime he gains fascinating insights. When as an author he writes a best selling book that does not pull its punches or dumb down its content it is well worth reading.
It is not my purpose to review this excellent book - and it does have its defects as it balances scientific reporting with popular culture.

The book cover is drawn by an artist and not a botanist!

It is Peter Wohlleben’s ability to think ‘out of the box’ and challenge conventional opinion that grabbed my attention. A reviewing professor said that each chapter had it’s ‘aha’ moments - I know what she means.
I want today to highlight just three things that I want to explore. One in particular excited me greatly!

1. Connections
Ever heard of the World Wood Web? Most of the plants in a forest are connected - as are some of the plants in your garden. It is now mainstream science how plants share resources and information. Peter  Wohlleben says that they ‘talk’ to each other.
  * In ancient beech woods many roots are naturally grafted together and trees are joined. (Root grafting across species barriers is on the other hand rare).

  * Most plants form symbiotic root associations with a very wide range of mycorrhizal fungi. Such fungi produce huge nets of mycelium and make connections between multiple  and diverse plants. I have previously suggested that all the bluebells in a natural forest would not be there without tree friendly fungi. Sugars, amino acids and numerous other organic compounds such as plant hormones are exchanged and without mycorrhiza many plants would find insufficient inorganic nutrients.
 * Electrical connections act much more slowly than in animals. Information is transmitted at a rate of about an inch in three seconds. Never-the-less it surely gets there. I read in a recent ‘New Scientist’ that the effects of different electric potentials in living organisms are of much greater importance than previously realised. Very significant epigenetic effects are being discovered. One scientist  suggested this area of study is a huge new domain.
 * Volatile chemicals move through the air. Pheromones can signal imminent attack of pests to the neighbours and astoundingly will invite predators and parasites of the pests to a tasty meal.
 * We will say nothing today about the sex life of plants facilitated by scent
 * Touch. Branches on trees and their neighbours grow neatly together. Have you ever pruned out a huge piece of a shrub (en passant excellent pruning) and when it is gone you would hardly know? Good vibrations!

Peter Wohlleben enumerates numerous examples of connectivity in action.
 *Trees help their neighbours that have fallen on bad times.      
 *Protecting the unity of a forest is in every tree’s interest.
 *Trees do their damnedest to look after their children
 *Trees can associate across species barriers via mycorrhiza to their mutual benefit. In the appendix Canadian Professor of Forest Ecology, Dr. Suzanne Sinard quotes how young paper birch and conifers have been shown to have a symbiotic relationship. 
 *Ancient tree stumps sometimes (albeit rarely) are kept alive for many years by their neighbours.
*Synchronisation of high fruiting 'mast years' in beech and oak
Each new chapter introduces a new topic. In one he discusses nature’s densities of planting

2. Stem flow
You won’t find much in the botany books about how plants can intercept and direct rain. Not all do, and a beech and a spruce are completely different. You might have noticed spruce and firs are a great place to stay dry if the heavens open. (Perhaps not in thunder). They cast off water out to the fringes or retain water in the canopy. This is an adaptation to the wet and cold places they grow in their natural habitat. As much as half inch of rain might be held in the canopy and will be lost to evaporation later.
The beech which often grows in dry habitat wants to keep all the water it can get. Peter Wohlleben  says that in storms you can watch heavy rain drain out of the branches and channel down the trunk to the roots.
Scientist Peter Williams put his spoke in this one when he pointed out to me that tree roots go much further than a tree canopy although he did give some ground (pun partly intended) when I reminded him we were talking about forest and not necessarily about an individual tree.
To be fair to the author he does state that even the beech will waste to evaporation half of the water that falls on its canopy.

I once read in the New Scientist about certain plants that grow in very dry infertile arid soil and rely on stem flow not only to channel water but also flush in nutrients from such as bird lime.

I have mentioned stem flow myself in relation to rosette forming weeds such as epilobiums when you spray them with glyphosate. If light  rain (unfortunately) follows application, the weedkiller is washed straight to the roots and the weed is still killed.
I first observed stem flow when on my knees under a potato in extremely light rain. I noticed every drop was channeled down grooves on its surfaces direct to the roots. (Rude comments invited).

