In Praise of Brush Piles (part 3)
Some science to back up what I am seeing with my own two eyes
I think most of us — at some point or another — feel the need to corroborate what we are seeing with our own two eyes, with a scientific explanation. While I knew intuitively that my woody debris collecting habits, and that my ever-growing brush piles were having a noticeable and positive impact on the growth rate of my hedge of spruce trees truth be told, I really didn’t understand why.
Oh, I had a few ideas why. I conjectured that, as they decomposed, all those sticks, logs, and leaves were adding nutrients back into the soil, while also moderating moisture around the roots of my little spruce trees. But I also laboured under a long held assumption that piling woody debris around trees, or any other plant for that matter, was a bad idea. I had been cautioned that woody debris (such as wood chips and bark mulch) actually stunted plant growth. I was told that during the process of wood decomposition, nutrients — namely nitrogen— are extracted from the soil and would impede plant growth. And yet, my healthy, happy row of thriving spruce trees seemed to be telling me the exact opposite. My spruce trees seemed to be shouting how much they loved the woody bits I had been piling around them over the years. Those trees continued getting bigger and bigger! And the deep, robust green of their needles told me that they were healthy and would likely be far more resilient to adverse environmental conditions — like the droughty summers we were starting to get more of in Nova Scotia.
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Like many folks from the baby-boomer generation, I grew up with the notion that most problems could be solved by something you could buy in a store. In the realm of gardening or farming that meant the problem of stunted plants could be solved if you just added a bit more NPK fertilizer (N -nitrogen; P-phosphorus; K-potassium) to your tilled-up soil.
Those three letters —NPK — found on the label of every commercial fertilizer you can buy have promised miracles of abundance to millions of people since the early 20th century. NPK is practically synonymous with the Green Revolution and it’s purportedly unstoppable explosion of agricultural growth.
As a kid, I remember driving with my Dad to the local farm co-op store, in Stevensville, Ontario, to buy bags of NPK fertilizer. We would then go home and apply said fertilizer to our worn out Willoughby Clay Plain garden soil —the very same soil that had at one time been home to one of the most extensive Carolinian forest complexes in the eastern section of the Niagara Peninsula. My Dad and I, however, had our sights on growing sweet corn and tomatoes - non-native species that had not evolved with the ecology of the land. It struck me then, as it horrifies me now, how those multi-coloured granules of NPK fertilizer made the earthworms writhe in pain, even as they burned my skin, sifting through my fingers into the long, hand-hoed rows where we planted our corn seed.
Its turning out that this purported miracle of abundance is, in fact, a mirage. Plants (even corn and tomatoes) need more than NPK if they are going to thrive. Even more importantly, our precious and uniquely evolved soil, which my Dad and I were unwittingly and continuously denaturing, needed a whole lot more than a yearly drenching of NPK, if it wasn’t going to degrade permanently into hardpan.
What our garden soil really needed was a return to the forest cycles that had co-evolved the complex relationship of organic and inorganic substances — unique to that place. At one time, these complex relationships had enabled botanicals to flourish, such as spring beauty (Claytonia virginica), pin oak (Quercus palustris), wild geranium (Geranium maculatum) and shagbark hickory (Carya ovata). And while there were still a few small pockets of Carolinian flora that grew near my home, most of the native plants had vanished, as farmland and other land clearing activities intensified. And as the native trees and plants disappeared, so too did their innate possibilities of food, fodder, and medicine.
While NPK still holds a significant sway with a lot of folks in the agri-industry sector, within the past few decades more and more scientists have started asking different question about how best to bolster soil fertility, and regenerate the ecological integrity of denatured land. Not surprisingly, the sector most interested in investigating the merits of using woody debris to restore degraded soil is the forest industry. Increasingly, forest science is showing that woody debris is integral to restoring soil health and ecological integrity after disturbances - such as fire, forest clearing and landslides.
Following a landslide in the Karpaty Mountains, in Poland, researchers learned that the microbial activity in soil can be effectively stimulated by leaving deadwood on the landslide surface. These same microorganisms were positively correlated with the rate of nutrient release, in particular that of carbon — an essential component for building tree and plant tissue (1). Similarly, another study noted that decomposing coarse woody debris (CWD) released “carbon rich, acidic dissolved organic matter (DOM). Leaving coarse woody debris on land that had been deforested had the effect of creating ‘islands of fertility” (2).
