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The growth pulse: why pruning stimulates neighbouring plants

For those of you who have sometimes wondered or have been asked the question: "where can I find out more about this syntropic pulsing effect?", I will share here two peer-reviewed papers that might be useful. I get asked this question quite often. Most people have never heard the idea that pruning or harvesting a plant (that is, mimicking herbivory or destructive weather) actively stimulates the growth of the neighbouring plants (especially in an undisturbed, no-till soil). In my experience, if you have worked with mixed systems of annuals and perennials, chances are you will have noticed this enough times to find this concept quite believable. But not everybody has, and not everybody is prone to trust anecdotal evidence in their practice - which is fair enough.

So, here is how I tell this story to fellow growers or scientific/academically-minded folks who question the validity of this claim. Essentially there are two steps:

  1. Plants respond to pruning or herbivory by reallocating carbohydrates to the below-ground parts of the plant and increasing exudation rates. This, in turn, increases microbial activity in the rhizosphere. This effect is accompanied by a rejuvenation of the whole plant, which can live a lot longer by delaying senescence.

  2. The increased microbial activity benefits neighbouring plants, via increased mineralisation (more microbes make more soluble nutrients available). Some people believe that this is also mediated by the mycorrhizal network via gibberellin (a plant hormone released by roots) - more on this in Ernst Gotsch article linked below.

  3. This processed is hormonally reversed when plants develop flowers and start to ripen their seeds. At this stage, the plant becomes more demanding and competitive.

Step 1 has been examined and confirmed in several studies (two of these are linked below). Steps 2 and 3 are commonly observed in real-life situations, and there is currently very limited academic evidence.

Why this happens though, is a lot easier to understand. Ecologically, herbivory is a disturbance stimulus that introduces light to lower strata/layers of the ecosystem, and this stimulates neighbouring plants to increase photosynthetic rates. It can be assumed that a radical equivalent of this effect has also developed in co-evolution, so as to allow gaps to be filled by existing surrounding diversity. Disturbance creates gaps and increased root activity, and this in turn "pulses" or "triggers" the entire patch. Thus, when the canopy of a plant is pruned, not only light is introduced to the lower layers, but a “growth pulse” is propagated into the system, and felt by the neighbouring plants.

When the first westerners colonised North America, they reported that after clearing a forested area, for a few years they were blessed with the best soil and growing conditions they had and would ever experience. What they were doing was creating a gap in a well-established ecosystem. In the gap, soil is at its peak fertility, the extra light creates the perfect conditions for vegetable and fruit production, and the pulse generated by the felling and the pruning enhances this effect. (More on this in one of my articles, linked below)






REFERENCES
PHOTOS

- Living Soil Garden, High density interplanting of biennial and annual vegetables. (facebook.com/goffinLSG)

- Terragenera, young Syntropic food forest designed by Matteo Siroli


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