3. Peter Wohlleben rubbishes the standard explanation of how water gets to the top of a 300 foot tree.

 American Redwoods achieve more than 300 foot high, but not here in Oxford
You might understand my excitement after my recent post on this topic which promoted former student Alan Warwick’s previously unpublished and highly unorthodox theory. 
My post went down like a lead balloon! Five hundred loyal followers read it perhaps out of habit but there were no comments and it has not been read since!
It was not of one of my better efforts. I completely bodged my attempt to  accommodate the standard theory with Alan’s proposal of a ‘push' rather than a ‘pull’.  
Cathi described my post as a complete dog’s breakfast.

The standard explanation of upward water movement enrols transpiration as the driving force and enlists the alleged phenomenon of cohesion of water molecules as they hold together up complete water columns all the way from the roots. 

Metasequoia can growth fifty metres high in the wild. As a deciduous plant where are the leaves to drive upward flow in Spring?

Like my friend Alan Warwick, Peter Wohlleben does not believe the standard theory.
What got me excited was Wohlleben’s report about research which listened to the columns of water within a tree trunk. The researchers detected thousands of bubbles. This was enough for my hero to reject the cohesion theory. He does not go further however and leaves the phenomenon a mystery.

Alan’s theory, although it does not depend exactly on physical bubbles, does depend on the kind of pressures that occur in a bottle of champagne.
I have added an addenda to my previous post and invite you to go there and read more!

I wrote about mycorrhiza

Sunday, 5 February 2017

Old, cold and beautiful

Peter’s Pictures

This old abandoned house
once filled with love and laughter
now sits alone at the end of the road
with no one to tell its tales to
ultimate panic queen

Peter lost his old spade in his wood ten years ago
Not much use now
Actually a shovel
by God, the old man could handle a spade. 
Just like his old man.
The son does something different
But I've no spade to follow men like them. 
Seamus Hainey

Cosmopolitan mosses likewise salve
sidewalk cracks, crumbling walls.
Pattiann Rogers

The spider, dropping down from twig,
Unfolds a plan of her devising,
A thin premeditated rig
To use in rising.


The leaf is dead, the yearning past away;
Alfred Tennyson

Foxglove seeds
Nature's candelabra
In life, a mere weed

The wall is builded of field-stones great and small
Tumbled about by frost and storm
Shaped and polished by ice and rain and sun:
Some flattened, grooved, and chiseled
Helen Keller

Over the years, I oft have wondered
was I right to leave the old place
where I found security and life.
Unca Goat

Peter Williams was inspired by January’s hoar frost. He  provides lots of pictures that appear in my posts. You can find them in the theme column articles he has written.

His garden is not always as pristine as I pretend

Friday, 27 January 2017

Epigenetics in the Garden

When this caterpillar metamorphoses into a butterfly it retains an unchanged genome
Epigenetic's can claim to be the biggest biological advance in the last twenty years and new implications in plant and animal biology become apparent every day. Scarcely anything that happens to the plants and organisms in our garden is not epigenetically controlled.

What is epigenetics?

I am indebted to Nessa Carey’s fine exposition in her book ‘The Epigenetics Revolution’. It becomes heavy going as you turn later pages but never have I read such clear explanations.

In simple terms epigenetics is about how and when genes impart their information. In a sense the term ‘gene’ is too crude a term in reading DNA. When the human genome was first fully encoded scientists were amazed that gene numbers barely reached twenty five thousand. This is slightly less than a rice plant and a small fraction of paris, the woodland weed! (The latter being as yet the biggest genome that has so far been counted).

The Paris japonica genome is fifty times bigger than a human one

The really significant numbers are that in humans, for example, we are coded by about three billion base pairs of genetic code. The number of cells in our body are measured in trillions. Every single one of these cells is shaped and performs under epigenetic influence.