But what about the concern that decomposing woody debris ties up nutrients critical for plant growth? Here is what I found out in my literature review: When microbes become active in decomposing woody debris, they do draw upon plant-available forms of nitrogen in the soil to fuel their own activities. However, as decomposition proceeds and the decomposing material becomes exhausted, microbial populations decline. At this point nitrogen is once again available to vascular plants. Within this natural cycle, nitrogen, which is critical for plant growth is only temporarily immobilized. Woody debris can, in fact, serve as a nutrient reservoir and assist in regulating nutrient cycling and productivity in forest ecosystems (3).
The size and form of woody debris is also an important consideration. There is a big difference between applying chipped wood or using larger sized woody debris to achieve the desired results of slowly releasing nutrients back into the soil:
“Harvey (1993) also recommended providing coarse, large down wood as sources of soil wood for future nitrogen and nutrient sources, and leaf litter, small twigs, and roots as more immediate sources of nitrogen. But it is only as large chunks does decaying wood provide its most beneficial, long-term (“time-released”) ecological services. Large decaying wood provides an acidic, high-phenolic, lignin matrix that best serves conifers and certain soil microbes (but not herbaceous plants and other microbes). Coarse wood in the soil is a very unique and critical element for forest productivity.
Chipping of fuel wood and distributing the chips on site does not seem to be an ecologically viable way of reducing excess fuels. In one experiment in a high-elevation forest in Wyoming, it was found that rainfall leached large amounts of toxic, water-soluble phenolics from the chips, and as a result all tree seedlings dies on the site (Harvey 1993). This also caused blocked soil structure following winter freeze” (4).
But can the same re-generated soil ecologies that grow healthy forests, also grow vegetables for human consumption? This question is a bit more difficult to answer, because forest soils have more variables than just a complex of chemical properties. True forest soils exhibit physical properties —eg. distinct soil horizons — that are quite different from the homogeneous properties most vegetable gardeners strive for. Trees and herbaceous forest dwelling botanicals have deep, tenacious root systems that are capable of extending well beyond what is visible above ground. Most vegetable plants have root systems that are more limited in their scope of growth.
In addition, most of the vegetables we like to grow in our gardens are sun loving plants, which do not thrive under true forest conditions. To address these differences of botanical preferences while simultaneously doing all that we can to revitalize forest/woodland ecosystems requires us to think outside the box. It is this kind of thinking that has led many to explore the benefits of forest garden design, and biomimicry.
Within the realm of forest gardening there are a host of tools we can employ that offer opportunities for regenerating soil. By using woody debris in our garden (and crop field) designs — such as brush piles/windrows and hugelkultur structures — we can create vital nutrient and microbial dense sites wherein we can plant fruit or nut trees, or other perennials plants. We can use our woody debris piles to make windbreaks and create microclimates or more tender crops. These are just a couple of ways that we can realize the benefits of forest ecosystems, while maintaining the qualities of a vegetable garden (eg. access to sunlight) required for growing annual food crops. As with our gardens and forest lands, our own creativity can grow, when we pay closer attention to what we see with our own two eyes.
Wojciech Piaszczyk,Jarosław Lasota , Grzegorz Gaura,and Ewa Błonska. (2021). Effect of Deadwood Decomposition on the Restoration of Soil Cover in Landslide Areas of the Karpaty Mountains, Poland. (Forests, vol 2).
J.D.H. SPEARS* and K. LAJTHA. ( 2004) .The imprint of coarse woody debris on soil chemistry in the western Oregon Cascades. Biogeochemistry 71: 163–175.
J. K. Zimmerman,W.M. Pulliam,D. J. Lodge,V.Quifiones-Orfila,N. Fetcher,S. Guzman-Grajales,J. A. Parrotta, C. E. Asbury (t), L. R. Walker, and R. B. Waide
Oikos 72: 314-322 (1995). Nitrogen immbolization by decomposing woody debris and the recovery of tropical wet forest from hurricane damage. Oikos vol. 72, 314-322.
Bruce G. Marcot, (2003). Ecosystem Processes Related to Wood Decay. https://apps.fs.usda.gov/r6_decaid/legacy/decaid/pages/Ecosystem-Processes.html.
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Thank you again, Ruth, for sharing such valuable insights and information, and putting this together in a fascinating way!
Thx Ruth for putting this together. Brush piles make so much sense. We really do have to learn very different ways of managing nutrients. Another aspect of "NPK addiction": pests. Nitrogen fertilizer especially stimulates many pests and diseases, also fast growing weeds, so we in turn 'need' pesticides, fungicides, herbicides.