Every cell in our body contains a complete set of the genetic code in our genome and yet most of the code is not expressed in the individual cell. The code is not turned on. For example the information specific to making a liver cell is not turned on in a skin one!
A dramatic illustration of epigenetics in action is the development of a new embryo in the womb. The first divisions from a human embryo are undifferentiated  and potentially can divide to create any human cell. Once epigenetically tagged their divisional destination might be for ever predetermined. A primordial skin cell might only ever divide to produce skin. Some cells such as neurones once laid down might never divide again and yet serve us for the rest of our life.
Some cells are more versatile and may permit change. A great challenge in this new science is to discover cells that retain their versatility and to explore ways of returning certain cells to the undifferentiated state. Already significant advances have been made. To replace our body’s damaged cells by generating new ones is for certain medical conditions a holy grail.
Reading the script
A genome might be regarded as a script rather than recipe. It is reassembled from one sexual generation to another as male and female components combine but essentially it retains the same structure. In asexual reproduction it barely changes at all. The actual genome script is, at the present level of understanding, not changed at all by epigenetic action. What does change is its interpretation.
‘Genes’ are epigenetically turned off or on by a process of tagging. You may have read about temporary methyl, ethyl and histone attachments to the genetic machinery. The presence of a tag might mean genetic code is switched on or off - or some partial combination. Some tags remain a lifetime or longer, some tags are ephemeral. In the life of a cell some tags might be on or off like a yo yo.
An aspect of epigenetics that has caught the public imaginations is that in some cases the modified action of genes by the process of tagging can be passed on from one generation to the next. Has Lamarck’s theory of the inheritance of acquired characteristics been reinstated as a source of genetic diversity? Seemingly not, but long term epigenetic change is not without significance.
Normally at birth a new germ cell’s tagging is wiped clean. We now know in some cases this does not happen and the environmental influences on one generation are passed on to the next and in some cases beyond.

What initiates tagging?
Significantly the environment. The environment in the womb, the environment within the body and the effects of the surrounding world. 
In truth almost anything might drive the process of tagging. All the cells of the body working together constantly effect one another. The action of pathogens, symbiotic entities, physical damage to cells, pheromone signals from afar are all included. 

Relevance to the gardener 
I have intended setting down a few thoughts about epigenetics in gardening for some time and reading Nessa Carey has emboldened me to to do so.

I need to explain that many plant cells retain pluripotency (the ability to differentiate into any new cell) throughout the plant’s life. Pluripotent cells occur in the primary meristems at the shoot and root tips and in the secondary meristems such as cambium. These cells as well as reproducing themselves in their undifferentiated form can differentiate into any plant part. Plants unlike most animals can completely renew themselves - either as  extension growth or when detached as new entities. We gardeners call the latter vegetative propagation. Even simple cells like parenchyma can become pluripotent.
Scientists exploit the regenerative abilities of plant cells in meristem culture.

You might say cells of the stem are stem cells in respect of rooting!

Certain differentiated cells in a simple plant stem are able to generate root cells when we take stem cuttings. On the other hand there are fewer examples of plant root cells having the ability to produce stems. Some of the exceptions can be quite a nuisance when they appear as suckers! Such roots with the capacity for pluripotent action are the basis for root cuttings. 

Gardeners and ecologists have been reading for some time about research showing plant genes been turned on or off. Not infrequently we read how plants from a distance warn their neighbours with pheromone signals of the presence of a parasite or predators. Supplied with this information a plant mounts it defence - as diverse as producing distasteful tannins, growing more spines, producing natural toxins or even sending out its own signals to beneficial predatory insects.
All these defences come at a cost to the plant’s resources and being epigenetically ‘switched off’ when not needed is part of a plant’s good housekeeping.

I sometimes get irritated about researchers pontificating about the significance of genes being working or not. I have previously indicated my irritation when two years ago researchers on biochar demonstrated significant growth improvements by using biochar as a growing media and yet damned this material by reporting the plant genes that fight pathogens were switched off. I suggest you might expect them to be switched off if the plant was thriving without them.
In actual practice plants grown in the field using biochar and indeed in historic terra preta produces very healthy plants indeed.

My clivia has never been healthier since being potted into charcoal

Scientists recognise that a single genome can in different environments  produce different ‘phenotypes’. (A phenotype can be recognised on the basis of an organism’s external features). In animals different phenotypes produced from the same genome might be as different as a caterpillar and butterfly or as ephemeral as the colour of a fur coat in the Winter.

Different phenotypes of genetically identical plants is not uncommon and are epigenetically controlled. Certain aquatics look very different if growing on dry land rather than in water. 
Plant leaves on the same plant vary in shape, size and chloroplast density dependant on whether grown in full light or shade or varying temperature. Its all epigenetic.

Varying phenotypes are a characteristic of juvenility. A young beech tree retains its leaves in Winter whilst an old one doesn’t unless it has been kept young by clipping. Brenda’s son did not recognise the flowering ivy in his new garden. It looks so different to the juvenile kind.
Some conifers permanently retain their juvenile phase and this is a source of dwarf forms.

This variety of hamamelis unfortunately retains its dead leaves.

Peter Williams’ picture shows ivy in its mature form

I wrote recently about hard and soft growth. Surely this is epigenetically controlled?

A well researched epigenetic phenomenon is a plant’s response to cold in relation to flowering and the timing of seed germination. Vernalisation (preparing for Spring) controls for example a plant cell’s ability to differentiate flowers. Scientists have identified how genetic tags control this phenomenon. Extreme in its sophistication it combines classic epigenetic phenomenon such as  environmental stimulus, delayed timing, long lasting action and at the end of a season ‘scrubbing’ tags clean.

In animal studies there is great interest in developing profitable pharmaceuticals that encode epigenetic actions. I wonder if ever there will be chemicals sold to promote flowering?  At least that might stifle the garden pundit’s feeble resource of suggesting you give your none flowering plants a dose of potassium!

Queen bee

Royal Jelly is epigenetically potent
Another remarkable example of phenotypically distinct organisms that share exactly the same genome is the thousands of worker bees and their very few sisters that become queens. 
Widely researched their epigenetic story is now almost unravelled. 
The ruling queen after several fertilisations by drones on her maiden flight settles down to a sedentary existence of as long as eighteen months and in that time lays tens of thousands of eggs. Thousands of females will be genetically identical (Not all the bees, distinct fertilisations will give rise to different clones and the very few males have their own special duty).

The epigenetic destiny of sisters is controlled by their nutrition. For the first two days from hatching all females are fed with ‘Royal Jelly’. This substance is produced by specialist workers - themselves phenotypically gifted to exude this material. Future queens continue to be fed Royal Jelly for the rest of their life.
The content of this material is a complex rich mixture and contains epigenetically active components. Crucial genetic elements in the growth processes have now been elucidated, methylation enzymes have been identified and it interesting to note that to become a queen a specific gene is turned off.

Must tell the others

Apparently in bees there is another well examined example of epigenetic activity that involves a worker’s memory of food sources classically communicated in the famed ‘waggle dance’. Not only does it need to remember it needs to forget a previous site. It’s all in the tagging.

Brave New World
New Scientist last month reported in a major feature that there were huge developments in plant epigenetics just round the corner. Many of the developments will relate to yield and pest and disease control in Agriculture. What comes to farming gets passed on to horticulture and one day we might see at the garden centre epigenetic products that change the colours of flowers.
Much of the epigenetic action in potential farming products revolves around a switching mechanism that involves RNA interference. Like all the above examples there are no permanent changes to the genome.

Respite for the bees?
The bees will be pleased that an epigenetic product that controls varroa mite has been announced to be in its final stage of development

Relevant links

I get precious about my biochar

I now now wonder if soft growth involves epigenetic change

More about varroa mite

paris genome

Tuesday, 17 January 2017

The benefits of mulch

Much ado about mulching

Mulch is a thin cover on the soil surface. Mulch gets thin cover in the horticultural press too. Reporting is very shallow.
You probably know that mulching makes plants grow better, conserves water, looks good, insulates the soil and can suppress weed. You might add that it protects the soil from erosion in heavy rain and keeps the mud splashes off the strawberries.
For many gardeners that is sufficient but you won’t get away with so little today. As is my wont, I want to dig deeper!
Except I don’t dig. A  perceptive student once inquired why as I am so keen on minimum cultivation I don’t just put a mulch on top of the soil and get a cavalcade of further advantages. He was right of course, it was a very shrewd insight. Although I have often rather snidely declared that the main advantage of mulching is to prevent the over enthusiastic gardener disturbing the soil it might actually be true.
As I so often promote the benefits of none cultivation I will spare you that today!

I bark mulched last May. Three cubic metres delivered from a local nursery £120

Mulching materials
The actual properties and gardening merits vary with the material. The very best for water conservation are gravel and stones. Include in this list larger rocks and un-cemented paving. How often do you hear that clematis likes a ‘cool root run’ under stones. Partially true. The active principle is that loose stones and gravel do not intercept water and even light rain nearly all penetrates through. Like most mulches, as intended, they do reduce direct evaporation from the surface. Permeable plastics and substances such as polythene add to the list of good water conservers. 

Bark mulch gives all year round cover
Mulches are usually fairly long lasting surface materials although those such as bark and wood shreddings need topping up every few years as they slowly degrade. Discuss with the rabbits and moles the permanence of stone chippings.

Peter was mulching last week (I think this post reminded him!)
We also use the term mulching to include surface organic materials such as farmyard manure and garden compost. In actual fact these materials are usually intended to be incorporated by the worms and only remain at the surface for a while. Manure mulched rose beds annually topped up might last the whole year. Lawn mowings and Autumn leaves are perhaps a half way house in this crude classification. Mulching as a very effective means of adding organic matter is not really my subject today.

I will use my plastic scarifier to sweep the leaves off the the grass path on to my borders

Weed control 
Other than plastics, mulching against existing established perennial weed is almost useless. The weeds will love the mulch as much as the plants. We will say nothing about gardener’s attempts to smother perennials with newspaper and cardboard!
Against weeds coming from seed mulch’s measure of control is really quite useful. It seems to be generally agreed that mulches need to be about two inches thick to stop weed seed germinating. This cover will be enough to suppress light levels at the soil surface to inhibit germination and enough to prevent emergence of small seeds. It won’t help very much with seed that blows onto the surface or is shed in situ (shame on you).
In my own case the thinness of my wallet reduces the thickness of my mulch. My weeds are well enough controlled by my glyphosate spraying and I actually want many of my garden plant to sow themselves around. (This is why my mulches never overlay plastic - I want my garden plants to self seed).

Hand weeding anyone?

My Scilla biflora feely seeds itself around
This cyclamen would have not sowed itself if the gravel was under-laid with plastic
People tell me that weeds that do sow themselves in mulch pull out very easily!
This year I newly mulched a large part of my borders with bark. This was to pacify Brenda who can’t stand the liverwort! 
The freshened up borders looked very nice.

An insulating layer. Be careful what you wish for
In general a mulch’s insulation might be thought to be a good thing if it keeps soil warm in Winter and cool in Summer. 
Not all mulches are insulators and plastics are only thin layers. Some colours are good heat absorbers and/or emitters. The relationship between day and night temperatures might be just as significant as those between seasons. It might for example be a difference between crop failure and success if night air above mulch is too cold and fruit blossom is frost damaged.
It is really quite complicated - what do you want? If a Winter laid mulch keeps the soil colder this might be thought to be a bad thing. On the other hand if my frost delicate Dicentra spectabilis emerges a little later I will be very pleased.
In practice in normal UK conditions I ignore potential exceptions, lay mulches at almost any time of the year and in terms of heat transfer hope they are beneficial!

Three case studies about soil temperature
*In my earlier post about gravel mulching a lady wrote in that in her tropical climate gravel got too warm in the hot Summer sun. I must say that on our subsequent holiday in Costa Rica I did not notice this effect on the weeds growing in the hotel’s gravel roads! On the other hand the dry sand on the beach was too hot to walk on. I imagine in such climates the soil is much cooler under stones and although hot for a plant to clamber over keep roots happy?

*A neglected exploitation of mulch’s insulation is putting an extra layer over dormant tender plants such as dahlias in Winter. The mulch might even be extra soil!
This year we had five months of spectacular colour of dahlias overwintered in the ground - it had been a very mild Winter. To increase my chances next year I have carefully folded over the large frost-dead tops of the dahlias and supplemented that with a layer of my miscanthus prunings and scattered amongst it mole soil from the lawn! I will remove it all in March! It does look a mess but the dahlias are worth it.

 Dahlias advertise my vulgar taste next to the road
The mulch will have confirmed my reputation

This mulch was an accident
I have only limited hopes for my effort! Insulation might be successful if Winter cold comes in short sharp spells.
If we get a prolonged period of penetrating frost there will only be a very small benefit. With only a very small heat source from deep soil my mulch will be of very little value in stopping the soil freezing.
As an analogy in our extreme 2010 Winter the inside of my unheated greenhouse was as cold as outside. Your own Winter coat only keeps you warm because of the heat from your body. Blog sleuth and myth buster Robert Pavlis writes about this phenomenon when he tested soil insulating cones.

The best insulation for your dahlias is to plant them very deeply at original planting!

 *When I went to see Peter he showed me the graphs he had drawn when he wrote his PhD thesis. They showed how soil surface temperatures fluctuated throughout a sunny Summer’s day. It is really quite astounding. Most mulches would dampen such fluctuations although thin plastics might increase them.

Drawn by hand before computer graphics, Peter measured soil surface temperatures on successive Summer days.
(Ignore the two horizontal lines)
Water retention
My old Head of Department, a soil scientist, contended that mulches contributed very little to soil water conservation and I used to argue that they did.
I thought before finalising this section I would have a word with soil scientist Peter Williams and found that he was not very enthusiastic either. He emphasised that never-the less he thought mulches in the round were a very good thing.  He went on to mention that he had recently assessed the soil moisture under the mulch in his garden after heavy rain that had followed a dry period and found that very little water had penetrated through. Mulches reduce water evaporative loss from a soil surface but they sometimes intercept rain before it can penetrate in.

At that point I scrubbed my text for this section and started again!

I think a key attribute of a water conserving mulch is that it should let water pass through without soaking it up! I have long argued that none absorptive materials such as gravel and to a lesser extent coarse bark act like a one way valve for light rain that otherwise would evaporate away.

A significant thing to understand is that an un-mulched wet soil surface becomes dry very quickly in drying weather. It rapidly becomes its own mulch in terms of water retention and soil surface evaporation stops. It will start again only when the soil is re-wetted and in the case of light showers the water will be very soon gone. 
Gardening books will correctly tell you that the best time to put on a new mulch is when the soil is already very wet. I put this to Peter and he explained that although the water retained by such a mulch would be a very useful equivalent of perhaps half an inch of rain that this was not very much in relation to the soil’s total available water content.
Leaving gravel and plastic aside, I think a mulch’s greatest contribution to soil water relations might be to keep the soil surface ‘in play’ in dry weather. When a dry un-mulched soil has reached the stage of ‘self mulching’ it is dry several inches down. In that zone root water absorption and such as mycorrhizal activity will temporarily shut down. Considering that the most fertile soil is usually at the surface this is not a good thing. Peter added that such dry surface soil can also get harmfully hot on a warm Summer’s day (see his graph).

If roots come right up to the surface and into a mulch so much the better

My Sternbergia lutea is growing in the gravel

Cosmetic value  - disguise. I have my own problems when I try to grow ferns on my hydrophobic sandy soil! I plunge ten litre pots to give me a watering lip. 
Things people ask.
Do mulches such as wood chips deplete the soil of nitrogen?

One might counter with the question, how could a layer at the surface do so? 
It can! Most of the decay of woody mulch is a result of fungal action. Fungus mycelium grows down into the soil to draw up nitrogen to maintain a suitable carbon/nitrogen ratio.
The good news is that the nitrogen extraction is very small and for most practical purposes can be ignored.
Not so if woody mulch is worked into the ground. If it is a problem it can be corrected by a light dressing of a nitrogen containing general fertiliser.

Peter’s recent three inch mulch of composted mowings and leaves won’t deplete nitrogen 
Can fresh wood chips, bark or shreddings be used without composting?

There might sometimes be slightly toxic content of fresh material that might damage very delicate plants.
In practice with my well established sturdy perennial borders if I find an arboriculturist anxious to rid himself of a lorry load of his shreddings I ask him to dump them and I use them straight away.

Is a gravel mulch suitable for herbaceous borders?

Not if your border is the kind where you are aways transplanting. I personally prefer bark for a herbaceous border
On the other hand I love gravel mulch around free standing specimen herbaceous plants - and shrubs too


My previous post on mulching
Hardcore gardening

Related Posts Plugin for WordPress, Blogger...